QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::begin: Paint device returned engine == 0, type: 3

class Widget : public QLabel
{
    Q_OBJECT
public:
    Widget():QLabel()
    {
        if (pix.load("logo.png"))
            qDebug() << "pixmap loaded";
        else
            qDebug() << "failed to load pixmap";

        setPixmap(pix);
    }

protected:
    void mousePressEvent(QMouseEvent *ev)
    {
        QMimeData *mimeData = new QMimeData;
        mimeData->setImageData(pix.toImage());

        QDrag *drag = new QDrag(this);
        drag->setMimeData(mimeData);
        //drag->setPixmap(pix); // add this line to suppress QPainter warnings 

        drag->exec();
    }

private:
    QPixmap pix;

};

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

    Widget wid;
    wid.show();
    
    return a.exec();
}