Currently if the second argument to isinstance has type Any, we don't narrow the type down. This doesn't work as expected with ignored imports. Example (assume that foo has a missing stub):

from typing import Union

from foo import A  # type: ignore

def f(x: Union[A, str]) -> None:
    if isinstance(x, A):
        x.method_only_in_a()  # Item "str" of "Union[Any, str]" has no attribute "method_only_in_a"

I'd expect the type of x to be 'narrowed' (actually widened) to Any, and mypy should probably not generate an error.