Build Failed — but no reason!

Did you perform a build clean (option clean) ? And possibly restart XCode to reopen the project. XCode seemingly caches a lot of information that may confuse the compiler.

Yes, of course.

What does the build transcript say about what it was doing and what the outcome was?

Sorry, that information got lost when I found the error manually. I can't provide logs anymore 😟

Yep.4 years later, tneyears later, doesn't matter with Apple. I just get build failed, Command SwiftCompile failed with nonzero exit code, or just nothing but build failed. Restarted Xcode twice, deleted deprived data once. Googling this shows it has happened since 2013. I guess they just don't have enough money to have tools on par with the industry. I have to just create a new project and do a ton of extra work because of the poor tooling. JS sucks, but god its tools actually work. I'll reboot my Mac and pretend I'm a PC. Its not like Apple ever acknowledges these problems anyway and will never fix them. I'm tired of losing work to the tooling.