Giving Union of types as `bound` argument of TypeVar and calling overloaded function that requires one of the types, type checker reports an error

Question

Checking the following code with mypy, it says 30: error: Argument 1 to "f" has incompatible type "T"; expected "X".

But there are decorated declarations of f so we should be able to pass value of class X or Y as the argument a.

Question 1: Could not we use @overload and TypeVar(bound=Union) at the same time ? Or I misunderstand something ?

from typing import TypeVar, Union, overload


class X:
    pass


class Y:
    pass


@overload
def f(a: X):
    ...


@overload
def f(a: Y):
    ...


def f(a):
    return


T = TypeVar("T", bound=Union[X, Y])


def func(a: T) -> T:
    f(a)
    return a

I tried another example as following and it does not make type errors.

Question 2: What makes the difference between overloaded function and classes having same method ?

from typing import TypeVar, Union


class X:
    def method(self) -> float:
        return 1.0


class Y:
    def method(self) -> float:
        return 1.0


T = TypeVar("T", bound=Union[X, Y])


def func(a: T) -> T:
    a.method()
    return a