Merge from two differently typed channels

EitherMerge.scala

import akka.stream.scaladsl._
import akka.stream._
import scala.util._

import FanInShape._
class EitherMergeShape[L, R]( _init: Init[Either[L, R]] = Name( "EitherMerge" ) )
    extends FanInShape[Either[L, R]]( _init ) {

  val left = newInlet[L]( "left" )
  val right = newInlet[R]( "right" )
  protected override def construct( i: Init[Either[L, R]] ) = new EitherMergeShape( i )
}

class EitherMerge[L, R] extends FlexiMerge[Either[L, R], EitherMergeShape[L, R]](
  new EitherMergeShape, Attributes.name( "EitherMerge" )
) {
  import FlexiMerge._
  override def createMergeLogic( p: PortT ) = new MergeLogic[Either[L, R]] {
    override def initialState =
      State[Any]( ReadPreferred[Any]( p.right, p.left ) ) { 
               // ^-- Here I get a compilation error. ReadPreferered inlets must have the same type. Why this constraint?!
        ( ctx, _, element ) ⇒

          val either = element match {
            case left: L  ⇒ Left( left )
            case right: R ⇒ Right( right )
          }
          ctx.emit( either )
          SameState
      }
  }
}

EitherMergeCompiles.scala

package apoyo
package storage

import akka.stream.scaladsl._
import akka.stream._
import scala.util._
import scala.reflect._

import FanInShape._
class EitherMergeShape[L, R]( _init: Init[Either[L, R]] = Name( "EitherMerge" ) )
    extends FanInShape[Either[L, R]]( _init ) {

  val left = newInlet[L]( "left" )
  val right = newInlet[R]( "right" )
  protected override def construct( i: Init[Either[L, R]] ) = new EitherMergeShape( i )
}

class EitherMerge[L: ClassTag, R: ClassTag] extends FlexiMerge[Either[L, R], EitherMergeShape[L, R]](
  new EitherMergeShape, Attributes.name( "EitherMerge" )
) {
  import FlexiMerge._
  override def createMergeLogic( p: PortT ) = new MergeLogic[Either[L, R]] {

    override def initialState =
      State[Any]( ReadPreferred[Nothing]( p.right, p.left ).asInstanceOf[ReadPreferred[Any]] ) {
        ( ctx, _, element ) ⇒

          val either = element match {
            case left: L  ⇒ Left( left )
            case right: R ⇒ Right( right )
          }
          ctx.emit( either )
          SameState
      }
  }
}

Added modified version based on Joe Edwards answer on akka-users list.