There is a similar issue with `functools.singledispatch`

```
>>> from functools import singledispatch
>>> @singledispatch
... def flip(x: str) -> int:
... 	"""Signature when given a string"""
... 	return int(x)
... 
>>> @flip.register
... def _(x: int) -> str:
... 	"""Signature when given an int"""
... 	return str(x)
... 
>>> flip(5)
'5'
>>> flip('5')
5
>>> help(flip)
Help on function flip in module __main__:
flip(x: str) -> int
    Signature when given a string
```