Trying to iterate over typing.Union results in hang

[mrahtz]

If you accidentally iterate over a Union, you get a hang:

from typing import Union
list(Union)
# ...hang

It looks like it's an infinite loop:

from typing import Union
for i in Union:
    print(i)
# 0, 1, 2, ...

As far as I can tell, this happens because typing.Union is implemented using typing._SpecialForm, and _SpecialForm is implemented using __getitem__ to support parameterisation like Union[int]. Unfortunately, because _SpecialForm doesn't implement __iter__, __getitem__ is also used for iteration - meaning that when trying to iterate, we effectively do Union[0], Union[1], etc.

[JelleZijlstra]

We should deprecate itertools.count because it's now redundant.

More seriously, agree that we should disallow this.

[mrahtz]

Fixed in 4739997.