slice().indices() returns incorrect values

[misianne]

BPO	42467
Nosy	@mdickinson
Superseder	bpo-11842: slice.indices with negative step and default stop

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-11-25.21:10:34.758>
created_at = <Date 2020-11-25.20:54:21.047>
labels = ['interpreter-core', '3.8']
title = 'slice().indices() returns incorrect values'
updated_at = <Date 2020-11-25.21:10:34.757>
user = 'https://bugs.python.org/misianne'

bugs.python.org fields:

activity = <Date 2020-11-25.21:10:34.757>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-25.21:10:34.758>
closer = 'mark.dickinson'
components = ['Interpreter Core']
creation = <Date 2020-11-25.20:54:21.047>
creator = 'misianne'
dependencies = []
files = []
hgrepos = []
issue_num = 42467
keywords = []
message_count = 2.0
messages = ['381862', '381864']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'misianne']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '11842'
type = None
url = 'https://bugs.python.org/issue42467'
versions = ['Python 3.8']

[misianne]

I have a problem with slice().indices():

x=[0,1,2,3,4,5,6,7,8,9]*10
x[-91:-101:-3] # == [9, 6, 3, 0] OK!
x[slice(*slice(-91,-101,-3).indices(len(x)))] # == [] WRONG!

This is correct:

x[-91:-100:-3] # == [9, 6, 3] OK!
x[slice(*slice(-91,-100,-3).indices(len(x)))] # == [9, 6, 3] OK!

This is not:

x[-91:-101:-3] # == [9, 6, 3, 0] OK!
x[slice(*slice(-91,-101,-3).indices(len(x)))] # == [] WRONG!

The problem is that
slice(-91,-101,-3).indices(len(x))
returns:
(9,-1,-3)
which result in an empty list, while:
(9,None,-3)
gives the correct result.

[mdickinson]

This is essentially a duplicate of bpo-11842. In short, slice.indices is working as intended here. It's best to think of slice.indices as start, stop and step arguments that could be provided to the range built-in function to get the set of relevant indices.