I get that they're the same command-line tool, but as a bystander that doesn't tell me whether it's implemented underneath as two separate incompatible compilers that are toggled between with a flag. And even if it's a single compiler with mostly shared components, that doesn't have to imply that the binaries that are produced are ABI-compatible between the two. I'm not implying that I cannot believe that this use-case will work, only asking for some assurance from the compiler devs that it does.
-swift-version 3 really just turns on some different paths through the parser and type checker to allow previously-valid Swift 3 code to survive to code generation time.
