style/argbhelper.cpp:#include "private/qwidget_p.h" This could silently break even between Qt point releases.
As with bug 523616, we can introduce a subslot in qt5 and you can depend on ":5=", if you think it's worth it.
FTR, qt5 has a subslot since 5.6.0
I haven't seen any evidence of breakage yet that might indicate the need to use the subslot.