https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67064
--- Comment #14 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Ville Voutilainen from comment #13)
> It is correct that currently none of the pedantic-flags diagnose the use of
> this extension; perhaps that should be fixed while fixing this bug...
'-Wpedantic' does not cause warning messages for use of the
alternate keywords whose names begin and end with '__'. Pedantic
warnings are also disabled in the expression that follows
'__extension__'. However, only system header files should use
these escape routes; application programs should avoid them.
