Hi @phranck,
We're planning on doing this work in the master branch of swift-corelibs-foundation, as part of the work for Swift 5.
Back-porting it all to the Swift 3.1 branch may be possible - I don't think we are using anything new and fancy in the Swift for UserDefaults -- although the rest of swift-corelibs-foundation does.
We have to leave this as an exercise to the reader though. :)