1.scala

$ scala -Xprint:typer -e "for { a <- Option(2) ; b <- Option(a) } yield b"
[[syntax trees at end of                     typer]] // scalacmd15102494186904250690.scala
package <empty> {
  object Main extends scala.AnyRef {
    def <init>(): Main.type = {
      Main.super.<init>();
      ()
    };
    def main(args: Array[String]): Unit = {
      final class $anon extends scala.AnyRef {
        def <init>(): <$anon: AnyRef> = {
          $anon.super.<init>();
          ()
        };
        scala.Option.apply[Int](2).flatMap[Int](((a: Int) => scala.Option.apply[Int](a).map[Int](((b: Int) => b))))
      };
      {
        new $anon();
        ()
      }
    }
  }
}

2.scala

$ scala -Xprint:typer -e "for { a <- Option(2) ; b = a } yield b"
[[syntax trees at end of                     typer]] // scalacmd460138560044168679.scala
package <empty> {
  object Main extends scala.AnyRef {
    def <init>(): Main.type = {
      Main.super.<init>();
      ()
    };
    def main(args: Array[String]): Unit = {
      final class $anon extends scala.AnyRef {
        def <init>(): <$anon: AnyRef> = {
          $anon.super.<init>();
          ()
        };
        scala.Option.apply[Int](2).map[(Int, Int)](((a: Int) => {
  val b: Int = a;
  scala.Tuple2.apply[Int, Int](a, b)
})).map[Int](((x$1: (Int, Int)) => (x$1: (Int, Int) @unchecked) match {
          case (_1: Int, _2: Int): (Int, Int)((a @ _), (b @ _)) => b
        }))
      };
      {
        new $anon();
        ()
      }
    }
  }
}