Swift日期（byAdding：to :)返回nil以进行REPL中的琐碎计算 [英] Swift date(byAdding:to:) returns nil for trivial calculation in REPL

问题描述

我希望它返回一个 Date 对象，该对象表示从现在开始一小时的时间：

I expect this to return a Date object representing the time one hour from now:

Calendar.current.date(byAdding: DateComponents(hour: 1), to: Date()) 

但是它返回 nil 。这似乎是API的一种非常直接的用法，所以也许我对API的工作方式有一些基本的误解？

However it returns nil. This seems like a rather straightforward use of the API so perhaps I have some fundamental misconception about how this API should work?

仅 影响Xcode随附的 swift 命令行REPL（在macOS Catalina 10.15.3上的版本11.3.1）。

This only appears to affect the swift command line REPL included with Xcode (version 11.3.1 on macOS Catalina 10.15.3).

有趣的是，将上述代码包装在 print（）调用中会强制其打印正确的结果。那么为什么显示 nil ？

Interestingly, wrapping the above code in a print() call forces it to print the correct result. So why is it showing nil?

Apple Swift version 5.1.3 (swiftlang-1100.0.282.1 clang-1100.0.33.15)
Target: x86_64-apple-darwin19.3.0

推荐答案

这是Swift REPL，请参见

This is a bug in the Swift REPL, see

• SR-11593 REPL错误地将URL报告为nil


• SR-12172日期说明？即使非nil表达式也会打印为nil

• SR-11593 REPL incorrectly reports URL as nil
• SR-12172 Description of Date? expression is printed as nil even when non-nil

它会影响某些Swift覆盖的可选值和非可选值类型（例如日期，日历， URL ）。示例：

It affects both optional and non-optional values of some Swift overlay types (such as Date, Calendar, URL). Example:

$ swift
Welcome to Apple Swift version 5.1.3 (swiftlang-1100.0.282.1 clang-1100.0.33.15).
Type :help for assistance.
  1> import Foundation
  2>  
  3> let d1 = Date()
d1: Date = {}
  4> let d2: Date? = Date()
d2: Date? = nil
  5> let url = URL(string: "https://www.google.com")
url: URL? = nil

作为解决方法，您可以打印值：

As a workaround you can print the values:

  7> print(d1)
2020-03-03 10:17:00 +0000
  8> print(d2)
Optional(2020-03-03 10:17:07 +0000)
  9> print(url)
Optional(https://www.google.com)

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