Xcode 7 command failed due to signal: illegal instruction 4

Question

I just used Xcode 7's migration tool to migrate a project from Swift 1.2 to 2. After fixing up errors missed and such, all is well except for an error which prevents me from even building: Command failed due to signal: illegal instruction 4.

I have tried the help in these articles (Xcode 7 and Swift 2.0 : Command failed due to signal: Abort trap: 6, and Command failed due to signal: Abort trap: 6) which are not identical issues to mine, but nevertheless they were not able to fix the issue.

I have cleaned the build and removed the derived data folder. I have up to date CocoaPods installation, Xcode tools are at 7.0, and I Swift compile optimization is at None. Is there anything else I'm missing?

Thanks!

I had the same issue, in my case I had two classes and a protocol: class A: B, P but class B had a property enforced by protocol P. For some reason, not including the protocol in the class that really had the property was causing this error. I just made class B include the protocol and the error went away! — dcestari
Thanks @dcestari, was there a build error showing up for you that helped you find this? Or was it just random chance? — BJ Miller
Yes @bj-miller but they were not clear, I just saw similar things to ref myAttribute (assuming the myAttribute was the name of the property) and ref B. — dcestari
I see. I did see in the build error that it was referring to a particular file that is a superclass to 5 subclasses, as it has a few methods shared among all children. So now I'm re-watching Protocol-Oriented Programming to see if I can rewrite it ;-) Thanks for the reply @dcestari — BJ Miller
Please file a bug at bugreport.apple.com and include the crashlog, and if possible the project or a reduced case which causes the crash; then please post the bug number here. — Flash Sheridan

BJ Miller BJ Miller · Accepted Answer · 2015-06-24T18:34:14

So an answer was found thanks to the help of a coworker. We found the offending file in the build error, but there was no line provided. Through process of elimination, we found it to be a line that was declaring a new constant to the result of getting a JSON dictionary ([String : AnyObject], typealiased to JSONDictionary), from inside an optional dictionary. Here is the line:

let objectsDictionary = maybeJSON?[key] as? JSONDictionary

Changed this to two guard statements:

guard let goodJSON = maybeJSON as? JSONDictionary else { return ... }
guard let objectsDictionary = goodJSON[key] as? JSONDictionary else { return ... }

This line worked in Xcode 6.3.2 as it would just provide an optional value, but for some reason, some change in Xcode 7 didn't like this. I hope this can help anyone else who runs across this.

Xcode 7 command failed due to signal: illegal instruction 4

2 Answers