TypeVarTuple with Callable and default arguments

Previous discussion: https://github.com/microsoft/pyright/issues/3775

I've been playing around with TypeVarTuples recently, in particular together with Callable and args.
https://peps.python.org/pep-0646/#type-variable-tuples-with-callable

One issue became obvious early on which isn't defined in the PEP itself.

How should default arguments be handled?

A common pattern, especially in async code, is to pass a callable and it's args to a function

from typing import Callable
from typing_extensions import TypeVarTuple

Ts = TypeVarTuple("Ts")

def call_later(cb: Callable[[*Ts], None], *args: *Ts) -> None: ...

That works well if all arguments are required

def func1(x: int, y: int, z: int) -> None: ...
call_later(func1, 0, 0, 0)  # ok

However, what should happen if y and z have default arguments?

def func2(x: int, y: int = 0, z: int = 0) -> None: ...
call_later(func2, 0)

Instinctively, I would think that it should work, too. As @erictraut did point out though, there are at least a few cases where the behavior would need to be further specified / or explicitly forbidden.

Start by reading the previous discussion in issue 3775 and the PEP 646 section on TypeVarTuples with Callable. Compare the required-argument and default-argument examples, along with the edge cases raised in the discussion, and establish a specified or explicitly forbidden behavior for default arguments.