And it doesn't really need to. Consider this:

    from functools import singledispatch
    from typing import Union

    @singledispatch
    def myfunc(arg:Union[int, str]):
        print("Called with something else")
    
    @myfunc.register
    def _(arg:int):
        print("Called the one with int")
    
    @myfunc.register
    def _(arg:str):
        print("Called the one with str")

    myfunc(123)
    myfunc("123")

Everything works as you'd expect, and the IDE's type hints are accurate as long as you actually put the correct type hint

    def myfunc(arg:Union[int, str]):