The following code tries to mimic Polymorphic Embedding of DSLs: rather than giving the behavior in Inner, it is encoded in the useInner method of its enclosing class. I added the enclosing method so that user has only to keep a reference to Inner instances, but can always get their enclosing instance. By doing this, all Inner instances from a specific Outer instance are bound to only one behavior (but it is wanted here).

abstract class Outer {
  sealed class Inner {
    def enclosing = Outer.this
  }
 def useInner(x:Inner) : Boolean
}

def toBoolean(x:Outer#Inner) : Boolean = x.enclosing.useInner(x)

It does not compile and scala 2.8 complains about:

type mismatch; found: sandbox.Outer#Inner
               required: _81.Inner where val _81:sandbox.Outer

From Programming Scala: Nested classes and A Tour of Scala: Inner Classes, it seems to me that the problem is that useInnerexpects as argument an Inner instance from a specific Outer instance.

What is the true explanation and how to solve this problem ?