CHANGELOG.md

# CHANGELOG

> [!NOTE]
> This is in reverse chronological order, so newer entries are added to the top.

## Swift 6.2

* [SE-0472][]:
  Introduced new `Task.immediate` and `taskGroup.addImmediateTask` APIs, which allow a task to run "immediately" in the
  calling context if its isolation is compatible with the enclosing one. This can be used to create tasks which execute 
  without additional scheduling overhead, and allow for finer-grained control over where a task begins running.

  The canonical example for using this new API is using an unstructured immediate task like this:
  
  ```swift
  func synchronous() { // synchronous function
  // executor / thread: "T1"
  let task: Task<Void, Never> = Task.immediate {
  // executor / thread: "T1"
  guard keepRunning() else { return } // synchronous call (1)
  
      // executor / thread: "T1"
      await noSuspension() // potential suspension point #1 // (2)
      
      // executor / thread: "T1"
      await suspend() // potential suspension point #2 // (3), suspend, (5)
      // executor / thread: "other"
  }
  
  // (4) continue execution
  // executor / thread: "T1"
  }
  ```

* [SE-0471][]:
  Actor and global actor annotated types may now declare a synchronous `isolated deinit`, which allows such deinitializer
  to access actor isolated state while deinitializing the actor. This enables actor deinitializers to safely access
  and shut down or close resources during an actors deinitialization, without explicitly resorting to unstructured 
  concurrency tasks.

  ```swift
  class NonSendableAhmed { 
    var state: Int = 0
  }

  @MainActor
  class Maria {
    let friend: NonSendableAhmed

    init() {
      self.friend = NonSendableAhmed()
    }

    init(sharingFriendOf otherMaria: Maria) {
      // While the friend is non-Sendable, this initializer and
      // and the otherMaria are isolated to the MainActor. That is,
      // they share the same executor. So, it's OK for the non-Sendable value
      // to cross between otherMaria and self.
      self.friend = otherMaria.friend
    }
    
    isolated deinit {
      // Used to be a potential data race. Now, deinit is also
      // isolated on the MainActor, so this code is perfectly
      // correct.
      friend.state += 1
    }
  }
    
  func example() async {
    let m1 = await Maria()
    let m2 = await Maria(sharingFriendOf: m1)
    doSomething(m1, m2)
  }
  ```

* [SE-0469][]:
  Swift concurrency tasks (both unstructured and structured, via the TaskGroup `addTask` APIs) may now be given 
  human-readable names, which can be used to support debugging and identifying tasks.

  ```swift
  let getUsers = Task("Get Users for \(accountID)") {
    await users.get(accountID)
  }
  ```

* [SE-0462][]:
  Task priority escalation may now be explicitly caused to a `Task`, as well as reacted to using the new task priority escalation handlers:      

  ```swift
  // priority: low
  // priority: high!
  await withTaskPriorityEscalationHandler {
  await work()
  } onPriorityEscalated: { newPriority in // may not be triggered if ->high escalation happened before handler was installed
  // do something
  }
  ```
* [SE-0461][]:
  Nonisolated asynchronous functions may now execute on the calling actor, when the upcoming feature `NonisolatedNonsendingByDefault`
  is enabled, or when explicitly opted-into using the `nonisolated(nonsending)` keywords. This allows for fine grained control
  over where nonisolated asynchronous functions execute, and allows for the default behavior of their execution to be changed
  from always executing on the global concurrent pool, to the calling actor, which can yield noticeable performance improvements 
  thanks to less executor hopping when nonisolated and isolated code is invoked in sequence. 
  
  This also allows for safely using asynchronous functions on non-sendable types from actors, like so:

  ```swift
  class NotSendable {
    func performSync() { ... }

    nonisolated(nonsending)
    func performAsync() async { ... }
  }

  actor MyActor {
    let x: NotSendable

    func call() async {
      x.performSync() // okay

      await x.performAsync() // okay
    }
  }
  ```

* The Swift compiler no longer diagnoses references to declarations that are
  potentially unavailable because the platform version might not be new enough
  when those references occur inside of contexts that are also unavailable to
  that platform. This addresses a long-standing nuisance for multi-platform
  code. However, there is also a chance that existing source code may become
  ambiguous as a result:

  ```swift
  struct A {}
  struct B {}

  func potentiallyAmbiguous(_: A) {}

  @available(macOS 99, *)
  func potentiallyAmbiguous(_: B) {}

  @available(macOS, unavailable)
  func unavailableOnMacOS() {
    potentiallyAmbiguous(.init()) // error: ambiguous use of 'init()'
  }
  ```

  Code that is now ambiguous as a result should likely be restructured since
  disambiguation based on platform introduction alone has never been a reliable
  strategy, given that the code would eventually become ambiguous anyways when
  the deployment target is raised.

* [SE-0470][]:
  A protocol conformance can be isolated to a specific global actor, meaning that the conformance can only be used by code running on that actor. Isolated conformances are expressed by specifying the global actor on the conformance itself:

  ```swift
  protocol P {
    func f()
  }

  @MainActor
  class MyType: @MainActor P {
    /*@MainActor*/ func f() {
      // must be called on the main actor
    }
  }
  ```

  Swift will produce diagnostics if the conformance is directly accessed in code that isn't guaranteed to execute in the same global actor. For example:

  ```swift
  func acceptP<T: P>(_ value: T) { }

  /*nonisolated*/ func useIsolatedConformance(myType: MyType) {
    acceptP(myType) // error: main actor-isolated conformance of 'MyType' to 'P' cannot be used in nonisolated context
  }
  ```

  To address such issues, only use an isolated conformance from code that executes on the same global actor.

* [SE-0419][]:
  Introduced the new `Runtime` module, which contains a public API that can
  generate backtraces, presently supported on macOS and Linux.  Capturing a
  backtrace is as simple as

  ```swift
  import Runtime

  func foo() {
    // Without symbols
    let backtrace = try! Backtrace.capture()

    print(backtrace)

    // With symbol lookup
    let symbolicated = backtrace.symbolicated()!

    print(symbolicated)
  }
  ```

* [SE-0458][]:
  Introduced an opt-in mode for strict checking of memory safety, which can be
  enabled with the compiler flag `-strict-memory-safety`. In this mode,
  the Swift compiler will produce warnings for uses of memory-unsafe constructs
  and APIs. For example, 

  ```swift
  func evilMalloc(size: Int) -> Int {
    // use of global function 'malloc' involves unsafe type 'UnsafeMutableRawPointer'
    return Int(bitPattern: malloc(size))
  }
  ```

  These warnings are in their own diagnostic group (`StrictMemorySafety`) and can
  be suppressed by ackwnowledging the memory-unsafe behavior, for
  example with an `unsafe` expression:

  ```swift
  func evilMalloc(size: Int) -> Int {
    return unsafe Int(bitPattern: malloc(size)) // no warning
  }
  ```

## Swift 6.1

* [#78389][]:
  Errors pertaining to the enforcement of [`any` syntax][SE-0335] on boxed
  protocol types (aka existential types), including those produced by enabling
  the upcoming feature `ExistentialAny`, are downgraded to warnings until a
  future language mode.

  These warnings can be escalated back to errors with `-Werror ExistentialAny`.

* Previous versions of Swift would incorrectly allow Objective-C `-init...`
  methods with custom Swift names to be imported as initializers, but with base 
  names other than `init`. The compiler now diagnoses these attributes and
  infers a name for the initializer as though they are not present.

* Projected value initializers are now correctly injected into calls when
  an argument exactly matches a parameter with an external property wrapper.

  For example:

  ```swift
  struct Binding {
    ...
	init(projectedValue: Self) { ... }
  }

  func checkValue(@Binding value: Int) {}

  func use(v: Binding<Int>) {
    checkValue($value: v)
	// Transformed into: `checkValue(value: Binding(projectedValue: v))`
  }
  ```

  Previous versions of the Swift compiler incorrectly omitted projected value
  initializer injection in the call to `checkValue` because the argument type
  matched the parameter type exactly.

* [SE-0444][]:
  When the upcoming feature `MemberImportVisibility` is enabled, Swift will
  require that a module be directly imported in a source file when resolving
  member declarations from that module:
  
  ```swift
  let recipe = "2 slices of bread, 1.5 tbs peanut butter".parse()
  // error: instance method 'parse()' is inaccessible due to missing import of
  //        defining module 'RecipeKit'
  // note:  add import of module 'RecipeKit'
  ```
  
  This new behavior prevents ambiguities from arising when a transitively
  imported module declares a member that conflicts with a member of a directly
  imported module.

* Syntactic SourceKit queries no longer attempt to provide information
  within the inactive `#if` regions. For example, given:

  ```swift
  #if DEBUG
  extension MyType: CustomDebugStringConvertible {
    var debugDescription: String { ... }
  }
  #endif
  ```

  If `DEBUG` is not set, SourceKit results will not involve the
  inactive code. Clients should use either SourceKit-LSP or
  swift-syntax for syntactic queries that are independent of the
  specific build configuration.

* [SE-0442][]:
  TaskGroups can now be created without explicitly specifying their child task's result types:

  Previously the child task type would have to be specified explicitly when creating the task group:

  ```swift
  await withTaskGroup(of: Int.self) { group in 
    group.addTask { 12 }

    return await group.next()
  } 
  ```

  Now the type is inferred based on the first use of the task group within the task group's body:

  ```swift
  await withTaskGroup { group in 
    group.addTask { 12 }

    return await group.next()
  } 
  ```


## Swift 6.0

### 2024-09-17 (Xcode 16.0)

* Swift 6 comes with a new language mode that prevents the risk of data races
  at compile time. This guarantee is accomplished through _data isolation_; the
  compiler will validate that data passed over a boundary between concurrently
  executing code is either safe to reference concurrently, or mutually
  exclusive access to the value is enforced.

  The data-race safety checks were previously available in Swift 5.10 through
  the `-strict-concurrency=complete` compiler flag. Complete concurrency
  checking in Swift 5.10 was overly restrictive, and Swift 6 removes many
  false-positive data-race warnings through better `Sendable` inference,
  new analysis that proves mutually exclusive access when passing values with
  non-`Sendable` type over isolation boundaries, and more.

  You can enable the Swift 6 language mode using the `-swift-version 6`
  compiler flag.

* [SE-0428][]:
  Distributed actors now have the ability to support complete split server / 
  client systems, thanks to the new `@Resolvable` macro and runtime changes.

  It is now possible to share an "API module" between a client and server 
  application, declare a resolvable distributed actor protocol with the expected 
  API contract and perform calls on it, without knowing the specific type the 
  server is implementing those actors as. 

  Declaring such protocol looks like this:

```swift
import Distributed 

@Resolvable
protocol Greeter where ActorSystem: DistributedActorSystem<any Codable> {
  distributed func greet(name: String) -> String
}
```

And the module structure to support such applications looks like this:

```
                         ┌────────────────────────────────────────┐
                         │                API Module              │
                         │========================================│
                         │ @Resolvable                            │
                         │ protocol Greeter: DistributedActor {   │
                 ┌───────┤   distributed func greet(name: String) ├───────┐
                 │       │ }                                      │       │
                 │       └────────────────────────────────────────┘       │
                 │                                                        │
                 ▼                                                        ▼   
┌────────────────────────────────────────────────┐      ┌──────────────────────────────────────────────┐
│             Client Module                      │      │               Server Module                  │
│================================================│      │==============================================│
│ let g = try $Greeter.resolve(...) /*new*/      │      │ distributed actor EnglishGreeter: Greeter {  │
│ try await greeter.hello(name: ...)             │      │   distributed func greet(name: String) {     │
└────────────────────────────────────────────────┘      │     "Greeting in english, for \(name)!"      │
/* Client cannot know about EnglishGreeter type */      │   }                                          │      
                                                        │ }                                            │
                                                        └──────────────────────────────────────────────┘
```

* [SE-0424][]:
  Serial executor gains a new customization point `checkIsolation()`, which can be
  implemented by custom executor implementations in order to provide a last resort  
  check before the isolation asserting APIs such as `Actor.assumeIsolated` or
  `assertIsolated` fail and crash.

  This specifically enables Dispatch to implement more sophisticated isolation
  checking, and now even an actor which is "on a queue which is targeting 
  another specific queue" can be properly detected using these APIs.

* Closures can now appear in pack expansion expressions, which allows you to
  construct a parameter pack of closures where each closure captures the
  corresponding element of some other parameter pack. For example:

  ```swift
  struct Manager<each T> {
    let fn: (repeat () -> (each T))

    init(_ t: repeat each T) {
      fn = (repeat { each t })
    }
  }
  ```

* [SE-0431][]:
  You can now require a function value to carry its actor isolation
  dynamically in a way that can be directly read by clients:

  ```swift
  func apply<R>(count: Int,
                operation: @isolated(any) async () -> R) async -> [R]
      where R: Sendable {
    // implementation
  }
  ```

  The isolation can read with the `.isolation` property, which has type
  `(any Actor)?`:

  ```swift
  let iso = operation.isolation
  ```

  This capability has been adopted by the task-creation APIs in the
  standard library.  As a result, creating a task with an actor-isolated
  function will now synchronously enqueue the task on the actor, which
  can be used for transitive event-ordering guarantees if the actor
  guarantees that jobs will be run in the order they are enqueued, as
  `@MainActor` does.  If the function is not explicitly isolated, Swift
  still retains the right to optimize enqueues for functions that actually
  start by doing work with different isolation from their formal isolation.

* [SE-0423][]:
  You can now use `@preconcurrency` attribute to replace static actor isolation
  checking with dynamic checks for witnesses of synchronous nonisolated protocol
  requirements when the witness is isolated. This is common when Swift programs
  need to interoperate with frameworks written in C/C++/Objective-C whose
  implementations cannot participate in static data race safety.

  ```swift
  public protocol ViewDelegateProtocol {
    func respondToUIEvent()
  }
  ```

  It's now possible for a `@MainActor`-isolated type to conform to
  `ViewDelegateProtocol` by marking conformance declaration as `@preconcurrency`:

  ```swift
  @MainActor
  class MyViewController: @preconcurrency ViewDelegateProtocol {
    func respondToUIEvent() {
      // implementation...
    }
  }
  ```

  The compiler would emit dynamic checks into the `respondToUIEvent()` witness
  to make sure that it's always executed in `@MainActor` isolated context.

  Additionally, the compiler would emit dynamic actor isolation checks for:

  - `@objc` thunks of synchronous actor-isolated members of classes.

  - Synchronous actor-isolated function values passed to APIs that
    erase actor isolation and haven't yet adopted strict concurrency checking.

  - Call-sites of synchronous actor-isolated functions imported from Swift 6 libraries.

  The dynamic actor isolation checks can be disabled using the flag
  `-disable-dynamic-actor-isolation`.

* [SE-0420][]:
  `async` functions can now explicitly inherit the isolation of their caller
  by declaring an `isolated` parameter with the default value of `#isolation`:

  ```swift
  func poll(isolation: isolated (any Actor)? = #isolation) async -> [Item] {
    // implementation
  }
  ```

  When the caller is actor-isolated, this allows it to pass isolated state
  to the function, which would otherwise have concurrency problems.  The
  function may also be able to eliminate unwanted scheduling changes, such
  as when it can quickly return in a fast path without needing to suspend.

* [SE-0418][]:

  The compiler would now automatically employ `Sendable` on functions
  and key path literal expressions that cannot capture non-Sendable values.

  This includes partially-applied and unapplied instance methods of `Sendable`
  types, as well as non-local functions. Additionally, it is now disallowed
  to utilize `@Sendable` on instance methods of non-Sendable types.

  Let's use the following type to illustrate the new inference rules:

  ```swift
  public struct User {
    var name: String

    func getAge() -> Int { ... }
  }
  ```

  Key path `\User.name` would be inferred as `WritableKeyPath<User, String> & Sendable`
  because it doesn't capture any non-Sendable values.

  The same applies to keypath-as-function conversions:

  ```swift
  let _: @Sendable (User) -> String = \User.name // Ok
  ```

  A function value produced by an un-applied reference to `getAge`
  would be marked as `@Sendable` because `User` is a `Sendable` struct:

  ```swift
  let _ = User.getAge // Inferred as `@Sendable (User) -> @Sendable () -> Int`

  let user = User(...)
  user.getAge // Inferred as `@Sendable () -> Int`
  ```

* [SE-0432][]:
  Noncopyable enums can be pattern-matched with switches without consuming the
  value you switch over:

  ```swift
  enum Lunch: ~Copyable {
    case soup
    case salad
    case sandwich
  }
  
  func isSoup(_ lunch: borrowing Lunch) -> Bool {
    switch lunch {
      case .soup: true
      default: false
    }
  }
  ```


* [SE-0429][]:
  The noncopyable fields of certain types can now be consumed individually:

  ```swift
  struct Token: ~Copyable {}

  struct Authentication: ~Copyable {
    let id: Token
    let name: String

    mutating func exchange(_ new: consuming Token) -> Token {
      let old = self.id  // <- partial consumption of 'self'
      self = .init(id: new, name: self.name)
      return old
    }
  }
  ```

* [SE-0430][]:

  Region-Based Isolation is now extended to enable the application of an
  explicit `sending` annotation to function parameters and results. A function
  parameter or result that is annotated with `sending` is required to be
  disconnected at the function boundary and thus possesses the capability of
  being safely sent across an isolation domain or merged into an actor-isolated
  region in the function's body or the function's caller respectively. Example:
  
  ```swift
  func parameterWithoutSending(_ x: NonSendableType) async {
    // Error! Cannot send a task-isolated value to the main actor!
    await transferToMainActor(x)
  }
  
  func parameterWithSending(_ x: sending NonSendableType) async {
    // Ok since `x` is `sending` and thus disconnected.
    await transferToMainActor(x)
  }
  ```

* [SE-0414][]:

  The compiler is now capable of determining whether or not a value that does
  not conform to the `Sendable` protocol can safely be sent over an isolation
  boundary. This is done by introducing the concept of *isolation regions* that
  allows the compiler to reason conservatively if two values can affect each
  other. Through the usage of isolation regions, the compiler can now prove that
  sending a value that does not conform to the `Sendable` protocol over an
  isolation boundary cannot result in races because the value (and any other
  value that might reference it) is not used in the caller after the point of
  sending allowing code like the following to compile:
  
  ```swift
  actor MyActor {
      init(_ x: NonSendableType) { ... }
  }
  
  func useValue() {
    let x = NonSendableType()
    let a = await MyActor(x) // Error without Region-Based Isolation!
  }
  ```

* [SE-0427][]:
  You can now suppress `Copyable` on protocols, generic parameters, 
  and existentials:

  ```swift
  // Protocol does not require conformers to be Copyable.
  protocol Flower: ~Copyable {
    func bloom()
  }

  // Noncopyable type
  struct Marigold: Flower, ~Copyable {
    func bloom() { print("Marigold blooming!") }
  }

  // Copyable type
  struct Hibiscus: Flower {
    func bloom() { print("Hibiscus blooming!") }
  }

  func startSeason(_ flower: borrowing some Flower & ~Copyable) {
    flower.bloom()
  }

  startSeason(Marigold())
  startSeason(Hibiscus())
  ```

  By writing `~Copyable` on a generic type, you're suppressing a default
  `Copyable` constraint that would otherwise appear on that type. This permits
  noncopyable types, which have no `Copyable` conformance, to conform to such 
  protocols and be substituted for those generic types. Full functionality of this
  feature requires the newer Swift 6 runtime.

* Since its introduction in Swift 5.1 the @TaskLocal property wrapper was used to   
  create and access task-local value bindings. Property wrappers introduce mutable storage,
  which was now properly flagged as potential source of concurrency unsafety.
 
  In order for Swift 6 language mode to not flag task-locals as potentially thread-unsafe,
  task locals are now implemented using a macro. The macro has the same general semantics 
  and usage patterns, however there are two source-break situations which the Swift 6 
  task locals cannot handle:

  Using an implicit default `nil` value for task local initialization, when combined with a type alias:
  ```swift
  // allowed in Swift 5.x, not allowed in Swift 6.x
  
  typealias MyValue = Optional<Int> 
  
  @TaskLocal
  static var number: MyValue // Swift 6: error, please specify default value explicitly
  
  // Solution 1: Specify the default value
  @TaskLocal
  static var number: MyValue = nil
  
  // Solution 2: Avoid the type-alias
  @TaskLocal
  static var number: Optional<Int>
  ```

  At the same time, task locals can now be declared as global properties, which wasn't possible before.

* Swift 5.10 missed a semantic check from [SE-0309][]. In type context, a reference to a
  protocol `P` that has associated types or `Self` requirements should use
  the `any` keyword, but this was not enforced in nested generic argument positions.
  This is now an error as required by the proposal:

  ```swift
  protocol P { associatedtype A }
  struct Outer<T> { struct Inner<U> { } }
  let x = Outer<P>.Inner<P>()  // error
  ```
  To correct the error, add `any` where appropriate, for example
  `Outer<any P>.Inner<any P>`.

* Swift 5.10 accepted certain invalid opaque return types from [SE-0346][].
  If a generic argument of a constrained opaque return type did not
  satisfy the requirements on the primary associated type, the generic
  argument was silently ignored and type checking would proceed as if it
  weren't stated. This now results in a diagnostic:

  ```swift
  protocol P<A> { associatedtype A: Sequence }
  struct G<A: Sequence>: P {}

  func f() -> some P<Int> { return G<Array<Int>>() }  // error
  ```

  The return type above should be written as `some P<Array<Int>>` to match
  the return statement. The old broken behavior in this situation can also
  be restored, by removing the erroneous constraint and using the more general
  upper bound `some P`.

* [SE-0408][]:
  A `for`-`in` loop statement can now accept a pack expansion expression,
  enabling iteration over the elements of its respective value pack. This form
  supports pattern matching, control transfer statements, and other features
  available to a `Sequence`-driven `for`-`in` loop, except for the `where`
  clause. Below is an example implementation of the equality operator for
  tuples of arbitrary length using pack iteration:

  ```swift
  func == <each Element: Equatable>(lhs: (repeat each Element),
                                    rhs: (repeat each Element)) -> Bool {

    for (left, right) in repeat (each lhs, each rhs) {
      guard left == right else { return false }
    }
    return true
  }
  ```

  The elements of the value pack corresponding to the pack expansion expression
  are evaluated on demand, meaning the i<sup>th</sup> element is evaluated on
  the i<sup>th</sup> iteration:

  ```swift
  func doSomething(_: some Any) {}

  func evaluateFirst<each T>(_ t: repeat each T) {
    for _ in repeat doSomething(each t) {
      break
    }
  }

  evaluateFirst(1, 2, 3) 
  // 'doSomething' will be called only on the first element of the pack.
  ```

* [SE-0352][]:
  The Swift 6 language mode will open existential values with
  "self-conforming" types (such as `any Error` or `@objc` protocols)
  passed to generic functions. For example:

  ```swift
  func takeError<E: Error>(_ error: E) { }

  func passError(error: any Error) {
    takeError(error)  // Swift 5 does not open `any Error`, Swift 6 does
  }
  ```

  This behavior can be enabled prior to the Swift 6 language mode
  using the upcoming language feature `ImplicitOpenExistentials`.

* [SE-0422][]:
  Non-built-in expression macros can now be used as default arguments that
  expand at each call site. For example, a custom `#CurrentFile` macro used as
  a default argument in 'Library.swift' won't be expanded to `"Library.swift"`:

  ```swift
  @freestanding(expression)
  public macro CurrentFile() -> String = ...

  public func currentFile(name: String = #CurrentFile) { name }
  ```

  Instead, it will be expanded at where the function is called:
  
  ```swift
  print(currentFile())
  // Prints "main.swift"
  ```

  The expanded code can also use declarations from the caller side context:

  ```swift
  var person = "client"
  greetPerson(/* greeting: #informalGreeting */)
  // Prints "Hi client" if macro expands to "Hi \(person)"
  ```

* [SE-0417][]:
  Tasks now gain the ability to respect Task Executor preference.
  This allows tasks executing default actors (which do not declare a custom executor),
  and nonisolated asynchronous functions to fall back to a preferred executor, rather than always
  executing on the default global pool.

  The executor preference may be stated using the `withTaskExecutorPreference` function:

  ```swift
  nonisolated func doSomething() async { ... }
  
  await withTaskExecutorPreference(preferredExecutor) {
    doSomething()
  ```

  Or when creating new unstructured or child-tasks (e.g. in a task group):

  ```swift
  Task(executorPreference: preferredExecutor) {
    // executes on 'preferredExecutor'
    await doSomething() // doSomething body would execute on 'preferredExecutor'
  }
  ```

* [SE-0413][]:

  Functions can now specify the type of error that they throw as part of the
  function signature. For example:

  ```swift
  func parseRecord(from string: String) throws(ParseError) -> Record { ... }
  ```

  A call to `parseRecord(from:)` will either return a `Record` instance or throw
  an error of type `ParseError`. For example, a `do..catch` block will infer
  the `error` variable as being of type `ParseError`:

  ```swift
  do {
    let record = try parseRecord(from: myString)
  } catch {
    // error has type ParseError
  }
  ```

  Typed throws generalizes over throwing and non-throwing functions. A function
  that is specified as `throws` (without an explicitly-specified error type) is
  equivalent to one that specifies `throws(any Error)`, whereas a non-throwing
  is equivalent to one that specifies `throws(Never)`. Calls to functions that
  are `throws(Never)` are non-throwing.

  Typed throws can also be used in generic functions to propagate error types
  from parameters, in a manner that is more precise than `rethrows`. For
  example, the `Sequence.map` operation can propagate the thrown error type from
  its closure parameter, indicating that it only throws errors of the same type
  as that closure does:

  ```swift
  extension Sequence {
    func map<T, E>(_ body: (Element) throws(E) -> T) throws(E) -> [T] { ... }
  }
  ```

  When given a non-throwing closure as a parameter, `map` will not throw.

* [#70065][]:

  With the implementation of [SE-0110][], a closure parameter syntax consisting
  of only a parameter type — and no parameter name — was accidentally made legal
  for certain unambiguous type syntaxes in Swift 4. For example:

  ```swift
  let closure = { ([Int]) in }
  ```

  Having been [gated](https://github.com/apple/swift/pull/28171) behind a
  compiler warning since at least Swift 5.2, this syntax is now rejected.

* [#71075][]:

  \_SwiftConcurrencyShims used to declare the `exit` function, even though it
  might not be available. The declaration has been removed, and must be imported
  from the appropriate C library module (e.g. Darwin or SwiftGlibc)
  
* [SE-0270][]:

  The Standard Library now provides APIs for performing collection operations
  over noncontiguous elements. For example:
  
  ```swift
  var numbers = Array(1...15)

  // Find the indices of all the even numbers
  let indicesOfEvens = numbers.indices(where: { $0.isMultiple(of: 2) })

  // Perform an operation with just the even numbers
  let sumOfEvens = numbers[indicesOfEvens].reduce(0, +)
  // sumOfEvens == 56

  // You can gather the even numbers at the beginning
  let rangeOfEvens = numbers.moveSubranges(indicesOfEvens, to: numbers.startIndex)
  // numbers == [2, 4, 6, 8, 10, 12, 14, 1, 3, 5, 7, 9, 11, 13, 15]
  // numbers[rangeOfEvens] == [2, 4, 6, 8, 10, 12, 14]
  ```
  
  The standard library now provides a new `indices(where:)` function which creates
  a `RangeSet` - a new type representing a set of discontiguous indices. `RangeSet`
  is generic over its index type and can be used to execute operations over
  noncontiguous indices such as collecting, moving, or removing elements from a
  collection. Additionally, `RangeSet` is generic over any `Comparable` collection
  index and can be used to represent a selection of items in a list or a refinement
  of a filter or search result.

## Swift 5.10

### 2024-03-05 (Xcode 15.3)

* Swift 5.10 closes all known static data-race safety holes in complete strict
concurrency checking.

  When writing code against `-strict-concurrency=complete`, Swift 5.10 will
  diagnose all potential for data races at compile time unless an explicit
  unsafe opt out, such as `nonisolated(unsafe)` or `@unchecked Sendable`, is
  used.

  For example, in Swift 5.9, the following code crashes at runtime due to a
  `@MainActor`-isolated initializer being evaluated outside the actor, but it
  was not diagnosed under `-strict-concurrency=complete`:

  ```swift
  @MainActor
  class MyModel {
    init() {
      MainActor.assertIsolated()
    }

    static let shared = MyModel()
  }

  func useShared() async {
    let model = MyModel.shared
  }

  await useShared()
  ```

  The above code admits data races because a `@MainActor`-isolated static
  variable, which evaluates a `@MainActor`-isolated initial value upon first
  access, is accessed synchronously from a `nonisolated` context. In Swift
  5.10, compiling the code with `-strict-concurrency=complete` produces a
  warning that the access must be done asynchronously:

  ```
  warning: expression is 'async' but is not marked with 'await'
    let model = MyModel.shared
                ^~~~~~~~~~~~~~
                await
  ```

  Swift 5.10 fixed numerous other bugs in `Sendable` and actor isolation
  checking to strengthen the guarantees of complete concurrency checking.

  Note that the complete concurrency model in Swift 5.10 is conservative.
  Several Swift Evolution proposals are in active development to improve the
  usability of strict concurrency checking ahead of Swift 6.

* [SE-0412][]:

  Global and static variables are prone to data races because they provide memory that can be accessed from any program context.  Strict concurrency checking in Swift 5.10 prevents data races on global and static variables by requiring them to be either:

    1. isolated to a global actor, or
    2. immutable and of `Sendable` type.

  For example:

  ```swift
  var mutableGlobal = 1
  // warning: var 'mutableGlobal' is not concurrency-safe because it is non-isolated global shared mutable state
  // (unless it is top-level code which implicitly isolates to @MainActor)

  @MainActor func mutateGlobalFromMain() {
    mutableGlobal += 1
  }

  nonisolated func mutateGlobalFromNonisolated() async {
    mutableGlobal += 10
  }

  struct S {
    static let immutableSendable = 10
    // okay; 'immutableSendable' is safe to access concurrently because it's immutable and 'Int' is 'Sendable'
  }
  ```

  A new `nonisolated(unsafe)` modifier can be used to annotate a global or static variable to suppress data isolation violations when manual synchronization is provided:

  ```swift
  // This global is only set in one part of the program
  nonisolated(unsafe) var global: String!
  ```

  `nonisolated(unsafe)` can be used on any form of storage, including stored properties and local variables, as a more granular opt out for `Sendable` checking, eliminating the need for `@unchecked Sendable` wrapper types in many use cases:

  ```swift
  import Dispatch

  // 'MutableData' is not 'Sendable'
  class MutableData { ... } 

  final class MyModel: Sendable {
    private let queue = DispatchQueue(...)
    // 'protectedState' is manually isolated by 'queue'
    nonisolated(unsafe) private var protectedState: MutableData
  }
  ```

  Note that without correct implementation of a synchronization mechanism to achieve data isolation, dynamic run-time analysis from exclusivity enforcement or tools such as the Thread Sanitizer could still identify failures.

* [SE-0411][]:

  Swift 5.10 closes a data-race safety hole that previously permitted isolated
  default stored property values to be synchronously evaluated from outside the
  actor. For example, the following code compiles warning-free under
  `-strict-concurrency=complete` in Swift 5.9, but it will crash at runtime at
  the call to `MainActor.assertIsolated()`:

  ```swift
  @MainActor func requiresMainActor() -> Int {
    MainActor.assertIsolated()
    return 0
  }

  @MainActor struct S {
    var x = requiresMainActor()
    var y: Int
  }

  nonisolated func call() async {
    let s = await S(y: 10)
  }

  await call()
  ```

  This happens because `requiresMainActor()` is used as a default argument to
  the member-wise initializer of `S`, but default arguments are always
  evaluated in the caller. In this case, the caller runs on the generic
  executor, so the default argument evaluation crashes.

  Under `-strict-concurrency=complete` in Swift 5.10, default argument values
  can safely share the same isolation as the enclosing function or stored
  property. The above code is still valid, but the isolated default argument is
  guaranteed to be evaluated in the callee's isolation domain.

## Swift 5.9.2

### 2023-12-11 (Xcode 15.1)

* [SE-0407][]:

  Member macros can specify a list of protocols via the `conformances` argument to the macro role. The macro implementation will be provided with those protocols that are listed but have not already been implemented by the type to which the member macro is attached, in the same manner as extension macros.

  ```swift
  @attached(member, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
  @attached(extension, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
  macro Codable() = #externalMacro(module: "MyMacros", type: "CodableMacro")
  ```

## Swift 5.9

### 2023-09-18 (Xcode 15.0)

* [SE-0382][], [SE-0389][], [SE-0394][], [SE-0397][]:

  Swift 5.9 includes a new macro system that can be used to eliminate boilerplate and provide new forms of expressive APIs. Macros are declared with the new `macro` introducer:

  ```swift
  @freestanding(expression)
  macro assert(_ condition: Bool) = #externalMacro(module: "PowerAssertMacros", type: "AssertMacro")
  ```

  Macros have parameter and result types, like functions, but are defined as separate programs that operate on syntax trees (using [swift-syntax][]) and produce new syntax trees that are incorporated into the program. Freestanding macros, indicated with the `@freestanding` attribute, are expanded in source code with a leading `#`:

  ```swift
  #assert(x + y == z) // expands to check the result of x + y == z and report failure if it's false
  ```

  Macros can also be marked as `@attached`, in which case they will be meaning that they will be expanded using custom attribute syntax. For example:

  ```swift
  @attached(peer, names: overloaded)
  macro AddCompletionHandler() = #externalMacro(
    module: "ConcurrencyHelperMacros",
    type: "AddCompletionHandlerMacro"
  )
  
  @AddCompletionHandler
  func fetchAvatar(from url: URL) throws -> Image { ... }
  
  // expands to...
  func fetchAvatar(from url: URL, completionHandler: @escaping (Result<Image, Error>) -> Void) {
    Task.detached {
      do {
        let result = try await fetchAvatar(from: url)
        completionHandler(.success(result))
      } catch {
        completionHandler(.failure(error))
      }
    }
  }
  ```

  Macros are implemented in separate programs, which are executed by the Swift compiler. The Swift Package Manager's manifest provides a new `macro` target type to describe macros:

  ```swift
  import PackageDescription
  import CompilerPluginSupport
  
  let package = Package(
      name: "ConcurrencyHelpers",
      dependencies: [
          .package(url: "https://github.com/apple/swift-syntax", from: "509.0.0"),
      ],
      targets: [
          .macro(name: "ConcurrencyHelperMacros",
                 dependencies: [
                     .product(name: "SwiftSyntaxMacros", package: "swift-syntax"),
                     .product(name: "SwiftCompilerPlugin", package: "swift-syntax")
                 ]),
          .target(name: "ConcurrencyHelpers", dependencies: ["ConcurrencyHelperMacros"]),
          .testTarget(name: "ConcurrencyHelperMacroTests", dependencies: ["ConcurrencyHelperMacros"]),
      ]
  )
  ```

* [SE-0380][]:

  `if` and `switch` statements may now be used as expressions to:

  * Return values from functions, properties, and closures (either with
    implicit or explicit `return`)
  * Throw errors using `throw`
  * Assign values to variables
  * Declare variables

  Each branch of the `if` or `switch` must be a single expression, the value
  of which becomes the value of the overall expression when that branch is
  chosen.

  ```swift
  let bullet =
    if isRoot && (count == 0 || !willExpand) { "" }
    else if count == 0 { "- " }
    else if maxDepth <= 0 { "▹ " }
    else { "▿ " }
  ```

  ```swift
  public static func width(_ x: Unicode.Scalar) -> Int {
    switch x.value {
      case 0..<0x80: 1
      case 0x80..<0x0800: 2
      case 0x0800..<0x1_0000: 3
      default: 4
    }
  }
  ```

* [#64927][]:

  Swift 5.9 introduces warnings that catch conversions from an inout
  argument in the caller to an `UnsafeRawPointer` in the callee
  whenever the original type contains an object reference.

  ```swift
  func inspectString(string: inout String) {
    readBytes(&string)
    // warning: forming an 'UnsafeRawPointer' to an inout variable of type String
    // exposes the internal representation rather than the string contents.
  }
  ```

  ```swift
  func inspectData(data: inout Data) {
    readBytes(&data)
    // warning: forming an 'UnsafeRawPointer' to a variable of type 'T';
    // this is likely incorrect because 'T' may contain an object reference.
  }
  ```

  Please see the "Workarounds for common cases" section link in github
  issue #64927.

* Marking stored properties as unavailable with `@available` has been banned,
  closing an unintentional soundness hole that had allowed arbitrary
  unavailable code to run and unavailable type metadata to be used at runtime:
  
  ```swift
  @available(*, unavailable)
  struct Unavailable {
    init() {
      print("Unavailable.init()")
    }
  }

  struct S {
    @available(*, unavailable)
    var x = Unavailable()
  }

  _ = S() // prints "Unavailable.init()"
  ```
  
  Marking `deinit` as unavailable has also been banned for similar reasons.

* [SE-0366][]:

  The lifetime of a local variable value can be explicitly ended using the
  `consume` operator, forwarding ownership to the surrounding call, assignment,
  or initialization without copying:

  ```swift
  var x: [String] = []
  x.append("apples")
  x.append("bananas")
  x.append("oranges")

  process(consume x) // forward the current value, without copying

  x = [] // start building a new value
  x.append("broccoli")
  x.append("cauliflower")
  x.append("asparagus")
  ...
  ```

* [SE-0377][]:

  Functions can now declare whether they take value parameters by `borrowing`
  access to a value provided by the caller, or by `consuming` a value that the
  callee is allowed to take ownership of:

  ```swift
  struct HealthyFoods {
    var values: [String] = []

    // Ask to `consume` the parameter, since we want to use it
    // to incorporate into our own `values` array
    mutating func add(_ value: consuming String) {
        values.append(value)
    }
  }
  ```

## Swift 5.8

### 2023-03-30 (Xcode 14.3)

* [SE-0376][]:

  The `@backDeployed(before:)` attribute may now be used to extend the availability of a function to OS releases prior to the introduction of that function as ABI.
  
  For example, suppose that `struct Temperature` was introduced in a macOS SDK framework in macOS 12. Later in macOS 13 the framework authors decided to add a `degreesFahrenheit` property as a convenience:
  
  ```swift
  @available(macOS 12, *)
  public struct Temperature {
    public var degreesCelsius: Double
    
    // ...
  }
  
  extension Temperature {
    @available(macOS 12, *)
    @backDeployed(before: macOS 13)
    public var degreesFahrenheit: Double {
      return (degreesCelsius * 9 / 5) + 32
    }
  }
  ```
  
  Adding the `@backDeployed` attribute to `degreesFahrenheit` enables the framework author to make this new declaration available to apps with a minimum deployment target of macOS 12, even though the ABI entry point for `degreesFahrenheit` is only present in macOS 13 and up.
  
  When a function with `@backDeployed` is called, the compiler wraps the invocation of the function in a thunk. The thunk checks whether the library entry point for the declaration is available at runtime, and invokes it if it is. Otherwise, a copy of the function that was emitted into the client is called instead.

* [#56139][]:

  Сollection downcasts in cast patterns are now supported. For example:

  ```swift
  func collectionDowncast(_ arr: [Any]) {
    switch arr {
    case let ints as [Int]:
      // ...
    case is [Bool]:
      // ...
    }
  }
  ``` 
* [SE-0370][]:

  The API of `UnsafeMutableRawPointer`, `UnsafeMutableBufferPointer`, `UnsafeMutableRawBufferPointer` were improved, adding previously missing initialization (and deinitialization) methods, including more performant initialization from `Collection` types.

  For `UnsafeMutablePointer<T>` and `UnsafeMutableBufferPointer<T>`, method names containing the word "assign" were renamed to use the word "update", and many more were added. Every multi-element initialization method of `UnsafeMutablePointer` and `UnsafeMutableBufferPointer` now has a corresponding "update" method.

  Slices of `UnsafeBufferPointer`, `UnsafeRawBufferPointer`, `UnsafeMutableBufferPointer` and `UnsafeMutableRawBufferPointer` now share the collection-like API of their base type. For example, given an initialized `b: UnsafeMutableBufferPointer<Int>`, the following lines are synonymous:
  ```swift
  b.update(repeating: 0)
  b[b.startIndex..<b.endIndex].update(repeating: 0)
  ```

* [SE-0365][]:
 
  Implicit `self` is now permitted for `weak self` captures, after `self` is unwrapped.

  For example, the usage of implicit `self` below is permitted:

  ```swift
  class ViewController {
    let button: Button

    func setup() {
        button.tapHandler = { [weak self] in
            guard let self else { return }
            dismiss() // refers to `self.dismiss()`
        }
    }

    func dismiss() { ... }
  }
  ```

  In Swift 5 language modes, implicit `self` is permitted for `weak self` captures in _non-escaping_ closures even before `self` is unwrapped. For example, this code compiles successfully in Swift 5 language mode:

  ```swift
  class ExampleClass {
    func makeArray() -> [String] {
      // `Array.map` takes a non-escaping closure:
      ["foo", "bar", "baaz"].map { [weak self] string in
        double(string) // implicitly refers to `self!.double(string)`
      }
    } 

    func double(_ string: String) -> String {
      string + string
    }
  }
  ```

  In Swift 6, the above code will no longer compile. `weak self` captures in non-escaping closures now have the same behavior as captures in escaping closures (as described in [SE-0365][]). Code relying on the previous behavior will need to be updated to either unwrap `self` (e.g. by adding a `guard let self else return` statement), or to use a different capture method (e.g. using `[self]` or `[unowned self]` instead of `[weak self]`).

* [SE-0362][]:

  The compiler flag `-enable-upcoming-feature X` can now be used to enable a specific feature `X` that has been accepted by the evolution process, but whose introduction into the language is waiting for the next major version (e.g., version 6). The `X` is specified by any proposal that falls into this category:
  * `ConciseMagicFile` enables the new `#file` semantics in [SE-0274][].
  * `ForwardTrailingClosures` disables the "backward" scanning behavior of [SE-0286][].
  * `BareSlashRegexLiterals` enables the regex literal syntax of [SE-0354][].

  Features can be detected in source code with `#if hasFeature(X)`.

## Swift 5.7

### 2022-09-12 (Xcode 14.0)

* [SE-0327][]:

  There are a few notable changes in Swift 5.7 with respect to SE-0327.

  First, the deinitializer and most kinds of initializers for `actor` types, and types constrained by a global actor like the `@MainActor`, have revised rules about what expressions are permitted in their body. The goal of these revisions has been to improve language expressivity and safety. In particular, many more programming patterns are now permitted in these initializers.

  For example, a non-async initializer of an `actor` prior to Swift 5.7 would raise a diagnostic any time `self` escapes the initializer before returning. That diagnostic's purpose was to protect against a possible data race when accessing isolated stored proeprties. But, that diagnostic was emitted even if there was no dangerous racy access.

  In Swift 5.7, the compiler now checks these initializers for dangerous accesses to isolated stored properties that occur after an escape of `self`:

  ```swift
  actor Database {
    // ... other properties ...
    var rows: Int = 0

    init(_ world: DataUser) {
      defer {
        print("last = \(self.rows)") // ❌ this access to 'rows' is illegal.
      }

      print("before = \(self.rows)") // ✅ this access to 'rows' is OK
      world.publishDatabase(self)    // ✅ passing 'self' is OK in Swift 5.7+
      print("after = \(self.rows)")  // ❌ this access to 'rows' is illegal. 

      Task { [weak self] in          // ✅ capturing 'self' is OK in Swift 5.7+
        while let db = self { await db.prune() }
      }
    }
  }
  ```

  This is a control-flow sensitive check, meaning an illegal access does not necessarily appear on a source line after an escape of `self` (in the example above, consider _when_ the `defer` is executed). The compiler will always point out one of the escapes of `self` that is causing an access to become illegal.

  Next, delegating initializers of an actor are no longer always non-isolated. This means an `async` delegating initializer can do the same things as a non-delegating one.

  Finally, the diagnostic about non-isolated default-value expressions introduced for Swift 5.6 in the Xcode 13.3 release has been removed. The proposed rule was not precise enough to avoid flagging an innocuous yet common pattern in SwiftUI code involving `@StateObject` properties and `@MainActor`.

* The Swift compiler no longer warns about redundant requirements in generic declarations. For example,
  the following code diagnosed a warning in Swift 5.6 about the `T.Iterator : IteratorProtocol`
  requirement being redundant, because it is implied by `T : Sequence`:

  ```swift
  func firstElement<T: Sequence>(_: T) -> T.Element where T.Iterator: IteratorProtocol {...}
  ```

  A redundant requirement does not indicate a coding error, and sometimes it is desirable to spell them
  out for documentation purposes. For this reason these warnings are now disabled by default.

  To restore the previous behavior, pass the `-Xfrontend -warn-redundant-requirements`
  compiler flag.

* [SE-0338][]:

  Non-isolated async functions now always execute on the global concurrent pool,
  so calling a non-isolated async function from actor-isolated code will leave
  the actor. For example:

  ```swift
  class C { }

  func f(_: C) async { /* always executes on the global concurrent pool */ }

  actor A {
    func g(c: C) async {
      /* always executes on the actor */
      print("on the actor")

      await f(c)
    }
  }
  ```

  Prior to this change, the call from `f` to `g` might have started execution of
  `g` on the actor, which could lead to actors being busy longer than strictly
  necessary. Now, the non-isolated async function will always hop to the global
  cooperative pool, not run on the actor. This can result in a behavior change
  for programs that assumed that a non-isolated async function called from a
  `@MainActor` context will be executed on the main actor, although such
  programs were already technically incorrect.

  Additionally, when leaving an actor to execution on the global cooperative
  pool, `Sendable` checking will be performed, so the compiler will emit a
  diagnostic in the call to `f` if `c` is not of `Sendable` type.

* [SE-0350][]:

  The standard library has a new `Regex<Output>` type.

  This type represents an _extended regular expression_, allowing more fluent
  string processing operations. A `Regex` may be created by
  [initialization from a string][SE-0355]:

  ```swift
  let pattern = "a[bc]+" // matches "a" followed by one or more instances
                         // of either "b" or "c"
  let regex = try! Regex(pattern)
  ```

  Or via a [regex literal][SE-0354]:

  ```swift
  let regex = #/a[bc]+/#
  ```

  In Swift 6, `/` will also be supported as a delimiter for `Regex` literals.
  You can enable this mode in Swift 5.7 with the `-enable-bare-slash-regex`
  flag. Doing so will cause some existing expressions that use `/` as an 
  operator to no longer compile; you can add parentheses or line breaks as a
  workaround.

  There are [new string-processing algorithms][SE-0357] that support
  `String`, `Regex` and arbitrary `Collection` types.

* [SE-0329][]:

  New types representing time and clocks were introduced. This includes a protocol `Clock` defining clocks which allow for defining a concept of now and a way to wake up after a given instant. Additionally a new protocol `InstantProtocol` for defining instants in time was added. Furthermore a new protocol `DurationProtocol` was added to define an elapsed duration between two given `InstantProtocol` types. Most commonly the `Clock` types for general use are the `SuspendingClock` and `ContinuousClock` which represent the most fundamental clocks for the system. The `SuspendingClock` type does not progress while the machine is suspended whereas the `ContinuousClock` progresses no matter the state of the machine. 

  ```swift
  func delayedHello() async throws {
    try await Task.sleep(until: .now + .milliseconds(123), clock: .continuous)
    print("hello delayed world")
  }
  ```

  `Clock` also has methods to measure the elapsed duration of the execution of work. In the case of the `SuspendingClock` and `ContinuousClock` this measures with high resolution and is suitable for benchmarks.

  ```swift
  let clock = ContinuousClock()
  let elapsed = clock.measure {
    someLongRunningWork()
  }
  ```

* [SE-0309][]:

  Protocols with associated types and `Self` requirements can now be used as the
  types of values with the `any` keyword.

  Protocol methods that return associated types can be called on an `any` type;
  the result is type-erased to the associated type's upper bound, which is another
  `any` type having the same constraints as the associated type. For example:

  ```swift
  protocol Surface {...}
  
  protocol Solid {
    associatedtype SurfaceType: Surface
    func boundary() -> SurfaceType
  }
  
  let solid: any Solid = ...
  
  // Type of 'boundary' is 'any Surface'
  let boundary = solid.boundary()
  ```

  Protocol methods that take an associated type or `Self` cannot be used with `any`,
  however in conjunction with [SE-0352][], you can pass the `any` type to a function
  taking a generic parameter constrained to the protocol. Within the generic context,
  type relationships are explicit and all protocol methods can be used.

* [SE-0346][]:

  Protocols can now declare a list of one or more _primary associated types_, which enable writing same-type requirements on those associated types using angle bracket syntax:

  ```swift
  protocol Graph<Vertex, Edge> {
    associatedtype Vertex
    associatedtype Edge
  }
  ```

  You can now write a protocol name followed by type arguments in angle brackets, like
  `Graph<Int, String>`, anywhere that a protocol conformance requirement may appear:

  ```swift
  func shortestPath<V, E>(_: some Graph<V, E>, from: V, to: V) -> [E]

  extension Graph<Int, String> {...}

  func build() -> some Graph<Int, String> {}
  ```

  A protocol name followed by angle brackets is shorthand for a conformance requirement,
  together with a same-type requirement for the protocol's primary associated types.
  The first two examples above are equivalent to the following:

  ```swift
  func shortestPath<V, E, G>(_: G, from: V, to: V) -> [E]
    where G: Graph, G.Vertex == V, G.Edge == E

  extension Graph where Vertex == Int, Edge == String {...}
  ```

  The `build()` function returning `some Graph<Int, String>` can't be written using a
  `where` clause; this is an example of a constrained opaque result type, which is new expressivity in Swift 5.7.

* [SE-0353][]:

  Protocols with primary associated types can now be used in existential types,
  enabling same-type constraints on those associated types.

  ```swift
  let strings: any Collection<String> = [ "Hello" ]
  ```

  Note that language features requiring runtime support like dynamic casts
  (`is`, `as?`, `as!`), as well as generic usages of parameterized existentials
  in generic types (e.g. `Array<any Collection<Int>>`) involve additional
  availability checks to use. Back-deploying usages in generic position can be
  worked around with a generic type-erasing wrapper struct, which is now much
  simpler to implement:

  ```swift
  struct AnyCollection<T> {
    var wrapped: any Collection<T>
  }

  let arrayOfCollections: [AnyCollection<T>] = [ /**/ ]
  ```

* [SE-0358][]:

  Various protocols in the standard library now declare primary associated types, for
  example `Sequence` and `Collection` declare a single primary associated type `Element`.
  For example, this allows writing down the types `some Collection<Int>` and
  `any Collection<Int>`.

* References to `optional` methods on a protocol metatype, as well as references to dynamically looked up methods on `AnyObject` are now supported on par with other function references. The type of such a reference (formerly an immediate optional by mistake) has been altered to that of a function that takes a single argument and returns an optional value of function type:

  ```swift
  class Object {
    @objc func getTag() -> Int { ... }
  }

  let getTag: (AnyObject) -> (() -> Int)? = AnyObject.getTag

  @objc protocol Delegate {
    @objc optional func didUpdateObject(withTag tag: Int)
  }

  let didUpdateObjectWithTag: (Delegate) -> ((Int) -> Void)? = Delegate.didUpdateObject
  ```

  > **Warning**  
  > Due to the type change, selectors for aforementioned method references that require writing out their type explicitly for disambiguation will no longer compile. To fix this, simply adjust the written type, or resort to a `#if swift(<5.7)` directive when compatibility with older compiler versions is warranted. For example:
  >
  > ```swift
  > #if swift(<5.7)
  > let decidePolicyForNavigationAction = #selector(WKNavigationDelegate.webView(_:decidePolicyFor:decisionHandler:) as ((WKNavigationDelegate) -> (WKWebView, WKNavigationAction, @escaping (WKNavigationActionPolicy) -> Void) -> Void)?)
  > #else
  > let decidePolicyForNavigationAction = #selector(WKNavigationDelegate.webView(_:decidePolicyFor:decisionHandler:) as (WKNavigationDelegate) -> ((WKWebView, WKNavigationAction, @escaping (WKNavigationActionPolicy) -> Void) -> Void)?)
  > #endif
  > ```

* [SE-0349][]:

  Loading data from raw memory represented by `UnsafeRawPointer`,
  `UnsafeRawBufferPointer` and their mutable counterparts now supports unaligned
  accesses. This previously required a workaround involving an intermediate
  copy:

  ```swift
  let result = unalignedData.withUnsafeBytes { buffer -> UInt32 in
    var storage = UInt32.zero
    withUnsafeMutableBytes(of: &storage) {
      $0.copyBytes(from: buffer.prefix(MemoryLayout<UInt32>.size))
    }
    return storage
  }
  ```
  Now:
  ```swift
  let result = unalignedData.withUnsafeBytes { $0.loadUnaligned(as: UInt32.self) }
  ```
  Additionally, the counterpart `storeBytes(of:toByteOffset:as:)` had its
  alignment restriction lifted, so that storing to arbitrary offsets of raw
  memory can now succeed.

* [SE-0334][]:

  - `UnsafeRawPointer` and `UnsafeMutableRawPointer` have new functionality for
  pointer arithmetic, adding functions to obtain a pointer advanced to the next
  or previous alignment boundary:

    ```swift
    extension UnsafeRawPointer {
      public func alignedUp<T>(for: T.type) -> UnsafeRawPointer
      public func alignedDown<T>(for: T.type) -> UnsafeRawPointer
      public func alignedUp(toMultipleOf alignment: Int) -> UnsafeRawPointer
      public func alignedDown(toMultipleOf alignment: Int) -> UnsafeRawPointer
    }
    ```
  - It is now possible to use a pointer to `struct` to obtain a pointer to one
  of its stored properties:

    ```swift
    withUnsafeMutablePointer(to: &myStruct) {
      let interiorPointer = $0.pointer(to: \.myProperty)!
      return myCFunction(interiorPointer)
    }
    ```
  - Comparisons between pointers have been simplified by being more permissive.
  Since pointers are representations of memory locations within a single pool of
  underlying memory, Swift now allows comparing pointers without requiring type
  conversions with the `==`, `!=`, `<`,`<=`,`>`, and `>=` operators. 

* [SE-0333][]:

  It is now possible to use the `withMemoryRebound<T>()` method on raw memory,
  that is `UnsafeRawPointer` , `UnsafeRawBufferPointer` and their mutable
  counterparts. Additionally, we clarified the semantics of
  `withMemoryRebound<T>()` when used on typed memory (`UnsafePointer<Pointee>`,
  `UnsafeBufferPointer<Pointee>` and their mutable counterparts). Whereas
  `Pointee` and `T` were previously required to have the same stride, you can
  now rebind in cases where `Pointee` is an aggregate of `T` or vice-versa. For
  example, given an `UnsafeMutableBufferPointer<CGPoint>`, you can now use
  `withMemoryRebound` to operate temporarily on a
  `UnsafeMutableBufferPointer<CGFloat>`, because `CGPoint` is an aggregate of
  `CGFloat`.

* [SE-0352][]:

  It's now possible to call a generic function with a value of protocol type
  in places that would previously fail because `any` types do not conform
  to their protocols. For example:

  ```swift
  protocol P {
    associatedtype A
    func getA() -> A
  }

  func takeP<T: P>(_ value: T) { }

  func test(p: any P) {
    takeP(p) // was an error "type 'any P' cannot conform to 'P'", now accepted
  }
  ```

  This operates by "opening" the value of protocol type and passing the
  underlying type directly to the generic function.

* [SE-0347][]:

  It's now possible to use a default value expression with a generic parameter type
  to default the argument and its type:

  ```swift
  func compute<C: Collection>(_ values: C = [0, 1, 2]) {
    ...
  }
  ```

  `compute` is now accepted by compiler and `[Int]` is going to be inferred
  for `C` at call sites that do not provide the argument explicitly.

* [SE-0326][]:

  It's now possible to infer parameter and result types from the body of a multi-statement
  closure. The distinction between single- and multi-statement closures has been removed.

  Use of closures becomes less cumbersome by removing the need to constantly specify explicit
  closure types which sometimes could be pretty large e.g. when there are multiple parameters
  or a complex tuple result type.

  For example:

  ```swift
  func map<T>(fn: (Int) -> T) -> T {
    return fn(42)
  }

  func computeResult<U: BinaryInteger>(_: U) -> U { /* processing */ }

  let _ = map {
    if let $0 < 0 {
       // do some processing
    }

    return computeResult($0)
  }
  ```

  The result type of `map` can now be inferred from the body of the trailing closure
  passed as an argument.

* [SE-0345][]:

  It is now possible to unwrap optional variables with a shorthand syntax that
  shadows the existing declaration. For example, the following:
  
  ```swift
  let foo: String? = "hello world"
  
  if let foo {
    print(foo) // prints "hello world"
  }
  ```
  
  is equivalent to:
  
  ```swift
  let foo: String? = "hello world"
  
  if let foo = foo {
    print(foo) // prints "hello world"
  }
  ```

* [SE-0340][]:

  It is now possible to make declarations unavailable from use in asynchronous
  contexts with the `@available(*, noasync)` attribute.

  This is to protect the consumers of an API against undefined behavior that can
  occur when the API uses thread-local storage, or encourages using thread-local
  storage, across suspension points, or protect developers against holding locks
  across suspension points which may lead to undefined behavior, priority
  inversions, or deadlocks.

* [SE-0343][]:

  Top-level scripts support asynchronous calls.

  Using an `await` by calling an asynchronous function or accessing an isolated
  variable transitions the top-level to an asynchronous context. As an
  asynchronous context, top-level variables are `@MainActor`-isolated and the
  top-level is run on the `@MainActor`.

  Note that the transition affects function overload resolution and starts an
  implicit run loop to drive the concurrency machinery.

  Unmodified scripts are not affected by this change unless `-warn-concurrency` is
  passed to the compiler invocation. With `-warn-concurrency`, variables in the
  top-level are isolated to the main actor and the top-level context is isolated
  to the main actor, but is not an asynchronous context.

* [SE-0336][]:

  It is now possible to declare `distributed actor` and `distributed func`s inside of them.

  Distributed actors provide stronger isolation guarantees than "local" actors, and enable additional checks to be made on return types and parameters of distributed methods, e.g. checking if they conform to `Codable`. Distributed methods can be called on "remote" references of distributed actors, turning those invocations into remote procedure calls, by means of pluggable and user extensible distributed actor system implementations. 
  
  Swift does not provide any specific distributed actor system by itself, however, packages in the ecosystem fulfill the role of providing those implementations.
  
  ```swift
  distributed actor Greeter { 
    var greetingsSent = 0
    
    distributed func greet(name: String) -> String {
      greetingsSent += 1
      return "Hello, \(name)!"
    }
  }
  
  func talkTo(greeter: Greeter) async throws {
    // isolation of distributed actors is stronger, it is impossible to refer to
    // any stored properties of distributed actors from outside of them:
    greeter.greetingsSent // distributed actor-isolated property 'name' can not be accessed from a non-isolated context
    
    // remote calls are implicitly throwing and async, 
    // to account for the potential networking involved:
    let greeting = try await greeter.greet(name: "Alice")
    print(greeting) // Hello, Alice!
  }
  ```

* The compiler now emits a warning when a non-final class conforms to a protocol that imposes a same-type requirement between `Self` and an associated type. This is because such a requirement makes the conformance unsound for subclasses.

  For example, Swift 5.6 would allow the following code, which at runtime would construct an instance of `C` and not `SubC` as expected:

  ```swift
  protocol P {
    associatedtype A : Q where Self == Self.A.B
  }
  
  protocol Q {
    associatedtype B
  
    static func getB() -> B
  }
  
  class C : P {
    typealias A = D
  }
  
  class D : Q {
    typealias B = C
  
    static func getB() -> C { return C() }
  }
  
  extension P {
    static func getAB() -> Self {
      // This is well-typed, because `Self.A.getB()` returns
      // `Self.A.B`, which is equivalent to `Self`.
      return Self.A.getB()
    }
  }
  
  class SubC : C {}
  
  // P.getAB() declares a return type of `Self`, so it should
  // return `SubC`, but it actually returns a `C`.
  print(SubC.getAB())
  ```

  To make the above example correct, either the class `C` needs to become `final` (in which case `SubC` cannot be declared) or protocol `P` needs to be re-designed to not include the same-type requirement `Self == Self.A.B`.

* [SE-0341][]:

  Opaque types can now be used in the parameters of functions and subscripts, when they provide a shorthand syntax for the introduction of a generic parameter. For example, the following:

  ```swift
  func horizontal(_ v1: some View, _ v2: some View) -> some View {
    HStack {
      v1
      v2
    }
  }
  ```

  is equivalent to

  ```swift
  func horizontal<V1: View, V2: View>(_ v1: V1, _ v2: V2) -> some View {
    HStack {
      v1
      v2
    }
  }
  ```

  With this, `some` in a parameter type provides a generalization where the
  caller chooses the parameter's type as well as its value, whereas `some` in
  the result type provides a generalization where the callee chooses the
  resulting type and value.

* The compiler now correctly emits errors for `@available` attributes on stored properties with the `lazy` modifier or with attached property wrappers. Previously, the attribute was accepted on this subset of stored properties but the resulting binary would crash at runtime when type metadata was unavailable.

  ```swift
  struct S {
    @available(macOS 99, *) // error: stored properties cannot be marked potentially unavailable with '@available'
    lazy var a: Int = 42

    @available(macOS 99, *) // error: stored properties cannot be marked potentially unavailable with '@available'
    @Wrapper var b: Int
  }
  ```

* The compiler now correctly emits warnings for more kinds of expressions where a protocol conformance is used and may be unavailable at runtime. Previously, member reference expressions and type erasing expressions that used potentially unavailable conformances were not diagnosed, leading to potential crashes at runtime.

  ```swift
  struct Pancake {}
  protocol Food {}

  extension Food {
    var isGlutenFree: Bool { false }
  }

  @available(macOS 12.0, *)
  extension Pancake: Food {}

  @available(macOS 11.0, *)
  func eatPancake(_ pancake: Pancake) {
    if (pancake.isGlutenFree) { // warning: conformance of 'Pancake' to 'Food' is only available in macOS 12.0 or newer
      eatFood(pancake) // warning: conformance of 'Pancake' to 'Food' is only available in macOS 12.0 or newer
    }
  }

  func eatFood(_ food: Food) {}
  ```

* [SE-0328][]:

  Opaque types (expressed with `some`) can now be used in structural positions
  within a result type, including having multiple opaque types in the same
  result. For example:

  ```swift
  func getSomeDictionary() -> [some Hashable: some Codable] {
    return [ 1: "One", 2: "Two" ]
  }
  ```

## Swift 5.6

### 2022-03-14 (Xcode 13.3)

* [SE-0327][]:

  In Swift 5 mode, a warning is now emitted if the default-value expression of an
  instance-member property requires global-actor isolation. For example:

  ```swift
  @MainActor
  func partyGenerator() -> [PartyMember] { fatalError("todo") }
  
  class Party {
    @MainActor var members: [PartyMember] = partyGenerator()
    //                                      ^~~~~~~~~~~~~~~~
    // warning: expression requiring global actor 'MainActor' cannot
    //          appear in default-value expression of property 'members'
  }
  ```
  
  Previously, the isolation granted by the type checker matched the isolation of
  the property itself, but at runtime that is not guaranteed. In Swift 6, 
  such default-value expressions will become an error if they require isolation.

* Actor isolation checking now understands that `defer` bodies share the isolation of their enclosing function.

  ```swift
  // Works on global actors
  @MainActor
  func runAnimation(controller: MyViewController) async {
    controller.hasActiveAnimation = true
    defer { controller.hasActiveAnimation = false }

    // do the animation here...
  }

  // Works on actor instances
  actor OperationCounter {
    var activeOperationCount = 0

    func operate() async {
      activeOperationCount += 1
      defer { activeOperationCount -= 1 }

      // do work here...
    }
  }
  ```

* [SE-0335][]:

  Swift now allows existential types to be explicitly written with the `any`
  keyword, creating a syntactic distinction between existential types and
  protocol conformance constraints. For example:

  ```swift
  protocol P {}

  func generic<T>(value: T) where T: P {
    ...
  }

  func existential(value: any P) {
     ...
  }
  ```

* [SE-0337][]:

  Swift now provides an incremental migration path to data race safety, allowing
  APIs to adopt concurrency without breaking their clients that themselves have
  not adopted concurrency. An existing declaration can introduce
  concurrency-related annotations (such as making its closure parameters
  `@Sendable`) and use the `@preconcurrency` attribute to maintain its behavior
  for clients who have not themselves adopted concurrency:

  ```swift
  // module A
  @preconcurrency func runOnSeparateTask(_ workItem: @Sendable () -> Void)

  // module B
  import A

  class MyCounter {
    var value = 0
  }

  func doesNotUseConcurrency(counter: MyCounter) {
    runOnSeparateTask {
      counter.value += 1 // no warning, because this code hasn't adopted concurrency
    }
  }

  func usesConcurrency(counter: MyCounter) async {
    runOnSeparateTask {
      counter.value += 1 // warning: capture of non-Sendable type 'MyCounter'
    }
  }
  ```

  One can enable warnings about data race safety within a module with the
  `-warn-concurrency` compiler option. When using a module that does not yet
  provide `Sendable` annotations, one can suppress warnings for types from that
  module by marking the import with `@preconcurrency`:

  ```swift
  /// module C
  public struct Point {
    public var x, y: Double
  }

  // module D
  @preconcurrency import C

  func centerView(at location: Point) {
    Task {
      await mainView.center(at: location) // no warning about non-Sendable 'Point' because the @preconcurrency import suppresses it
    }
  }
  ```

* [SE-0302][]:

  Swift will now produce warnings to indicate potential data races when
  non-`Sendable` types are passed across actor or task boundaries. For
  example:

  ```swift
  class MyCounter {
    var value = 0
  }

  func f() -> MyCounter {
    let counter = MyCounter()
    Task {
      counter.value += 1  // warning: capture of non-Sendable type 'MyCounter'
    }
    return counter
  }
  ```

* [SE-0331][]:

  The conformance of the unsafe pointer types (e.g., `UnsafePointer`,
  `UnsafeMutableBufferPointer`) to the `Sendable` protocols has been removed,
  because pointers cannot safely be transferred across task or actor boundaries.

* References to `Self` or so-called "`Self` requirements" in the type signatures
  of protocol members are now correctly detected in the parent of a nested type.
  As a result, protocol members that fall under this overlooked case are no longer
  available on values of protocol type:

  ```swift
  struct Outer<T> {
    struct Inner {}
  }

  protocol P {}
  extension P {
    func method(arg: Outer<Self>.Inner) {}
  }

  func test(p: P) {
    // error: 'method' has a 'Self' requirement and cannot be used on a value of
    // protocol type (use a generic constraint instead).
    _ = p.method
  }
  ```

* [SE-0324][]:

  Relax diagnostics for pointer arguments to C functions. The Swift
  compiler now accepts limited pointer type mismatches when directly
  calling functions imported from C as long as the C language allows
  those pointer types to alias. Consequently, any Swift
  `Unsafe[Mutable]Pointer<T>` or `Unsafe[Mutable]RawPointer` may be
  passed to C function arguments declared as `[signed|unsigned] char
  *`. Swift `Unsafe[Mutable]Pointer<T>` can also be passed to C
  function arguments with an integer type that differs from `T` only
  in its signedness.

  For example, after importing a C function declaration:
  ```c
  long long decode_int64(const char *ptr_to_int64);
  ```
  Swift can now directly pass a raw pointer as the function argument:
  ```swift
  func decodeAsInt64(data: Data) -> Int64 {
      data.withUnsafeBytes { (bytes: UnsafeRawBufferPointer) in
          decode_int64(bytes.baseAddress!)
      }
  }
  ```

* [SE-0322][]:

  The standard library now provides a new operation
  `withUnsafeTemporaryAllocation` which provides an efficient temporarily
  allocation within a limited scope, which will be optimized to use stack
  allocation when possible.

* [SE-0320][]:

  Dictionaries with keys of any type conforming to the new protocol
  `CodingKeyRepresentable` can now be encoded and decoded. Formerly, encoding
  and decoding was limited to keys of type `String` or `Int`.

* [SE-0315][]:

  Type expressions and annotations can now include "type placeholders" which
  directs the compiler to fill in that portion of the type according to the usual
  type inference rules. Type placeholders are spelled as an underscore ("`_`") in
  a type name. For instance:
  
  ```swift
  // This is OK--the compiler can infer the key type as `Int`.
  let dict: [_: String] = [0: "zero", 1: "one", 2: "two"]
  ```

* [SE-0290][]:

  It is now possible to write inverted availability conditions by using the new `#unavailable` keyword:

  ```swift
  if #unavailable(iOS 15.0) {
      // Old functionality
  } else {
      // iOS 15 functionality 
  }
  ```

## Swift 5.5

### 2021-09-20 (Xcode 13.0)

* [SE-0323][]:

  The main function is executed with `MainActor` isolation applied, so functions
  and variables with `MainActor` isolation may be called and modified
  synchronously from the main function. If the main function is annotated with a
  global actor explicitly, it must be the main actor or an error is emitted. If
  no global actor annotation is present, the main function is implicitly run on
  the main actor.

  The main function is executed synchronously up to the first suspension point.
  Any tasks enqueued by initializers in Objective-C or C++ will run after the
  main function runs to the first suspension point. At the suspension point, the
  main function suspends and the tasks are executed according to the Swift
  concurrency mechanisms.

* [SE-0313][]:

  Parameters of actor type can be declared as `isolated`, which means that they
  represent the actor on which that code will be executed. `isolated` parameters
  extend the actor-isolated semantics of the `self` parameter of actor methods
  to arbitrary parameters. For example:

  ```swift
  actor MyActor {
    func f() { }
  }

  func g(actor: isolated MyActor) {
    actor.f()   // okay, this code is always executing on "actor"
  }

  func h(actor: MyActor) async {
    g(actor: actor)        // error, call must be asynchronous
    await g(actor: actor)  // okay, hops to "actor" before calling g
  }
  ```

  The `self` parameter of actor methods are implicitly `isolated`. The
  `nonisolated` keyword makes the `self` parameter no longer `isolated`.

* [#57081][]:

  The compiler now correctly rejects the application of generic arguments to the
  special `Self` type:

  ```swift
  struct Box<T> {
    // previously interpreted as a return type of Box<T>, ignoring the <Int> part;
    // now we diagnose an error with a fix-it suggesting replacing `Self` with `Box`
    static func makeBox() -> Self<Int> {...}
  }
  ```

* [#57225][]:

  The compiler now correctly rejects `@available` annotations on enum cases with
  associated values with an OS version newer than the current deployment target:

  ```swift
  @available(macOS 12, *)
  public struct Crayon {}

  public enum Pen {
    case pencil

    @available(macOS 12, *)
    case crayon(Crayon)
  }
  ```

  While this worked with some examples, there is no way for the Swift runtime to
  perform the requisite dynamic layout needed to support this in general, which
  could cause crashes at runtime.

  Note that conditional availability on stored properties in structs and classes
  is not supported for similar reasons; it was already correctly detected and
  diagnosed.

* [SE-0311][]:

  Task local values can be defined using the new `@TaskLocal` property wrapper.
  Such values are carried implicitly by the task in which the binding was made,
  as well as any child-tasks, and unstructured task created from the tasks context.
  
  ```swift
  struct TraceID { 
    @TaskLocal
    static var current: TraceID? 
  }
  
  func printTraceID() {
    if let traceID = TraceID.current {
      print("\(traceID)")
    } else {
      print("nil")
    }
  }
  
  func run() async { 
    printTraceID()    // prints: nil
    TraceID.$current.withValue("1234-5678") { 
      printTraceID()  // prints: 1234-5678
      inner()         // prints: 1234-5678
    }
    printTraceID()    // prints: nil
  }
  
  func inner() {
    // if called from a context in which the task-local value
    // was bound, it will print it (or 'nil' otherwise)
    printTraceID()
  }
  ```

* [SE-0316][]:

	A type can be defined as a global actor. Global actors extend the notion
	of actor isolation outside of a single actor type, so that global state
	(and the functions that access it) can benefit from actor isolation,
	even if the state and functions are scattered across many different
	types, functions and modules. Global actors make it possible to safely
	work with global variables in a concurrent program, as well as modeling
	other global program constraints such as code that must only execute on
  the "main thread" or "UI thread". A new global actor can be defined with
  the `globalActor` attribute:

  ```swift
  @globalActor
  struct DatabaseActor {
    actor ActorType { }

    static let shared: ActorType = ActorType()
  }
  ```

  Global actor types can be used as custom attributes on various declarations,
  which ensures that those declarations are only accessed on the actor described
  by the global actor's `shared` instance. For example:

  ```swift
  @DatabaseActor func queryDB(query: Query) throws -> QueryResult

  func runQuery(queryString: String) async throws -> QueryResult {
    let query = try Query(parsing: queryString)
    return try await queryDB(query: query) // 'await' because this implicitly hops to DatabaseActor.shared
  }
  ```

  The concurrency library defines one global actor, `MainActor`, which
  represents the main thread of execution. It should be used for any code that
  must execute on the main thread, e.g., for updating UI.

* [SE-0313][]:

  Declarations inside an actor that would normally be actor-isolated can
  explicitly become non-isolated using the `nonisolated` keyword. Non-isolated
  declarations can be used to conform to synchronous protocol requirements:

  ```swift
  actor Account: Hashable {
    let idNumber: Int
    var balance: Double

    nonisolated func hash(into hasher: inout Hasher) { // okay, non-isolated satisfies synchronous requirement
      hasher.combine(idNumber) // okay, can reference idNumber from outside the let
      hasher.combine(balance) // error: cannot synchronously access actor-isolated property
    }
  }
  ```

* [SE-0300][]:

  Async functions can now be suspended using the `withUnsafeContinuation`
  and `withUnsafeThrowingContinuation` functions. These both take a closure,
  and then suspend the current async task, executing that closure with a
  continuation value for the current task. The program must use that
  continuation at some point in the future to resume the task, passing in
  a value or error, which then becomes the result of the `withUnsafeContinuation`
  call in the resumed task.

* Type names are no longer allowed as an argument to a subscript parameter that expects a metatype type

  ```swift
  struct MyValue {
  }

  struct MyStruct {
    subscript(a: MyValue.Type) -> Int { get { ... } }
  }

  func test(obj: MyStruct) {
    let _ = obj[MyValue]
  }
  ```

  Accepting subscripts with `MyValue` as an argument was an oversight because `MyValue` requires explicit `.self`
  to reference its metatype, so correct syntax would be to use `obj[MyValue.self]`.

* [SE-0310][]:
  
  Read-only computed properties and subscripts can now define their `get` accessor to be `async` and/or `throws`, by writing one or both of those keywords between the `get` and `{`.  Thus, these members can now make asynchronous calls or throw errors in the process of producing a value:
  ```swift
  class BankAccount: FinancialAccount {
    var manager: AccountManager?

    var lastTransaction: Transaction {
      get async throws {
        guard manager != nil else { throw BankError.notInYourFavor }
        return await manager!.getLastTransaction()
      }
    }

    subscript(_ day: Date) -> [Transaction] {
      get async {
        return await manager?.getTransactions(onDay: day) ?? []
      }
    }
  }

  protocol FinancialAccount {
    associatedtype T
    var lastTransaction: T { get async throws }
    subscript(_ day: Date) -> [T] { get async }
  }
  ```
  Accesses to such members, like `lastTransaction` above, will require appropriate marking with `await` and/or `try`:
  ```swift
  extension BankAccount {
    func meetsTransactionLimit(_ limit: Amount) async -> Bool {
      return try! await self.lastTransaction.amount < limit
      //                    ^~~~~~~~~~~~~~~~ this access is async & throws
    }                
  }

    
  func hadWithdrawalOn(_ day: Date, from acct: BankAccount) async -> Bool {
    return await !acct[day].allSatisfy { $0.amount >= Amount.zero }
    //            ^~~~~~~~~ this access is async
  }
  ```

* [SE-0306][]:

  Swift 5.5 includes support for actors, a new kind of type that isolates its instance data to protect it from concurrent access. Accesses to an actor's instance declarations from outside the must be asynchronous:

  ```swift
  actor Counter {
    var value = 0

    func increment() {
      value = value + 1
    }
  }

  func useCounter(counter: Counter) async {
    print(await counter.value) // interaction must be async
    await counter.increment()  // interaction must be async
  }
  ```

* The determination of whether a call to a `rethrows` function can throw now considers default arguments of `Optional` type.

  In Swift 5.4, such default arguments were ignored entirely by `rethrows` checking. This meant that the following example was accepted:

  ```swift
  func foo(_: (() throws -> ())? = nil) rethrows {}
  foo()  // no 'try' needed
  ```

  However, it also meant that the following was accepted, even though the call to `foo()` can throw and the call site is not marked with `try`:

  ```swift
  func foo(_: (() throws -> ())? = { throw myError }) rethrows {}
  foo()  // 'try' *should* be required here
  ```

  The new behavior is that the first example is accepted because the default argument is syntactically written as `nil`, which is known not to throw. The second example is correctly rejected, on account of missing a `try` since the default argument *can* throw.

* [SE-0293][]:

  Property wrappers can now be applied to function and closure parameters:

  ```swift
  @propertyWrapper
  struct Wrapper<Value> {
    var wrappedValue: Value

    var projectedValue: Self { return self }

    init(wrappedValue: Value) { ... }

    init(projectedValue: Self) { ... }
  }

  func test(@Wrapper value: Int) {
    print(value)
    print($value)
    print(_value)
  }

  test(value: 10)

  let projection = Wrapper(wrappedValue: 10)
  test($value: projection)
  ```

  The call-site can pass a wrapped value or a projected value, and the property wrapper will be initialized using `init(wrappedValue:)` or `init(projectedValue:)`, respectively.

* [SE-0299][]:

  It is now possible to use leading-dot syntax in generic contexts to access static members of protocol extensions where `Self` is constrained to a fully concrete type:

  ```swift
  public protocol ToggleStyle { ... }

  public struct DefaultToggleStyle: ToggleStyle { ... }

  extension ToggleStyle where Self == DefaultToggleStyle {
    public static var `default`: Self { .init() }
  }

  struct Toggle {
    func applyToggle<T: ToggleStyle>(_ style: T) { ... }
  }

  Toggle(...).applyToggle(.default)
  ```

* Whenever a reference to `Self` does not impede the usage of a protocol as a value type, or a protocol member on a value of protocol type, the same is now true for references to `[Self]` and `[Key : Self]`:

  ```swift
  protocol Copyable {
    func copy() -> Self
    func copy(count: Int) -> [Self]
  }

  func test(c: Copyable) {
    let copy: Copyable = c.copy() // OK
    let copies: [Copyable] = c.copy(count: 5) // also OK
  }
  ```

* [SE-0296][]:

  Asynchronous programming is now natively supported using async/await. Asynchronous functions can be defined using `async`:

  ```swift
  func loadWebResource(_ path: String) async throws -> Resource { ... }
  func decodeImage(_ r1: Resource, _ r2: Resource) async throws -> Image
  func dewarpAndCleanupImage(_ i : Image) async -> Image
  ```

  Calls to `async` functions may suspend, meaning that they give up the thread on which they are executing and will be scheduled to run again later. The potential for suspension on asynchronous calls requires the `await` keyword, similarly to the way in which `try` acknowledges a call to a `throws` function:

  ```swift
  func processImageData() async throws -> Image {
    let dataResource  = try await loadWebResource("dataprofile.txt")
    let imageResource = try await loadWebResource("imagedata.dat")
    let imageTmp      = try await decodeImage(dataResource, imageResource)
    let imageResult   = await dewarpAndCleanupImage(imageTmp)
    return imageResult
  }
  ```

* The `lazy` keyword now works in local contexts, making the following valid:

  ```swift
  func test(useIt: Bool) {
    lazy var result = getPotentiallyExpensiveResult()
    if useIt {
      doIt(result)
    }
  }
  ```

* [SE-0297][]:

  An Objective-C method that delivers its results asynchronously via a completion handler block will be translated into an `async` method that directly returns the result (or throws). For example, the following Objective-C method from [CloudKit](https://developer.apple.com/documentation/cloudkit/ckcontainer/1640387-fetchshareparticipantwithuserrec):

  ```objc
  - (void)fetchShareParticipantWithUserRecordID:(CKRecordID *)userRecordID
      completionHandler:(void (^)(CKShareParticipant * _Nullable, NSError * _Nullable))completionHandler;
  ```

  will be translated into an `async throws` method that returns the participant instance:

  ```swift
  func fetchShareParticipant(
      withUserRecordID userRecordID: CKRecord.ID
  ) async throws -> CKShare.Participant
  ```

  Swift callers can invoke this `async` method within an `await` expression:

  ```swift
  guard let participant = try? await container.fetchShareParticipant(withUserRecordID: user) else {
      return nil
  }
  ```

* [SE-0298][]:

  The "for" loop can be used to traverse asynchronous sequences in asynchronous code:

  ```swift
  for try await line in myFile.lines() {
    // Do something with each line
  }
  ```

  Asynchronous for loops use asynchronous sequences, defined by the protocol
  `AsyncSequence` and its corresponding `AsyncIterator`.

## Swift 5.4

### 2021-04-26 (Xcode 12.5)

* Protocol conformance checking now considers `where` clauses when evaluating if a `typealias` is a suitable witness for an associated type requirement. The following code is now rejected:

  ```swift
  protocol Holder {
    associatedtype Contents
  }

  struct Box<T> : Holder {}
  // error: type 'Box<T>' does not conform to protocol 'Holder'

  extension Box where T : Hashable {
    typealias Contents = T
  }
  ```

  In most cases, the compiler would either crash or produce surprising results when making use of a `typealias` with an unsatisfied `where` clause, but it is possible that some previously-working code is now rejected. In the above example, the conformance can be fixed in one of various ways:

  1) making it conditional (moving the `: Holder` from the definition of `Box` to the extension)
  2) moving the `typealias` from the extension to the type itself
  3) relaxing the `where` clause on the extension

* Availability checking now rejects protocols that refine less available protocols. Previously, this was accepted by the compiler but could result in linker errors or runtime crashes:

  ```swift
  @available(macOS 11, *)
  protocol Base {}

  protocol Bad : Base {}
  // error: 'Base' is only available in macOS 11 or newer

  @available(macOS 11, *)
  protocol Good : Base {} // OK
  ```

* The `@available` attribute is no longer permitted on generic parameters, where it had no effect:

  ```swift
  struct Bad<@available(macOS 11, *) T> {}
  // error: '@available' attribute cannot be applied to this declaration

  struct Good<T> {} // equivalent
  ```

* If a type is made to conform to a protocol via an extension, the availability of the extension is now taken into account when forming generic types that use this protocol conformance. For example, consider a `Box` type whose conformance to `Hashable` uses features only available on macOS 11:

  ```swift
  public struct Box {}

  @available(macOS 11, *)
  extension Box : Hashable {
    func hash(into: inout Hasher) {
      // call some new API to hash the value...
    }
  }

  public func findBad(_: Set<Box>) -> Box {}
  // warning: conformance of 'Box' to 'Hashable' is only available in macOS 11 or newer

  @available(macOS 11, *)
  public func findGood(_: Set<Box>) -> Box {} // OK
  ```

  In the above code, it is not valid for `findBad()` to take a `Set<Box>`, since `Set` requires that its element type conform to `Hashable`; however the conformance of `Box` to `Hashable` is not available prior to macOS 11.

  Note that using an unavailable protocol conformance is a warning, not an error, to avoid potential source compatibility issues. This is because it was technically possible to write code in the past that made use of unavailable protocol conformances but worked anyway, if the optimizer had serendipitously eliminated all runtime dispatch through this conformance, or the code in question was entirely unreachable at runtime.

  Protocol conformances can also be marked as completely unavailable or deprecated, by placing an appropriate `@available` attribute on the extension:

  ```swift
  @available(*, unavailable, message: "Not supported anymore")
  extension Box : Hashable {}

  @available(*, deprecated, message: "Suggest using something else")
  extension Box : Hashable {}
  ```

  If a protocol conformance is defined on the type itself, it inherits availability from the type. You can move the protocol conformance to an extension if you need it to have narrower availability than the type.

* When `swift` is run with no arguments, it starts a REPL (read eval print loop) that uses LLDB. The compiler also had a second REPL implementation, known as the "integrated REPL", formerly accessible by running `swift -frontend -repl`. The "integrated REPL" was only intended for use by compiler developers, and has now been removed.

  Note that this does not take away the ability to put Swift code in a script and run it with `swift myScript.swift`. This so-called "script mode" is distinct from the integrated REPL, and continues to be supported.

* Property wrappers now work in local contexts, making the following valid:

  ```swift
  @propertyWrapper
  struct Wrapper<T> {
    var wrappedValue: T
  }

  func test() {
    @Wrapper var value = 10
  }
  ```

* [#52471][]:

  Function overloading now works in local contexts, making the following valid:

  ```swift
  func outer(x: Int, y: String) {
    func doIt(_: Int) {}
    func doIt(_: String) {}

    doIt(x) // calls the first 'doIt(_:)' with an Int value
    doIt(y) // calls the second 'doIt(_:)' with a String value
  }
  ```

* [SE-0284][]:

  Functions, subscripts, and initializers may now have more than one variadic parameter, as long as all parameters which follow variadic parameters are labeled. This makes declarations like the following valid:

  ```swift
  func foo(_ a: Int..., b: Double...) { }

  struct Bar {
    subscript(a: Int..., b b: Int...) -> [Int] { a + b }

    init(a: String..., b: Float...) { }
  }
  ```

* [SE-0287][]:

  Implicit member expressions now support chains of member accesses, making the following valid:
  
  ```swift
  let milky: UIColor = .white.withAlphaComponent(0.5)
  let milky2: UIColor = .init(named: "white")!.withAlphaComponent(0.5)
  let milkyChance: UIColor? = .init(named: "white")?.withAlphaComponent(0.5)
  ```
  
  As is the case with the existing implicit member expression syntax, the resulting type of the chain must be the same as the (implicit) base, so it is not well-formed to write:
  
  ```swift
  let cgMilky: CGColor = .white.withAlphaComponent(0.5).cgColor
  ```
  
  (Unless, of course, appropriate `white` and `withAlphaComponent` members were defined on `CGColor`.)
  
  Members of a "chain" can be properties, method calls, subscript accesses, force unwraps, or optional chaining question marks. Furthermore, the type of each member along the chain is permitted to differ (again, as long as the base of the chain matches the resulting type) meaning the following successfully typechecks:
  
  ```swift
  struct Foo {
    static var foo = Foo()
    static var bar = Bar()
    
    var anotherFoo: Foo { Foo() }
    func getFoo() -> Foo { Foo() }
    var optionalFoo: Foo? { Foo() }
    subscript() -> Foo { Foo() }
  }
  
  struct Bar {
    var anotherFoo = Foo()
  }

  let _: Foo? = .bar.anotherFoo.getFoo().optionalFoo?.optionalFoo![]
  ```

## Swift 5.3

### 2020-09-16 (Xcode 12.0)

* [SE-0279][] & [SE-0286][]:

  Trailing closure syntax has been extended to allow additional labeled closures to follow the initial unlabeled closure:
  
  ```swift
  // Single trailing closure argument
  UIView.animate(withDuration: 0.3) {
    self.view.alpha = 0
  }
  // Multiple trailing closure arguments
  UIView.animate(withDuration: 0.3) {
    self.view.alpha = 0
  } completion: { _ in
    self.view.removeFromSuperview()
  }
  ```
  
  Additionally, trailing closure arguments now match the appropriate parameter according to a forward-scan rule (as opposed to the previous backward-scan rule):
  
  ```swift
  func takesClosures(first: () -> Void, second: (Int) -> Void = { _ in }) {}
  
  takesClosures {
    print("First")
  }
  ```
  
  In the above example, the trailing closure argument matches parameter `first`, whereas pre-Swift-5.3 it would have matched `second`. In order to ease the transition to this new rule, cases in which the forward-scan and backward-scan match a single trailing closure to different parameters, the backward-scan result is preferred and a warning is emitted. This is expected to be upgraded to an error in the next major version of Swift.

* [#49631][]:

  Property observers such as `willSet` and `didSet` are now supported on `lazy` properties:

  ```swift
  class C {
    lazy var property: Int = 0 {
      willSet { print("willSet called!") } // Okay
      didSet { print("didSet called!") } // Okay
    }
  }
  ```

  Note that the initial value of the property will be forced and made available as the `oldValue` for the `didSet` observer, if the property hasn't been accessed yet.
  
  ```swift
  class C {
    lazy var property: Int = 0 {
      didSet { print("Old value: ", oldValue) }
    }
  }

  let c = C()
  c.property = 1 // Prints 'Old value: 0'
  ```

  This could have side-effects, for example if the lazy property's initializer is doing other work.

* [#54108][]:

  Exclusivity violations within code that computes the `default`
  argument during Dictionary access are now diagnosed.

  ```swift
  struct Container {
     static let defaultKey = 0

     var dictionary = [defaultKey:0]

     mutating func incrementValue(at key: Int) {
       dictionary[key, default: dictionary[Container.defaultKey]!] += 1
     }
  }
  // error: overlapping accesses to 'self.dictionary', but modification requires exclusive access; consider copying to a local variable
  //     dictionary[key, default: dictionary[Container.defaultKey]!] += 1
  //     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // note: conflicting access is here
  //     dictionary[key, default: dictionary[Container.defaultKey]!] += 1
  //                              ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
  ```

  The exclusivity violation can be avoided by precomputing the `default`
  argument using a local variable.

  ```swift
  struct Container {
    static let defaultKey = 0

    var dictionary = [defaultKey:0]

    mutating func incrementValue(at key: Int) {
      let defaultValue = dictionary[Container.defaultKey]!
      dictionary[key, default: defaultValue] += 1
    }
  }
  // No error.
  ```

* [SE-0268][]:
  
  A `didSet` observer which does not refer to the `oldValue` in its body or does not explicitly request it by placing it in the parameter list (i.e. `didSet(oldValue)`) will no longer trigger a call to the property getter to fetch the `oldValue`.
  
  ```swift
  class C {
    var value: Int = 0 {
      didSet { print("didSet called!") }
    }
  }
  
  let c = C()
  // This does not trigger a call to the getter for 'value'
  // because the 'didSet' observer on 'value' does not
  // refer to the 'oldValue' in its body, which means
  // the 'oldValue' does not need to be fetched.
  c.value = 1
  ```
  
* [SE-0276][]:

  Catch clauses in a `do`-`catch` statement can now include multiple patterns in a comma-separated list. The body of a `catch` clause will be executed if a thrown error matches any of its patterns.

  ```swift
  do {
    try performTask()
  } catch TaskError.someFailure(let msg),
          TaskError.anotherFailure(let msg) {
    showMessage(msg)
  }
  ```

* [SE-0280][]:
  
  Enum cases can now satisfy static protocol requirements. A static get-only property of type `Self` can be witnessed by an enum case with no associated values and a static function with arguments and returning `Self` can be witnessed by an enum case with associated values.
  
  ```swift
  protocol P {
    static var foo: Self { get }
    static func bar(value: Int) -> Self
  }
  
  enum E: P {
    case foo // matches 'static var foo'
    case bar(value: Int) // matches 'static func bar(value:)'
  }
  ```

* [SE-0267][]:
  
  Non-generic members that support a generic parameter list, including nested type declarations, are now allowed to carry a contextual `where` clause against outer generic parameters. Previously, such declarations could only be expressed by placing the member inside a dedicated constrained extension.

  ```swift
  struct Box<Wrapped> {
    func boxes() -> [Box<Wrapped.Element>] where Wrapped: Sequence { ... }
  }
  ```
  Since contextual `where` clauses are effectively visibility constraints, overrides adopting this feature must be at least as visible as the overridden method. In practice, this implies any instance of `Derived` that can access `Base.foo` must also be able to access `Derived.foo`.
  
  ```swift
  class Base<T> {
    func foo() where T == Int { ... }
  }
  
  class Derived<U>: Base<U> {
    // OK, <U where U: Equatable> has broader visibility than <T where T == Int>
    override func foo() where U: Equatable { ... } 
  }

* [#42697][]:

  Unapplied references to protocol methods are now supported. Previously this
  only worked for methods defined in structs, enums and classes.

  ```swift
  protocol Cat {
    func play(catToy: Toy)
  }

  let fn = Cat.play(catToy:)
  fn(myCat)(myToy)
  ```

* [SE-0266][]:
  
  Enumerations with no associated values, or only `Comparable` associated values, can opt-in to synthesized `Comparable` conformance by declaring conformance to the `Comparable` protocol. The synthesized implementation orders the cases first by case-declaration order, and then by lexicographic order of the associated values (if any).
  
  ```swift
  enum Foo: Comparable {
    case a(Int), b(Int), c
  }
  
  // .a(0) < .a(1) < .b(0) < .b(1) < .c
  ```

* [SE-0269][]:

  When an escaping closure explicitly captures `self` in its capture list, the
  use of implicit `self` is enabled within that closure. This means that the
  following code is now valid:
  
  ```swift
  func doStuff(_ stuff: @escaping () -> Void) {}
  
  class C {
    var x = 0

    func method() {
      doStuff { [self] in
        x += 1
      }
    }
  }
  ```
  
  This proposal also introduces new diagnostics for inserting `self` into the
  closure's capture list in addition to the existing 'use `self.` explicitly'
  fix-it.

## Swift 5.2

### 2020-03-24 (Xcode 11.4)

* [#54246][]:

  When chaining calls to `filter(_:)` on a lazy sequence or collection, the
  filtering predicates will now be called in the same order as eager filters.
  
  ```swift
  let evens = (1...10).lazy
      .filter { $0.isMultiple(of: 2) }
      .filter { print($0); return true }
  _ = evens.count
  // Prints 2, 4, 6, 8, and 10 on separate lines
  ```
  
  Previously, the predicates were called in reverse order.
  
* [apple/swift-corelibs-foundation#4326][]:

  The compiler will now emit a warning when attempting to pass a temporary
  pointer argument produced from an array, string, or inout argument to a
  parameter which is known to escape it. This includes the various initializers 
  for the `UnsafePointer`/`UnsafeBufferPointer` family of types, as well as
  memberwise initializers.

  ```swift
  struct S {
    var ptr: UnsafePointer<Int8>
  }

  func foo() {
    var i: Int8 = 0
    let ptr = UnsafePointer(&i)
    // warning: initialization of 'UnsafePointer<Int8>' results in a 
    // dangling pointer
    
    let s1 = S(ptr: [1, 2, 3]) 
    // warning: passing '[Int8]' to parameter, but argument 'ptr' should be a
    // pointer that outlives the call to 'init(ptr:)'
    
    let s2 = S(ptr: "hello")
    // warning: passing 'String' to parameter, but argument 'ptr' should be a
    // pointer that outlives the call to 'init(ptr:)'
  }
  ```

  All 3 of the above examples are unsound because each argument produces a
  temporary pointer only valid for the duration of the call they are passed to.
  Therefore the returned value in each case references a dangling pointer.

* [#44797][]:

  The compiler now supports local functions whose default arguments capture
  values from outer scopes.

  ```swift
  func outer(x: Int) -> (Int, Int) {
    func inner(y: Int = x) -> Int {
      return y
    }

    return (inner(), inner(y: 0))
  }
  ```

* [#53830][]:

  The compiler will now correctly strip argument labels from function references
  used with the `as` operator in a function call. As a result, the `as` operator
  can now be used to disambiguate a call to a function with argument labels. 
  
  ```swift
  func foo(x: Int) {}
  func foo(x: UInt) {}
  
  (foo as (Int) -> Void)(5)  // Calls foo(x: Int)
  (foo as (UInt) -> Void)(5) // Calls foo(x: UInt)
  ```
  
  Previously this was only possible for functions without argument labels.
  
  This change also means that a generic type alias can no longer be used to
  preserve the argument labels of a function reference through the `as`
  operator. The following is now rejected:
  
  ```swift
  typealias Magic<T> = T
  func foo(x: Int) {}
  (foo as Magic)(x: 5) // error: Extraneous argument label 'x:' in call
  ```
  
  The function value must instead be called without argument labels:
  
  ```swift
  (foo as Magic)(5)
  ```

* [#53699][]:

  A class-constrained protocol extension, where the extended protocol does
  not impose a class constraint, will now infer the constraint implicitly.

  ```swift
  protocol Foo {}
  class Bar: Foo {
    var someProperty: Int = 0
  }

  // Even though 'Foo' does not impose a class constraint, it is automatically
  // inferred due to the Self: Bar constraint.
  extension Foo where Self: Bar {
    var anotherProperty: Int {
      get { return someProperty }
      // As a result, the setter is now implicitly nonmutating, just like it would
      // be if 'Foo' had a class constraint.
      set { someProperty = newValue }
    }
  }
  ```

* [SE-0253][]:

  Values of types that declare `func callAsFunction` methods can be called
  like functions. The call syntax is shorthand for applying
  `func callAsFunction` methods.

  ```swift
  struct Adder {
    var base: Int
    func callAsFunction(_ x: Int) -> Int {
      return x + base
    }
  }
  var adder = Adder(base: 3)
  adder(10) // returns 13, same as `adder.callAsFunction(10)`
  ```

  * `func callAsFunction` argument labels are required at call sites.
  * Multiple `func callAsFunction` methods on a single type are supported.
  * `mutating func callAsFunction` is supported.
  * `func callAsFunction` works with `throws` and `rethrows`.
  * `func callAsFunction` works with trailing closures.
  
* [SE-0249][]:

  A `\Root.value` key path expression is now allowed wherever a `(Root) -> Value` 
  function is allowed. Such an expression is implicitly converted to a key path 
  application of `{ $0[keyPath: \Root.value] }`.
  
  For example:
  
  ```swift
  struct User {
    let email: String
    let isAdmin: Bool
  }
  
  users.map(\.email) // this is equivalent to: users.map { $0[keyPath: \User.email] }
  ```

* [#46789][]:

  A method override is no longer allowed to have a generic signature with
  requirements not imposed by the base method. For example:

  ```swift
  protocol P {}
  
  class Base {
    func foo<T>(arg: T) {}
  }
  
  class Derived: Base {
    override func foo<T: P>(arg: T) {}
  }
  ```

  will now be diagnosed as an error.

* [#48673][]:

  Subscripts can now declare default arguments:

  ```swift
  struct Subscriptable {
    subscript(x: Int, y: Int = 0) {
      ...
    }
  }

  let s = Subscriptable()
  print(s[0])
  ```

## Swift 5.1

### 2019-09-20 (Xcode 11.0)

* [#51478][]:

  Duplicate tuple element labels are no longer allowed, because it leads
  to incorrect behavior. For example:

  ```swift
  let dupLabels: (foo: Int, foo: Int) = (foo: 1, foo: 2)

  enum Foo { case bar(x: Int, x: Int) }
  let f: Foo = .bar(x: 0, x: 1)
  ```

  will now be diagnosed as an error. 

  Note: You can still use duplicate argument labels when declaring functions and
  subscripts, as long as the internal parameter names are different. For example:

  ```swift
  func foo(bar x: Int, bar y: Int) {}
  subscript(a x: Int, a y: Int) -> Int {}
  ```

* [SE-0244][]:

  Functions can now hide their concrete return type by declaring what protocols
  it conforms to instead of specifying the exact return type:

  ```swift
  func makeMeACollection() -> some Collection {
    return [1, 2, 3]
  }
  ```

  Code that calls the function can use the interface of the protocol, but
  does not have visibility into the underlying type.

* [SE-0254][]:

  Subscripts can now be declared `static` or (inside classes) `class`.

* [SE-0252][]:

  The existing `@dynamicMemberLookup` attribute has been extended with a
  support for strongly-typed keypath implementations:

  ```swift
  @dynamicMemberLookup
  struct Lens<T> {
    let getter: () -> T
    let setter: (T) -> Void

    var value: T {
      get {
        return getter()
      }
      set {
        setter(newValue)
      }
    }

    subscript<U>(dynamicMember keyPath: WritableKeyPath<T, U>) -> Lens<U> {
      return Lens<U>(
          getter: { self.value[keyPath: keyPath] },
          setter: { self.value[keyPath: keyPath] = $0 })
    }
  }
  ```

* [#51064][], [#51546][]:

  More thorough checking has been implemented for restrictions around
  escaping closures capturing `inout` parameters or values of noescape type.
  While most code should not be affected, there are edge cases where
  the Swift 5.0 compiler would accept code violating these restrictions.
  This could result in runtime crashes or silent data corruption.
  
  An example of invalid code which was incorrectly accepted by the Swift 5.0
  compiler is an `@escaping` closure calling a local function which
  references an `inout` parameter from an outer scope:

  ```swift
  struct BadCaptureExample {
    var escapingClosure: () -> ()

    mutating func takesInOut(_ x: inout Int) {
      func localFunction() {
        x += 1
      }

      escapingClosure = { localFunction() }
    }
  }
  ```

  The compiler now correctly diagnoses the above code by pointing out that
  the capture of `x` by `localFunction()` is invalid, since `localFunction()`
  is referenced from an `@escaping` closure.

  This also addresses certain cases where the compiler incorrectly diagnosed
  certain code as invalid, when in fact no violation of restrictions had
  taken place. For example,

  ```swift
  func takesNoEscape(_ fn: () -> ()) {
    func localFunction() {
      fn()
    }

    { localFunction() }()
  }
  ```

* [#45277][]:

  Conversions between tuple types are now fully implemented.
  Previously, the following would diagnose an error:

  ```swift
  let values: (Int, Int) = (10, 15)
  let converted: (Int?, Any) = values

* [SE-0242][]:

  The memberwise initializer for structures now provide default values for variables that hold default expressions.

  ```swift
  struct Dog {
    var name = "Generic dog name"
    var age = 0

    // The synthesized memberwise initializer
    init(name: String = "Generic dog name", age: Int = 0)
  }

  let sparky = Dog(name: "Sparky") // Dog(name: "Sparky", age: 0)
  ```

* [SE-0068][]:

  It is now possible to use `Self` to refer to the innermost nominal
  type inside struct, enum and class declarations. For example, the
  two method declarations inside this struct are equivalent:
  
  ```swift
  struct Box<Value> {
    func transform1() -> Self { return self }
    func transform2() -> Box<Value> { return self }
  }
  ```

  In classes, `Self` is the dynamic type of the `self` value, as before.
  Existing restrictions on `Self` in declaration types still apply;
  that is, `Self` can only appear as the return type of a method.
  However, `Self` can now be used inside the body of a method
  without limitation.

* [#50338][]:

  Enum cases can now be matched against an optional enum without
  requiring a '?' at the end of the pattern.

  ```swift
  enum Foo { case zero, one }

  let foo: Foo? = .zero

  switch foo {
    case .zero: break
    case .one: break
    case .none: break
  }
  ```

* [#52244][]:

  `weak` and `unowned` stored properties no longer inhibit the
   automatic synthesis of `Equatable` or `Hashable` conformance.

* [#45293][]:

  An `@autoclosure` parameter can now be declared with a typealias type.

  ```swift
  class Foo {
    typealias FooClosure = () -> String
    func fooFunction(closure: @autoclosure FooClosure) {}
  }
  ```

* [#50143][]:

  Methods declared `@objc` inside a class can now return `Self`:

  ```swift
  class MyClass : NSObject {
    @objc func clone() -> Self { return self }
  }
  ```

* [#44784][]:

  Assigning '.none' to an optional enum which also has a 'none' case
  or comparing such an enum with '.none' will now warn. Such expressions
  create an ambiguity because the compiler chooses Optional.none
  over Foo.none.

  ```swift
  enum Foo { case none }

  // Assigned Optional.none instead of Foo.none
  let foo: Foo? = .none
  // Comparing with Optional.none instead of Foo.none
  let isEqual = foo == .none
  ```

  The compiler will provide a warning along with a fix-it to
  replace '.none' with 'Optional.none' or 'Foo.none' to resolve
  the ambiguity.

* Key path expressions can now include references to tuple elements.

* Single-parameter functions accepting values of type `Any` are no
  longer preferred over other functions.

  ```swift
  func foo(_: Any) { print("Any") }
  func foo<T>(_: T) { print("T") }
  foo(0) // prints "Any" in Swift < 5.1, "T" in Swift 5.1
  ```

* [SE-0245][]:

  `Array` and `ContiguousArray` now have `init(unsafeUninitializedCapacity:initializingWith:)`,
  which provides access to the array's uninitialized storage.

## Swift 5.0

### 2019-03-25 (Xcode 10.2)

* [SE-0235][]:

  The standard library now contains a `Result` type for manually propagating errors.
  
  ```swift
  enum Result<Success, Failure: Error> {
      case success(Success)
      case failure(Failure)
  }
  ```
  
  This type serves a complementary role to that of throwing functions and initializers. 
  Use `Result` in situations where automatic error propagation or `try`-`catch` 
  blocks are undesirable, such as in asynchronous code or when accumulating the 
  results of successive error-producing operations.
  
* `Error` now conforms to itself. This allows for the use of `Error` itself as 
  the argument for a generic parameter constrained to `Error`.

* Swift 3 mode has been removed. Supported values for the `-swift-version`
  flag are `4`, `4.2`, and `5`.

* [SE-0228][]:

  String interpolation has been overhauled to improve its performance,
  clarity, and efficiency.

  Note that the old `_ExpressibleByStringInterpolation` protocol has been
  removed; any code making use of this protocol will need to be updated
  for the new design. An `#if compiler` block can be used to conditionalize
  code between 4.2 and 5.0, for example:
  
  ```swift
  #if compiler(<5.0)
  extension MyType : _ExpressibleByStringInterpolation { ... }
  #else
  extension MyType : ExpressibleByStringInterpolation { ... }
  #endif
  ```

* [SE-0213][]:

  If `T` conforms to one of the `ExpressibleBy*` protocols and `literal` is a
  literal expression, then `T(literal)` will construct a literal of type `T`
  using the corresponding protocol, rather than calling a constructor member
  of `T` with a value of  the protocol's default literal type.

  For example, expressions like `UInt64(0xffff_ffff_ffff_ffff)` are now valid,
  where previously they would overflow the default integer literal type of `Int`.

* [SE-0230][]:

  In Swift 5 mode, `try?` with an expression of Optional type will flatten the
  resulting Optional, instead of returning an Optional of an Optional.

* [#48289][]:

  In Swift 5 mode, `@autoclosure` parameters can no longer be forwarded to
  `@autoclosure` arguments in another function call. Instead, you must explicitly
  call the function value with `()`; the call itself is wrapped inside an
  implicit closure, guaranteeing the same behavior as in Swift 4 mode.

  Example:

  ```swift
  func foo(_ fn: @autoclosure () -> Int) {}
  func bar(_ fn: @autoclosure () -> Int) {
    foo(fn)   // Incorrect, `fn` can't be forwarded and has to be called
    foo(fn()) // Ok
  }
  ```

* [#50641][]:

  Single-element labeled tuple expressions, for example `(label: 123)`, were
  allowed in some contexts but often resulted in surprising, inconsistent
  behavior that varied across compiler releases. They are now completely
  disallowed.

  Note that single-element labeled _types_, for example `var x: (label: Int)`,
  have already been prohibited since Swift 3.

* [#43310][]:

  In Swift 5 mode, a class method returning `Self` can no longer be overridden
  with a method returning a non-final concrete class type. Such code is not
  type safe and will need to be updated.

  For example,

  ```swift
  class Base {
    class func factory() -> Self { ... }
  }

  class Derived : Base {
    class override func factory() -> Derived { ... }
  }
  ```

* In Swift 5 mode, the type of `self` in a convenience initializer of a non-final
  class is now the dynamic `Self` type, and not the concrete class type.

* [#48153][]:

  Protocols can now constrain their conforming types to those that subclasses a
  given class. Two equivalent forms are supported:

  ```swift
  protocol MyView : UIView { ... }
  protocol MyView where Self : UIView { ... }
  ```

  Note that Swift 4.2 accepted the second form, but it was not fully implemented
  and could sometimes crash at compile time or run time.

* [#43248][]:

  Extension binding now supports extensions of nested types which themselves are
  defined inside extensions. Previously this could fail with some declaration orders,
  producing spurious "undeclared type" errors.

* [#49687][]:

  Exclusive memory access is now enforced at runtime by default in
  optimized (`-O`/`-Osize`) builds. Programs that violate exclusivity will
  trap at runtime with an "overlapping access" diagnostic
  message. This can be disabled via a command line flag:
  `-enforce-exclusivity=unchecked`, but doing so may result in undefined
  behavior.

  Runtime violations of exclusivity typically result from
  simultaneous access of class properties, global variables (including
  variables in top-level code), or variables captured by escaping
  closures.

* [SE-0216][]:

  The `@dynamicCallable` attribute enables nominal types to be "callable" via a
  simple syntactic sugar. The primary use case is dynamic language
  interoperability.

  Toy example:

  ```swift
  @dynamicCallable
  struct ToyCallable {
    func dynamicallyCall(withArguments: [Int]) {}
    func dynamicallyCall(withKeywordArguments: KeyValuePairs<String, Int>) {}
  }
  let x = ToyCallable()
  x(1, 2, 3) // desugars to `x.dynamicallyCall(withArguments: [1, 2, 3])`
  x(label: 1, 2) // desugars to `x.dynamicallyCall(withKeywordArguments: ["label": 1, "": 2])`
  ```

* [#49799][]:

  In Swift 5 mode, attempting to declare a static property with the same name as a
  nested type is now always correctly rejected. Previously, it was possible to
  perform such a redeclaration in an extension of a generic type.
  
  For example:
  ```swift
  struct Foo<T> {}
  extension Foo {
    struct i {}
    
    // compiler error: Invalid redeclaration of 'i'
    // (prior to Swift 5, this did not produce an error)
    static var i: Int { return 0 }
  }
  ```

* [#46831][]:

  In Swift 5 mode, when casting an optional value to a generic placeholder type,
  the compiler will be more conservative with the unwrapping of the value. The 
  result of such a cast now more closely matches the result you would get in a
  non-generic context.
  
  For example:
  ```swift
  func forceCast<U>(_ value: Any?, to type: U.Type) -> U {
    return value as! U
  }

  let value: Any? = 42
  print(forceCast(value, to: Any.self)) 
  // prints: Optional(42)
  // (prior to Swift 5, this would print: 42)
  
  print(value as! Any)                  
  // prints: Optional(42)
  ```

* [SE-0227][]:

  Key paths now support the `\.self` keypath, which is a `WritableKeyPath`
  that refers to its entire input value:

    ```swift
    let id = \Int.self

    var x = 2
    print(x[keyPath: id]) // prints 2
    x[keyPath: id] = 3
    print(x[keyPath: id]) // prints 3
    ```

* [SE-0214][]:

  The `DictionaryLiteral` type has been renamed to `KeyValuePairs`.
  A typealias preserves the old name for compatibility.

* [#45213][]

  Default arguments are now printed in SourceKit-generated interfaces for Swift
  modules, instead of just using a placeholder `default`.

* `unowned` and `unowned(unsafe)` variables now support Optional types.

* Designated initializers with variadic parameters are now correctly inherited
  in subclasses.

* Extensions of concrete subclasses of generic classes can now contain
  `@objc` members.

* Complex recursive type definitions involving classes and generics that would
  previously cause deadlocks at run time are now fully supported.

* [#43036][]

  In Swift 5 mode, when setting a property from within its own `didSet` or
  `willSet` observer, the observer will now only avoid being recursively called
  if the property is set on `self` (either implicitly or explicitly).

  For example:
  ```swift
  class Node {
    var children = [Node]()

    var depth: Int = 0 {
      didSet {
        if depth < 0 {
          // Will not recursively call didSet, as setting depth on self (same
          // with `self.depth = 0`).
          depth = 0
        }

        // Will call didSet for each of the children, as we're not setting the
        // property on self (prior to Swift 5, this did not trigger property
        // observers to be called again).
        for child in children {
          child.depth = depth + 1
        }
      }
    }
  }
  ```

## Swift 4.2

### 2018-09-17 (Xcode 10.0)

* [SE-0202][]

  The standard library now provides a unified set of randomization functionality.
  Integer types, floating point types, and Bool all introduce a new static
  method that creates a random value.
  
  ```swift
  let diceRoll = Int.random(in: 1 ... 6)
  let randomUnit = Double.random(in: 0 ..< 1)
  let randomBool = Bool.random()
  ```
  
  There are also additions to select a random element from a collection or
  shuffle its contents.
  
  ```swift
  let greetings = ["hey", "hello", "hi", "hola"]
  let randomGreeting = greetings.randomElement()! // This returns an Optional
  let newGreetings = greetings.shuffled() // ["hola", "hi", "hey", "hello"]
  ```

  Core to the randomization functionality is a new `RandomNumberGenerator`
  protocol. The standard library defines its own random number generator
  called `SystemRandomNumberGenerator` which is backed by a secure and
  thread-safe random number generator on each platform. All the randomization
  functions have a `using:` parameter that take a `RandomNumberGenerator` that
  users can pass in their own random number generator.
  
  ```swift
  struct MersenneTwister: RandomNumberGenerator {
    func next() -> UInt64 {
      // implementation
    }
  }
  
  var mt = MersenneTwister()
  let diceRoll = Int.random(in: 1 ... 6, using: &mt)
  ```

* [SE-0194][]

  The new CaseIterable protocol describes types which have a static
  “allCases” property that is used to describe all of the cases of the
  type. Swift will synthesize this “allCases” property for enums that
  have no associated values. For example:

  ```swift
  enum Suit: CaseIterable {
    case heart
    case club
    case diamond
    case spade
  }

  print(Suit.allCases) // prints [Suit.heart, Suit.club, Suit.diamond, Suit.spade]
  ```

* [SE-0185][]

  Protocol conformances are now able to be synthesized in extensions in the same
  file as the type definition, allowing automatic synthesis of conditional
  conformances to `Hashable`, `Equatable` and `Codable` (both `Encodable` and
  `Decodable`). For instance, if there is a generic wrapper type that can only
  be `Equatable` when its wrapped type is also `Equatable`, the `==` method can
  be automatically constructed by the compiler:

  ```swift
  struct Generic<Param> {
    var property: Param
  }

  extension Generic: Equatable where Param: Equatable {}
  // Automatically synthesized inside the extension:
  // static func ==(lhs: Generic, rhs: Generic) -> Bool {
  //   return lhs.property == rhs.property
  // }
  ```

  Code that wants to be as precise as possible should generally not
  conditionally conform to `Codable` directly, but rather its two constituent
  protocols `Encodable` and `Decodable`, or else one can only (for instance)
  decode a `Generic<Param>` if `Param` is `Encodable` in addition to
  `Decodable`, even though `Encodable` is likely not required:

  ```swift
  // Unnecessarily restrictive:
  extension Generic: Codable where Param: Codable {}

  // More precise:
  extension Generic: Encodable where Param: Encodable {}
  extension Generic: Decodable where Param: Decodable {}
  ```

  Finally, due to `Decodable` having an `init` requirement, it is not possible
  to conform to `Decodable` in an extension of a non-final class: such a class
  needs to have any `init`s from protocols be `required`, which means they need
  to be in the class definition.


* [SE-0054][]

  `ImplicitlyUnwrappedOptional<T>` is now an unavailable typealias of `Optional<T>`.
  Declarations annotated with `!` have the type `Optional<T>`. If an
  expression involving one of these values will not compile successfully with the
  type `Optional<T>`, it is implicitly unwrapped, producing a value of type `T`.

  In some cases this change will cause code that previously compiled to
  need to be adjusted. Please see [this blog post](https://swift.org/blog/iuo/)
  for more information.

* [SE-0206][]

    The standard library now uses a high-quality, randomly seeded, universal
    hash function, represented by the new public `Hasher` struct.
  
    “Random seeding” varies the result of `hashValue` on each execution of a
    Swift program, improving the reliability of the standard library's hashed
    collections such as `Set` and `Dictionary`. In particular, random seeding
    enables better protection against (accidental or deliberate) hash-flooding
    attacks.
    
    This change fulfills a long-standing prophecy in Hashable's documentation:

    > Hash values are not guaranteed to be equal across different executions of
    > your program. Do not save hash values to use during a future execution.
    
    As a consequence of random seeding, the elements in `Set` and `Dictionary`
    values may have a different order on each execution. This may expose some
    bugs in existing code that accidentally relies on repeatable ordering.

    Additionally, the `Hashable` protocol now includes an extra function
    requirement, `hash(into:)`. The new requirement is designed to be much
    easier to implement than the old `hashValue` property, and it generally
    provides better hashing. To implement `hash(into:)`, simply feed the exact
    same components of your type that you compare in `Equatable`'s `==`
    implementation to the supplied `Hasher`:
    
    ```swift
    struct Foo: Hashable {
      var a: String?
      var b: [Int]
      var c: [String: Int]
      
      static func ==(lhs: Foo, rhs: Foo) -> Bool {
        return lhs.a == rhs.a && lhs.b == rhs.b && lhs.c == rhs.c
      }
      
      func hash(into hasher: inout Hasher) {
        hasher.combine(a)
        hasher.combine(b)
        hasher.combine(c)
      }
    }
    ```
    
    Automatic synthesis for `Hashable` ([SE-0185]) has been updated to generate
    `hash(into:)` implementations. For example, the `==` and `hash(into:)`
    implementations above are equivalent to the ones synthesized by the
    compiler, and can be removed without changing the meaning of the code.
    
    Synthesis has also been extended to support deriving `hashValue` from
    `hash(into:)`, and vice versa. Therefore, code that only implements
    `hashValue` continues to work in Swift 4.2. This new compiler functionality
    works for all types that can implement `Hashable`, including classes.
    
    Note that these changes don't affect Foundation's hashing interface. Classes
    that subclass `NSObject` should override the `hash` property, like before.

    In certain controlled environments, such as while running particular tests,
    it may be helpful to selectively disable hash seed randomization, so that
    hash values and the order of elements in `Set`/`Dictionary` values remain
    consistent across executions. You can disable hash seed randomization by
    defining the environment variable `SWIFT_DETERMINISTIC_HASHING` with the
    value of `1`. The Swift runtime looks at this variable during process
    startup and, if it is defined, replaces the random seed with a constant
    value.
  
* [#42728][]

  The behavior of `.description` and `.debugDescription` for floating-point
  numbers has been changed. Previously these unconditionally printed a fixed
  number of decimal digits (e.g. 15 and 17 for Double, respectively). They now
  print exactly as many digits as are needed for the resulting string to
  convert back to the original source value, and no more. For more details,
  see the original bug report and the linked pull request.

* [SE-0193][]

  Various function-like declarations can now be marked as `@inlinable`,
  making their bodies available for optimizations from other modules.

  Inlinable function bodies must only reference public declarations, unless
  the referenced declaration is marked as `@usableFromInline`.

  Note that the presence of the attribute itself does not force inlining or
  any other optimization to be performed, nor does it have any effect on
  optimizations performed within a single module.

* The C `long double` type is now imported as `Float80` on i386 and x86_64
  macOS and Linux. The tgmath functions in the Darwin and glibc modules now
  support `Float80` as well as `Float` and `Double`. Several tgmath
  functions have been made generic over `[Binary]FloatingPoint` so that they
  will automatically be available for any conforming type.

* [SE-0143][]

  The standard library types `Optional`, `Array`, `ArraySlice`,
  `ContiguousArray`, `Dictionary`, `Range`, and `ClosedRange` now conform to the
  `Hashable` protocol when their element or bound types (as the case may be)
  conform to `Hashable`.  This makes synthesized `Hashable` implementations
  available for types that include stored properties of these types.

* [SE-0196][]
  
  Custom compile-time warnings or error messages can be emitted using the
  `#warning(_:)` and `#error(_:)` directives.

  ```swift
  #warning("this is incomplete")

  #if MY_BUILD_CONFIG && MY_OTHER_BUILD_CONFIG
    #error("MY_BUILD_CONFIG and MY_OTHER_BUILD_CONFIG cannot both be set")
  #endif
  ```

* Public classes may now have internal `required` initializers. The rule for
  `required` initializers is that they must be available everywhere the class
  can be subclassed, but previously we said that `required` initializers on
  public classes needed to be public themselves. (This limitation is a holdover
  from before the introduction of the open/public distinction in Swift 3.)

* C macros containing casts are no longer imported to Swift if the type in the
  cast is unavailable or deprecated, or produces some other diagnostic when
  referenced. (These macros were already only imported under very limited
  circumstances with very simple values, so this is unlikely to affect
  real-world code.)

* [SE-0143][]

  Runtime query of conditional conformances is now implemented. Therefore,
  a dynamic cast such as `value as? P`, where the dynamic type of `value`
  conditionally conforms to `P`, will succeed when the conditional
  requirements are met.

## Swift 4.1

### 2018-03-29 (Xcode 9.3)

* [SE-0075][]

  Compile-time testing for the existence and importability of modules is now
  implemented as a build configuration test.  The `canImport` test allows
  the development of features that require a possibly-failing import 
  declaration across multiple platforms.  

  ```swift
  #if canImport(UIKit)
    import UIKit
    class MyView : UIView {}
  #elseif canImport(AppKit)
    import AppKit
    class MyView : NSView {}
  #else
    class MyView : CustomView {}
  #endif
  ```

* [SE-0189][]

  If an initializer is declared in a different module from a struct, it must
  use `self.init(…)` or `self = …` before returning or accessing `self`.
  Failure to do so will produce a warning in Swift 4 and an error in Swift 5.
  This is to keep a client app from accidentally depending on a library's
  implementation details, and matches an existing restriction for classes,
  where cross-module initializers must be convenience initializers.

  This will most commonly affect code that extends a struct imported from C.
  However, most imported C structs are given a zeroing no-argument initializer,
  which can be called as `self.init()` before modifying specific properties.

  Swift library authors who wish to continue allowing initialization on a
  per-member basis should explicitly declare a public memberwise initializer
  for clients in other modules to use.

* [SE-0166][] / [SE-0143][]

  The standard library now defines the conformances of `Optional`,
  `Array`, `Dictionary`, and `Set` to `Encodable` and `Decodable` as
  conditional conformances, available only when their type parameters
  conform to `Encodable` or `Decodable`, respectively.

* [SE-0188][] 
  
  Index types for most standard library collections now conform to `Hashable`. 
  These indices can now be used in key-path subscripts and hashed collections:
  
  ```swift
  let s = "Hashable"
  let p = \String.[s.startIndex]
  s[keyPath: p] // "H"
  ```

* [SE-0143][] The standard library types `Optional`, `Array`, `ArraySlice`,
  `ContiguousArray`, and `Dictionary` now conform to the `Equatable` protocol
  when their element types conform to `Equatable`. This allows the `==` operator
  to compose (e.g., one can compare two values of type `[Int : [Int?]?]` with
  `==`), as well as use various algorithms defined for `Equatable` element
  types, such as `index(of:)`.

* [SE-0157][] is implemented. Associated types can now declare "recursive"
  constraints, which require that the associated type conform to the enclosing
  protocol. The standard library protocols have been updated to make use of
  recursive constraints. For example, the `SubSequence` associated type of
  `Sequence` follows the enclosing protocol:

  ```swift
  protocol Sequence {
    associatedtype Element
    associatedtype SubSequence: Sequence
      where SubSequence.Element == Element,
            SubSequence.SubSequence == SubSequence
    // ...
  }

  protocol Collection: Sequence where Self.SubSequence: Collection {
    // ...
  }
  ```

  As a result, a number of new constraints have been introduced into the
  standard library protocols:

  * Make the `Indices` associated type have the same traversal requirements as
    its enclosing protocol, e.g., `Collection.Indices` conforms to
    `Collection`, `BidirectionalCollection.Indices` conforms to
    `BidirectionalCollection`, and so on
  * Make `Numeric.Magnitude` conform to `Numeric`
  * Use more efficient `SubSequence` types for lazy filter and map
  * Eliminate the `*Indexable` protocols

* [SE-0161][] is fully implemented. KeyPaths now support subscript, optional
  chaining, and optional force-unwrapping components.

* [SE-0186][]

  It is no longer valid to use the ownership keywords `weak` and `unowned` for property declarations in protocols. These keywords are meaningless and misleading when used in a protocol as they don't have any effect.

  In Swift 3 and 4 mode the following example will produce a warning with a fix-it to remove the keyword. In Swift 5 mode and above an error will be produced.

  ```swift
  class A {}

  protocol P {
      weak var weakVar: A? { get set }
      unowned var unownedVar: A { get set }
  }
  ```

* [SE-0185][]

  Structs and enums that declare a conformance to `Equatable`/`Hashable` now get an automatically synthesized implementation of `==`/`hashValue`. For structs, all stored properties must be `Equatable`/`Hashable`. For enums, all enum cases with associated values must be `Equatable`/`Hashable`.

  ```swift
  public struct Point: Hashable {
    public let x: Int
    public let y: Int

    public init(x: Int, y: Int) {
      self.x = x
      self.y = y
    }
  }

  Point(3, 0) == Point(0, 3)  // false
  Point(3, 0) == Point(3, 0)  // true
  Point(3, 0).hashValue       // -2942920663782199421

  public enum Token: Hashable {
    case comma
    case identifier(String)
    case number(Int)
  }

  Token.identifier("x") == .number(5)        // false
  Token.identifier("x") == .identifier("x")  // true
  Token.number(50).hashValue                 // -2002318238093721609
  ```

  If you wish to provide your own implementation of `==`/`hashValue`, you still can; a custom implementation will replace the one synthesized by the compiler.

## Swift 4.0

### 2017-09-19 (Xcode 9.0)

* [SE-0165][] and [SE-0154][]

  The standard library's `Dictionary` and `Set` types have some new features. You can now create a new dictionary from a sequence of keys and values, and merge keys and values into an existing dictionary.

  ```swift
  let asciiTable = Dictionary(uniqueKeysWithValues: zip("abcdefghijklmnopqrstuvwxyz", 97...))
  // ["w": 119, "n": 110, "u": 117, "v": 118, "x": 120, "q": 113, ...]

  let vegetables = ["tomato", "carrot", "onion", "onion", "carrot", "onion"]
  var vegetableCounts = Dictionary(zip(vegetables, repeatElement(1, count: Int.max)),
                                   uniquingKeysWith: +)
  vegetableCounts.merge([("tomato", 1)], uniquingKeysWith: +)
  // ["tomato": 2, "carrot": 2, "onion": 3]
  ```

  Filtering a set or a dictionary now results in the same type. You can also now transform just the values of a dictionary, keeping the same keys, using the `mapValues(_:)` method.

  ```swift
  let vowels: Set<Character> = ["a", "e", "i", "o", "u"]
  let asciiVowels = asciiTable.filter({ vowels.contains($0.key) })
  asciiVowels["a"]  // 97
  asciiVowels["b"]  // nil

  let asciiHexTable = asciiTable.mapValues({ "0x" + String($0, radix: 16) })
  // ["w": "0x77", "n": "0x6e", "u": "0x75", "v": "0x76", "x": "0x78", ...]
  ```

  When using a key as a dictionary subscript, you can now supply a default value to be returned if the key is not present in the dictionary.

  ```swift
  for veg in ["tomato", "cauliflower"] {
      vegetableCounts[veg, default: 0] += 1
  }
  // ["tomato": 3, "carrot": 2, "onion": 3, "cauliflower": 1]
  ```

  Use the new `init(grouping:by:)` initializer to convert an array or other sequence into a dictionary, grouped by a particular trait.

  ```swift
  let buttons = // an array of button instances
  let buttonsByStatus = Dictionary(grouping: buttons, by: { $0.isEnabled })
  // How many enabled buttons?
  print("Enabled:", buttonsByStatus[true]?.count ?? 0)
  ```

  Additionally, dictionaries and sets now have a visible `capacity` property and a `reserveCapacity(_:)` method similar to arrays, and a dictionary's `keys` and `values` properties are represented by specialized collections.

* [SE-0161][] is partially implemented. Swift now natively supports key path
  objects for properties. Similar to KVC key path strings in Cocoa, key path
  objects allow a property to be referenced independently of accessing it
  from a value:

    ```swift
    struct Point {
      var x, y: Double
    }
    let x = \Point.x
    let y = \Point.y

    let p = Point(x: 3, y: 4)
    p[keyPath: x] // gives 3
    p[keyPath: y] // gives 4
    ```

* Core Foundation types implicitly conform to Hashable (and Equatable), using
  CFHash and CFEqual as the implementation. This change applies even to "Swift
  3 mode", so if you were previously adding this conformance yourself, use
  `#if swift(>=3.2)` to restrict the extension to Swift 3.1 and below.
  ([#44995][])

* [SE-0156][]

  Protocol composition types can now contain one or more class type terms,
  forming a class-constrained protocol composition.

  For example:

  ```swift
  protocol Paintable {
    func paint()
  }

  class Canvas {
    var origin: CGPoint
  }

  class Wall : Canvas, Paintable {
    func paint() { ... }
  }

  func render(_: Canvas & Paintable) { ... }

  render(Wall())
  ```

  Note that class-constrained protocol compositions can be written and
  used in both Swift 3 and Swift 4 mode.

  Generated headers for Swift APIs will map class-constrained protocol
  compositions to Objective-C protocol-qualified class types in both
  Swift 3 and Swift 4 mode (for instance, `NSSomeClass & SomeProto &
  OtherProto` in Swift becomes `NSSomeClass <SomeProto, OtherProto>`
  in Objective-C).

  Objective-C APIs which use protocol-qualified class types differ in
  behavior when imported by a module compiled in Swift 3 mode and
  Swift 4 mode. In Swift 3 mode, these APIs will continue to import as
  protocol compositions without a class constraint
  (eg, `SomeProto & OtherProto`).

  In Swift 4 mode, protocol-qualified class types import as
  class-constrained protocol compositions, for a more faithful mapping
  of APIs from Objective-C to Swift.

  Note that the current implementation of class-constrained protocol
  compositions lacks three features outlined in the Swift evolution proposal:

  - In the evolution proposal, a class-constrained is permitted to contain
    two different classes as long as one is a superclass of the other.
    The current implementation only allows multiple classes to appear in
    the composition if they are identical.

  - In the evolution proposal, associated type and class inheritance clauses
    are generalized to allow class-constrained protocol compositions. The
    current implementation does not allow this.

  - In the evolution proposal, protocol inheritance clauses are allowed to
    contain a class, placing a requirement that all conforming types are
    a subclass of the given class. The current implementation does not
    allow this.

  These missing aspects of the proposal can be introduced in a future
  release without breaking source compatibility with existing code.

* [SE-0142][]

  Protocols and associated types can now contain `where` clauses that
  provide additional restrictions on associated types. For example:

    ```swift
    protocol StringRepresentable: RawRepresentable
    where RawValue == String { }

    protocol RawStringWrapper {
      associatedtype Wrapped: RawRepresentable
        where Wrapper.RawValue == String
    }
    ```

* [SE-0160][]

  In Swift 4 mode, a declaration is inferred to be `@objc` where it is required for semantic consistency of the programming model. Specifically, it is inferred when:

    * The declaration is an override of an `@objc` declaration
    * The declaration satisfies a requirement in an `@objc` protocol
    * The declaration has one of the following attributes: `@IBAction`, `@IBOutlet`, `@IBInspectable`, `@GKInspectable`, or `@NSManaged`

  Additionally, in Swift 4 mode, `dynamic` declarations that don't
  have `@objc` inferred based on the rules above will need to be
  explicitly marked `@objc`.

  Swift 3 compatibility mode retains the more-permissive Swift 3
  rules for inference of `@objc` within subclasses of
  `NSObject`. However, the compiler will emit warnings about places
  where the Objective-C entry points for these inference cases are
  used, e.g., in a `#selector` or `#keyPath` expression, via
  messaging through `AnyObject`, or direct uses in Objective-C code
  within a mixed project. The warnings can be silenced by adding an
  explicit `@objc`. Uses of these entrypoints that are not
  statically visible to the compiler can be diagnosed at runtime by
  setting the environment variable
  `SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT` to a value between 1 and 3
  and testing the application. See the [migration discussion in
  SE-0160](https://github.com/apple/swift-evolution/blob/main/proposals/0160-objc-inference.md#minimal-migration-workflow).

* [SE-0138](https://github.com/apple/swift-evolution/blob/main/proposals/0138-unsaferawbufferpointer.md#amendment-to-normalize-the-slice-type):

  Slicing a raw buffer no longer results in the same raw buffer
  type. Specifically, `Unsafe[Mutable]BufferPointer.SubSequence` now has type
  `[Mutable]RandomAccessSlice<Unsafe[Mutable]RawBufferPointer>`. Therefore,
  indexing into a raw buffer slice is no longer zero-based. This is required for
  raw buffers to fully conform to generic `Collection`. Changing the slice type
  resulted in the following behavioral changes:

  Passing a region within buffer to another function that takes a buffer can no
  longer be done via subscript:

  Incorrect: `takesRawBuffer(buffer[i..<j])`

  This now requires explicit initialization, using a `rebasing:` initializer,
  which converts from a slice to a zero-based `Unsafe[Mutable]RawBufferPointer`:

  Correct: `takesRawBuffer(UnsafeRawBufferPointer(rebasing: buffer[i..<j]))`

  Subscript assignment directly from a buffer no longer compiles:

  Incorrect: `buffer[n..<m] = smaller_buffer`

  This now requires creation of a slice from the complete source buffer:

  Correct: `buffer[n..<m] = smaller_buffer.suffix(from: 0)`

  `UnsafeRawBufferPointer`'s slice type no longer has a nonmutating subscript
  setter. So assigning into a mutable `let` buffer no longer compiles:

  ```swift
  let slice = buffer[n..<m]
  slice[i..<j] = buffer[k..<l]
  ```

  The assigned buffer slice now needs to be a `var`.

  ```swift
  var slice = buffer[n..<m]
  slice[i..<j] = buffer[k..<l]
  ```

* [#44138][]:

  Covariant method overrides are now fully supported, fixing many crashes
  and compile-time assertions when defining or calling such methods.
  Examples:

  ```swift
  class Bed {}
  class Nook : Bed {}

  class Cat<T> {
    func eat(snack: T) {}
    func play(game: String) {}
    func sleep(where: Nook) {}
  }

  class Dog : Cat<(Int, Int)> {
    // 'T' becomes concrete
    override func eat(snack: (Int, Int)) {}

    // 'game' becomes optional
    override func play(game: String?) {}

    // 'where' becomes a superclass
    override func sleep(where: Bed) {}
  }
  ```

* [SE-0148][]:

  Subscript declarations can now be defined to have generic parameter lists.
  Example:

  ```swift
  extension JSON {
    subscript<T>(key: String) -> T?
        where T : JSONConvertible {
      // ...
    }
  }
  ```

* [SE-0110][]:

  In Swift 4 mode, Swift's type system properly distinguishes between functions that
  take one tuple argument, and functions that take multiple arguments.

* More types of C macros which define integer constants are supported by the
  importer. Specifically the `+, -, *, /, ^, >>, ==, <, <=, >, >=` operators
  are now recognized, and the previously-supported `<<, &&, ||, &, |`
  operators always look through importable macros on each side of the operator.
  Logical AND and OR macros (`&&` and `||`) are now imported as Boolean
  constants, rather than integers of value 0 or 1.

  ```c
  #define HIGHER    (5 + 5)
  #define THE_EDGE  (INT64_MAX - 1)
  #define FORTY_TWO (6 * 9)
  #define SPLIT     (THE_EDGE / FORTY_TWO)

  #define HALF_AND_HALF (UINT64_MAX ^ UINT32_MAX)

  #define SMALL   (BITWIDTH == 32)
  #define TINY    (BITWIDTH <= 16)
  #define LIMITED (SMALL || TINY)   // now imported as Bool.
  ```

## Swift 3.1

### 2017-03-27 (Xcode 8.3)

* [SE-0080][]:

  Adds a new family of conversion initializers to all numeric types that
  either complete successfully without loss of information or return nil.

* Swift will now warn when an `NSObject` subclass attempts to override the
  class `initialize` method. Swift doesn't guarantee that references to class
  names trigger Objective-C class realization if they have no other
  side effects, leading to bugs when Swift code attempted to override
  `initialize`.

* [#45001][]

  C functions that "return twice" are no longer imported into Swift. Instead,
  they are explicitly made unavailable, so attempting to reference them will
  result in a compilation error.

  Examples of functions that "return twice" include `vfork` and `setjmp`.
  These functions change the control flow of a program in ways that Swift
  has never supported. For example, definitive initialization of variables,
  a core Swift language feature, could not be guaranteed when these functions
  were used.

  Swift code that references these functions will no longer compile. Although
  this could be considered a source-breaking change, it's important to note that
  any use of these functions would have most likely crashed at runtime. Now,
  the compiler will prevent them from being used in the first place.

* Indirect fields from C structures and unions are now always imported, while
  they previously weren't imported if they belonged to a union. This is done by
  naming anonymous fields. For example:

  ```c
  typedef struct foo_t {
    union {
      int a;
      double b;
    };
  } foo_t;
  ```

  Get imported as:

  ```swift
  struct foo_t {
    struct __Unnamed_union___Anonymous_field0 {
      var a : Int { get set }
      var b : Double { get set }
    }
    var __Anonymous_field0 : foo_t.__Unnamed_union___Anonymous_field0

    // a and b are computed properties accessing the content of __Anonymous_field0
    var a : Int { get set }
    var b : Double { get set }
  }
  ```

  Since new symbols are exposed from imported structure/unions, this may conflict
  with existing code that extended C types in order to provide their own accessors
  to the indirect fields.

* The `withoutActuallyEscaping` function from [SE-0103][] has been implemented.
  To pass off a non-escaping closure to an API that formally takes an
  `@escaping` closure, but which is used in a way that will not in fact
  escape it in practice, use `withoutActuallyEscaping` to get an escapable
  copy of the closure and delimit its expected lifetime. For example:

  ```swift
  func doSimultaneously(_ f: () -> (), and g: () -> (), on q: DispatchQueue) {
    // DispatchQueue.async normally has to be able to escape its closure
    // since it may be called at any point after the operation is queued.
    // By using a barrier, we ensure it does not in practice escape.
    withoutActuallyEscaping(f) { escapableF in
      withoutActuallyEscaping(g) { escapableG in
        q.async(escapableF)
        q.async(escapableG)
        q.sync(flags: .barrier) {}
      }
    }
    // `escapableF` and `escapableG` must be dequeued by the point
    // `withoutActuallyEscaping` returns.
  }
  ```

  The old workaround of using `unsafeBitCast` to cast to an `@escaping` type
  is not guaranteed to work in future versions of Swift, and will
  now raise a warning.

* [#44055][]

  Nested types may now appear inside generic types, and nested types may have their own generic parameters:

  ```swift
  struct OuterNonGeneric {
      struct InnerGeneric<T> {}
  }

  struct OuterGeneric<T> {
      struct InnerNonGeneric {}

      struct InnerGeneric<T> {}
  }

  extension OuterNonGeneric.InnerGeneric {}
  extension OuterGeneric.InnerNonGeneric {}
  extension OuterGeneric.InnerGeneric {}
  ```

* [#43621][]:

  Constrained extensions allow same-type constraints between generic parameters and concrete types. This enables you to create extensions, for example, on `Array` with `Int` elements:

  ```swift
  extension Array where Element == Int { }
  ```

* [SE-0045][]:

  The `Sequence` protocol adds two new members `prefix(while:)` and
  `drop(while:)` for common utility. `prefix(while:)` requests the longest subsequence
  satisfying a predicate.  `drop(while:)` requests the remaining
  subsequence after dropping the longest subsequence satisfying a
  predicate.

## Swift 3.0

### 2016-09-13 (Xcode 8.0)

* [SE-0101][]:

  The functions `sizeof()`, `strideof()`, and `alignof()` have been removed.
  Memory layout properties for a type `T` are now spelled
  `MemoryLayout<T>.size`, `MemoryLayout<T>.stride`, and
  `MemoryLayout<T>.alignment`, respectively.

* [SE-0136][]:

  The functions `sizeofValue()`, `strideofValue()`, and `alignofValue()` have been renamed to `MemoryLayout.size(ofValue:)`, `MemoryLayout.stride(ofValue:)`,
  and `MemoryLayout.alignment(ofValue:)`.

* [SE-0125][]:

  The functions `isUniquelyReferenced()` and `isUniquelyReferencedNonObjC()`
  have been removed. Call the function `isKnownUniquelyReferenced()` instead.

  Classes using `isUniquelyReferenced()` needed to inherit from `NonObjectiveCBase`. The `NonObjectiveCBase` class has been removed.

  The method `ManagedBufferPointer.holdsUniqueReference` has been renamed to
  `ManagedBufferPointer.isUniqueReference`.

  ```swift
  // old
  class SwiftKlazz : NonObjectiveCBase {}
  expectTrue(isUniquelyReferenced(SwiftKlazz()))

  var managedPtr : ManagedBufferPointer = ...
  if !managedPtr.holdsUniqueReference() {
    print("not unique")
  }

  // new
  class SwiftKlazz {}
  expectTrue(isKnownUniquelyReferenced(SwiftKlazz()))

  var managedPtr : ManagedBufferPointer = ...
  if !managedPtr.isUniqueReference() {
    print("not unique")
  }
  ```

* [SE-0124][]:

  Initializers on `Int` and `UInt` that accept an `ObjectIdentifier` must now use an explicit `bitPattern` label.

  ```swift
  let x: ObjectIdentifier = ...

  // old
  let u = UInt(x)
  let i = Int(x)

  // new
  let u = UInt(bitPattern: x)
  let i = Int(bitPattern: x)
  ```

* [SE-0120][]:

  The collection methods `partition()` and `partition(isOrderedBefore:)` have been removed from Swift. They are replaced by the method `partition(by:)` which takes a unary predicate.

  Calls to the `partition()` method can be replaced by the following code.

  ```swift
  // old
  let p = c.partition()

  // new
  let p = c.first.flatMap({ first in
      c.partition(by: { $0 >= first })
  }) ?? c.startIndex
  ```

* [SE-0103][]:

  Closure parameters are now non-escaping by default and do not require `@noescape` annotation. Use `@escaping` to indicate that a closure parameter can escape. `@autoclosure(escaping)` is now spelled `@autoclosure @escaping`. `@noescape` and `@autoclosure(escaping)` are deprecated.

* [SE-0115][]:

  To clarify their roles, `*LiteralConvertible` protocols have been renamed to `ExpressibleBy*Literal`.  The protocol requirements are unchanged.

* [SE-0107][]:

  An `Unsafe[Mutable]RawPointer` type has been introduced. It replaces
  `Unsafe[Mutable]Pointer<Void>`. Conversion from `UnsafePointer<T>`
  to `UnsafePointer<U>` has been disallowed. `Unsafe[Mutable]RawPointer`
  provides an API for untyped memory access, and an API for binding memory
  to a type. Binding memory allows for safe conversion between pointer types.

  For detailed instructions on how to migrate your code to the new API refer to the [UnsafeRawPointer migration guide](https://swift.org/migration-guide/se-0107-migrate.html). See also: See `bindMemory(to:capacity:)`, `assumingMemoryBound(to:)`, and
  `withMemoryRebound(to:capacity:)`.

* [SE-0096][]:

  The `dynamicType` keyword has been removed from Swift.  It's replaced by a new primitive function `type(of:)`.  Existing code
using the `.dynamicType` member to retrieve the type of an expression should migrate to this new primitive.  Code using `.dynamicType` in conjunction with `sizeof` should migrate to the `MemoryLayout` structure introduced by [SE-0101][].

* [SE-0113][]:

  The following two methods were added to `FloatingPoint`:

  ```swift
  func rounded(_ rule: FloatingPointRoundingRule) -> Self
  mutating func round( _ rule: FloatingPointRoundingRule)
  ```

  These methods bind the IEEE 754 roundToIntegral operations. They provide the functionality of the C / C++ `round()`, `ceil()`, `floor()`, and `trunc()` functions along with other rounding operations.

  Following onto [SE-0113][] and [SE-0067][], the following `Darwin.C` and `glibc` module mathematical operations now operate on any type conforming to `FloatingPoint`: `fabs`, `sqrt`, `fma`,
  `remainder`, `fmod`, `ceil`, `floor`, `round`, and `trunc`.

  See also: the changes associated with [SE-0067][].

* [SE-0067][]:

  The `FloatingPoint` protocol has been expanded to include most IEEE 754
  required operations. A number of useful properties have been added to the
  protocol, representing quantities like the largest finite value or
  the smallest positive normal value (these correspond to the macros such as
  FLT_MAX defined in C).

  While almost all of the changes are additive, four changes impact existing code:

  - The `%` operator is no longer available for `FloatingPoint` types. It
  was difficult to use correctly and its semantics did not match
  those of the corresponding integer operation. This made it something of an attractive nuisance. The new method `formTruncatingRemainder(dividingBy:)`
  provides the old semantics if they are needed.

  - The static property `.NaN` has been renamed `.nan`.

  - The static property `.quietNaN` was redundant and has been removed. Use
  `.nan` instead.

  - The predicate `isSignaling` has been renamed `isSignalingNaN`.

  See also: the changes associated with [SE-0113][].

* [SE-0111][]:

  Argument labels have been removed from Swift function types. They are now
  part of the name of a function, subscript, or initializer. Calls to a function or initializer, and subscript uses, still require argument labels as they always have:

  ```swift
  func doSomething(x: Int, y: Int) { }
  doSomething(x: 0, y: 0)     // argument labels are required
  ```

  Unapplied references to functions or initializers no longer carry argument labels. For example:

  ```swift
  let f = doSomething(x:y:)     // inferred type is now (Int, Int) -> Void
  ```

  Explicitly-written function types can no longer carry argument labels. You can still provide parameter names for documentation purposes using the '_' in the argument label position:

  ```swift
  typealias CompletionHandler =
     (token: Token, error: Error?) -> Void   // error: function types cannot have argument labels

  typealias CompletionHandler =
     (_ token: Token, _ error: Error?) -> Void   // okay: names are for documentation purposes
  ```

* [SE-0025][]:

  The access level formerly known as `private` is now called `fileprivate`. A Swift 3 declaration marked `private` can no longer be accessed outside its lexical scope (essentially its enclosing curly braces `{}`). A `private` declaration at the top level of a file can be accessed anywhere within the same file, as it could in Swift 2.

* [SE-0131][]:

  The standard library introduces the `AnyHashable` type for use in hashed heterogeneous collections. Untyped `NSDictionary` and `NSSet` Objective-C APIs now import as `[AnyHashable: Any]` and `Set<AnyHashable>`.

* [SE-0102][]:

  Swift removes the `@noreturn` attribute on function declarations and replaces the attribute with an empty `Never` type:

  ```swift
  @noreturn func fatalError(msg: String) { ... }  // old
  func fatalError(msg: String) -> Never { ... }   // new

  func performOperation<T>(continuation: @noreturn T -> ()) { ... }  // old
  func performOperation<T>(continuation: T -> Never) { ... }         // new
  ```

* [SE-0116][]:

  Swift now imports Objective-C `id` APIs as `Any`. In Swift 2, `id` imported as `AnyObject`. Swift also imports untyped `NSArray` and `NSDictionary` as `[Any]` and `[AnyHashable: Any]`, respectively.

* [SE-0072][]:

  Swift eliminates implicit bridging conversions. Use `as` to force the conversion from a Swift value type to its corresponding object. For example, use `string as NSString`. Use `as AnyObject` to convert a Swift value to its boxed `id` representation.

* Collection subtype conversions and dynamic casts now work with protocol types:

    ```swift
    protocol P {}; extension Int: P {}
    var x: [Int] = [1, 2, 3]
    var p: [P] = x
    var x2 = p as! [Int]
    ```

* [#44739][]:

  The `hasPrefix` and `hasSuffix` functions now consider the empty string to be a prefix and suffix of all strings.

* [SE-0128][]:

  Some non-failable UnicodeScalar initializers now return an Optional. When a UnicodeScalar cannot be constructed, these initializers return nil.

  ```swift
  // Old
  var string = ""
  let codepoint: UInt32 = 55357 // Invalid
  let ucode = UnicodeScalar(codepoint) // Program crashes here.
  string.append(ucode)
  ```

  The updated initializers allow users to write code that safely works around invalid codepoints, like this example:

  ```swift
  // New
  var string = ""
  let codepoint: UInt32 = 55357 // Invalid
  if let ucode = UnicodeScalar(codepoint) {
      string.append(ucode)
  } else {
      // do something else
  }
  ```

* [SE-0095][]:

  Swift removes the `protocol<...>` composition construct and introduces an infix type operator `&` in its place.

  ```swift
  let a: Foo & Bar
  let b = value as? A & B & C
  func foo<T : Foo & Bar>(x: T) { ... }
  func bar(x: Foo & Bar) { ... }
  typealias G = GenericStruct<Foo & Bar>
  ```

  Swift previously defined the empty protocol composition (the `Any` type) as `protocol<>`. This definition has been removed from the standard library. The `Any` keyword behavior remains unchanged.

* [SE-0091][]:

  Swift permits you to define operators within types or their extensions. For example:

  ```swift
  struct Foo: Equatable {
    let value: Int

    static func ==(lhs: Foo, rhs: Foo) -> Bool {
      return lhs.value == rhs.value
    }
  }
  ```

  You must declare these operators as `static` (or, within a class, `class
  final`) and they must use the same signature as their global counterparts. As part of this change, protocol-declared operator requirements must be declared `static` explicitly:

  ```swift
  protocol Equatable {
    static func ==(lhs: Self, rhs: Self) -> Bool
  }
  ```

  Note: The type checker performance optimization described by [SE-0091][]
  is not yet implemented.

* [SE-0099][]:

  Condition clauses in `if`, `guard`, and `while` statements now use a more
  regular syntax. Each pattern or optional binding must be prefixed with `case`
  or `let` respectively, and all conditions are separated by `,` instead of
  `where`.

  ```swift
  // before
  if let a = a, b = b where a == b { }

  // after
  if let a = a, let b = b, a == b { }
  ```

* [SE-0112][]:

  The `NSError` type now bridges to the Swift `Error` protocol type (formerly `ErrorProtocol` in Swift 3, `ErrorType` in Swift 2)
  in Objective-C APIs. `NSError` now bridges like other Objective-C types, e.g., `NSString` bridges to `String`.

  For
  example, the `UIApplicationDelegate` method
  `applicate(_:didFailToRegisterForRemoteNotificationsWithError:)`
  previously accepted an `NSError` argument:

  ```swift
  optional func application(_ application: UIApplication,
    didFailToRegisterForRemoteNotificationsWithError error: NSError)
  ```

  Now it accepts an `Error` argument:

  ```swift
  optional func application(_ application: UIApplication,
    didFailToRegisterForRemoteNotificationsWithError error: Error)
  ```

  Error types imported from Cocoa[Touch] maintain all of
  the information in the corresponding `NSError`. You no longer `catch let as NSError` to extract, for example, the user-info
  dictionary.

  Specific error types now contain typed accessors for
  their common user-info keys. For example:

  ```swift
  catch let error as CocoaError where error.code == .fileReadNoSuchFileError {
    print("No such file: \(error.url)")
  }
  ```

  Swift-defined error types can now provide localized error
  descriptions by adopting the new `LocalizedError` protocol, e.g.,

  ```swift
  extension HomeworkError : LocalizedError {
    var errorDescription: String? {
      switch self {
      case .forgotten: return NSLocalizedString("I forgot it")
      case .lost: return NSLocalizedString("I lost it")
      case .dogAteIt: return NSLocalizedString("The dog ate it")
      }
    }
  }
  ```

  New `RecoverableError` and `CustomNSError` protocols
  allow additional control over the handling of the error.

* [SE-0060][]:

  Function parameters with defaulted arguments are specified in
  declaration order. Call sites must now supply those arguments using that order:

    ```swift
    func requiredArguments(a: Int, b: Int, c: Int) {}
    func defaultArguments(a: Int = 0, b: Int = 0, c: Int = 0) {}

    requiredArguments(a: 0, b: 1, c: 2)
    requiredArguments(b: 0, a: 1, c: 2) // error
    defaultArguments(a: 0, b: 1, c: 2)
    defaultArguments(b: 0, a: 1, c: 2) // error
    ```

    Labeled parameters with default arguments may still be elided, so long as included arguments follow declaration order:

    ```swift
    defaultArguments(a: 0) // ok
    defaultArguments(b: 1) // ok
    defaultArguments(c: 2) // ok
    defaultArguments(a: 1, c: 2) // ok
    defaultArguments(b: 1, c: 2) // ok
    defaultArguments(c: 1, b: 2) // error
    ```

* Traps from force-unwrapping nil `Optional`s now show the source location of the force unwrap operator.

* [SE-0093][]:

  Slice types add a `base` property that allows public readonly access to their base collections.

* Nested generic functions may now capture bindings from the environment, for example:

    ```swift
    func outer<T>(t: T) -> T {
      func inner<U>(u: U) -> (T, U) {
        return (t, u)
      }
      return inner(u: (t, t)).0
    }
    ```

* Initializers are now inherited even if the base class or derived class is generic:

    ```swift
    class Base<T> {
      let t: T

      init(t: T) {
        self.t = t
      }
    }

    class Derived<T> : Base<T> {
      // init(t: T) is now synthesized to call super.init(t: t)
    }
    ```

* [SE-0081][]:

  "Move `where` clause to end of declaration" is now implemented. This change allows you to write `where` clauses after a declaration signature and before its body.  For example, before this change was implemented, you'd write:

    ```swift
    func anyCommonElements<T : SequenceType, U : SequenceType
        where T.Generator.Element: Equatable, T.Generator.Element == U.Generator.Element>
        (lhs: T, _ rhs: U) -> Bool
    {
        ...
    }
    ```

  Now the `where` clause appears just before the body:

    ```swift
    func anyCommonElements<T : SequenceType, U : SequenceType>(lhs: T, _ rhs: U) -> Bool
        where T.Generator.Element: Equatable, T.Generator.Element == U.Generator.Element
    {
        ...
    }
    ```

  The old form is currently accepted for compatibility. It will eventually be rejected.

* [SE-0071][]:

  "Allow (most) keywords in member references" is implemented.  This change allows the use of members after a dot without backticks, e.g. "foo.default", even though `default` is a keyword for `switch` statements.

* [SE-0057][]:

  Objective-C lightweight generic classes are now imported as generic types
  in Swift. Some limitations apply because Objective-C generics are not represented at runtime:

  - When an ObjC generic class is used in a checked `as?`, `as!`, or `is` cast, the generic parameters are not checked at runtime. The cast succeeds if the operand is an instance of the ObjC class, regardless of parameters.

    ```swift
    let x = NSFoo<NSNumber>(value: NSNumber(integer: 0))
    let y: AnyObject = x
    let z = y as! NSFoo<NSString> // Succeeds
    ```

  - Swift subclasses can only inherit from an ObjC generic class when its generic parameters are fully specified.

    ```swift
    // Error: Can't inherit ObjC generic class with unbound parameter T
    class SwiftFoo1<T>: NSFoo<T> { }

    // OK: Can inherit ObjC generic class with specific parameters
    class SwiftFoo2<T>: NSFoo<NSString> { }
    ```

  - Swift can extend ObjC generic classes but the extensions cannot be constrained, and definitions inside the extension don't have access to the class's generic parameters.

    ```swift
    extension NSFoo {
      // Error: Can't access generic param T
      func foo() -> T {
        return T()
      }
    }

    // Error: extension can't be constrained
    extension NSFoo where T: NSString {
    }
    ```

  - Foundation container classes `NS[Mutable]Array`, `NS[Mutable]Set`, and `NS[Mutable]Dictionary` are still imported as nongeneric classes for the time being.

* [SE-0036][]:

  Enum elements can no longer be accessed as instance members in instance methods.

  * As part of the changes for [SE-0055][] (see below), the *pointee* types of imported pointers (e.g. the `id` in `id *`) are no longer assumed to always be `_Nullable` even if annotated otherwise.
  * An implicit or explicit annotation of `_Null_unspecified` on a pointee type still imports as `Optional`.

* [SE-0055][]:

  The types `UnsafePointer`, `UnsafeMutablePointer`,
  `AutoreleasingUnsafeMutablePointer`, `OpaquePointer`, `Selector`, and `Zone` (formerly `NSZone`) now represent non-nullable pointers, i.e. pointers that are never `nil`. A nullable pointer is now represented using `Optional`, e.g. `UnsafePointer<Int>?` For types imported from C, non-object pointers (such as `int *`) now have their nullability taken into account.

  One possible area of difficulty is passing a nullable pointer to a function that uses C variadics. Swift will not permit this directly. As a workaround, use the following idiom to pass a pointer-sized integer value instead:

  ```swift
  unsafeBitCast(nullablePointer, to: Int.self)
  ```

* [SE-0046][]:

  Function parameters adopt consistent labeling across all function parameters. With this update, first parameter declarations match the existing behavior of the second and later parameters. This change makes the language simpler.

  Functions that were written and called as follows:

  ```swift
  func foo(x: Int, y: Int) {}
  foo(1, y: 2)

  func bar(a a: Int, b: Int) {}
  bar(a: 3, b: 4)
  ```

  Are now written as follows with the same behavior at call sites:

  ```swift
  func foo(_ x: Int, y: Int) {}
  foo(1, y: 2)

  func bar(a: Int, b: Int) {}
  bar(a: 3, b: 4)
  ```

* [SE-0037][]:

  Comments are now treated as whitespace when determining whether an operator is prefix, postfix, or binary. For example, these now work:

  ```swift
  if /*comment*/!foo { ... }
  1 +/*comment*/2
  ```

  Comments can no longer appear between a unary operator and its argument.

  ```swift
  foo/* comment */! // no longer works
  ```

  Parse errors resulting from this change can be resolved by moving the comment outside the expression.

* [SE-0031][]:

  The location of the inout attribute moves to after the colon (`:`) and before the parameter type.

  ```swift
  func foo(inout x: Int) {
  }
  ```

  will now be written as:

  ```swift
  func foo(x: inout Int) {
  }
  ```

* [SE-0053][]:

  `let` is no longer accepted as a parameter attribute for functions. The compiler provides a fixit to remove it from the function declaration.

* [SE-0003][]:

  `var` is no longer accepted as a parameter attribute for functions. The compiler provides a fixit to create a shadow copy in the function body.

  ```swift
  func foo(var x: Int) {
  }
  ```

  will now be written as:

  ```swift
  func foo(x: Int) {
    var x = x
  }
  ```

* The "none" members of imported NS_OPTIONS option sets are marked as unavailable when they are imported.  Use `[]` to make an empty option set, instead of a None member.

* [SE-0043][]

  Adds the ability to declare variables in multiple patterns in cases.

* [SE-0005][]

  Allows the Clang importer to import ObjC symbols using substantially different Swift-like naming paradigms:

  * These updates generalize the use of `swift_name`, allowing arbitrary C and Objective-C entity import names. This adds fine-grained control over the import process.
  * Redundant type names are pruned (`documentForURL(_: NSURL)` becomes `document(for: URL)`). Selectors are guaranteed to never be empty, to be transformed into Swift keywords, to be vacuously named (like `get`, `set`, `with`, `for`). Additional pruning rules preserve readability and sense.
  * Common arguments are sensibly defaulted where the Objective-C API strongly hints at the need for a default argument. (For example,  nullable trailing closures default to `nil`, option sets to `[]`, and `NSDictionary` parameters to `[:]`.) First argument labels are added for defaulted arguments.
  * Boolean properties are prepended with `is`, and read as assertions on the receiver.
  * Non-type values, including enumerators, are lowercased.
  * Classes that implement `compare(_:) -> NSComparisonResult` automatically import as `Comparable`.

* [SE-0040][]

  Attributes change from using `=` in parameters lists
  to using `:`, aligning with function call syntax.

  ```swift
  // before
  @available(*, unavailable, renamed="MyRenamedProtocol")

  // after
  @available(*, unavailable, renamed: "MyRenamedProtocol")
  ```

* [SE-0048][]

  Generic typealiases are now supported. For example:

  ```swift
  typealias StringDictionary<T> = Dictionary<String, T>
  typealias IntFunction<T> = (T) -> Int
  typealias MatchingTriple<T> = (T, T, T)
  typealias BackwardTriple<T1, T2, T3> = (T3, T2, T1)
  ```

  etc.

* [SE-0049][]

  The `@noescape` attribute is extended to be a more general type attribute. You can now declare values of `@noescape` function type, e.g. in manually curried function signatures.  You can now also declare local variables of `@noescape` type, and use `@noescape` in `typealiases`.  For example, this is now valid code:

  ```swift
  func apply<T, U>(@noescape f: T -> U,
                   @noescape g: (@noescape T -> U) -> U) -> U {
    return g(f)
  }
  ```

* [SE-0034][]

  The `#line` directive (which resets the logical
  source location for diagnostics and debug information) is renamed to `#sourceLocation`.

* [SE-0002][]

  Curried function syntax (with successive parenthesized groups of arguments) is removed, and now produces a compile-time error. Use chained functional return types instead.

  ```swift
  // Before
  public func project(function f: FunctionType)(p0: CGPoint, p1: CGPoint)(x: CGFloat) -> CGPoint

  // After
  public func project(function f: FunctionType) -> (p0: CGPoint, p1: CGPoint) -> (x: CGFloat) -> CGPoint
  ```

* Generic signatures can now contain superclass requirements with generic parameter types, for example:

  ```swift
  func f<Food : Chunks<Meat>, Meat : Molerat>(f: Food, m: Meat) {}
  ```

* Section markers are created in ELF binaries through special objects during link time. These objects allow for the deletion of `swift.ld` and the use of non-BFD linkers. A new argument to swiftc is provided to select the linker used, and the gold linker is set as the default for arm-based platforms.

* Catch blocks in `rethrows` functions may now `throw` errors. For example:

    ```swift
    func process(f: () throws -> Int) rethrows -> Int {
        do {
            return try f()
        } catch is SomeError {
            throw OtherError()
        }
    }
    ```

* Throwing closure arguments of a rethrowing function may now be optional. For example:

    ```swift
    func executeClosureIfNotNil(closure: (() throws -> Void)?) rethrows {
        try closure?()
    }
    ```

* [SE-0064][]:

  The Objective-C selectors for the getter or setter of a property can now be referenced with `#selector`. For example:

    ```swift
    let sel1 = #selector(getter: UIView.backgroundColor) // sel1 has type Selector
    let sel2 = #selector(setter: UIView.backgroundColor) // sel2 has type Selector
    ```

* [SE-0062][]:

  A key-path can now be formed with `#keyPath`. For example:

    ```swift
    person.valueForKeyPath(#keyPath(Person.bestFriend.lastName))
    ```

## Swift 2.2

### 2016-03-21 (Xcode 7.3)

* [SE-0011][]:

  Associated types in protocols can now be specified with a new `associatedtype`
  declaration, to replace the use of `typealias`:

    ```swift
    protocol P {
      associatedtype Ty
    }
    ```

  The `typealias` keyword is still allowed (but deprecated and produces a warning)
  in Swift 2.2. This warning will become an error in Swift 3.0.

* [SE-0002][]:

  Curried function syntax has been deprecated, and is slated to be removed in
  Swift 3.0.

* [SE-0004][]:

  The `++` and `--` operators have been deprecated, and are slated to be removed in
  Swift 3.0.  As a replacement, please use `x += 1` on integer or floating point
  types, and `x = x.successor()` on Index types.

* [SE-0029][]:

  The implicit tuple splat behavior in function application has been deprecated
  and will be removed in Swift 3.0.  For example, this code:

    ```swift
    func foo(a : Int, b : Int) { ... }
    let x = (1, b: 2)
    foo(x)   // Warning, deprecated.
    ```

  should move to being written as:

    ```swift
    foo(x.0, x.b)
    ```

* [SE-0028][]:

  New `#file`, `#line`, `#column`, and `#function` expressions have been introduced to
  replace the existing `__FILE__`, `__LINE__`, `__COLUMN__`, and `__FUNCTION__` symbols.
  The `__FILE__`-style symbols have been deprecated, and will be removed in
  Swift 3.0.

* The operator identifier lexer grammar has been revised to simplify the rules
  for operators that start with a dot (".").  The new rule is that an operator
  that starts with a dot may contain other dots in it, but operators that start
  with some other character may not contain dots.  For example:

    ```swift
    x....foo   --> "x" "...." "foo"
    x&%^.foo   --> "x" "&%^"  ".foo"
    ```

  This eliminates a special case for the `..<` operator, folding it into a simple
  and consistent rule.

* [SE-0007][]:

  The "C-style for loop", which is spelled `for init; comparison; increment {}`
  has been deprecated and is slated for removal in Swift 3.0.

* Three new doc comment fields, namely `- keyword:`, `- recommended:`
  and `- recommendedover:`, allow Swift users to cooperate with code
  completion engine to deliver more effective code completion results.
  The `- keyword:` field specifies concepts that are not fully manifested in
  declaration names. `- recommended:` indicates other declarations are preferred
  to the one decorated; to the contrary, `- recommendedover:` indicates
  the decorated declaration is preferred to those declarations whose names
  are specified.

* Designated class initializers declared as failable or throwing may now
  return `nil` or throw an error, respectively, before the object has been
  fully initialized. For example:

    ```swift
    class Widget : Gadget {
      let complexity: Int

      init(complexity: Int, elegance: Int) throws {
        if complexity > 3 { throw WidgetError.TooComplex }
        self.complexity = complexity

        try super.init(elegance: elegance)
      }
    }
    ```

* All slice types now have `removeFirst()` and `removeLast()` methods.

* `ArraySlice.removeFirst()` now preserves element indices.

* Global `anyGenerator()` functions have been changed into initializers on
  `AnyGenerator`, making the API more intuitive and idiomatic.  They have been
  deprecated in Swift 2.2, and will be removed in Swift 3.0.

* Closures appearing inside generic types and generic methods can now be
  converted to C function pointers, as long as no generic type parameters
  are referenced in the closure's argument list or body. A conversion of
  a closure that references generic type parameters now produces a
  diagnostic instead of crashing.

  **(rdar://problem/22204968)**

* Anonymously-typed members of C structs and unions can now be accessed
  from Swift. For example, given the following struct 'Pie', the 'crust'
  and 'filling' members are now imported:

    ```swift
    struct Pie {
      struct { bool crispy; } crust;
      union { int fruit; } filling;
    }
    ```

  Since Swift does not support anonymous structs, these fields are
  imported as properties named `crust` and `filling` having nested types
  named `Pie.__Unnamed_crust` and `Pie.__Unnamed_filling`.

  **(rdar://problem/21683348)**

* [SE-0001][]:

  Argument labels and parameter names can now be any keyword except
  `var`, `let`, or `inout`. For example:

  ```swift
  NSURLProtectionSpace(host: "somedomain.com", port: 443, protocol: "https",
                       realm: "Some Domain", authenticationMethod: "Basic")
  ```

  would previously have required `protocol` to be surrounded in
  back-ticks.

* [SE-0015][]:

  Tuples (up to arity 6) whose elements are all `Comparable` or `Equatable` now
  implement the full set of comparison/equality operators. The comparison
  operators are defined in terms of
  [lexicographical order](https://en.wikipedia.org/wiki/Lexicographical_order).

* The `@objc(SomeName)` attribute is now supported on enums and enum cases to
  rename the generated Objective-C declaration.

  **(rdar://problem/21930334)**

* [SE-0021][]:

  When referencing a function or initializer, one can provide the
  complete name, including argument labels. For example:

  ```swift
  let fn1 = someView.insertSubview(_:at:)
  let fn2 = someView.insertSubview(_:aboveSubview:)

  let buttonFactory = UIButton.init(type:)
  ```

* [SE-0020][]:

  There is a new build configuration function, `#if swift(>=x.y)`, which
  tests if the current Swift language version is at least `x.y`. This
  allows you to conditionally compile code for multiple language
  versions in the same file, even with different syntax, by deactivating
  parsing in inactive code blocks. For example:

  ```swift
  #if swift(>=2.2)
    // Only this code will be parsed in Swift 3.0
    func foo(x: Int) -> (y: Int) -> () {}
  #else
    // This code is ignored entirely.
    func foo(x: Int)(y: Int) {}
  #endif
  ```

* [SE-0022][]:

  The Objective-C selector of a Swift method can now be determined
  directly with the #selector expression, e.g.,:

  ```swift
  let sel = #selector(insertSubview(_:aboveSubview:)) // sel has type Selector
  ```

  Along with this change, the use of string literals as selectors has
  been deprecated, e.g.,

  ```swift
  let sel: Selector = "insertSubview:aboveSubview:"
  ```

  Generally, such string literals should be replaced with uses of
  `#selector`, and the compiler will provide Fix-Its that use
  `#selector`. In cases where this is not possible (e.g., when referring
  to the getter of a property), one can still directly construct
  selectors, e.g.:

  ```swift
  let sel = Selector("propertyName")
  ```

  Note that the compiler is now checking the string literals used to
  construct Selectors to ensure that they are well-formed Objective-C
  selectors and that there is an `@objc` method with that selector.

## Swift 2.1

### 2015-10-21 (Xcode 7.1)

* Enums imported from C now automatically conform to the `Equatable` protocol,
  including a default implementation of the `==` operator. This conformance
  allows you to use C enum pattern matching in switch statements with no
  additional code. **(17287720)**

* The `NSNumber.unsignedIntegerValue` property now has the type `UInt` instead
  of `Int`, as do other methods and properties that use the `NSUInteger` type
  in Objective-C and whose names contain `unsigned..`. Most other uses of
  `NSUInteger` in system frameworks are imported as `Int` as they were in
  Xcode 7. **(19134055)**

* Field getters and setters are now created for named unions imported from C.
  In addition, an initializer with a named parameter for the field is provided.
  For example, given the following Objective-C `typedef`:

  ```objc
  typedef union IntOrFloat {
    int intField;
    float floatField;
  } IntOrFloat;
  ```

  Importing this `typedef` into Swift generates the following interface:

  ```swift
  struct IntOrFloat {
    var intField: Int { get set }
    init(intField: Int)

    var floatField: Float { get set }
    init(floatField: Float)
  }
  ```

  **(19660119)**

* Bitfield members of C structs are now imported into Swift. **(21702107)**

* The type `dispatch_block_t` now refers to the type
  `@convention(block) () -> Void`, as it did in Swift 1.2.
  This change allows programs using `dispatch_block_create` to work as expected,
  solving an issue that surfaced in Xcode 7.0 with Swift 2.0.

  **Note:** Converting to a Swift closure value and back is not guaranteed to
  preserve the identity of a `dispatch_block_t`.
  **(22432170)**

* Editing a file does not trigger a recompile of files that depend upon it if
  the edits only modify declarations marked private. **(22239821)**

* Expressions interpolated in strings may now contain string literals.
  For example, `My name is \(attributes["name"]!)` is now a valid expression.
  **(14050788)**

* Error messages produced when the type checker cannot solve its constraint
  system continue to improve in many cases.

  For example, errors in the body of generic closures (for instance, the
  argument closure to `map`) are much more usefully diagnosed. **(18835890)**

* Conversions between function types are supported, exhibiting covariance in
  function result types and contravariance in function parameter types.
  For example, it is legal to assign a function of type `Any -> Int` to a
  variable of type `String -> Any`. **(19517003)**

## Swift 2.0

### 2015-09-17 (Xcode 7.0)

#### Swift Language Features

* New `defer` statement. This statement runs cleanup code when the scope is
  exited, which is particularly useful in conjunction with the new error
  handling model. For example:

  ```swift
  func xyz() throws {
     let f = fopen("x.txt", "r")
     defer { fclose(f) }
     try foo(f)                    // f is closed if an error is propagated.
     let f2 = fopen("y.txt", "r")
     defer { fclose(f2) }
     try bar(f, f2)                // f2 is closed, then f is closed if an error is propagated.
  }                                // f2 is closed, then f is closed on a normal path
  ```

  **(17302850)**

* Printing values of certain `enum` types shows the enum `case` and payload, if
  any. This is not supported for `@objc` enums or certain enums with multiple
  payloads. **(18334936)**

* You can specify availability information on your own declarations with the
  `@available()` attribute.

  For example:

  ```swift
  @available(iOS 8.0, OSX 10.10, *)
  func startUserActivity() -> NSUserActivity {
    ...
  }
  ```

  This code fragment indicates that the `startUserActivity()` method is
  available on iOS 8.0+, on OS X v10.10+, and on all versions of any other
  platform. **(20938565)**

* A new `@nonobjc` attribute is introduced to selectively suppress ObjC export
  for instance members that would otherwise be `@objc`. **(16763754)**

* Nongeneric classes may now inherit from generic classes. **(15520519)**

* Public extensions of generic types are now permitted.

  ```swift
  public extension Array { ... }
  ```

  **(16974298)**

* Enums now support multiple generic associated values, for example:

  ```swift
  enum Either<T, U> {
     case Left(T), Right(U)
  }
  ```

  **(15666173)**

* **Protocol extensions**: Extensions can be written for protocol types.
  With these extensions, methods and properties can be added to any type that
  conforms to a particular protocol, allowing you to reuse more of your code.
  This leads to more natural caller side "dot" method syntax that follows the
  principle of "fluent interfaces" and that makes the definition of generic
  code simpler (reducing "angle bracket blindness"). **(11735843)**

* **Protocol default implementations**: Protocols can have default
  implementations for requirements specified in a protocol extension, allowing
  "mixin" or "trait" like patterns.

* **Availability checking**. Swift reports an error at compile time if you call an
  API that was introduced in a version of the operating system newer than the
  currently selected deployment target.

  To check whether a potentially unavailable API is available at runtime, use
  the new `#available()` condition in an if or guard statement. For example:

  ```swift
  if #available(iOS 8.0, OSX 10.10, *) {
    // Use Handoff APIs when available.
    let activity =
      NSUserActivity(activityType:"com.example.ShoppingList.view")
    activity.becomeCurrent()
  } else {
    // Fall back when Handoff APIs not available.
  }
  ```

  **(14586648)**

* Native support for C function pointers: C functions that take function pointer
  arguments can be called using closures or global functions, with the
  restriction that the closure must not capture any of its local context.
  For example, the standard C qsort function can be invoked as follows:

  ```swift
  var array = [3, 14, 15, 9, 2, 6, 5]
  qsort(&array, array.count, sizeofValue(array[0])) { a, b in
    return Int32(UnsafePointer<Int>(a).memory - UnsafePointer<Int>(b).memory)
  }
  print(array)
  ```

  **(16339559)**

* **Error handling**. You can create functions that `throw`, `catch`, and manage
  errors in Swift.

  Using this capability, you can surface and deal with recoverable
  errors, such as "file-not-found" or network timeouts. Swift's error handling
  interoperates with `NSError`. **(17158652)**

* **Testability**: Tests of Swift 2.0 frameworks and apps are written without
  having to make internal routines public.

  Use `@testable import {ModuleName}` in your test source code to make all
  public and internal routines usable. The app or framework target needs to be
  compiled with the `Enable Testability` build setting set to `Yes`. The `Enable
  Testability` build setting should be used only in your Debug configuration,
  because it prohibits optimizations that depend on not exporting internal
  symbols from the app or framework. **(17732115)**

* if statements can be labeled, and labeled break statements can be used as a
  jump out of the matching if statement.

  An unlabeled break does not exit the if statement. It exits the enclosing
  loop or switch statement, or it is illegal if none exists. (19150249)

* A new `x?` pattern can be used to pattern match against optionals as a
  synonym for `.Some(x)`. **(19382878)**

* Concatenation of Swift string literals, including across multiple lines, is
  now a guaranteed compile-time optimization, even at `-Onone`. **(19125926)**

* Nested functions can now recursively reference themselves and other nested
  functions. **(11266246)**

* Improved diagnostics:
  - A warning has been introduced to encourage the use of let instead of var
    when appropriate.
  - A warning has been introduced to signal unused variables.
  - Invalid mutation diagnostics are more precise.
  - Unreachable switch cases cause a warning.
  - The switch statement "exhaustiveness checker" is smarter.
  **(15975935,20130240)**

* Failable convenience initializers are allowed to return `nil` before calling
  `self.init`.

  Designated initializers still must initialize all stored properties before
  returning `nil`; this is a known limitation. **(20193929)**

* A new `readLine()` function has been added to the standard library.
  **(15911365)**

* **SIMD Support**: Clang extended vectors are imported and usable in Swift.

  This capability enables many graphics and other low-level numeric APIs
  (for example, `simd.h`) to be usable in Swift.

* New `guard` statement: This statement allows you to model an early exit out
  of a scope.

  For example:

  ```swift
  guard let z = bar() else { return }
  use(z)
  ```

  The `else` block is required to exit the scope (for example, with `return`,
  `throw`, `break`, `continue`, and so forth) or end in a call to a `@noreturn`
  function. **(20109722)**

* Improved pattern matching: `switch`/`case` pattern matching is available to
  many new conditional control flow statements, including `if`/`case`,
  `while`/`case`, `guard`/`case`, and `for-in`/`case`. `for-in` statements can
  also have `where` clauses, which combine to support many of the features of
  list comprehensions in other languages.

* A new `do` statement allows scopes to be introduced with the `do` statement.

  For example:

  ```swift
  do {
      //new scope
      do {
          //another scope
      }
  }
  ```

#### Swift Enhancements and Changes

* A new keyword `try?` has been added to Swift.

  `try?` attempts to perform an operation that may throw. If the operation
  succeeds, the result is wrapped in an optional; if it fails (that is, if an
  error is thrown), the result is `nil` and the error is discarded.

  For example:

  ```swift
  func produceGizmoUsingTechnology() throws -> Gizmo { ... }
  func produceGizmoUsingMagic() throws -> Gizmo { ... }

  if let result = try? produceGizmoUsingTechnology() { return result }
  if let result = try? produceGizmoUsingMagic() { return result }
  print("warning: failed to produce a Gizmo in any way")
  return nil
  ```

  `try?` always adds an extra level of `Optional` to the result type of the
  expression being evaluated. If a throwing function's normal return type is
  `Int?`, the result of calling it with `try?` will be `Int??`, or
  `Optional<Optional<Int>>`. **(21692467)**

* Type names and enum cases now print and convert to `String` without
  qualification by default. `debugPrint` or `String(reflecting:)` can still be
  used to get fully qualified names. For example:

  ```swift
  enum Fruit { case Apple, Banana, Strawberry }
  print(Fruit.Apple)      // "Apple"
  debugPrint(Fruit.Apple) // "MyApp.Fruit.Apple")
  ```

  **(21788604)**

* C `typedef`s of block types are imported as `typealias`s for Swift closures.

  The primary result of this is that `typedef`s for blocks with a parameter of
  type `BOOL` are imported as closures with a parameter of type `Bool` (rather
  than `ObjCBool` as in the previous release). This matches the behavior of
  block parameters to imported Objective-C methods. **(22013912)**

* The type `Boolean` in `MacTypes.h` is imported as `Bool` in contexts that allow
  bridging between Swift and Objective-C types.

  In cases where the representation is significant, `Boolean` is imported as a
  distinct `DarwinBoolean` type, which is `BooleanLiteralConvertible` and can be
  used in conditions (much like the `ObjCBool` type). **(19013551)**

* Fields of C structs that are marked `__unsafe_unretained` are presented in
  Swift using `Unmanaged`.

  It is not possible for the Swift compiler to know if these references are
  really intended to be strong (+1) or unretained (+0). **(19790608)**

* The `NS_REFINED_FOR_SWIFT` macro can be used to move an Objective-C
  declaration aside to provide a better version of the same API in Swift,
  while still having the original implementation available. (For example, an
  Objective-C API that takes a `Class` could offer a more precise parameter
  type in Swift.)

  The `NS_REFINED_FOR_SWIFT` macro operates differently on different declarations:

  - `init` methods will be imported with the resulting Swift initializer having
    `__` prepended to its first external parameter name.

    ```objc
    // Objective-C
    - (instancetype)initWithClassName:(NSString *)name NS_REFINED_FOR_SWIFT;
    ```

    ```swift
    // Swift
    init(__className: String)
    ```

  - Other methods will be imported with `__` prepended to their base name.

    ```objc
    // Objective-C
    - (NSString *)displayNameForMode:(DisplayMode)mode NS_REFINED_FOR_SWIFT;
    ```

    ```swift
    // Swift
    func __displayNameForMode(mode: DisplayMode) -> String
    ```

  - Subscript methods will be treated like any other methods and will not be
    imported as subscripts.

  - Other declarations will have `__` prepended to their name.

    ```objc
    // Objective-C
    @property DisplayMode mode NS_REFINED_FOR_SWIFT;
    ```

    ```swift
    // Swift
    var __mode: DisplayMode { get set }
    ```

  **(20070465)**

* Xcode provides context-sensitive code completions for enum elements and
  option sets when using the shorter dot syntax. **(16659653)**

* The `NSManaged` attribute can be used with methods as well as properties,
  for access to Core Data's automatically generated Key-Value-Coding-compliant
  to-many accessors.

  ```swift
  @NSManaged var employees: NSSet

  @NSManaged func addEmployeesObject(employee: Employee)
  @NSManaged func removeEmployeesObject(employee: Employee)
  @NSManaged func addEmployees(employees: NSSet)
  @NSManaged func removeEmployees(employees: NSSet)
  ```

  These can be declared in your `NSManagedObject` subclass. **(17583057)**

* The grammar has been adjusted so that lines beginning with '.' are always
  parsed as method or property lookups following the previous line, allowing
  for code formatted like this to work:

  ```swift
  foo
    .bar
    .bas = 68000
  ```

  It is no longer possible to begin a line with a contextual static member
  lookup (for example, to say `.staticVar = MyType()`). **(20238557)**

* Code generation for large `struct` and `enum` types has been improved to reduce
  code size. **(20598289)**

* Nonmutating methods of structs, enums, and protocols may now be partially
  applied to their self parameter:

  ```swift
  let a: Set<Int> = [1, 2, 3]
  let b: [Set<Int>] = [[1], [4]]
  b.map(a.union) // => [[1, 2, 3], [1, 2, 3, 4]]
  ```

  **(21091944)**

* Swift documentation comments recognize a new top-level list
  item: `- Throws: ...`

  This item is used to document what errors can be thrown and why. The
  documentation appears alongside parameters and return descriptions in Xcode
  QuickHelp. **(21621679)**

* Unnamed parameters now require an explicit `_:` to indicate that they are
  unnamed. For example, the following is now an error:

  ```swift
  func f(Int) { }
  ```

  and must be written as:

  ```swift
  func f(_: Int) { }
  ```

  This simplifies the argument label model and also clarifies why cases like
  `func f((a: Int, b: Int))` do not have parameters named `a` and `b`.
  **(16737312)**

* It is now possible to append a tuple to an array. **(17875634)**

* The ability to refer to the 0 element of a scalar value (producing the
  scalar value itself) has been removed. **(17963034)**

* Variadic parameters can now appear anywhere in the parameter list for a
  function or initializer. For example:

  ```swift
  func doSomethingToValues(values: Int... , options: MyOptions = [], fn: (Int) -&gt; Void) { ... }
  ```

  **(20127197)**

* Generic subclasses of Objective-C classes are now supported. **(18505295)**

* If an element of an enum with string raw type does not have an explicit raw
  value, it will default to the text of the enum's name. For example:

  ```swift
  enum WorldLayer : String {
      case Ground, BelowCharacter, Character
  }
  ```

  is equivalent to:

  ```swift
  enum WorldLayer : String {
      case Ground = "Ground"
      case BelowCharacter = "BelowCharacter"
      case Character = "Character"
  }
  ```

  **(15819953)**

* The `performSelector` family of APIs is now available for Swift code.
  **(17227475)**

* When delegating or chaining to a failable initializer (for example, with
  `self.init(...)` or `super.init(...)`), one can now force-unwrap the result with
  `!`. For example:

  ```swift
  extension UIImage {
    enum AssetIdentifier: String {
      case Isabella
      case William
      case Olivia
    }

    convenience init(assetIdentifier: AssetIdentifier) {
      self.init(named: assetIdentifier.rawValue)!
    }
  }
  ```

  **(18497407)**

* Initializers can now be referenced like static methods by referring to
  `.init` on a static type reference or type object. For example:

  ```swift
  let x = String.init(5)
  let stringType = String.self
  let y = stringType.init(5)

  let oneTwoThree = [1, 2, 3].map(String.init).reduce("", combine: +)
  ```

  `.init` is still implicit when constructing using a static type, as in
  `String(5)`. `.init` is required when using dynamic type objects or when
  referring to the initializer as a function value. **(21375845)**

* Enums and cases can now be marked indirect, which causes the associated
  value for the enum to be stored indirectly, allowing for recursive data
  structures to be defined. For example:

  ```swift
    enum List<T> {
    case Nil
    indirect case Cons(head: T, tail: List<T>)
  }

  indirect enum Tree<T> {
    case Leaf(T)
    case Branch(left: Tree<T>, right: Tree<T>)
  }
  ```

  **(21643855)**

* Formatting for Swift expression results has changed significantly when
  using `po` or `expr -O`. Customization that was introduced has been refined
  in the following ways:

  - The formatted summary provided by either `debugDescription` or
    `description` methods will always be used for types that conform to
    `CustomDebugStringConvertible` or `CustomStringConvertible` respectively.
    When neither conformance is present, the type name is displayed and
    reference types also display the referenced address to more closely mimic
    existing behavior for Objective-C classes.

  - Value types such as enums, tuples, and structs display all members
    indented below the summary by default, while reference types will not. This
    behavior can be customized for all types by implementing
    `CustomReflectable`.

  These output customizations can be bypassed by using `p` or `expr` without
  the `-O` argument to provide a complete list of all fields and their values.
  **(21463866)**

* Properties and methods using `Unmanaged` can now be exposed to Objective-C.
  **(16832080)**

* Applying the `@objc` attribute to a class changes that class's compile-time
  name in the target's generated Objective-C header as well as changing its
  runtime name. This applies to protocols as well. For example:

  ```swift
  // Swift
  @objc(MyAppDelegate)
  class AppDelegate : NSObject, UIApplicationDelegate {
    // ...
  }
  ```

  ```objc
  // Objective-C
  @interface MyAppDelegate : NSObject <UIApplicationDelegate>
    // ...
  @end
  ```

  **(17469485)**

* Collections containing types that are not Objective-C compatible are no
  longer considered Objective-C compatible types themselves.

  For example, previously `Array<SwiftClassType>` was permitted as the type
  of a property marked `@objc`; this is no longer the case. **(19787270)**

* Generic subclasses of Objective-C classes, as well as nongeneric classes
  that inherit from such a class, require runtime metadata instantiation and
  cannot be directly named from Objective-C code.

  When support for generic subclasses of Objective-C classes was first added,
  the generated Objective-C bridging header erroneously listed such classes,
  which, when used, could lead to incorrect runtime behavior or compile-time
  errors. This has been fixed.

  The behavior of the `@objc` attribute on a class has been clarified such that
  applying `@objc` to a class which cannot appear in a bridging header is now
  an error.

  Note that this change does not result in a change of behavior with valid
  code because members of a class are implicitly `@objc` if any superclass of
  the class is an `@objc` class, and all `@objc` classes must inherit from
  NSObject. **(21342574)**

* The performance of `-Onone` (debug) builds has been improved by using
  prespecialized instances of generics in the standard library. It produces
  significantly faster executables in debug builds in many cases, without
  impacting compile time. **(20486658)**

* `AnyObject` and `NSObject` variables that refer to class objects can be cast
  back to class object types. For example, this code succeeds:

  ```swift
  let x: AnyObject = NSObject.self
  let y = x as! NSObject.Type
  ```

  Arrays, dictionaries, and sets that contain class objects successfully
  bridge with `NSArray`, `NSDictionary`, and `NSSet` as well. Objective-C APIs
  that provide `NSArray<Class> *` objects, such as `-[NSURLSessionConfiguration
  protocolClasses]`, now work correctly when used in Swift. **(16238475)**

* `print()` and reflection via Mirrors is able to report both the current
  case and payload for all enums with multiple payload types. The only
  remaining enum types that do not support reflection are `@objc` enums and
  enums imported from C. **(21739870)**

* Enum cases with payloads can be used as functions. For example:

  ```swift
  enum Either<T, U> { case Left(T), Right(U) }
  let lefts: [Either<Int, String>] = [1, 2, 3].map(Either.Left)
  let rights: [Either<Int, String>] = ["one", "two", "three"].map(Either.Right)
  ```

  **(19091028)**

* `ExtensibleCollectionType` has been folded into
  `RangeReplaceableCollectionType`. In addition, default implementations have
  been added as methods, which should be used instead of the free Swift
  module functions related to these protocols. **(18220295)**

#### Swift Standard Library

* The standard library moved many generic global functions (such as `map`,
  `filter`, and `sort`) to be methods written with protocol extensions. This
  allows those methods to be pervasively available on all sequence and
  collection types and allowed the removal of the global functions.

* Deprecated enum elements no longer affect the names of nondeprecated
  elements when an Objective-C enum is imported into Swift. This may cause
  the Swift names of some enum elements to change. **(17686122)**

* All enums imported from C are `RawRepresentable`, including those not
  declared with `NS_ENUM` or `NS_OPTIONS`. As part of this change, the value
  property of such enums has been renamed `rawValue`. **(18702016)**

* Swift documentation comments use a syntax based on the Markdown format,
  aligning them with rich comments in playgrounds.

  - Outermost list items are interpreted as special fields and are highlighted
    in Xcode QuickHelp.

  - There are two methods of documenting parameters: parameter outlines and
    separate parameter fields. You can mix and match these forms as you see
    fit in any order or continuity throughout the doc comment. Because these
    are parsed as list items, you can nest arbitrary content underneath them.

  - Parameter outline syntax:

    ```swift
    - Parameters:
      - x: ...
      - y: ...
    ```

  - Separate parameter fields:

    ```swift
    - parameter x: ...
    - parameter y: ...
    ```

  - Documenting return values:

    ```swift
    - returns: ...
    ```

  Other special fields are highlighted in QuickHelp, as well as rendering
  support for all of Markdown. (20180161)

* The `CFunctionPointer<T -> U>` type has been removed. C function types are
  specified using the new `@convention(c)` attribute. Like other function
  types, `@convention(c) T -> U` is not nullable unless made optional. The
  `@objc_block` attribute for specifying block types has also been removed and
  replaced with `@convention(block)`.

* Methods and functions have the same rules for parameter names. You can omit
  providing an external parameter name with `_`. To further simplify the model,
  the shorthand `#` for specifying a parameter name has been removed, as have
  the special rules for default arguments.

  ```swift
  // Declaration
    func printFunction(str: String, newline: Bool)
    func printMethod(str: String, newline: Bool)
    func printFunctionOmitParameterName(str: String, _  newline: Bool)

  // Call
    printFunction("hello", newline: true)
    printMethod("hello", newline: true)
    printFunctionOmitParameterName("hello", true)
  ```

  **(17218256)**

* `NS_OPTIONS` types get imported as conforming to the `OptionSetType` protocol,
  which presents a set-like interface for options. Instead of using bitwise
  operations such as:

  ```swift
  // Swift 1.2:
  object.invokeMethodWithOptions(.OptionA | .OptionB)
  object.invokeMethodWithOptions(nil)

  if options @ .OptionC == .OptionC {
    // .OptionC is set
  }
  ```

  Option sets support set literal syntax, and set-like methods such as contains:

  ```swift
  object.invokeMethodWithOptions([.OptionA, .OptionB])
  object.invokeMethodWithOptions([])

  if options.contains(.OptionC) {
    // .OptionC is set
  }
  ```

  A new option set type can be written in Swift as a struct that conforms to
  the `OptionSetType` protocol. If the type specifies a `rawValue` property and
  option constants as `static let` constants, the standard library will provide
  default implementations of the rest of the option set API:

  ```swift
  struct MyOptions: OptionSetType {
    let rawValue: Int

    static let TuringMachine  = MyOptions(rawValue: 1)
    static let LambdaCalculus = MyOptions(rawValue: 2)
    static let VonNeumann     = MyOptions(rawValue: 4)
  }

  let churchTuring: MyOptions = [.TuringMachine, .LambdaCalculus]
  ```

  **(18069205)**

* Type annotations are no longer allowed in patterns and are considered part
  of the outlying declaration. This means that code previously written as:

  ```swift
  var (a : Int, b : Float) = foo()
  ```

  needs to be written as:

  ```swift
  var (a, b) : (Int, Float) = foo()
  ```

  if an explicit type annotation is needed. The former syntax was ambiguous
  with tuple element labels. **(20167393)**

* The `do`/`while` loop is renamed to `repeat`/`while` to make it obvious
  whether a statement is a loop from its leading keyword.

  In Swift 1.2:

  ```swift
  do {
  ...
  } while <condition>
  ```

  In Swift 2.0:

  ```swift
  repeat {
  ...
  } while <condition>
  ```

  **(20336424)**

* `forEach` has been added to `SequenceType`. This lets you iterate over
  elements of a sequence, calling a body closure on each. For example:

  ```swift
  (0..<10).forEach {
    print($0)
  }
  ```

  This is very similar to the following:

  ```swift
  for x in 0..<10 {
    print(x)
  }
  ```

  But take note of the following differences:

  - Unlike for-in loops, you can't use `break` or `continue` to exit the current
    call of the body closure or skip subsequent calls.

  - Also unlike for-in loops, using `return` in the body closure only exits from
    the current call to the closure, not any outer scope, and won't skip
    subsequent calls.

  **(18231840)**

* The `Word` and `UWord` types have been removed from the standard library; use
  `Int` and `UInt` instead. **(18693488)**

* Most standard library APIs that take closures or `@autoclosure` parameters
  now use `rethrows`. This allows the closure parameters to methods like `map`
  and `filter` to throw errors, and allows short-circuiting operators like `&&`,
  `||`, and `??` to work with expressions that may produce errors.
  **(21345565)**

* SIMD improvements: Integer vector types in the simd module now only support
  unchecked arithmetic with wraparound semantics using the `&+`, `&-`, and `&*`
  operators, in order to better support the machine model for vectors.
  The `+`, `-`, and `*` operators are unavailable on integer vectors, and Xcode
  automatically suggests replacing them with the wrapping operators.

  Code generation for vector types in the simd module has been improved to
  better utilize vector hardware, leading to dramatically improved performance
  in many cases. **(21574425)**

* All `CollectionType` objects are now sliceable. `SequenceType` now has a notion
  of `SubSequence`, which is a type that represents only some of the values but
  in the same order. For example, the `ArraySubSequence` type is `ArraySlice`,
  which is an efficient view on the `Array` type's buffer that avoids copying as
  long as it uniquely references the `Array` from which it came.

  The following free Swift functions for splitting/slicing sequences have been
  removed and replaced by method requirements on the `SequenceType` protocol
  with default implementations in protocol extensions. `CollectionType` has
  specialized implementations, where possible, to take advantage of efficient
  access of its elements.

  ```swift
  /// Returns the first `maxLength` elements of `self`,
  /// or all the elements if `self` has fewer than `maxLength` elements.
  prefix(maxLength: Int) -> SubSequence

  /// Returns the last `maxLength` elements of `self`,
  /// or all the elements if `self` has fewer than `maxLength` elements.
  suffix(maxLength: Int) -> SubSequence

  /// Returns all but the first `n` elements of `self`.
  dropFirst(n: Int) -> SubSequence

  /// Returns all but the last `n` elements of `self`.
  dropLast(n: Int) -> SubSequence

  /// Returns the maximal `SubSequence`s of `self`, in order, that
  /// don't contain elements satisfying the predicate `isSeparator`.
  split(maxSplits maxSplits: Int, allowEmptySlices: Bool, @noescape isSeparator: (Generator.Element) -> Bool) -> [SubSequence]
  ```

  The following convenience extension is provided for `split`:

  ```swift
  split(separator: Generator.Element, maxSplit: Int, allowEmptySlices: Bool) -> [SubSequence]
  ```

  Also, new protocol requirements and default implementations on
  `CollectionType` are now available:

  ```swift
  /// Returns `self[startIndex..<end]`
  prefixUpTo(end: Index) -> SubSequence

  /// Returns `self[start..<endIndex]`
  suffixFrom(start: Index) -> SubSequence

  /// Returns `prefixUpTo(position.successor())`
  prefixThrough(position: Index) -> SubSequence
  ```

  **(21663830)**

* The `print` and `debugPrint` functions are improved:
  - Both functions have become variadic, and you can print any number of items
    with a single call.
  - `separator: String = " "` was added so you can control how the items are
    separated.
  - `terminator: String = "\n"` replaced `appendNewline: bool = true.`  With
    this change, `print(x, appendNewline: false)` is expressed as
    `print(x, terminator: "")`.

  - For the variants that take an output stream, the argument label `toStream`
    was added to the stream argument.

  The `println` function from Swift 1.2 has been removed. **(21788540)**

* For consistency and better composition of generic code, `ArraySlice` indices
  are no longer always zero-based but map directly onto the indices of the
  collection they are slicing and maintain that mapping even after mutations.

  Before:

  ```swift
  var a = Array(0..<10)
  var s = a[5..<10]
  s.indices        // 0..<5
  s[0] = 111
  s                // [111, 6, 7, 8, 9]
  s.removeFirst()
  s.indices        // 1..<5
  ```

  After:

  ```swift
  var a = Array(0..<10)
  var s = a[5..<10]
  s.indices        // 5..<10
  s[5] = 99
  s                // [99, 6, 7, 8, 9]
  s.removeFirst()
  s.indices        // 6..<10
  ```

  Rather than define variants of collection algorithms that take explicit
  subrange arguments, such as `a.sortSubrangeInPlace(3..<7)`, the Swift
  standard library provides "slicing," which composes well with algorithms.
  This enables you to write `a[3..<7].sortInPlace()`, for example. With most
  collections, these algorithms compose naturally.

  For example, before this change was incorporated:

  ```swift
  extension MyIntCollection {
    func prefixThroughFirstNegativeSubrange() -> SubSequence {
      // Find the first negative element
      let firstNegative = self.indexOf { $0 < 0 } ?? endIndex

      // Slice off non-negative prefix
      let startsWithNegative = self.suffixFrom(firstNegative)

      // Find the first non-negative position in the slice
      let end = startsWithNegative.indexOf { $0 >= 0 } ?? endIndex
      return self[startIndex..<end]
    }
  }
  ```

  The above code would work for any collection of `Int`s unless the collection
  is an `Array<Int>`. Unfortunately, when array slice indices are zero-based,
  the last two lines of the method need to change to:

  ```swift
  let end = startsWithNegative.indexOf { $0 >= 0 }
    ?? startsWithNegative.endIndex
  return self[startIndex..<end + firstNegative]
  ```

  These differences made working with slices awkward, error-prone, and
  nongeneric.

  After this change, Swift collections start to provide a guarantee that, at
  least until there is a mutation, slice indices are valid in the collection
  from which they were sliced, and refer to the same elements. **(21866825)**

* The method `RangeReplaceableCollectionType.extend()` was renamed to
  `appendContentsOf()`, and the `splice()` method was renamed to
  `insertContentsOf()`. **(21972324)**

* `find` has been renamed to `indexOf()`, sort has been renamed to
  `sortInPlace()`, and `sorted()` becomes `sort()`.

* `String.toInt()` has been renamed to a failable `Int(String)` initializer,
  since initialization syntax is the preferred style for type conversions.

* `String` no longer conforms to `SequenceType` in order to prevent non-Unicode
  correct sequence algorithms from being prominently available on String. To
  perform grapheme-cluster-based, UTF-8-based, or UTF-16-based algorithms, use
  the `.characters`, `.utf8`, and `.utf16` projections respectively.

* Generic functions that declare type parameters not used within the generic
  function's type produce a compiler error. For example:

  ```swift
  func foo<T>() { } // error: generic parameter 'T' is not used in function signature
  ```

* The `Dictionary.removeAtIndex(_:)` method now returns the key-value pair
  being removed as a two-element tuple (rather than returning `Void`).
  Similarly, the `Set.removeAtIndex(_:)` method returns the element being
  removed. **(20299881)**

* Generic parameters on types in the Swift standard library have been renamed
  to reflect the role of the types in the API. For example, `Array<T>` became
  `Array<Element>`, `UnsafePointer<T>` became `UnsafePointer<Memory>`, and so
  forth. **(21429126)**

* The `SinkType` protocol and `SinkOf` struct have been removed from the standard
  library in favor of `(T) -> ()` closures. **(21663799)**

## Swift 1.2

### 2015-04-08 (Xcode 6.3)

#### Swift Language Changes

* The notions of guaranteed conversion and "forced failable" conversion are now
  separated into two operators. Forced failable conversion now uses the `as!`
  operator. The `!` makes it clear to readers of code that the cast may fail and
  produce a runtime error. The `as` operator remains for upcasts
  (e.g. `someDerivedValue as Base`) and type annotations (`0 as Int8`) which
  are guaranteed to never fail. **(19031957)**

* Immutable (`let`) properties in `struct` and `class` initializers have been
  revised to standardize on a general "`let`s are singly initialized but never
  reassigned or mutated" model. Previously, they were completely mutable
  within the body of initializers. Now, they are only allowed to be assigned
  to once to provide their value. If the property has an initial value in its
  declaration, that counts as the initial value for all initializers.
  **(19035287)**

* The implicit conversions from bridged Objective-C classes
  (`NSString`/`NSArray`/`NSDictionary`) to their corresponding Swift value types
  (`String`/`Array`/`Dictionary`) have been removed, making the Swift type
  system simpler and more predictable.

  This means that the following code will no longer work:

  ```swift
  import Foundation
  func log(s: String) { println(x) }
  let ns: NSString = "some NSString" // okay: literals still work
  log(ns)     // fails with the error
              // "'NSString' is not convertible to 'String'"
  ```

  In order to perform such a bridging conversion, make the conversion explicit
  with the as keyword:

  ```swift
  log(ns as String) // succeeds
  ```

  Implicit conversions from Swift value types to their bridged Objective-C
  classes are still permitted. For example:

  ```swift
  func nsLog(ns: NSString) { println(ns) }
  let s: String = "some String"
  nsLog(s) // okay: implicit conversion from String to NSString is permitted
  ```

  Note that these Cocoa types in Objective-C headers are still automatically
  bridged to their corresponding Swift type, which means that code is only
  affected if it is explicitly referencing (for example) `NSString` in a Swift
  source file. It is recommended you use the corresponding Swift types (for
  example, `String`) directly unless you are doing something advanced, like
  implementing a subclass in the class cluster. **(18311362)**

* The `@autoclosure` attribute is now an attribute on a parameter, not an
  attribute on the parameter's type.

  Where before you might have used

  ```swift
  func assert(predicate : @autoclosure () -> Bool) {...}
  ```

  you now write this as

  ```swift
  func assert(@autoclosure predicate : () -> Bool) {...}
  ```

  **(15217242)**

* The `@autoclosure` attribute on parameters now implies the new `@noescape`
  attribute.

* Curried function parameters can now specify argument labels.

  For example:

  ```swift
  func curryUnnamed(a: Int)(_ b: Int) { return a + b }
  curryUnnamed(1)(2)

  func curryNamed(first a: Int)(second b: Int) -> Int { return a + b }
  curryNamed(first: 1)(second: 2)
  ```

  **(17237268)**

* Swift now detects discrepancies between overloading and overriding in the
  Swift type system and the effective behavior seen via the Objective-C runtime.

  For example, the following conflict between the Objective-C setter for
  `property` in a class and the method `setProperty` in its extension is now
  diagnosed:

  ```swift
  class A : NSObject {
  var property: String = "Hello" // note: Objective-C method 'setProperty:'
      // previously declared by setter for
      // 'property' here
  }

  extension A {
  func setProperty(str: String) { }     // error: method 'setProperty'
      // redeclares Objective-C method
      //'setProperty:'
  }
  ```

  Similar checking applies to accidental overrides in the Objective-C runtime:

  ```swift
  class B : NSObject {
  func method(arg: String) { }     // note: overridden declaration
      // here has type '(String) -> ()'
  }

  class C : B {
  func method(arg: [String]) { } // error: overriding method with
      // selector 'method:' has incompatible
      // type '([String]) -> ()'
  }
  ```

  as well as protocol conformances:

  ```swift
  class MyDelegate : NSObject, NSURLSessionDelegate {
  func URLSession(session: NSURLSession, didBecomeInvalidWithError:
      Bool){ } // error: Objective-C method 'URLSession:didBecomeInvalidWithError:'
      // provided by method 'URLSession(_:didBecomeInvalidWithError:)'
      // conflicts with optional requirement method
      // 'URLSession(_:didBecomeInvalidWithError:)' in protocol
      // 'NSURLSessionDelegate'
  }
  ```

  **(18391046, 18383574)**

* The precedence of the Nil Coalescing Operator (`??`) has been raised to bind
  tighter than short-circuiting logical and comparison operators, but looser
  than as conversions and range operators. This provides more useful behavior
  for expressions like:

  ```swift
  if allowEmpty || items?.count ?? 0 > 0 {...}
  ```

* The `&/` and `&%` operators were removed, to simplify the language and
  improve consistency.

  Unlike the `&+`, `&-`, and `&*` operators, these operators did not provide
  two's-complement arithmetic behavior; they provided special case behavior
  for division, remainder by zero, and `Int.min/-1`. These tests should be
  written explicitly in the code as comparisons if needed. **(17926954)**

* Constructing a `UInt8` from an ASCII value now requires the ascii keyword
  parameter. Using non-ASCII unicode scalars will cause this initializer to
  trap. **(18509195)**

* The C `size_t` family of types are now imported into Swift as `Int`, since
  Swift prefers sizes and counts to be represented as signed numbers, even if
  they are non-negative.

  This change decreases the amount of explicit type conversion between `Int`
  and `UInt`, better aligns with `sizeof` returning `Int`, and provides safer
  arithmetic properties. **(18949559)**

* Classes that do not inherit from `NSObject` but do adopt an `@objc` protocol
  will need to explicitly mark those methods, properties, and initializers
  used to satisfy the protocol requirements as `@objc`.

  For example:

  ```swift
  @objc protocol SomethingDelegate {
      func didSomething()
  }

  class MySomethingDelegate : SomethingDelegate {
      @objc func didSomething() { ... }
  }
  ```

#### Swift Language Fixes

* Dynamic casts (`as!`, `as?` and `is`) now work with Swift protocol types, so
  long as they have no associated types. **(18869156)**

* Adding conformances within a Playground now works as expected.

  For example:

  ```swift
  struct Point {
    var x, y: Double
  }

  extension Point : Printable {
    var description: String {
      return "(\(x), \(y))"
    }
  }

  var p1 = Point(x: 1.5, y: 2.5)
  println(p1) // prints "(1.5, 2.5)"
  ```

* Imported `NS_ENUM` types with undocumented values, such as
  `UIViewAnimationCurve`, can now be converted from their raw integer values
  using the `init(rawValue:)` initializer without being reset to `nil`. Code
  that used `unsafeBitCast` as a workaround for this issue can be written to
  use the raw value initializer.

  For example,

  ```swift
  let animationCurve =
    unsafeBitCast(userInfo[UIKeyboardAnimationCurveUserInfoKey].integerValue,
    UIViewAnimationCurve.self)
  ```

  can now be written instead as

  ```swift
  let animationCurve = UIViewAnimationCurve(rawValue:
    userInfo[UIKeyboardAnimationCurveUserInfoKey].integerValue)!
  ```

  **(19005771)**

* Negative floating-point literals are now accepted as raw values in enums.
  **(16504472)**

* Unowned references to Objective-C objects, or Swift objects inheriting from
  Objective-C objects, no longer cause a crash if the object holding the
  unowned reference is deallocated after the referenced object has been
  released. **(18091547)**

* Variables and properties with observing accessors no longer require an
  explicit type if it can be inferred from the initial value expression.
  **(18148072)**

* Generic curried functions no longer produce random results when fully
  applied. **(18988428)**

* Comparing the result of a failed `NSClassFromString` lookup against `nil` now
  behaves correctly. **(19318533)**

* Subclasses that override base class methods with co- or contravariance in
  Optional types no longer cause crashes at runtime.

  For example:

  ```swift
  class Base {
    func foo(x: String) -> String? { return x }
  }
  class Derived: Base {
    override func foo(x: String?) -> String { return x! }
  }
  ```

  **(19321484)**

#### Swift Language Enhancements

* Swift now supports building targets incrementally, i.e. not rebuilding
  every Swift source file in a target when a single file is changed.

  The incremental build capability is based on a conservative dependency
  analysis, so you may still see more files rebuilding than absolutely
  necessary. If you find any cases where a file is not rebuilt when it should
  be, please file a bug report. Running Clean on your target afterwards should
  allow you to complete your build normally. **(18248514)**

* A new `Set` data structure is included which provides a generic collection of
  unique elements with full value semantics. It bridges with `NSSet`, providing
  functionality analogous to `Array` and `Dictionary`. **(14661754)**

* The `if-let` construct has been expanded to allow testing multiple optionals
  and guarding conditions in a single `if` (or `while`) statement using syntax
  similar to generic constraints:

  ```swift
  if let a = foo(), b = bar() where a < b,
     let c = baz() {
  }
  ```

  This allows you to test multiple optionals and include intervening boolean
  conditions, without introducing undesirable nesting (for instance, to avoid
  the optional unwrapping _"pyramid of doom"_).

  Further, `if-let` now also supports a single leading boolean condition along
  with optional binding `let` clauses. For example:

  ```swift
  if someValue > 42 && someOtherThing < 19, let a = getOptionalThing() where a > someValue {
  }
  ```

  **(19797158, 19382942)**

* The `if-let` syntax has been extended to support a single leading boolean
  condition along with optional binding `let` clauses.

  For example:

  ```swift
  if someValue > 42 && someOtherThing < 19, let a = getOptionalThing() where a > someValue {
  }
  ```

  **(19797158)**

* `let` constants have been generalized to no longer require immediate
  initialization. The new rule is that a `let` constant must be initialized
  before use (like a `var`), and that it may only be initialized: not
  reassigned or mutated after initialization. This enables patterns such as:

  ```swift
  let x: SomeThing
  if condition {
    x = foo()
  } else {
    x = bar()
  }
  use(x)
  ```

  which formerly required the use of a `var`, even though there is no mutation
  taking place. **(16181314)**

* `static` methods and properties are now allowed in classes (as an alias for
  `class final`). You are now allowed to declare static stored properties in
  classes, which have global storage and are lazily initialized on first
  access (like global variables). Protocols now declare type requirements as
  static requirements instead of declaring them as class requirements.
  **(17198298)**

* Type inference for single-expression closures has been improved in several ways:
  - Closures that are comprised of a single return statement are now type
    checked as single-expression closures.
  - Unannotated single-expression closures with non-`Void` return types can now
    be used in `Void` contexts.
  - Situations where a multi-statement closure's type could not be inferred
    because of a missing return-type annotation are now properly diagnosed.

* Swift enums can now be exported to Objective-C using the `@objc` attribute.
  `@objc` enums must declare an integer raw type, and cannot be generic or use
  associated values. Because Objective-C enums are not namespaced, enum cases
  are imported into Objective-C as the concatenation of the enum name and
  case name.

  For example, this Swift declaration:

  ```swift
  // Swift
  @objc
  enum Bear: Int {
     case Black, Grizzly, Polar
  }
  ```

  imports into Objective-C as:

  ```objc
  // Objective-C
  typedef NS_ENUM(NSInteger, Bear) {
     BearBlack, BearGrizzly, BearPolar
  };
  ```

  **(16967385)**

* Objective-C language extensions are now available to indicate the nullability
  of pointers and blocks in Objective-C APIs, allowing your Objective-C APIs
  to be imported without `ImplicitlyUnwrappedOptional`. (See items below for
  more details.) **(18868820)**

* Swift can now partially import C aggregates containing unions, bitfields,
  SIMD vector types, and other C language features that are not natively
  supported in Swift. The unsupported fields will not be accessible from
  Swift, but C and Objective-C APIs that have arguments and return values of
  these types can be used in Swift. This includes the Foundation `NSDecimal`
  type and the `GLKit` `GLKVector` and `GLKMatrix` types, among others.
  **(15951448)**

* Imported C structs now have a default initializer in Swift that initializes
  all of the struct's fields to zero.

  For example:

  ```swift
  import Darwin
  var devNullStat = stat()
  stat("/dev/null", &devNullStat)
  ```

  If a structure contains fields that cannot be correctly zero initialized
  (i.e. pointer fields marked with the new `__nonnull` modifier), this default
  initializer will be suppressed. **(18338802)**

* New APIs for converting among the `Index` types for `String`,
  `String.UnicodeScalarView`, `String.UTF16View`, and `String.UTF8View` are
  available, as well as APIs for converting each of the `String` views into
  `String`s. **(18018911)**

* Type values now print as the full demangled type name when used with
  `println` or string interpolation.

  ```swift
  toString(Int.self)          // prints "Swift.Int"
  println([Float].self)       // prints "Swift.Array&lt;Swift.Float&gt;"
  println((Int, String).self) // prints "(Swift.Int, Swift.String)"
  ```

  **(18947381)**

* A new `@noescape` attribute may be used on closure parameters to functions.
  This indicates that the parameter is only ever called (or passed as an
  `@noescape` parameter in a call), which means that it cannot outlive the
  lifetime of the call. This enables some minor performance optimizations,
  but more importantly disables the `self.` requirement in closure arguments.
  This enables control-flow-like functions to be more transparent about their
  behavior. In a future beta, the standard library will adopt this attribute
  in functions like `autoreleasepool()`.

  ```swift
  func autoreleasepool(@noescape code: () -> ()) {
     pushAutoreleasePool()
     code()
     popAutoreleasePool()
  }
  ```

  **(16323038)**

* Performance is substantially improved over Swift 1.1 in many cases. For
  example, multidimensional arrays are algorithmically faster in some cases,
  unoptimized code is much faster in many cases, and many other improvements
  have been made.

* The diagnostics emitted for expression type check errors are greatly
  improved in many cases. **(18869019)**

* Type checker performance for many common expression kinds has been greatly
  improved. This can significantly improve build times and reduces the number
  of "expression too complex" errors. **(18868985)**

* The `@autoclosure` attribute has a second form, `@autoclosure(escaping)`, that
  provides the same caller-side syntax as `@autoclosure` but allows the
  resulting closure to escape in the implementation.

  For example:

  ```swift
  func lazyAssertion(@autoclosure(escaping) condition: () -> Bool,
                     message: String = "") {
    lazyAssertions.append(condition) // escapes
    }
  lazyAssertion(1 == 2, message: "fail eventually")
  ```

  **(19499207)**

#### Swift Performance

* A new compilation mode has been introduced for Swift called Whole Module
  Optimization. This option optimizes all of the files in a target together
  and enables better performance (at the cost of increased compile time). The
  new flag can be enabled in Xcode using the `Whole Module Optimization` build
  setting or by using the `swiftc` command line tool with the flag
  `-whole-module-optimization`. **(18603795)**

#### Swift Standard Library Enhancements and Changes

* `flatMap` was added to the standard library. `flatMap` is the function that
  maps a function over something and returns the result flattened one level.
  `flatMap` has many uses, such as to flatten an array:

  ```swift
  [[1,2],[3,4]].flatMap { $0 }
  ```

  or to chain optionals with functions:

  ```swift
  [[1,2], [3,4]].first.flatMap { find($0, 1) }
  ```

  **(19881534)**

* The function `zip` was added. It joins two sequences together into one
  sequence of tuples. **(17292393)**

* `utf16Count` is removed from `String`. Instead use count on the `UTF16` view
  of the `String`.

  For example:

  ```swift
  count(string.utf16)
  ```

  **(17627758)**

## Swift 1.1

### 2014-12-02 (Xcode 6.1.1)

* Class methods and initializers that satisfy protocol requirements now properly
  invoke subclass overrides when called in generic contexts. For example:

    ```swift
    protocol P {
      class func foo()
    }

    class C: P {
      class func foo() { println("C!") }
    }

    class D: C {
      override class func foo() { println("D!") }
    }

    func foo<T: P>(x: T) {
      x.dynamicType.foo()
    }

    foo(C()) // Prints "C!"
    foo(D()) // Used to incorrectly print "C!", now prints "D!"
    ```

  **(18828217)**

### 2014-10-09 (Xcode 6.1)

* Values of type `Any` can now contain values of function type. **(16406907)**

* Documentation for the standard library (displayed in quick help and in the
  synthesized header for the Swift module) is improved. **(16462500)**

* Class properties don't need to be marked final to avoid `O(n)` mutations on
  value semantic types. **(17416120)**

* Casts can now be performed between `CF` types (such as `CFString`, `CGImage`,
  and `SecIdentity`) and AnyObject. Such casts will always succeed at run-time.
  For example:

  ```swift
  var cfStr: CFString = ...
  var obj: AnyObject = cfStr as AnyObject
  var cfStr = obj as CFString
  ```

  **(18088474)**

* `HeapBuffer<Value, Element>`, `HeapBufferStorage<Value, Element>`, and
  `OnHeap<Value>` were never really useful, because their APIs were
  insufficiently public.  They have been replaced with a single class,
  `ManagedBuffer<Value, Element>`.  See also the new function
  `isUniquelyReferenced(x)` which is often useful in conjunction with
  `ManagedBuffer`.

* The `Character` enum has been turned into a struct, to avoid
  exposing its internal implementation details.

* The `countElements` function has been renamed `count`, for better
  consistency with our naming conventions.

* Mixed-sign addition and subtraction operations, that were
  unintentionally allowed in previous versions, now cause a
  compilation error.

* OS X apps can now apply the `@NSApplicationMain` attribute to their app delegate
  class in order to generate an implicit `main` for the app. This works like
  the `@UIApplicationMain` attribute for iOS apps.

* Objective-C `init` and factory methods are now imported as failable
  initializers when they can return `nil`. In the absence of information
  about a potentially-`nil` result, an Objective-C `init` or factory
  method will be imported as `init!`.

  As part of this change, factory methods that have NSError**
  parameters, such as `+[NSString
  stringWithContentsOfFile:encoding:error:]`, will now be imported as
  (failable) initializers, e.g.,

    ```swift
    init?(contentsOfFile path: String,
          encoding: NSStringEncoding,
          error: NSErrorPointer)
    ```

* Nested classes explicitly marked `@objc` will now properly be included in a
  target's generated header as long as the containing context is also
  (implicitly or explicitly) `@objc`. Nested classes not explicitly marked
  `@objc` will never be printed in the generated header, even if they extend an
  Objective-C class.

* All of the `*LiteralConvertible` protocols, as well as
  `StringInterpolationConvertible`, now use initializers for their
  requirements rather than static methods starting with
  `convertFrom`. For example, `IntegerLiteralConvertible` now has the
  following initializer requirement:

    ```swift
    init(integerLiteral value: IntegerLiteralType)
    ```
  Any type that previously conformed to one of these protocols will
  need to replace its `convertFromXXX` static methods with the
  corresponding initializer.

## Swift 1.0

### 2014-09-15 (Xcode 6.0)

* Initializers can now fail by returning `nil`. A failable initializer is
  declared with `init?` (to return an explicit optional) or `init!` (to return
  an implicitly-unwrapped optional). For example, you could implement
  `String.toInt` as a failable initializer of `Int` like this:

    ```swift
    extension Int {
      init?(fromString: String) {
        if let i = fromString.toInt() {
          // Initialize
          self = i
        } else {
          // Discard self and return 'nil'.
          return nil
        }
      }
    }
    ```

  The result of constructing a value using a failable initializer then becomes
  optional:

    ```swift
    if let twentytwo = Int(fromString: "22") {
      println("the number is \(twentytwo)")
    } else {
      println("not a number")
    }
    ```

  In the current implementation, struct and enum initializers can return `nil`
  at any point inside the initializer, but class initializers can only return
  `nil` after all of the stored properties of the object have been initialized
  and `self.init` or `super.init` has been called. If `self.init` or
  `super.init` is used to delegate to a failable initializer, then the `nil`
  return is implicitly propagated through the current initializer if the
  called initializer fails.

* The `RawRepresentable` protocol that enums with raw types implicitly conform
  to has been redefined to take advantage of failable initializers. The
  `fromRaw(RawValue)` static method has been replaced with an initializer
  `init?(rawValue: RawValue)`, and the `toRaw()` method has been replaced with
  a `rawValue` property. Enums with raw types can now be used like this:

    ```swift
    enum Foo: Int { case A = 0, B = 1, C = 2 }
    let foo = Foo(rawValue: 2)! // formerly 'Foo.fromRaw(2)!'
    println(foo.rawValue) // formerly 'foo.toRaw()'
    ```

### 2014-09-02

* Characters can no longer be concatenated using `+`.  Use `String(c1) +
  String(c2)` instead.

### 2014-08-18

* When force-casting between arrays of class or `@objc` protocol types
  using `a as [C]`, type checking is now deferred until the moment
  each element is accessed.  Because bridging conversions from NSArray
  are equivalent to force-casts from `[NSArray]`, this makes certain
  Array round-trips through Objective-C code `O(1)` instead of `O(N)`.

### 2014-08-04

* `RawOptionSetType` now implements `BitwiseOperationsType`, so imported
  `NS_OPTIONS` now support the bitwise assignment operators `|=`, `&=`,
  and `^=`. It also no longer implements `BooleanType`; to check if an option
  set is empty, compare it to `nil`.

* Types implementing `BitwiseOperationsType` now automatically support
  the bitwise assignment operators `|=`, `&=`, and `^=`.

* Optionals can now be coalesced with default values using the `??` operator.
  `??` is a short-circuiting operator that takes an optional on the left and
  a non-optional expression on the right. If the optional has a value, its
  value is returned as a non-optional; otherwise, the expression on the right
  is evaluated and returned:

    ```swift
    var sequence: [Int] = []
    sequence.first ?? 0 // produces 0, because sequence.first is nil
    sequence.append(22)
    sequence.first ?? 0 // produces 22, the value of sequence.first
    ```

* The optional chaining `?` operator can now be mutated through, like `!`.
  The assignment and the evaluation of the right-hand side of the operator
  are conditional on the presence of the optional value:

    ```swift
    var sequences = ["fibonacci": [1, 1, 2, 3, 4], "perfect": [6, 28, 496]]
    sequences["fibonacci"]?[4]++ // Increments element 4 of key "fibonacci"
    sequences["perfect"]?.append(8128) // Appends to key "perfect"

    sequences["cubes"]?[3] = 3*3*3 // Does nothing; no "cubes" key
    ```

  Note that optional chaining still flows to the right, so prefix increment
  operators are *not* included in the chain, so this won't type-check:

    ```swift
    ++sequences["fibonacci"]?[4] // Won't type check, can't '++' Int?
    ```

### 2014-07-28

* The swift command line interface is now divided into an interactive driver
  `swift`, and a batch compiler `swiftc`:

  ```
  swift [options] input-file [program-arguments]
    Runs the script 'input-file' immediately, passing any program-arguments
    to the script. Without any input files, invokes the repl.

  swiftc [options] input-filenames
    The familiar swift compiler interface: compiles the input-files according
    to the mode options like -emit-object, -emit-executable, etc.
  ```

* For greater clarity and explicitness when bypassing the type system,
  `reinterpretCast` has been renamed `unsafeBitCast`, and it has acquired
  a (required) explicit type parameter.  So

    ```swift
    let x: T = reinterpretCast(y)
    ```

  becomes

    ```swift
    let x = unsafeBitCast(y, T.self)
    ```

* Because their semantics were unclear, the methods `asUnsigned` (on
  the signed integer types) and `asSigned` (on the unsigned integer
  types) have been replaced.  The new idiom is explicit construction
  of the target type using the `bitPattern:` argument label.  So,

    ```swift
    myInt.asUnsigned()
    ```

  has become

    ```swift
    UInt(bitPattern: myInt)
    ```

* To better follow Cocoa naming conventions and to encourage
  immutability, The following pointer types were renamed:

  | Old Name                        | New Name                               |
  |---------------------------------|----------------------------------------|
  | `UnsafePointer<T>`              | `UnsafeMutablePointer<T>`              |
  | `ConstUnsafePointer<T>`         | `UnsafePointer<T>`                     |
  | `AutoreleasingUnsafePointer<T>` | `AutoreleasingUnsafeMutablePointer<T>` |

  Note that the meaning of `UnsafePointer` has changed from mutable to
  immutable. As a result, some of your code may fail to compile when
  assigning to an `UnsafePointer.memory` property.  The fix is to
  change your `UnsafePointer<T>` into an `UnsafeMutablePointer<T>`.

* The optional unwrapping operator `x!` can now be assigned through, and
  mutating methods and operators can be applied through it:

    ```swift
    var x: Int! = 0
    x! = 2
    x!++

    // Nested dictionaries can now be mutated directly:
    var sequences = ["fibonacci": [1, 1, 2, 3, 0]]
    sequences["fibonacci"]![4] = 5
    sequences["fibonacci"]!.append(8)
    ```

* The `@auto_closure` attribute has been renamed to `@autoclosure`.

* There is a new `dynamic` declaration modifier. When applied to a method,
  property, subscript, or initializer, it guarantees that references to the
  declaration are always dynamically dispatched and never inlined or
  devirtualized, and that the method binding can be reliably changed at runtime.
  The implementation currently relies on the Objective-C runtime, so `dynamic`
  can only be applied to `@objc-compatible` declarations for now. `@objc` now
  only makes a declaration visible to Objective-C; the compiler may now use
  vtable lookup or direct access to access (non-dynamic) `@objc` declarations.

    ```swift
    class Foo {
      // Always accessed by objc_msgSend
      dynamic var x: Int

      // Accessed by objc_msgSend from ObjC; may be accessed by vtable
      // or by static reference in Swift
      @objc var y: Int

      // Not exposed to ObjC (unless Foo inherits NSObject)
      var z: Int
    }
    ```

  `dynamic` enables KVO, proxying, and other advanced Cocoa features to work
  reliably with Swift declarations.

* Clang submodules can now be imported:

    ```swift
    import UIKit.UIGestureRecognizerSubclass
    ```

* The numeric optimization levels `-O[0-3]` have been removed in favor of the
  named levels `-Onone` and `-O`.

* The `-Ofast` optimization flag has been renamed to `-Ounchecked`. We will accept
  both names for now and remove `-Ofast` in a later build.

* An initializer that overrides a designated initializer from its
  superclass must be marked with the `override` keyword, so that all
  overrides in the language consistently require the use of
  `override`. For example:

    ```swift
    class A {
      init() { }
    }

    class B : A {
      override init() { super.init() }
    }
    ```

* Required initializers are now more prominent in several ways. First,
  a (non-final) class that conforms to a protocol that contains an
  initializer requirement must provide a required initializer to
  satisfy that requirement. This ensures that subclasses will also
  conform to the protocol, and will be most visible with classes that
  conform to `NSCoding`:

    ```swift
    class MyClass : NSObject, NSCoding {
      required init(coder aDecoder: NSCoder!) { /*... */ }
      func encodeWithCoder(aCoder: NSCoder!) { /* ... */ }
    }
    ```
  Second, because `required` places a significant requirement on all
  subclasses, the `required` keyword must be placed on overrides of a
  required initializer:

    ```swift
    class MySubClass : MyClass {
      var title: String = "Untitled"

      required init(coder aDecoder: NSCoder!) { /*... */ }
      override func encodeWithCoder(aCoder: NSCoder!) { /* ... */ }
    }
    ```
  Finally, required initializers can now be inherited like any other
  initializer:

    ```swift
    class MySimpleSubClass : MyClass { } // inherits the required init(coder:).
    ```

### 2014-07-21

* Access control has been implemented.

  - `public` declarations can be accessed from any module.
  - `internal` declarations (the default) can be accessed from within the
    current module.
  - `private` declarations can be accessed only from within the current file.

  There are still details to iron out here, but the model is in place.
  The general principle is that an entity cannot be defined in terms of another
  entity with less accessibility.

  Along with this, the generated header for a framework will only include
  public declarations. Generated headers for applications will include public
  and internal declarations.

* `CGFloat` is now a distinct floating-point type that wraps either a
  `Float` (on 32-bit architectures) or a `Double` (on 64-bit
  architectures). It provides all of the same comparison and
  arithmetic operations of Float and Double, and can be created using
  numeric literals.

* The immediate mode `swift -i` now works for writing `#!` scripts that take
  command line arguments. The `-i` option to the swift driver must now come at
  the end of the compiler arguments, directly before the input filename. Any
  arguments that come after `-i` and the input filename are treated as arguments
  to the interpreted file and forwarded to `Process.arguments`.

* Type inference for `for..in` loops has been improved to consider the
  sequence along with the element pattern. For example, this accepts
  the following loops that were previously rejected:

    ```swift
    for i: Int8 in 0..<10 { }
    for i: Float in 0.0...10.0 { }
    ```

* Introduced the new `BooleanLiteralConvertible` protocol, which allows
  user-defined types to support Boolean literals. `true` and `false`
  are now `Boolean` constants and keywords.

* The `@final`, `@lazy`, `@required` and `@optional` attributes are now
  considered to be declaration modifiers - they no longer require (or allow) an
  `@` sign.

* The `@prefix`, `@infix`, and `@postfix` attributes have been changed to
  declaration modifiers, so they are no longer spelled with an `@` sign.
  Operator declarations have been rearranged from `operator prefix +` to
  `prefix operator +` for consistency.

### 2014-07-03

* C function pointer types are now imported as `CFunctionPointer<T>`, where `T`
  is a Swift function type. `CFunctionPointer` and `COpaquePointer` can be
  explicitly constructed from one another, but they do not freely convert, nor
  is `CFunctionPointer` compatible with Swift closures.

  Example: `int (*)(void)` becomes `CFunctionPointer<(Int) -> Void>`.

* The interop model for pointers in C APIs has been simplified. Most code that
  calls C functions by passing arrays, UnsafePointers, or the addresses of
  variables with `&x` does not need to change. However, the `CConstPointer` and
  `CMutablePointer` bridging types have been removed, and functions and methods
  are now imported as and overridden by taking UnsafePointer and
  `ConstUnsafePointer` directly. `Void` pointers are now imported as
  `(Const)UnsafePointer<Void>`; `COpaquePointer` is only imported for opaque
  types now.

* `Array` types are now spelled with the brackets surrounding the
  element type. For example, an array of `Int` is written as:

    ```swift
    var array: [Int]
    ```

* `Dictionary` types can now be spelled with the syntax `[K : V]`, where `K`
  is the key type and `V` is the value type. For example:

    ```swift
    var dict: [String : Int] = ["Hello" : 1, "World" : 2]
    ```

  The type `[K : V]` is syntactic sugar for `Dictionary<K, V>`; nothing
  else has changed.

* The `@IBOutlet` attribute no longer implicitly (and invisibly) changes the type
  of the declaration it is attached to.  It no longer implicitly makes variables
  be an implicitly unwrapped optional and no longer defaults them to weak.

* The `\x`, `\u` and `\U` escape sequences in string literals have been
  consolidated into a single and less error prone `\u{123456}` syntax.

### 2014-06-23

* The half-open range operator has been renamed from `..` to `..<` to reduce
  confusion.  The `..<` operator is precedented in Groovy (among other languages)
  and makes it much more clear that it doesn't include the endpoint.

* Class objects such as `NSObject.self` can now be converted to `AnyObject` and
  used as object values.

* Objective-C protocol objects such as `NSCopying.self` can now be used as
  instances of the `Protocol` class, such as in APIs such as XPC.

* Arrays now have full value semantics: both assignment and
  initialization create a logically-distinct object

* The `sort` function and array method modify the target in-place.  A
  new `sorted` function and array method are non-mutating, creating
  and returning a new collection.

### 2014-05-19

* `sort`, `map`, `filter`, and `reduce` methods on `Array`s accept trailing
  closures:

    ```swift
    let a = [5, 6, 1, 3, 9]
    a.sort{ $0 > $1 }
    println(a)                                 // [9, 6, 5, 3, 1]
    println(a.map{ $0 * 2 })                   // [18, 12, 10, 6, 2]
    println(a.map{ $0 * 2 }.filter{ $0 < 10})  // [6, 2]
    println(a.reduce(1000){ $0 + $1 })         // 1024 (no kidding)
    ```

* A lazy `map()` function in the standard library works on any `Sequence`.
  Example:

    ```swift
    class X {
      var value: Int

      init(_ value: Int) {
        self.value = value
        println("created X(\(value))")
      }
    }

    // logically, this sequence is X(0), X(1), X(2), ... X(50)
    let lazyXs = map(0..50){ X($0) }

    // Prints "created X(...)" 4 times
    for x in lazyXs {
      if x.value == 4 {
        break
      }
    }
    ```

* There's a similar lazy `filter()` function:

    ```swift
    // 0, 10, 20, 30, 40
    let tens = filter(0..50) { $0 % 10 == 0 }
    let tenX = map(tens){ X($0) }    // 5 lazy Xs
    let tenXarray = Array(tenX)      // Actually creates those Xs
    ```

* Weak pointers of classbound protocol type work now.

* `IBOutlets` now default to weak pointers with implicit optional type (`T!`).

* `NSArray*` parameters and result types of Objective-C APIs are now
  imported as `AnyObject[]!`, i.e., an implicitly unwrapped optional
  array storing `AnyObject` values. For example, `NSView`'s constraints
  property

    ```objc
    @property (readonly) NSArray *constraints;
    ```

  is now imported as

    ```swift
    var constraints: AnyObject[]!
    ```

  Note that one can implicitly convert between an `AnyObject[]` and an
  `NSArray` (in both directions), so (for example) one can still
  explicitly use `NSArray` if desired:

    ```swift
    var array: NSArray = view.constraints
    ```

  Swift arrays bridge to `NSArray` similarly to the way Swift
  strings bridge to `NSString`.

* `ObjCMutablePointer` has been renamed `AutoreleasingUnsafePointer`.

* `UnsafePointer` (and `AutoreleasingUnsafePointer`)'s `set()` and `get()`
  have been replaced with a property called `memory`.

  - Previously you would write:

    ```swift
    val = p.get()
    p.set(val)
    ```

  - Now you write:

    ```swift
    val = p.memory
    p.memory = val
    ```

* Removed shorthand `x as T!`; instead use `(x as T)!`

  - `x as T!` now means "x as implicitly unwrapped optional".

* Range operators `..` and `...` have been switched.

  - `1..3` now means 1,2
  - `1...3` now means 1,2,3

* The pound sign (`#`) is now used instead of the back-tick (\`) to mark
  an argument name as a keyword argument, e.g.,

    ```swift
    func moveTo(#x: Int, #y: Int) { ... }
    moveTo(x: 5, y: 7)
    ```

* Objective-C factory methods are now imported as initializers. For
  example, `NSColor`'s `+colorWithRed:green:blue:alpha` becomes

    ```swift
    init(red: CGFloat, green: CGFloat, blue: CGFloat, alpha: CGFloat)
    ```

  which allows an `NSColor` to be created as, e.g.,

    ```swift
    NSColor(red: 0.5, green: 0.25, blue: 0.25, alpha: 0.5)
    ```

  Factory methods are identified by their kind (class methods), name
  (starts with words that match the words that end the class name),
  and result type (`instancetype` or the class type).

* Objective-C properties of some `CF` type are no longer imported as `Unmanaged`.

* REPL mode now uses LLDB, for a greatly-expanded set of features. The colon
  prefix now treats the rest of the line as a command for LLDB, and entering
  a single colon will drop you into the debugging command prompt. Most
  importantly, crashes in the REPL will now drop you into debugging mode to
  see what went wrong.

  If you do have a need for the previous REPL, pass `-integrated-repl`.

* In a UIKit-based application, you can now eliminate your 'main.swift' file
  and instead apply the `@UIApplicationMain` attribute to your
  `UIApplicationDelegate` class. This will cause the `main` entry point to the
  application to be automatically generated as follows:

    ```swift
    UIApplicationMain(argc, argv, nil,
                      NSStringFromClass(YourApplicationDelegate.self))
    ```

  If you need nontrivial logic in your application entry point, you can still
  write out a `main.swift`. Note that `@UIApplicationMain` and `main.swift` are
  mutually exclusive.

### 2014-05-13

* weak pointers now work with implicitly unchecked optionals, enabling usecases
  where you don't want to `!` every use of a weak pointer.  For example:

     ```swift
     weak var myView : NSView!
     ```

  of course, they still work with explicitly checked optionals like `NSView?`

* Dictionary subscripting now takes/returns an optional type.  This allows
  querying a dictionary via subscripting to gracefully fail.  It also enables
  the idiom of removing values from a dictionary using `dict[key] = nil`.
  As part of this, `deleteKey` is no longer available.

* Stored properties may now be marked with the `@lazy` attribute, which causes
  their initializer to be evaluated the first time the property is touched
  instead of when the enclosing type is initialized.  For example:

    ```swift
    func myInitializer() -> Int { println("hello\n"); return 42 }
    class MyClass {
      @lazy var aProperty = myInitializer()
    }

    var c = MyClass()     // doesn't print hello
    var tmp = c.aProperty // prints hello on first access
    tmp = c.aProperty     // doesn't print on subsequent loads.

    c = MyClass()         // doesn't print hello
    c.aProperty = 57      // overwriting the value prevents it from ever running
    ```

  Because lazy properties inherently rely on mutation of the property, they
  cannot be `let`s.  They are currently also limited to being members of structs
  and classes (they aren't allowed as local or global variables yet) and cannot
  be observed with `willSet`/`didSet` yet.

* Closures can now specify a capture list to indicate with what strength they
  want to capture a value, and to bind a particular field value if they want to.

  Closure capture lists are square-bracket delimited and specified before the
  (optional) argument list in a closure.  Each entry may be specified as `weak`
  or `unowned` to capture the value with a weak or unowned pointer, and may
  contain an explicit expression if desired.  Some examples:

    ```swift
    takeClosure { print(self.title) }                    // strong capture
    takeClosure { [weak self] in print(self!.title) }    // weak capture
    takeClosure { [unowned self] in print(self.title) }  // unowned capture
    ```

  You can also bind arbitrary expression to named values in the capture list.
  The expression is evaluated when the closure is formed, and captured with the
  specified strength.  For example:

    ```swift
    // weak capture of "self.parent"
    takeClosure { [weak tmp = self.parent] in print(tmp!.title) }
    ```

  The full form of a closure can take a signature (an argument list and
  optionally a return type) if needed.  To use either the capture list or the
  signature, you must specify the context sensitive `in` keyword.  Here is a
  (weird because there is no need for `unowned`) example of a closure with both:

    ```swift
    myNSSet.enumerateObjectsUsingBlock { [unowned self] (obj, stop) in
      self.considerWorkingWith(obj)
    }
    ```

* The word `with` is now removed from the first keyword argument name
  if an initialized imported from Objective-C. For example, instead of
  building `UIColor` as:

    ```swift
    UIColor(withRed: r, green: g, blue: b, alpha: a)
    ```

  it will now be:

    ```swift
    UIColor(red: r, green: g, blue: b, alpha: a)
    ```

* `Dictionary` can be bridged to `NSDictionary` and vice versa:

  - `NSDictionary` has an implicit conversion to `Dictionary<NSObject,
    AnyObject>`.  It bridges in O(1), without memory allocation.

  - `Dictionary<K, V>` has an implicit conversion to `NSDictionary`.
    `Dictionary<K, V>` bridges to `NSDictionary` iff both `K` and `V` are
    bridged.  Otherwise, a runtime error is raised.

    Depending on `K` and `V` the operation can be `O(1)` without memory
    allocation, or `O(N)` with memory allocation.

* Single-quoted literals are no longer recognized.  Use double-quoted literals
  and an explicit type annotation to define `Characters` and `UnicodeScalars`:

    ```swift
    var ch: Character = "a"
    var us: UnicodeScalar = "a"
    ```

### 2014-05-09

* The use of keyword arguments is now strictly enforced at the call
  site. For example, consider this method along with a call to it:

    ```swift
    class MyColor {
      func mixColorWithRed(red: Float, green: Float, blue: Float) { /* ... */ }
    }

    func mix(color: MyColor, r: Float, g: Float, b: Float) {
      color.mixColorWithRed(r, g, b)
    }
    ```

  The compiler will now complain about the missing `green:` and
  `blue:` labels, with a Fix-It to correct the code:

    ```
    color.swift:6:24: error: missing argument labels 'green:blue:' in call
      color.mixColorWithRed(r, g, b)
                           ^
                               green:  blue:
    ```

  The compiler handles missing, extraneous, and incorrectly-typed
  argument labels in the same manner. Recall that one can make a
  parameter a keyword argument with the back-tick or remove a keyword
  argument with the underscore.

    ```swift
    class MyColor {
      func mixColor(`red: Float, green: Float, blue: Float) { /* ... */ }
      func mixColorGuess(red: Float, _ green: Float, _ blue: Float) { /* ... */ }
    }

    func mix(color: MyColor, r: Float, g: Float, b: Float) {
      color.mixColor(red: r, green: g, blue: b) // okay: all keyword arguments
      color.mixColorGuess(r, g, b) // okay: no keyword arguments
    }
    ```

  Arguments cannot be re-ordered unless the corresponding parameters
  have default arguments. For example, given:

    ```swift
    func printNumber(`number: Int, radix: Int = 10, separator: String = ",") { }
    ```

  The following three calls are acceptable because only the arguments for
  defaulted parameters are re-ordered relative to each other:

    ```swift
    printNumber(number: 256, radix: 16, separator: "_")
    printNumber(number: 256, separator: "_")
    printNumber(number: 256, separator: ",", radix: 16)
    ```

  However, this call:

    ```swift
    printNumber(separator: ",", radix: 16, number: 256)
    ```

  results in an error due to the re-ordering:

    ```
    printnum.swift:7:40: error: argument 'number' must precede argument 'separator'
    printNumber(separator: ",", radix: 16, number: 256)
                ~~~~~~~~~~~~~~             ^       ~~~
    ```

* `;` can no longer be used to demarcate an empty case in a switch statement,
  use `break` instead.

### 2014-05-07

* The compiler's ability to diagnose many common kinds of type check errors has
  improved. (`expression does not type-check` has been retired.)

* Ranges can be formed with floating point numbers, e.g. `0.0 .. 100.0`.

* Convenience initializers are now spelled as `convenience init` instead of with
  the `-> Self` syntax.  For example:

    ```swift
    class Foo {
      init(x : Int) {}  // designated initializer

      convenience init() { self.init(42) } // convenience initializer
    }
    ```

  You still cannot declare designated initializers in extensions, only
  convenience initializers are allowed.

* Reference types using the CoreFoundation runtime are now imported as
  class types.  This means that Swift will automatically manage the
  lifetime of a `CFStringRef` the same way that it manages the lifetime
  of an `NSString`.

  In many common cases, this will just work.  Unfortunately, values
  are returned from `CF`-style APIs in a wide variety of ways, and
  unlike Objective-C methods, there simply isn't enough consistency
  for Swift to be able to safely apply the documented conventions
  universally.  The framework teams have already audited many of the
  most important `CF`-style APIs, and those APIs should be imported
  without a hitch into Swift.  For all the APIs which haven't yet
  been audited, we must import return types using the `Unmanaged` type.
  This type allows the programmer to control exactly how the object
  is passed.

  For example:

    ```swift
    // CFBundleGetAllBundles() returns an Unmanaged<CFArrayRef>.
    // From the documentation, we know that it returns a +0 value.
    let bundles = CFBundleGetAllBundles().takeUnretainedValue()

    // CFRunLoopCopyAllModes() returns an Unmanaged<CFArrayRef>.
    // From the documentation, we know that it returns a +1 value.
    let modes = CFRunLoopCopyAllModes(CFRunLoopGetMain()).takeRetainedValue()
    ```

  You can also use `Unmanaged` types to pass and return objects
  indirectly, as well as to generate unbalanced retains and releases
  if you really require them.

  The API of the Unmanaged type is still in flux, and your feedback
  would be greatly appreciated.

### 2014-05-03

* The `@NSManaged` attribute can be applied to the properties of an
  `NSManagedObject` subclass to indicate that they should be handled by
  CoreData:

    ```swift
    class Employee : NSManagedObject {
      @NSManaged var name: String
      @NSManaged var department: Department
    }
    ```

* The `@weak` and `@unowned` attributes have become context sensitive keywords
  instead of attributes.  To declare a `weak` or `unowned` pointer, use:

    ```swift
    weak var someOtherWindow : NSWindow?
    unowned var someWindow : NSWindow
    ```
  ... with no `@` on the `weak`/`unowned`.

### 2014-04-30

* Swift now supports a `#elseif` form for build configurations, e.g.:

    ```swift
    #if os(OSX)
      typealias SKColor = NSColor
    #elseif os(iOS)
      typealias SKColor = UIColor
    #else
      typealias SKColor = Green
    #endif
    ```

* You can now use the `true` and `false` constants in build configurations,
  allowing you to emulate the C idioms of `#if 0` (but spelled `#if false`).

* `break` now breaks out of switch statements.

* It is no longer possible to specify `@mutating` as an attribute, you may only
  use it as a keyword, e.g.:

    ```swift
    struct Pair {
      var x, y : Int
      mutating func nuke() { x = 0; y = 0 }
    }
    ```
  The former `@!mutating` syntax used to mark setters as non-mutating is now
  spelled with the `nonmutating` keyword.  Both mutating and nonmutating are
  context sensitive keywords.

* `NSLog` is now available from Swift code.

* The parser now correctly handles expressions like `var x = Int[]()` to
  create an empty array of integers.  Previously you'd have to use syntax like
  `Array<Int>()` to get this.  Now that this is all working, please prefer to
  use `Int[]` consistently instead of `Array<Int>`.

* `Character` is the new character literal type:

    ```swift
    var x = 'a' // Infers 'Character' type
    ```

  You can force inference of `UnicodeScalar` like this:

    ```swift
    var scalar: UnicodeScalar = 'a'
    ```

  `Character` type represents a Unicode extended grapheme cluster (to put it
  simply, a grapheme cluster is what users think of as a character: a base plus
  any combining marks, or other cases explained in
  [Unicode Standard Annex #29](http://unicode.org/reports/tr29/)).

### 2014-04-22

* Loops and switch statements can now carry labels, and you can
  `break`/`continue` to those labels.  These use conventional C-style label
  syntax, and should be dedented relative to the code they are in.  An example:

    ```swift
    func breakContinue(x : Int) -> Int {
    Outer:
      for a in 0..1000 {

      Switch:
        switch x {
        case 42: break Outer
        case 97: continue Outer
        case 102: break Switch
        case 13: continue // continue always works on loops.
        case 139: break   // break will break out of the switch (but see below)
        }
      }
    }
    ```

* We are changing the behavior of `break` to provide C-style semantics, to allow
  breaking out of a switch statement.  Previously, break completely ignored
  switches so that it would break out of the nearest loop. In the example above,
  `case 139` would break out of the `Outer` loop, not the `Switch`.

  In order to avoid breaking existing code, we're making this a compile time
  error instead of a silent behavior change.  If you need a solution for the
  previous behavior, use labeled break.

  This error will be removed in a week or two.

* Cocoa methods and properties that are annotated with the
  `NS_RETURNS_INNER_POINTER` attribute, including `-[NSData bytes]` and
  `-[{NS,UI}Color CGColor]`, are now safe to use and follow the same lifetime
  extension semantics as ARC.

### 2014-04-18

* Enabling/disabling of asserts

    ```swift
    assert(condition, msg)
    ```

  is enabled/disabled dependent on the optimization level. In debug mode at
  `-O0` asserts are enabled. At higher optimization levels asserts are disabled
  and no code is generated for them. However, asserts are always type checked
  even at higher optimization levels.

  Alternatively, assertions can be disabled/enabled by using the frontend flag
  `-assert-config Debug`, or `-assert-config Release`.

* Added optimization flag `-Ofast`. It disables all assertions (`assert`), and
  runtime overflow and type checks.

* The "selector-style" function and initializer declaration syntax is
  being phased out. For example, this:

    ```
    init withRed(red: CGFloat) green(CGFloat) blue(CGFloat) alpha(CGFloat)
    ```

  will now be written as:

    ```swift
    init(withRed red: CGFloat, green: CGFloat, blue: CGFloat, alpha: CGFloat)
    ```

  For each parameter, one can have both an argument API name (i.e.,
  `withRed`, which comes first and is used at the call site) and an
  internal parameter name that follows it (i.e. `red`, which comes
  second and is used in the implementation). When the two names are
  the same, one can simply write the name once and it will be used for
  both roles (as with `green`, `blue`, and `alpha` above). The
  underscore (`_`) can be used to mean "no name", as when the
  following function/method:

    ```
    func murderInRoom(room:String) withWeapon(weapon: String)
    ```

  is translated to:

    ```swift
    func murderInRoom(_ room: String, withWeapon weapon: String)
    ```

  The compiler now complains when it sees the selector-style syntax
  and will provide Fix-Its to rewrite to the newer syntax.

  Note that the final form of selector syntax is still being hammered
  out, but only having one declaration syntax, which will be very
  close to this, is a known.

* Stored properties can now be marked with the `@NSCopying` attribute, which
  causes their setter to be synthesized with a copy to `copyWithZone:`.  This may
  only be used with types that conform to the `NSCopying` protocol, or option
  types thereof.  For example:

    ```swift
    @NSCopying var myURL : NSURL
    ```

  This fills the same niche as the (`copy`) attribute on Objective-C properties.

### 2014-04-16

* Optional variables and properties are now default-initialized to `nil`:

    ```swift
    class MyClass {
      var cachedTitle: String?       // "= nil" is implied
    }
    ```

* `@IBOutlet` has been improved in a few ways:

  - `IBOutlets` can now be `@unchecked` optional.

  - An `IBOutlet` declared as non-optional, i.e.,

    ```swift
    @IBOutlet var button: NSButton
    ```

    will be treated as an `@unchecked` optional.  This is considered to
    be the best practice way to write an outlet, unless you want to explicitly
    handle the null case - in which case, use `NSButton?` as the type. Either
    way, the `= nil` that was formerly required is now implicit.

* The precedence of `is` and `as` is now higher than comparisons, allowing the
  following sorts of things to be written without parens:

    ```swift
    if x is NSButton && y is NSButtonCell { ... }

    if 3/4 as Float == 6/8 as Float { ... }
    ```

* Objective-C blocks are now transparently bridged to Swift closures. You never
  have to write `@objc_block` when writing Objective-C-compatible methods anymore.
  Block parameters are now imported as unchecked optional closure types,
  allowing `nil` to be passed.

### 2014-04-09

* `Dictionary` changes:

  - `Elements` are now tuples, so you can write

    ```swift
    for (k, v) in d {
      // ...
    }
    ```

  - `keys` and `values` properties, which are `Collections` projecting
    the corresponding aspect of each element.  `Dictionary` indices are
    usable with their `keys` and `values` properties, so:

    ```swift
    for i in indices(d) {
      let (k, v) = d[i]
      assert(k == d.keys[i])
      assert(v == d.values[i])
    }
    ```

* Semicolon can be used as a single no-op statement in otherwise empty cases in
  `switch` statements:

    ```swift
    switch x {
    case 1, 2, 3:
      print("x is 1, 2 or 3")
    default:
      ;
    }
    ```

* `override` is now a context sensitive keyword, instead of an attribute:

    ```swift
    class Base {
      var property: Int { return 0 }
      func instanceFunc() {}
      class func classFunc() {}
    }
    class Derived : Base {
      override var property: Int { return 1 }
      override func instanceFunc() {}
      override class func classFunc() {}
    }
    ```

### 2014-04-02

* Prefix splitting for imported enums has been revised again due to feedback:
  - If stripping off a prefix would leave an invalid identifier (like `10_4`),
    leave one more word in the result than would otherwise be there
    (`Behavior10_4`).
  - If all enumerators have a `k` prefix (for `constant`) and the enum doesn't,
    the `k` should not be considered when finding the common prefix.
  - If the enum name is a plural (like `NSSomethingOptions`) and the enumerator
    names use the singular form (`NSSomethingOptionMagic`), this is considered
    a matching prefix (but only if nothing follows the plural).

* Cocoa APIs that take pointers to plain C types as arguments now get imported
  as taking the new `CMutablePointer<T>` and `CConstPointer<T>` types instead
  of `UnsafePointer<T>`. These new types allow implicit conversions from
  Swift `inout` parameters and from Swift arrays:

    ```swift
    let rgb = CGColorSpaceCreateDeviceRGB()
    // CGColorRef CGColorCreate(CGColorSpaceRef, const CGFloat*);
    let white = CGColorCreate(rgb, [1.0, 1.0, 1.0])

    var s = 0.0, c = 0.0
    // void sincos(double, double*, double*);
    sincos(M_PI/2, &s, &c)
    ```

  Pointers to pointers to ObjC classes, such as `NSError**`, get imported as
  `ObjCMutablePointer<NSError?>`. This type doesn't work with arrays, but
  accepts inouts or `nil`:

    ```swift
    var error: NSError?
    let words = NSString.stringWithContentsOfFile("/usr/share/dict/words",
      encoding: .UTF8StringEncoding,
      error: &error)
    ```

  `Void` pointer parameters can be passed an array or inout of any type:

    ```swift
    // + (NSData*)dataWithBytes:(const void*)bytes length:(NSUInteger)length;
    let data = NSData.dataWithBytes([1.5, 2.25, 3.125],
                                    length: sizeof(Double.self) * 3)
    var fromData = [0.0, 0.0, 0.0]
    // - (void)getBytes:(void*)bytes length:(NSUInteger)length;
    data.getBytes(&fromData, length: sizeof(Double.self) * 3)
    ```

  Note that we don't know whether an API reads or writes the C pointer, so
  you need to explicitly initialize values (like `s` and `c` above) even if
  you know that the API overwrites them.

  This pointer bridging only applies to arguments, and only works with well-
  behaved C and ObjC APIs that don't keep the pointers they receive as
  arguments around or do other dirty pointer tricks. Nonstandard use of pointer
  arguments still requires `UnsafePointer`.

* Objective-C pointer types now get imported by default as the `@unchecked T?`
  optional type.  Swift class types no longer implicitly include `nil`.

  A value of `@unchecked T?` can be implicitly used as a value of `T`.
  Swift will implicitly cause a reliable failure if the value is `nil`,
  rather than introducing undefined behavior (as in Objective-C ivar
  accesses or everything in C/C++) or silently ignoring the operation
  (as in Objective-C message sends).

  A value of `@unchecked T?` can also be implicitly used as a value of `T?`,
  allowing you explicitly handle the case of a `nil` value.  For example,
  if you would like to just silently ignore a message send a la Objective-C,
  you can use the postfix `?` operator like so:

    ```swift
    fieldsForKeys[kHeroFieldKey]?.setEditable(true)
    ```

  This design allows you to isolate and handle `nil` values in Swift code
  without requiring excessive "bookkeeping" boilerplate to use values that
  you expect to be non-`nil`.

  For now, we will continue to import C pointers as non-optional
  `UnsafePointer` and `C*Pointer` types; that will be evaluated separately.

  We intend to provide attributes for Clang to allow APIs to opt in to
  importing specific parameters, return types, etc. as either the
  explicit optional type `T?` or the simple non-optional type `T`.

* The "separated" call syntax, i.e.,

    ```
    NSColor.colorWithRed(r) green(g) blue(b) alpha(a)
    UIColor.init withRed(r) green(g) blue(b) alpha(a)
    ```

  is being removed. The compiler will now produce an error and provide
  Fix-Its to rewrite calls to the "keyword-argument" syntax:

    ```swift
    NSColor.colorWithRed(r, green: g, blue: b, alpha: a)
    UIColor(withRed: r, green:g, blue:b, alpha: a)
    ```

* The `objc` attribute now optionally accepts a name, which can be
  used to provide the name for an entity as seen in Objective-C. For
  example:

    ```swift
    class MyType {
      var enabled: Bool {
        @objc(isEnabled) get {
          // ...
        }
      }
    }
    ```

  The `@objc` attribute can be used to name initializers, methods,
  getters, setters, classes, and protocols.

* Methods, properties and subscripts in classes can now be marked with the
  `@final` attribute.  This attribute prevents overriding the declaration in any
  subclass, and provides better performance (since dynamic dispatch is avoided
  in many cases).

### 2014-03-26

* Attributes on declarations are no longer comma separated.

  Old syntax:

    ```
    @_silgen_name("foo"), @objc func bar() {}
    ```

  New syntax:

    ```swift
    @_silgen_name("foo") @objc
    ```

  The `,` was vestigial when the attribute syntax consisted of bracket lists.

* `switch` now always requires a statement after a `case` or `default`.

  Old syntax:

    ```swift
    switch x {
    case .A:
    case .B(1):
      println(".A or .B(1)")
    default:
      // Ignore it.
    }
    ```

  New syntax:

    ```swift
    switch x {
    case .A, .B(1):
      println(".A or .B(1)")
    default:
      () // Ignore it.
    }
    ```

  The following syntax can be used to introduce guard expressions for patterns
  inside the `case`:

    ```swift
    switch x {
    case .A where isFoo(),
         .B(1) where isBar():
      ...
    }
    ```

* Observing properties can now `@override` properties in a base class, so you
  can observe changes that happen to them.

     ```swift
     class MyAwesomeView : SomeBasicView {
      @override
      var enabled : Bool {
        didSet {
          println("Something changed")
        }
      }
      ...
    }
    ```

  Observing properties still invoke the base class getter/setter (or storage)
  when accessed.

* An `as` cast can now be forced using the postfix `!` operator without using
  parens:

    ```swift
    class B {}
    class D {}

    let b: B = D()

    // Before
    let d1: D = (b as D)!
    // After
    let d2: D = b as D!
    ```

  Casts can also be chained without parens:

    ```swift
    // Before
    let b2: B = (((D() as B) as D)!) as B
    // After
    let b3: B = D() as B as D! as B
    ```

* `as` can now be used in `switch` cases to match the result of a checked cast:

    ```swift
    func printHand(hand: Any) {
      switch hand {
      case 1 as Int:
        print("ace")
      case 11 as Int:
        print("jack")
      case 12 as Int:
        print("queen")
      case 13 as Int:
        print("king")
      case let numberCard as Int:
        print("\(numberCard)")
      case let (a, b) as (Int, Int) where a == b:
        print("two ")
        printHand(a)
        print("s")
      case let (a, b) as (Int, Int):
        printHand(a)
        print(" and a ")
        printHand(b)
      case let (a, b, c) as (Int, Int, Int) where a == b && b == c:
        print("three ")
        printHand(a)
        print("s")
      case let (a, b, c) as (Int, Int, Int):
        printHand(a)
        print(", ")
        printHand(b)
        print(", and a ")
        printHand(c)
      default:
        print("unknown hand")
      }
    }
    printHand(1, 1, 1) // prints "three aces"
    printHand(12, 13) // prints "queen and a king"
    ```

* Enums and option sets imported from C/Objective-C still strip common
  prefixes, but the name of the enum itself is now taken into consideration as
  well. This keeps us from dropping important parts of a name that happen to be
  shared by all members.

    ```objc
    // NSFileManager.h
    typedef NS_OPTIONS(NSUInteger, NSDirectoryEnumerationOptions) {
        NSDirectoryEnumerationSkipsSubdirectoryDescendants = 1UL << 0,
        NSDirectoryEnumerationSkipsPackageDescendants      = 1UL << 1,
        NSDirectoryEnumerationSkipsHiddenFiles             = 1UL << 2
    } NS_ENUM_AVAILABLE(10_6, 4_0);
    ```

    ```swift
    // Swift
    let opts: NSDirectoryEnumerationOptions = .SkipsPackageDescendants
    ```

* `init` methods in Objective-C protocols are now imported as
  initializers. To conform to `NSCoding`, you will now need to provide

    ```swift
    init withCoder(aDecoder: NSCoder) { ... }
    ```

  rather than

    ```swift
    func initWithCoder(aDecoder: NSCoder) { ... }
    ```

### 2014-03-19

* When a class provides no initializers of its own but has default
  values for all of its stored properties, it will automatically
  inherit all of the initializers of its superclass. For example:

    ```swift
    class Document {
      var title: String

      init() -> Self {
        self.init(withTitle: "Default title")
      }

      init withTitle(title: String) {
        self.title = title
      }
    }

    class VersionedDocument : Document {
      var version = 0

      // inherits 'init' and 'init withTitle:' from Document
    }
    ```

  When one does provide a designated initializer in a subclass, as in
  the following example:

    ```swift
    class SecureDocument : Document {
      var key: CryptoKey

      init withKey(key: CryptoKey) -> Self {
        self.init(withKey: key, title: "Default title")
      }

      init withKey(key: CryptoKey) title(String) {
        self.key = key
        super.init(withTitle: title)
      }
    }
    ```

  the compiler emits Objective-C method stubs for all of the
  designated initializers of the parent class that will abort at
  runtime if called, and which indicate which initializer needs to be
  implemented. This provides memory safety for cases where an
  Objective-C initializer (such as `-[Document init]` in this example)
  appears to be inherited, but isn't actually implemented.

* `nil` may now be used as a Selector value. This allows calls to Cocoa methods
  that accept `nil` selectors.

* `[]` and `[:]` can now be used as the empty array and dictionary literal,
  respectively.  Because these carry no information about their element types,
  they may only be used in a context that provides this information through type
  inference (e.g. when passing a function argument).

* Properties defined in classes are now dynamically dispatched and can be
  overridden with `@override`.  Currently `@override` only works with computed properties
  overriding other computed properties, but this will be enhanced in coming weeks.

### 2014-03-12

* The `didSet` accessor of an observing property now gets passed in the old value,
  so you can easily implement an action for when a property changes value.  For
  example:

    ```swift
    class MyAwesomeView : UIView {
      var enabled : Bool = false {
      didSet(oldValue):
        if oldValue != enabled {
          self.needsDisplay = true
        }
      }
      ...
    }
    ```

* The implicit argument name for set and willSet property specifiers has been
  renamed from `(value)` to `(newValue)`.  For example:

    ```swift
    var i : Int {
      get {
        return 42
      }
      set {  // defaults to (newValue) instead of (value)
        print(newValue)
      }
    }
    ```

* The magic identifier `__FUNCTION__` can now be used to get the name of the
  current function as a string. Like `__FILE__` and `__LINE__`, if
  `__FUNCTION__` is used as a default argument, the function name of the caller
  is passed as the argument.

    ```swift
    func malkovich() {
      println(__FUNCTION__)
    }
    malkovich() // prints "malkovich"

    func nameCaller(name: String = __FUNCTION__) -> String {
      return name
    }

    func foo() {
      println(nameCaller()) // prints "foo"
    }

    func foo(x: Int) bar(y: Int) {
      println(nameCaller()) // prints "foo:bar:"
    }
    ```

  At top level, `__FUNCTION__` gives the module name:

    ```swift
    println(nameCaller()) // prints your module name
    ```

* Selector-style methods can now be referenced without applying arguments
  using member syntax `foo.bar:bas:`, for instance, to test for the availability
  of an optional protocol method:

    ```swift
    func getFrameOfObjectValueForColumn(ds: NSTableViewDataSource,
                                        tableView: NSTableView,
                                        column: NSTableColumn,
                                        row: Int) -> AnyObject? {
      if let getObjectValue = ds.tableView:objectValueForTableColumn:row: {
        return getObjectValue(tableView, column, row)
      }
      return nil
    }
    ```

* The compiler now warns about cases where a variable is inferred to have
  `AnyObject`, `AnyClass`, or `()` type, since type inference can turn a simple
  mistake (e.g. failing to cast an `AnyObject` when you meant to) into something
  with ripple effects.  Here is a simple example:

    ```
    t.swift:4:5: warning: variable 'fn' inferred to have type '()', which may be unexpected
    var fn = abort()
        ^
    t.swift:4:5: note: add an explicit type annotation to silence this warning
    var fn = abort()
        ^
          : ()
    ```

  If you actually did intend to declare a variable of one of these types, you
  can silence this warning by adding an explicit type (indicated by the Fixit).
  See **rdar://15263687 and rdar://16252090** for more rationale.

* `x.type` has been renamed to `x.dynamicType`, and you can use `type` as a
  regular identifier again.

### 2014-03-05

* C macros that expand to a single constant string are now imported as global
  constants. Normal string literals are imported as `CString`; `NSString` literals
  are imported as `String`.

* All values now have a `self` property, exactly equivalent to the value
  itself:

    ```swift
    let x = 0
    let x2 = x.self
    ```

  Types also have a `self` property that is the type object for that
  type:

    ```swift
    let theClass = NSObject.self
    let theObj = theClass()
    ```

  References to type names are now disallowed outside of a constructor call
  or member reference; to get a type object as a value, `T.self` is required.
  This prevents the mistake of intending to construct an instance of a
  class but forgetting the parens and ending up with the class object instead:

    ```swift
    let x = MyObject // oops, I meant MyObject()...
    return x.description() // ...and I accidentally called +description
                           //    instead of -description
    ```

* Initializers are now classified as **designated initializers**, which
  are responsible for initializing the current class object and
  chaining via `super.init`, and **convenience initializers**, which
  delegate to another initializer and can be inherited. For example:

    ```swift
    class A {
      var str: String

      init() -> Self { // convenience initializer
        self.init(withString: "hello")
      }

      init withString(str: String) { // designated initializer
        self.str = str
      }
    }
    ```

  When a subclass overrides all of its superclass's designated
  initializers, the convenience initializers are inherited:

    ```swift
    class B {
      init withString(str: String) { // designated initializer
        super.init(withString: str)
      }

      // inherits A.init()
    }
    ```

  Objective-C classes that provide `NS_DESIGNATED_INITIALIZER`
  annotations will have their init methods mapped to designated
  initializers or convenience initializers as appropriate; Objective-C
  classes without `NS_DESIGNATED_INITIALIZER` annotations have all of
  their `init` methods imported as designated initializers, which is
  safe (but can be verbose for subclasses). Note that the syntax and
  terminology is still somewhat in flux.

* Initializers can now be marked as `required` with an attribute,
  meaning that every subclass is required to provide that initializer
  either directly or by inheriting it from a superclass. To construct

    ```swift
    class View {
      @required init withFrame(frame: CGRect) { ... }
    }

    func buildView(subclassObj: View.Type, frame: CGRect) -> View {
      return subclassObj(withFrame: frame)
    }

    class MyView : View {
      @required init withFrame(frame: CGRect) {
        super.init(withFrame: frame)
      }
    }

    class MyOtherView : View {
      // error: must override init withFrame(CGRect).
    }
    ```

* Properties in Objective-C protocols are now correctly imported as properties.
  (Previously the getter and setter were imported as methods.)

* Simple enums with no payloads, including `NS_ENUM`s imported
  from Cocoa, now implicitly conform to the Equatable and Hashable protocols.
  This means they can be compared with the `==` and `!=` operators and can
  be used as `Dictionary` keys:

    ```swift
    enum Flavor {
      case Lemon, Banana, Cherry
    }

    assert(Flavor.Lemon == .Lemon)
    assert(Flavor.Banana != .Lemon)

    struct Profile {
      var sweet, sour: Bool
    }

    let flavorProfiles: Dictionary<Flavor, Profile> = [
      .Lemon:  Profile(sweet: false, sour: true ),
      .Banana: Profile(sweet: true,  sour: false),
      .Cherry: Profile(sweet: true,  sour: true ),
    ]
    assert(flavorProfiles[.Lemon].sour)
    ```

* `val` has been removed.  Long live `let`!

* Values whose names clash with Swift keywords, such as Cocoa methods or
  properties named `class`, `protocol`, `type`, etc., can now be defined and
  accessed by wrapping reserved keywords in backticks to suppress their builtin
  meaning:

    ```swift
    let `class` = 0
    let `type` = 1
    let `protocol` = 2
    println(`class`)
    println(`type`)
    println(`protocol`)

    func foo(Int) `class`(Int) {}
    foo(0, `class`: 1)
    ```

### 2014-02-26

* The `override` attribute is now required when overriding a method,
  property, or subscript from a superclass. For example:

    ```swift
    class A {
      func foo() { }
    }

    class B : A {
      @override func foo() { } // 'override' is required here
    }
    ```

* We're renaming `val` back to `let`.  The compiler accepts both for this week,
  next week it will just accept `let`.  Please migrate your code this week, sorry
  for the back and forth on this.

* Swift now supports `#if`, `#else` and `#endif` blocks, along with target
  configuration expressions, to allow for conditional compilation within
  declaration and statement contexts.

  Target configurations represent certain static information about the
  compile-time build environment.  They are implicit, hard-wired into the
  compiler, and can only be referenced within the conditional expression of an
  `#if` block.

  Target configurations are tested against their values via a pseudo-function
  invocation expression, taking a single argument expressed as an identifier.
  The argument represents certain static build-time information.

  There are currently two supported target configurations:
    `os`, which can have the values `OSX` or `iOS`
    `arch`, which can have the values `i386`, `x86_64`, `arm` and `arm64`

  Within the context of an `#if` block's conditional expression, a target
  configuration expression can evaluate to either `true` or `false`.

  For example:

    ```swift
    #if arch(x86_64)
      println("Building for x86_64")
    #else
      println("Not building for x86_64")
    #endif

    class C {
    #if os(OSX)
      func foo() {
        // OSX stuff goes here
      }
    #else
      func foo() {
        // non-OSX stuff goes here
      }
    #endif
    }
    ```

  The conditional expression of an `#if` block can be composed of one or more of
  the following expression types:
    - A unary expression, using `!`
    - A binary expression, using `&&` or `||`
    - A parenthesized expression
    - A target configuration expression

  For example:

    ```swift
    #if os(iOS) && !arch(I386)
    ...
    #endif
    ```

  Note that `#if`/`#else`/`#endif` blocks do not constitute a preprocessor, and
  must form valid and complete expressions or statements. Hence, the following
  produces a parser error:

    ```swift
    class C {

    #if os(iOS)
      func foo() {}
    }
    #else
      func bar() {}
      func baz() {}
    }
    #endif
    ```

  Also note that "active" code will be parsed, typechecked and emitted, while
  "inactive" code will only be parsed.  This is why code in an inactive `#if` or
  `#else` block will produce parser errors for malformed code.  This allows the
  compiler to detect basic errors in inactive regions.

  This is the first step to getting functionality parity with the important
  subset of the C preprocessor.  Further refinements are planned for later.

* Swift now has both fully-closed ranges, which include their endpoint, and
  half-open ranges, which don't.

    ```swift
    (swift) for x in 0...5 { print(x) } ; print('\n') // half-open range
    01234
    (swift) for x in 0..5 { print(x) } ; print('\n')  // fully-closed range
    012345
    ```

* Property accessors have a new brace-based syntax, instead of using the former
  "label like" syntax.  The new syntax is:

  ```swift
  var computedProperty: Int {
    get {
      return _storage
    }
    set {
      _storage = value
    }
  }

  var implicitGet: Int {    // This form still works.
    return 42
  }

  var storedPropertyWithObservingAccessors: Int = 0 {
    willSet { ... }
    didSet { ... }
  }
  ```

* Properties and subscripts now work in protocols, allowing you to do things
  like:

    ```swift
    protocol Subscriptable {
      subscript(idx1: Int, idx2: Int) -> Int { get set }
      var prop: Int { get }
    }

    func foo(s: Subscriptable) {
      return s.prop + s[42, 19]
    }
    ```

  These can be used for generic algorithms now as well.

* The syntax for referring to the type of a type, `T.metatype`, has been
  changed to `T.Type`. The syntax for getting the type of a value, `typeof(x)`,
  has been changed to `x.type`.

* `DynamicSelf` is now called `Self`; the semantics are unchanged.

* `destructor` has been replaced with `deinit`, to emphasize that it
  is related to `init`. We will refer to these as
  `deinitializers`. We've also dropped the parentheses, i.e.:

    ```swift
    class MyClass {
      deinit {
        // release any resources we might have acquired, etc.
      }
    }
    ```

* Class methods defined within extensions of Objective-C classes can
  now refer to `self`, including using `instancetype` methods. As a
  result, `NSMutableString`, `NSMutableArray`, and `NSMutableDictionary`
  objects can now be created with their respective literals, i.e.,

    ```swift
    var dict: NSMutableDictionary = ["a" : 1, "b" : 2]
    ```

### 2014-02-19

* The `Stream` protocol has been renamed back to `Generator,` which is
  precedented in other languages and causes less confusion with I/O
  streaming.

* The `type` keyword was split into two: `static` and `class`.  One can define
  static functions and static properties in structs and enums like this:

    ```swift
    struct S {
      static func foo() {}
      static var bar: Int = 0
    }
    enum E {
      static func foo() {}
    }
    ```

  `class` keyword allows one to define class properties and class methods in
  classes and protocols:

    ```swift
    class C {
      class func foo() {}
      class var bar: Int = 0
    }
    protocol P {
      class func foo() {}
      class var bar: Int = 0
    }
    ```

  When using `class` and `static` in the extension, the choice of keyword
  depends on the type being extended:

    ```swift
    extension S {
      static func baz() {}
    }
    extension C {
      class func baz() {}
    }
    ```

* The `let` keyword is no longer recognized.  Please move to `val`.

* The standard library has been renamed to `Swift` (instead of `swift`) to be
  more consistent with other modules on our platforms.

* `NSInteger` and other types that are layout-compatible with Swift standard
  library types are now imported directly as those standard library types.

* Optional types now support a convenience method named "cache" to cache the
  result of a closure. For example:

  ```swift
  class Foo {
    var _lazyProperty: Int?
    var property: Int {
      return _lazyProperty.cache { computeLazyProperty() }
    }
  }
  ```

### 2014-02-12

* We are experimenting with a new message send syntax. For example:

    ```swift
    SKAction.colorizeWithColor(SKColor.whiteColor()) colorBlendFactor(1.0) duration(0.0)
    ```

  When the message send is too long to fit on a single line, subsequent lines
  must be indented from the start of the statement or declaration. For
  example, this is a single message send:

    ```swift
    SKAction.colorizeWithColor(SKColor.whiteColor())
             colorBlendFactor(1.0)
             duration(0.0)
    ```

  while this is a message send to colorizeWithColor: followed by calls
  to `colorBlendFactor` and `duration` (on self or to a global function):

    ```swift
    SKAction.colorizeWithColor(SKColor.whiteColor())
    colorBlendFactor(1.0) // call to 'colorBlendFactor'
    duration(0.0) // call to 'duration'
    ```

* We are renaming the `let` keyword to `val`.  The `let` keyword didn't work
  out primarily because it is not a noun, so "defining a let" never sounded
  right.  We chose `val` over `const` and other options because `var` and `val`
  have similar semantics (making syntactic similarity useful), because `const`
  has varied and sordid connotations in C that we don't want to bring over, and
  because we don't want to punish the "preferred" case with a longer keyword.

  For migration purposes, the compiler now accepts `let` and `val` as synonyms,
  `let` will be removed next week.

* Selector arguments in function arguments with only a type are now implicitly
  named after the selector chunk that contains them.  For example, instead of:

    ```swift
    func addIntsWithFirst(first : Int) second(second : Int) -> Int {
      return first+second
    }
    ```

  you can now write:

    ```swift
    func addIntsWithFirst(first : Int) second(Int) -> Int {
      return first+second
    }
    ```

  if you want to explicitly want to ignore an argument, it is recommended that
  you continue to use the `_` to discard it, as in:

    ```swift
    func addIntsWithFirst(first : Int) second(_ : Int) -> Int {...}
    ```

* The `@inout` attribute in argument lists has been promoted to a
  context-sensitive keyword.  Where before you might have written:

    ```swift
    func swap<T>(a : @inout T, b : @inout T) {
      (a, b) = (b, a)
    }
    ```

  You are now required to write:

    ```swift
    func swap<T>(inout a : T, inout b : T) {
      (a, b) = (b, a)
    }
    ```

  We made this change because `inout` is a fundamental part of the type
  system, which attributes are a poor match for.  The inout keyword is
  also orthogonal to the `var` and `let` keywords (which may be specified in
  the same place), so it fits naturally there.

* The `@mutating` attribute (which can be used on functions in structs,
  enums, and protocols) has been promoted to a context-sensitive keyword.
  Mutating struct methods are now written as:

    ```swift
    struct SomeStruct {
      mutating func f() {}
    }
    ```

* Half-open ranges (those that don't include their endpoint) are now
  spelled with three `.`s instead of two, for consistency with Ruby.

    ```swift
    (swift) for x in 0...5 { print(x) } ; print('\n') // new syntax
    01234
    ```

  Next week, we'll introduce a fully-closed range which does include
  its endpoint.  This will provide:

    ```swift
    (swift) for x in 0..5 { print(x) } ; print('\n')  // coming soon
    012345
    ```

  These changes are being released separately so that users have a
  chance to update their code before its semantics changes.

* Objective-C properties with custom getters/setters are now imported
  into Swift as properties. For example, the Objective-C property

    ```swift
    @property (getter=isEnabled) BOOL enabled;
    ```

  was previously imported as getter (`isEnabled`) and setter
  (`setEnabled`) methods. Now, it is imported as a property (`enabled`).

* `didSet`/`willSet` properties may now have an initial value specified:

    ```swift
    class MyAwesomeView : UIView {
      var enabled : Bool = false {       // Initial value.
      didSet: self.needsDisplay = true
      }
      ...
    }
    ```

  they can also be used as non-member properties now, e.g. as a global
  variable or a local variable in a function.

* Objective-C instancetype methods are now imported as methods that
  return Swift's `DynamicSelf` type. While `DynamicSelf` is not
  generally useful for defining methods in Swift, importing to it
  eliminates the need for casting with the numerous `instancetype` APIs,
  e.g.,

    ```swift
    let tileNode: SKSpriteNode = SKSpriteNode.spriteNodeWithTexture(tileAtlas.textureNamed("tile\(tileNumber).png"))!
    ```

  becomes

    ```swift
    let tileNode = SKSpriteNode.spriteNodeWithTexture(tileAtlas.textureNamed("tile\(tileNumber).png"))
    ```

  `DynamicSelf` will become more interesting in the coming weeks.

### 2014-02-05

* `if` and `while` statements can now conditionally bind variables. If the
  condition of an `if` or `while` statement is a `let` declaration, then the
  right-hand expression is evaluated as an `Optional` value, and control flow
  proceeds by considering the binding to be `true` if the `Optional` contains a
  value, or `false` if it is empty, and the variables are available in the true
  branch. This allows for elegant testing of dynamic types, methods, nullable
  pointers, and other Optional things:

    ```swift
    class B : NSObject {}
    class D : B {
      func foo() { println("we have a D") }
    }
    var b: B = D()
    if let d = b as D {
      d.foo()
    }
    var id: AnyObject = D()
    if let foo = id.foo {
      foo()
    }
    ```

* When referring to a member of an `AnyObject` (or `AnyClass`) object
  and using it directly (such as calling it, subscripting, or
  accessing a property on it), one no longer has to write the `?` or
  `!`. The run-time check will be performed implicitly. For example:

    ```swift
    func doSomethingOnViews(views: NSArray) {
      for view in views {
          view.updateLayer() // no '!' needed
      }
    }
    ```

  Note that one can still test whether the member is available at
  runtime using `?`, testing the optional result, or conditionally
  binding a variable to the resulting member.

* The `swift` command line tool can now create executables and libraries
  directly, just like Clang. Use `swift main.swift` to create an executable and
  `swift -emit-library -o foo.dylib foo.swift` to create a library.

* Object files emitted by Swift are not debuggable on their own, even if you
  compiled them with the `-g` option. This was already true if you had multiple
  files in your project. To produce a debuggable Swift binary from the command
  line, you must compile and link in a single step with `swift`, or pass object
  files AND swiftmodule files back into `swift` after compilation.
  (Or use Xcode.)

* `import` will no longer import other source files, only built modules.

* The current directory is no longer implicitly an import path. Use `-I .` if
  you have modules in your current directory.

### 2014-01-29

* Properties in structs and classes may now have `willSet:` and `didSet:`
  observing accessors defined on them:

  For example, where before you may have written something like this in a class:

    ```swift
    class MyAwesomeView : UIView {
      var _enabled : Bool  // storage
      var enabled : Bool { // computed property
      get:
        return _enabled
      set:
        _enabled = value
        self.needDisplay = true
      }
      ...
    }
    ```

  you can now simply write:

    ```swift
    class MyAwesomeView : UIView {
      var enabled : Bool {  // Has storage & observing methods
      didSet: self.needDisplay = true
      }
      ...
    }
    ```

  Similarly, if you want notification before the value is stored, you can use
  `willSet`, which gets the incoming value before it is stored:

    ```swift
    var x : Int {
    willSet(value):  // value is the default and may be elided, as with set:
      println("changing from \(x) to \(value)")
    didSet:
      println("we've got a value of \(x) now.\n")
    }
    ```

  The `willSet`/`didSet` observers are triggered on any store to the property,
  except stores from `init()`, destructors, or from within the observers
  themselves.

  Overall, a property now may either be "stored" (the default), "computed"
  (have a `get:` and optionally a `set:` specifier), or an observed
  (`willSet`/`didSet`) property.  It is not possible to have a custom getter
  or setter on an observed property, since they have storage.

  Two known-missing bits are:
  - **(rdar://problem/15920332) didSet/willSet variables need to allow initializers**
  - **(rdar://problem/15922884) support non-member didset/willset properties**

  Because of the first one, for now, you need to explicitly store an initial
  value to the property in your `init()` method.

* Objective-C properties with custom getter or setter names are (temporarily)
  not imported into Swift; the getter and setter will be imported individually
  as methods instead. Previously, they would appear as properties within the
  Objective-C class, but attempting to use the accessor with the customized
  name would result in a crash.

  The long-term fix is tracked as **(rdar://problem/15877160)**.

* Computed 'type' properties (that is, properties of types, rather
  than of values of the type) are now permitted on classes, on generic
  structs and enums, and in extensions.  Stored 'type' properties in
  these contexts remain unimplemented.

  The implementation of stored 'type' properties is tracked as
  **(rdar://problem/15915785)** (for classes) and **(rdar://problem/15915867)**
  (for generic types).

* The following command-line flags have been deprecated in favor of new
  spellings. The old spellings will be removed in the following week's build:

  | Old Spelling             | New Spelling                  |
  |--------------------------|-------------------------------|
  | `-emit-llvm`             | `-emit-ir`                    |
  | `-triple`                | `-target`                     |
  | `-serialize-diagnostics` | `-serialize-diagnostics-path` |

* Imported `NS_OPTIONS` types now have a default initializer which produces a
  value with no options set. They can also be initialized to the empty set with
  `nil`. These are equivalent:

    ```swift
    var x = NSMatchingOptions()
    var y: NSMatchingOptions = nil
    ```

### 2014-01-22

* The swift binary no longer has an SDK set by default. Instead, you must do
  one of the following:
    - pass an explicit `-sdk /path/to/sdk`
    - set `SDKROOT` in your environment
    - run `swift` through `xcrun`, which sets `SDKROOT` for you

* `let` declarations can now be used as struct/class properties.  A `let`
  property is mutable within `init()`, and immutable everywhere else.

    ```swift
    class C {
      let x = 42
      let y : Int
      init(y : Int) {
        self.y = y   // ok, self.y is mutable in init()
      }

      func test() {
        y = 42       // error: 'y' isn't mutable
      }
    }
    ```

* The immutability model for structs and enums is complete, and arguments are
  immutable by default.  This allows the compiler to reject mutations of
  temporary objects, catching common bugs.  For example, this is rejected:

    ```swift
    func setTo4(a : Double[]) {
      a[10] = 4.0     // error: 'a' isn't mutable
    }
    ...
    setTo4(someArray)
    ```

  since `a` is semantically a copy of the array passed into the function.  The
  proper fix in this case is to mark the argument is `@inout`, so the effect is
  visible in the caller:

    ```swift
    func setTo4(a : @inout Double[]) {
      a[10] = 4.0     // ok: 'a' is a mutable reference
    }
    ...
    setTo4(&someArray)
    ```

  Alternatively, if you really just want a local copy of the argument, you can
  mark it `var`.  The effects aren't visible in the caller, but this can be
  convenient in some cases:

    ```swift
    func doStringStuff(var s : String) {
      s += "foo"
      print(s)
    }
    ```

* Objective-C instance variables are no longer imported from headers written in
  Objective-C. Previously, they would appear as properties within the
  Objective-C class, but trying to access them would result in a crash.
  Additionally, their names can conflict with property names, which confuses
  the Swift compiler, and there are no patterns in our frameworks that expect
  you to access a parent or other class's instance variables directly. Use
  properties instead.

* The `NSObject` protocol is now imported under the name
  `NSObjectProtocol` (rather than `NSObjectProto`).

### 2014-01-15

* Improved deallocation of Swift classes that inherit from Objective-C
  classes: Swift destructors are implemented as `-dealloc` methods that
  automatically call the superclass's `-dealloc`. Stored properties are
  released right before the object is deallocated (using the same
  mechanism as ARC), allowing properties to be safely used in
  destructors.

* Subclasses of `NSManagedObject` are now required to provide initial
  values for each of their stored properties. This permits
  initialization of these stored properties directly after +alloc to
  provide memory safety with CoreData's dynamic subclassing scheme.

* `let` declarations are continuing to make slow progress. Curried
  and selector-style arguments are now immutable by default, and
  `let` declarations now get proper debug information.

### 2014-01-08

* The `static` keyword changed to `type`. One can now define "type
  functions" and "type variables" which are functions and variables
  defined on a type (rather than on an instance of the type), e.g.,

    ```swift
    class X {
      type func factory() -> X { ... }

      type var version: Int
    }
    ```

  The use of `static` was actively misleading, since type methods
  on classes are dynamically dispatched (the same as Objective-C
  `+` methods).

  Note that `type` is a context-sensitive keyword; it can still be
  used as an identifier.

* Strings have a new native UTF-16 representation that can be
  converted back and forth to `NSString` at minimal cost. String
  literals are emitted as UTF-16 for string types that support it
  (including Swift's `String`).

* Initializers can now delegate to other initializers within the same
  class by calling `self.init`. For example:

    ```swift
    class A { }

    class B : A {
      var title: String

      init() {
        // note: cannot access self before delegating
        self.init(withTitle: "My Title")
      }

      init withTitle(title: String) {
        self.title = title
        super.init()
      }
    }
    ```

* Objective-C protocols no longer have the `Proto` suffix unless there
  is a collision with a class name. For example, `UITableViewDelegate` is
  now imported as `UITableViewDelegate` rather than
  `UITableViewDelegateProto`. Where there is a conflict with a class,
  the protocol will be suffixed with `Proto`, as in `NSObject` (the
  class) and `NSObjectProto` (the protocol).

### 2014-01-01

* Happy New Year

* Division and remainder arithmetic now trap on overflow. Like with the other
  operators, one can use the "masking" alternatives to get non-trapping
  behavior. The behavior of the non-trapping masking operators is defined:

    ```swift
    x &/ 0 == 0
    x &% 0 == 0
    SIGNED_MIN_FOR_TYPE &/ -1 == -1 // i.e. Int8: -0x80 / -1 == -0x80
    SIGNED_MIN_FOR_TYPE &% -1 == 0
    ```

* Protocol conformance checking for `@mutating` methods is now implemented: an
  `@mutating` struct method only fulfills a protocol requirement if the protocol
  method was itself marked `@mutating`:

    ```swift
    protocol P {
      func nonmutating()
      @mutating
      func mutating()
    }

    struct S : P {
      // Error, @mutating method cannot implement non-@mutating requirement.
      @mutating
      func nonmutating() {}

      // Ok, mutating allowed, but not required.
      func mutating() {}
    }
    ```

  As before, class methods never need to be marked `@mutating` (and indeed, they
  aren't allowed to be marked as such).

### 2013-12-25

* Merry Christmas

* The setters of properties on value types (structs/enums) are now `@mutating` by
  default.  To mark a setter non-mutating, use the `@!mutating` attribute.

* Compiler inserts calls to `super.init()` into the class initializers that do
  not call any initializers explicitly.

* A `map` method with the semantics of Haskell's `fmap` was added to
  `Array<T>`.  Map applies a function `f: T->U` to the values stored in
  the array and returns an `Array<U>`.  So,

    ```swift
    (swift) func names(x: Int[]) -> String[] {
              return x.map { "<" + String($0) + ">" }
            }
    (swift) names(Array<Int>())
    // r0 : String[] = []
    (swift) names([3, 5, 7, 9])
    // r1 : String[] = ["<3>", "<5>", "<7>", "<9>"]
    ```

### 2013-12-18

* Global variables and static properties are now lazily initialized on first
  use. Where you would use `dispatch_once` to lazily initialize a singleton
  object in Objective-C, you can simply declare a global variable with an
  initializer in Swift. Like `dispatch_once`, this lazy initialization is thread
  safe.

  Unlike C++ global variable constructors, Swift global variables and
  static properties now never emit static constructors (and thereby don't
  raise build warnings). Also unlike C++, lazy initialization naturally follows
  dependency order, so global variable initializers that cross module
  boundaries don't have undefined behavior or fragile link order dependencies.

* Swift has the start of an immutability model for value types. As part of this,
  you can now declare immutable value bindings with a new `let` declaration,
  which is semantically similar to defining a get-only property:

    ```swift
    let x = foo()
    print(x)        // ok
    x = bar()       // error: cannot modify an immutable value
    swap(&x, &y)    // error: cannot pass an immutable value as @inout parameter
    x.clear()       // error: cannot call mutating method on immutable value
    getX().clear()  // error: cannot mutate a temporary
    ```

  In the case of bindings of class type, the bound object itself is still
  mutable, but you cannot change the binding.

    ```swift
    let r = Rocket()
    r.blastOff()    // Ok, your rocket is mutable.
    r = Rocket()    // error: cannot modify an immutable binding.
    ```

  In addition to the `let` declaration itself, `self` on classes, and a few
  other minor things have switched to immutable bindings.

  A pivotal part of this is that methods of value types (structs and enums) need
  to indicate whether they can mutate self - mutating methods need to be
  disallowed on let values (and get-only property results, temporaries, etc) but
  non-mutating methods need to be allowed.  The default for a method is that it
  does not mutate `self`, though you can opt into mutating behavior with a new
  `@mutating` attribute:

    ```swift
    struct MyWeirdCounter {
      var count : Int

      func empty() -> Bool { return count == 0 }

      @mutating
      func reset() {
        count = 0
      }
      ...
    }

    let x = MyWeirdCounter()
    x.empty()   // ok
    x.reset()   // error, cannot mutate immutable 'let' value
    ```

   One missing piece is that the compiler does not yet reject mutations of self
   in a method that isn't marked `@mutating`.  That will be coming soon.  Related
   to methods are properties.  Getters and setters can be marked mutating as
   well:

   ```swift
   extension MyWeirdCounter {
      var myproperty : Int {
      get:
        return 42

      @mutating
      set:
        count = value*2
      }
    }
    ```

  The intention is for setters to default to mutating, but this has not been
  implemented yet.  There is more to come here.

* A `map` method with the semantics of Haskell's `fmap` was added to
  `Optional<T>`.  Map applies a function `f: T->U` to any value stored in
  an `Optional<T>`, and returns an `Optional<U>`.  So,

    ```swift
   (swift) func nameOf(x: Int?) -> String? {
             return x.map { "<" + String($0) + ">" }
           }
   (swift)
   (swift) var no = nameOf(.None) // Empty optional in...
   // no : String? = <unprintable value>
   (swift) no ? "yes" : "no"      // ...empty optional out
   // r0 : String = "no"
   (swift)
   (swift) nameOf(.Some(42))      // Non-empty in
   // r1 : String? = <unprintable value>
   (swift) nameOf(.Some(42))!     // Non-empty out
   // r2 : String = "<42>"
   ```

* Cocoa types declared with the `NS_OPTIONS` macro are now available in Swift.
  Like `NS_ENUM` types, their values are automatically shortened based
  on the common prefix of the value names in Objective-C, and the name can
  be elided when type context provides it. They can be used in `if` statements
  using the `&`, `|`, `^`, and `~` operators as in C:

    ```swift
    var options: NSJSONWritingOptions = .PrettyPrinted
    if options & .PrettyPrinted {
      println("pretty-printing enabled")
    }
    ```

  We haven't yet designed a convenient way to author `NS_OPTIONS`-like types
  in Swift.

### 2013-12-11

* Objective-C `id` is now imported as `AnyObject` (formerly known as
 `DynamicLookup`), Objective-C `Class` is imported as `AnyClass`.

* The casting syntax `x as T` now permits both implicit conversions
  (in which case it produces a value of type `T`) and for
  runtime-checked casts (in which case it produces a value of type `T?`
  that will be `.Some(casted x)` on success and `.None` on failure). An
  example:

    ```swift
    func f(x: AnyObject, y: NSControl) {
      var view = y as NSView                  // has type 'NSView'
      var maybeView = x as NSView             // has type NSView?
    }
    ```

* The precedence levels of binary operators has been redefined, with a much
  simpler model than C's.  This is with a goal to define away classes of bugs
  such as those caught by Clang's `-Wparentheses` warnings, and to make it
  actually possible for normal humans to reason about the precedence
  relationships without having to look them up.

  We ended up with 6 levels, from tightest binding to loosest:
    ```
    exponentiative: <<, >>
    multiplicative: *, /, %, &
    additive: +, -, |, ^
    comparative: ==, !=, <, <=, >=, >
    conjunctive: &&
    disjunctive: ||
    ```

* The `Enumerable` protocol has been renamed `Sequence`.

* The `Char` type has been renamed `UnicodeScalar`.  The preferred
  unit of string fragments for users is called `Character`.

* Initialization semantics for classes, structs and enums init methods are now
  properly diagnosed by the compiler.  Instance variables now follow the same
  initialization rules as local variables: they must be defined before use.  The
  initialization model requires that all properties with storage in the current
  class be initialized before `super.init` is called (or, in a root class, before
  any method is called on `self,` and before the final return).

  For example, this will yield an error:

    ```swift
    class SomeClass : SomeBase {
      var x : Int

      init() {
        // error: property 'self.x' not initialized at super.init call
        super.init()
      }
    }
    ```

  A simple fix for this is to change the property definition to `var x = 0`,
  or to explicitly assign to it before calling `super.init()`.

* Relatedly, the compiler now diagnoses incorrect calls to `super.init()`.  It
  validates that any path through an initializer calls `super.init()` exactly once,
  that all ivars are defined before the call to super.init, and that any uses
  which require the entire object to be initialized come after the `super.init`
  call.

* Type checker performance has improved considerably (but we still
  have much work to do here).

### 2013-12-04

* The "slice" versus "array" subtlety is now dead. `Slice<T>` has been folded
  into `Array<T>` and `T[]` is just sugar for `Array<T>`.

### 2013-11-20

* Unreachable code warning has been added:

    ```swift
    var y: Int = 1
    if y == 1 { // note: condition always evaluates to true
      return y
    }
    return 1 // warning: will never be executed
    ```

* Overflows on integer type conversions are now detected at runtime and, when
  dealing with constants, at compile time:

    ```swift
    var i: Int = -129
    var i8 = Int8(i)
    // error: integer overflows when converted from 'Int' to 'Int8'

    var si = Int8(-1)
    var ui = UInt8(si)
    // error: negative integer cannot be converted to unsigned type 'UInt8'
    ```

* `def` keyword was changed back to `func`.

### 2013-11-13

* Objective-C-compatible protocols can now contain optional
  requirements, indicated by the `@optional` attribute:

    ```swift
    @class_protocol @objc protocol NSWobbling {
      @optional def wobble()
    }
    ```

  A class that conforms to the `NSWobbling` protocol above can (but does
  not have to) implement `wobble`. When referring to the `wobble`
  method for a value of type `NSWobbling` (or a value of generic type
  that is bounded by `NSWobbling`), the result is an optional value
  indicating whether the underlying object actually responds to the
  given selector, using the same mechanism as messaging `id`. One can
  use `!` to assume that the method is always there, `?` to chain the
  optional, or conditional branches to handle each case distinctly:

    ```swift
    def tryToWobble(w : NSWobbling) {
      w.wobble()   // error: cannot call a value of optional type
      w.wobble!()  // okay: calls -wobble, but fails at runtime if not there
      w.wobble?()  // okay: calls -wobble only if it's there, otherwise no-op
      if w.wobble {
        // okay: we know -wobble is there
      } else {
        // okay: we know -wobble is not there
      }
    }
    ```

* Enums from Cocoa that are declared with the `NS_ENUM` macro are now imported
  into Swift as Swift enums. Like all Swift enums, the constants of the Cocoa
  enum are scoped as members of the enum type, so the importer strips off the
  common prefix of all of the constant names in the enum when forming the Swift
  interface. For example, this Objective-C declaration:

    ```objc
    typedef NS_ENUM(NSInteger, NSComparisonResult) {
      NSOrderedAscending,
      NSOrderedSame,
      NSOrderedDescending,
    };
    ```

  shows up in Swift as:

    ```swift
    enum NSComparisonResult : Int {
      case Ascending, Same, Descending
    }
    ```

  The `enum` cases can then take advantage of type inference from context.
  In Objective-C, you would write:

    ```objc
    NSNumber *foo = [NSNumber numberWithInt: 1];
    NSNumber *bar = [NSNumber numberWithInt: 2];

    switch ([foo compare: bar]) {
    case NSOrderedAscending:
      NSLog(@"ascending\n");
      break;
    case NSOrderedSame:
      NSLog(@"same\n");
      break;
    case NSOrderedDescending:
      NSLog(@"descending\n");
      break;
    }
    ```

  In Swift, this becomes:

    ```swift
    var foo: NSNumber = 1
    var bar: NSNumber = 2

    switch foo.compare(bar) {
    case .Ascending:
      println("ascending")
    case .Same:
      println("same")
    case .Descending:
      println("descending")
    }
    ```

* Work has begun on implementing static properties. Currently they are supported
  for nongeneric structs and enums.

    ```swift
    struct Foo {
      static var foo: Int = 2
    }
    enum Bar {
      static var bar: Int = 3
    }
    println(Foo.foo)
    println(Bar.bar)
    ```

### 2013-11-06

* `func` keyword was changed to `def`.

* Implicit conversions are now allowed from an optional type `T?` to another
  optional type `U?` if `T` is implicitly convertible to `U`. For example,
  optional subclasses convert to their optional base classes:

    ```swift
    class Base {}
    class Derived : Base {}

    var d: Derived? = Derived()
    var b: Base? = d
    ```

### 2013-10-30

* Type inference for variables has been improved, allowing any
  variable to have its type inferred from its initializer, including
  global and instance variables:

    ```swift
    class MyClass {
      var size = 0 // inferred to Int
    }

    var name = "Swift"
    ```

  Additionally, the arguments of a generic type can also be inferred
  from the initializer:

    ```swift
    // infers Dictionary<String, Int>
    var dict: Dictionary = ["Hello": 1, "World": 2]
    ```

### 2013-10-23

* Missing return statement from a non-`Void` function is diagnosed as an error.

* `Vector<T>` has been replaced with `Array<T>`. This is a complete rewrite to use
  value-semantics and copy-on-write behavior. The former means that you never
  need to defensively copy again (or remember to attribute a property as "copy")
  and the latter yields better performance than defensive copying. `Dictionary<T>`
  is next.

* `switch` can now pattern-match into structs and classes, using the syntax
  `case Type(property1: pattern1, property2: pattern2, ...):`.

    ```swift
    struct Point { var x, y: Double }
    struct Size { var w, h: Double }
    struct Rect { var origin: Point; var size: Size }

    var square = Rect(Point(0, 0), Size(10, 10))

    switch square {
    case Rect(size: Size(w: var w, h: var h)) where w == h:
      println("square")
    case Rect(size: Size(w: var w, h: var h)) where w > h:
      println("long rectangle")
    default:
      println("tall rectangle")
    }
    ```

  Currently only stored properties ("ivars" in ObjC terminology) are
  supported by the implementation.

* Array and dictionary literals allow an optional trailing comma:

    ```swift
    var a = [1, 2,]
    var d = ["a": 1, "b": 2,]
    ```

### 2013-10-16

* Unlike in Objective-C, objects of type `id` in Swift do not
  implicitly convert to any class type. For example, the following
  code is ill-formed:

    ```swift
    func getContentViewBounds(window : NSWindow) -> NSRect {
      var view : NSView = window.contentView() // error: 'id' doesn't implicitly convert to NSView
     return view.bounds()
    }
    ```

  because `contentView()` returns an `id`. One can now use the postfix
  `!` operator to allow an object of type `id` to convert to any class
  type, e.g.,

    ```swift
    func getContentViewBounds(window : NSWindow) -> NSRect {
      var view : NSView = window.contentView()! // ok: checked conversion to NSView
     return view.bounds()
    }
    ```

  The conversion is checked at run-time, and the program will fail if
  the object is not an NSView. This is shorthand for

    ```swift
    var view : NSView = (window.contentView() as NSView)!
    ```

  which checks whether the content view is an `NSView` (via the `as
  NSView`). That operation returns an optional `NSView` (written
  `NSView?`) and the `!` operation assumes that the cast succeeded,
  i.e., that the optional has a value in it.

* The unconditional checked cast syntax `x as! T` has been removed. Many cases
  where conversion from `id` is necessary can now be handled by postfix `!`
  (see above). Fully general unconditional casts can still be expressed using
  `as` and postfix `!` together, `(x as T)!`.

* The old "square bracket" attribute syntax has been removed.

* Overflows on construction of integer and floating point values from integer
  literals that are too large to fit the type are now reported by the compiler.
  Here are some examples:

    ```swift
    var x = Int8(-129)
    // error: integer literal overflows when stored into 'Int8'

    var y: Int = 0xFFFF_FFFF_FFFF_FFFF_F
    // error: integer literal overflows when stored into 'Int'
    ```

  Overflows in constant integer expressions are also reported by the compiler.

    ```swift
    var x: Int8 = 125
    var y: Int8 = x + 125
    // error: arithmetic operation '125 + 125' (on type 'Int8') results in
    //        an overflow
    ```

* Division by zero in constant expressions is now detected by the compiler:

    ```swift
    var z: Int = 0
    var x = 5 / z  // error: division by zero
    ```

* Generic structs with type parameters as field types are now fully supported.

    ```swift
    struct Pair<T, U> {
      var first: T
      var second: U
    }
    ```

### 2013-10-09

* Autorelease pools can now be created using the `autoreleasepool` function.

    ```swift
    autoreleasepool {
      // code
    }
    ```

  Note that the wrapped code is a closure, so constructs like `break` and
  `continue` and `return` do not behave as they would inside an Objective-C
  `@autoreleasepool` statement.

* Enums can now declare a "raw type", and cases can declare "raw values",
  similar to the integer underlying type of C enums:

    ```swift
    // Declare the underlying type as in Objective-C or C++11, with
    // ': Type'
    enum AreaCode : Int {
      // Assign explicit values to cases with '='
      case SanFrancisco = 415
      case EastBay = 510
      case Peninsula = 650
      case SanJose = 408
      // Values are also assignable by implicit auto-increment
      case Galveston // = 409
      case Baltimore // = 410
    }
    ```

  This introduces `fromRaw` and `toRaw` methods on the enum to perform
  conversions from and to the raw type:

    ```swift
    /* As if declared:
        extension AreaCode {
          // Take a raw value, and produce the corresponding enum value,
          // or None if there is no corresponding enum value
          static func fromRaw(raw:Int) -> AreaCode?

          // Return the corresponding raw value for 'self'
          func toRaw() -> Int
        }
     */

    AreaCode.fromRaw(415) // => .Some(.SanFrancisco)
    AreaCode.fromRaw(111) // => .None
    AreaCode.SanJose.toRaw() // => 408
    ```

  Raw types are not limited to integer types--they can additionally be
  character, floating-point, or string values:

    ```swift
    enum State : String {
      case CA = "California"
      case OR = "Oregon"
      case WA = "Washington"
    }

    enum SquareRootOfInteger : Float {
      case One = 1.0
      case Two = 1.414
      case Three = 1.732
      case Four = 2.0
    }
    ```

  Raw types are currently limited to simple C-like enums with no payload cases.
  The raw values are currently restricted to simple literal values; expressions
  such as `1 + 1` or references to other enum cases are not yet supported.
  Raw values are also currently required to be unique for each case in an enum.

  Enums with raw types implicitly conform to the `RawRepresentable` protocol,
  which exposes the fromRaw and toRaw methods to generics:

    ```swift
    protocol RawRepresentable {
      typealias RawType
      static func fromRaw(raw: RawType) -> Self?
      func toRaw() -> RawType
    }
    ```

* Attribute syntax has been redesigned (see **(rdar://10700853)** and
  **(rdar://14462729)**) so that attributes now precede the declaration and use
  the `@` character to signify them.  Where before you might have written:

    ```swift
    func [someattribute=42] foo(a : Int) {}
    ```

  you now write:

    ```swift
    @someattribute=42
    func foo(a : Int) {}
    ```

  This flows a lot better (attributes don't push the name for declarations away),
  and means that square brackets are only used for array types, collection
  literals, and subscripting operations.

* The `for` loop now uses the Generator protocol instead of the `Enumerator`
  protocol to iterate a sequence. This protocol looks like this:

    ```swift
    protocol Generator {
      typealias Element
      func next() -> Element?
    }
    ```

  The single method `next()` advances the generator and returns an
  Optional, which is either `.Some(value)`, wrapping the next value out
  of the underlying sequence, or `.None` to signal that there are no
  more elements. This is an improvement over the previous Enumerator
  protocol because it eliminates the separate `isEmpty()` query and
  better reflects the semantics of ephemeral sequences like
  un-buffered input streams.

### 2013-10-02

* The `[byref]` attribute has been renamed to `[inout]`.  When applied to a logical
  property, the getter is invoked before a call and the setter is applied to
  write back the result.  `inout` conveys this better and aligns with existing
  Objective-C practice better.

* `[inout]` arguments can now be captured into closures. The semantics of a
  inout capture are that the captured variable is an independent local variable
  of the callee, and the inout is updated to contain the value of that local
  variable at function exit.

  In the common case, most closure arguments do not outlive the duration of
  their callee, and the observable behavior is unchanged.  However, if the
  captured variable outlives the function, you can observe this.  For example,
  this code:

    ```swift
    func foo(x : [inout] Int) -> () -> Int {
      func bar() -> Int {
        x += 1
        return x
      }
      // Call 'bar' once while the inout is active.
      bar()
      return bar
    }

    var x = 219
    var f = foo(&x)
    // x is updated to the value of foo's local x at function exit.
    println("global x = \(x)")
    // These calls only update the captured local 'x', which is now independent
    // of the inout parameter.
    println("local x = \(f())")
    println("local x = \(f())")
    println("local x = \(f())")

    println("global x = \(x)")
    ```

  will print:

    ```
    global x = 220
    local x = 221
    local x = 222
    local x = 223
    global x = 220
    ```

  In no case will you end up with a dangling pointer or other unsafe construct.

* `x as T` now performs a checked cast to `T?`, producing `.Some(t)` if the
  cast succeeds, or `.None` if the cast fails.

* The ternary expression (`x ? y : z`) now requires whitespace between the
  first expression and the question mark.  This permits `?` to be used
  as a postfix operator.

* A significant new piece of syntactic sugar has been added to ease working
  with optional values.  The `?` postfix operator is analogous to `!`, but
  instead of asserting on None, it causes all the following postfix
  operators to get skipped and return `None`.

  In a sense, this generalizes (and makes explicit) the Objective-C behavior
  where message sends to `nil` silently produce the zero value of the result.

  For example, this code

    ```swift
    object?.parent.notifyChildEvent?(object!, .didExplode)
    ```

  first checks whether `object` has a value; if so, it drills to its
  parent and checks whether that object implements the `notifyChildEvent`
  method; if so, it calls that method.  (Note that we do not yet have
  generalized optional methods.)

  This code:

    ```swift
    var titleLength = object?.title.length
    ```

  checks whether `object` has a value and, if so, asks for the length of
  its title.  `titleLength` will have type `Int?`, and if `object` was
  missing, the variable will be initialized to None.

* Objects with type `id` can now be used as the receiver of property
  accesses and subscript operations to get (but not set) values. The
  result is of optional type. For example, for a variable `obj` of
  type `id`, the expression

    ```swift
    obj[0]
    ```

  will produce a value of type `id`, which will either contain the
  result of the message send objectAtIndexedSubscript(0) (wrapped in an
  optional type) or, if the object does not respond to
  `objectAtIndexedSubscript:`, an empty optional. The same approach
  applies to property accesses.

* `_` can now be used not only in `var` bindings, but in assignments as well,
  to ignore elements of a tuple assignment, or to explicitly ignore values.

    ```swift
    var a = (1, 2.0, 3)
    var x = 0, y = 0
    _ = a           // explicitly load and discard 'a'
    (x, _, y) = a   // assign a.0 to x and a.2 to y
    ```

### 2013-09-24

* The `union` keyword has been replaced with `enum`.  Unions and enums
  are semantically identical in swift (the former just has data
  associated with its discriminators) and `enum` is the vastly more
  common case.  For more rationale, please see
  [docs/proposals/Enums.rst](https://github.com/apple/swift/blob/main/docs/proposals/Enums.rst)

* The Optional type `T?` is now represented as an `enum`:

    ```swift
    enum Optional<T> {
      case None
      case Some(T)
    }
    ```

  This means that, in addition to the existing Optional APIs, it can be
  pattern-matched with switch:

    ```swift
    var x : X?, y : Y?
    switch (x, y) {
    // Both are present
    case (.Some(var a), .Some(var b)):
      println("both")

    // One is present
    case (.Some, .None):
    case (.None, .Some):
      println("one")

    // Neither is present
    case (.None, .None):
      println("neither")
    }
    ```

* Enums now allow multiple cases to be declared in a comma-separated list
  in a single `case` declaration:

    ```swift
    enum Color {
      case Red, Green, Blue
    }
    ```

* The Objective-C `id` and `Class` types now support referring to
  methods declared in any class or protocol without a downcast. For
  example, given a variable `sender` of type `id`, one can refer to
  `-isEqual: with:`

    ```swift
    sender.isEqual
    ```

  The actual object may or may not respond to `-isEqual`, so this
  expression returns result of optional type whose value is determined via a
  compiler-generated `-respondsToSelector` send. When it succeeds, the
  optional contains the method; when it fails, the optional is empty.

  To safely test the optional, one can use, e.g.,

    ```swift
    var senderIsEqual = sender.isEqual
    if senderIsEqual {
      // this will never trigger an "unrecognized selector" failure
      var equal = senderIsEqual!(other)
    } else {
      // sender does not respond to -isEqual:
    }
    ```

  When you *know* that the method is there, you can use postfix `!` to
  force unwrapping of the optional, e.g.,

    ```swift
    sender.isEqual!(other)
    ```

  This will fail at runtime if in fact sender does not respond to `-isEqual:`.
  We have some additional syntactic optimizations planned for testing
  an optional value and handling both the success and failure cases
  concisely. Watch this space.

* Weak references now always have optional type.  If a weak variable
  has an explicit type, it must be an optional type:

    ```swift
    var [weak] x : NSObject?
    ```

  If the variable is not explicitly typed, its type will still be
  inferred to be an optional type.

* There is now an implicit conversion from `T` to `T?`.

### 2013-09-17

* Constructor syntax has been improved to align better with
  Objective-C's `init` methods. The `constructor` keyword has been
  replaced with `init`, and the selector style of declaration used for
  func declarations is now supported. For example:

    ```swift
    class Y : NSObject {
      init withInt(i : Int) string(s : String) {
        super.init() // call superclass initializer
      }
    }
    ```

  One can use this constructor to create a `Y` object with, e.g.,

    ```swift
    Y(withInt:17, string:"Hello")
    ```

  Additionally, the rules regarding the selector corresponding to such
  a declaration have been revised. The selector for the above
  initializer is `initWithInt:string:`; the specific rules are
  described in the documentation.

  Finally, Swift initializers now introduce Objective-C entry points,
  so a declaration such as:

    ```swift
    class X : NSObject {
      init() {
        super.init()
      }
    }
    ```

  Overrides `NSObject`'s `-init` method (which it calls first) as well
  as introducing the 'allocating' entry point so that one can create a
  new `X` instance with the syntax `X()`.

* Variables in top-level code (i.e. scripts, but not global variables in
  libraries) that lack an initializer now work just like local variables:
  they must be explicitly assigned-to sometime before any use, instead of
  being default constructed.  Instance variables are still on the TODO
  list.

* Generic unions with a single payload case and any number of empty cases
  are now implemented, for example:

    ```swift
    union Maybe<T> {
      case Some(T)
      case None
    }

    union Tristate<T> {
      case Initialized(T)
      case Initializing
      case Uninitialized
    }
    ```

  Generic unions with multiple payload cases are still not yet implemented.

### 2013-09-11

* The implementation now supports partial application of class and struct
  methods:

    ```swift
    (swift) class B { func foo() { println("B") } }
    (swift) class D : B { func foo() { println("D") } }
    (swift) var foo = B().foo
    // foo : () -> () = <unprintable value>
    (swift) foo()
    B
    (swift) foo = D().foo
    (swift) foo()
    D
    ```

  Support for partial application of Objective-C class methods and methods in
  generic contexts is still incomplete.

### 2013-09-04

* Local variable declarations without an initializer are no longer implicitly
  constructed.  The compiler now verifies that they are initialized on all
  paths leading to a use of the variable.  This means that constructs like this
  are now allowed:

    ```swift
    var p : SomeProtocol
    if whatever {
      p = foo()
    } else {
      p = bar()
    }
    ```

  where before, the compiler would reject the definition of `p` saying that it
  needed an initializer expression.

  Since all local variables must be initialized before use, simple things like
  this are now rejected as well:

    ```swift
    var x : Int
    print(x)
    ```

  The fix is to initialize the value on all paths, or to explicitly default
  initialize the value in the declaration, e.g. with `var x = 0` or with
  `var x = Int()` (which works for any default-constructible type).

* The implementation now supports unions containing protocol types and weak
  reference types.

* The type annotation syntax, `x as T`, has been removed from the language.
  The checked cast operations `x as! T` and `x is T` still remain.

### 2013-08-28

* `this` has been renamed to `self`.  Similarly, `This` has been renamed to
  `Self`.

* Swift now supports unions. Unlike C unions, Swift's `union` is type-safe
  and always knows what type it contains at runtime. Union members are labeled
  using `case` declarations; each case may have a different set of
  types or no type:

    ```swift
    union MaybeInt {
      case Some(Int)
      case None
    }

    union HTMLTag {
      case A(href:String)
      case IMG(src:String, alt:String)
      case BR
    }
    ```

  Each `case` with a type defines a static constructor function for the union
  type. `case` declarations without types become static members:

    ```swift
    var br = HTMLTag.BR
    var a = HTMLTag.A(href:"http://www.apple.com/")
    // 'HTMLTag' scope deduced for '.IMG' from context
    var img : HTMLTag = .IMG(src:"http://www.apple.com/mac-pro.png",
                             alt:"The new Mac Pro")
    ```

  Cases can be pattern-matched using `switch`:

    ```swift
    switch tag {
    case .BR:
      println("<br>")
    case .IMG(var src, var alt):
      println("<img src=\"\(escape(src))\" alt=\"\(escape(alt))\">")
    case .A(var href):
      println("<a href=\"\(escape(href))\">")
    }
    ```

  Due to implementation limitations, recursive unions are not yet supported.

* Swift now supports autolinking, so importing frameworks or Swift libraries
  should no longer require adding linker flags or modifying your project file.

### 2013-08-14

* Swift now supports weak references by applying the `[weak]` attribute to a
  variable declaration.

    ```swift
    (swift) var x = NSObject()
    // x : NSObject = <NSObject: 0x7f95d5804690>
    (swift) var [weak] w = x
    // w : NSObject = <NSObject: 0x7f95d5804690>
    (swift) w == nil
    // r2 : Bool = false
    (swift) x = NSObject()
    (swift) w == nil
    // r3 : Bool = true
    ```

  Swift also supports a special form of weak reference, called `[unowned]`, for
  references that should never be `nil` but are required to be weak to break
  cycles, such as parent or sibling references. Accessing an `[unowned]`
  reference asserts that the reference is still valid and implicitly promotes
  the loaded reference to a strong reference, so it does not need to be loaded
  and checked for nullness before use like a true `[weak]` reference.

    ```swift
    class Parent {
      var children : Array<Child>

      func addChild(c:Child) {
        c.parent = this
        children.append(c)
      }
    }

    class Child {
      var [unowned] parent : Parent
    }
    ```

### 2013-07-31

* Numeric literals can now use underscores as separators. For example:

    ```swift
    var billion = 1_000_000_000
    var crore = 1_00_00_000
    var MAXINT = 0x7FFF_FFFF_FFFF_FFFF
    var SMALLEST_DENORM = 0x0.0000_0000_0000_1p-1022
    ```

* Types conforming to protocols now must always declare the conformance in
  their inheritance clause.

* The build process now produces serialized modules for the standard library,
  greatly improving build times.

### 2013-07-24

* Arithmetic operators `+`, `-`, `*`, and `/` on integer types now do
  overflow checking and trap on overflow. A parallel set of masking operators,
  `&+`, `&-`, `&*`, and `&/`, are defined to perform two's complement wrapping
  arithmetic for all signed and unsigned integer types.

* Debugger support. Swift has a `-g` command line switch that turns on
  debug info for the compiled output. Using the standard lldb debugger
  this will allow single-stepping through Swift programs, printing
  backtraces, and navigating through stack frames; all in sync with
  the corresponding Swift source code. An unmodified lldb cannot
  inspect any variables.

  Example session:

    ```
    $ echo 'println("Hello World")' >hello.swift
    $ swift hello.swift -c -g -o hello.o
    $ ld hello.o "-dynamic" "-arch" "x86_64" "-macosx_version_min" "10.9.0" \
         -framework Foundation lib/swift/libswift_stdlib_core.dylib \
         lib/swift/libswift_stdlib_posix.dylib -lSystem -o hello
    $ lldb hello
    Current executable set to 'hello' (x86_64).
    (lldb) b top_level_code
    Breakpoint 1: where = hello`top_level_code + 26 at hello.swift:1, addre...
    (lldb) r
    Process 38592 launched: 'hello' (x86_64)
    Process 38592 stopped
    * thread #1: tid = 0x1599fb, 0x0000000100000f2a hello`top_level_code + ...
        frame #0: 0x0000000100000f2a hello`top_level_code + 26 at hello.shi...
    -> 1         println("Hello World")
    (lldb) bt
    * thread #1: tid = 0x1599fb, 0x0000000100000f2a hello`top_level_code + ...
        frame #0: 0x0000000100000f2a hello`top_level_code + 26 at hello.shi...
        frame #1: 0x0000000100000f5c hello`main + 28
        frame #2: 0x00007fff918605fd libdyld.dylib`start + 1
        frame #3: 0x00007fff918605fd libdyld.dylib`start + 1
    ```

  Also try `s`, `n`, `up`, `down`.

### 2013-07-17

* Swift now has a `switch` statement, supporting pattern matching of
  multiple values with variable bindings, guard expressions, and range
  comparisons. For example:

    ```swift
    func classifyPoint(point:(Int, Int)) {
      switch point {
      case (0, 0):
        println("origin")

      case (_, 0):
        println("on the x axis")

      case (0, _):
        println("on the y axis")

      case (var x, var y) where x == y:
        println("on the y = x diagonal")

      case (var x, var y) where -x == y:
        println("on the y = -x diagonal")

      case (-10..10, -10..10):
        println("close to the origin")

      case (var x, var y):
        println("length \(sqrt(x*x + y*y))")
      }
    }
    ```

### 2013-07-10

* Swift has a new closure syntax. The new syntax eliminates the use of
  pipes. Instead, the closure signature is written the same way as a
  function type and is separated from the body by the `in`
  keyword. For example:

    ```swift
    sort(fruits) { (lhs : String, rhs : String) -> Bool in
      return lhs > rhs
    }
    ```

  When the types are omitted, one can also omit the parentheses, e.g.,

    ```swift
    sort(fruits) { lhs, rhs in lhs > rhs }
    ```

  Closures with no parameters or that use the anonymous parameters
  (`$0`, `$1`, etc.) don't need the `in`, e.g.,

    ```swift
    sort(fruits) { $0 > $1 }
    ```

* `nil` can now be used without explicit casting. Previously, `nil` had
  type `NSObject`, so one would have to write (e.g.) `nil as! NSArray`
  to create a `nil` `NSArray`. Now, `nil` picks up the type of its
  context.

* `POSIX.EnvironmentVariables` and `swift.CommandLineArguments` global variables
  were merged into a `swift.Process` variable.  Now you can access command line
  arguments with `Process.arguments`.  In order to access environment variables
  add `import POSIX` and use `Process.environmentVariables`.

<!-- References -->

[SE-0001]: <https://github.com/apple/swift-evolution/blob/main/proposals/0001-keywords-as-argument-labels.md>
[SE-0002]: <https://github.com/apple/swift-evolution/blob/main/proposals/0002-remove-currying.md>
[SE-0003]: <https://github.com/apple/swift-evolution/blob/main/proposals/0003-remove-var-parameters.md>
[SE-0004]: <https://github.com/apple/swift-evolution/blob/main/proposals/0004-remove-pre-post-inc-decrement.md>
[SE-0005]: <https://github.com/apple/swift-evolution/blob/main/proposals/0005-objective-c-name-translation.md>
[SE-0006]: <https://github.com/apple/swift-evolution/blob/main/proposals/0006-apply-api-guidelines-to-the-standard-library.md>
[SE-0007]: <https://github.com/apple/swift-evolution/blob/main/proposals/0007-remove-c-style-for-loops.md>
[SE-0008]: <https://github.com/apple/swift-evolution/blob/main/proposals/0008-lazy-flatmap-for-optionals.md>
[SE-0009]: <https://github.com/apple/swift-evolution/blob/main/proposals/0009-require-self-for-accessing-instance-members.md>
[SE-0010]: <https://github.com/apple/swift-evolution/blob/main/proposals/0010-add-staticstring-unicodescalarview.md>
[SE-0011]: <https://github.com/apple/swift-evolution/blob/main/proposals/0011-replace-typealias-associated.md>
[SE-0012]: <https://github.com/apple/swift-evolution/blob/main/proposals/0012-add-noescape-to-public-library-api.md>
[SE-0013]: <https://github.com/apple/swift-evolution/blob/main/proposals/0013-remove-partial-application-super.md>
[SE-0014]: <https://github.com/apple/swift-evolution/blob/main/proposals/0014-constrained-AnySequence.md>
[SE-0015]: <https://github.com/apple/swift-evolution/blob/main/proposals/0015-tuple-comparison-operators.md>
[SE-0016]: <https://github.com/apple/swift-evolution/blob/main/proposals/0016-initializers-for-converting-unsafe-pointers-to-ints.md>
[SE-0017]: <https://github.com/apple/swift-evolution/blob/main/proposals/0017-convert-unmanaged-to-use-unsafepointer.md>
[SE-0018]: <https://github.com/apple/swift-evolution/blob/main/proposals/0018-flexible-memberwise-initialization.md>
[SE-0019]: <https://github.com/apple/swift-evolution/blob/main/proposals/0019-package-manager-testing.md>
[SE-0020]: <https://github.com/apple/swift-evolution/blob/main/proposals/0020-if-swift-version.md>
[SE-0021]: <https://github.com/apple/swift-evolution/blob/main/proposals/0021-generalized-naming.md>
[SE-0022]: <https://github.com/apple/swift-evolution/blob/main/proposals/0022-objc-selectors.md>
[SE-0023]: <https://github.com/apple/swift-evolution/blob/main/proposals/0023-api-guidelines.md>
[SE-0024]: <https://github.com/apple/swift-evolution/blob/main/proposals/0024-optional-value-setter.md>
[SE-0025]: <https://github.com/apple/swift-evolution/blob/main/proposals/0025-scoped-access-level.md>
[SE-0026]: <https://github.com/apple/swift-evolution/blob/main/proposals/0026-abstract-classes-and-methods.md>
[SE-0027]: <https://github.com/apple/swift-evolution/blob/main/proposals/0027-string-from-code-units.md>
[SE-0028]: <https://github.com/apple/swift-evolution/blob/main/proposals/0028-modernizing-debug-identifiers.md>
[SE-0029]: <https://github.com/apple/swift-evolution/blob/main/proposals/0029-remove-implicit-tuple-splat.md>
[SE-0030]: <https://github.com/apple/swift-evolution/blob/main/proposals/0030-property-behavior-decls.md>
[SE-0031]: <https://github.com/apple/swift-evolution/blob/main/proposals/0031-adjusting-inout-declarations.md>
[SE-0032]: <https://github.com/apple/swift-evolution/blob/main/proposals/0032-sequencetype-find.md>
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[#64927]: <https://github.com/apple/swift/issues/64927>
[#42697]: <https://github.com/apple/swift/issues/42697>
[#42728]: <https://github.com/apple/swift/issues/42728>
[#43036]: <https://github.com/apple/swift/issues/43036>
[#43248]: <https://github.com/apple/swift/issues/43248>
[#43310]: <https://github.com/apple/swift/issues/43310>
[#43621]: <https://github.com/apple/swift/issues/43621>
[#44055]: <https://github.com/apple/swift/issues/44055>
[#44138]: <https://github.com/apple/swift/issues/44138>
[#44739]: <https://github.com/apple/swift/issues/44739>
[#44784]: <https://github.com/apple/swift/issues/44784>
[#44797]: <https://github.com/apple/swift/issues/44797>
[#44995]: <https://github.com/apple/swift/issues/44995>
[#45001]: <https://github.com/apple/swift/issues/45001>
[#45213]: <https://github.com/apple/swift/issues/45213>
[#45277]: <https://github.com/apple/swift/issues/45277>
[#45293]: <https://github.com/apple/swift/issues/45293>
[apple/swift-corelibs-foundation#4326]: <https://github.com/apple/swift-corelibs-foundation/issues/4326>
[#46789]: <https://github.com/apple/swift/issues/46789>
[#46831]: <https://github.com/apple/swift/issues/46831>
[#48153]: <https://github.com/apple/swift/issues/48153>
[#48289]: <https://github.com/apple/swift/issues/48289>
[#48673]: <https://github.com/apple/swift/issues/48673>
[#49631]: <https://github.com/apple/swift/issues/49631>
[#49687]: <https://github.com/apple/swift/issues/49687>
[#49799]: <https://github.com/apple/swift/issues/49799>
[#50143]: <https://github.com/apple/swift/issues/50143>
[#50338]: <https://github.com/apple/swift/issues/50338>
[#50641]: <https://github.com/apple/swift/issues/50641>
[#51064]: <https://github.com/apple/swift/issues/51064>
[#51478]: <https://github.com/apple/swift/issues/51478>
[#51546]: <https://github.com/apple/swift/issues/51546>
[#52244]: <https://github.com/apple/swift/issues/52244>
[#52471]: <https://github.com/apple/swift/issues/52471>
[#53699]: <https://github.com/apple/swift/issues/53699>
[#53830]: <https://github.com/apple/swift/issues/53830>
[#54108]: <https://github.com/apple/swift/issues/54108>
[#54246]: <https://github.com/apple/swift/issues/54246>
[#57081]: <https://github.com/apple/swift/issues/57081>
[#57225]: <https://github.com/apple/swift/issues/57225>
[#56139]: <https://github.com/apple/swift/issues/56139>
[#70065]: <https://github.com/apple/swift/pull/70065>
[#71075]: <https://github.com/apple/swift/pull/71075>
[#78389]: <https://github.com/swiftlang/swift/pull/78389>
[swift-syntax]: https://github.com/apple/swift-syntax