Type checking support for new.target and import.meta as mixed

Summary:
Rather than give an `unsupported-syntax` error when `new.target` or `import.meta` are used, type them as `mixed`.
This is a bit more useful (and still safe).

Fixes #1152
Reference to #6913

Changelog: [feature] Added type checking support for `new.target` and `import.meta` as `mixed`

Reviewed By: mroch

Differential Revision: D30745294

fbshipit-source-id: 1026ddde29936b7c5c8f81daf3a7b2515a514679

src/typing/statement.ml

@@ -3643,6 +3643,22 @@ and expression_ ~cond ~annot cx loc e : (ALoc.t, ALoc.t * Type.t) Ast.Expression
   | Generator _ ->
     Flow.add_output cx Error_message.(EUnsupportedSyntax (loc, GeneratorExpression));
     Tast_utils.error_mapper#expression ex
+  | MetaProperty
+      {
+        MetaProperty.meta = (_, { Ast.Identifier.name = "new"; _ }) as meta;
+        property = (_, { Ast.Identifier.name = "target"; _ }) as property;
+        comments;
+      } ->
+    let t = bogus_trust () |> MixedT.at loc in
+    ((loc, t), MetaProperty { MetaProperty.meta; property; comments })
+  | MetaProperty
+      {
+        MetaProperty.meta = (_, { Ast.Identifier.name = "import"; _ }) as meta;
+        property = (_, { Ast.Identifier.name = "meta"; _ }) as property;
+        comments;
+      } ->
+    let t = bogus_trust () |> MixedT.at loc in
+    ((loc, t), MetaProperty { MetaProperty.meta; property; comments })
   | MetaProperty _ ->
     Flow.add_output cx Error_message.(EUnsupportedSyntax (loc, MetaPropertyExpression));
     Tast_utils.error_mapper#expression ex

tests/meta_property/import_meta.js

@@ -0,0 +1,4 @@
+// @flow
+
+(import.meta: mixed); // OK
+(import.meta: 1); // Error

tests/meta_property/meta_property.exp

@@ -0,0 +1,45 @@
+Error ----------------------------------------------------------------------------------------------- import_meta.js:4:2
+
+Cannot cast `import.meta` to number literal `1` because mixed [1] is incompatible with number literal `1` [2].
+[incompatible-cast]
+
+   import_meta.js:4:2
+   4| (import.meta: 1); // Error
+       ^^^^^^^^^^^ [1]
+
+References:
+   import_meta.js:4:15
+   4| (import.meta: 1); // Error
+                    ^ [2]
+
+
+Error ------------------------------------------------------------------------------------------------ new_target.js:5:4
+
+Cannot cast `new.target` to boolean because mixed [1] is incompatible with boolean [2]. [incompatible-cast]
+
+   new_target.js:5:4
+   5|   (new.target: boolean); // Error
+         ^^^^^^^^^^ [1]
+
+References:
+   new_target.js:5:16
+   5|   (new.target: boolean); // Error
+                     ^^^^^^^ [2]
+
+
+Error ----------------------------------------------------------------------------------------------- new_target.js:13:6
+
+Cannot cast `new.target` to boolean because mixed [1] is incompatible with boolean [2]. [incompatible-cast]
+
+   new_target.js:13:6
+   13|     (new.target: boolean); // Error
+            ^^^^^^^^^^ [1]
+
+References:
+   new_target.js:13:18
+   13|     (new.target: boolean); // Error
+                        ^^^^^^^ [2]
+
+
+
+Found 3 errors

tests/meta_property/new_target.js

@@ -0,0 +1,15 @@
+// @flow
+
+function Foo() {
+  (new.target: mixed); // OK
+  (new.target: boolean); // Error
+
+  if (!new.target) {} // OK
+}
+
+class A {
+  constructor() {
+    (new.target: mixed); // OK
+    (new.target: boolean); // Error
+  }
+}