推断的类型参数

Question

编译此代码段时，为什么会出现错误？

trait ID[R <: Record[R] with KeyedRecord[Long]] {

  this: R =>

  val idField = new LongField(this)
}

ERROR：

inferred type arguments [ID[R] with R] do not conform to class LongField's
type parameter bounds [OwnerType <: net.liftweb.record.Record[OwnerType]]

我该如何解决这个问题？

---

LongField定义：

class LongField[OwnerType <: Record[OwnerType]](rec: OwnerType)
  extends Field[Long, OwnerType] with MandatoryTypedField[Long] with LongTypedField {

Answer 1

转换

val idField = new LongField(this)

到

val idField = new LongField[R](this)

如果您未指定类型R，则LongField无法检查该类型是否与Record[OwnerType]共同变体。明确提到它应该可以解决目的。

PS：我不知道要重新确认的其他类声明，但以下声明有效：

case class Record[R]
class KeyedRecord[R] extends Record[R]
class LongField[OwnerType <: Record[OwnerType]](rec: OwnerType)
trait ID[R <: Record[R] with KeyedRecord[Long]] {
  this: R =>
  val idField = new LongField[R](this)
}