[flow] better handing of unions in type guard checks

Summary:
This change tries to prevent performance regression in code like the following
```
declare var x: T;
declare var foo: (x: mixed) => x is T
if (foo(x)) {}
```
where T is a large enum-like union.

Before this change, `predicate_no_concretization` on a `LatentP` predicate would do concretize all types, including unions, leading to the breakup of the type of the large union `T`. In `types_differ`, each member of `T` is compared against each member of the type guard type, which also happens to be `T`. This effectively yields a O(n^2) operation which becomes too slow on large enough `T`s.

This diff tries to address this by introducing a layer of faster checks before we get to the slow check above. Specifically,
1. we make the initial concretization preserve enum-like unions (which tend to be large)
2. we use fast comparison of this concretized type against the guard (`try_intersect`)
3. if we don't reach a good result, we proceed to concretize the input again, this time using full concretization on unions.
4. for each part of the newly concretized input, we run the same comparison against the guard (`try_intersect`)

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D69762017

fbshipit-source-id: 39f0af1541f3646c977cdc8860290a2c298fd622

Author: panagosg7

src/typing/flow_js.ml

@@ -472,7 +472,7 @@ struct
             ConcretizeT
               {
                 reason = _;
-                kind = ConcretizeForPredicate ConcretizeForMaybeOrExistPredicateTest;
+                kind = ConcretizeForPredicate ConcretizeKeepOptimizedUnions;
                 seen = _;
                 collector;
               }

src/typing/predicate_kit.ml

@@ -30,8 +30,10 @@ let concretization_variant_of_predicate = function
   | MaybeP
   | NotP MaybeP
   | TruthyP
-  | NotP TruthyP ->
-    ConcretizeForMaybeOrExistPredicateTest
+  | NotP TruthyP
+  | LatentP _
+  | NotP (LatentP _) ->
+    ConcretizeKeepOptimizedUnions
   | _ -> ConcretizeForGeneralPredicateTest
 
 type predicate_result_mut =
@@ -615,23 +617,56 @@ and intersect =
   in
   fun cx t1 t2 ->
     let reason1 = TypeUtil.reason_of_t t1 in
+    (* Pre-processing of t1 has concretized it up to optimized unions. It is
+     * important to keep it this way to prevent expensive checks right away.
+     * Consider for example the code:
+     *
+     *   declare var x: T;
+     *   declare var foo: (x: mixed) => x is T
+     *   if (foo(x)) {}
+     *
+     * where T is a really large enum-like union. `try_intersect` will quickly
+     * return `t1` as the result here, without us having to try to fully
+     * concretize `t1`.
+     *)
     (* Input t1 is already concretized as input to the predicate mechanism *)
     match try_intersect cx reason1 (C.wrap_unsafe t1) t2 with
     | Some t -> t
     | None ->
-      let r = update_desc_reason invalidate_rtype_alias reason1 in
-      IntersectionT (r, InterRep.make t2 t1 [])
+      (* No definitive refinement found. We fall back to more expensive
+       * concretization that breaks up all unions (including optimized ones). *)
+      possible_concrete_types_for_inspection cx reason1 t1
+      |> Base.List.map ~f:(fun t1 ->
+             (* t1 was just concretized *)
+             match try_intersect cx reason1 (C.wrap_unsafe t1) t2 with
+             | Some t -> t
+             | None ->
+               let r = update_desc_reason invalidate_rtype_alias reason1 in
+               IntersectionT (r, InterRep.make t2 t1 [])
+         )
+      |> TypeUtil.union_of_ts (update_desc_reason invalidate_rtype_alias reason1)
 
 (* This utility is expected to be used when negating the refinement of a type [t1]
  * with a type guard `x is t2`. The only case considered here is that of t1 <: t2.
  * This means that the positive branch will always be taken, and so we are left with
  * `empty` in the negated case. *)
 and type_guard_diff cx t1 t2 =
+  let reason1 = TypeUtil.reason_of_t t1 in
   if TypeUtil.quick_subtype t1 t2 || speculative_subtyping_succeeds cx t1 t2 then
-    let r = update_desc_reason invalidate_rtype_alias (TypeUtil.reason_of_t t1) in
+    let r = update_desc_reason invalidate_rtype_alias reason1 in
     Type_filter.TypeFilterResult { type_ = DefT (r, EmptyT); changed = true }
   else
-    Type_filter.TypeFilterResult { type_ = t1; changed = false }
+    let t1s_conc = possible_concrete_types_for_inspection cx reason1 t1 in
+    let (ts_rev, changed) =
+      Base.List.fold t1s_conc ~init:([], false) ~f:(fun (acc, changed) t1 ->
+          if TypeUtil.quick_subtype t1 t2 || speculative_subtyping_succeeds cx t1 t2 then
+            (acc, changed)
+          else
+            (t1 :: acc, true)
+      )
+    in
+    let r1 = update_desc_reason invalidate_rtype_alias reason1 in
+    Type_filter.TypeFilterResult { type_ = TypeUtil.union_of_ts r1 (List.rev ts_rev); changed }
 
 and prop_exists_test cx key sense obj result_collector =
   match has_prop cx (OrdinaryName key) obj with

src/typing/type.ml

@@ -1013,7 +1013,7 @@ module rec TypeTerm : sig
 
   and predicate_concretizer_variant =
     | ConcretizeForGeneralPredicateTest
-    | ConcretizeForMaybeOrExistPredicateTest
+    | ConcretizeKeepOptimizedUnions
     | ConcretizeRHSForInstanceOfPredicateTest
     | ConcretizeRHSForLiteralPredicateTest
 
@@ -4141,7 +4141,7 @@ let string_of_use_op_rec : use_op -> string =
 
 let string_of_predicate_concretizer_variant = function
   | ConcretizeForGeneralPredicateTest -> "ConcretizeForGeneralPredicateTest"
-  | ConcretizeForMaybeOrExistPredicateTest -> "ConcretizeForMaybeOrExistPredicateTest"
+  | ConcretizeKeepOptimizedUnions -> "ConcretizeKeepOptimizedUnions"
   | ConcretizeRHSForInstanceOfPredicateTest -> "ConcretizeRHSForInstanceOfPredicateTest"
   | ConcretizeRHSForLiteralPredicateTest -> "ConcretizeRHSForLiteralPredicateTest"
 

tests/type_guards/refinement.js

@@ -125,7 +125,7 @@ function sentinel_multi_tag() {
 }
 
 
-function negation() {
+function negation1() {
   declare function isNumber(x: mixed): x is number;
   declare function isString(x: mixed): x is string;
 
@@ -296,3 +296,24 @@ function getRaccoon(s: string): ?Raccoon {
   }
   return null;
 }
+
+function negation2() {
+  declare var isA: (x: mixed) => x is 'a';
+  declare var x: 'a' | 'c';
+
+  if (isA(x)) { return; }
+  x as 'c'; // okay
+}
+
+function negation3() {
+  declare function isFalsey(value: ?mixed): value is null | void | false | '' | 0;
+
+  function foo(arr: ?Array<mixed> | $ReadOnlyArray<mixed>): void {
+    if (isFalsey(arr)) {
+      return;
+    }
+    arr as Array<mixed> | $ReadOnlyArray<mixed>; // okay
+  }
+
+  foo([]); // triggrers union optimization
+}