Class typevars lose information from `isinstance` checks

[msullivan]

from typing import Generic, TypeVar

T = TypeVar('T')

class C(Generic[T]):
    def foo(self, x: T) -> None:
        if isinstance(x, int):
            self.bar(x)

    def bar(self, x: T) -> None:
        pass

fails with Argument 1 to "bar" of "C" has incompatible type "int"; expected "T", even though x is obviously a T (that was its annotated type!)

Arguably this is just the Dreaded Intersection Bug (#3603) in a different manifestation?

[ilevkivskyi]

Yes, and this is probably a hardest aspect of the problem, because I don't see an easy way to work around this using some fake TypeInfos for intersections.