[flow][match] Propagate hints through match expressions case bodies

Summary:
We need to propagate hints through match expressions case bodies.
The simple case is when we have some external hint. More interesting is getting hints from sibling case bodies. This can come out of order, and indeed this might be a common pattern with match expressions as people like to list their edge cases first. E.g.

```
declare const x: number;
const out = match (x) {
  0: [],
  const x: [x],
};
```
So, we consider every sibling case body, from top to bottom, as a "best effort hint".

The logic for hints diverges the match statement and match expression logic, so I break these up again into seperate methods.

Changelog: [internal]

Reviewed By: panagosg7

Differential Revision: D70140524

fbshipit-source-id: 499d5e8864964c32c6bce372743dec2adb4781cf

src/analysis/env_builder/name_def.ml

@@ -2871,8 +2871,7 @@ class def_finder ~autocomplete_hooks ~react_jsx env_info toplevel_scope =
       | Ast.Expression.Conditional expr -> this#visit_conditional ~hints expr
       | Ast.Expression.AsConstExpression { Ast.Expression.AsConstExpression.expression; _ } ->
         this#visit_expression ~hints ~cond expression
-      | Ast.Expression.Match x ->
-        this#visit_match ~on_case_body:(fun e -> ignore @@ this#expression e) x
+      | Ast.Expression.Match x -> this#visit_match_expression ~hints x
       | Ast.Expression.Class _
       | Ast.Expression.Identifier _
       | Ast.Expression.Import _
@@ -3146,32 +3145,58 @@ class def_finder ~autocomplete_hooks ~react_jsx env_info toplevel_scope =
 
     method! match_expression loc _ = fail loc "Should be visited by visit_match_expression"
 
+    method private visit_match_expression ~hints x =
+      let open Ast.Match in
+      let { arg; cases; match_keyword_loc; comments = _ } = x in
+      this#add_ordinary_binding
+        match_keyword_loc
+        (mk_reason RMatch match_keyword_loc)
+        (Binding (Root (Value { hints = []; expr = arg })));
+      let value_hints =
+        Base.List.foldi cases ~init:IMap.empty ~f:(fun i acc (_, { Case.body; _ }) ->
+            if expression_is_definitely_synthesizable ~autocomplete_hooks body then
+              let hint = Hint_t (ValueHint body, BestEffortHint) in
+              IMap.add i hint acc
+            else
+              acc
+        )
+      in
+      Base.List.iteri cases ~f:(fun i (case_loc, { Case.pattern; body; guard; comments = _ }) ->
+          let match_root =
+            (case_loc, Ast.Expression.Identifier (Flow_ast_utils.match_root_ident case_loc))
+          in
+          ignore @@ this#expression match_root;
+          let acc = Value { hints = []; expr = match_root } in
+          this#add_match_destructure_bindings acc pattern;
+          ignore @@ super#match_pattern pattern;
+          run_opt this#expression guard;
+          (* We use best-effort value hints for cases other than the current case.
+             Hints are ordered as the cases are in source, top to bottom. *)
+          let value_hints = value_hints |> IMap.remove i |> IMap.values |> List.rev in
+          let hints = Base.List.append hints value_hints in
+          this#visit_expression ~hints ~cond:NonConditionalContext body
+      )
+
     method! match_statement _ x =
-      this#visit_match ~on_case_body:(fun b -> run_loc this#block b) x;
+      let open Ast.Match in
+      let { arg; cases; match_keyword_loc; comments = _ } = x in
+      this#add_ordinary_binding
+        match_keyword_loc
+        (mk_reason RMatch match_keyword_loc)
+        (Binding (Root (Value { hints = []; expr = arg })));
+      Base.List.iter cases ~f:(fun (case_loc, { Case.pattern; body; guard; comments = _ }) ->
+          let match_root =
+            (case_loc, Ast.Expression.Identifier (Flow_ast_utils.match_root_ident case_loc))
+          in
+          ignore @@ this#expression match_root;
+          let acc = Value { hints = []; expr = match_root } in
+          this#add_match_destructure_bindings acc pattern;
+          ignore @@ super#match_pattern pattern;
+          run_opt this#expression guard;
+          run_loc this#block body
+      );
       x
 
-    method private visit_match
-        : 'B. on_case_body:('B -> unit) -> ('loc, 'loc, 'B) Ast.Match.t -> unit =
-      fun ~on_case_body x ->
-        let open Ast.Match in
-        let { arg; cases; match_keyword_loc; comments = _ } = x in
-        this#add_ordinary_binding
-          match_keyword_loc
-          (mk_reason RMatch match_keyword_loc)
-          (Binding (Root (Value { hints = []; expr = arg })));
-        Base.List.iter cases ~f:(function
-            | (case_loc, { Case.pattern; body; guard; comments = _ }) ->
-            let match_root =
-              (case_loc, Ast.Expression.Identifier (Flow_ast_utils.match_root_ident case_loc))
-            in
-            ignore @@ this#expression match_root;
-            let acc = Value { hints = []; expr = match_root } in
-            this#add_match_destructure_bindings acc pattern;
-            ignore @@ super#match_pattern pattern;
-            run_opt this#expression guard;
-            on_case_body body
-            )
-
     method add_match_destructure_bindings root pattern =
       let visit_binding loc name binding =
         let binding = this#mk_hooklike_if_necessary (Flow_ast_utils.hook_name name) binding in

tests/match/hints.js

@@ -0,0 +1,83 @@
+// Annotation hint
+type F = string => boolean;
+{
+  declare const x: 'a';
+
+  const out: F = match (x) {
+      'a': y => true, // OK
+  };
+}
+
+// Sibling before
+{
+  declare const x: 'a' | 'b';
+
+  const out = match (x) {
+      'a': [1],
+      'b': [], // Should be `Array<number>`
+  };
+  out as Array<number>; // OK
+}
+
+// Sibling after
+{
+  declare const x: 'a' | 'b';
+
+  const out = match (x) {
+      'a': [], // Should be `Array<number>`
+      'b': [1],
+  };
+  out as Array<number>; // OK
+}
+
+// Multiple siblings, one valid
+{
+  declare const x: 'a' | 'b' | 'c' | 'd';
+
+  const out = match (x) {
+      'a': 1,
+      'b': {},
+      'c': [1],
+      'd': [], // Should be `Array<number>`
+  };
+  out as number | {} | Array<number>; // OK
+}
+
+// Multiple siblings, multiple valid
+{
+  declare const x: 'a' | 'b' | 'c';
+
+  const out = match (x) {
+      'a': [true],
+      'b': [1],
+      'c': [],
+  };
+  out as Array<boolean> | Array<number>; // OK
+}
+{
+  declare const x: 'a' | 'b' | 'c';
+
+  const out = match (x) {
+      'a': (x: number) => 1,
+      'b': (x: string) => true,
+      'c': x => x as number, // OK
+  };
+}
+
+// Cycles avoided
+{
+  declare const x: 'a' | 'b';
+
+  const out: (x: number) => void = match (x) { // OK
+    'a': (x) => {},
+    'b': (x) => {},
+  };
+}
+{
+  declare const x: 'a' | 'b';
+
+  const out: Array<number> = match (x) { // OK
+    'a': [],
+    'b': [],
+  };
+}