无法解析受约束 class 类型参数的隐式

Question

package ir.ashkan.shahnameh

object Implicit {
  sealed trait A
  class B extends A
  class C extends A

  def listOf[T <: A](implicit ts: List[T]): List[T] = ts

  class Module[T <: A] {
    implicit val bList: List[B] = ???
    implicit val cList: List[C] = ???

    listOf[T].toSet // HERE
  }
}

编译失败：

could not find implicit value for parameter ts: List[T] (No implicit view available from Int => T.)
[error]     listOf[T].toSet
[error]           ^

Answer 1

Module 需要隐式 List[T]。您可以为其定义隐式构造函数参数：

object Implicit {
  sealed trait A
  class B extends A
  class C extends A

  def listOf[T <: A](implicit ts: List[T]): List[T] = ts
  
  implicit val aList: List[A] = List.empty[A]
  implicit val bList: List[B] = List.empty[B]

  class Module[T <: A](implicit ts: List[T]) {    
    listOf[T].toSet
  }
}

val x = new Implicit.Module[Implicit.B]

无法解析受约束 class 类型参数的隐式

Can't resolve the implicit for a constrained class type parameter

scala

implicit

type-parameter