Xcode not displaying compile errors

Besides the DerivedData folder, also try cleaning Xcode caches completely:

rm -rf $HOME/Library/Caches/com.apple.dt.Xcode/

I would recommend also killing the SourceKit process — always a source of issues these days! — but since you are already did a full reboot, so looks like you are good there.

If everything fails, you might consider upgrading to the latest Xcode, version 8.3. Just be aware that:

Xcode 8.3 no longer supports Swift 2.3. Please migrate your projects containing Swift 2.3 code to Swift 3 syntax by opening the project and choosing Edit > Convert > To Current Swift Syntax.

Project Settings, "Show all issues" fixed it for me.