Appiied suggestion to refactor to template sfinae

dune/istl/io.hh

@@ -425,11 +425,11 @@ namespace Dune {
    * #include <dune/istl/io.hh>
    * \endcode
    */
-  template <class MatrixType>
+  template <class MatrixType,
+    std::enable_if_t<not Dune::IsNumber<MatrixType>::value, int> = 0>
   void writeMatrixToMatlabHelper(const MatrixType& matrix,
                                  int externalRowOffset, int externalColOffset,
-                                 std::ostream& s,
-                                 [[maybe_unused]] typename std::enable_if_t<!Dune::IsNumber<MatrixType>::value>* sfinae = nullptr)
+                                 std::ostream& s)
   {
     // Precompute the accumulated sizes of the columns
     std::vector<typename MatrixType::size_type> colOffset(matrix.M());