Вывод типа std::bind

class engine
{
    //...
   protected:
   void log(log_level level, const char *description)
   {
    if (m_log_fn)
        m_log_fn(level, description);
   }
   log_callback m_log_fn;
 
   class timed_log_event
   {
    decltype(std::bind(std::declval<decltype(&engine::log)>(), std::declval<engine*>(), std::declval<log_level>(), std::placeholders::_1)) m_log_f;
public:
    timed_log_event(engine * i_engine, const std::string i_operation, worker_log_level i_level = WLL_INFO)
        : m_log_f(std::bind(&engine::log, this, 0, std::placeholders::_1))
    {
        m_log_f("AAA");
 
    }
    };
 
};

class engine
{
    //...
   protected:
   void log(log_level level, const char *description)
   {
    if (m_log_fn)
        m_log_fn(level, description);
   }
   log_callback m_log_fn;
 
   class timed_log_event
   {
    decltype(std::bind(&engine::log, std::declval<engine*>(), std::declval<log_level>(), std::placeholders::_1)) m_log_f;
public:
    timed_log_event(engine * i_engine, const std::string& i_operation, worker_log_level i_level = WLL_INFO)
        : m_log_f(std::bind(&engine::log, i_engine, i_level, std::placeholders::_1))
    {
        m_log_f("AAA");
 
    }
    };
 
};