Matching generic return type of `SupportsGetItem`

[cmp0xff]

Describe the bug

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

Code or Screenshots

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()
    else:
        return x[0]

VS Code extension or command-line

Type "Series" is not assignable to return type "T@f"
  Type "Series" is not assignable to type "T@f"  (reportReturnType)

• mypy works fine: https://mypy-play.net/?gist=3aaea716bd3429a90a96457884f46712
• pyright reports an error: https://pyright-play.net/?strict=true&code=GYJw9gtgBA%2BjwFcAuCQFM5QJYQA5hCSgEMA7UsJYpLMUgZwFgAoF0SKJAT1y1IHNseAkQAqATQAKAURgBhABLS5AaQCSAOQDiAGigBBUlz2ieaAGrEQLFlmBQJM%2BUtWatALhZRvUdtBjcuGj0ABZoACZC%2BIRQAMoIuNFI9FpoSGpIaBA2zADGADbE9PRxaCBYwe5QAHS1OQVFJQAi1MQAYiDEEGiezD5Q4Wj2cPxpWJkQcAAU9Gj5wHoA1mhcVWAARgBWaLlIAJRQALQAfKXllV79PugoIKRnFfRTezmiUAC8DmaWIFMARKI-i9WMxBvZgFMAB5VeKJEQpNIZLIAbUMxgcAF0DicHL1%2BnZsPQ%2BPQqKRcmgoXoWlQOl00Hs8VdvDdUPdYmVHs9Lt45rNGUyWXcoJDkQAGDEsIA