[Bug libstdc++/50871] libstdc++ should be built with -Wpedantic and/or -Wsystem-headers

["jason at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50871

--- Comment #15 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #14)
> It's better than what we have today, but I'm not sure it's where we want to
> get to in the long term.
> 
> I've been trying a patch  that allows the system_header pragma to be
> disabled when building and testing.

How about using 

#pragma GCC diagnostic warning "-Wsystem-headers"

instead?

> This causes testsuite failures due to
> warnings that can't be controlled by options or diagnostic pragmas:
> 
> /home/jwakely/build/powerpc64le-unknown-linux-gnu/libstdc++-v3/include/cmath:
> 47: error: #include_next is a GCC extension

Maybe leave off -Wpedantic for now.

> /home/jwakely/build/powerpc64le-unknown-linux-gnu/libstdc++-v3/include/
> complex:1951: warning: floating point suffix 'if' shadowed by implementation

This seems like a real issue.  Perhaps for C++14 and up we should disable the
built-in complex suffixes.