how to specialization a template class which declare in a template class?

I use gcc in linux.

template<class T>
class A
{
public:
    template<class N>
    class B
    {
    };
};

template<>
template<class T>
class A<T>::B<int>
{
    void test();
    T t;
};



int main()
{
    A<int>::B<int> b;
    //b.test();

    exit(0);
}

the complier said : in partial specialization didn't use template
parameter 'T'.
Why?
How to modify the code?
Thanks.

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