Code like this currently fails but should be okay: ``` python def f(x: Union[int, str]) -> None: if not isinstance(x, int): return x + 2 # x must be int here so this is okay, but mypy complains ``` Note that this works currently (without the `not` operator): ``` python def f(x: Union[int, str]) -> None: if isinstance(x, str): return x + 2 # this is considered valid, as it should be ```