I hope this makes sense. :) Recently rebuilt the entire system, which was the usual nightmare collection of portage failures, and trying to find small groups of packages that would build successfully. Throughout this process, I kept adjusting the USE flags, removing flags that were no longer used, and attempting to remove obsolete packages from the system. After I finished, I did an emerge -e world, which completed without apparent error (and with no warnings of any kind). However, when checking some dependencies on a different machine, I noticed something odd. Two of the dependencies (for quiterss) did not get rebuilt. I eventually tracked this down to the fact that I had not enabled the qt5 (or qt4, but I didn't want that) USE flag for those packages. I added lines to /etc/portage/package.use enabling the qt5 USE flag for them, and emerge -ep world confirmed that both of them (dev-qt/qtlockedfile and dev-qt/qtsingleapplication) were now in the build list. However, this is not (in my opinion) what is supposed to happen. The emerge man page clearly states (for the -e option) "Reinstalls target atoms and their entire deep dependency tree, as though *no* (emphasis added) packages are currently installed." If those packages were not installed, then portage would tell me: "The following USE changes are necessary to proceed: (see "package.use" in the portage(5) man page for more details) # required by net-news/quiterss-0.18.8::gentoo # required by quiterss (argument) >=dev-qt/qtsingleapplication-2.6.1_p20150629 qt5 # required by dev-qt/qtsingleapplication-2.6.1_p20150629::gentoo # required by net-news/quiterss-0.18.8::gentoo # required by quiterss (argument) >=dev-qt/qtlockedfile-2.4.1_p20150629 qt5" In fact, that is when I get (with the qt5 USE flag not set for those packages) when I type emerge -ep quiterss on a machine that does not have either of them installed. If the packages are installed on the machine, portage should issue the same warning, but instead, it just silently skips rebuilding them.
*** This bug has been marked as a duplicate of bug 651018 ***
