scala generic function `not found: type ?`

Question

When I use scala to create a function like this,It told me that not found: type ?

scala
    def save[
      K: SpatialComponent: TypeTag,
      V <: CellGrid: TypeTag: ? => TileMergeMethods[V]: ? => TilePrototypeMethods[V]
      ](id: LayerId, rdd: RDD[(K, V)] with Metadata[TileLayerMetadata[K]], method: KeyIndexMethod[K])

Can somebody told me how to resolve this?

Answer 1

I got it,must add this code to build.sbt.

resolvers += Resolver.sonatypeRepo("releases")
addCompilerPlugin("org.spire-math" % "kind-projector" % "0.8.0" cross CrossVersion.binary)

https://github.com/non/kind-projector