generic descriptor and inheritance

generic_descriptors.py

from typing import (
    Generic, Optional, TYPE_CHECKING,
    Type, TypeVar, Union, overload,
)

T = TypeVar("T", bound="A")  # noqa

class Descr(Generic[T]):

    @overload
    def __get__(self: "Descr[T]", instance: None, owner: Type[T]) -> "Descr[T]": ...

    @overload
    def __get__(self: "Descr[T]", instance: T, owner: Type[T]) -> T: ...

    def __get__(self: "Descr[T]", instance: Optional[T], owner: Type[T]) -> Union["Descr[T]", T]:
        if instance is None:
            return self
        return instance


class A:

    @classmethod
    def construct(cls: Type[T]) -> T:
        return cls()

    def me(self: T) -> T:
        return self

    attr: int = 123

    descr = Descr[T]()  # doesn't work 


class B(A):
    new_attr: int = 123
    qwerty: str = "qwe"


if __name__ == "__main__":
    a = A().construct()
    if TYPE_CHECKING:
        reveal_type(a)  # A*, ok
        reveal_type(a.me())  # A*, ok
        reveal_type(a.descr)  # T?
    print("a.attr =", a.descr.attr)  # T? has no attribute "attr"
                                     # no runtime error
    b = B().construct()
    if TYPE_CHECKING:
        reveal_type(b)  # B*, ok
        reveal_type(b.me())  # B*, ok
        reveal_type(b.descr)  #T?

    print("b.new_attr =", b.descr.new_attr)  # T? has no attribute "new_attr"
                                             # no runtime error
    print("b.qwerty =", b.descr.qwerty)  # T? has no attribute "qwerty"
                                         # no runtime error