fix #9179: ensure enum values are singleton with serialisation

[bishabosha]

depends on #9524 for correct semantics merged

fixes #9179

this also adds the values and valueOf methods to companions of enums parameterised by a type

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[bishabosha]

I guess the issue now is that if someone has the wrong parents for their enum, then they get a cryptic error related to an object of the wrong type being passed to the EnumValues.register method, e.g.:

scala> enum Tag[+T]:
     |   case Int extends AnyRef
     | 
2 |  case Int extends AnyRef
  |                         ^
  |              Found:    ($anon.this : Object with runtime.EnumValue {...})
  |              Required: Tag[?]

Edit: this can be overcome by a cast at the register call that will be eliminated and refchecks will sanity check for unsafe casts
Second Edit: asInstanceOf is rejected by init checker

[smarter]

I guess the issue now is that if someone has the wrong parents for their enum

Can't we do something like:
When typechecking a case, verify that it's a subtype of the enum, if not emit an error and artificially add the enum as a parent of the class anyway to avoid follow-on errors

[bishabosha]

When typechecking a case, verify that it's a subtype of the enum, if not emit an error and artificially add the enum as a parent of the class anyway to avoid follow-on errors

for

enum Foo[F[_]]:
  case Bar extends AnyRef

what's the best way to invent type arguments for Foo that are well formed?

as an alternative which I don't really like, I am replacing the template body with an empty list if the parents are bad

[smarter]

what's the best way to invent type arguments for Foo that are well formed?

Nothing is poly-kinded so Foo[Nothing] should work.

[bishabosha]

@smarter thank you for your suggestion about adding in the parent, it seems to not cause any knock on effects

[bishabosha]

I was experimenting with removing the EnumValueSerializationProxy class and was filtering on the companion to look for the .values method, but because it is has no parameters its info is ExprType and so its signature is Signature.NotAMethod, should that be correct?, is filtering by signature not possible? (.values is guaranteed anyway where I am doing this check so not necessary)

[smarter]

its info is ExprType and so its signature is Signature.NotAMethod, should that be correct?

That's normal yes, and you can filter by .signature == Signature.NotAMethod to find it

[bishabosha]

I added another case to prevent anonymous classes extending an enum if they were defined within a term called $new (where $new isn't synthetic)

[bishabosha]

rebased

[bishabosha]

I'm thinking that adding .values when there are only non-simple enum cases is actually a bug fix, not a new feature, without it java.util.EnumSet crashes whenever you try to construct it:

scala> enum Foo[T] extends java.lang.Enum[Foo[_]] { case A extends Foo[Unit] }
// defined class Foo

scala> java.util.EnumSet.allOf(classOf[Foo[?]])
java.lang.ClassCastException: class rs$line$1$Foo not an enum
	at java.util.EnumSet.noneOf(EnumSet.java:112)
	at java.util.EnumSet.allOf(EnumSet.java:131)
	at rs$line$2$.<init>(rs$line$2:1)
	at rs$line$2$.<clinit>(rs$line$2)
	at rs$line$2.res0(rs$line$2)
        ...

and now:

scala> java.util.EnumSet.allOf(classOf[Foo[?]])
val res0: java.util.EnumSet[Foo[?]] = [A]

[smarter]

Agreed (the alternative bugfix would be to reject java.lang.Enum parents for parameterized enums)