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

include/pybind11/stl_bind.h

@@ -286,18 +286,24 @@ 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((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) {
+                // For a positive step, erasing an element shifts the remaining
+                // (later) elements down by one, so the next index to erase is
+                // ``start + step - 1``. For a negative step the visited indices
+                // are strictly decreasing, so erasing never shifts them and the
+                // next index is simply ``start + step``.
+                ssize_t offset = step > 0 ? step - 1 : step;
+                for (ssize_t i = 0; i < slicelength; ++i) {
                     v.erase(v.begin() + DiffType(start));
-                    start += step - 1;
+                    start += offset;
                 }
             }
         },

tests/test_stl_binders.py

@@ -67,6 +67,29 @@ def test_vector_int():
     assert len(v_int2) == 0
 
 
+def test_vector_delitem_slice():
+    slice_cases = [
+        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),
+    ]
+    for n in range(8):
+        for s in slice_cases:
+            ref = list(range(n))
+            got = m.VectorInt(range(n))
+            del ref[s]
+            del got[s]
+            assert list(got) == ref, f"n={n} slice={s}"
+
+
 # Older PyPy's failed here, related to the PyPy's buffer protocol.
 def test_vector_buffer():
     b = bytearray([1, 2, 3, 4])