Incorrect "incompatible default for argument" when the hint is a generic type with a TypeVar

[Zac-HD]

I would expect mypy to understand that (None,) is a valid default for Tuple[T, ...], but instead I get

error: Incompatible default for argument "a"
       (default has type "Tuple[None]", argument has type "Tuple[T, ...]")

from typing import List, Tuple, TypeVar
T = TypeVar("T")  # or covariant, or contravariant

def f(a: Tuple[T, ...] = (None,)) -> List[T]:  # Or Tuple[T], Set[T], etc.
    return list(a)

This was actually found while working on HypothesisWorks/hypothesis#167, where the signature eventually desugars to (Callable[..., T], SearchStrategy[T]) -> SearchStrategy[Callable[..., T]], but the tuple-to-list example here has the same problem and is much easier to follow.

[JelleZijlstra]

This is a duplicate of #3737, which has a simpler example.

[ilevkivskyi]

Note that supporting this would require lower bounds for type variables (currently mypy supports only upper bounds). For None it may sound silly, but technically it is still needed. Current workaround is to use overloads, see the original issue.