gh-118772: Allow TypeVars without a default to follow those with a default when constructing aliases

[JelleZijlstra]

• Issue: Relax TypeVar default ordering constraints #118772

[miss-islington-app]

Thanks @JelleZijlstra for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-118776 is a backport of this pull request to the 3.13 branch.

[AlexWaygood]

Unfortunately I think it's infeasible to backport this to typing_extensions, unless we can think of some other approach to implement the same behaviour at typing_extensions. Here's the beginnings of a typing_extensions patch: https://github.com/python/typing_extensions/compare/main...AlexWaygood:typing_extensions:646-default-before-nondefault?expand=1. It passes tests on Python 3.11 and 3.12, but starts interfering with the typing.py internals in some quite fundamental ways on lower versions of Python due to the extensiveness of the monkeypatching. It's plausibly fixable, but it would require even more invasive monkeypatching, and I think it would make the code too fragile.

[AlexWaygood]

Alright, here we go... python/typing_extensions#392