Scala泛型隐式值重载时不明确？

Question

object Test {
  implicit def x = List(1, 2)

  trait A[T] {
    def res(): Int = 0
    def makeResPlusOne(): Int = res() + 1    // All right
  }
  trait B[T] extends A[T] {
    def res()(implicit y: List[T]) = y.size
  }
  class AA extends A[Int]
  class BB extends B[Int]
}
val x: Test.A[Int] = new Test.BB
x.res() // Outputs 0 instead of 2.

object Test {
  implicit def x = List(1, 2)

  trait A[T] {
    def res()(implicit y: List[T]): Int = 0  // Added the implicit keyword.
    def makeResPlusOne(): Int = res() + 1    // Fails to compile.
  }
  trait B[T] extends A[T] {
    override def res()(implicit y: List[T]) = y.size
  }
  class AA extends A[Int]
  class BB extends B[Int]
}
val x: Test.A[Int] = new Test.BB
x.res() // Error

error: could not find implicit value for parameter y: List[Int]

编辑

如果我导入一个隐式的。但是，我有MakeresplusOne()方法，它在第二种情况下触发错误，但在第一种情况下不触发。请告诉我如何正确定义这个方法，它通常在编译时不需要隐式。_

 error: could not find implicit value for parameter y: List[T]
    def makeResPlusOne(): Int = res() + 1
                                ^

Answer 1

你很亲密：

object Test {
  implicit def list = List(1, 2)

  trait A[T] {
    def res()(implicit y: List[T]): Int = 0  // Added the implicit keyword.
  }
  trait B[T] extends A[T] {
    override def res()(implicit y: List[T]) = y.size
  }
  class AA extends A[Int]
  class BB extends B[Int]
}
import Test.list
val x: Test.A[Int] = new Test.BB
x.res() // Works!

您忘记导入隐式列表。请注意，我已经重命名了隐式，以避免在导入变量x时发生冲突。

编辑：

object Test {
  implicit def xx = List(1, 2)

  trait A[T] {
    def res()(implicit y: List[T]): Int = 0
    def makeResPlusOne()(implicit y: List[T]): Int = res() + 1 // Works now.
  }
  trait B[T] extends A[T] {
    override def res()(implicit y: List[T]) = y.size
  }
  class AA extends A[Int]
  class BB extends B[Int]
}
import Test.xx // import the implicit list
val x: Test.A[Int] = new Test.BB
x.res() // Works