Assert that ignoreNodes are set before dereferencing.

dune/solvers/common/canignore.hh

@@ -3,6 +3,8 @@
 #ifndef DUNE_SOLVERS_COMMON_CANIGNORE_HH
 #define DUNE_SOLVERS_COMMON_CANIGNORE_HH
 
+#include <cassert>
+
 /** \brief Abstract base class for solvers and iterationsteps that are
  *      able to ignore degrees of freedom described by a bit field.
  */
@@ -59,6 +61,7 @@ public:
      */
     const BitVector& ignore() const
     {
+        assert(hasIgnore());
         return *ignoreNodes_;
     }