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kotlinlang

Is there some workaround for this? When `a` is a var I get this error:
```Type of 'a' doesn't match the type of the overridden 'var' property 'var a: Letter' defined in '/Base'```
It doesn't occur when it's a val
```open class Base {
    open var a: Letter = Letter()
}

class BaseExt : Base() {
    override var a: A = A()
}

open class Letter

class A : Letter()```

`A` is a subtype of `Letter`, so if it's a `val` all is good. The problem when it's a `var` is that someone could assign any subtype of `Letter` to `a`, which would break instances of `BaseExt`:

```val ext: Base = BaseExt()
ext.a = Letter() // respects the interface of Base, yet shouldn't be allowed for BaseExt```