Mixing OpenGL and QPainter in QOpenGLWidget

main.cpp

#include <QApplication>
#include <QMainWindow>
#include <QHBoxLayout>
#include <QGLWidget>
#include <QOpenGLContext>

#include <QDebug>
#include <QPainter>
#include <QOpenGLWidget>

class Widget : public QOpenGLWidget
{
Q_OBJECT
public:
    Widget(QWidget *parent=nullptr)
            : QOpenGLWidget(parent)
    { }

public:
    void initializeGL() {
        glClearColor(0, 0, 0, 1);
        glEnable(GL_DEPTH_TEST);
        glEnable(GL_LIGHT0);
        glEnable(GL_LIGHTING);
        glColorMaterial(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE);
        glEnable(GL_COLOR_MATERIAL);
    }

    void resizeGL(int width, int height) {
        glViewport(0, 0, width, height);
    }

    void paintGL() {
        qDebug() << "Error 1: " << glGetError();

        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glBegin(GL_TRIANGLES);
        glColor3f(1.0f, 0.0, 0.0);
        glVertex3f(-0.5f, -0.5f, 0);
        glColor3f(0.0, 1.0f, 0.0);
        glVertex3f(0.5f, -0.5f, 0.0);
        glColor3f(0.0, 0.0, 1.0);
        glVertex3f(0.0, 0.5f, 0);
        glEnd();

        QPainter p(this);
        p.setPen(Qt::red);
        p.drawLine(rect().topLeft(), rect().bottomRight());
        qDebug() << "Error 2: " << glGetError();
    }
};


int main(int argc, char *argv[])
{
    /* Note: Calling QSurfaceFormat::setDefaultFormat() before
       constructing the QApplication instance is mandatory on some
       platforms (for example, OS X) when an OpenGL core profile context
       is requested. This is to ensure that resource sharing between
       contexts stays functional as all internal contexts are created
       using the correct version and profile. */
    QSurfaceFormat fmt;
    if (QOpenGLContext::openGLModuleType() == QOpenGLContext::LibGL) {
        fmt.setVersion(2,1);
        fmt.setProfile(QSurfaceFormat::CoreProfile);
        fmt.setProfile(QSurfaceFormat::CompatibilityProfile);
        fmt.setOption(QSurfaceFormat::DebugContext);
    } else {
        fmt.setVersion(3, 0);
    }

    QSurfaceFormat::setDefaultFormat(fmt);

    QApplication::setDesktopSettingsAware(false);
    QApplication app(argc, argv);

    Widget win;
    win.show();

    return app.exec();
}