Scala for-comprehension with tuple decomposition

for {
  a <- Some(1)
  b <- Some(2)
} yield (a,b)

返回一些((1,2))

for {
  a <- Right(1).right
  b <- Left(2).left
} yield (a,b)

返回左((1,2))

现在我想在理解中分解元组.

for {
  (a,b) <- Some((1,2))
  (c,d) <- Some((3,4))
} yield (a,b,c,d)

返回一些((1,2,3,4))

for {
  (a,b) <- Right((1,2)).right
  (c,d) <- Left((3,4)).left
} yield (a,d)

无法编译：

error: constructor cannot be instantiated to expected type;
found   : (T1,T2)
required: scala.util.Either[Nothing,(Int,Int)]
                   (a,2)).right

error: constructor cannot be instantiated to expected type;
found   : (T1,T2)
required: scala.util.Either[(Int,Int),Nothing]

为什么这个最后一个例子不起作用？有什么不同？