Allow implicit matches to bind type variables

[odersky]

This test case currently fails:

object Mapper {

  class MapTo[A, B](val f: A => B)

  inline def mapOne[X](x: X) = implicit match {
    case m: MapTo[X, y] => m.f(x)
  }

  inline def mapTuple[Xs <: Tuple](xs: Xs) <: Tuple = xs match {
    case xs: (x1 *: xs1) =>
      implicit match {
        case m: MapTo[`x1`, y] => m.f(xs.head.asInstanceOf[x1]) *: mapTuple(xs.tail : xs1)
      }
    case () =>
      ()
  }
}
object Test extends App {
  import Mapper._

  val xs = (1, "hello")

  locally {
    implicit val i1: MapTo[Int, Double] = new MapTo[Int, Double](_.toDouble)
    implicit val i2: MapTo[String, Boolean] = new MapTo[String, Boolean](_.length > 3)

    val r1 = mapOne[X = Int](1)

    val r2 = mapTuple(xs)
  }

}

The problem is that in the type pattern MapTo[X, y] of the implicit match case

   case m: MapTo[X, y] => m.f(x)

the y is bound to a GADT bound TypeRef. But the implicit search only knows how to constrain TypeParamRefs.

Hopefully, we get closer to a solution by identifying GADT bound and constrained variables.