Imported From: https://issues.scala-lang.org/browse/SI-4333?orig=1
Reporter: @oxbowlakes
See #5211

@retronym said:
"If the selector of a pattern match is an instance of a sealed class (�5.2), the compilation of pattern matching can emit warnings which diagnose that a given set of
patterns is not exhaustive, i.e. that there is a possibility of a MatchError being raised at run-time."

That doesn't apply for Enumeration.

@oxbowlakes said:
Can this be upped to an enhancement request? I think that:

a) it breaks the principle of least surprise
b) it is something that can quite clearly be checked at compile-time

@retronym said:
For better or for worse, scala.Enumeration is provided by the library, not the language. As such, it isn't afforded special treatment by the pattern matcher.

Perhaps this could provided by an IDE inspection. But I doubt you'll convince anyone that it could or should go in the pattern matcher, circa 2011.

@acruise said:
I agree that special-casing Enumeration in the compiler is unlikely to happen, but it's interesting to think about how this kind of thing could be done in a general way.

CODE SKETCH, DO NOT COMPILE

case class sealed(value: Seq[_ <: Enumeration#Value] extends Annotation

object Foo extends Enumeration {
  @sealed(List(Foo1,Foo2)) // Probably doable, but smelly repetition
  @sealed(values)          // Too bad values is a def, not a stable identifier
  case class FooValue(i: Int, s: String, d: Double) extends Value(i, s)

  type Foo = FooValue

  val Foo1 = Foo(1,"One",1.0)
  val Foo2 = Foo(2,"Two",2.0)
}

@acruise said:
Replying to [comment:4 acruise]:

case class sealed(value: Seq[_ <: Enumeration#Value] extends Annotation

I guess that should be Seq[Any] or Seq[_] to avoid special-casing enumerations in the compiler. :)

@retronym said:
Well, you can't pass arbitrary objects to annotations, they must be Contant Expressions (like literals, arrays, other annotations, or final vals, in an object, without type annotation, with a constant expression as the RHS.

You can pass type parameters though.

https://gist.github.com/867149