What is the default Swift version for auto suggestion on Xcode version 9.0 beta?
Is it Swift 3 or Swift 4?
What is the default Swift version for auto suggestion on Xcode version 9.0 beta?
Is it Swift 3 or Swift 4?
It is Swift 4. Apple always encourages to use the latest language and its features.
for more reference you can get the document from apple
You can use Xcode 9 to build targets that are written in either Swift 4 or Swift 3.
When the Swift 4 compiler is working with Swift 3 code, it identifies its language version as 3.2. As a result, you can use conditional compilation blocks like #if swift(>=3.2) to write code that’s compatible with multiple versions of the Swift compiler.