Naming of enumeration fields relies on unspecified behaviour of the JVM

[scabug]

On Oracle JDK:

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0).
Type in expressions to have them evaluated.
Type :help for more information.

scala> object State extends Enumeration {
     |   val IDLE, STARTED = Value
     |   var x = IDLE
     | }
defined module State

scala> State.x
res0: State.Value = IDLE

scala> State.IDLE
res1: State.Value = IDLE 

On OpenJDK, it prints "x" instead of "IDLE" because the name of the var is accidentally picked up instead of the intended field name. This is implementation-dependent: "The elements in the array returned are not sorted and are not in any particular order" ([http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Class.html#getMethods()]).

There's probably not much we can do about this unless we can use Scala reflection instead of Java reflection to get the fields in the correct order.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-5462?orig=1
Reporter: @szeiger
Affected Versions: 2.9.1
See #5211

[som-snytt]

Result also changes from run to run, although on this jvm 2.11 usually produces one order for the accessors and 2.12 produces the reverse order.

$ scala
Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_144).
Type in expressions for evaluation. Or try :help.

scala> :pa
// Entering paste mode (ctrl-D to finish)

object Xab extends Enumeration {
  type Xab = Value
  val Aab, Bab = Value
  val Cab = Aab
}

// Exiting paste mode, now interpreting.

defined object Xab

scala> Xab.values
res0: Xab.ValueSet = Xab.ValueSet(Aab, Bab)

scala> :quit
$ scala
Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_144).
Type in expressions for evaluation. Or try :help.

scala> :pa
// Entering paste mode (ctrl-D to finish)

object Xab extends Enumeration {
  type Xab = Value
  val Aab, Bab = Value
  val Cab = Aab
}

// Exiting paste mode, now interpreting.

defined object Xab

scala> Xab.values
res0: Xab.ValueSet = Xab.ValueSet(Cab, Bab)

[sjrd]

This could be completely fixed by a macro that looks at the left-hand-side to which it is assigned, similarly to settingKey in sbt.

This would also remove all uses of reflection in Enumeration, which would make it fit for Scala.js out of the box. Currently we have to special-case it in our compiler plugin (and we basically do the job that a macro should be doing).