trait Container[S <: Seq[Int] with Singleton] {
  val s: S
}

trait Builder {
  def withoutDefault(s0: Seq[Int]): Container[s0.type] = new Container[s0.type] {
    val s: s0.type = s0
  }
  def withDefault(s0: Seq[Int], i: Int = 0): Container[s0.type] = withoutDefault(s0)
}

object Builder {
  val instance1 = new Builder { }
  def instance2 = new Builder { }
}

object Builder1 {
  def withoutDefault(s0: Seq[Int]): Container[s0.type] = new Container[s0.type] {
    val s: s0.type = s0
  }
  def withDefault(s0: Seq[Int], i: Int = 0): Container[s0.type] = withoutDefault(s0)
}

object Test {
  val seq = Seq(1, 2, 3, 4)
  val works1: Container[seq.type] = Builder1.withoutDefault(seq)
  val works2: Container[seq.type] = Builder1.withDefault(seq, 0)
  val works3: Container[seq.type] = Builder1.withDefault(seq)

  val works4: Container[seq.type] = Builder.instance1.withoutDefault(seq)
  val works5: Container[seq.type] = Builder.instance1.withDefault(seq, 0)
  val works6: Container[seq.type] = Builder.instance1.withDefault(seq)

  val works7: Container[seq.type] = Builder.instance2.withoutDefault(seq)
  val works8: Container[seq.type] = Builder.instance2.withDefault(seq, 0)
  val fails: Container[seq.type] = Builder.instance2.withDefault(seq)
}