I know Swift can't compile for iOS 6 as I've gone down that road in the past — you could, at one point, force it to happen, but there were all kinds of ABI issues and even slightly complex code would segfault. iOS 6 was never officially supported by Swift (though the betas of Swift 1.0 did work, unofficially). But it's kinda nice to know that modern Swift is still back-compatible all the way to iOS 7.0, the first OS Swift supported