Ah, I see, it's a headonly install. In this case, we would need to `depends_on "homebrew/headonly/gccxml"` so that it tells the user to tap homebrew/headonly. I don't think we have ever depended on headonly before, but it has been considered. However, since this is awaiting a pull request from that tap, we cannot do anything about this until it is approved. In the meantime, it would be nice if there was an alternative that made gccxml not a dependency.