TypeVars should be allowed to be the same

Two unbound TypeVars A and B can potentially be the same:

from typing import Callable, Iterable, TypeVar

A = TypeVar("A")
B = TypeVar("B")

def identity(x: A) -> A:
    return x

# ok
def _map(queue: Iterable[B], function: Callable[[B], A]) -> Iterable[A]:
    return map(function, queue)

# fails
def _map2(queue: Iterable[B], function: Callable[[B], A] = identity) -> Iterable[A]:
    return map(function, queue)

The introduced default keyword argument leads to the following unexpected error:
error: Incompatible default for argument "function" (default has type "Callable[[A], A]", argument has type "Callable[[B], A]")

I'm using mypy 0.740.

Start by running the two _map examples from the issue and compare how mypy checks the default Callable[[A], A] against Callable[[B], A]. Trace the type-variable compatibility and default-argument checking involved. Done means _map2 is accepted and the reported incompatibility has a regression test.