https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64864
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- The line markers allows the compiler to properly distinguish between what tokens come from where, e.g. system headers vs. normal headers (should we warn about issues in there if -Wsystem-headers is not used and it is some warning not enabled by default in system headers), or e.g. for #pragma GCC diagnostics tracking.
