Question about alias declarations in template class

I am confusing about alias declarations in template situation. Example:(Just toy for "demo")

template<class T>
class A {
private:
    using t = T;

public:
    t add(T a, T b);
};

// alias declarations
template<class T>
using tt = typename A<T>::t;

template<class T>
typename A<T>::t A<T>::add(T a, T b){
    return a+b;
}

// error !!!!
template<class T>
tt<T> A<T>::add(T a, T b) {
    return a + b;
}

Is any method to write in alias simplify form ?