[완료][C++] 왜 연산자를 호출하지 못할까요?

template <typename T>
class List 
{
    public:
        class Node 
        {    
            friend class List;
 
            template <typename _T>
            friend typename List<_T>::Node* operator +(const int index, const typename List<_T>::Node &node);
 
 
 
            public:
                T data;
                Node *previous, *next;
 
            public:
                Node() : data(0), previous(NULL), next(NULL) {}
 
 
 
            ......(생략)
 
 
        };
 
 
 
......(생략)
 
 
 
};
 
 
template <typename T>
typename List<T>::Node* operator +(const int index, const typename List<T>::Node &node)
{
    int i;
    typename List<T>::Node *current_node;
 
    for(i=0,current_node=&node;(current_node!=NULL)&&(i<index);++i)
        current_node=current_node->next;
 
    return current_node;
}
 
 
int main()
{
    List<double> *a=List<double>::Create_a_List();
 
    a->Add_front(123.123);
    List<double>::Node *node=a->Add_Node(200.0);
 
    printf("%g\n", node->data);
    node=--(*node);
    printf("%g\n", node->data);
    node=++(*node);
    printf("%g\n", node->data);
 
    node=*a>>0;
    printf("%g\n", node->data);
 
    node=1+(*node);                            <======== 20번 line
    printf("%g\n", node->data);
 
    List<double>::Destroy_a_List(a);
}