Fix roundtrip unit test for derived Python objects

As expected, the unit test was failing because args were passed by-value
and not by reference. However, it wasn't possible to just change the policy
to reference or reference_internal.
reference gave errors, because const unique_ptr& only accepts reference_internal.
reference_internal gives an error, because the parent required for keep_alive is null.
Hence, I decided to introduce a new policy to explicitly indicate passing from
from Python to C++ via an overridden method. That's the cleanest solution to correctly
react to the special needs of this use case.

Author: rhaschke

include/pybind11/detail/common.h

@@ -434,7 +434,11 @@ enum class return_value_policy : uint8_t {
         collected while Python is still using the child. More advanced
         variations of this scheme are also possible using combinations of
         return_value_policy::reference and the keep_alive call policy */
-    reference_internal
+    reference_internal,
+
+    /*  This internally-only used policy applies to C++ arguments passed
+        to virtual methods overridden in Python to allow reference passing. */
+    automatic_override
 };
 
 PYBIND11_NAMESPACE_BEGIN(detail)

include/pybind11/detail/smart_holder_type_casters.h

@@ -472,6 +472,10 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
     }
 
     static handle cast(T const &src, return_value_policy policy, handle parent) {
+        return cast(const_cast<T &>(src), policy, parent);
+    }
+
+    static handle cast(T &src, return_value_policy policy, handle parent) {
         // type_caster_base BEGIN
         // clang-format off
         if (policy == return_value_policy::automatic || policy == return_value_policy::automatic_reference)
@@ -481,11 +485,13 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
         // type_caster_base END
     }
 
-    static handle cast(T &src, return_value_policy policy, handle parent) {
-        return cast(const_cast<T const &>(src), policy, parent); // Mutbl2Const
+    static handle cast(T const *src, return_value_policy policy, handle parent) {
+        return cast(const_cast<T *>(src), policy, parent);
     }
 
-    static handle cast(T const *src, return_value_policy policy, handle parent) {
+    static handle cast(T *src, return_value_policy policy, handle parent) {
+        if (policy == return_value_policy::automatic_override)
+            policy = return_value_policy::reference;
         auto st = type_caster_base<T>::src_and_type(src);
         return cast_const_raw_ptr( // Originally type_caster_generic::cast.
             st.first,
@@ -496,10 +502,6 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
             make_constructor::make_move_constructor(src));
     }
 
-    static handle cast(T *src, return_value_policy policy, handle parent) {
-        return cast(const_cast<T const *>(src), policy, parent); // Mutbl2Const
-    }
-
 #if defined(_MSC_VER) && _MSC_VER < 1910
     // Working around MSVC 2015 bug. const-correctness is lost.
     // SMART_HOLDER_WIP: IMPROVABLE: make common code work with MSVC 2015.
@@ -723,7 +725,9 @@ 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::automatic_override)
+            ;
+        else if (policy != return_value_policy::reference_internal)
             throw cast_error("Invalid return_value_policy for unique_ptr&");
         return smart_holder_type_caster<T>::cast(src.get(), policy, parent);
     }

include/pybind11/detail/type_caster_base.h

@@ -899,6 +899,8 @@ template <typename type> class type_caster_base : public type_caster_generic {
     }
 
     static handle cast(const itype *src, return_value_policy policy, handle parent) {
+        if (policy == return_value_policy::automatic_override)
+            policy = return_value_policy::reference;
         auto st = src_and_type(src);
         return type_caster_generic::cast(
             st.first, policy, parent, st.second,

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::automatic_override, 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::automatic_override, typename... Args>
     PYBIND11_DEPRECATED("call(...) was deprecated in favor of operator()(...)")
         object call(Args&&... args) const;