Changed underlying type of RefinedThis

Now: The underlying refined type. Was: The parent of the type.
We need the change because RefinedThis is used as a narrowed version
of the underlying refinedType (e.g. in TypeComparer rebase), and the old
scheme would lose a binding of that type.

Author: odersky

src/dotty/tools/dotc/core/Types.scala

@@ -2131,7 +2131,7 @@ object Types {
 
   case class RefinedThis(binder: RefinedType) extends BoundType with SingletonType {
     type BT = RefinedType
-    override def underlying(implicit ctx: Context) = binder.parent
+    override def underlying(implicit ctx: Context) = binder
     def copyBoundType(bt: BT) = RefinedThis(bt)
 
     // need to customize hashCode and equals to prevent infinite recursion for