Fixes infinite streams in #3091. No review.

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@21188 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/library/scala/collection/immutable/Stream.scala

@@ -119,6 +119,16 @@ self =>
      else new Stream.Cons(head, (tail ++ that).asInstanceOf[Stream[A]])).asInstanceOf[That]
   }	
   
+  /**
+   * Create a new stream which contains all intermediate results of applying the operator
+   * to subsequent elements left to right.
+   * @note This works because the target type of the Builder That is a Stream.
+   */
+  override final def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
+    (if (this.isEmpty) Stream(z)
+    else new Stream.Cons(z, tail.scanLeft(op(z, head))(op).asInstanceOf[Stream[B]])).asInstanceOf[That]
+  }
+  
   /** Create a new stream which contains all elements of this stream
    *  followed by all elements of Iterator `that'
    */

test/files/pos/scan.scala

@@ -13,6 +13,11 @@ object Test {
     val emp = List[Int]()
     assert(emp.scanLeft(0)(_ + _) == List(0))
     assert(emp.scanRight(0)(_ + _) == List(0))
+    
+    val stream = Stream(1, 2, 3, 4, 5)
+    assert(stream.scanLeft(0)(_ + _) == Stream(0, 1, 3, 6, 10, 15))
+    
+    assert(Stream.from(1).scanLeft(0)(_ + _).take(5) == Stream(0, 1, 3, 6, 10))
   }
 
 }