C++ why is `this` necessary?

With GCC, we can't access the member in the base class without writing this explicitly, but it works on MSVC, what is going on? Is it because of the CRTP?

#include <iostream>

template<class T>
struct Base {
protected:
    T* a;
};

template<class U>
struct Derived : Base<Derived<U>> {
    void print_a() {
        std::cout << a << std::endl; // doesn't work on GCC
        std::cout << this->a << std::endl; // works on GCC
    }
};

int main() {
    Derived<float> d;
    d.print_a();
}