object TestCase { //now matter if you put (abstract) class or trait it will fail in all cases trait MapOps[T] //if fs was reduced to List (generic type with one parameter) then the code compiles //if you inherit from MapOps[T] instead of MapOps[F] then code compiles fine implicit def map2ops[T,F](fs: Map[T,F]) = new MapOps[F] { //if you remove this line, then code compiles lazy val m: Manifest[T] = sys.error("just something to make it compile") def is(xs: List[T]) = List(xs) } def main(args: Array[String]) { println(Map(1 -> "2") is List(2)) } }