Use ParamSpec when the wrapper has more keyword-only arguments than wrapped function

[crusaderky]

PEP-612 explains how Concatenate can be used to write a wrapper where the output function requires less arguments than the wrapped one.
The opposite, where the output function requires more arguments than the wrapped one, is not clarified by the PEP, nor in my opinion it should, as it seems to be fairly non-controversial.

These work fine with mypy 1.7.0:

from typing import Callable, ParamSpec, TypeVar
P = ParamSpec("P")
R = TypeVar("R")

def w1(f: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> R:
    return f(*args, **kwargs)

def w2(f: Callable[P, R], extra_arg: int, *args: P.args, **kwargs: P.kwargs) -> R:
    return f(*args, **kwargs)

This however doesn't:

def w3(f: Callable[P, R], *args: P.args, extra_arg: int, **kwargs: P.kwargs) -> R:
    return f(*args, **kwargs)

error: ParamSpec must have "*args" typed as "P.args" and "**kwargs" typed as "P.kwargs"  [valid-type]

[erictraut]

PEP 612 expressly forbids the signature in w3 in your example. Here's the exact quote:

Placing keyword-only parameters between the *args and **kwargs is forbidden.

The "rejected alternatives" section of the PEP gives some indication of why this limitation was added.

Mypy should generate an error for w3, but it doesn't currently.

[ilevkivskyi]

Duplicate of #13966