The attached sbt project makes the compiler run into a dead loop at scala.tools.nsc.transform.SpecializeTypes.specializedTypeVars(SpecializeTypes.scala:432)

I later found out a previous version of the package (monocle) was also being pulled in (via argonaut), see: argonaut-io/argonaut#163 and some code was moved about causing binary incompatabilities, but it seems either an unexpected code path is being traversed, or a possibility somewhere isn't being considered.