namespace std {
  template<class CompletionFunction = /* see description */>
    class barrier;
}

namespace std {
  template<class CompletionFunction = /* see description */>
  class barrier {
  public:
    using arrival_token =  /* see description */;
 
    static constexpr ptrdiff_t max() noexcept;
 
    constexpr explicit barrier(ptrdiff_t expected,
                               CompletionFunction f = CompletionFunction());
    ~barrier();
 
    barrier(const barrier&) = delete;
    barrier& operator=(const barrier&) = delete;
 
    [[nodiscard]] arrival_token arrive(ptrdiff_t update = 1);
    void wait(arrival_token&& arrival) const;
 
    void arrive_and_wait();
    void arrive_and_drop();
 
  private:
    CompletionFunction completion;      // exposition only
  };
}