Confusing type error with covariance and view bounds

type parameter name shadowing ;)

there in a class type parameter named T and a method type parameter named T. so T != T, which leads to a confusing message

Am 05.02.2012 19:40, schrieb Jordan Lewis:

Hi,
I'm trying to write a simple implementation of a functional binary tree in Scala. I've run into a type error that I don't quite understand.

sealed abstract class UnbalancedSet[+T] {
  def insert[U >: T <% Ordered[U]](x: U): UnbalancedSet[U]
  def member(x: T): Boolean
}

case object EmptyUnbalancedSet extends UnbalancedSet[Nothing] {
  def insert[U <% Ordered[U]](x: U) = NonEmptyUnbalancedSet(this, x, this)
  def member(x: Any) = false
}

final case class NonEmptyUnbalancedSet[+T]
    (left: UnbalancedSet[T], value: T, right: UnbalancedSet[T]) extends UnbalancedSet[T] {
  def insert[U >: T <% Ordered[U]](x: U) =
    if      (x < value) NonEmptyUnbalancedSet(left insert x, value, right<
span class="pun" style="outline-style: none; outline-width: initial; outline-color: initial; color: #5a5a5a; ">)
    else if (x > value) NonEmptyUnbalancedSet(left, value, right insert x)
    else this
  def member[T <% Ordered[T]](x: T) =
    if      (x < value) left member x
    else if (x > value) right member x
    else true
}

This program fails to compile:
[error] /Users/jordan/PFDScala/src/main/scala/UnbalancedSet.scala:24: type mismatch;[error]  found   : NonEmptyUnbalancedSet.this.value.type (with underlying type T)[error]  required: T[error]     if      (x < value) left member x[error]                  ^[error] /Users/jordan/PFDScala/src/main/scala/UnbalancedSet.scala:24: type mismatch;[error]  found   : x.type (with underlying type T)[error]  required: T[error]     if      (x < value) left member x<
/font>
... and likewise for the opposite case, if (x > value). 
What is going wrong here? What does the compiler mean by "underlying type", and why is it the case that a type with underlying type T is a mismatch with type T?
Thanks,Jordan Lewis