Capturing existential typeclasses in Scala

gistfile1.scala

scala> trait Pet[A] {
     | def blah(a: A): A
     | }
defined trait Pet

scala> object IntPet extends Pet[Int] {
     | def blah(i: Int) = i
     | }
defined module IntPet

scala> object StringPet extends Pet[String] {
     | def blah(s: String) = s + ", Esq."
     | }
defined module StringPet

scala> trait Pet0 { type A; def a: A; def tc: Pet[A] }
defined trait Pet0

scala> case class Pet1[X](a: X, tc: Pet[X]) extends Pet0 { type A = X }
defined class Pet1

scala> val pets : List[Pet0] = List(Pet1(1, IntPet), Pet1("a", StringPet))
pets: List[Pet0] = List(Pet1(1,IntPet$@6cbcf243), Pet1(a,StringPet$@29e6eb25))

scala> pets.map { p => p.tc.blah(p.a) }
res2: List[Pet0#A] = List(1, a, Esq.)