Understanding bound TypeVars with generic parameters

Question

I'm trying to understand how bound variables work in TypeVars. I understand that any subclass of the bound class is allowed, but once I make the bound class a generic, things that I would expect to work don't:

from typing import Generic, TypeVar

class X:
    pass
class Y(X):
    pass

T = TypeVar("T", bound=X)

class A(Generic[T]):
    def __init__(self, param: T):
        self.param = param
class B(A[T]):
    pass

S = TypeVar("S", bound=A[X])

def foo(bar: S) -> S:
    return bar

foo(B(Y())) # Type "B[Y]" cannot be assigned to type "A[X]"

Could someone explain why this doesn't work, and if any workarounds are known?