improve assert in FEFaceEvaluation

diff --git a/include/deal.II/matrix_free/fe_evaluation.h b/include/deal.II/matrix_free/fe_evaluation.h
index 45de596e71ddb3c9e45438ae28a1bb6e4dc0dfe0..0dd7d3d89436aff54acab092e236faf1ae32cefc 100644 (file)
--- a/include/deal.II/matrix_free/fe_evaluation.h
+++ b/include/deal.II/matrix_free/fe_evaluation.h
@@ -8493,7 +8493,10 @@ FEFaceEvaluation<dim,
       face_index <
         this->matrix_info->get_task_info().boundary_partition_data.back())
     Assert(this->is_interior_face,
-           ExcMessage("Boundary faces do not have a neighbor"));
+           ExcMessage(
+             "Boundary faces do not have a neighbor. When looping over "
+             "boundary faces use FEFaceEvaluation with the parameter "
+             "is_interior_face set to true. "));
 
   this->face_no =
     (this->is_interior_face ? faces.interior_face_no : faces.exterior_face_no);