std::noop_coroutine_handle(C++20)

namespace std {
  struct noop_coroutine_promise {};

  template<>
  struct coroutine_handle<noop_coroutine_promise>
  {
    constexpr operator coroutine_handle<>() const noexcept;

    constexpr explicit operator bool() const noexcept;
    constexpr bool done() const noexcept;

    constexpr void operator()() const noexcept;
    constexpr void resume() const noexcept;
    constexpr void destroy() const noexcept;

    noop_coroutine_promise& promise() const noexcept;

    constexpr void* address() const noexcept;

  private:
    coroutine_handle(unspecified);
  };

  using noop_coroutine_handle = coroutine_handle<noop_coroutine_promise>;
}