Matching generic return type of `SupportsGetItem`

[cmp0xff]

Bug Report

mypy does not match the generic return type of SupportsGetItem properly.

This was proposed in astral-sh/ty#3988 (comment).

To Reproduce

https://mypy-play.net/?gist=b8bb3085f195314a6c7c89ed2bdd2576

from __future__ import annotations

from typing import TYPE_CHECKING, Any, TypeVar

if TYPE_CHECKING:
    from _typeshed import SupportsGetItem

class Series: ...

class DataFrame:
    def __getitem__(self, key: object) -> Series:
        return Series()

T = TypeVar("T")

def f(x: SupportsGetItem[Any, T]) -> T:
    if isinstance(x, DataFrame):
        # return Series()
        return 1  # false error-free
    else:
        return x[0]

Expected Behaviour

Report an error about return type

Actual Behaviour

No error

Your Environment

mypy playground

[Jbansal2]

Hi! I'd like to investigate and work on fixing this issue. If no one else is already working on it, could you please assign it to me?