#include <memory>
 
template<typename T, typename ...Args>
    std::unique_ptr<T> make_unique(Args&& ...args) {
    return std::unique_ptr<T>(new T{std::forward<Args>(args)...});
}
 
struct S {
    template<class... Args>
    static std::unique_ptr<S> create(Args&&... args) {
        return std::unique_ptr<S>(new S(std::forward<Args>(args)...));
    }
private: // if I remove this line, then the compilation is OK
    S(int) {}
    S() = default;
};
 
int main() {
    auto s1 = S::create(); // OK
    auto s2 = S::create(0); // Now OK as well
}

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