[SR-7783] Cannot subclass `NSString` due to `ExpressibleByStringLiteral` initializer

[michelf]

Previous ID	SR-7783
Radar	rdar://problem/40636610
Original Reporter	@michelf
Type	Bug

Additional Detail from JIRA

Votes	1
Component/s	Compiler, Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: 7ca07235bdf43a633e747405b099e091

Issue Description:

Normally, you can't add conformance to `ExpressibleByStringLiteral` in a class extension for a non-final class. But somehow Foundation manages to do it. As a consequence, you can't create a subclass of `NSString` or `NSMutableString` because you can't override the required initializer because it comes from an extension.

If you write this:

class C: NSString {
    override init() { super.init() }
}

You get fix-its telling you to add these:

required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}
required init?(pasteboardPropertyList propertyList: Any, ofType type: NSPasteboard.PasteboardType) {
    fatalError("init(pasteboardPropertyList:ofType:) has not been implemented")
}
required convenience init(stringLiteral value: StaticString) {
    fatalError("init(stringLiteral:) has not been implemented")
}

But there's no way to get the last one to compile because:

"Overriding declarations in extensions is not supported"

Xcode 9.3 (9E145)

[belkadan]

If you override all of NSString's designated initializers, you'll be able to inherit the required convenience initializer.

[michelf]

While the following works for NSString:

class C: NSString {

   override init() { fatalError() }
   required init?(coder aDecoder: NSCoder) { fatalError() }

   required init?(pasteboardPropertyList propertyList: Any, ofType type: NSPasteboard.PasteboardType) { fatalError() }

}

It doesn't work when subclassing NSMutableString.

class D: NSMutableString {

override init() { fatalError() }
required init?(coder aDecoder: NSCoder) { fatalError() }

required init?(pasteboardPropertyList propertyList: Any, ofType type: NSPasteboard.PasteboardType) { fatalError() }

}

'required' initializer 'init(stringLiteral🙂' must be provided by subclass of 'NSString'

[belkadan]

NSMutableString adds init(capacity:)…and doesn't mark it as a designated initializer, which means init() is still needed as well. (Also, that AppKit one makes me very sad. That's a bug we probably can't fix.)

The compiler-side change here is probably to add a note about why init(stringLiteral:) can't be inherited, i.e. listing the designated initializers that haven't been overridden.

[michelf]

Agree. Offering a fix-it with the missing designated initializers would be great. They're hard to find because they can be scattered in many places.

[belkadan]

@swift-ci create