Accessing dunders via type object produces TypeVar clashes

[oremanj]

Consider the following code:

from typing import AsyncIterator, TypeVar

T = TypeVar("T")

async def anext_via_instance(it: AsyncIterator[T]) -> T:
    return await it.__anext__()

async def anext_via_type(it: AsyncIterator[T]) -> T:
    anext = type(it).__anext__
    return await anext(it)

mypy happily accepts anext_via_instance, but in anext_via_type it fails with:

main.py:10: error: Incompatible return value type (got "_T_co", expected "T")  [return-value]
main.py:10: error: Argument 1 has incompatible type "AsyncIterator[T]"; expected "AsyncIterator[_T_co]"  [arg-type]

If I add a reveal_type(anext) in anext_via_type:

main.py:10: note: Revealed type is "def (self: typing.AsyncIterator[_T_co`1]) -> typing.Awaitable[_T_co`1]"

This behavior doesn't seem to be a recent change; it showed up on every version I spot-checked on the playground, and is still present on master. From the error message it looks like some kind of issue with TypeVar inference, but I'm not an expert.

(I'm trying to write something like anext_via_type in order to mimic the interpreter's behavior accessing a magic method in a specific circumstance; it won't accept an instance attribute, only a class attribute, so I don't want to accept an instance attribute either.)