Swift linker error in release build fixable by adding code

问题

In Xcode 8, a Swift project of mine works in Debug mode but fails to link in Release mode:

Undefined symbols for architecture x86_64:
  "Swift.UnsafeMutableBufferPointer.(subscript.materializeForSet : (Swift.Int) -> A).(closure #1)"

I can fix the error by changing this line:

let nextPeriod = currentSchedule.periods.filter({ $0.startDate > now }).sorted(by: { $0.startDate < $1.startDate }).first

to:

let nextPeriod = currentSchedule.periods.filter({ $0.startDate > now }).first

but that obviously alters the behavior of my app. Luckily, I can also fix the error by adding the following line to almost any method, initializer, or property observer:

let _ = [""].sorted(by: {$0 < $1})

(I say "almost" any because it doesn't seem to fix the linker error when added to property observers on properties with enum types I defined myself.)

...What?

回答1:

This is a compiler bug, which has been fixed in Xcode 8.1.

回答2:

I had the same problem using sorted(by: method in one project that I was migrating to Swift 3.

It seems a bug in the compiler and I found this following references:

• Swift 2.1 Error sorting in place, only on release builds (same problem in an old Xcode/Swift version)
  
• https://forums.developer.apple.com/thread/63213 (problem on Xcode 8)
  

For now the only workaround that worked for me was change the swift compiler optimization level to None on Build Settings.

回答3:

At first i thought is that weird code apple inserted in few places with Comparable stuffs... But didn't worked when i added it back. What worked was to step down the optimization level to: fast, single file optimization They messed up this xcode version, storyboards also have huge problems.

回答4:

Had the same problem when migrating to Swift 3.

I got it to work by letting the object that calls the code inherit from NSObject.

来源：https://stackoverflow.com/questions/39381045/swift-linker-error-in-release-build-fixable-by-adding-code