removes std::invoke as we're rolling back to c++1y

value.hpp

@@ -116,7 +116,7 @@ std::vector<R> t2v(const std::tuple<ArgTypes...>& t){
 namespace detail {
     template <class F, class Tuple, std::size_t... I>
     constexpr decltype(auto) call_impl(F &&f, Tuple &&t, std::index_sequence<I...>){
-        return std::invoke(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...);
+        return f(std::get<I>(std::forward<Tuple>(t))...);
     }
 }