Connection to TypeVar bound to Union is lost after returning Self from method #16826

Bug Report

A variable of which the type is determined by a TypeVar can normally return an object of type Self from a method and have the returned object recognized as an instance of the TypeVar type. This no longer works of the TypeVar is bound to a Union, even if the method always returns a Self object for every type in the Union.

To Reproduce

from typing import Self, TypeVar


class A:
    def new_version(self) -> Self:
        return self


class B:
    def new_version(self) -> Self:
        return self


T = TypeVar("T", bound=A)
S = TypeVar("S", bound=A | B)


def f(x: T) -> T:
    return x.new_version()


def g(x: S) -> S:
    return x.new_version()

Mypy playground gist: https://mypy-play.net/?mypy=latest&python=3.12&gist=5e9aa0a21d090f7f21e4e3b643cd9941

Expected Behavior

mypy should approve both function f and g.

Actual Behavior

mypy considers function g to be erroneous:

main.py:23: error: Incompatible return value type (got "A | B", expected "S")  [return-value]

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.12