Xcode 7 command failed due to signal: illegal instruction 4

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.

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  • 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?


    2 Solutions Collect From Internet About “Xcode 7 command failed due to signal: illegal instruction 4”

    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.

    ‘Illegal instruction’ simply means that your binary contains instructions that are invalid for the type of architecture you are trying to run the code with. Start looking at the minimum version in your project build settings.