Disable implicit conversion of 0 to pybind11::handle.

include/pybind11/pytypes.h

@@ -190,6 +190,11 @@ class object_api : public pyobject_tag {
     bool rich_compare(object_api const &other, int value) const;
 };
 
+template <typename T>
+using is_pyobj_ptr_or_nullptr_t = detail::any_of<std::is_same<T, PyObject *>,
+                                                 std::is_same<T, PyObject *const>,
+                                                 std::is_same<T, std::nullptr_t>>;
+
 PYBIND11_NAMESPACE_END(detail)
 
 #if !defined(PYBIND11_HANDLE_REF_DEBUG) && !defined(NDEBUG)
@@ -211,9 +216,23 @@ class handle : public detail::object_api<handle> {
 public:
     /// The default constructor creates a handle with a ``nullptr``-valued pointer
     handle() = default;
-    /// Creates a ``handle`` from the given raw Python object pointer
+
+    /// Enable implicit conversion from ``PyObject *`` and ``nullptr``.
+    /// Not using ``handle(PyObject *ptr)`` to avoid implicit conversion from ``0``.
+    template <typename T,
+              detail::enable_if_t<detail::is_pyobj_ptr_or_nullptr_t<T>::value, int> = 0>
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    handle(T ptr) : m_ptr(ptr) {}
+
+    /// Enable implicit conversion through ``T::operator PyObject *()``.
+    template <
+        typename T,
+        detail::enable_if_t<detail::all_of<detail::none_of<std::is_base_of<handle, T>,
+                                                           detail::is_pyobj_ptr_or_nullptr_t<T>>,
+                                           std::is_convertible<T, PyObject *>>::value,
+                            int> = 0>
     // NOLINTNEXTLINE(google-explicit-constructor)
-    handle(PyObject *ptr) : m_ptr(ptr) {} // Allow implicit conversion from PyObject*
+    handle(T &obj) : m_ptr(obj) {}
 
     /// Return the underlying ``PyObject *`` pointer
     PyObject *ptr() const { return m_ptr; }

tests/test_pytypes.cpp

@@ -39,7 +39,68 @@ class float_ : public py::object {
 };
 } // namespace external
 
+namespace implicit_conversion_from_0_to_handle {
+// Uncomment to trigger compiler error. Note: Before PR #4008 this used to compile successfully.
+// void expected_to_trigger_compiler_error() { py::handle(0); }
+} // namespace implicit_conversion_from_0_to_handle
+
+// Used to validate systematically that PR #4008 does/did NOT change the behavior.
+void pure_compile_tests_for_handle_from_PyObject_pointers() {
+    {
+        PyObject *ptr = Py_None;
+        py::handle{ptr};
+    }
+    {
+        PyObject *const ptr = Py_None;
+        py::handle{ptr};
+    }
+    // Uncomment to trigger compiler errors.
+    // PyObject const *               ptr = Py_None; py::handle{ptr};
+    // PyObject const *const          ptr = Py_None; py::handle{ptr};
+    // PyObject volatile *            ptr = Py_None; py::handle{ptr};
+    // PyObject volatile *const       ptr = Py_None; py::handle{ptr};
+    // PyObject const volatile *      ptr = Py_None; py::handle{ptr};
+    // PyObject const volatile *const ptr = Py_None; py::handle{ptr};
+}
+
+namespace handle_from_move_only_type_with_operator_PyObject {
+
+// Reduced from
+// https://github.com/pytorch/pytorch/blob/279634f384662b7c3a9f8bf7ccc3a6afd2f05657/torch/csrc/utils/object_ptr.h
+struct operator_ncnst {
+    operator_ncnst() = default;
+    operator_ncnst(operator_ncnst &&) = default;
+    operator PyObject *() /* */ { return Py_None; } // NOLINT(google-explicit-constructor)
+};
+
+struct operator_const {
+    operator_const() = default;
+    operator_const(operator_const &&) = default;
+    operator PyObject *() const { return Py_None; } // NOLINT(google-explicit-constructor)
+};
+
+bool from_ncnst() {
+    operator_ncnst obj;
+    auto h = py::handle(obj);  // Critical part of test: does this compile?
+    return h.ptr() == Py_None; // Just something.
+}
+
+bool from_const() {
+    operator_const obj;
+    auto h = py::handle(obj);  // Critical part of test: does this compile?
+    return h.ptr() == Py_None; // Just something.
+}
+
+void m_defs(py::module_ &m) {
+    m.def("handle_from_move_only_type_with_operator_PyObject_ncnst", from_ncnst);
+    m.def("handle_from_move_only_type_with_operator_PyObject_const", from_const);
+}
+
+} // namespace handle_from_move_only_type_with_operator_PyObject
+
 TEST_SUBMODULE(pytypes, m) {
+    handle_from_move_only_type_with_operator_PyObject::m_defs(m);
+
     // test_bool
     m.def("get_bool", [] { return py::bool_(false); });
     // test_int

tests/test_pytypes.py

@@ -9,6 +9,11 @@
 from pybind11_tests import pytypes as m
 
 
+def test_handle_from_move_only_type_with_operator_PyObject():  # noqa: N802
+    assert m.handle_from_move_only_type_with_operator_PyObject_ncnst()
+    assert m.handle_from_move_only_type_with_operator_PyObject_const()
+
+
 def test_bool(doc):
     assert doc(m.get_bool) == "get_bool() -> bool"