Singleton enum cases (marked `Serializable`) are incorrectly serialized

[akouryy]

Minimized code

Running twice with 0.25.0-RC1:

import java.io._

enum A:
  case B

object Main extends App:
  val file = File("/tmp/EnumSerialization.dat")

  if file.exists
    val a = ObjectInputStream(FileInputStream(file)).readObject().asInstanceOf[A]
    println(a == A.B)
    a match
      case A.B => // passes exhaustivity check
  else
    ObjectOutputStream(FileOutputStream(file)).writeObject(A.B)

Output

On second execution:

false

[error] java.lang.ExceptionInInitializerError
...
[error] Caused by: scala.MatchError: B (of class A$$anon$1)

Expectation

I expected a == A.B because it is marked Serializable by default.

I think there are two options:

• to stop Enum (with singleton cases) extending Serializable, which seems to go against Mirror.Product is available for enum values that are not subtypes of Product #9011, or
• to generate custom writeObject() and readObject() when desugaring enum, though I'm not sure if it's possible and worthwhile.

[odersky]

I think we should try to generate readObject and writeObject methods. It's natural for enumerations to be serializable.