c – 为什么会收到编译错误？

#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 make_unique<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); // Compilation error
}

任何人都可以从编译器中解释这个错误的原因？

main.cpp: In instantiation of ‘std::unique_ptr make_unique(Args&&
…) [with T = S; Args = {int}]’:

main.cpp:11:58: required from ‘static std::unique_ptr
S::create(Args&& …) [with Args = {int}]’

main.cpp:20:26: required from here

main.cpp:14:5: error: ‘S::S(int)’ is private

06001

main.cpp:5:65: error: within this context
return std::unique_ptr(new T{std::forward(args)…});

06002