I'm working in a specific Xcode project and deliberatly making errors in my code (like using undeclared variables), but Xcode doesnt seem to show any compile errors in my code.
It shows errors only when its build or clean.
Tried clean and build, cleaning derived data and restarted both Xcode and system, none of it works.
Using Xcode 8.2.1 (8C1002) and Swift 3
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.