builtins.slice: more precise __new__ overloads and let StopT default to StartT.

[randolf-scholz]

Different overloads than #12899, testing mypy primer.

• added check_slice.py stubtest, testing both slice.__new__ constructor and slice[T] annotation
• changed StopT to default to StartT instead of Any. For example:
  • slice[int] = slice[int, int, Any] represents finite intervals
  • slice[int | None] = slice[int | None, int | None, Any] arbitrary intervals
• Added more comprehensive suite of overloads for __new__, removed special casing for int types.

slice.__new__ now follows a very simply rule: any None argument will result in the corresponding Generic Type to be considered as Any.

[AlexWaygood]

Looks promising!

[AlexWaygood]

Nice! Looks like you now just need to update some assertions in the test-case file to use Union[int, Any] rather than Any in order to make the test case pass.

[randolf-scholz]

One doubt I have is whether we should change it so that slice[A] = slice[A, A, A] after all.

Without this convention, describing the most commonly used slice type, which I think is

slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex | None]

Probably requires an alias since it is so long.

Moreover, I think we need to clarify whether the collections.abc.Sequence protocol requires __getitem__ to support slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex | None], or if slice[int | None, int | None, int | None] is sufficient. This is likely a critical case as many users want to subclass Sequence, and it shouldn't be too inconvenient to type hint the implementation.

[randolf-scholz]

@AlexWaygood #13008 is the same as this, but defaults step to start | stop, so that slice[SupportsIndex | None] gives us the most commonly used slice type. In this case, people who want to use datetime/timedelta need to annotate with 3 values.

A decision needs to be made between:

1. Use this patch. For type hinting index-slices there are 3 options:
   1. Guide users to type hint slice[SupportsIndex | None], which means step will not be checked.
   2. Guide users to use an IndexSlice type alias provided by typing_extensions / typing.
   3. Guide users to use their own alias.
2. Use builtins.slice: more precise __new__ overloads and defaults for StopT and StepT. #13008, and guide users to annotate index-slices with slice[SupportsIndex | None], and users who want to use non-standard slices to provide all 3 type variables.

[github-actions]

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

[srittau]

Closing in favor of #13008.