Q_FALLTHROUGH was only introduced in Qt 5.8, so it shouldn't be used in the Qt 4 version.
It is not used in qt4 version.
Yes, it is. qt4/common/common.cpp line 242.
It turns out it's because it's a symlink to the qt5 version.
Git commit 14eec32771fc3384009d74c13c3557925a93c749 by Yichao Yu. Committed on 11/12/2017 at 14:17. Pushed by yuyichao into branch 'master'. Introduce QTC_FALLTHROUGH (from LLVM) * Fix compilation on gtk2, qt4, and possibly old version qt5 (< 5.8). * Fix a few more fallthrough warnings. M +10 -0 lib/utils/utils.h M +9 -4 qt4/style/qtcurve.cpp M +19 -19 qt5/common/common.cpp M +2 -2 qt5/style/qtcurve.cpp M +1 -1 qt5/style/qtcurve_api.cpp https://commits.kde.org/qtcurve/14eec32771fc3384009d74c13c3557925a93c749