Empty enum bodies are accepted

[Blaisorblade]

After enum A extends AnyRef the REPL thinks the line is not over and requires braces. The REPL should instead do the same it does for classes:

scala> class A extends AnyRef
// defined class A

scala> enum A extends AnyRef
     | {}
// defined class A

(noticed while testing #5008).

EDIT: enum A extends AnyRef {} is forbidden by the grammar, see below.

[allanrenucci]

This is a parser issue:

enum Foo extends AnyRef

-- [E040] Syntax Error: tests/allan/Test.scala:99:23 ---------------------------
99 |enum Foo extends AnyRef
   |                       ^
   |                       '{' expected, but eof found

[allanrenucci]

Shouldn't we require enum to be non empty anyway?

[Blaisorblade]

Oh right, that's actually required by the grammar (so this is a parser issue):

EnumDef           ::=  id ClassConstr [‘extends’ [ConstrApps]] EnumBody
EnumBody          ::=  [nl] ‘{’ [SelfType] EnumStat {semi EnumStat} ‘}’
EnumStat          ::=  TemplateStat
                    |  {Annotation [nl]} {Modifier} EnumCase
EnumCase          ::=  ‘case’ (id ClassConstr [‘extends’ ConstrApps]] | ids)

We should additionally require at least one EnumCase (in either Parser or Desugar), lest people write

enum Root {
  case class Foo()
}

Defining case classes inside enums is legal but is most likely a typo. It seems hard to forbid (for consistency reasons) since enums can have arbitrary members, tho maybe we should anyway — this looks very much likely a future wart.