Union of ParamSpec?

Feature

Currently it's possible to concatenate a type to a paramspec to get a new paramspec.

I think I may need the ability to concatenate two paramspecs, like:

P = ParamSpec('P')
Q = ParamSpec('Q')
R = TypeVar("R")
Callable[Concatenate[P, Q], R]

Pitch

I'm trying to describe partial-like functions, I spotted https://github.com/python/mypy/pull/16939 which solves if for functools.partial (which is good) but still not giving a way to describe other partial-like functions.

Currently I can successfully describe the "removal" of one or multiple parameters:

from collections.abc import Callable
from typing import Concatenate, Generic, ParamSpec, TypeVar

P = ParamSpec('P')
Q = ParamSpec('Q')
S = TypeVar('S')
T = TypeVar('T')
U = TypeVar('U')


def substract_one_parameter(given: T, func: Callable[Concatenate[T, P], U]) -> Callable[P, U]:
    def inner(*args: P.args, **kwargs: P.kwargs) -> U:
        return func(given, *args, **kwargs)
    return inner


def substract_two_parameters(first: S, second: T, func: Callable[Concatenate[S, T, P], U]) -> Callable[P, U]:
    def inner(*args: P.args, **kwargs: P.kwargs) -> U:
        return func(first, second, *args, **kwargs)
    return inner


def int_int_int(a: int, b: int) -> int:
    return a + b


c = substract_one_parameter(1, int_int_int)
reveal_type(c)  # def (b: builtins.int) -> builtins.int
print(c(2))


d = substract_two_parameters(1, 2, int_int_int)
reveal_type(d)  # def () -> builtins.int
print(d())

But what I think I need is:

# P are the "removed" parameters
# Q are the "kept" parameters
def substract_n_parameters(func: Callable[Concatenate[P, Q], U], *outer_args: P.args, **outer_kwargs: P.kwargs) -> Callable[Q, U]:
    def inner(*args: Q.args, **kwargs: Q.kwargs) -> U:
        return func(*outer_args, *args, **outer_kwargs, **kwargs)
    return inner

I feel that would cleanly express the "removal" of P from [P, Q], without the need to introduce a new function like Difference[P, Q].

I feel like this could also be used for the real functools.partial, maybe simplifying the current (working, yeah) implementation.

No source files or tests are named. Start by comparing the existing ParamSpec and Concatenate behavior with the provided examples, then review the referenced functools.partial implementation and pull request. Done means determining whether two ParamSpecs can be concatenated and validating the proposed substract_n_parameters typing behavior.