Fix warning for non-const comparator

diff --git a/include/deal.II/matrix_free/face_setup_internal.h b/include/deal.II/matrix_free/face_setup_internal.h
index 368364ca4a053f22dcc6d24015433554b5c12c1c..8a1c71c4e4caa9e26c0f0852a562f840be1ceb8f 100644 (file)
--- a/include/deal.II/matrix_free/face_setup_internal.h
+++ b/include/deal.II/matrix_free/face_setup_internal.h
@@ -854,7 +854,7 @@ namespace internal
     struct FaceComparator
     {
       bool operator() (const FaceToCellTopology<length> &face1,
-                       const FaceToCellTopology<length> &face2)
+                       const FaceToCellTopology<length> &face2) const
       {
         for (unsigned int i=0; i<length; ++i)
           if (face1.cells_interior[i] < face2.cells_interior[i])