"Unused function" build failure(s) — clang 6.0

-Wall -Werror are including the option -Wunused-function. Without not the case with g++
namespace { 
   void dummy_for_semicolon_do_not_use();
}
PackageVersionSupposed error messageFull log Bug report
ddcutil 0.8.6-1vcp_feature_set.c:225:1: error: unused function 'unopaque_feature_set' [-Werror,-Wunused-function] Log
infinipath-psm 3.3+20.604758e7-5ptl.c:539:1: error: unused function 'ips_lock_shared_context' [-Werror,-Wunused-function] Log
libcerf 1.3-2w_of_z.c:79:21: error: unused function 'cpolar' [-Werror,-Wunused-function] Log

3 errors