Swift：具有多个模式的Switch case无法绑定到变量

这是允许的：

let somePoint = (1,1)
switch somePoint {
// Case with multiple patterns without binding
case (0,_),(_,0):
    println("((somePoint.0),(somePoint.1)) is on an axis")
default:
    println("((somePoint.0),(somePoint.1)) is not of an axis")
}

这也是允许的：

let somePoint = (1,1)
switch somePoint {
// Case with single patterns with binding
case (0,let y):
    println("(0,(y)) is on an axis")
case (let x,0):
    println("((x),0) is on an axis")
default:
    println("((somePoint.0),(somePoint.1)) is not of an axis")
}

但是,这是禁止的：

let somePoint = (1,1)
switch somePoint {
// Case with multiple patterns that have bindings
case (0,let y),(let x,(y)) is on an axis")
default:
    println("((somePoint.0),(somePoint.1)) is not of an axis")
}

以上产生错误：

error: 'case' labels with multiple patterns cannot declare variables