Enum Improvements Epic

[bishabosha]

• enums should have sensible flags: Lazy enums. #5034
• forbid extending java.lang.Enum outside of an enum Prevent extending java.lang.Enum except from an enum #7499
• allow extending java.lang.Enum arbitrarily with 3.0-migration Problem extending java.lang.Enum #7174
• desugar parent java.lang.Enum before TASTy extending java.lang.Enum should be desugared before TASTy  #7478
• toString ergonomics Enum implementation overrides custom toString #7227
• .values on enum companion is not in order: 0.16.0-RC3 Enum .values does not return enums in order #6663
• drop scala.runtime.EnumValues: Drop EnumValues from Enums' modules' API #6620
• Require that the first class in the extends clause of an enum case is the enum class: enum branches can extend any class #6601
• variance not copied from base class type parameters: Variance not copied into enum case class type parameters #9260, Recursive extension method using implicit argument with generic fails to infer correct types #9509
• serialization incorrect for simple enum cases: Singleton enum cases (marked Serializable) are incorrectly serialized #9179
• bytecode issues
  • Bad type operand on stack: Invoke compiled enum class file from dotr failed #7424, Compiling code with Enums in without doing a clean compile causes runtime VerifyError #6664
  • java.lang.NoSuchFieldError: NoSuchFieldError When Using enum Variants from the REPL #7410, NoSuchFieldError when changing type parameter on enum case (incremental compilation) #6677, enum case object not accessible in package object at runtime #7287, Cannot access enum from dependency at runtime #7991
• Decide if we drop scala.Enum Should we remove scala.Enum trait as a parent of enums? #9873
  • Answer for this is we rename it to scala.reflect.Enum
• Allowing new mixins in individual cases for java enums: java style enums should not allow mixins in cases #9162 (decided not really a problem)
• Decide if .apply on enum companion should give precise type or not. (@smarter)
  • Additionally if so, decide if simple enum constants become case object not val.
• crash with java enum and varargs: Enum compiler crash with -Ycheck:all #7734
• implement precise return type on .apply for companion objects of class enum cases, and widen the class to the base enum when inferring types.
• drop enumLabel
• keep values array and valueOf only when singleton values appear
• make $fromOrdinal companion method public and rename to fromOrdinal

[nicolasstucki]

Discussion should go in individual issues.

[odersky]

Additionally if so, decide if simple enum constants become case object not val.

They should definitely stay vals. case objects generate way too much bytecode. The point of simple enum constants is that you can have hundreds of them in a type. I don't want a bloated jar because of that.

[abgruszecki]

@bishabosha do you think we can close this?

[bishabosha]

yes I think so