If you override all of NSString's designated initializers, you'll be able to inherit the required convenience initializer.

While the following works for NSString:

class C: NSString {

   override init() { fatalError() }
   required init?(coder aDecoder: NSCoder) { fatalError() }

   required init?(pasteboardPropertyList propertyList: Any, ofType type: NSPasteboard.PasteboardType) { fatalError() }

}

It doesn't work when subclassing NSMutableString.

class D: NSMutableString {

override init() { fatalError() }
required init?(coder aDecoder: NSCoder) { fatalError() }

required init?(pasteboardPropertyList propertyList: Any, ofType type: NSPasteboard.PasteboardType) { fatalError() }

}

'required' initializer 'init(stringLiteral🙂' must be provided by subclass of 'NSString'

NSMutableString adds init(capacity:)…and doesn't mark it as a designated initializer, which means init() is still needed as well. (Also, that AppKit one makes me very sad. That's a bug we probably can't fix.)

The compiler-side change here is probably to add a note about why init(stringLiteral:) can't be inherited, i.e. listing the designated initializers that haven't been overridden.

Agree. Offering a fix-it with the missing designated initializers would be great. They're hard to find because they can be scattered in many places.

@swift-ci create