[bug#12121] Add test for inserting an ArrayBuffer into itself

Author: NthPortal

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

@@ -167,6 +167,10 @@ class ArrayBuffer[A] private (initialElements: Array[AnyRef], initialSize: Int)
  size0 = size0 + elemsLength
  elems match {
  case elems: ArrayBuffer[_] =>
+ // if `elems eq this`, this works because `elems.array eq this.array`,
+ // we didn't overwrite the values being inserted after moving them in
+ // the previous copy a few lines up, and `System.arraycopy` will
+ // effectively "read" all the values before overwriting any of them.
  Array.copy(elems.array, 0, array, index, elemsLength)
  case _ =>
  var i = 0