fix: use std::addressof in type_caster_base.h (#5189)

* Add tests only. Fails to build with this error message:

```
g++ -o pybind11/tests/test_copy_move.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/python3.11 -isystem /usr/include/eigen3 -DPYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX -DPYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD_IF_AVAILABLE -DPYBIND11_TEST_BOOST -DPYBIND11_INTERNALS_VERSION=10000000 -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_copy_move.cpp
In file included from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../cast.h:15,
                 from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../attr.h:14,
                 from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/class.h:12,
                 from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/pybind11.h:13,
                 from /usr/local/google/home/rwgk/forked/pybind11/include/pybind11/stl.h:12,
                 from /usr/local/google/home/rwgk/forked/pybind11/tests/test_copy_move.cpp:11:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h: In instantiation of ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(const itype&, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../cast.h:1230:37:   required from ‘pybind11::object pybind11::cast(T&&, return_value_policy, handle) [with T = const UnusualOpRef&; typename std::enable_if<(! std::is_base_of<detail::pyobject_tag, typename std::remove_reference<_Tp>::type>::value), int>::type <anonymous> = 0]’
/usr/local/google/home/rwgk/forked/pybind11/tests/test_copy_move.cpp:162:80:   required from here
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1110:20: error: no matching function for call to ‘pybind11::detail::type_caster_base<UnusualOpRef>::cast(const UnusualOpRef::NonTrivialType, pybind11::return_value_policy&, pybind11::handle&)’
 1110 |         return cast(&src, policy, parent);
      |                ~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1105:19: note: candidate: ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(const itype&, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’
 1105 |     static handle cast(const itype &src, return_value_policy policy, handle parent) {
      |                   ^~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1105:37: note:   no known conversion for argument 1 from ‘const UnusualOpRef::NonTrivialType’ {aka ‘const std::shared_ptr<int>’} to ‘const pybind11::detail::type_caster_base<UnusualOpRef>::itype&’ {aka ‘const UnusualOpRef&’}
 1105 |     static handle cast(const itype &src, return_value_policy policy, handle parent) {
      |                        ~~~~~~~~~~~~~^~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1113:19: note: candidate: ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(itype&&, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’
 1113 |     static handle cast(itype &&src, return_value_policy, handle parent) {
      |                   ^~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1113:32: note:   no known conversion for argument 1 from ‘const UnusualOpRef::NonTrivialType’ {aka ‘const std::shared_ptr<int>’} to ‘pybind11::detail::type_caster_base<UnusualOpRef>::itype&&’ {aka ‘UnusualOpRef&&’}
 1113 |     static handle cast(itype &&src, return_value_policy, handle parent) {
      |                        ~~~~~~~~^~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1142:19: note: candidate: ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(const itype*, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’
 1142 |     static handle cast(const itype *src, return_value_policy policy, handle parent) {
      |                   ^~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1142:37: note:   no known conversion for argument 1 from ‘const UnusualOpRef::NonTrivialType’ {aka ‘const std::shared_ptr<int>’} to ‘const pybind11::detail::type_caster_base<UnusualOpRef>::itype*’ {aka ‘const UnusualOpRef*’}
 1142 |     static handle cast(const itype *src, return_value_policy policy, handle parent) {
      |                        ~~~~~~~~~~~~~^~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h: In instantiation of ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(itype&&, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../cast.h:1230:37:   required from ‘pybind11::object pybind11::cast(T&&, return_value_policy, handle) [with T = UnusualOpRef; typename std::enable_if<(! std::is_base_of<detail::pyobject_tag, typename std::remove_reference<_Tp>::type>::value), int>::type <anonymous> = 0]’
/usr/local/google/home/rwgk/forked/pybind11/tests/test_copy_move.cpp:163:74:   required from here
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1114:20: error: no matching function for call to ‘pybind11::detail::type_caster_base<UnusualOpRef>::cast(UnusualOpRef::NonTrivialType, pybind11::return_value_policy, pybind11::handle&)’
 1114 |         return cast(&src, return_value_policy::move, parent);
      |                ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1105:19: note: candidate: ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(const itype&, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’
 1105 |     static handle cast(const itype &src, return_value_policy policy, handle parent) {
      |                   ^~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1105:37: note:   no known conversion for argument 1 from ‘UnusualOpRef::NonTrivialType’ {aka ‘std::shared_ptr<int>’} to ‘const pybind11::detail::type_caster_base<UnusualOpRef>::itype&’ {aka ‘const UnusualOpRef&’}
 1105 |     static handle cast(const itype &src, return_value_policy policy, handle parent) {
      |                        ~~~~~~~~~~~~~^~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1113:19: note: candidate: ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(itype&&, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’
 1113 |     static handle cast(itype &&src, return_value_policy, handle parent) {
      |                   ^~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1113:32: note:   no known conversion for argument 1 from ‘UnusualOpRef::NonTrivialType’ {aka ‘std::shared_ptr<int>’} to ‘pybind11::detail::type_caster_base<UnusualOpRef>::itype&&’ {aka ‘UnusualOpRef&&’}
 1113 |     static handle cast(itype &&src, return_value_policy, handle parent) {
      |                        ~~~~~~~~^~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1142:19: note: candidate: ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast(const itype*, pybind11::return_value_policy, pybind11::handle) [with type = UnusualOpRef; itype = UnusualOpRef]’
 1142 |     static handle cast(const itype *src, return_value_policy policy, handle parent) {
      |                   ^~~~
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/detail/../detail/type_caster_base.h:1142:37: note:   no known conversion for argument 1 from ‘UnusualOpRef::NonTrivialType’ {aka ‘std::shared_ptr<int>’} to ‘const pybind11::detail::type_caster_base<UnusualOpRef>::itype*’ {aka ‘const UnusualOpRef*’}
 1142 |     static handle cast(const itype *src, return_value_policy policy, handle parent) {
      |                        ~~~~~~~~~~~~~^~~
```

* Replace `&src` with `std::addressof(src)` to fix the error building the added tests.

* Fix accident (not sure how the `const` slipped in here when transferring the diff from pybind11k).

include/pybind11/detail/type_caster_base.h

@@ -1107,11 +1107,11 @@ class type_caster_base : public type_caster_generic {
             || policy == return_value_policy::automatic_reference) {
             policy = return_value_policy::copy;
         }
-        return cast(&src, policy, parent);
+        return cast(std::addressof(src), policy, parent);
     }
 
     static handle cast(itype &&src, return_value_policy, handle parent) {
-        return cast(&src, return_value_policy::move, parent);
+        return cast(std::addressof(src), return_value_policy::move, parent);
     }
 
     // Returns a (pointer, type_info) pair taking care of necessary type lookup for a

tests/pybind11_tests.h

@@ -3,6 +3,8 @@
 #include <pybind11/eval.h>
 #include <pybind11/pybind11.h>
 
+#include <memory>
+
 namespace py = pybind11;
 using namespace pybind11::literals;
 
@@ -52,6 +54,17 @@ union IntFloat {
     float f;
 };
 
+class UnusualOpRef {
+public:
+    using NonTrivialType = std::shared_ptr<int>; // Almost any non-trivial type will do.
+    // Overriding operator& should not break pybind11.
+    NonTrivialType operator&() { return non_trivial_member; }
+    NonTrivialType operator&() const { return non_trivial_member; }
+
+private:
+    NonTrivialType non_trivial_member;
+};
+
 /// Custom cast-only type that casts to a string "rvalue" or "lvalue" depending on the cast
 /// context. Used to test recursive casters (e.g. std::tuple, stl containers).
 struct RValueCaster {};

tests/test_copy_move.cpp

@@ -157,6 +157,13 @@ struct type_caster<CopyOnlyInt> {
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(pybind11)
 
+namespace {
+
+py::object CastUnusualOpRefConstRef(const UnusualOpRef &cref) { return py::cast(cref); }
+py::object CastUnusualOpRefMovable(UnusualOpRef &&mvbl) { return py::cast(std::move(mvbl)); }
+
+} // namespace
+
 TEST_SUBMODULE(copy_move_policies, m) {
     // test_lacking_copy_ctor
     py::class_<lacking_copy_ctor>(m, "lacking_copy_ctor")
@@ -293,6 +300,11 @@ TEST_SUBMODULE(copy_move_policies, m) {
 
     // 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_>(); });
+
+    py::class_<UnusualOpRef>(m, "UnusualOpRef");
+    m.def("CallCastUnusualOpRefConstRef",
+          []() { return CastUnusualOpRefConstRef(UnusualOpRef()); });
+    m.def("CallCastUnusualOpRefMovable", []() { return CastUnusualOpRefMovable(UnusualOpRef()); });
 }
 
 /*

tests/test_copy_move.py

@@ -132,3 +132,9 @@ def test_pytype_rvalue_cast():
 
     value = m.get_pytype_rvalue_castissue(1.0)
     assert value == 1
+
+
+def test_unusual_op_ref():
+    # Merely to test that this still exists and built successfully.
+    assert m.CallCastUnusualOpRefConstRef().__class__.__name__ == "UnusualOpRef"
+    assert m.CallCastUnusualOpRefMovable().__class__.__name__ == "UnusualOpRef"