SI-8691 SeqView throws exception when prepending a collection

Prepend was throwing segfaults as it was handled differently from Append.  This brings the two into line with each other.

There are various optimizations that could be applied that have not been, however, such as intercepting sequential prepends and generating one multi-prepend instead of nested single-element prepends.

Unit test added to verify minimally that bug is gone.

1 files changed, 10 insertions, 12 deletions

diff --git a/src/library/scala/collection/SeqViewLike.scala b/src/library/scala/collection/SeqViewLike.scala
index e719f19c78..92d5547fd2 100644
--- a/src/library/scala/collection/SeqViewLike.scala
+++ b/src/library/scala/collection/SeqViewLike.scala
@@ -95,6 +95,14 @@ trait SeqViewLike[+A,
       if (idx < self.length) self(idx) else restSeq(idx - self.length)
   }
 
+  trait Prepended[B >: A] extends super.Prepended[B] with Transformed[B] {
+    protected[this] lazy val fstSeq = fst.toSeq
+    def length: Int = fstSeq.length + self.length
+    def apply(idx: Int): B =
+      if (idx < fstSeq.length) fstSeq(idx)
+      else self.apply(idx - fstSeq.length)
+  }
+
   trait Filtered extends super.Filtered with Transformed[A] {
     protected[this] lazy val index = {
       var len = 0
@@ -178,21 +186,12 @@ trait SeqViewLike[+A,
     final override protected[this] def viewIdentifier = "P"
   }
 
-  trait Prepended[B >: A] extends Transformed[B] {
-    protected[this] val fst: B
-    override def iterator: Iterator[B] = Iterator.single(fst) ++ self.iterator
-    def length: Int = 1 + self.length
-    def apply(idx: Int): B =
-      if (idx == 0) fst
-      else self.apply(idx - 1)
-    final override protected[this] def viewIdentifier = "A"
-  }
-
   /** Boilerplate method, to override in each subclass
    *  This method could be eliminated if Scala had virtual classes
    */
   protected override def newForced[B](xs: => GenSeq[B]): Transformed[B] = new { val forced = xs } with AbstractTransformed[B] with Forced[B]
   protected override def newAppended[B >: A](that: GenTraversable[B]): Transformed[B] = new { val rest = that } with AbstractTransformed[B] with Appended[B]
+  protected override def newPrepended[B >: A](that: GenTraversable[B]): Transformed[B] = new { protected[this] val fst = that } with AbstractTransformed[B] with Prepended[B]
   protected override def newMapped[B](f: A => B): Transformed[B] = new { val mapping = f } with AbstractTransformed[B] with Mapped[B]
   protected override def newFlatMapped[B](f: A => GenTraversableOnce[B]): Transformed[B] = new { val mapping = f } with AbstractTransformed[B] with FlatMapped[B]
   protected override def newFiltered(p: A => Boolean): Transformed[A] = new { val pred = p } with AbstractTransformed[A] with Filtered
@@ -211,7 +210,6 @@ trait SeqViewLike[+A,
     val patch = _patch
     val replaced = _replaced
   } with AbstractTransformed[B] with Patched[B]
-  protected def newPrepended[B >: A](elem: B): Transformed[B] = new { protected[this] val fst = elem } with AbstractTransformed[B] with Prepended[B]
 
   // see comment in IterableViewLike.
   protected override def newTaken(n: Int): Transformed[A] = newSliced(SliceInterval(0, n))
@@ -241,7 +239,7 @@ trait SeqViewLike[+A,
   }
 
   override def +:[B >: A, That](elem: B)(implicit bf: CanBuildFrom[This, B, That]): That =
-    newPrepended(elem).asInstanceOf[That]
+    newPrepended(elem :: Nil).asInstanceOf[That]
 
   override def :+[B >: A, That](elem: B)(implicit bf: CanBuildFrom[This, B, That]): That =
     ++(Iterator.single(elem))(bf)