make compilation more explicit when Eigen < 3.2 is used

Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/fair.h

@@ -29,6 +29,7 @@
 #include <CGAL/Eigen_solver_traits.h>  // for sparse linear system solver
 #endif
 
+#include <boost/type_traits/is_same.hpp>
 
 namespace CGAL {
 
@@ -122,6 +123,12 @@ namespace internal {
       //if no solver is provided and Eigen version < 3.2
 #endif
 
+    typedef typename GetSolver<NamedParameters, Default_solver>::type SparseLinearSolver;
+    if (boost::is_same<SparseLinearSolver, bool>::value)
+    {
+      BOOST_STATIC_ASSERT_MSG(EIGEN_VERSION_AT_LEAST(3, 2, 0),
+                              "The function `fair` requires Eigen3 version 3.2 or later.");
+    }
     typedef typename GetVertexPointMap < PolygonMesh, NamedParameters>::type VPMap;
     typedef CGAL::internal::Cotangent_weight_with_voronoi_area_fairing<PolygonMesh, VPMap>
       Default_Weight_calculator;

Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/internal/named_params_helper.h

@@ -100,6 +100,17 @@ public:
   > ::type  type;
 };
 
+template<typename NamedParameters, typename DefaultSolver>
+class GetSolver
+{
+public:
+  typedef typename boost::lookup_named_param_def <
+    CGAL::sparse_linear_solver_t,
+    NamedParameters,
+    DefaultSolver
+  > ::type type;
+};
+
 
 
 #endif //CGAL_NAMED_PARAMETERS_HELPERS_H