随播对象中的Scala隐式类型类优先级

问题描述

trait Eq[-A] {
  def eq(a: A, b: A): Boolean
}
object Eq {
  implicit object IntEq extends Eq[Int] {
    def eq(a: Int, b: Int) = a == b
  }
}

trait Supertrait[+A]
object Supertrait {
  implicit def Eq[A: Eq]: Eq[Supertrait[A]] = ???
}

trait Subtrait[+A] extends Supertrait[A]
object Subtrait {
  implicit def Eq[A: Eq]: Eq[Subtrait[A]] = ???
}

def f[A](x: Subtrait[A])(implicit ev: Eq[Subtrait[A]]) = ???

f(new Subtrait[Int] {})

编译此代码时，会发生以下错误:

When compiling this code, the following error occurs:

Error:(32, 4) ambiguous implicit values:
 both method Eq in object Supertrait of type [A](implicit evidence$1: Eq[A])Eq[Supertrait[A]]
 and method Eq in object Subtrait of type [A](implicit evidence$2: Eq[A])Eq[Subtrait[A]]
match expected type Eq[Subtrait[Int]]
  f(new Subtrait[Int] {})
   ^

为什么Subtrait随播对象中的implicit def的优先级比Supertrait中的优先级高?

Why doesn't the implicit def in the Subtrait companion object has higher precedence than the one in Supertrait?

我希望子特性的伴随对象中的implicit def的优先级高于父特性中的implicit def的优先级.

I'd like that implicit defs in the companion object of subtraits have higher precedence than those in supertraits.

LowPriorityImplicits技巧也不起作用.请参见在Scala中的隐式实例中强制执行优先级.

The LowPriorityImplicits trick does not work either. See Enforcing precedence in implicit instances in Scala.

推荐答案

您的代码似乎违反了非歧义规则的隐式要求.

It looks like your code violates the Non-Ambiguity Rule for implicits.

来自 Scala编程，第一版:

可以在此处找到完整的文本..

The complete text can be found here.

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