Xcode error: "Command CompileSwiftSources failed with nonzero exit code", is this Xcode or Swift compiler?

For iOS project, I get this error very consistently. But if I do "Clean build folder" and compile, it's fine. But then if I go on to run on device, it's this same error. Then I "Clean build folder" and directly "run" as a single step, it compile and run on device fine.

Where to report this error? Feedback Assistant for Xcode or here for Swift compiler bug?