QBoxLayout embedded in QGridLayout spacing bug

In this example I have a QBoxLayout embedded in a QGridLayout. The QGridLayout has the verticalSpacing set, but not the horizontalSpacing. I would expect the QBoxLayout spacing to fall back to QStyle::PM_LayoutHorizontalSpacing. It does not, instead a spacing of 0 is used.

By stepping through this I found I could fix it by overriding QStyle::layoutSpacing, but that directly contradicts that documentation for layoutSpacing, which claims it is ignored unless PM_LayoutHorizontalSpacing is negative.

It can also, obviously, be worked around by manually setting the spacing for the QBoxLayout.