It seems like, at a minimum, this would need to support v4.5.6 and 6.7.8 - ie, static versions, both with and without the v.

It would certainly not support aliases, since a) that varies widely by version management tool, and b) the point of an "x-version" file is to target one specific version.

This raises the question, however, about io.js versions, which it should also support. Should iojs-v1.2.3 be allowed? If not, v1.2.3 would still unambiguously point to an io.js version. What about a future where there's more than the version number to target - like, "engine" or "architecture"?