return structural types (or use aux pattern) to hang on to path dependent types

StructuralReturnType.scala

package broken

import broken.Tag._

sealed trait Tag[A]
object Tag {
  case object IntTag    extends Tag[Int]
  case object StringTag extends Tag[String]
}

trait BrokenTestThing {
  type A
  def feedMyTag(tag: Tag[A]) = ???
}

object BrokenTestThing {
  def apply[B](thing: B): BrokenTestThing /*{ //uncomment to fix
    type A = B
  }*/ =
    new BrokenTestThing {
      type A = B
    }
}

object Test extends App {
  val brokenTestThing1 = BrokenTestThing("String")
  brokenTestThing1.feedMyTag(StringTag)
  val brokenTestThing2 = BrokenTestThing(42)
  brokenTestThing2.feedMyTag(IntTag)
}