check if Kernel provides the nested type

Frechet_distance/include/CGAL/Frechet_distance/internal/curve.h

@@ -21,6 +21,7 @@
 #include <CGAL/Interval_nt.h>
 #include <CGAL/Kernel/Type_mapper.h>
 #include <CGAL/Cartesian_converter.h>
+#include <CGAL/STL_Extension/internal/Has_nested_type_Has_filtered_predicates_tag.h>
 #include <vector>
 
 namespace CGAL {
@@ -59,14 +60,16 @@ public:
 
     static auto get_type()
     {
+      if constexpr (::CGAL::internal::Has_nested_type_Has_filtered_predicates_tag<K>::value)
+      {
         if constexpr (K::Has_filtered_predicates_tag::value) {
             return typename K::Exact_kernel{};
-        } else {
-            if constexpr (std::is_floating_point_v<typename K::FT>)
-                return CGAL::Simple_cartesian<CGAL::Exact_rational>{};
-            else
-                return K{};
         }
+      }
+      if constexpr (std::is_floating_point_v<typename K::FT>)
+          return CGAL::Simple_cartesian<CGAL::Exact_rational>{};
+      else
+          return K{};
     }
 
     using Rational_kernel = decltype(get_type());

STL_Extension/include/CGAL/STL_Extension/internal/Has_nested_type_Has_filtered_predicates_tag.h

@@ -0,0 +1,24 @@
+// Copyright (c) 2024 GeometryFactory Sarl (France)
+// All rights reserved.
+//
+// This file is part of CGAL (www.cgal.org)
+//
+// $URL$
+// $Id$
+// SPDX-License-Identifier: LGPL-3.0-or-later OR LicenseRef-Commercial
+//
+// Author(s)     : Sébastien Loriot
+
+#ifndef CGAL_HAS_NESTED_TYPE_HAS_FILTERED_PREDICATES_TAG_H
+#define CGAL_HAS_NESTED_TYPE_HAS_FILTERED_PREDICATES_TAG_H
+
+#include <boost/mpl/has_xxx.hpp>
+
+namespace CGAL {
+namespace internal {
+
+BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(Has_nested_type_Has_filtered_predicates_tag, Has_filtered_predicates_tag, false)
+
+} } // end namespace CGAL::internal
+
+#endif // CGAL_HAS_NESTED_TYPE_HAS_FILTERED_PREDICATES_TAG_H