add signed overload for `py::slice::compute`

include/pybind11/pytypes.h

@@ -1133,6 +1133,13 @@ public:
                                     (ssize_t *) stop, (ssize_t *) step,
                                     (ssize_t *) slicelength) == 0;
     }
+    bool compute(ssize_t length, ssize_t *start, ssize_t *stop, ssize_t *step,
+      ssize_t *slicelength) const {
+      return PySlice_GetIndicesEx((PYBIND11_SLICE_OBJECT *) m_ptr,
+          length, start,
+          stop, step,
+          slicelength) == 0;
+    }
 };
 
 class capsule : public object {

tests/test_sequences_and_iterators.cpp

@@ -71,6 +71,25 @@ py::list test_random_access_iterator(PythonType x) {
 }
 
 TEST_SUBMODULE(sequences_and_iterators, m) {
+    // test_sliceable
+    class Sliceable{
+    public:
+      Sliceable(int n): size(n) {}
+      int start,stop,step;
+      int size;
+    };
+    py::class_<Sliceable>(m,"Sliceable")
+        .def(py::init<int>())
+        .def("__getitem__",[](const Sliceable &s, py::slice slice) {
+          ssize_t start, stop, step, slicelength;
+          if (!slice.compute(s.size, &start, &stop, &step, &slicelength))
+              throw py::error_already_set();
+          int istart = static_cast<int>(start);
+          int istop =  static_cast<int>(stop);
+          int istep =  static_cast<int>(step);
+          return std::make_tuple(istart,istop,istep);
+        })
+        ;
 
     // test_sequence
     class Sequence {

tests/test_sequences_and_iterators.py

@@ -33,6 +33,19 @@ def test_generalized_iterators():
             next(it)
 
 
+def test_sliceable():
+    sliceable = m.Sliceable(100)
+    assert sliceable[::] == (0, 100, 1)
+    assert sliceable[10::] == (10, 100, 1)
+    assert sliceable[:10:] == (0, 10, 1)
+    assert sliceable[::10] == (0, 100, 10)
+    assert sliceable[-10::] == (90, 100, 1)
+    assert sliceable[:-10:] == (0, 90, 1)
+    assert sliceable[::-10] == (99, -1, -10)
+    assert sliceable[50:60:1] == (50, 60, 1)
+    assert sliceable[50:60:-1] == (50, 60, -1)
+
+
 def test_sequence():
     cstats = ConstructorStats.get(m.Sequence)