lubs.scala

package scratch

sealed trait Sub1[-F[_],+G[_]] {
  def apply[A](f: F[A]): G[A]
}

object Sub1 {
  implicit def sub1[F[_]]: Sub1[F,F] = new Sub1[F,F] { def apply[A](f: F[A]) = f }
}

object TypeMember {

  sealed trait Lub1[-F[_],-G[_]] {
    type Lub[_]
    implicit def subF: Sub1[F,Lub]
    implicit def subG: Sub1[G,Lub]
  }
  trait Lub1Instances0 {
    implicit def lub1[F[_],G[_],Lub0[_]](implicit S1: Sub1[F,Lub0], S2: Sub1[G,Lub0]): Lub1[F,G] { type Lub[x] = Lub0[x] }= new Lub1[F,G] {
      type Lub[x] = Lub0[x]
      def subF = S1
      def subG = S2
    }
  }
  object Lub1 extends Lub1Instances0 {
    implicit def id[F[_]]: Lub1[F,F] { type Lub[x] = F[x] } = new Lub1[F,F] {
      type Lub[x] = F[x]
      def subF = implicitly
      def subG = implicitly
    }
  }


  def foo[F[_], G[_], A, B](x: F[A], y: G[B])(implicit L: Lub1[F,G]): L.Lub[(A, B)] = ???
  def bar = foo(List(1), List(2))
  def baz = foo(List(1), List(2))(implicitly)
}

object TypeParameter {

  sealed trait Lub1[-F[_],-G[_],+Lub[_]] {
    implicit def subF: Sub1[F,Lub]
    implicit def subG: Sub1[G,Lub]
  }
  trait Lub1Instances0 {
    implicit def lub1[F[_],G[_],Lub0[_]](implicit S1: Sub1[F,Lub0], S2: Sub1[G,Lub0]): Lub1[F,G,Lub0] = new Lub1[F,G,Lub0] {
      def subF = S1
      def subG = S2
    }
  }
  object Lub1 extends Lub1Instances0 {
    implicit def id[F[_]]: Lub1[F,F,F] = new Lub1[F,F,F] {
      def subF = implicitly
      def subG = implicitly
    }
  }


  def foo[F[_], G[_], Lub[_], A, B](x: F[A], y: G[B])(implicit L: Lub1[F,G,Lub]): Lub[(A, B)] = ???
  def bar = foo(List(1), List(2))
  def baz = foo(List(1), List(2))(implicitly)
}

// [error] /Users/mpilquist/Development/scratch/lubs/src/main/scala/scratch/lubs.scala:61: ambiguous implicit values:
// [error]  both value StringCanBuildFrom in object Predef of type => scala.collection.generic.CanBuildFrom[String,Char,String]
// [error]  and method $conforms in object Predef of type [A]=> <:<[A,A]
// [error]  match expected type T
// [error]   def baz = foo(List(1), List(2))(implicitly)
// [error]                                   ^
// [error] one error found