[1.20 regression] mypy no longer detects variable type narrowing via reassignment

[abrahammurciano]

Bug Report

Mypy v1.20.0 no longer detects variable type narrowing via reassignment in this case.

To Reproduce

from typing import Iterable

def foo(args: Iterable[str | int] | str | int) -> Iterable[str]:
    if isinstance(args, (str, int)):
        args = (args,)
    args = (
        arg if isinstance(arg, str) else str(arg)
        for arg in args
    )
    return args

• With mypy 1.19.1: https://mypy-play.net/?mypy=1.19.1&python=3.14&gist=b8a1cf65bb99945451f64e846e18f245
• With mypy 1.20.0: https://mypy-play.net/?mypy=1.20.0&python=3.14&gist=b8a1cf65bb99945451f64e846e18f245

Expected Behavior

I expect this code to type check successfully with mypy 1.20.0 like it did with 1.19.1

Actual Behavior

main.py:10: error: Incompatible return value type (got "Generator[str | int, None, None]", expected "Iterable[str]")  [return-value]

Your Environment

• Mypy version used: 1.19.1 and 1.20.0
• Python version used: 3.14

[hauntsaninja]

Bisects to #20622 cc @ilevkivskyi

[ilevkivskyi]

Ah, I guess walrus is not the only thing that can change binder version while accepting an expression, ternary can do that as well. It seems to me ternary should be fine however, i.e, it shouldn't cause an issue similar to the issue fixed by that PR.

I think we can simply explicitly check for walrus instead of using the binder version.