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, 39 insertions, 0 deletions

diff --git a/test/files/run/SD-290.scala b/test/files/run/SD-290.scala
new file mode 100644
index 0000000000..0af9cb7cfa
--- /dev/null
+++ b/test/files/run/SD-290.scala
@@ -0,0 +1,39 @@
+object p1 {
+  class B
+  object B
+
+  class C extends java.io.Serializable
+  object C
+
+  type D = DD
+  object D
+}
+package object p2 {
+  class B
+  object B
+
+  class C extends java.io.Serializable
+  object C
+
+  type D = DD
+  object D
+
+}
+class DD extends java.io.Serializable
+
+object Test {
+  def main(args: Array[String]): Unit = {
+
+    // This is the behaviour that was intended and was unchanged by this commmit.
+    assert(!(p1.B : Object).isInstanceOf[scala.Serializable])
+    assert(p1.C.isInstanceOf[scala.Serializable])
+    assert(!(p1.D: Object).isInstanceOf[scala.Serializable])
+
+    assert(!(p2.B : Object).isInstanceOf[scala.Serializable])
+    assert(p2.C.isInstanceOf[scala.Serializable])
+
+    // this behaviour was different in 2.12.1 and earlier due to a bug
+    // in companionSymbolOf
+    assert(!(p2.D: Object).isInstanceOf[scala.Serializable])
+  }
+}