bpo-46685: improve test coverage of Self and Never in typing

[sobolevn]

https://bugs.python.org/issue46685

[JelleZijlstra]

While you're fixing tests, would you mind renaming the AllTests class to DunderAllTests? Every time I see it that name confuses me into thinking it has all of the tests.

[JelleZijlstra]

On second thought, maybe not: if we make that change we should do it in the bugfix branches too, but the rest of this PR is only applicable to 3.11.

[sobolevn]

@JelleZijlstra by the way, while working on __all__ in typeshed, I've noticed that many modules with __all__ don't have check__all__ test.

I can send a PR with it, if others think it is a good idea.

I personally think, that it is! Because it can help us catching problems with public / not-exported names in the future.

[GBeauregard]

Thanks.

There are other typeforms where the _type_check path is untested too, and not just special forms. For the most part you can test all of them with stringification+get_type_hints. I'd been thinking lately about the best idea for fixing this coverage gap but haven't gotten to it.

The only one I'm currently aware of that fails such a check are ParamSpec's P.args and P.kwargs. I have a bpo open for this from the Annotated side of the bug, but it's the same problem. I will fix it when that stuff gets hashed out. But, I fixed 3 or so other bugs that would have been caught by these types of tests over the past 2 weeks, so there may be more 👀

[JelleZijlstra]

I am planning to merge in a few days (cc @gvanrossum).

[gvanrossum]

+1