#include <iostream>
using namespace std;
 
#ifndef TEST_H
#define TEST_H
 
class User;
 
template <class T>
class Wrapper {
public:
    template<class U>
    auto Callback(...) {};
};
 
template<class T = User>
class Library {
public:
    T* node = nullptr;
    void utility() {
        T::Callback(node);
    }
};
 
class User : public Wrapper<User> {
public:
    static void Callback( class Wrapper* ) { std::cout << "Callback was called.\n";  }
};
 
#endif // TEST_H
 
int main() {
 
    Library<User> l;
    l.utility();
 
    return 0;
}

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