Inferring constraints for callable types doesn't verify argument counts and kinds

[JukkaL]

The method mypy.constraints.ConstraintBuildervisitor.visit_callable_type is pretty broken. It assumes that there are no *args, **kwargs and that the callable types are generally compatible.

This issue may be already filed, but adding it now so that I won't forget about it.

[ddfisher]

@JukkaL can you clarify with some concrete problems?

[JukkaL]

Here is an example that produces incorrect output (though the example isn't realistic):

from typing import TypeVar, Callable, Tuple

T = TypeVar('T')
S = TypeVar('S')

def f(x: Callable[[T, S], None]) -> Tuple[T, S]:
    pass

def g(*x: int) -> None: pass

reveal_type(f(g))  # Tuple[int, <nothing>]

The expected output is Tuple[int, int].