Subclassing Annotated generic class fails

[eltoder]

Bug report

Checklist

• I am confident this is a bug in CPython, not a bug in a third-party project
• I have searched the CPython issue tracker,
  and am confident this bug has not been reported before

CPython versions tested on:

3.9, 3.10, 3.11

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.11.4 (main, Jul 24 2023, 08:22:29) [GCC 8.3.0]

A clear and concise description of the bug:

According to #96771, inheriting from typing.Annotated is a feature and is expected to work. However, it breaks when used with generic classes. Minimal example:

import typing

T = typing.TypeVar("T")

class C(typing.Generic[T]):
    pass

class D(typing.Annotated[C[int], 123]):
    pass

fails with

TypeError: _GenericAlias.__init__() takes 3 positional arguments but 4 were given

However, it works if I use C without arguments:

class D(typing.Annotated[C, 123]):
    pass

AFAICT, the reason is that Annotated[C[int], 123] sets __origin__ to C[int] instead of C like all other generics. This change in _AnnotatedAlias.__init__ fixes the issue:

@@ -2112,7 +2112,7 @@ class _AnnotatedAlias(_NotIterable, _GenericAlias, _root=True):
         if isinstance(origin, _AnnotatedAlias):
             metadata = origin.__metadata__ + metadata
             origin = origin.__origin__
-        super().__init__(origin, origin)
+        super().__init__(getattr(origin, "__origin__", origin), origin)
         self.__metadata__ = metadata

     def copy_with(self, params):

but I did not check if it breaks anything else. (This makes __origin__ is C and __args__ == (C[int],), so no information is lost.)

CC @AlexWaygood @sobolevn

Linked PRs

• gh-108394: Fix subclassing annotated generic alias #108403