模糊地使用运算符＆＃39; - ＆＃39;有两个Uint的

Question

在XCode beta 5中，当我在操场上执行以下代码时：

import Swift

let a = UInt(0)
let b = UInt(0)
var string = ""

string += a + b > 1 ?
    "true" :
    "false"

let bool = a - b > 1
string += bool ?
    "true" :
    "false"

string = a - b > 1 ?
    "true" :
    "false"

string += a - b > 1 ? //ERROR
    "true" :
    "false"

我收到以下错误：

Playground execution failed: /var/folders/ws/cpskvst94cq5fb0vjmljzmkw0000gn/T/./lldb/41335/playground29.swift:10:13: error: ambiguous use of operator '-'
string += a - b > 1 ?
            ^
Swift.-:1:6: note: found this candidate
func -<T : Strideable>(lhs: T, rhs: T) -> T.Stride
     ^
Swift.-:1:6: note: found this candidate
func -<T : _DisallowMixedSignArithmetic>(lhs: T, rhs: T) -> T._DisallowMixedSignArithmetic

这是打算吗？为什么只有最后一行产生错误？