Modules w. serializable type alias "companions" are not serializable

The behaviour changed in #5550, this commit adapts to the change so
that we'll be binary compatible after boostrapping.

MiMa alerted us to a change in the parentage of two objects in the
forkjoin package object.

In Scala 2.12.0/1, they implemented `scala.Serializable`. Recently,
this (synthetically added) parent was absent. This appears to be
due to a bug fix in `companionSymbolOf`, which no longer treats
objects and same-named type aliases to be companions.

This commit manually adds the formerly-synthetic parents to these
objects, and documents the change in compiler behaviour with a test.

Fixes scala/scala-dev#290

1 files changed, 2 insertions, 2 deletions

diff --git a/src/library/scala/concurrent/forkjoin/package.scala b/src/library/scala/concurrent/forkjoin/package.scala
index 1915e25d7b..889890e30b 100644
--- a/src/library/scala/concurrent/forkjoin/package.scala
+++ b/src/library/scala/concurrent/forkjoin/package.scala
@@ -25,7 +25,7 @@ package object forkjoin {
   @deprecated("use java.util.concurrent.ForkJoinTask directly, instead of this alias",         "2.12.0")
   type ForkJoinTask[T] = juc.ForkJoinTask[T]
   @deprecated("use java.util.concurrent.ForkJoinTask directly, instead of this alias",         "2.12.0")
-  object ForkJoinTask {
+  object ForkJoinTask extends scala.Serializable {
     def adapt(runnable: Runnable): ForkJoinTask[_]                           = juc.ForkJoinTask.adapt(runnable)
     def adapt[T](callable: juc.Callable[_ <: T]): ForkJoinTask[T]            = juc.ForkJoinTask.adapt(callable)
     def adapt[T](runnable: Runnable, result: T): ForkJoinTask[T]             = juc.ForkJoinTask.adapt(runnable, result)
@@ -51,7 +51,7 @@ package object forkjoin {
   @deprecated("use java.util.concurrent.ThreadLocalRandom directly, instead of this alias",    "2.12.0")
   type ThreadLocalRandom      = juc.ThreadLocalRandom
   @deprecated("use java.util.concurrent.ThreadLocalRandom directly, instead of this alias",    "2.12.0")
-  object ThreadLocalRandom {
+  object ThreadLocalRandom extends scala.Serializable {
     // For source compatibility, current must declare the empty argument list.
     // Having no argument list makes more sense since it doesn't have any side effects,
     // but existing callers will break if they invoked it as `current()`.