It's also probably worth distinguishing between "fully supported and shipped" versions and "versions which are accepted but aren't fully baked yet". By that I mean the Xcode 10 (Swift 4.2) compiler actually accepts -swift-version 5, but version 5 is still in development and tooling needs to know that even though version 5 is accepted by the compiler, it shouldn't be chosen by the tooling.

I'm still a little confused on the use case here. Any particular sources have an intrinsic Swift version; that can't just change because the compiler is different.

Sure they can. What do you think #if swift(>=4.2) is for?

There's also plenty of Swift source out there that is simple enough that it is compatible with multiple major Swift versions without any source-level changes. I'm pretty sure at least one of my Swift libraries required zero source changes to go from Swift 3 to Swift 4.

I guess that's true, but I still don't see how it translates to a feature here. If my file supports Swift 4.2 and Swift 5, I always want to use Swift 5 mode when it's available. If it just supports Swift 4.2, I don't have a choice. That makes me think that the actual compiler version is what's important, not the list of supported versions.

…except for 4.2, which throws off the whole "just use the major version thing", and now that it's happened once I can't really say it'll never happen again. Hm.

It's also unclear what'll happen with Swift versions in the future, after Swift 5.

I'm also not comfortable relying on the idea that the Swift compiler version tracks the Swift Language version.