Comment (by Roland Haas):

To further add to the confusion, for C code (but the affected code in VolumeIntegrals_vacuum is C++), we seem to rely on the old pre-C99 behaviour of inline which would make inline appear externally as well (and then should complain about multiply defined symbols). See https://bitbucket.org/einsteintoolkit/tickets/issues/484/cctk_check_c_inline-can-define-away-the . The safest solution is most likely to always declare an inline functions as static inline which agrees between old and new usage.

--
Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/1974/add-wvuthorns_diagnostics-to-the-et