from typing import overload, Callable

@overload
def asdf(fn: Callable[[], str]) -> str: ...


@overload
def asdf(fn: Callable[[], None]) -> None: ...


def asdf(fn: Callable[[], str | None]) -> str | None:
    return ""


@asdf
def foo() -> None: ...


reveal_type(foo)  # expected: `None`, actual: `None | Unknown`