If you try to build x11-libs/libva with all USE disabled it fails with: >configure: error: Please select at least one backend (DRM, X11, Wayland) Reproducible: Always
Created attachment 406448 [details] build.log
+ 14 Jul 2015; Alexis Ballier <aballier@gentoo.org> libva-1.6.0.ebuild, + libva-9999.ebuild: + fix regression overwriting REQUIRED_USE, bug #554398 + thanks, it was there but overwritten later on in the ebuild...