Swift date(byAdding:to:) returns nil for trivial calculation in REPL

问题

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

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

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?

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

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

回答1:

This is a bug in the Swift REPL, see

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

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

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)

来源：https://stackoverflow.com/questions/60459151/swift-datebyaddingto-returns-nil-for-trivial-calculation-in-repl