diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_curve_data_traits_2.h b/Arrangement_on_surface_2/include/CGAL/Arr_curve_data_traits_2.h
index 6fbf49afb5eb..b4ecc559815d 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_curve_data_traits_2.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_curve_data_traits_2.h
@@ -248,7 +248,6 @@ class Arr_curve_data_traits_2 : public Traits_ {
     const Base_traits_2& m_base;
 
     template <typename T>
-    bool are_mergeable_data(const T& cv1, const T& cv2, long) const {
     bool are_mergeable_data(const T& /* cv1 */, const T& /* cv2 */, long) const {
       CGAL_error_msg("Equality operator is not supported.");
       return false;
diff --git a/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h b/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h
index 4dad5388eaa2..35c1c0ebcb88 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arrangement_on_surface_2.h
@@ -39,7 +39,7 @@
 #include <CGAL/HalfedgeDS_iterator.h>
 #include <CGAL/Arrangement_2/Arrangement_2_iterators.h>
 #include <CGAL/In_place_list.h>
-#include <CGAL/Arr_observer.h>
+#include <CGAL/Aos_observer.h>
 #include <CGAL/Arr_accessor.h>
 #include <CGAL/Arrangement_2/Arr_traits_adaptor_2.h>
 #include <CGAL/function_objects.h>
diff --git a/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h b/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h
index 539025ee9ac3..2bc19cf65c70 100644
--- a/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h
+++ b/Envelope_3/include/CGAL/Envelope_3/Envelope_divide_and_conquer_3.h
@@ -113,8 +113,7 @@ class Envelope_divide_and_conquer_3 {
   using Halfedge_around_vertex_circulator =
     typename Minimization_diagram_2::Halfedge_around_vertex_circulator;
 
-  using Md_observer = Arr_observer<Minimization_diagram_2>;
-
+  using Md_observer = typename Minimization_diagram_2::Observer;
   using Face = typename Minimization_diagram_2::Face;
   using Envelope_data_iterator = typename Face::Data_iterator;