namespace std {
  // class template generator
  template<class Ref, class V = void, class Allocator = void>
  class generator;
 
  namespace pmr {
    template<class Ref, class V = void>
    using generator = std::generator<Ref, V, polymorphic_allocator<>>;
  }
}

namespace std {
  template<class Ref, class V = void, class Allocator = void>
  class generator : public ranges::view_interface<generator<Ref, V, Allocator>> {
  private:
    using value = conditional_t<is_void_v<V>, 
                                remove_cvref_t< Ref >, V>;          // exposition only
    using reference = conditional_t<is_void_v<V>, Ref&&, Ref>;      // exposition only
 
    // class generator::iterator
    class iterator;                                                 // exposition only
 
  public:
    using yielded =
      conditional_t<is_reference_v<reference>, reference, const reference&>;
 
    // class generator::promise_type
    class promise_type;
 
    generator(const generator&) = delete;
    generator(generator&& other) noexcept;
 
    ~generator();
 
    generator& operator=(generator other) noexcept;
 
    iterator begin();
    default_sentinel_t end() const noexcept;
 
  private:
    coroutine_handle<promise_type> coroutine_ = nullptr;  // exposition only
    unique_ptr<stack<coroutine_handle<>>> active_;        // exposition only
  };
}

namespace std {
  template<class Ref, class V, class Allocator>
  class generator<Ref, V, Allocator>::promise_type {
  public:
    generator get_return_object() noexcept;
 
    suspend_always initial_suspend() const noexcept { return {}; }
    auto final_suspend() noexcept;
 
    suspend_always yield_value(yielded val) noexcept;
 
    auto yield_value(const remove_reference_t<yielded>& lval)
      requires is_rvalue_reference_v<yielded> &&
        constructible_from<remove_cvref_t<yielded>, const remove_reference_t<yielded>&>;
 
    template<class R2, class V2, class Alloc2, class Unused>
      requires same_as<typename generator<T2, V2, Alloc2>::yielded, yielded>
      auto yield_value(ranges::elements_of<generator<T2, V2, Alloc2>&&,
                       Unused> g) noexcept;
 
    template<ranges::input_range R, class Alloc>
      requires convertible_to<ranges::range_reference_t<R>, yielded>
      auto yield_value(ranges::elements_of<R, Alloc> r) noexcept;
 
    void await_transform() = delete;
 
    void return_void() const noexcept {}
    void unhandled_exception();
 
    void* operator new(size_t size)
      requires same_as<Allocator, void> || default_initializable<Allocator>;
 
    template<class Alloc, class... Args>
      requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>
      void* operator new(size_t size, allocator_arg_t, const Alloc& alloc,
                         const Args&...);
 
    template<class This, class Alloc, class... Args>
      requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>
      void* operator new(size_t size, const This&, allocator_arg_t, const Alloc& alloc,
                         const Args&...);
 
    void operator delete(void* pointer, size_t size) noexcept;
 
  private:
    add_pointer_t<yielded> value_ = nullptr;    // exposition only
    exception_ptr except_;                      // exposition only
  };
}

namespace std {
  template<class Ref, class V, class Allocator>
  class generator<Ref, V, Allocator>::iterator {
  public:
    using value_type = value;
    using difference_type = ptrdiff_t;
 
    iterator(iterator&& other) noexcept;
    iterator& operator=(iterator&& other) noexcept;
 
    reference operator*() const noexcept(is_nothrow_copy_constructible_v<reference>);
    iterator& operator++();
    void operator++(int);
 
    friend bool operator==(iterator i, default_sentinel_t);
 
  private:
    coroutine_handle<promise_type> coroutine_; // exposition only
  };
}