2.12: case objects are not serializable when extending class

[RustedBones]

when declaring case objects that extends class instead of trait, serialization fails with scala 2.12.
case objects are supposed to be serializable.

Reproduction steps

Scala version: 2.12.19 (also testing on 2.12.x scala branch)

package scala

import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4

import java.io.{ByteArrayInputStream, ByteArrayOutputStream, ObjectInputStream, ObjectOutputStream}

sealed abstract class Status(val code: Int)
case object On extends Status(1)
case object Off extends Status(0)

@RunWith(classOf[JUnit4])
class CaseObjectSerializationTest {

  @Test
  def canSerializeMatchError = {
    val barrayOut = new ByteArrayOutputStream()
    new ObjectOutputStream(barrayOut).writeObject(On)
    val barrayIn = new ByteArrayInputStream(barrayOut.toByteArray)
    val status = new ObjectInputStream(barrayIn).readObject().asInstanceOf[Status]
    assert(status == On)
  }
}

Added the test from scala/2.12.x branch: diff

Problem

The test fails with

[error] Test scala.CaseObjectSerializationTest.canSerializeMatchError failed: java.io.InvalidClassException: scala.On$; no valid constructor, took 0.003 sec
[error]     at java.io.ObjectStreamClass$ExceptionInfo.newInvalidClassException(ObjectStreamClass.java:158)
[error]     at java.io.ObjectStreamClass.checkDeserialize(ObjectStreamClass.java:746)
[error]     at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2202)
[error]     at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1687)
[error]     at java.io.ObjectInputStream.readObject(ObjectInputStream.java:489)
[error]     at java.io.ObjectInputStream.readObject(ObjectInputStream.java:447)
[error]     at scala.CaseObjectSerializationTest.canSerializeMatchError(CaseObjectSerializationTest.scala:21)
[error]     at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]     at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error]     at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]     at java.lang.reflect.Method.invoke(Method.java:566)
[error]     ...

[RustedBones]

Did some digging and found the cause of the issue / resolution in scala 2.13 here

• scala 2.12: case object have synthetic method readResolve. This replaces the de-serialized object with the singleton instance. However, the de-serialization of the original object fails before the replacement. Indeed, a Serializable class must have access to the no-arg constructor of its first non-serializable superclass.

• scala 2.13: case object have synthetic method writeReplace and uses a proxy to de-serialize the object to return the singleton instance. No failure here.

[SethTisue]

right... fundamentally a duplicate of #10412 , I think? though the failure mode is different. in any case, closing since it's fixed as of 2.13.0 and we don't keep 2.12-only tickets open