type mismatch with example

[rongpenl]

Hi,

I am following this tutorial. Awesome work. However, I couldn't get this example working with the code provided.

trait Stream[+T] {
    def prepend[U >: T](elem: U) : Stream[U] = Stream.cons(elem, this)
}

Compiling the code above gives me error as following.

polymorphism.scala:15: error: type mismatch;
 found   : Stream[T]
 required: scala.collection.immutable.Stream[?]
           def prepend[U >: T](elem: U) : Stream[U] = Stream.cons(elem, this)

Did I miss something?

exercises-scalatutorial/src/main/scala/scalatutorial/sections/PolymorphicTypes.scala

Line 433 in 38531b1

* def prepend [U >: T](elem: U): Stream[U] = Stream.cons(elem, this)