template<typename _Lock>
  void
  wait(_Lock& __lock)
  {
shared_ptr<mutex> __mutex = _M_mutex; // <-- Extend lifetime of mutex.
unique_lock<mutex> __my_lock(*__mutex);
_Unlock<_Lock> __unlock(__lock);
// *__mutex must be unlocked before re-locking __lock so move
// ownership of *__mutex lock to an object with shorter lifetime.
unique_lock<mutex> __my_lock2(std::move(__my_lock));
_M_cond.wait(__my_lock2);
  }