Qt::Popup window flag and QDialog modailty

It seems that when a QDialog is given the Qt::PopUp window flag, it will never actually be modal.

Documentation for Qt::PopUp clearly states that this flag "Indicates that the widget is a popup top-level window, i.e. that it is modal, but has a window system frame appropriate for popup menus."

Also, QDialog::modal also mentions "exec() ignores the value of this property and always pops up the dialog as modal."

However, if I were to display a dialog as follows:

QDialog dia(this, Qt::Popup);
dia.exec();

the dialog is not modal. It is possible to interact with the calling window.

This is either a bug or a documentation error.