Using bare Union as annotation is not handled properly

[KotlinIsland]

from typing import Union
def foo(a: Union): ...

Tc
    foo    1

👉 python -m robot.run test.robot
==============================================================================
Test                                                                          
==============================================================================
Tc                                                                    | FAIL |
AttributeError: __args__
------------------------------------------------------------------------------

[pekkaklarck]

The reason for this issue is that when you have a: Union, our code recognizes it as union but unlike Union[...] the plain Union doesn't have __args__. That's easy to fix by changing union.__args__ to getattr(union, '__args__', ()) and after that the failure changes to same as if you use a: () (#4646). It's clearly not valid to have a union without any types and we could make it an explicit error. Alternatively we could just return the value as-is in this case, but I believe failure is better.