[Bug c/105499] inconsistency between -Werror=c++-compat and g++ in __extension__ block

[vincent-gcc at vinc17 dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105499

--- Comment #8 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
It is bad that __extension__ does two completely different things:
1. Disable warnings associated with GNU extensions, like ({ ... }).
2. Disable compatibility warnings that do not correspond to GNU extensions,
like invalid conversions in C++.