trait ProjectReference
trait ProjectRef
trait AutoPlugin
private abstract class ProjectDef[PR <: ProjectReference](val autoPlugins: Seq[AutoPlugin]) extends ProjectDefinition[PR]
sealed trait ResolvedProject extends ProjectDefinition[ProjectReference] {
  /** The [[AutoPlugin]]s enabled for this project as computed from [[natures]].*/
  def autoPlugins: Seq[AutoPlugin]
}

sealed trait ProjectDefinition[PR <: ProjectReference] {
  private[sbt] def autoPlugins: Seq[AutoPlugin]
}


object Test {
  new ProjectDef[ProjectReference](null) with ResolvedProject
}