Ref<> objects must be nested by reference because they potentially store a temporary object

(transplanted from 6719e56b5b
)

test/ref.cpp

@@ -14,9 +14,9 @@
 
 static int nb_temporaries;
 
-inline void on_temporary_creation(int size) {
+inline void on_temporary_creation(int) {
   // here's a great place to set a breakpoint when debugging failures in this test!
-  if(size!=0) nb_temporaries++;
+  nb_temporaries++;
 }