ParamSpec: No support for adding keyword-only argument

I've been looking through PEP 612 but I just wasn't able to find any way to implement behavior that would allow me to do this:

P = ParamSpec("P")
R = TypeVar("R")

def foo(f: Callable[P, R]) -> Callable[P, List[R]]:  # Here, the parameters are actually P + optional kw-only int x = 3
    def inner(*args: P.args, x: int = 3, **kwargs: P.kwargs) -> List[R]:
        results = []
        for _ in range(x):
          ret = f(*args, **kwargs)
          results.append(ret)
        return results
    return inner

@foo
def bar(p: int):
    return p + 2

bar(5)  # Acceptable
bar(5, x=8)  # Should also be Acceptable

Proposed solution:

Callable[[P.args, x: int = 3, P.kwargs], List[R]]

EDIT: I didn't notice that this was already in rejected alternatives section (I didn't scroll that far), however I still think an implementation like this would be worth adding since I have already encountered the need for this twice and there's simply absolutely no way to handle it.

I'm fine with the proposed syntax suggestion of

Concatenate[("x", int), P]

Though it didn't mention anything about default values, but those could just be handled by a third value in that tuple. It would probably be a lot easier to implement than my proposed suggestion, though to be fair, at least to me, my suggestion looks a bit cleaner.

Start by reading PEP 612's rejected alternatives and the existing ParamSpec and Concatenate specifications. The issue names no implementation files or tests, so first identify the relevant typing-spec entry points; done requires an agreed syntax and defined behavior for optional keyword-only parameters.