dotty implicit function types

Main_fails1.scala

object ImplicitFunShowTest {
  import Showing._
  def showList[T](list: List[T]) = list.map(t => show(t): String)
  // no implicit argument of type Showing.Show[T] found for parameter x$0 of method apply in trait ImplicitFunction1
  
  def main(args: Array[String]): Unit = {
    println( showList(List("a","b","c")) )
    println( showList(List(1,2,3)) )
  }
}

Main_fails2.scala

object ImplicitFunShowTest {
  import Showing._
  def showList[T](list: List[T]): Showable[T, List[String]] = list.map(t => show(t))
  // found:    implicit Showing.Show[T] => 
  //   scala.collection.immutable.List[Showing.Showable[T, String] | String]
  // required: Showing.Showable[T, scala.collection.immutable.List[String]]
  
  def main(args: Array[String]): Unit = {
    println( showList(List("a","b","c")) )
    println( showList(List(1,2,3)) )
  }
}

Main_works1.scala

object ImplicitFunShowTest {
  import Showing._
  def showList[T](list: List[T]): Showable[T, List[String]] = list.map(t => thisShow.show(t))
  
  def main(args: Array[String]): Unit = {
    println( showList(List("a","b","c")) )
    println( showList(List(1,2,3)) )
  }
}

Main_works2.scala

object ImplicitFunShowTest {
  import Showing._
  def showList[T](list: List[T]): Showable[T, List[String]] = list.map(t => show(t): String)
  
  def main(args: Array[String]): Unit = {
    println( showList(List("a","b","c")) )
    println( showList(List(1,2,3)) )
  }
}

Main_works3.scala

object ImplicitFunShowTest {
  import Showing._
  def showList[T](list: List[T]) = { implicit _: Show[T] =>
    list.map(t => show(t))
  }
  
  def main(args: Array[String]): Unit = {
    println( showList(List("a","b","c")) )
    println( showList(List(1,2,3)) )
  }
}

Showing.scala

object Showing {
  trait Show[T] { def show(t: T): String } 
  
  object Show {
    implicit val showString: Show[String] = new Show[String] {
      def show(t: String) = t
    }
    implicit val showInt: Show[Int] = new Show[Int] {
      def show(t: Int) = t.toString
    }
  }
  
  type Showable[T,R] = implicit Show[T] => R
  def thisShow[T]: Showable[T,Show[T]] = implicitly[Show[T]] 
  def show[T](t: T): Showable[T,String] = thisShow.show(t)
}