Add T cast(object &&obj) overload as suggested by @Skylion007

The original suggestion leads to `error: call to 'cast' is ambiguous` (full error message below), therefore SFINAE guarding is needed.

```
clang++ -o pybind11/tests/test_type_caster_pyobject_ptr.os -c -std=c++17 -fPIC -fvisibility=hidden -O0 -g -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -Wunused-result -Werror -isystem /usr/include/python3.10 -isystem /usr/include/eigen3 -DPYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX -DPYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD_IF_AVAILABLE -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/local/google/home/rwgk/forked/pybind11/include -I/usr/local/google/home/rwgk/clone/pybind11/include /usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_pyobject_ptr.cpp
In file included from /usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_pyobject_ptr.cpp:1:
In file included from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/functional.h:12:
In file included from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/pybind11.h:13:
In file included from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/class.h:12:
In file included from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/attr.h:14:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/cast.h:1165:12: error: call to 'cast' is ambiguous
    return pybind11::cast<T>(std::move(*this));
           ^~~~~~~~~~~~~~~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/functional.h:109:70: note: in instantiation of function template specialization 'pybind11::object::cast<_object *>' requested here
                return hfunc.f(std::forward<Args>(args)...).template cast<Return>();
                                                                     ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/functional.h:103:16: note: in instantiation of member function 'pybind11::detail::type_caster<std::function<_object *(int)>>::load(pybind11::handle, bool)::func_wrapper::operator()' requested here
        struct func_wrapper {
               ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/cast.h:1456:47: note: in instantiation of member function 'pybind11::detail::type_caster<std::function<_object *(int)>>::load' requested here
        if ((... || !std::get<Is>(argcasters).load(call.args[Is], call.args_convert[Is]))) {
                                              ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/cast.h:1434:50: note: in instantiation of function template specialization 'pybind11::detail::argument_loader<const std::function<_object *(int)> &, int>::load_impl_sequence<0UL, 1UL>' requested here
    bool load_args(function_call &call) { return load_impl_sequence(call, indices{}); }
                                                 ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/pybind11.h:227:33: note: in instantiation of member function 'pybind11::detail::argument_loader<const std::function<_object *(int)> &, int>::load_args' requested here
            if (!args_converter.load_args(call)) {
                                ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/pybind11.h:101:9: note: in instantiation of function template specialization 'pybind11::cpp_function::initialize<(lambda at /usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_pyobject_ptr.cpp:50:9), _object *, const std::function<_object *(int)> &, int, pybind11::name, pybind11::scope, pybind11::sibling, pybind11::return_value_policy>' requested here
        initialize(
        ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/pybind11.h:1163:22: note: in instantiation of function template specialization 'pybind11::cpp_function::cpp_function<(lambda at /usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_pyobject_ptr.cpp:50:9), pybind11::name, pybind11::scope, pybind11::sibling, pybind11::return_value_policy, void>' requested here
        cpp_function func(std::forward<Func>(f),
                     ^
/usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_pyobject_ptr.cpp:48:7: note: in instantiation of function template specialization 'pybind11::module_::def<(lambda at /usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_pyobject_ptr.cpp:50:9), pybind11::return_value_policy>' requested here
    m.def(
      ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/cast.h:1077:3: note: candidate function [with T = _object *, $1 = 0]
T cast(object &&obj) {
  ^
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/cast.h:1149:1: note: candidate function [with T = _object *]
cast(object &&object) {
^
1 error generated.
```

Author: rwgk

include/pybind11/cast.h

@@ -1068,10 +1068,24 @@ T cast(const handle &handle) {
 // It is the responsibility of the caller to ensure that the reference count
 // is decremented.
 template <typename T,
-          detail::enable_if_t<detail::is_same_ignoring_cvref<T, PyObject *>::value, int> = 0>
-T cast(const handle &handle) {
+          typename Handle,
+          detail::enable_if_t<detail::is_same_ignoring_cvref<T, PyObject *>::value
+                                  && detail::is_same_ignoring_cvref<Handle, handle>::value,
+                              int>
+          = 0>
+T cast(Handle &&handle) {
     return handle.inc_ref().ptr();
 }
+// To optimize way an inc_ref/dec_ref cycle:
+template <typename T,
+          typename Object,
+          detail::enable_if_t<detail::is_same_ignoring_cvref<T, PyObject *>::value
+                                  && detail::is_same_ignoring_cvref<Object, object>::value,
+                              int>
+          = 0>
+T cast(Object &&obj) {
+    return obj.release().ptr();
+}
 
 // C++ type -> py::object
 template <typename T, detail::enable_if_t<!detail::is_pyobject<T>::value, int> = 0>

tests/test_type_caster_pyobject_ptr.cpp

@@ -25,7 +25,7 @@ TEST_SUBMODULE(type_caster_pyobject_ptr, m) {
         PyObject *ptr = PyLong_FromLongLong(6758L);
         return py::cast(ptr, py::return_value_policy::take_ownership);
     });
-    m.def("cast_to_pyobject_ptr", [](py::handle obj) {
+    m.def("cast_handle_to_pyobject_ptr", [](py::handle obj) {
         auto rc1 = obj.ref_count();
         auto *ptr = py::cast<PyObject *>(obj);
         auto rc2 = obj.ref_count();
@@ -34,6 +34,27 @@ TEST_SUBMODULE(type_caster_pyobject_ptr, m) {
         }
         return 100 - py::reinterpret_steal<py::object>(ptr).attr("value").cast<int>();
     });
+    m.def("cast_object_to_pyobject_ptr", [](py::object obj) {
+        py::handle hdl = obj;
+        auto rc1 = hdl.ref_count();
+        auto *ptr = py::cast<PyObject *>(std::move(obj));
+        auto rc2 = hdl.ref_count();
+        if (rc2 != rc1) {
+            return -1;
+        }
+        return 300 - py::reinterpret_steal<py::object>(ptr).attr("value").cast<int>();
+    });
+    m.def("cast_list_to_pyobject_ptr", [](py::list lst) {
+        // This is to cover types implicitly convertible to object.
+        py::handle hdl = lst;
+        auto rc1 = hdl.ref_count();
+        auto *ptr = py::cast<PyObject *>(std::move(lst));
+        auto rc2 = hdl.ref_count();
+        if (rc2 != rc1) {
+            return -1;
+        }
+        return 400 - static_cast<int>(py::len(py::reinterpret_steal<py::list>(ptr)));
+    });
 
     m.def(
         "return_pyobject_ptr",

tests/test_type_caster_pyobject_ptr.py

@@ -13,8 +13,16 @@ def test_cast_from_pyobject_ptr():
     assert m.cast_from_pyobject_ptr() == 6758
 
 
-def test_cast_to_pyobject_ptr():
-    assert m.cast_to_pyobject_ptr(ValueHolder(24)) == 76
+def test_cast_handle_to_pyobject_ptr():
+    assert m.cast_handle_to_pyobject_ptr(ValueHolder(24)) == 76
+
+
+def test_cast_object_to_pyobject_ptr():
+    assert m.cast_object_to_pyobject_ptr(ValueHolder(43)) == 257
+
+
+def test_cast_list_to_pyobject_ptr():
+    assert m.cast_list_to_pyobject_ptr([1, 2, 3, 4, 5]) == 395
 
 
 def test_return_pyobject_ptr():