For what it is worth, the equivalent Scala code is trait Pong[T] {} class Ping[T] extends Pong[Pong[X forSome { type X >: Ping[Ping[T]]}]] { def Ping(

For what it is worth, the equivalent Scala code is trait Pong[T] {} class Ping[T] extends Pong[Pong[X forSome { type X >: Ping[Ping[T]]}]] { def Ping() { val Ping : Pong[X forSome { type X >: Ping[Long]}] = new Ping[Long](); } } which produces the errors Test.scala:3: error: illegal cyclic reference involving class Ping class Ping[T] extends Pong[Pong[X forSome { type X >: Ping[Ping[T]]}]] { ^ Test.scala:5: error: type mismatch; found : Ping[Long] required: Pong[X forSome { type X >: Ping[Long] }] Note: Long <: X forSome { type X >: Ping[Long] }, but trait Pong is invariant in type T. You may wish to define T as +T instead. (SLS 4.5) val Ping : Pong[X forSome { type X >: Ping[Long]}] = new Ping[Long]();