Issue Description:

If a class inheriting NSObject declares their own inits without overriding super's parameter-less init, and any one of these inits has default values for all parameters, calling the init with default parameter values may result in a runtime crash.

Basically, if I have:

class SomeObject: NSObject {
     var someValue: String

     init(value: String = "SomeObject") {
         self.someValue = value
         super.init()
     }
}

Somewhere the app crashes at a call site SomeObject() with:

Fatal error: Use of unimplemented initializer 'init()'...

Many third-party libraries do this, for example:

Alamofire: SessionDelegate.swift @ Line 38

PhoneNumberKit: PhoneNumberKit.swift @ Line 24

I cannot reproduce this in a brand-new project, but at least two iOS apps I wrote had encountered this problem. The same project that is compiled several times on different machines may get different results. I think I've found some clues to why this is happening, and a way to reproduce it although not entirely straightforward.

When BUILD_LIBRARY_FOR_DISTRIBUTION is turned on for Alamofire, build that crashes has the following Swift Interface generated:

@objc open class SessionDelegate : ObjectiveC.NSObject {
  public init(fileManager: Foundation.FileManager = .default)
  @objc override dynamic public init()
  @objc deinit
}

While the build that doesn't crash is missing the dynamic init in Swift Interface:

@objc open class SessionDelegate : ObjectiveC.NSObject {
 public init(fileManager: Foundation.FileManager = .default)
 @objc deinit
}

So to reproduce, download or write a framework that has a class with all-default-values init. Then archive and generate an XCFramework of it. For Alamofire 5.5.0 the command to create XCFramework from its source directory is:

(Note that you need to turn Build Library for Distribution ON and Skip Intall OFF first for the intended Alamofire target.)

xcodebuild archive -project "Alamofire.xcodeproj" -scheme "Alamofire iOS" -sdk iphoneos -archivePath ./iOS.xcarchive

xcodebuild archive -project "Alamofire.xcodeproj" -scheme "Alamofire iOS" -sdk iphonesimulator -archivePath ./Simulator.xcarchive

xcodebuild -create-xcframework -archive ./Simulator.xcarchive -framework "Alamofire.framework" -debug-symbols "$(pwd)/Simulator.xcarchive/dSYMs" -archive ./iOS.xcarchive -framework "Alamofire.framework" -debug-symbols "$(pwd)/iOS.xcarchive/dSYMs" -output "./Alamofire.xcframework"

Observe if any *.swiftinterface has

@objc override dynamic public init()

for SessionDelegate declaration. If it does, integrating this framework and call

_ = SessionDelegate()

somewhere in the app will crash at runtime.

If the generated framework doesn't have that line, try running the command again on another machine (or at some other time). I have not found exactly what is causing this and how to reproduce it consistently. But since the exactly same code generates different interfaces, I'm fairly certain this is compiler's wrong-doing.