I have checked the question related to how to find the current Swift version by the command swift --version, but how can I upgrade my Swift version from 3.1 to 3.2? (because the Xcode 9 only supports Swift 4.0 & Swift 3.2)
To build successfully both in Xcode 8 & Xcode 9, it's better to upgrade Swift from 3.1 to 3.2. But how?
My target was to make sure build successfully both in Xcode 8 & Xcode 9, So that publish with Xcode 8 would be OK, and try out the feature of Xcode 9 is OK too. So I will not upgrade code to swift 4.0 yet until Xcode 9.0 official edition is published.
After my test, I can't convert my code from swift 3.1 to 3.2 by Xcode 8.3.3's 'edit->convert' function.
The reason why I try to convert the code to Swift 3.2 is that I will get error of "Module compiled with Swift 3.1 cannot be imported in Swift 4.0" if I build the code with Xcode 9.
Swift 3.2 is a stopgap solution to continue using Swift 3 code under Xcode 9 without fully converting projects to Swift 4.
The Swift version included in Xcode 8.3.3 is 3.1. Converting your code to Swift 3.2 under Xcode 9 does not guarantee that it will still compile under Xcode 8.
I'm not sure compiling Swift 3.1 code as Swift 3.2 under Xcode 9 requires any conversion at all, since they both share the same syntax. You may have to fix some API calls to compensate for changes in the new SDKs.
Apple has a little more information here:
(updated)
Regarding your last image, if you have the source code for the Runes module, you'll have to recompile it under Xcode 9. That will make use of Swift 3.2 and allow it to be imported in Swift 4.
You can try this:
- Build Settings -> Swift Langauge Version -> Swift 3.2.
- Edit -> Convert -> To Current Swift Syntax.
Noted: These step using Xcode 9
Dependency modules/ frameworks in swift have to be recompiled w/ xcode9 since swift is not ABI compatible/ stable. If you have included dependencies using package distribution systems like carthage or cocoa pods, the modules would be recompiled w/ xcode9 and you shouldn't see this issue. If you are linking packages built distributed in binary format, then you can run into the issue
You do not have to convert it.
Swift 3.2 is not an actual version. It is how Xcode 9 (and the Swift 4 compiler) calls Swift 3. (I guess to differentiate from Swift 3 built with Xcode 8).
From Option Menu: Edit -> Convert -> To Current Swift Syntax
Above steps help you to convert into current swift syntax.
来源：https://stackoverflow.com/questions/44431282/how-can-i-update-swift-from-3-1-to-3-2-but-not-to-4-0