Well I think that's because they aren't yet an official part of the language until that proposal is accepted, and Apple is shipping them as a custom extension t

Well I think that's because they aren't yet an official part of the language until that proposal is accepted, and Apple is shipping them as a custom extension to Swift that they hope to remove at some point. Edit: Also the current version of the proposal is here: https://github.com/apple/swift-evolution/blob/master/proposals/0289-function-builders.md https://github.com/apple/swift-evolution/blob/master/proposa...