really use the type of the returned expression

Classification/include/CGAL/Classification/Point_set_neighborhood.h

@@ -75,7 +75,7 @@ class Point_set_neighborhood
 
     // we did not put `reference` here on purpose as the recommanded default
     // is `Identity_property_map<Point_3>` and not `Identity_property_map<const Point_3>`
-    friend auto get (const My_point_property_map& ppmap, key_type i)
+    friend decltype(auto) get (const My_point_property_map& ppmap, key_type i)
     { return get(ppmap.point_map, *(ppmap.input->begin()+std::size_t(i))); }
   };
 

Property_map/include/CGAL/property_map.h

@@ -411,7 +411,7 @@ struct Property_map_to_unary_function{
   {}
 
   template <class KeyType>
-  auto
+  decltype(auto)
   operator()(const KeyType& a) const
   {
     return get(map,a);