Default parameters for arguments of generic type do not typecheck

[rkr-at-dbx]

In the following example, the marked line does not typecheck:

from typing import Generic, TypeVar

_U = TypeVar('_U')

class Thing(Generic[_U]):
    def __init__(self, u):
        # type: (_U) -> None
        pass

def foo(param=3):
    # type: (_U) -> Thing[_U]
    return Thing(param)  # <--

This results in:
error: Incompatible default for argument "param" (default has type "int", argument has type "_U"), even though int is a valid value for _U.

[ilevkivskyi]

Duplicate of #3737.

(#4236 is also related).