[flow] Fix incorrect ObjT fast path logic

Summary: Changelog: [errors] We fixed a bug that leads to Flow sometimes ignore the differences in call props of an object. New errors might be exposed. [example](https://flow.org/try/#1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAztvhgE6UYCe+JADpdhgCYowa5kA0I2KAFcAtiRQAXSkOz9sADwxgJ+NPTbYuQ3BMnTZA+Y2yU4IwRO4A6SFBIrGVDGM7c+h46fNRLuKxJIGWh8MeT0ZfhYlCStpHzNsFBAMIQkIEQwJODAQfiEyfBE4eWw2fDgofDBMsAALfAA3KjgsXGxxZC4eAw0G-GhcWn9aY3wWZldu-g1mbGqJUoBaCRHEzrcDEgBrbAk62kXhXFxJ923d-cPRHEpTgyEoMDaqZdW7vKgoOfaSKgOKpqmDA+d4gB5fMA-P6LCCMLLQbiLOoYCqgh6-GDYRYIXYLSgkRZkCR4jpddwPfJLZjpOBkO4AX34kA0SRWxgABAAVWoPLac1YAHgASgA+dkAXnZwHZAAoAJTIdlC9n0gDcAB0fKt2QBZWjc3n84zCsWSg1QPmC0UaqAaJ5UbDs2z2XX6nkWxV682W43CEQ3EU2r3urbsjAkLkho3YAX2UxQBCB9kAemT7JMlAglEVfpu7IAfmK4xUkPwGiYSHBoEkMvYTCB6UA)

Reviewed By: panagosg7

Differential Revision: D55970196

fbshipit-source-id: ea9642eab3ff6921ae25a3ca33a0f5f18bd3a5f7

src/typing/subtyping_kit.ml

@@ -1666,13 +1666,13 @@ module Make (Flow : INPUT) : OUTPUT = struct
     (*********************************************)
 
     (* ObjT -> ObjT *)
-    | ( DefT (lreason, ObjT ({ props_tmap = lflds; _ } as l_obj)),
-        DefT (ureason, ObjT ({ props_tmap = uflds; _ } as u_obj))
+    | ( DefT (lreason, ObjT ({ props_tmap = lflds; call_t = l_call_id; _ } as l_obj)),
+        DefT (ureason, ObjT ({ props_tmap = uflds; call_t = r_call_id; _ } as u_obj))
       ) ->
       let u_deft = u in
       Type_inference_hooks_js.dispatch_obj_to_obj_hook cx l u_deft;
       let print_fast_path = Context.is_verbose cx in
-      if Properties.equal_id lflds uflds then (
+      if Properties.equal_id lflds uflds && l_call_id = r_call_id then (
         if print_fast_path then prerr_endline "ObjT ~> ObjT fast path: yes"
       ) else (
         if print_fast_path then prerr_endline "ObjT ~> ObjT fast path: no";

tests/objects/different_call_prop_no_fast_path.js

@@ -0,0 +1,8 @@
+// This is a regression test for incorrect fast-path for objects
+// Previously, we enable ObjT ~> ObjT fast path if props_ids are the same,
+// which causes us to incorrectly enter the fast path when the call prop has changed.
+
+type ThunkType<R> = { (): R };
+type MyThunkType<R> = ThunkType<R>;
+declare const MyThunk: MyThunkType<number>;
+MyThunk as ThunkType<string>; // error: number ~> string

tests/objects/objects.exp

@@ -167,6 +167,24 @@ References:
                                ^^^^^^ [1]
 
 
+Error -------------------------------------------------------------------------- different_call_prop_no_fast_path.js:8:1
+
+Cannot cast `MyThunk` to `ThunkType` because number [1] is incompatible with string [2] in the return value.
+[incompatible-cast]
+
+   different_call_prop_no_fast_path.js:8:1
+   8| MyThunk as ThunkType<string>; // error: number ~> string
+      ^^^^^^^
+
+References:
+   different_call_prop_no_fast_path.js:7:36
+   7| declare const MyThunk: MyThunkType<number>;
+                                         ^^^^^^ [1]
+   different_call_prop_no_fast_path.js:8:22
+   8| MyThunk as ThunkType<string>; // error: number ~> string
+                           ^^^^^^ [2]
+
+
 Error --------------------------------------------------------------------------------------------------- objects.js:4:3
 
 Cannot get `x.foo` because property `foo` is missing in object type [1]. [prop-missing]
@@ -364,4 +382,4 @@ References:
 
 
 
-Found 23 errors
+Found 24 errors