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

Beginne mit den Callable- und generischen ParamSpec-Beispielen des Vorschlags und überprüfe anschließend, wie Standardargumente derzeit durch Callback-Protokolle dargestellt werden. Lege das Typisierungsverhalten für WithDefault fest, einschließlich der Akzeptanz weggelassener Argumente und der Ablehnung erforderlicher Argumente, und validiere beide Beispiele, bevor du die Funktion als vollständig betrachtest.