3

考虑：

sealed trait A

case object B extends A
case object C extends A
case object D extends A

sealed trait Test {
  type AA <: A
  val aa: AA
}

object Test {
  type Aux[AAA <: A] = Test { type AA = AAA }
}

def compilesOk(t: Test) = t.aa match {
  case _: B.type =>
    println("B")
  case _: C.type =>
    println("C")
  case _: D.type =>
    println("D")
}

def compileError(t: Test) = t.aa match {
  case B => println("B")
  case C => println("C")
  case D => println("D")
}

该compileError函数无法编译并出现以下错误：

Error:(47, 10) pattern type is incompatible with expected type;
 found   : B.type
 required: t.AA
    case B => println("B")
Error:(48, 10) pattern type is incompatible with expected type;
 found   : C.type
 required: t.AA
    case C => println("C")
Error:(49, 10) pattern type is incompatible with expected type;
 found   : D.type
 required: t.AA
    case D => println("D")

我真的不明白不兼容错误。所有B，C并且D是 的实例A，它有什么问题以及为什么compilesOk函数编译得很好？

4

0 回答 0