Allow `override enum`

[japgolly]

Minimized code

//> using scala 3.7.4

sealed trait A:
  type Blah

case object B extends A:
  override class Blah()

case object C extends A:
  override enum Blah:
    case Testing

Output

[error] Only access modifiers are allowed on enum definitions
[error]   override enum Blah:
[error]                 ^^^^

Expectation

The C case should compile like the B case does.

[soronpo]

But you actually don't need the override keyword for this to compile.

[som-snytt]

This is interesting because the enum desugars to abstract class which is incompatible with override.

sealed trait A:
  type Blah <: reflect.Enum

case object B extends A:
  override class Blah extends reflect.Enum

case object C extends A:
  override enum Blah:
    case Testing

case object D extends A:
  override abstract class Blah extends reflect.Enum

If the intention is to check my spelling of Blah, then that is guaranteed by any term that is overridden and has a signature that implicates that type.

But maybe the intention was to override a term, much as the collections supply a factory companion. The companion of an enum is not "secondary" to the abstract class.

Maybe an exception for the synthetic class will prove harmless.

Improvement on 8.0 was #23795
and previously on Christmas Day 2019 #7850
which surface interesting questions about flag checking and the nature of enums.

[som-snytt]

I see it doesn't currently complain about override class X.

Scala 2 rejects the modifier; then refchecks does not check it in Scala 3.

[som-snytt]

The argument in favor of the status quo is simplicity (as mentioned in previous discussions). As noted above, what does a desugared enum override? It's easier to require overriding with an alias.