[regression] Type not narrowed after isinstance inside or expression

[p-sawicki]

Bug Report

This seems to have regressed in ef28d9c cc @hauntsaninja. On the previous commit the revealed type is Any.

To Reproduce

from typing import Any

def f(val: Any | str) -> None:
    if val is None or isinstance(val, str):
        return
    reveal_type(val)  # Revealed type is "Any | str"

Expected Behavior

note: Revealed type is "Any"

Actual Behavior

note: Revealed type is "Any | str"

Your Environment

• Mypy version used: mypy 2.0.0+dev.ef28d9c2ca6476a9cffeb6c1e8dbe1a9432fda90
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.13.5

[hauntsaninja]

Thanks, patch for this in #21231