Merge pull request scala#10810 from som-snytt/issue/13019-treeseqmap-empty

Empty iterator must not mutate

Author: lrytz

src/library/scala/collection/immutable/TreeSeqMap.scala

@@ -392,10 +392,11 @@ object TreeSeqMap extends MapFactory[TreeSeqMap] {
 
       if (it != Zero) push(it)
 
-      def hasNext = index != 0
+      def hasNext = index > 0
       @tailrec
       def next(): V =
-        pop match {
+        if (!hasNext) scala.collection.Iterator.empty.next()
+        else pop match {
           case Bin(_,_, Tip(_, v), right) =>
             push(right)
             v

test/junit/scala/collection/immutable/TreeSeqMapTest.scala

@@ -174,6 +174,12 @@ class TreeSeqMapTest {
       assertEquals(s"modification empty from instance keeps modification order", List(1 -> 3, 3 -> 4), e3.toList)
     }
   }
+  @Test
+  def `t13019 empty.iterator is idempotent`: Unit = {
+    val m = util.Try(TreeSeqMap.empty.iterator.next())
+    assertTrue(m.isFailure)
+    assertFalse("empty iterator does not have next", TreeSeqMap.empty.iterator.hasNext)
+  }
 }
 object TreeSeqMapTest extends App {
   import TreeSeqMap.Ordering._