Does C++ std::advance implementation, Need to judge the sign of "Distance" parameter?

Question

I am recently learnign about some std::algorithm internals std::advance receives a number type "Distance", I suppose its internal implementation should look lie this: (tag-dispatch is suggested by [Effective C++])

template<class InputIt, class Distance>
void advance(InputIt& it, Distance n>
{
     advance_impl(it, n, Iterator_trait<InputIt>::categary)
}

The Iterator_trait should help to do a tag-dispatch and tell the category of iterator, whether it's random access or sequential
So for random access category, the "impl" should look like

template<class InputIt, class Distance>
void advance_impl(InputIt& it, Distance n>
{
  it = it +n
}

And for none-random_access category, impl should look like this? (just my guess)

template<class InputIt, class Distance>
void advance_impl(InputIt& it, Distance n>
{
  if(n==0)return
  else if(n>0)
  {
     for(Distance d=0;d<n;++d)
         ++it
  }
  else
  {
     for(Distance d=0;d<n;++d)
         --it;
  }
}

It's just my guess, I wonder if STL implementation really need to judge if n=0,>0,<0 as for different cases.

My guessed implementation should be much too complex. But what is a more graceful implementation, when the iterator is not random-access category.

I suppose a for loop is needed there, how about the sign of "Distance"?