NotContains and IsDistinct constraints for HList

[vpavkin]

There was a conversation on gitter regarding these, but some urgent stuff weighted in short after.
So I leave it here for better times to be either merged or rejected :)

Note:
An implementation of IsDistinctConstraint without use of NotContainsConstraint can be following (with a use of ambiguous implicits trick):

trait IsDistinctConstraint[L <: HList]

object IsDistinctConstraint {
  def apply[T <: HList](implicit ev: IsDistinctConstraint[T]): IsDistinctConstraint[T] = ev

  // we have to build chain here so that ambiguous breaker works for smth like A :: B :: B :: HNil
  implicit def unique0 = new IsDistinctConstraint[HNil] {}
  implicit def unique1[H, T <: HList](implicit ev: IsDistinctConstraint[T]) = new IsDistinctConstraint[H :: T] {}

  implicit def uniqueAmb1[H <: HList, T](implicit ev: Selector[H, T]): IsDistinctConstraint[T :: H] = 
    shapeless.unexpected
  implicit def uniqueAmb2[H <: HList, T](implicit ev: Selector[H, T]): IsDistinctConstraint[T :: H] = 
    shapeless.unexpected
}

Maybe it's a better choice...

[milessabin]

This looks very promising, but the RC cycle for 2.3.0 has already started so I'm afraid I'm going to have to punt it until 2.3.1.

[vpavkin]

Sure, would be great! :)

[milessabin]

LGTM! Many thanks ... merging :-)