Cannot override mutable property 'a1' of type 'A' with covariant type 'B'

Using Swift
1

Overview:

I have mentioned below a simplified example that causes the error Cannot override mutable property 'a1' of type 'A' with covariant type 'B'

Real Problem:

I have an iOS project in which when similar code is compiled on Xcode (for the simulator)

  • Sometimes it compiles ok (not sure if it was the cache)
  • Sometimes I get Command CompileSwift failed with a nonzero exit code
  • Sometimes I get Cannot override mutable property 'a1' of type 'A' with covariant type 'B'

Sorry the above explanation is very vague, when I created a new project with the below classes, I consistently get Cannot override mutable property 'a1' of type 'A' with covariant type 'B'

Note:

  • In my real world project when I change the overriding type to be the same, the code consistently compiles fine.
  • The overriding property was a subclass of UITextField (Based on the above pasted forum link, there was some pointers about how Objective-C might not be as strict as Swift, so I am not sure if it was because UITextField was written in Objective-C)

On the surface it seemed like valid code, but based on In a sub-class, is it possible to override a property of the super-class and constrain to a sub-type of the original property?, I understand that I shouldn't be overriding this way because of the example explained. (table view / scroll view delegate example).

Questions:

  1. Is the inconsistency in compilation error / success something that has been noticed by others too or is this a problem with my project ?
  2. Let me know if there is a way to find more from the non-zero error code.

Code:

class A {}
class B : A {}

class Car {
    private (set) var a1 = A()
}

class SpecialCar : Car {
    override var a1: B {
        return B()
    }
}
2

Change override var a1: B { ... } to override var a1: A { ... }

Also, I suggest you to read mikeash.com: Friday Q&A 2015-11-20: Covariance and Contravariance.

1 Like
3

Thanks @suyashsrijan on the surface my code seemed reasonable until I read the post on Swift Forums about a similar thread. I will read the link you posted.

I am just wondering what could be the inconsistency in compilation error and sometimes even compiled ok, not sure I am made a blunder or if it was some cache somewhere.

4

Cache issues are usually related to an error message not going away. In your case, CompileSwift failed tells me it might very well be a compiler crash. Try looking for a stack dump in Xcode's report navigator when the build fails. I suggest not just compiling, but running the app as well to be sure a build succeeded.

1 Like