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

**URL:** <https://forums.swift.org/t/cannot-override-mutable-property-a1-of-type-a-with-covariant-type-b/22809>\
**Category:** Using Swift\
**Created:** [April 8, 2019, 12:45pm UTC](https://forums.swift.org/t/cannot-override-mutable-property-a1-of-type-a-with-covariant-type-b/22809 "2019-04-08T12:45:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![somu](https://avatars.discourse-cdn.com/v4/letter/s/ce73a5/32.png) [@somu](https://forums.swift.org/u/somu)\
**Post date:** [April 8, 2019, 12:45pm UTC](https://forums.swift.org/t/cannot-override-mutable-property-a1-of-type-a-with-covariant-type-b/22809/1 "2019-04-08T12:45:25Z")

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## 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?](https://forums.swift.org/t/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/10747), 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:

```swift
class A {}
class B : A {}

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

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

```

---

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**Author:** ![suyashsrijan](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/suyashsrijan/32/5292_2.png) [@suyashsrijan](https://forums.swift.org/u/suyashsrijan)\
**Post date:** [April 8, 2019, 1:15pm UTC](https://forums.swift.org/t/cannot-override-mutable-property-a1-of-type-a-with-covariant-type-b/22809/2 "2019-04-08T13:15:11Z")

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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](https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html).

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<div class="post-metadata">

**Author:** ![somu](https://avatars.discourse-cdn.com/v4/letter/s/ce73a5/32.png) [@somu](https://forums.swift.org/u/somu)\
**Post date:** [April 8, 2019, 1:21pm UTC](https://forums.swift.org/t/cannot-override-mutable-property-a1-of-type-a-with-covariant-type-b/22809/3 "2019-04-08T13:21:56Z")

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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.

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<div class="post-metadata">

**Author:** ![anthonylatsis](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/anthonylatsis/32/39232_2.png) [@anthonylatsis](https://forums.swift.org/u/anthonylatsis)\
**Post date:** [April 8, 2019, 8:05pm UTC](https://forums.swift.org/t/cannot-override-mutable-property-a1-of-type-a-with-covariant-type-b/22809/4 "2019-04-08T20:05:42Z")

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> [@somu](#):
>
> 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.

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.