DbRep.scala

#!/usr/bin/env scala
trait DbRep[A] {
  type Rep
}

class Email

object EmailRep extends DbRep[Email] {
  override type Rep = String
}

class Foo

object FooRep extends DbRep[Foo] {
  override type Rep = Int
}

object Main extends App {
  // error: an implicit parameter section must be last
  // def lower[A](a: A)(implicit dbRep: DbRep[A])(implicit eq: dbRep.Rep =:= String): A = ???

  def lower[A](a: A, dbRep: DbRep[A])(implicit eq: dbRep.Rep =:= String): A = a

  println(lower[Email](new Email, EmailRep))

  // error: Cannot prove that FooRep.Rep =:= String.
  // println(lower[Foo](new Foo, FooRep))

}