gh-92261: Disallow iteration of Union (and other _SpecialForms)

[mrahtz]

As #92261 describes, list(Union) currently results in a hang because _SpecialForm.__getitem__ is used for iteration, meaning that list(Union) effectively does Union[0], Union[1], ....

This PR fixes it by implementing _SpecialForm.__iter__, which takes priority over _SpecialForm.__getitem__ when iterating.

Edit: and now also explicitly setting __iter__ = None on a number of other things in typing.py, to prevent similar errors with other things.

[carljm]

Looks good!

[serhiy-storchaka]

What about list(list)?

[mrahtz]

[Serhiy]

What about list(list)?

I've added a couple of tests to test_genericalias.py.

[serhiy-storchaka]

For me, the style is too verbose, but the code does everything right, and I don't want to be too picky.

[JelleZijlstra]

Should we apply this to 3.11 too?

[gvanrossum]

For a pure bugfix it seems rather involved. And since it is only intending to report a better error for something that's clearly invalid, I am not inclined to backport it. (Even though it has so many pieces that it will likely cause a fair amount of merge issues for future, simpler, bugfixes that do deserve being backported.

[JelleZijlstra]

It's not just an error message though: on 3.11 list(Union) will be an infinite loop that consumes more and more memory. Before 3.10 it would fail quickly because typing._type_check rejects ints. It's unfortunate if <=3.10 and >=3.12 fail quickly on list(Union) but 3.11 produces an infinite hang until you use all available memory.

[gvanrossum]

Okay, then I'll leave it to your judgment (and that of @pablogsal). Be sure to also backport the one-line fix for slots. :-)

[miss-islington]

Thanks @mrahtz for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-92582 is a backport of this pull request to the 3.11 branch.