classmethod decorator transforms callable into callable rather than producing a classmethod?

At the point at which the decorator is applied, the class method isn't a callable yet. It's a classmethod object that replies to __get__ to produce the bound method. Yet, MyPy already thinks it's a callable:

from __future__ import annotations
from typing import Callable

def decorate(c: classmethod[None]) -> Callable[[X], None]:
    def g(x: X) -> None:
        return c.__func__(type(x))
    return g

class X:
    @decorate  # error: Argument 1 to "decorate" has incompatible type "Callable[[Type[X]], None]"; expected "classmethod[None]"
    @classmethod
    def f(cls) -> None:
        print("okay")

x = X()
x.f()  # error: Invalid self argument "Type[X]" to class attribute function "f" with type "Callable[[X], None]"

まず、提供された例を mypy で再現し、checker が積み重なった @decorate と @classmethod デコレーターをどのように型付けするかを調べます。例が decorate に渡された classmethod と、後続の x.f() 呼び出しの両方を、どちらのエラーも報告せずに受け入れれば、修正は完了です。