Make NoReturn a SpecialForm

[JelleZijlstra]

This is consistent with the runtime definition: https://github.com/python/cpython/blob/main/Lib/typing.py#L434.

The previous definition was wrong; NoReturn is not and should not be equivalent to None. This fixes an issue in pyanalyze where it was interpreting NoReturn as equivalent to None.

cc @erictraut in case this affects pyright. mypy-primer will tell us what mypy thinks about it.

[JelleZijlstra]

Thanks!

[erictraut]

I've made the necessary changes in pyright, and I'll be publishing a new version within the next few hours. We can then update your PR to point to the new version, which should eliminate the CI errors.

[erictraut]

I've published pyright 1.1.187, which fixes this issue. Typeshed currently uses 1.1.184. If you update this version (in both workflows/tests.yml and tests/pyright_test.py) within your PR, the CI errors should disappear.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JelleZijlstra]

Thanks for the very quick fix! CI passes now.

[KotlinIsland]

I think the original author's intent would be much more accurately described with:

NoReturn: TypeAlias = Union[()] # "<nothing>" 

😳😳😳😳😳😳😳😳😳😳😳😳

This works correctly at type time:

from typing import TypeAlias, Union

Never: TypeAlias = Union[()]

a: Never = 1  # error: Incompatible types in assignment (expression has type "int", variable has type <nothing>)

But not at runtime 😁. Good thing it's just a stub.