Option to type default default arguments in Callable types - `WithDefault`

At the moment it's only possible to type callables with default arguments by using a Callback protocol . This adds a lot of additional code for just a few default arguments. Moreover it requires that users have at least a basic understanding of Protocols which is one of the more advanced typing concepts. Lastly, a Callback protocol can't be used to type a generic ParamSpec argument.

Proposal

Add a new WithDefault special type which can be used to annotate arguments in callable types.

Examples

from typing import Callable, TypeAlias, WithDefault

def func(a: str, b: int = 0) -> None: ...
def other(a: str, b: int) -> None: ...

def g(f: Callable[[str, WithDefault[int]], None]) -> None:
    f("Hello")  # ok
    f("World", 2)  # ok

g(func)  # ok
g(other)  # error

For generic ParamSpec types

class Job(Generic[_P]):
    def __init__(self, target: Callable[_P, None]) -> None:
        self.target = target

def g(job: Job[str, WithDefault[int]]) -> None:
    job.target("Hello")  # ok
    job.target("Hello", 2)  # ok

Start with the proposal's Callable and generic ParamSpec examples, then review how default arguments are currently represented through callback protocols. Define the typing behavior for WithDefault, including acceptance of omitted arguments and rejection of required ones, and validate both examples before considering the feature complete.