Add TypeAliasType

src/test_typing_extensions.py

@@ -4494,6 +4494,11 @@ def test_pickle(self):
                 unpickled = pickle.loads(pickled)
                 self.assertIs(unpickled, Alias)
 
+    def test_no_instance_subclassing(self):
+        with self.assertRaises(TypeError):
+            class MyAlias(TypeAliasType):
+                pass
+
 
 if __name__ == '__main__':
     main()

src/typing_extensions.py

@@ -2678,6 +2678,11 @@ def __getitem__(self, parameters):
         def __reduce__(self):
             return self.__name__
 
+        def __init_subclass__(cls, *args, **kwargs):
+            raise TypeError(
+                "type 'typing_extensions.TypeAliasType' is not an acceptable base type"
+            )
+
         # The presence of this method convinces typing._type_check
         # that TypeAliasTypes are types.
         def __call__(self):