I was doing a little bit of scouting on macOS 15 / Xcode 16 and ran in to a compiler error on Xcode 16.

The gist of the error is

  <instantiation>:5:1: error: non-private labels cannot appear between .cfi_startproc / .cfi_endproc pairs

Repeated many times. The full build log is available at https://gist.github.com/vcsjones/1649f2fb8b19ab86428582b6884229c0

Note this does not require a beta of macOS, only the developer tools.