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main.py

from __future__ import annotations
from typing import TypeVar, Concatenate, ParamSpec, Any

from collections.abc import Callable


F = TypeVar("F", bound = Callable[..., Any])
P = ParamSpec("P")
R = TypeVar("R")
S = TypeVar("S")


def copy_args(f: Callable[P, Any], /) -> Callable[[Callable[Concatenate[S, P], R]], Callable[Concatenate[S, P], R]]:
    """
    Capture the parameters type of `f`, then pretend that the decorated function's
    parameters are of this type.

    `f`'s return type is ignored, and the decorated function's return type is preserved.
    """

    return lambda _: _


def method(a: int, b: int) -> float:
    return a * b + 1.0


class Bar:
    def __init__(self, value: float) -> None:
        self.value = value


class Foo:

    @copy_args(method)
    def method(self, *args: Any, **kwargs: Any) -> Bar:
        return Bar(method(*args, **kwargs))


x = Foo().method(1, 2)
reveal_type(Foo().method)
reveal_type(x)