Fix py::cast from pytype rvalue to pytype (#3949)

* Fix py::cast from pytype rvalue to pytype

Previously, py::cast blindly assumed that the destination type was a C++
type rather than a python type when the source type was an rvalue.

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/pybind11/cast.h

@@ -1009,6 +1009,8 @@ struct return_value_policy_override<
 // Basic python -> C++ casting; throws if casting fails
 template <typename T, typename SFINAE>
 type_caster<T, SFINAE> &load_type(type_caster<T, SFINAE> &conv, const handle &handle) {
+    static_assert(!detail::is_pyobject<T>::value,
+                  "Internal error: type_caster should only be used for C++ types");
     if (!conv.load(handle, true)) {
 #if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
         throw cast_error("Unable to cast Python instance to C++ type (#define "
@@ -1099,21 +1101,30 @@ detail::enable_if_t<!detail::move_never<T>::value, T> move(object &&obj) {
 // - If both movable and copyable, check ref count: if 1, move; otherwise copy
 // - Otherwise (not movable), copy.
 template <typename T>
-detail::enable_if_t<detail::move_always<T>::value, T> cast(object &&object) {
+detail::enable_if_t<!detail::is_pyobject<T>::value && detail::move_always<T>::value, T>
+cast(object &&object) {
     return move<T>(std::move(object));
 }
 template <typename T>
-detail::enable_if_t<detail::move_if_unreferenced<T>::value, T> cast(object &&object) {
+detail::enable_if_t<!detail::is_pyobject<T>::value && detail::move_if_unreferenced<T>::value, T>
+cast(object &&object) {
     if (object.ref_count() > 1) {
         return cast<T>(object);
     }
     return move<T>(std::move(object));
 }
 template <typename T>
-detail::enable_if_t<detail::move_never<T>::value, T> cast(object &&object) {
+detail::enable_if_t<!detail::is_pyobject<T>::value && detail::move_never<T>::value, T>
+cast(object &&object) {
     return cast<T>(object);
 }
 
+// pytype rvalue -> pytype (calls converting constructor)
+template <typename T>
+detail::enable_if_t<detail::is_pyobject<T>::value, T> cast(object &&object) {
+    return T(std::move(object));
+}
+
 template <typename T>
 T object::cast() const & {
     return pybind11::cast<T>(*this);

tests/test_copy_move.cpp

@@ -289,4 +289,7 @@ TEST_SUBMODULE(copy_move_policies, m) {
         py::return_value_policy::move);
     m.def(
         "get_moveissue2", [](int i) { return MoveIssue2(i); }, py::return_value_policy::move);
+
+    // Make sure that cast from pytype rvalue to other pytype works
+    m.def("get_pytype_rvalue_castissue", [](double i) { return py::float_(i).cast<py::int_>(); });
 }

tests/test_copy_move.py

@@ -123,3 +123,10 @@ def test_move_fallback():
     assert m1.value == 1
     m2 = m.get_moveissue2(2)
     assert m2.value == 2
+
+
+def test_pytype_rvalue_cast():
+    """Make sure that cast from pytype rvalue to other pytype works"""
+
+    value = m.get_pytype_rvalue_castissue(1.0)
+    assert value == 1