Compile error "xxxx is private within this context" when build singleton class in qt

Question

I get compile errors when building my singleton class in qt5.15.0. The following minimal example can produce my problem.

Singleton.h

#ifndef SINGLETON_H
#define SINGLETON_H

#include <memory>

class Singleton
{
public:
  typedef std::shared_ptr<Singleton> Ptr;

  static Ptr GetInstance();

private:
  Singleton() = default;
  ~Singleton() = default;

  static Ptr _singleton;
};

#endif // SINGLETON_H

Singleton.cpp

#include "singleton.h"

Singleton::Ptr Singleton::_singleton = nullptr;

Singleton::Ptr Singleton::GetInstance()
{
  if(!_singleton)
  {
    _singleton = std::make_shared<Singleton>();
  }
  return _singleton;
}

And, a main.cpp for passing building process.

#include <QCoreApplication>

int main(int argc, char *argv[])
{
  QCoreApplication a(argc, argv);

  return a.exec();
}

When I build in QtCreator, I get following errors:

'constexpr Singleton::Singleton()' is private within this context
...
'Singleton::~Singleton()' is private within this context
...

What's wrong with my codes?