I1.scala

class Foo

object Main {
  def hoge[A](fooCompatible: A)(implicit converter: Converter[A]) = {
  }

  def main(args: Array[String]) {
    hoge(new Foo)(Converter.identity) // この行を消すと下の行もエラーになる
    hoge(new Foo)
  }
}

trait Converter[A] extends (A => Foo) {
}

object Converter {
  implicit val identity = new Converter[Foo] {
    def apply(foo: Foo) = foo
  }
}

I2.scala

// これでも通らない
class Foo

object Main {
  def hoge[A](fooCompatible: A)(implicit converter: Converter[A]) = {
  }

  def main(args: Array[String]) {
    println(Converter.x) 
    // hoge(new Foo)(Converter.identity)
    hoge(new Foo)
  }
}

trait Converter[A] extends (A => Foo) {
}

object Converter {
  val x = 1

  implicit val identity = new Converter[Foo] {
    def apply(foo: Foo) = foo
  }
}

I3.scala

// これは通る
class Foo

object Main {
  def hoge[A](fooCompatible: A)(implicit converter: Converter[A]) = {
  }

  def main(args: Array[String]) {
    // hoge(new Foo)(Converter.identity)
    hoge(new Foo)
  }
}

trait Converter[A] extends (A => Foo) {
}

object Converter {
  implicit val identity: Converter[Foo] = new Converter[Foo] {
    def apply(foo: Foo) = foo
  }
}

I4.scala

// これは通らない
class Foo

object Main {
  def hoge[A](fooCompatible: A)(implicit converter: Converter[A]) = {
  }

  def main(args: Array[String]) {
    // hoge(new Foo)(Converter.identity)
    hoge(new Foo)
  }
}

trait Converter[A] extends (A => Foo) {
}

object Converter {
  val i: Converter[Foo] = new Converter[Foo] {
    def apply(foo: Foo) = foo
  }

  implicit val identity = i
}