My PR has been pulled into the official MGRS library to fix this particular issue. However, this issue appears to still exist on Xcode 27.1 beta. So Apple has not fixed the problem yet... https://github.com/ngageoint/mgrs-ios

I used to be on the team that wrote that library. I have broken the crazy expression down just so that I can get my tests to compile, but I have not fully verified it. However, if this gets you further... https://github.com/danielbarela/mgrs-ios/blob/master/mgrs-ios/utm/UTM.swift