My goal is to get the new Xcode 10.2 compile and run our projects that are build using Swift 3 and 4.2 (some cocoapods too) and since Xcode 10.2 comes with Swift 5, an unsupported swift error came up when trying to run the projects out of the box (this is expected):

The solution here is to use an older toolchain as stated on blogs like this, which i did.

All good so far. However, after installing the Swift 3 toolchain from Swift.org, compiling the project will bring the following error:

Command /Library/Developer/Toolchains/swift-3.0-RELEASE.xctoolchain/usr/bin/swiftc failed with exit code 1

I already deleted Derived Data folder, cleanup pods (my podfile) and reinstalled them, restarted Xcode and Mac multiple times.

Running xcrun -f swift and swift --version will indeed print the correct swift 3.0 toolchain

Yes, I know I could just stay with Xcode 10.1 and don't have these issues. Code migration to Swift 4 is also an option but we don't have the time at hand for now so I'm trying to workout this walkaround.

Please help me to resolve this issue, I'm completely stuck. Thanks in advance.