Construct default BitVector for vectors with scalar entries

In that case the BitVector will be the input vector type, but with
entry type 'char'.

dune/solvers/common/defaultbitvector.hh

@@ -18,11 +18,18 @@ namespace Solvers {
 
 namespace Imp {
 
+/** \brief Construct a bitvector type with a nesting that matches the one of a given vector
+ *
+ * The default implementation implements the case where the vector is a number type
+ * (to end recursion), and returns 'char' in such cases.
+ *
+ * \tparam Vector The vector types whose nesting shall be matched
+ */
 template<class Vector>
 struct DefaultBitVector
 {
-  static_assert(AlwaysFalse<Vector>::value, "No DefaultBitVector known for this type.");
-  using type = void;
+  static_assert(IsNumber<Vector>::value, "No DefaultBitVector known for this type.");
+  using type = char;
 };
 
 template<class T, int i>