bugRepro.scala

trait Element[+R]
class NullElement[+R] extends Element[R]

trait BugRepro {
  def combineElements(actions: Element[_]*): Element[Unit]

  def sequence[R, M[+_] <: TraversableOnce[_]](in: M[Element[R]]): Element[M[R]]

  val mySeq = Seq(1,2,3)


  // bugRepro.scala:13: error: inferred type arguments [Nothing,Any] do not conform to method sequence's type parameter bounds [R,M[+_] <: TraversableOnce[_]]
  //       sequence(mySeq.map(i => new NullElement[Int]))
  //                  ^
  // bugRepro.scala:13: error: type mismatch;
  //  found   : Any
  //  required: M[Element[R]]
  //       sequence(mySeq.map(i => new NullElement[Int]))
  def typeError = {
    combineElements(
      sequence(mySeq.map(_ => new NullElement[Int]))
    )
  }

  // compiles fine, although equivalent to above code
  def compiles = {
    val outlined = mySeq.map(_ => new NullElement[Int])
    combineElements(
      sequence(outlined)
    )
  }
}