I'm in favor of this but as belatedly noted in the pitch thread we still need to use "4.1.50" (or similar) as the language version for -swift-version 4 in the Swift 4.2 compiler…so that 4.1 compilers can distinguish between Swift 4.1.x and Swift 4.2-in-4-mode! If you need to be compatible with a 4.0 or 4.1 compiler, you have to keep using #if swift a little longer.