Include GenericMeta unconditionally

Summary:
PEP 560 removed `typing.GenericMeta`, so it was conditionally removed from typeshed by python/typeshed#3933 .

However, we rely on that class to be the metaclass of `Generic`, all generic classes, and all protocols.  This meant that once typeshed was updated, lots of attributes disappeared.

This diff fixes that by special casing it back in.  This means we'll erroneously allow users to refer to it, but that's fine for now.

Reviewed By: grievejia

Differential Revision: D21854250

fbshipit-source-id: 294740a866800b70939f425e8919a3c275b36285

Author: mrkmndz

analysis/unannotatedGlobalEnvironment.ml

@@ -735,6 +735,36 @@ let missing_builtin_classes, missing_typing_classes, missing_typing_extensions_c
     ]
     |> List.map ~f:Node.create_with_default_location
   in
+  let generic_meta_body =
+    [
+      Statement.Define
+        {
+          signature =
+            {
+              name =
+                Reference.create "typing.GenericMeta.__getitem__"
+                |> Node.create_with_default_location;
+              parameters =
+                [
+                  { Parameter.name = "cls"; value = None; annotation = None }
+                  |> Node.create_with_default_location;
+                  { Parameter.name = "arg"; value = None; annotation = None }
+                  |> Node.create_with_default_location;
+                ];
+              decorators = [];
+              return_annotation = None;
+              async = false;
+              generator = false;
+              parent = Some (Reference.create "typing.GenericMeta");
+              nesting_define = None;
+            };
+          captures = [];
+          unbound_names = [];
+          body = [];
+        }
+      |> Node.create_with_default_location;
+    ]
+  in
 
   let typing_classes =
     [
@@ -750,6 +780,7 @@ let missing_builtin_classes, missing_typing_classes, missing_typing_extensions_c
       make ~metaclasses:[Primitive "typing.GenericMeta"] "typing.Generic";
       make "typing.ClassMethod" ~bases:single_unary_generic ~body:classmethod_body;
       make "typing.StaticMethod" ~bases:single_unary_generic ~body:staticmethod_body;
+      make "typing.GenericMeta" ~bases:[Primitive "type"] ~body:generic_meta_body;
     ]
   in
   let typing_extension_classes =
@@ -818,38 +849,6 @@ let register_class_definitions ({ Source.source_path = { SourcePath.qualifier; _
             Type.expression (Type.parametric "typing.Generic" [Single (Type.variable "typing._T")])
           in
           { definition with Class.bases = [{ name = None; value }] }
-      | "typing.GenericMeta" ->
-          let body =
-            [
-              Statement.Define
-                {
-                  signature =
-                    {
-                      name =
-                        Reference.create "typing.GenericMeta.__getitem__"
-                        |> Node.create_with_default_location;
-                      parameters =
-                        [
-                          { Parameter.name = "cls"; value = None; annotation = None }
-                          |> Node.create_with_default_location;
-                          { Parameter.name = "arg"; value = None; annotation = None }
-                          |> Node.create_with_default_location;
-                        ];
-                      decorators = [];
-                      return_annotation = None;
-                      async = false;
-                      generator = false;
-                      parent = Some (Reference.create "typing.GenericMeta");
-                      nesting_define = None;
-                    };
-                  captures = [];
-                  unbound_names = [];
-                  body = [];
-                }
-              |> Node.create_with_default_location;
-            ]
-          in
-          { definition with body }
       | _ -> definition
     in
     WriteOnly.set_class_definition

test/test.ml

@@ -943,7 +943,8 @@ let typeshed_stubs ?(include_helper_builtins = true) () =
         ClassVar: _SpecialForm = ...
         NoReturn: _SpecialForm = ...
 
-        class GenericMeta(type): ...
+        if sys.version_info < (3, 7):
+            class GenericMeta(type): ...
 
         @runtime
         class Sized(Protocol, metaclass=ABCMeta):