Most of the new warnings in 3.1 were with things that never should have worked in the first place (some stuff with public declarations referencing private typea

Most of the new warnings in 3.1 were with things that never should have worked in the first place (some stuff with public declarations referencing private typealiases in their type signatures and things like that). There were also some new warnings added for mixed type arithmetic which is a common source of surprising behavior. The initialize thing is as far as I understand due to a limitation in the Objective C runtime and overriding initialize in Swift never fully worked in the first place. If you feel the 3.0 to 3.1 migration was still too painful from a compatibility perspective, it is not too late to file bugs; Swift 4 will still remain compatible with Swift 3 source so we can make further adjustments to the type checker if needed.