This is caused in part by your use of using namespace std;.
There is a function in the standard library, std::distance, that is used to compute the distance between two iterators into a container. That function is being selected during overload resolution instead of your distance function.
If you don't use using namespace std;, you won't have this problem. Your use of using namespace std; brings all of the names from the std namespace into the global namespace. This is bad because there are a lot of common names in namespace std and it can cause convoluted issues during name lookup, as you've discovered here.
In general, prefer not to use using namespace, especially not at file scope. It's far easier and clearer in the long run to qualify each of the names that you want to use.
