Clarify the valid use locations of `typing.Concatenate`

Commencez par la section sur les classes génériques définies par l’utilisateur de la spécification de typing et la documentation de CPython pour typing.Concatenate, puis comparez les exemples et le comportement signalé de mypy, pyright et ty. Examinez les notes de recherche liées sur Concatenate et déterminez quels emplacements d’utilisation sont valides ; le travail est terminé lorsque la spécification de typing et la documentation de CPython décrivent clairement les formes prises en charge et la restriction concernant Concatenate imbriqué.

In python/cpython#142965, it was reported that the documentation of typing.Concatenate is "incorrect" (documentation that originates back to python/cpython#24000).

@A5rocks, in your example, Concatenate[int, P_2] somewhat finally lands as a first argument to Callable, just indirectly.
This snippet previously defined:

P_2 = ParamSpec("P_2")

class X(Generic[T, P]):
  f: Callable[P, int]
  x: T

Similar example is included in the typing spec:

https://typing.python.org/en/latest/spec/generics.html#user-defined-generic-classes

class X[T, **P]:
    f: Callable[P, int]
    x: T

# (...)
def accept_concatenate[**P](x: X[int, Concatenate[int, P]]) -> str: ...  # Accepted

However, it seems that the current situation (confirmed with mypy, pyright and ty) is that you can use Concatenate in all valid locations of ParamSpec except directly in a Concatenate. I.e., you can't do Concatenate[int, Concatenate[str, P]]), but you can (besides passing Concatenate form as the first argument to Callable):

1. Accumulate Concatenates as ParamSpecs

from collections.abc import Callable
from typing import Concatenate

type Y[**P] = Callable[Concatenate[int, P], None]
type X[**P] = Y[Concatenate[int, P]]

def foo(f: X[str]) -> None:
    reveal_type(f)
    # mypy: def (builtins.int, builtins.int, builtins.str)
    # pyright: (int, int, str) -> None
    # ty: (...) -> None

2. Bind Concatenates to user-defined generics as ParamSpecs

class X[T, **P]:
    f: Callable[P, int]
    x: T

def accept_concatenate[**P](x: X[int, Concatenate[int, P]]) -> str: ...

3. Use Concatenate as a type argument to tuple -- is this correct?

from typing import Concatenate

def c(t: tuple[Concatenate[int, ...]]) -> None:
    reveal_type(c)
    # mypy: def (t: tuple[[builtins.int, *Any, **Any]])
    # pyright: (t: tuple[Concatenate[int, ...]]) -> None
    # ty: def c(t: tuple[@Todo]) -> None

I've found this mostly by poking around -- I haven't analyzed the actual implementations (yet).
I'll continue to investigate this from these searches.

I think that the valid use locations of Concatenate should be clarified in the typing spec and then in the CPython docs.

Is there anything else I overlooked? CC @JelleZijlstra @AlexWaygood