Merge pull request scala#4153 from Ichoran/issue/8691

SI-8691  SeqView throws exception when prepending a collection

Author: lrytz

src/library/scala/collection/IterableViewLike.scala

@@ -69,6 +69,10 @@ trait IterableViewLike[+A,
   trait Appended[B >: A] extends super.Appended[B] with Transformed[B] {
     def iterator = self.iterator ++ rest
   }
+  
+  trait Prepended[B >: A] extends super.Prepended[B] with Transformed[B] {
+    def iterator = fst.toIterator ++ self
+  }
 
   trait Filtered extends super.Filtered with Transformed[A] {
     def iterator = self.iterator filter pred
@@ -110,6 +114,7 @@ trait IterableViewLike[+A,
   } with AbstractTransformed[(A1, B)] with ZippedAll[A1, B]
   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 { 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

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)

src/library/scala/collection/TraversableViewLike.scala

@@ -189,6 +189,15 @@ trait TraversableViewLike[+A,
     }
     final override protected[this] def viewIdentifier = "A"
   }
+  
+  trait Prepended[B >: A] extends Transformed[B] {
+    protected[this] val fst: GenTraversable[B]
+    def foreach[U](f: B => U) {
+      fst foreach f
+      self foreach f
+    }
+    final override protected[this] def viewIdentifier = "A"
+  }
 
   trait Filtered extends Transformed[A] {
     protected[this] val pred: A => Boolean
@@ -222,11 +231,15 @@ trait TraversableViewLike[+A,
     final override protected[this] def viewIdentifier = "D"
   }
 
-  override def ++[B >: A, That](xs: GenTraversableOnce[B])(implicit bf: CanBuildFrom[This, B, That]): That = {
+  override def ++[B >: A, That](xs: GenTraversableOnce[B])(implicit bf: CanBuildFrom[This, B, That]): That =
     newAppended(xs.seq.toTraversable).asInstanceOf[That]
-// was:    if (bf.isInstanceOf[ByPassCanBuildFrom]) newAppended(that).asInstanceOf[That]
-//         else super.++[B, That](that)(bf)
-  }
+  
+  override def ++:[B >: A, That](xs: TraversableOnce[B])(implicit bf: CanBuildFrom[This, B, That]): That = 
+    newPrepended(xs.seq.toTraversable).asInstanceOf[That]
+    
+  // Need second one because of optimization in TraversableLike
+  override def ++:[B >: A, That](xs: Traversable[B])(implicit bf: CanBuildFrom[This, B, That]): That = 
+    newPrepended(xs).asInstanceOf[That]
 
   override def map[B, That](f: A => B)(implicit bf: CanBuildFrom[This, B, That]): That = {
     newMapped(f).asInstanceOf[That]
@@ -253,6 +266,7 @@ trait TraversableViewLike[+A,
    */
   protected def newForced[B](xs: => GenSeq[B]): Transformed[B] = new { val forced = xs } with AbstractTransformed[B] with Forced[B]
   protected def newAppended[B >: A](that: GenTraversable[B]): Transformed[B] = new { val rest = that } with AbstractTransformed[B] with Appended[B]
+  protected def newPrepended[B >: A](that: GenTraversable[B]): Transformed[B] = new { val fst = that } with AbstractTransformed[B] with Prepended[B]
   protected def newMapped[B](f: A => B): Transformed[B] = new { val mapping = f } with AbstractTransformed[B] with Mapped[B]
   protected def newFlatMapped[B](f: A => GenTraversableOnce[B]): Transformed[B] = new { val mapping = f } with AbstractTransformed[B] with FlatMapped[B]
   protected def newFiltered(p: A => Boolean): Transformed[A] = new { val pred = p } with AbstractTransformed[A] with Filtered

src/library/scala/collection/immutable/StreamViewLike.scala

@@ -53,6 +53,7 @@ extends SeqView[A, Coll]
   /** boilerplate */
   protected override def newForced[B](xs: => scala.collection.GenSeq[B]): Transformed[B] = new { val forced = xs } with AbstractTransformed[B] with Forced[B]
   protected override def newAppended[B >: A](that: scala.collection.GenTraversable[B]): Transformed[B] = new { val rest = that } with AbstractTransformed[B] with Appended[B]
+  protected override def newPrepended[B >: A](that: scala.collection.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 => scala.collection.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
@@ -67,7 +68,6 @@ extends SeqView[A, Coll]
   protected override def newPatched[B >: A](_from: Int, _patch: scala.collection.GenSeq[B], _replaced: Int): Transformed[B] = {
     new { val from = _from; val patch = _patch; val replaced = _replaced } with AbstractTransformed[B] with Patched[B]
   }
-  protected override def newPrepended[B >: A](elem: B): Transformed[B] = new { protected[this] val fst = elem } with AbstractTransformed[B] with Prepended[B]
 
   override def stringPrefix = "StreamView"
 }

test/junit/scala/collection/SeqViewTest.scala

@@ -0,0 +1,16 @@
+package scala.collection
+
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.junit.Assert._
+import org.junit.Test
+
+@RunWith(classOf[JUnit4])
+class SeqViewTest {
+
+  @Test
+  def test_SI8691() {
+    // Really just testing to make sure ++: doesn't throw an exception
+    assert( Seq(1,2) ++: Seq(3,4).view == Seq(1,2,3,4) )
+  }
+}