sec-policy/selinux-base-policy-2.20120725-r12 introduced forced "unconfined" useflag, which enable dependency on sec-policy/selinux-unconfined, which itself depend on sec-policy/selinux-base-policy (by selinux-policy-2 eclass). So, it is impossible to instal them both on ~arch. Reproducible: Always
I marked the dependency as a PDEPEND, does that fix it? It's in the tree (repository) so probably hits the mirrors in an hour (give or take).
Seems to be fixed now. At least, installs fine.