Overload on Optional[str] and bytes fails with arg of type Union[str, bytes]

[gvanrossum]

This may well be a known issue. @Michael0x2a. (The real code used os.listdir(), but I'm simplifying it here and it fails the same way.)

@overload
def listdir(a: Optional[str] = None): pass
@overload
def listdir(a: bytes): pass
def listdir(a): pass

def f(a: Union[bytes, str]):
  listdir(a)  # error: Argument 1 to "listdir" has incompatible type "Union[bytes, str]"; expected "Optional[str]"

[ilevkivskyi]

Hm, if I remove the default value then it works:

@overload
def listdir(a: Optional[str]): pass
@overload
def listdir(a: bytes): pass
def listdir(a): pass

def f(a: Union[bytes, str]):
  listdir(a)  # OK

So must be some unnecessary restriction on arg kinds in union math and so probably easy to fix.

[Michael0x2a]

Yeah, union math currently tries to handle only signatures where the argument kinds are all the same -- I wasn't sure how to go about unioning together callables that contain some crazy mixture of keyword arguments, name-only arguments, *args, **kwargs so decided to go for simplicity.

I don't think it would be too hard to extend the current logic so it handles the case where one kind is a positional and the other is an optional. I can add that to my TODO list.