In gobex-transfer.c:306 [1]:

va_start(args, first_hdr_id);

where first_hdr_id is a guint8 and will be promoted. According to C99 and C++11, the behavior is undefined. Recent clang gives a warning of this:

warning: passing an object that undergoes default argument promotion to 'va_start' has undefined behavior [-Wvarargs]

which is in turn an error with -Werror given.

[1] https://cs.corp.google.com/chromeos_public/src/third_party/bluez/gobex/gobex-transfer.c?l=306