type inference is fragile for protocols like SupportsKeysAndGetItem

Repro: https://mypy-play.net/?mypy=latest&python=3.11&gist=d2f8dfa1fdfda5c0c286f1a73f5e39eb

Pasted below for convenience:

import typing as t

from _typeshed import SupportsKeysAndGetItem


KT = t.TypeVar("KT")
VT = t.TypeVar("VT")


def version1(obj: SupportsKeysAndGetItem[KT, VT] | t.Iterable[tuple[KT, VT]]):
    if hasattr(obj, "keys") and hasattr(obj, "__getitem__"):
        reveal_type(obj)  # SupportsKeysAndGetItem[KT, VT] | Iterable[tuple[KT, VT]] (as expected)
    else:
        reveal_type(obj)  # narrowed to Iterable[tuple[KT, VT]] as expected


def version2(obj: SupportsKeysAndGetItem[KT, VT] | t.Iterable[tuple[KT, VT]]):
    if hasattr(obj, "keys") and hasattr(obj, "__getitem__") and (obj_getitem := obj.__getitem__):
        reveal_type(obj)  # SupportsKeysAndGetItem[KT, VT] | Iterable[tuple[KT, VT]] (as expected)
    else:
        reveal_type(obj)  # NOT narrowed

I would expect version 2 to behave the same as version 1. Is that an improvement mypy could reasonably make?

I searched for relevant docs and existing issues, but didn't find anything specifically about this. Apologies if I missed something.

リンクされている mypy-play の再現コードを実行し、version1 と version2 の reveal_type の結果を比較します。組み合わせた hasattr チェックと walrus 代入に対する型の絞り込みロジックを追跡します。version2 が version1 と一貫して else ブランチを絞り込み、その例のリグレッションカバレッジがあることを完了条件とします。