c++11 variadic template method completely shadow base class method?

I have the following problem( see code below).

In a class that inherits from a base class, I want to be able to have two implementations of operator(). One that takes integers, one that takes "indices", see simple Index class. The operator(ints...) will be in the child class, while I need to have the operator(Index...) in the parent class (also with different return type.)

The code below is not the exact design but a minimal working example explaining the final problem.

Problem is, if I put operator(Index...) in the child class, everything is good. If I put it in the base class, i get a compilation error:

error: no match for call to ‘(Tensor<1, int>) (Index<'i'>&)’

when I call vec(i) and the end of the main. I understand the compiler doesn't find the good method, but why ? Is there some "shadowing" rule associated to variadic templates ?

Thanks !

#include <iostream>
#include <type_traits>

template<char i>
class Index{
public:
    Index(){};
};

template<int order, typename T, class tensor_type>
class Tensor_traits{
  public:

      //// here, doesn't compile ! !
    //template <char i>
    //T&operator()(Index<i> &ii){
    //std::cout << "puet" << std::endl;
    //}
};


template<int order, typename T>
class Tensor : public Tensor_traits<order, T, Tensor<order, T>> {
    private:
    int data[3] = {1,2,3};
    public:
    Tensor(){};

    template <typename... Idx>
        typename std::enable_if<std::is_same<Idx...,int>::value
        or std::is_same<Idx...,unsigned int>::value
        or std::is_same<Idx...,size_t>::value, T&>::type
        operator()(const Idx&... idx){
        return data[0]; //dummy here, actually i do other stuff !
        }

    // here, works!
    template <char i>
        T&operator()(Index<i> &ii){
        std::cout << "puet" << std::endl;
        }

};


int main() {

    Tensor<1,int> vec1;

    std::cout<< vec1(1) << std::endl;;

    Index<'i'> i;

    vec1(i);
}