Type narrowing / type guard fails to work in an inner function

[colinfang]

Mypy: v0.961
Python: v3.10

def f(g: Callable[[str], None] | None = None) -> None:
    g = g or print
    g('a') # type guard works, mypy ok
    def h() -> None:
        g('a') # type guard fails, mypy thinks it could be `None`
    h()

[JelleZijlstra]

Duplicate of #2608

The other issue has an explanation of why this is tricky.