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