Shared via mypy Playground

main.py

from collections.abc import Callable
from enum import Enum
from typing import Generic, Protocol, TypeVar, Type, overload


T = TypeVar("T", covariant=True)

class A(Generic[T]):
    pass


class B:
    pass


AT1 = TypeVar("AT1", bound=A)
AT2 = TypeVar("AT2", bound=A)


def my_decorator(func: Callable[[AT1], AT2]) -> Callable[[AT1 | B], AT2]: ...


@my_decorator
def f(a: A[float]) -> A[int]: ...

reveal_type(my_decorator)
reveal_type(f)