Ouch. Two flags that should reasonably stop this, and neither do. This feels a bit like the time I was told "No, -wAll does not in fact add all warnings".
-Wextra is also in Clang and is definitely not everything in either compiler.
-Weverything is everything but that is only really useful to discover new warning flags (in combination with lots of -Wno...) for which you are better off reading GCC/Clang's release notes and/or documentation.