Revert the merge of PR #2478

This reverts the merge commit 46cc91e787 onto
488c3e2879.

Cartesian_kernel/include/CGAL/Cartesian/function_objects.h

@@ -1043,6 +1043,7 @@ namespace CartesianKernelFunctors {
 
   };
 
+  // TODO ...
   template <typename K>
   class Compute_squared_radius_2
   {
@@ -1050,21 +1051,29 @@ namespace CartesianKernelFunctors {
     typedef typename K::Point_2     Point_2;
     typedef typename K::Circle_2    Circle_2;
   public:
-    typedef FT                      result_type;
+    template<class>
+    struct result {
+      typedef FT type;
+    };
+    
+    template<typename F>
+    struct result<F(Circle_2)> {
+      typedef const FT& type;
+    };
 
-    result_type
+    const FT&
     operator()( const Circle_2& c) const
     { return c.rep().squared_radius(); }
 
-    result_type
+    FT
     operator()( const Point_2& /*p*/) const
     { return FT(0); }
 
-    result_type
+    FT
     operator()( const Point_2& p, const Point_2& q) const
     { return squared_radiusC2(p.x(), p.y(), q.x(), q.y()); }
 
-    result_type
+    FT
     operator()( const Point_2& p, const Point_2& q, const Point_2& r) const
     { return squared_radiusC2(p.x(), p.y(), q.x(), q.y(), r.x(), r.y()); }
   };