Fix #7499: Prevent extending java.lang.Enum except from an enum

[TheElectronWill]

With this PR, extending java.lang.Enum from a class or trait gives an error.

class C1 extends jl.Enum[C1]
      ^
class C1 cannot extend java.lang.Enum: only enums defined with the enum syntax can

(one of) The Scala2 way(s) of declaring a java-compatible enum is also covered:

class C2(name: String, ordinal: Int) extends jl.Enum[C2](name, ordinal)
      ^
class C2 cannot extend java.lang.Enum: only enums defined with the enum syntax can

To emit a nice error for the above case I made the two-arguments constructor of Enum visible (if there's a better way I'll be glad to change that), and I introduced a check in CompleteJavaEnums to prevent that constructor from being explicitely called.

enum E extends java.lang.Enum[E]("name", 0) {
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            the constructor of java.lang.Enum cannot be called explicitly

[bishabosha]

Thanks for taking this on 💯 I just have a few comments

[TheElectronWill]

It's now possible to have a class that extends java.lang.Enum under -source 3.0-migration 🙃

[bishabosha]

LGTM, thanks :)

[bishabosha]

@TheElectronWill sorry, I made a mistake by suggesting Traits should be able to extend java.lang.Enum, e.g. in the test case

trait T extends java.lang.Enum[T]

enum MyEnum extends T {
  case A, B
}

something like EnumSet becomes unusable due to the F bound:

scala> java.util.EnumSet.allOf(classOf[MyEnum])
1 |java.util.EnumSet.allOf(classOf[MyEnum])
  |                        ^^^^^^^^^^^^^^^
  |        Found:    (classOf[MyEnum] : Class[MyEnum])
  |        Required: Class[E]
  |
  |        where:    E is a type variable with constraint <: Enum[LazyRef(E)]

[TheElectronWill]

Ah! It's easy to forbid traits again, I'll make a PR right now.