fix(stl_bind): correct __delitem__ for negative-step slices and re-enable contiguous erase fast path

include/pybind11/stl_bind.h

@@ -286,18 +286,28 @@ void vector_modifiers(
     cl.def(
         "__delitem__",
         [](Vector &v, const slice &slice) {
-            size_t start = 0, stop = 0, step = 0, slicelength = 0;
+            ssize_t start = 0, stop = 0, step = 0, slicelength = 0;
 
-            if (!slice.compute(v.size(), &start, &stop, &step, &slicelength)) {
+            if (!slice.compute(
+                    static_cast<ssize_t>(v.size()), &start, &stop, &step, &slicelength)) {
                 throw error_already_set();
             }
 
-            if (step == 1 && false) {
+            if (step == 1) {
                 v.erase(v.begin() + (DiffType) start, v.begin() + DiffType(start + slicelength));
-            } else {
-                for (size_t i = 0; i < slicelength; ++i) {
+            } else if (slicelength > 0) {
+                // Erase non-contiguous slices in descending index order so that
+                // erasing an element never shifts an index that remains to be erased.
+                if (step > 0) {
+                    start += (slicelength - 1) * step;
+                    step = -step;
+                }
+                while (true) {
                     v.erase(v.begin() + DiffType(start));
-                    start += step - 1;
+                    if (--slicelength == 0) {
+                        break;
+                    }
+                    start += step;
                 }
             }
         },

tests/test_stl_binders.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import sys
+
 import pytest
 
 from pybind11_tests import stl_binders as m
@@ -67,6 +69,40 @@ def test_vector_int():
     assert len(v_int2) == 0
 
 
+@pytest.mark.parametrize(
+    "s",
+    [
+        slice(1, 4),
+        slice(None, None, 2),
+        slice(1, None, 2),
+        slice(None, None, -1),
+        slice(None, None, -2),
+        slice(3, 1, -1),
+        slice(2, 2),
+        slice(None),
+        slice(5, 0, -2),
+        slice(-3, -1),
+        slice(None, None, -3),
+        slice(3, None, sys.maxsize),
+        slice(-2, -7, -2),
+    ],
+)
+def test_vector_delitem_slice(s):
+    for n in range(8):
+        ref = list(range(n))
+        got = m.VectorInt(range(n))
+        del ref[s]
+        del got[s]
+        assert list(got) == ref, f"n={n}"
+
+
+def test_vector_delitem_slice_step_zero():
+    v = m.VectorInt(range(8))
+    with pytest.raises(ValueError):
+        del v[::0]
+    assert list(v) == list(range(8))
+
+
 # Older PyPy's failed here, related to the PyPy's buffer protocol.
 def test_vector_buffer():
     b = bytearray([1, 2, 3, 4])