`typing.Annotated[int, ...]` is subclassable

[sobolevn]

You can subclass Annotated[int, "meta"]:

>>> import typing
>>> class Some(typing.Annotated[int, 'meta']): ...
... 
>>> Some.__mro__
(<class '__main__.Some'>, <class 'int'>, <class 'object'>)

But, here are a couple of problems:

1. All metadata is lost: so, no reason to use Annotated in the first place
2. https://peps.python.org/pep-0593/ never mentions this use-case
3. We assume that Annotated cannot be subclassed: https://github.com/python/cpython/blame/53a54b781d1f05f2d0b40ce88b3da92d5d23e9d2/Lib/test/test_typing.py#L6562-L6565 here

I think that Annotated should not be subclasses and this is a bug.
The PR is incoming :)

I found this while working on #96769

[sobolevn]

Related: #96080
This PR introduced explicit __mro_entries__ to _AnnotatedAlias.

[sobolevn]

But, Annotated was subclassable even in 3.10.0:

Python 3.10.0 (default, Nov  1 2021, 10:24:06) [Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import typing
>>> class Some(typing.Annotated[int, 'meta']): ...
... 
>>> Some.__mro__
(<class '__main__.Some'>, <class 'int'>, <class 'object'>)

[sobolevn]

Looks like there's a test for quite the opposite: https://github.com/python/cpython/blame/53a54b781d1f05f2d0b40ce88b3da92d5d23e9d2/Lib/test/test_typing.py#L6713-L6717

It ensures that Annotated can be subclassed. Related: #27841

[AlexWaygood]

I disagree that it is a bug that you can subclass Annotated[int, ...]. I think you're meant to be able to use Annotated[int, ...] at runtime at all sites where you could use int. Why should code like this be disallowed?

SpecialInt: TypeAlias = Annotated[int, "very special"]
class Foo(SpecialInt): ...

This works fine, and I don't see why the cases should be treated differently:

SpecialInt: TypeAlias = int
class Foo(SpecialInt): ...

[AlexWaygood]

In any case, it would surely be backwards-incompatible to change this behaviour now?

[sobolevn]

Yes, changing this will be backward incompatible. Even type-checkers can understand this:

from typing import Annotated

class Some(Annotated[int, 'meta']):
    pass

reveal_type(Some)
# ex.py:6: note: Revealed type is "Overload(def (Union[builtins.str, builtins.bytes, array.array[Any], mmap.mmap, ctypes._CData, pickle.PickleBuffer, typing.SupportsInt, typing_extensions.SupportsIndex, _typeshed.SupportsTrunc] =) -> ex.Some, def (Union[builtins.str, builtins.bytes], base: typing_extensions.SupportsIndex) -> ex.Some)"

So, it probably should stay as-is.