[error] knownDirectSubclasses of ... observed before subclass ... registered

I have a simple ADT setup:

sealed trait KeyStoreLocation
  final case class Resource(path: String) extends KeyStoreLocation
  final case class File(path: String)     extends KeyStoreLocation

This type is used as a member of another case-class that I load via pureconfig. It seemed to work, but then after a while it stopped compiling:

[error] knownDirectSubclasses of KeyStoreLocation observed before subclass Resource registered
[error] knownDirectSubclasses of KeyStoreLocation observed before subclass File registered

What is weird that I have not changed any of the classes involved here! It seems like a compilation order issue to me, but I don’t know how to fix it.

[briangordon]

Note for people who are googling this… this bug should be fixed in Scala 2.12.4 (scala/scala#6029)

In the meantime one easy workaround is to rename the file containing your ADT to have “AA” in front of the filename. That rejiggers the lexicographical order of the filenames and the compiler stops complaining.

[sirthias]

Nesting the ADT “cases” in the super types companion object, as is a popular name spacing technique, appears to trigger this problem. For instance

sealed trait Foo
object Foo {
  case object AFoo extends Foo
  case object BFoo extends Foo
}