Merge pull request scala/scala#9192 from retronym/t12140-map-merge

Fix NPE regression in immutable.HashMap.merge

Author: dwijnand

library/src/scala/collection/immutable/HashMap.scala

@@ -342,7 +342,7 @@ object HashMap extends ImmutableMapFactory[HashMap] with BitOperations.Int {
  }
 
  @deprecatedInheritance("This class will be made final in a future release.", "2.12.2")
- class HashMap1[A,+B](private[collection] val key: A, private[collection] val hash: Int, private[collection] val value: (B @uV), private[collection] var kv: (A,B @uV)) extends HashMap[A,B] {
+ class HashMap1[A,+B](private[collection] val key: A, private[collection] val hash: Int, private[collection] val value: (B @uV), private[this] var kvOrNull: (A,B @uV)) extends HashMap[A,B] {
  override def size = 1
 
  private[collection] def getKey = key
@@ -394,8 +394,8 @@ object HashMap extends ImmutableMapFactory[HashMap] with BitOperations.Int {
  override def foreach[U](f: ((A, B)) => U): Unit = f(ensurePair)
  override private[immutable] def foreachEntry[U](f: (A, B) => U): Unit = f(key, value)
  // this method may be called multiple times in a multi-threaded environment, but that's ok
- private[HashMap] def ensurePair: (A, B) = if (kv ne null) kv else {
- kv = (key, value); kv
+ private[HashMap] def ensurePair: (A, B) = if (kvOrNull ne null) kvOrNull else {
+ kvOrNull = (key, value); kvOrNull
  }
 
  protected[HashMap] override def merge0[B1 >: B](that: HashMap[A, B1], level: Int, merger: Merger[A, B1]): HashMap[A, B1] = {
@@ -405,10 +405,10 @@ object HashMap extends ImmutableMapFactory[HashMap] with BitOperations.Int {
  else if (this.hash == hm1.hash && this.key == hm1.key)
  if (merger eq HashMap.defaultMerger) this
  else if (merger eq HashMap.defaultMerger.invert) hm1
- else this.updated0(hm1.key, hm1.hash, level, hm1.value, hm1.kv, merger)
- else this.updated0(hm1.key, hm1.hash, level, hm1.value, hm1.kv, merger)
+ else this.updated0(hm1.key, hm1.hash, level, hm1.value, hm1.ensurePair, merger)
+ else this.updated0(hm1.key, hm1.hash, level, hm1.value, hm1.ensurePair, merger)
  case _ =>
- that.updated0(key, hash, level, value, kv, merger.invert)
+ that.updated0(key, hash, level, value, ensurePair, merger.invert)
  }
  }
 
@@ -504,7 +504,7 @@ object HashMap extends ImmutableMapFactory[HashMap] with BitOperations.Int {
  hm.merge0(this, level, merger.invert)
  case h1: HashMap1[A, B1] =>
  if (h1.hash != hash) makeHashTrieMap(hash, this, h1.hash, h1, level, size + 1)
- else updated0(h1.key, h1.hash, level, h1.value, h1.kv, merger)
+ else updated0(h1.key, h1.hash, level, h1.value, h1.ensurePair, merger)
  case c: HashMapCollision1[A, B1] =>
  if (c.hash != hash) makeHashTrieMap(hash, this, c.hash, c, level, c.size + size)
  else if (merger.retainIdentical && (c eq this)) this
@@ -760,7 +760,7 @@ object HashMap extends ImmutableMapFactory[HashMap] with BitOperations.Int {
 
  protected[HashMap] override def merge0[B1 >: B](that: HashMap[A, B1], level: Int, merger: Merger[A, B1]): HashMap[A, B1] = that match {
  case hm: HashMap1[A, B1] =>
- this.updated0(hm.key, hm.hash, level, hm.value.asInstanceOf[B1], hm.kv, merger)
+ this.updated0(hm.key, hm.hash, level, hm.value.asInstanceOf[B1], hm.ensurePair, merger)
  case that: HashTrieMap[A, B1] =>
  def mergeMaybeSubset(larger: HashTrieMap[A, B1], smaller: HashTrieMap[A, B1], merger: Merger[A, B1]):HashTrieMap[A, B1] = {
  var resultElems: Array[HashMap[A, B1]] = null