Appiied suggestion to refactor to template sfinae

dune/istl/io.hh

@@ -168,11 +168,11 @@ namespace Dune {
    * #include <dune/istl/io.hh>
    * \endcode
    */
-  template<class M>
+  template<class M,
+    std::enable_if_t<not Dune::IsNumber<M>::value, int> = 0>
   void print_row (std::ostream& s, const M& A, typename M::size_type I,
                   typename M::size_type J, typename M::size_type therow,
-                  int width, int precision,
-                  [[maybe_unused]] typename std::enable_if_t<!Dune::IsNumber<M>::value>* sfinae = nullptr)
+                  int width, int precision)
   {
     typename M::size_type i0=I;
     for (typename M::size_type i=0; i<A.N(); i++)