The array API specification stipulates that the slicing syntax needs to be equivalent to Pythons slice() syntax. That includes negative step sizes that PyTorch does not support:

import torch

t = torch.rand((3,))
t[::-1]

ValueError: step must be greater than zero

Additional context

This was already proposed in #229, but finally dismissed in #229 (comment) in favor of torch.flip(). Note that this is only a substitution for a negative step size of 1 (::-1) rather than negative step sizes in general.

cc @mruberry @rgommers @pmeier