Shared via mypy Playground

main.py

from typing import overload

class A[T]: ...

class B[T: (int, str)](A[T]):
    @overload
    def __new__(cls: type[B[int]]) -> B[int]: ...
    @overload
    def __new__(cls: type[B[str]]) -> B[str]: ...
    def __new__(cls): raise NotImplementedError
    
def foo(x: A[str]) -> None:
    if isinstance(x, B):
        reveal_type(x)