remove void_t from concepts

dune/vtk/function.hh

@@ -6,9 +6,9 @@
 #include <dune/common/typetraits.hh>
 #include <dune/common/version.hh>
 
-#include "localfunction.hh"
-#include "types.hh"
-#include "utility/arguments.hh"
+#include <dune/vtk/localfunction.hh>
+#include <dune/vtk/types.hh>
+#include <dune/vtk/utility/arguments.hh>
 
 namespace Dune
 {
@@ -29,14 +29,14 @@ namespace Dune
       using LocalDomain = typename Element::Geometry::LocalCoordinate;
 
       template <class GF>
-      using IsGridFunction = std::void_t<decltype(localFunction(std::declval<GF>()))>;
+      using IsGridFunction = decltype(localFunction(std::declval<GF>()));
 
       template <class LocalFunction, class LF = std::decay_t<LocalFunction>>
-      using IsLocalFunction = std::void_t<decltype((
+      using IsLocalFunction = decltype((
         std::declval<LF&>().bind(std::declval<Element>()),
         std::declval<LF&>().unbind(),
         std::declval<LF>()(std::declval<LocalDomain>()),
-      0))>;
+      0));
 
       template <class F, class D>
       using Range = std::decay_t<std::result_of_t<F(D)>>;