As you’ve probably already seen, the Swift team has announced the Swift 4.1 release process:
We’ve arranged to have swift-corelibs-foundation “auto-merge” from master for a longer period of time than the other projects. This is because we’re still doing more active development and we do not need as much time to converge for Linux as the rest of Swift does for all platforms. This should be in place until at least November 27.
After that, we’ll look at moving fixes to the branch on a case-by-case basis with new PRs. In the meantime, to target Swift 4.1 - put your changes up against the master branch.