Specific output type of default function argument incompatible with generic output

[Strilanc]

mypy believes that a function with a specific output type can not be used as the default value for a method expecting a function with a generic output type. It interprets the difference as a problem instead of as a constraint.

Repro code

T = TypeVar('T')

def make_zero() -> int:
    return 0

def produce_many(amount: int,
                 producer: Callable[[], T] = make_zero) -> Tuple[T, ...]:
    return tuple(producer() for _ in range(amount))

Expected output

No type error. When the default value for producer is used, the type variable T is inferred to be int.

Actual output

Incompatible default for argument "producer" (default has type "Callable[[], int]", argument has type "Callable[[], T]")

[gvanrossum]

As a work-around you can use two overloads -- one with the argument, one without. But I agree this could perhaps be done better.

[ilevkivskyi]

This is a duplicate of #3737