sample.scala

scala> for{(x,y) <- Right((3,4)).right} yield (x,y)
<console>:10: error: constructor cannot be instantiated to expected type;
 found   : (T1, T2)
 required: Either[Nothing,(Int, Int)]
              for{(x,y) <- Right((3,4)).right} yield (x,y)

scala> Right((3,4)).right map {case (x,y) => (x,y)}
res118: Product with Either[Nothing,(Int, Int)] with Serializable = Right((3,4))

scala> Right((2,3)).right.filter{case (x,y) => true;case _  => false}.get.right.map{case (x,y) => (x,y)}
res5: Product with Either[Nothing,(Int, Int)] with Serializable = Right((2,3))

filterが

Option[Either[Nothing,T]]

じゃなくて

Either.RightProjection[Nothing,T]

を返せばいいのに！