[flow] reposition type guard before use in refinement

Summary:
When refining with type guards it is possible that we end up using the type guard itself as the result of the refinement. In that case that type needs to be repositioned. If not if the type guard happens to be an opaque type then we risk having the reason associated with it point to a remote file, thus making it invalid.

Changelog: [fix] fixed a bug when using opaque types in type guards in files other than the one where the type guard function was defined.

Reviewed By: SamChou19815

Differential Revision: D67153109

fbshipit-source-id: 2702a2e670fcceef3d817349591db4c04d5373cb

src/typing/predicate_kit.ml

@@ -385,10 +385,10 @@ and call_latent_pred cx trace fun_t ~use_op ~reason ~targs ~argts ~sense ~idx ti
               (* This is not the refined parameter. *)
               Type_filter.TypeFilterResult { type_ = tin; changed = false }
             else if sense then
-              let type_ = intersect cx tin type_guard in
+              let type_ = intersect cx tin (reposition_reason cx ~trace reason type_guard) in
               Type_filter.TypeFilterResult { type_; changed = type_ != tin }
             else if not one_sided then
-              type_guard_diff cx tin type_guard
+              type_guard_diff cx tin (reposition_reason cx ~trace reason type_guard)
             else
               (* Do not refine else branch on one-sided type-guard *)
               Type_filter.TypeFilterResult { type_ = tin; changed = false }

tests/type_guards/opaque_exports.js

@@ -15,3 +15,9 @@ export opaque type OpaqueC = C;
 export opaque type OpaqueAs = As;
 export opaque type OpaqueT = void | OpaqueB | OpaqueC | OpaqueAs;
 export type OpaqueOrString = OpaqueT | string;
+
+export opaque type Animal: string = string;
+export opaque type Raccoon: Animal = string;
+
+declare export function isAnimal(_s: string): implies _s is Animal;
+declare export function isRaccoon(_s: string): implies _s is Raccoon;

tests/type_guards/refinement.js

@@ -285,3 +285,14 @@ function sentinel_refinement() {
     }
   }
 }
+
+import type {Animal, Raccoon} from './opaque_exports';
+import {isAnimal, isRaccoon} from './opaque_exports';
+
+function getRaccoon(s: string): ?Raccoon {
+  if (!isAnimal(s)) return null;
+  if (isRaccoon(s)) {
+    return s; // okay
+  }
+  return null;
+}