Matching generic return type of `SupportsGetItem`

Bug Report

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

This was proposed in https://github.com/astral-sh/ty/issues/3988#issuecomment-4960290236.

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

Start with the linked mypy-play reproduction and inspect how SupportsGetItem[Any, T] is handled when control flow narrows x to DataFrame. Confirm the expected diagnostic from the example, then trace the relevant type-checking behavior and add or update coverage for this case. Done means mypy reports the invalid return type instead of accepting return 1.