We want to apply the bold attribute to the font of a Button's label (the label only, not the whole control), but this is not possible as the following is ill-formed:

font: control.font
font.bold: true

We had to wrap IconLabel in a Control to allow the extra level of indirection by setting bold on the wrapping control and then explicitly propagating the new font to IconLabel:

contentItem: Control {
  id: ctrl
  font.bold: true
  background: IconLabel { font: ctrl.font }
}

A simple fix of changing the base class of QQuickIconLabel would probably do the trick and at the same time allow to simplify existing code of QQ.Controls.2 by removing those explicit propagations.