Properties `.start`, `.stop`, `. step` of (generic) `slice[...]` should be optional (`| None`)

Situation

#13008 made slice.__new__ more precise. Generic slice[T] accepts T | None as arguments when creating a slice.
However, properties .start, .stop, . step of (generic) slice[...] are typed as there respective types, i.e. T in above example.

Consequence

This leads to typecheckers accepting:

def test(s: slice[int]) -> None:
    assert_type(s.start, int)
    if s.start is None:
        assert_never(s.start)

whereas below obviously breaks:

test(slice(42))

Solution

Therefore, I suggest properties .start, .stop, . step should have optional (| None) return types:

    @property
    def start(self) -> _StartT_co | None: ...
    @property
    def step(self) -> _StepT_co | None: ...
    @property
    def stop(self) -> _StopT_co | None: ...

Alternative

Currently, typeshed uses slice[...] as slice[T | None] explicitly as per #13007, e.g.

class str:
    def __getitem__(self, key: SupportsIndex | slice[SupportsIndex | None], /) -> str: ...

While that works/typechecks correctly:

def test(s: slice[int | None]) -> None:
    assert_type(s.start, int | None)
    if s.start is None or isinstance(s.start, int):
        pass
    else:
        assert_never(s.start)

test(slice(42))   # okay
test(slice('x'))  # error: Argument 1 to "slice" has incompatible type "str"; expected "int | None"  [arg-type]

I see as downsides:

1. it's error-prone / less ergonomic
2. it renders part of #13008 __new__ overloading complexity superfluous
3. it made #13007 more complex

Question

@Sachaa-Thanasius: Was the proposed solution considered for #13007?

Parti dallo stub generico di slice di typeshed e rivedi le annotazioni di new discusse in #13008 insieme all’alternativa in #13007. Verifica gli esempi di slice(42) dell’issue rispetto ai tipi di .start, .stop e .step; il lavoro è completato quando le annotazioni preservano None e il comportamento di type-checking mostrato è corretto.