Yikes, prependToList was violating List immutab...

Yikes, prependToList was violating List immutability. Closes #4656, no
review.

3 files changed, 21 insertions, 2 deletions

diff --git a/src/library/scala/collection/mutable/ListBuffer.scala b/src/library/scala/collection/mutable/ListBuffer.scala
index 6a75108dfb..f1271627c1 100644
--- a/src/library/scala/collection/mutable/ListBuffer.scala
+++ b/src/library/scala/collection/mutable/ListBuffer.scala
@@ -245,9 +245,14 @@ final class ListBuffer[A]
    *
    *  @param xs   the list to which elements are prepended
    */
-  def prependToList(xs: List[A]): List[A] =
+  def prependToList(xs: List[A]): List[A] = {
     if (start.isEmpty) xs
-    else { last0.tl = xs; toList }
+    else {
+      if (exported) copy()
+      last0.tl = xs
+      toList
+    }
+  }
 
 // Overrides of methods in Buffer
 
diff --git a/test/files/run/bug4656.check b/test/files/run/bug4656.check
new file mode 100644
index 0000000000..15a62794a9
--- /dev/null
+++ b/test/files/run/bug4656.check
@@ -0,0 +1 @@
+List(1, 2, 3)
diff --git a/test/files/run/bug4656.scala b/test/files/run/bug4656.scala
new file mode 100644
index 0000000000..4f3d189c8f
--- /dev/null
+++ b/test/files/run/bug4656.scala
@@ -0,0 +1,13 @@
+object Test {
+  def f = {
+    val buf = new collection.mutable.ListBuffer[Int]
+    buf ++= List(1, 2, 3)
+    val l = buf.toList
+    buf prependToList List(4, 5, 6)
+    l
+  }
+
+  def main(args: Array[String]): Unit = {
+    println(f)
+  }
+}