> The two @overload definitions will be overwritten by 
> the non-overload one at runtime, and hence will ever 
> been seen by help().

We can fix this by adding an __overloads__ attribute.  The overload decorator can accumulate the chain in an external namespace and function creation can move that accumulation into the new attribute.

----- Proof of concept -----

from typing import Union, _overload_dummy

def create_function(func):
    namespace = func.__globals__
    key = f'__overload__{func.__qualname__}__'
    func.__overloads__ = namespace.pop(key, [])
    return func

def overload(func):
    namespace = func.__globals__
    key = f'__overload__{func.__qualname__}__'
    namespace[key] = func.__overloads__ + [func.__annotations__]
    return _overload_dummy

class Smudge(str):

    @overload
    @create_function
    def __getitem__(self, index: int) -> str:
        ...

    @overload
    @create_function
    def __getitem__(self, index: slice) -> 'Smudge':
        ...

    @create_function
    def __getitem__(self, index: Union[int, slice]) -> Union[str, 'Smudge']:
        'Return a smudged character or characters.' 
        if isinstance(index, slice):
            start, stop, step = index.indices(len(self))
            values = [self[i] for i in range(start, stop, step)]
            return Smudge(''.join(values))
        c = super().__getitem__(index)
        return chr(ord(c) ^ 1)

    @create_function
    def other_method(self, x:str) -> tuple:
        pass

print(f'{Smudge.__getitem__.__annotations__=}')
print(f'{Smudge.__getitem__.__overloads__=}')
print(f'{Smudge.other_method.__annotations__=}') 
print(f'{Smudge.other_method.__overloads__=}')