So far there are only a handful of small breaking changes between -swift-version 4 and -swift-version 5 on the master branch. The only ones I can think of are a

So far there are only a handful of small breaking changes between -swift-version 4 and -swift-version 5 on the master branch. The only ones I can think of are a type safety hole getting plugged and some warnings for corner cases around access control that got promoted to errors.