[swift-evolution] Casting Bug Swift

Joe Groff jgroff at apple.com
Thu Feb 25 15:06:48 CST 2016

> On Feb 25, 2016, at 1:01 PM, Антон Жилин via swift-evolution <swift-evolution at swift.org> wrote:
> Now imagine that covariance in generics was allowed. I will also use generic notation for arrays here.
> var apples: Array<Apple> = []
> apples.append(Apple())
> var fruits: Array<Fruit> = apples   // can we allow this?
> fruits.append(Orange())
> apples[0].tell()   //=> I'm an Orange!
> We obviously don't want this to happen.
> So we decide to automatically create an empty Array<Fruit> and call append() with all casted elements of the initial array whenever arrays are casted:
> var fruits: Array<Fruit> = copyThatArray(apples) as Array<Fruit>
> Now people will come and ask, why can't I do the same with Set<Fruit>? With Box<Fruit> from that library? But everything works on Array<Fruit>! Why??
> And so I end my story with conclusion that we should not allow covariant generics in any form.

This is a problem for reference-semantics arrays, but Swift's arrays have value semantics. `var fruits: Array<Fruit> = apples` creates a copy of the array, so adding Oranges to `fruits` doesn't affect `apples`.

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