QWidget has a x11Info() that returns non-existent QX11Info class

While looking for a way of porting some Qt4 code to Qt5, I noticed that QX11Info was deprecated. Nonetheless I get this:

grep -i x11info /opt/Qt/5/include/Qt*/*
/opt/Qt/5/include/QtWidgets/qwidget.h: const QX11Info &x11Info() const;