No public type alias allowed from private class parameters

The following snippet of code:

trait Define :
  type Test
  def work: Unit

class Class(define: Define) :
  type Test = define.Test
  def work =

gives a compiler error:

non-private type Test in class Class refers to private value define
in its type signature  = Class.this.define.Test

But why is that? What is the risk here? It seems to me that we make parts of private values public all the time, for example the method def work. When making define public with a val:

class Class (val define: Define)  

that of course helps, but exposes the rest of define at the same time.