SI-3346

SI-3346.scala

//
// An attempt to workaround SI-2712, foiled by SI-3346
//

trait TC[M[_]]

type EitherInt[A] = Either[Int, A]

implicit object EitherTC extends TC[EitherInt]

def foo[M[_]: TC, A](ma: M[A]) = ()

val eii: Either[Int, String] = Right("")

foo[EitherInt, String](eii)

// This one needs SI-2712 Higher Order Unification
//foo(eii) // not inferred


// A workaround is to provide a set of implicit conversions that take values
// based on type constructors of various shapes, and search for the
// type class instances.
//
// This is the approach taken by scalaz7.

trait TCValue[M[_], A] {
  implicit def self: M[A]
	def M: TC[M]

  // instead of `foo(eii)`, we'll try `eii.foo`
	def foo[M[_], A] = ()
}


implicit def ToTCValue[M[_], A](ma: M[A])(implicit M0: TC[M]) = new TCValue[M, A] {
  implicit val M = M0
  val self = ma
}
implicit def ToTCValueBin1[M[_, _], A, B](ma: M[A, B])(implicit M0: TC[({type λ[α]=M[A, α]})#λ]): TCValue[({type λ[α] = M[A, α]})#λ, B] = new TCValue[({type λ[α]=M[A, α]})#λ, B] {
  implicit val M = M0
  val self = ma
}
implicit def ToTCValueBin2[M[_, _], A, B](ma: M[A, B])(implicit M0: TC[({type λ[α]=M[α, B]})#λ]): TCValue[({type λ[α]=M[α, B]})#λ, A] = new TCValue[({type λ[α]=M[α, B]})#λ, A] {
  implicit val M = M0
  val self = ma
}


ToTCValueBin1(eii).foo

// as expected, could not find implicit parameter
// ToTCValueBin2(eii).bar

// error: implicit conversions are not applicable because they are ambiguous, both method ToTCValueBin1 ... and  method ToTCValueBin2
//        annoying!!
//        https://issues.scala-lang.org/browse/SI-3346
//
// Works if we remove ToTCValueBin2
// 
eii.bar