[1.20 regression] when same object is narrowed multiple times

[ktbarrett]

Bug Report

There was a regression in narrowing with isinstance checks from 1.19 to 1.20. This seems to only happen when the same object is narrowed multiple times. For example, this works correct if removing the outer is not None guard and the | None from the argument.

To Reproduce

mypy playground link

from __future__ import annotations


def _score_test(
    exc: BaseException | None, expected_excs: set[type[BaseException]]
) -> None:
    if exc is not None:
        reveal_type(exc)
        reveal_type(expected_excs)  # Revealed type is "BaseException"
        reveal_type(tuple(expected_excs))  # Revealed type is "tuple[type[BaseException], ...]"
        if isinstance(exc, tuple(expected_excs)):
            reveal_type(exc)  # Revealed type is "object"
            return logexc(exc)  # Argument 1 to "logexc" has incompatible type "object"; expected "BaseException | None"

def logexc(
    exc: BaseException | None
) -> None:
    ...