REPL-Fun.scala

scala> class Foo {
     |   type T = this.type
     |   def cloneit : T = {
     |     val x = new Foo
     |     x match {
     |       case y : T => y
     |       case _ => error("ZOMG")
     |     }
     |   }
     | }
<console>:11: error: type mismatch;
 found   : y.type (with underlying type Foo.this.T)
 required: Foo.this.T
       case y : T => y

Locking down the abstract type T still results in the type error!

scala> class Foo {            
   | final type T = this.type
   | def cloneit : T = {     
   |    val x = new Foo            
   |    x match {
   |      case y : T => y
   |      case _ => error("ZOMG")
   |    }
   | }
   | }
<console>:11: error: type mismatch;
 found   : y.type (with underlying type Foo.this.T)
 required: Foo.this.T
            case y : T => y