Swift：具有多种模式的开关大小写无法绑定到变量

在官方斯威夫特编程语言 https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/doc/uid/TP40014097-CH33-ID428指南中关于 switch case 有这样的说法： “...如果 case 包含多个与控制表达式匹配的模式，则这些模式都不能包含常量或变量绑定。”

包含多个模式是什么意思？

这意味着具有多种模式的 case 标签不能声明变量。

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, 0):
    println("(\(x), \(y)) is on an axis")
default:
    println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}

上面会产生一个错误：

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