README.md

This works:

scala> Typeclass[ADT].adtName(One)
res1: String = hi

However, this doesn't:

scala> Typeclass[One.type]
<console>:12: error: could not find implicit value for parameter evidence: Typeclass[One.type]
       Typeclass[One.type]

even though this does:

scala> implicit val typeclassForADT = Typeclass.typeclassFor[ADT]

scala> Typeclass[One.type]
res3: Typeclass[One.type] = Typeclass$$anon$1@3430e6a5

ADT.scala

sealed abstract trait ADT
case object One extends ADT
case object Two extends ADT

build.sbt

scalaVersion := "2.12.0"

crossScalaVersions := Seq("2.11.8", "2.12.0")

Typeclass.scala

import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context

trait Typeclass[-A] {
  def something(a: A): String
}

object Typeclass {
  implicit def typeclassFor[T](implicit ev: T =:= ADT): Typeclass[T] = new Typeclass[T] {
    def something(a: T): String = "hi"
  }

  def apply[A](implicit evidence: Typeclass[A]) =
    evidence
}