Members of enums are uninitialized at the time of val evaluation

[t9dupuy]

Compiler version

3.5.2

Minimized code

enum Color:
  case Red, Green, Blue

  val red = Red

println(Color.Red.red)

Output

null

Expectation

Value should be initialized and print Color.Red

[bishabosha]

it does at least warn with -Ysafe-init-global

-- Warning: --------------------------------------------------------------------
4 |  val red = Red
  |            ^^^
  |            Access uninitialized field value Red. Calling trace:
  |            ├── enum Color:	[ rs$line$1:1 ]
  |            │   ^
  |            ├── case Red, Green, Blue	[ rs$line$1:2 ]
  |            │                    ^
  |            ├── enum Color:	[ rs$line$1:1 ]
  |            │   ^
  |            └── val red = Red	[ rs$line$1:4 ]
  |                          ^^^

this isn't really possible to fix, it is exactly like doing this cyclic initialisation:

class Foo:
  val red = Foo.Red // constructor sets to value of uninitialised field

object Foo:
  val Red = Foo() // can't store field until constructor is finished

println(Foo.Red.red)

[Gedochao]

this isn't really possible to fix, it is exactly like doing this cyclic initialisation:

class Foo:
  val red = Foo.Red // constructor sets to value of uninitialised field

object Foo:
  val Red = Foo() // can't store field until constructor is finished

println(Foo.Red.red)

@bishabosha Hmm... If it is indeed a won'tfix, then we should have something better than a warning with a private flag such as -Ysafe-init-global, I think...
Could we just always warn, or even throw an error in this case?
cc @dwijnand @odersky @kasiaMarek