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#include <type_traits>
using namespace std;
 
template<typename T>
class singleton
{
public:
    singleton () {
        static_assert(!std::is_constructible<T>::value, "the constructor is not private");
    }
 
    static T& instance() {
        static T inst;
        return inst;
    }   
};
 
class class_with_public_constr : public singleton<class_with_public_constr> 
{
  friend class singleton<class_with_public_constr>;   
public:
  class_with_public_constr() {}
};
 
int main() 
{
  singleton<class_with_public_constr>::instance();
}
 

prog.cpp: In instantiation of 'singleton<T>::singleton() [with T = class_with_public_constr]':
prog.cpp:22:30:   required from here
prog.cpp:9:9: error: static assertion failed: the constructor is not private
         static_assert(!std::is_constructible<T>::value, "the constructor is not private");
         ^

Standard output is empty