message: Bring back lost meta function
diff: @@ -511,6 +511,11 @@ struct tl_exists {
  Pred<tl_head_t<List>>::value || tl_exists<tl_tail_t<List>, Pred>::value;
  };
  +template <template <class> class Pred>
  +struct tl_exists<empty_type_list, Pred> {
  + static constexpr bool value = false;
  +};
  +
  // Uncomment after having switched to C++14
  //template <class List, template <class> class Pred>
  //inline constexpr bool tl_exists_v = tl_exists<List, Pred>::value;
  