Get rid off unused-variable-warnings.

dune/istl/io.hh

@@ -407,11 +407,11 @@ namespace Dune {
    *
    * This specialization for numbers ends the recursion
    */
-  template <class FieldType>
+  template <class FieldType,
+    std::enable_if_t<Dune::IsNumber<FieldType>::value, int> = 0>
   void writeMatrixToMatlabHelper(const FieldType& value,
                                  int rowOffset, int colOffset,
-                                 std::ostream& s,
-                                 [[maybe_unused]] typename std::enable_if_t<Dune::IsNumber<FieldType>::value>* sfinae = nullptr)
+                                 std::ostream& s)
   {
     //+1 for Matlab numbering
     s << rowOffset + 1 << " " << colOffset + 1 << " ";