Merge pull request scala/scala#7009 from lrytz/merge-2.12-to-2.13-aug-7

Merge 2.12 to 2.13 aug 7 [ci: last-only]

Author: SethTisue

library/src/scala/Enumeration.scala

@@ -27,6 +27,11 @@ import scala.util.matching.Regex
  *  `Value` type member of the enumeration (`Value` selected on the stable
  *  identifier path of the enumeration instance).
  *
+ *  Values SHOULD NOT be added to an enumeration after its construction;
+ *  doing so makes the enumeration thread-unsafe. If values are added to an
+ *  enumeration from multiple threads (in a non-synchronized fashion) after
+ *  construction, the behavior of the enumeration is undefined.
+ *
  * @example {{{
  * // Define a new enumeration with a type alias and work with the full set of enumerated values
  * object WeekDay extends Enumeration {

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

@@ -312,7 +312,7 @@ object HashMap extends MapFactory[HashMap] {
     protected def merge0[V1 >: V](that: HashMap[K, V1], level: Int, merger: Merger[K, V1]): HashMap[K, V1] = {
       // this can be made more efficient by passing the entire ListMap at once
       var m = that
-      for (p <- kvs) m = m.updated0(p._1, this.hash, level, p._2, p, merger)
+      for (p <- kvs) m = m.updated0(p._1, this.hash, level, p._2, p, merger.invert)
       m
     }
 

library/src/scala/collection/mutable/ArrayBuilder.scala

@@ -98,9 +98,9 @@ object ArrayBuilder {
     protected var elems: Array[T] = _
 
     private def mkArray(size: Int): Array[T] = {
-      val newelems = new Array[T](size)
-      if (this.size > 0) Array.copy(elems, 0, newelems, 0, this.size)
-      newelems
+      if (capacity == size && capacity > 0) elems
+      else if (elems eq null) new Array[T](size)
+      else java.util.Arrays.copyOf[T](elems, size)
     }
 
     protected[this] def resize(size: Int): Unit = {