PETYAN

class C;

class A {
public:
    friend class C;
private:
    int a;
};

template<int A::* field>
class B {};

class C : public B<&A::a> {};

int main() {
    C c;
}

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template<typename T>
class C;

template<typename T>
class A {
public:
    template<typename U>
    friend class C;
private:
    int a;
};

template<typename T, int A<T>::* field>
class B {};

template<typename T>
class C : public B<T, &A<T>::a> {};

int main() {
    C<int> c;
}