milessabin/shapeless — comment on commit 447aae489dd1b7b47fc2d7cc027ca73844a76550
File: src/main/scala/shapeless/poly.scala line 72
By milessabin:

Definition site constraints on case shapes supported in the latest commit. For your scenarios you'll want to extend Pullback1[R] to limit the case result types uniformly to R. Let me know how you get on ...