one_of.h

#ifndef ONE_OF_H
#define ONE_OF_H

#include <tuple>
#include <utility>

template <class... T>
struct one_of {
  std::tuple<T...> t;
  constexpr one_of(T&&... values) : t{std::forward<T>(values)...} {}

  template <class U>
  constexpr bool operator==(U&& v) const&& {
    return comp_impl(std::forward<U>(v), t,
                     std::make_index_sequence<sizeof...(T)>{});
  }

 private:
  template <class U, class Tup, std::size_t... I>
  constexpr inline static bool comp_impl(U&& v, const Tup& tup,
                                         std::index_sequence<I...>) {
    return ((std::forward<U>(v) == std::get<I>(tup)) || ...);
  }
};

#endif  // ONE_OF_H