title: Expression of type Iterator[A] doesn't conform to expected type Iterator[A]
body: <p>I'm a total beginner in scala and I have a very simple code which implements a tree. I would like to make it Iterable, so that I can use iterator to go through every element in this tree. Unfortunately, IntelliJ shows error.</p>
  
  <blockquote>
    <p>Expression of type Iterator[A] doesn't conform to expected type Iterator[A]</p>
  </blockquote>
  
  <p>Which refers to iterator method in Node class.
  Below my code:</p>
  
  <pre><code>abstract class MyTreeItem[A] extends Iterable
  
  case class EmptyLeaf[A]() extends MyTreeItem[A] {
    override def iterator: Iterator[A] = Iterator.empty
  }
  
  case class Node[A](left: MyTreeItem[A], right: MyTreeItem[A], item: A) extends MyTreeItem[A] {
    *override def iterator: Iterator[A] = Iterator.single(item) ++ left.iterator ++ right.iterator*
  }
  </code></pre>
  
  <p>What is the problem here?</p>