Summary

Subscripting a deque with an index less than -len(d) wrapped around a second time instead of raising IndexError. On a length-5 deque, d[-6] returned d[4] rather than raising, and IndexError only appeared once the index went below -2*len. This affected reads, assignments, and deletions via the subscript operator. Fixes #923.

from collections import deque
d = deque([10, 20, 30, 40, 50])
d[-6]   # CPython: IndexError;  GraalPy (before): 50

Root cause

The negative index was normalized twice:

1. The subscript machinery (PyObjectGetItem / PyObjectSetItem / PyObjectDelItem → IndexForSqSlot) already adds len(self) to a negative index, mirroring CPython's PySequence_GetItem.
2. The deque slots DequeGetItemNode (sq_item) and DequeSetItemNode (sq_ass_item) then added len(self) again via NormalizeIndexCustomMessageNode.

So d[-6] on a length-5 deque became -6 + 5 = -1 (step 1) and then -1 + 5 = 4 (step 2), reading a valid slot instead of raising.

This is deque-specific: every other builtin sequence (list, tuple, str, bytes, array, …) also defines mp_subscript, so its subscripting goes through the mapping slot (normalized once). deque is the only builtin sequence that defines sq_item/sq_ass_item without the mp_* counterparts, so its subscripting goes through the sequence-slot path that double-normalized.

Fix

Aligns deque with CPython's division of labor (the caller normalizes; the slot only bounds-checks):

• DequeGetItemNode / DequeSetItemNode now only perform a bounds check and raise IndexError, matching _collectionsmodule.c:deque_item / deque_ass_item (and how list's sq_item slot, SequenceStorageSqItemNode, already behaves).
• The shared sq_* wrappers WrapSqItemBuiltinNode, WrapSqSetItemBuiltinNode, and WrapSqDelItemBuiltinNode now pass self (not the index object) to FixNegativeIndex, so the adjustment uses len(self), matching CPython's typeobject.c:getindex. Previously they passed the index object, making FixNegativeIndex compute len(index) — a no-op for integer indices. That latent bug was masked while the slot still normalized; once the slot stops normalizing, the wrapper must do the single normalization so the __getitem__/__setitem__/__delitem__ method path stays correct.

Testing

• New regression test test_negative_index_out_of_range in test_deque.py (covers d[i], d[i] = x, del d[i] for i < -len).