Tomorrow, we will be temporarily bringing down the CI, starting around 9 AM, California time, in order to upgrade to the latest beta of Xcode 9 and the associated SDKs. We will also be briefly locking master while we land some source changes in support of the updated SDKs on both master and the swift-4.0-branch. We expect the upgrade process to take an hour or two. We’ll send out a message when the upgrade is complete.
As a reminder, once the upgrade is complete, if you are doing development on a Mac, the minimum required Xcode to build Swift from source will be the latest Xcode 9 beta. The beta can be downloaded at: https://developer.apple.com/download/