I am an iOS developer not really well versed on compilers so forgive my ignorance.
From the discussion above I got the impression that I will not be able to deploy an app compiled with Swift 4 to an OS that comes with a Swift 5 ABI locked standard library?
Since we will not be able to ship the runtime and standard library with the app, the app compiled with Swift 4 will not be compatible with the OS provided runtime (Swift 5). Is this correct or am I totally off?