[flow][match] Guards can apply refinements

Summary:
Match pattern gaurds can apply refinements to values which are not the match argument.

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D67779047

fbshipit-source-id: f5ed21536484e92b8a0e58690294eb8584d8f077

src/analysis/env_builder/name_resolver.ml

@@ -2955,8 +2955,13 @@ module Make (Context : C) (FlowAPIUtils : F with type cx = Context.t) :
                        ignore @@ this#expression arg;
                        let completion_state =
                          this#run_to_completion (fun () ->
-                             Base.Option.iter guard ~f:(fun guard -> ignore @@ this#expression guard);
-                             ignore @@ this#expression body
+                             match guard with
+                             | Some guard ->
+                               this#push_refinement_scope empty_refinements;
+                               ignore @@ this#expression_refinement guard;
+                               ignore @@ this#expression body;
+                               this#pop_refinement_scope ()
+                             | None -> ignore @@ this#expression body
                          )
                        in
                        completion_states := completion_state :: !completion_states;

tests/match/body.js

@@ -67,3 +67,14 @@ function f2() {
 
   out as number; // OK
 }
+
+// Guards can refine values which are not the argument
+{
+  declare const y: number | string;
+
+  const out = match (x) {
+    1 if typeof y === 'number': y as number, // OK
+    const a if a === 1: a as 1,
+    _: 0,
+  };
+}