dev-lang/vala:0.18 is required by gnome-extra/evolution-data-server-3.8.3 dev-libs/gobject-introspection-1.36.0 is required by a lot of stuff. I changed the evolution-data-server to use vala:0.20. I'll update the bug report if this results in a build failure. Reproducible: Always
evolution-data-server-3.8 needs either vala-0.20.x or vala-0.18.x at build time. gobject-introspection-1.36 is incompatible with <vala-0.20 (see bug #463720) Therefore, to resolve the conflict, simply install vala:0.20 and remove earlier vala versions.