Here is the clang bug report with the testcase: https://bugs.llvm.org/show_bug.cgi?id=36916
Failing port: audio/caps-lv2 on 12 with clang60.
Upstream says the code in question is broken, but the diagnostic is lousy. I doubt we'll see a patch for llvm60 at this point so there isn't a clear point in keeping this open. If one appears I'm happy to apply it.