Welcome to Scala 2.13.0-pre-1f68c6c (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_181).
Type in expressions for evaluation. Or try :help.

scala> class A[B](val b: B) { def c: List[b.type] = List(b) }
defined class A

scala> class A[B](val b: B) { def c: List[b.type] = b :: Nil }
                                                    ^
       error: type mismatch;
        found   : rassoc$1.type (with underlying type B)
        required: A.this.b.type