From 3346aed52bfdaa3e384f3ca0ddf154af77657e27 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Loriot?= <sebastien.loriot@cgal.org>
Date: Tue, 17 Mar 2015 16:17:49 +0100
Subject: [PATCH] make Kernel_traits<T>::type::FT usable as return type of free
 function

in a free function that has such a return type, if there are several
overloads the return type of each should be instanciable.
This fix will make incorrect usages break later while enabling the use
in free function.
---
 Kernel_23/include/CGAL/Kernel_traits.h | 30 +++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/Kernel_23/include/CGAL/Kernel_traits.h b/Kernel_23/include/CGAL/Kernel_traits.h
index 2ab73d5a12a..0b44f743724 100644
--- a/Kernel_23/include/CGAL/Kernel_traits.h
+++ b/Kernel_23/include/CGAL/Kernel_traits.h
@@ -25,12 +25,40 @@
 #ifndef CGAL_KERNEL_TRAITS_H
 #define CGAL_KERNEL_TRAITS_H
 
+#include <boost/mpl/has_xxx.hpp>
+#include <boost/mpl/if.hpp>
+
 namespace CGAL {
 
+namespace internal_kernel_traits{
+  BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(Has_nested_R,R,false)
+
+  /// In case no specialization of CGAL::Kernel_traits is provided and the type used
+  /// is not coming from a CGAL Kernel, we provide a dummy Kernel just to break later
+  /// and only when it is used. This was introduced to avoid issues with a free function
+  /// return a nested type of the kernel through Kernel_traits. Even if the function
+  /// was not the one to consider, its return type should be instanciable.
+  template <class T>
+  struct Dummy_kernel
+  {
+    struct FT{};
+  };
+
+  template <class T, bool is_cgal_kernel = Has_nested_R<T>::value >
+  struct Kernel_traits{
+    typedef typename T::R type;
+  };
+
+  template <class T>
+  struct Kernel_traits<T, false>{
+    typedef Dummy_kernel<T> type;
+  };
+} // end of namespace internal_kernel_traits
+
 template <class T>
 struct Kernel_traits
 {
-  typedef typename T::R Kernel;
+  typedef typename internal_kernel_traits::Kernel_traits<T>::type Kernel;
   typedef Kernel type;
 };