scala 泛型函数 `not found: type ?`

Question

当我使用 scala 创建这样的函数时，它告诉我 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])

谁能告诉我如何解决这个问题？

Answer 1

我明白了，必须将此代码添加到 build.sbt。

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

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