Slice index false positive

[pkch]

from typing import Union, Any

d: Union[int, None]
q: Any
q[0:d]  # error: Slice index must be an integer or None

Reported by @chrish42.

[gvanrossum]

Probably a typeshed issue.

[JelleZijlstra]

I don't think so, this is mypy's general slicing check, not a specific __getitem__ override. Looks like the bug is in visit_slice_expr in checkexpr.py. The error message says that int or None is accepted, but the code only accepts int.

[chrish42]

Furthermore, None as a slice index works in at least some cases:

>>> range(10)[3:None]
range(3, 10)

So, at least for some cases, the code in visit_slice_expr should be more lenient and accept None as a slice index.

[gvanrossum]

OK, I thought mypy usually eventually defer to a dunder method, but if it doesn't in this case it's a mypy bug, perhaps related to strict optional.