Quick: in order to make x a dependent name, so that lookup is deferred until the template parameter is known

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Why do I have to access template base class members through the this pointer?

If the classes below were not templates I could simply have x in the derived class. However, with the code below, I have to use this->x. Why?

template <typename T>
class base {

protected:
    int x;
};

template <typename T>
class derived : public base<T> {

public:
    int f() { return this->x; }
};

int main() {
    derived<int> d;
    d.f();
    return 0;
}