Method __complete

include/exec/start_now.hpp:83–93

      {}

      constexpr void __complete() noexcept
      {
        if (__pending_.fetch_sub(1) == 1)
        {
          auto __joiner = __joiner_.exchange(nullptr);
          if (__joiner)
          {
            __joiner->join();
          }
        }
      }
    };

    template <class _Env>