title: Swift Concurrency announced for iOS 13 in Xcode 13.2 - how did they achieve this?
body: <p><a href="https://developer.apple.com/documentation/xcode-release-notes/xcode-13_2-release-notes" rel="noreferrer">Xcode 13.2 Beta release notes</a> features a promise for Swift Concurrency support for iOS 13.</p>
  <blockquote>
  <p>You can now use Swift Concurrency in applications that deploy to macOS 10.15, iOS 13, tvOS 13, and watchOS 6 or newer. This support includes async/await, actors, global actors, structured concurrency, and the task APIs. (70738378)</p>
  </blockquote>
  <p>However, back in Summer 2021 when it first appeared at WWDC it was hard constrained to be run on iOS 15+ only.</p>
  <p>My question is: what changed? How did they achieve backwards compatibility? Does it run in any way that is drastically different from the way it would run in iOS 15?</p>