我找到了Scala编译器错误吗？

Question

在Scala 2.9.1中，这不能编译，失败，未找到：值b ：

case class CaseClass(field: String)

object SomeObject {
  //val kludge = field

  def x(input: (CaseClass, String) => CaseClass): Unit = ()
  val field = x((a, b) => a.copy(field = b))
}

但是，这样做：

case class CaseClass(field: String)

object SomeObject {
  val kludge = field

  def x(input: (CaseClass, String) => CaseClass): Unit = ()
  val field = x((a, b) => a.copy(field = b))
}  

唯一的区别是注释行。如果这不是一个错误，为什么这是预期的行为？