Support class pointers in enum classes

Summary:
This was an oversight by me -- this function is never called on regular enums and the comment was wrong. Implementation is for legacy enums `extends Enum` and enum classes, which is why `enum E: class<C>` never raised the error observed on enum classes (see V1).

Some more cleanup is necessary to truly finish this, but for now this fixes enum class's intentional support for class pointers.

Reviewed By: viratyosin

Differential Revision: D79917193

fbshipit-source-id: d9e2b679dcaf19983eb324c894c187f21b0daff9

Author: vassilmladenov

hphp/hack/src/typing/typing_enum.ml

@@ -97,7 +97,8 @@ let enum_check_const ty_exp env cc t =
  * like any, or dynamic. The free status of type parameter is caught during
  * naming (Unbound name), so we only check the kind of type that is used.
  *)
-let enum_check_type env (pos : Pos_or_decl.t) ur ty_interface ty _on_error =
+let enum_class_check_type env (pos : Pos_or_decl.t) ur ty_interface ty _on_error
+    =
   let sd env =
     (* Allow pessimised enum class types when sound dynamic is enabled *)
     TypecheckerOptions.enable_sound_dynamic (Typing_env.get_tcopt env)
@@ -160,9 +161,7 @@ let enum_check_type env (pos : Pos_or_decl.t) ur ty_interface ty _on_error =
     | Tlabel _
     | Tneg _ ->
       false
-    | Tclass_ptr _ ->
-      (* TODO(T199606542) deprecate static ban on ::class for enums *)
-      false
+    | Tclass_ptr _ -> true
   in
   let (env, ty_err_opt) =
     match ty_interface with
@@ -225,17 +224,7 @@ let check_cyclic_constraint env (p, enum_name) constraint_ty =
 
 (** Check an enum declaration of the form
 
-     enum E : <ty_exp> as <ty_constraint>
-
-  or
-
-     class E extends Enum<ty_exp>
-
-  where the absence of `<ty_constraint>` is assumed to default to arraykey.
-
-  Check that `<ty_exp>` is int or string, and that
-
-    ty_exp <: ty_constraint <: arraykey
+     enum class E : <ty_exp> 
 
   Also that each type constant is of type `ty_exp`. *)
 let enum_class_check
@@ -292,7 +281,7 @@ let enum_class_check
       Option.iter ~f:(Typing_error_utils.add_typing_error ~env) ty_err2;
       (* Check that ty_exp <: arraykey *)
       let env =
-        enum_check_type
+        enum_class_check_type
           env
           pos
           Reason.URenum_underlying
@@ -311,7 +300,7 @@ let enum_class_check
           in
           Option.iter ~f:(Typing_error_utils.add_typing_error ~env) ty_err1;
           let env =
-            enum_check_type
+            enum_class_check_type
               env
               pos
               Reason.URenum_cstr

hphp/hack/test/typecheck/class_pointers/enums.php

@@ -0,0 +1,15 @@
+<?hh
+
+abstract class A {}
+class C extends A {}
+class D extends A {}
+
+enum BadEnum: class<A> {
+  C = C::class;
+  D = D::class;
+}
+
+enum class GoodEnumClass: class<A> {
+  class<A> C = C::class;
+  class<A> D = D::class;
+}

hphp/hack/test/typecheck/class_pointers/enums.php.exp

@@ -0,0 +1,4 @@
+ERROR: File "enums.php", line 8, characters 7-14:
+Using `C::class` in this position will trigger an implicit runtime conversion to string, please use `nameof C` (Typing[4482])
+ERROR: File "enums.php", line 9, characters 7-14:
+Using `D::class` in this position will trigger an implicit runtime conversion to string, please use `nameof D` (Typing[4482])