“前缀的歧义使用"Swift 3 的编译器错误 [英] "Ambiguous use of prefix" compiler error with Swift 3

问题描述

我刚刚使用 Xcode 8 beta 将我的项目从 Swift 2.2 迁移到 Swift 3.0.

I just migrated my project from Swift 2.2 to Swift 3.0 with Xcode 8 beta.

我有类似于以下代码的内容(您可以将其粘贴到 Playground 中):

I have something similar to the following code (you can paste this into a playground):

import Foundation

let datesWithCount: [(Date, Int)] = [(Date(), 1), (Date(), 2), (Date(), 3)]

let dates: [Date] = datesWithCount.sorted {
    $0.0 < $1.0
}.prefix(1).map {
    return $0.0
}

在 Swift 2.2 中编译得很好.但是，使用 Swift 3.0 时出现错误

In Swift 2.2 this compiled fine. However, with Swift 3.0 I get the error

'前缀'的歧义使用

让它在 Swift 3.0 中编译的唯一方法是将地图拆分成单独的一行:

The only way to get this to compile in Swift 3.0 is to split out the map into a separate line:

let sortedDatesWithCount = datesWithCount.sorted {
    $0.0 < $1.0
}.prefix(1)

let mappedDates = sortedDatesWithCount.map {
    return $0.0
}

顺便说一句，在实际代码中，我从 map 返回 NSNotification 对象，而不是 Date ，但错误是相同的.我只是在这里使用了 Date 来简化示例.

BTW, in the actual code I'm returning NSNotification objects from the map not Dates but the error is the same. I just used Date here for making the example simple.

有什么办法可以把它编译成单行吗?

Is there any way to get this to compile as a one liner?

更新:为 Swift 项目创建了一个 JIRA.

UPDATE: Created a JIRA for the Swift project.

推荐答案

如果在将 ArraySlice 传递给 map 之前将其制作成一个数组，它会起作用:

It works if you make the ArraySlice into an Array before passing it to map:

let dates: [Date] = Array(datesWithCount.sorted {
    $0.0 < $1.0
}.prefix(1)).map { return $0.0 }

这看起来是编译器中的类型推断错误.

This looks like a type inference bug in the compiler.

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