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/pytypes.h

@@ -105,9 +105,9 @@ class object_api : public pyobject_tag {
         function will throw a `cast_error` exception. When the Python function
         call fails, a `error_already_set` exception is thrown.
     \endrst */
-    template <return_value_policy policy = return_value_policy::automatic_reference, typename... Args>
+    template <return_value_policy policy = return_value_policy::reference, typename... Args>
     object operator()(Args &&...args) const;
-    template <return_value_policy policy = return_value_policy::automatic_reference, typename... Args>
+    template <return_value_policy policy = return_value_policy::reference, typename... Args>
     PYBIND11_DEPRECATED("call(...) was deprecated in favor of operator()(...)")
         object call(Args&&... args) const;