[Bug libstdc++/70722] New: include_next in cmath skips user-defined wrapper

[martin.thomson at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70722

            Bug ID: 70722
           Summary: include_next in cmath skips user-defined wrapper
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: martin.thomson at gmail dot com
  Target Milestone: ---

Firefox has a bunch of wrappers for system headers.  I'm not 100% on the
reasoning, but most just play with visibility.  For instance, the math.h
wrapper looks like:

   #pragma GCC system_header
   #pragma GCC visibility push(default)
   #include_next <math.h>
   #pragma GCC visibility pop

Firefox fails to link when built with libstdc++ 6.0.0 because it includes
<cmath> before it includes <math.h>.  This leads to inclusion of <math.h> with
different visibility than is desired.

The version of cmath I have includes this:

   #define _GLIBCXX_INCLUDE_NEXT_C_HEADERS
   #include_next <math.h>
   #undef _GLIBCXX_INCLUDE_NEXT_C_HEADERS

Removing the "_next" causes the build to succeed.

You can see my flailings regarding this issue here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1264534