WIP: invoke c++17 rework

include/utl/meta/typelist.h

@@ -265,7 +265,7 @@ namespace meta {
    //! @{
    namespace back_impl {
       template <typename List>
-      struct back_ { };
+      struct back_ { using type = nil_; };
 
       template <typename Head, typename... Tail>
       struct back_<typelist<Head, Tail...>> {

include/utl/utility/invoke.h

@@ -21,7 +21,7 @@
 //! @{
 namespace utl {
 
-#if !defined __cpp_lib_is_invocable
+//#if !defined __cpp_lib_is_invocable
    namespace detail {
 
       template <class T>
@@ -216,9 +216,10 @@ namespace utl {
    >;
    //! @}
 
-#else
-
-#endif
+//#else
+//using is_invocable = std::is_invocable;
+//
+//#endif
 }
 
 //! @}