#include <QApplication>
#include <QObject>

#include <set>

struct MyItem
{
  int value;
};

class MyClass : public QObject
{
  Q_OBJECT
signals:
  void itemsChanged(const std::set<MyItem>& items);
};

#include "test.moc" // only necessary as we have a single file here

int main(int argc, char* argv[])
{
  QApplication app(argc, argv);

  MyClass myClass;

  return QApplication::exec();
}

binary '<<': no operator found which takes a right-hand operand of type 'const MyItem' (or there is no acceptable conversion) C:\dev\dependencies\Qt\6.9.0\msvc2022_64\include\QtCore\qdebug.h