How to build Swift 3 project on Xcode 9?

Question

Xcode 9 Release Notes says that Swift 3 compilation is supported:

One compiler for Swift 4 and Swift 3, Swift 4 and Swift 3 targets c

Answer 1

All the above answers are answering the wrong question. When using Xcode 9, you are using the Swift 4 compiler (even if you are using it to compile Swift 3 code) to fix this, recompile using the appropriate compiler/Xcode.

If you are trying to do this via commandline, you can use sudo xcode-select -switch to switch between xcode versions. Good luck!