Made sliding/grouped throw an exception when read past the end.

Closes #3017.

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@20777 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/library/scala/collection/Iterator.scala

@@ -937,6 +937,8 @@ trait Iterator[+A] { self =>
       if (!filled)
         fill()
       
+      if (!filled)
+        throw new NoSuchElementException("next on empty iterator")
       filled = false
       buffer.toList
     }      

test/files/run/unittest_iterator.scala

@@ -33,5 +33,18 @@ object Test
     assertThat(1, (1 to 8).toList) { it.sliding(8, 8) withPartial false }
     assertThat(2, List(9, 10, -1, -1, -1)) { it.sliding(5, 8) withPadding -1 }
     assertThat(1, (1 to 5).toList) { it.sliding(5, 8) withPartial false }
+    
+    // make sure it throws past th end
+    val thrown = try {
+      val it = List(1,2,3).sliding(2)
+      it.next
+      it.next
+      it.next
+      false
+    }
+    catch {
+      case _: NoSuchElementException => true
+    }
+    assert(thrown)
   }
 }