When gcc-multilib and gcc are at the same pkgrel, the non-multilib gcc split packages (fortran, ada, go, objc) can be installed at the same time as gcc-multilib (since they depend on 'gcc' and gcc-multilib provides the same version of 'gcc'). If the gcc-multilib pkgrel is later advanced ahead of gcc's, as it is now with 7.1.1-2 vs 7.1.1-3, anyone with these side-by-side installs is suddenly unable to update because of version conflicts.

I don't think it should be allowed to install gcc-multilib alongside a non-multilib gcc-X regardless of version, but pacman does currently allow this when the versions match because there are no conflicts preventing it.

https://bugs.archlinux.org/task/54231 was closed with the reason "gcc-*-multilib is all or nothing", but that is not mentioned on the multilib wiki page and is not enforced by the package manager, so it is relatively easy to accidentally end up in a 'maybe-can't-upgrade-later' situation as described.