My /usr/lib64/qt4/libQtCLucene.so.4 was broken, but trying to rebuild the package didn't work since the ebuild tried to link against /usr/lib64/qt4/libQtCLucene.so.4 and therefore broke. I had to unmerge x11-libs/qt-assistant and then merge it again. Reproducible: Couldn't Reproduce
Ebuild version was: x11-libs/qt-assistant-4.7.2
Please post emerge --info x11-libs/qt-assistant. Also, if you happen to have the build log from the failed rebuild attempts, please attach that.
Created attachment 280221 [details] emerge --info x11-libs/qt-assistant
If confirmed, this means that assistant is linked against the installed version of libQtCLucene instead of the just-built one, which can lead to problems when upgrading Qt because of missing symbols in the old version of libQtCLucene.
This seems fixed in 4.8.0.