From Ratan: desugar a for-comprehension to see why assignment won't work with Either RightProjections

for-comp-either-right.scala

////
// Output of running:
//   $ scala -Xprint:parser -e "val e: Either[String, Int] = Right(1); for {i <- e.right; j = i + 1} yield j"
////
//
// Picked up _JAVA_OPTIONS: -Xss3m -XX:MaxPermSize=256M
// /tmp/scalacmd2064075741948415763.scala:1: error: value map is not a member 
// of Product with Either[String,(Int, Int)] with Serializable
// val e: Either[String, Int] = Right(1); for {i <- e.right; j = i + 1} yield j
//                                               ^
// one error found
// [[syntax trees at end of parser]]// Scala source: scalacmd2064075741948415763.scala
package <empty> {
  object Main extends scala.ScalaObject {
    def <init>() = {
      super.<init>();
      ()
    };
    def main(argv: Array[String]): scala.Unit = {
      val args = argv;
      {
        final class $anon extends scala.AnyRef {
          def <init>() = {
            super.<init>();
            ()
          };
          val e: Either[String, Int] = Right(1);
          e.right.map(((i) => {
  val j = i.$plus(1);
  scala.Tuple2(i, j)
})).map(((x$1) => x$1: @scala.unchecked match {
            case scala.Tuple2((i @ _), (j @ _)) => j
          }))
        };
        new $anon()
      }
    }
  }
}