Pass arguments from trampoline methods via return_value_policy::reference

Otherwise, modifications applied by Python-coded method overrides
would be applied to copies, even though the parameters were passed
by pointer or reference.

Author: rhaschke

include/pybind11/detail/smart_holder_type_casters.h

@@ -724,7 +724,9 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
             return none().release();
         if (policy == return_value_policy::automatic)
             policy = return_value_policy::reference_internal;
-        if (policy != return_value_policy::reference_internal)
+        else if (policy == return_value_policy::reference && !parent)
+            ; // passing from trampoline dispatcher: no parent available
+        else if (policy != return_value_policy::reference_internal)
             throw cast_error(
                 "Invalid return_value_policy: unique_ptr const& expects reference_internal");
         return smart_holder_type_caster<T>::cast(src.get(), policy, parent);

include/pybind11/pybind11.h

@@ -2243,7 +2243,7 @@ template <class T> function get_override(const T *this_ptr, const char *name) {
         pybind11::gil_scoped_acquire gil; \
         pybind11::function override = pybind11::get_override(static_cast<const cname *>(this), name); \
         if (override) { \
-            auto o = override(__VA_ARGS__); \
+            auto o = override.operator()<pybind11::return_value_policy::reference>(__VA_ARGS__); \
             if (pybind11::detail::cast_is_temporary_value_reference<ret_type>::value) { \
                 static pybind11::detail::override_caster_t<ret_type> caster; \
                 return pybind11::detail::cast_ref<ret_type>(std::move(o), caster); \