Variable type inference doesn't handle loops

[eltoder]

Apologies if this was already reported. I could not find an existing issue.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=7b4923efe4d5bad055733ce5e8f9a2f7

def fn(x: int | None) -> None:
    pass

def test(n: int) -> None:
    x = None
    for _ in range(n):
        fn(x)
        x = "123"

Expected Behavior

This should give an error because the type of x in test is str | None, but fn requires int | None.

Actual Behavior

No errors.

Your Environment

• Mypy version used: 1.5.1
• Python version used: 3.11

[ikonst]

We generally don't do anything sensible for loops - #14807, #14993, etc. We might want an issue label for those.

[eltoder]

I think #14807 is more ambitious because it needs a fixed point. #14993 might be the same issue, but the example there is more convoluted.