Method join

include/exec/start_now.hpp:56–62

    struct __joiner : STDEXEC::__immovable
    {
      constexpr void join() const noexcept
      {
        if (__op_)
        {
          __op_(__ptr_);
        }
      }

      void (*__op_)(void*) noexcept = nullptr;
      void* __ptr_                  = nullptr;