fix: provide useful behavior of default py::slice (#5620)

* Change behavior of default py::slice

* make clang-tidy happy

* Update tests/test_pytypes.py

---------

Co-authored-by: Bryn Lloyd <12702862+dyollb@users.noreply.github.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

Author: dyollb

include/pybind11/pytypes.h

@@ -1942,13 +1942,14 @@ class weakref : public object {
 
 class slice : public object {
 public:
-    PYBIND11_OBJECT_DEFAULT(slice, object, PySlice_Check)
+    PYBIND11_OBJECT(slice, object, PySlice_Check)
     slice(handle start, handle stop, handle step)
         : object(PySlice_New(start.ptr(), stop.ptr(), step.ptr()), stolen_t{}) {
         if (!m_ptr) {
             pybind11_fail("Could not allocate slice object!");
         }
     }
+    slice() : slice(none(), none(), none()) {}
 
 #ifdef PYBIND11_HAS_OPTIONAL
     slice(std::optional<ssize_t> start, std::optional<ssize_t> stop, std::optional<ssize_t> step)

tests/test_pytypes.cpp

@@ -671,6 +671,8 @@ TEST_SUBMODULE(pytypes, m) {
 
     m.def("test_list_slicing", [](const py::list &a) { return a[py::slice(0, -1, 2)]; });
 
+    m.def("test_list_slicing_default", [](const py::list &a) { return a[py::slice()]; });
+
     // See #2361
     m.def("issue2361_str_implicit_copy_none", []() {
         py::str is_this_none = py::none();

tests/test_pytypes.py

@@ -603,7 +603,8 @@ def test_number_protocol():
 
 def test_list_slicing():
     li = list(range(100))
-    assert li[::2] == m.test_list_slicing(li)
+    assert li[0:-1:2] == m.test_list_slicing(li)
+    assert li[::] == m.test_list_slicing_default(li)
 
 
 def test_issue2361():