Can you give a type argument to a TypeVar

Here is the situation I'm facing. I have class A and a generic type B. The type parameter of B bound to A.

from typing import Generic, TypeVar

AT = TypeVar("AT", bound="A")
BT = TypeVar("BT", bound="B")


class A:
    pass

class B(Generic[AT]):
    def __init__(self, a: AT):
        self._a = a

    def a(self) -> AT:
        return self._a

I also have a generic class C with type parameter bounded by B. I want this class to have a method bar that given an object of type B returns something of the same type as the type parameter of the B passed as argument. I tried

class C(Generic[BT[AT]]):
    def bar(self, b: BT) -> AT:
        return b.a()

However, type checking on the code below with mypy gives:

Returning Any from function declared to return "AT"

I also tried:

class C(Generic[BT]):
    def bar(self, b: BT[AT]) -> AT:
        return b.a()

but then I get the error

Type variable "BT" used with arguments

Is there a way to capture this idea with type annotation or a better way to structure the code to capture this constraint?

Any idea about that? Thank you!