close #3667.

scala> def ser(o: AnyRef) = new java.io.ObjectOutputStream(new java.io.ByteArrayOutputStream()).writeObject(o)
ser: (o: AnyRef)Unit

scala> @serializable class Outer {
     |   case class Inner(x: Int)
     | }
defined class Outer

scala> val o = new Outer
o: Outer = Outer@34469729

scala> ser(new o.Inner(1))

scala> o.Inner // initialize the Inner$module field of o

res1: o.Inner.type = Inner

scala> ser(new o.Inner(1))
java.io.NotSerializableException: Outer$Inner$

review by extempore.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala b/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
index 735cb4a3c4..57540fffef 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
@@ -147,14 +147,15 @@ trait Unapplies extends ast.TreeDSL
       TypeTree(),
       Literal(Constant(cdef.name.decode)))
 
-    companionModuleDef(cdef, parents ::: List(gen.scalaScalaObjectConstr), List(toString))
+    companionModuleDef(cdef, parents, List(toString))
   }
 
-  def companionModuleDef(cdef: ClassDef, parents: List[Tree], body: List[Tree] = Nil): ModuleDef = atPos(cdef.pos.focus) {
+  def companionModuleDef(cdef: ClassDef, parents: List[Tree] = Nil, body: List[Tree] = Nil): ModuleDef = atPos(cdef.pos.focus) {
+    val allParents = parents ::: List( gen.scalaScalaObjectConstr)
     ModuleDef(
       Modifiers(cdef.mods.flags & AccessFlags | SYNTHETIC, cdef.mods.privateWithin),
       cdef.name.toTermName,
-      Template(parents, emptyValDef, NoMods, Nil, List(Nil), body, cdef.impl.pos.focus))
+      Template(allParents, emptyValDef, NoMods, Nil, List(Nil), body, cdef.impl.pos.focus))
   }
 
   private val caseMods = Modifiers(SYNTHETIC | CASE)