[bug#12284] Ensure ArrayBufferView is consistent with buffer

src/library/scala/collection/mutable/ArrayBuffer.scala

@@ -114,7 +114,8 @@ class ArrayBuffer[A] private (initialElements: Array[AnyRef], initialSize: Int)
 
   def length = size0
 
-  override def view: ArrayBufferView[A] = new ArrayBufferView(array, size0, () => mutationCount)
+  // TODO: return `IndexedSeqView` rather than `ArrayBufferView`
+  override def view: ArrayBufferView[A] = new ArrayBufferView(this, () => mutationCount)
 
   override def iterableFactory: SeqFactory[ArrayBuffer] = ArrayBuffer
 
@@ -317,18 +318,30 @@ object ArrayBuffer extends StrictOptimizedSeqFactory[ArrayBuffer] {
   }
 }
 
-final class ArrayBufferView[A] private[mutable](val array: Array[AnyRef], val length: Int, mutationCount: () => Int)
+// TODO: use `CheckedIndexedSeqView.Id` once we can change the return type of `ArrayBuffer#view`
+final class ArrayBufferView[A] private[mutable](underlying: ArrayBuffer[A], mutationCount: () => Int)
   extends AbstractIndexedSeqView[A] {
   @deprecated("never intended to be public; call ArrayBuffer#view instead", since = "2.13.6")
   def this(array: Array[AnyRef], length: Int) = {
     // this won't actually track mutation, but it would be a pain to have the implementation
     // check if we have a method to get the current mutation count or not on every method and
     // change what it does based on that. hopefully no one ever calls this.
-    this(array, length, () => 0)
+    this({
+      val _array = array
+      val _length = length
+      new ArrayBuffer[A](0) {
+        this.array = _array
+        this.size0 = _length
+      }
+    }, () => 0)
   }
 
+  @deprecated("never intended to be public", since = "2.13.6")
+  def array: Array[AnyRef] = underlying.toArray[Any].asInstanceOf[Array[AnyRef]]
+
   @throws[IndexOutOfBoundsException]
-  def apply(n: Int): A = if (n < length) array(n).asInstanceOf[A] else throw new IndexOutOfBoundsException(s"$n is out of bounds (min 0, max ${length - 1})")
+  def apply(n: Int): A = underlying(n)
+  def length: Int = underlying.length
   override protected[this] def className = "ArrayBufferView"
 
   // we could inherit all these from `CheckedIndexedSeqView`, except this class is public

test/junit/scala/collection/mutable/ArrayBufferTest.scala

@@ -455,4 +455,21 @@ class ArrayBufferTest {
     buf.insertAll(1, buf)
     assertSameElements(List(1, 1, 2, 3, 2, 3), buf)
   }
+
+  // scala/bug#12284
+  @Test
+  def viewConsistency(): Unit = {
+    def check[U](op: ArrayBuffer[Int] => U): Unit = {
+      val buf = ArrayBuffer(1, 2, 3)
+      val view = buf.view
+      op(buf)
+      assertSameElements(buf, view)
+    }
+
+    check(_.clear())
+    check(_.dropRightInPlace(1))
+    check(_.dropInPlace(1))
+    check(_ ++= (1 to 100))
+    check(_.insertAll(1, 1 to 100))
+  }
 }