Class Is

sample/Extractor-Pattern.cpp:7–15

template <typename T>
struct Is
{
  template <typename Arg>
  Arg &&operator()(Arg &&arg) const
  {
    static_assert(std::is_same_v<T, std::decay_t<Arg>>);
    return std::forward<Arg>(arg);
  }
};

template <typename T>
inline constexpr Is<T> is;