scala – 使用Context Bound时无法找到隐式值

sealed trait Acceptable[T]

  object Acceptable {
    implicit object Int extends Acceptable[Int]

    implicit object String extends Acceptable[String]
  }

  case class Enc[T](f: T => Any)

  implicit def test[I,T](implicit f: I => T,a: Acceptable[T]): Enc[I] =
    Enc[I](f)

  val e = implicitly[Enc[Int]]

它成功编译.

如您所见：可接受的[T]参数应该很容易转换为context bound：

implicit def test[I,T: Acceptable](implicit f: I => T): Enc[I] =
    Enc[I](f)

但在那之后,更改编译开始失败并出现错误：

could not find implicit value for parameter e: app.Enc[Int]

为什么会这样？

更新：

我试过-Xlog-implicits编译器选项和编译日志给我：

[info] /path/to/ScalaTest/src/main/scala/app/Main.scala:60: test is not a valid implicit value for app.Enc[Int] because:
[info] hasMatchingSymbol reported error: ambiguous implicit values:
[info]  both object Int in object Acceptable of type app.Acceptable.Int.type
[info]  and object String in object Acceptable of type app.Acceptable.String.type
[info]  match expected type app.Acceptable[T]
[info]   val e = implicitly[Enc[Int]]
[info]                     ^
[info] /path/to/ScalaTest/src/main/scala/app/Main.scala:60: app.test is not a valid implicit value for app.Enc[Int] because:
[info] hasMatchingSymbol reported error: ambiguous implicit values:
[info]  both object Int in object Acceptable of type app.Acceptable.Int.type
[info]  and object String in object Acceptable of type app.Acceptable.String.type
[info]  match expected type app.Acceptable[T]
[info]   val e = implicitly[Enc[Int]]
[info]                     ^
[error] /path/to/ScalaTest/src/main/scala/app/Main.scala:60: could not find implicit value for parameter e: app.Enc[Int]
[error]   val e = implicitly[Enc[Int]]

好的,我理解这个输出.但是为什么它在隐式参数的情况下有效呢？