make $Values on a frozen object return singleton literals

Summary:
on a frozen object, StrT, NumT and BoolT values can't be changed so we
can treat them as singletons. this makes this pattern work:

```
const Enum = Object.freeze({
  X: 'x',
  Y: 'y',
});
type EnumT = $Values<typeof Enum>
('a': EnumT); // error instead of StrT ~> StrT
```

Reviewed By: avikchaudhuri

Differential Revision: D6232529

fbshipit-source-id: d24339d7a7a607e24a9312d637f83d5c92a97a15

src/typing/flow_js.ml

@@ -2801,15 +2801,29 @@ let rec __flow cx ((l: Type.t), (u: Type.use_t)) trace =
     (* values *)
     (**********)
 
-    | DefT (_, ObjT { props_tmap = tmap; dict_t; _ }), GetValuesT (reason, values) ->
+    | DefT (_, ObjT { props_tmap = tmap; dict_t; flags; _ }), GetValuesT (reason, values) ->
       (* Find all of the props. *)
       let props = Context.find_props cx tmap in
       (* Get the read type for all readable properties and discard the rest. *)
       let ts = SMap.fold (fun key prop ts ->
         match Property.read_t prop with
         (* Don't include the type for the internal "$call" property if one
            exists. *)
-        | Some t when key != "$call" -> t :: ts
+        | Some t when key != "$call" ->
+            let t = if flags.frozen then
+              match t with
+              | DefT (t_reason, StrT (Literal (_, lit))) ->
+                let t_reason = replace_reason_const (RStringLit lit) t_reason in
+                DefT (t_reason, SingletonStrT lit)
+              | DefT (t_reason, NumT (Literal (_, lit))) ->
+                let t_reason = replace_reason_const (RNumberLit (snd lit)) t_reason in
+                DefT (t_reason, SingletonNumT lit)
+              | DefT (t_reason, BoolT (Some lit)) ->
+                let t_reason = replace_reason_const (RBooleanLit lit) t_reason in
+                DefT (t_reason, SingletonBoolT lit)
+              | _ -> t
+            else t in
+            t :: ts
         | _ -> ts
       ) props [] in
       (* If the object has a dictionary value then add that to our types. *)

tests/values/.flowconfig

@@ -1,2 +1,3 @@
 [options]
 unsafe.enable_getters_and_setters=true
+no_flowlib=false

tests/values/object_types.js

@@ -80,3 +80,18 @@ magicTrick('DIAMONDS'); // Error: 'DIAMONDS' is a key, but not a value.
 magicTrick('Magic'); // Error: 'Magic' is not a value.
 magicTrick(('Diamonds': string)); // Error: the `string` type is to general and
                                   // not a value.
+
+// same as Suite above, but uses Object.freeze instead of needing an
+// annotation.
+const FrozenSuite = Object.freeze({
+  DIAMONDS: 'Diamonds',
+  CLUBS: 'Clubs',
+  HEARTS: 'Hearts',
+  SPADES: 'Spades',
+});
+type FrozenSuiteEnum = $Values<typeof FrozenSuite>
+('Diamonds': FrozenSuiteEnum); // ok
+(DIAMONDS: FrozenSuiteEnum); // ok
+('DIAMONDS': FrozenSuiteEnum); // Error: 'DIAMONDS' is a key, but not a value.
+('Magic': FrozenSuiteEnum); // Error: 'Magic' is not a value.
+(('Diamonds': string): FrozenSuiteEnum); // Error: `string` is too general

tests/values/values.exp

@@ -366,5 +366,23 @@ Error: object_types.js:81
  64: function magicTrick(suite: SuiteEnum) {
                                 ^^^^^^^^^ string enum
 
+Error: object_types.js:95
+ 95: ('DIAMONDS': FrozenSuiteEnum); // Error: 'DIAMONDS' is a key, but not a value.
+      ^^^^^^^^^^ string. This type is incompatible with
+ 95: ('DIAMONDS': FrozenSuiteEnum); // Error: 'DIAMONDS' is a key, but not a value.
+                  ^^^^^^^^^^^^^^^ string enum
+
+Error: object_types.js:96
+ 96: ('Magic': FrozenSuiteEnum); // Error: 'Magic' is not a value.
+      ^^^^^^^ string. This type is incompatible with
+ 96: ('Magic': FrozenSuiteEnum); // Error: 'Magic' is not a value.
+               ^^^^^^^^^^^^^^^ string enum
+
+Error: object_types.js:97
+ 97: (('Diamonds': string): FrozenSuiteEnum); // Error: `string` is too general
+                   ^^^^^^ string. This type is incompatible with
+ 97: (('Diamonds': string): FrozenSuiteEnum); // Error: `string` is too general
+                            ^^^^^^^^^^^^^^^ string enum
+
 
-Found 36 errors
+Found 39 errors