QDialog ignores FramelessWindowHint passed to constructor

#include <QApplication>
#include <QDialog>
#include <QComboBox>
#include <QHBoxLayout>
#include <QPushButton>

struct Widget : public QDialog
{
    // passing FramelessWindowHint to the constructor has no effect in Qt 5...
    Widget(QWidget *parent = 0) : QDialog(parent, Qt::FramelessWindowHint)
    {
        // ...but this seems to work
        setWindowFlags(Qt::FramelessWindowHint);

        QHBoxLayout *layout = new QHBoxLayout(this);

        QPushButton *quitButton = new QPushButton("Quit", this);
        layout->addWidget(quitButton);

        connect(quitButton, SIGNAL(pressed()), qApp, SLOT(quit()));
    }
};

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

    Widget w;
    w.show();

    return a.exec();
}