[Bug libstdc++/115885] New: Build errors when libstdc++ math.h included in extern "C" block

[kacper.slominski72 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115885

            Bug ID: 115885
           Summary: Build errors when libstdc++ math.h included in extern
                    "C" block
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kacper.slominski72 at gmail dot com
  Target Milestone: ---

Since r15-1857-gde19b516edbf919d31e9d22fdbf6066342d904a2 bits/cpp_type_traits.h
now includes <type_traits>. This means that <cmath>, and the libstdc++ <math.h>
now also transitively include <type_traits>.

This causes the following build errors if <math.h> is included somewhere inside
an `extern "C"` block:

In file included from
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits/cpp_type_traits.h:40,
                 from
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/cmath:44,
                 from
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/math.h:36,
                 from /usr/include/libavutil/common.h:36,
                 from /usr/include/libavutil/avutil.h:301,
                 from /usr/include/libavcodec/codec_par.h:26,
                 from /usr/include/libavformat/avformat.h:312,
                 from
/var/tmp/portage/dev-qt/qtmultimedia-6.7.2-r1/work/qtmultimedia-everywhere-src-6.7.2/src/plugins/multimedia/ffmpeg/qffmpegdefs_p.h:19,
                 from
/var/tmp/portage/dev-qt/qtmultimedia-6.7.2-r1/work/qtmultimedia-everywhere-src-6.7.2/src/plugins/multimedia/ffmpeg/qffmpeg_p.h:17,
                 from
/var/tmp/portage/dev-qt/qtmultimedia-6.7.2-r1/work/qtmultimedia-everywhere-src-6.7.2/src/plugins/multimedia/ffmpeg/qffmpeg.cpp:4,
                 from
/var/tmp/portage/dev-qt/qtmultimedia-6.7.2-r1/work/qtmultimedia-everywhere-src-6.7.2_build/src/plugins/multimedia/ffmpeg/CMakeFiles/QFFmpegMediaPlugin.dir/Unity/unity_0_cxx.cxx:7:
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/type_traits:69:3: error:
template with C linkage
   69 |   template<typename _Tp>
      |   ^~~~~~~~
/var/tmp/portage/dev-qt/qtmultimedia-6.7.2-r1/work/qtmultimedia-everywhere-src-6.7.2/src/plugins/multimedia/ffmpeg/qffmpegdefs_p.h:18:1:
note: ‘extern "C"’ linkage started here
   18 | extern "C" {
      | ^~~~~~~~~~


qtmultimedia does the following in qffmpegdefs_p.h:

extern "C" {
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libswresample/swresample.h>
#include <libavutil/avutil.h>
#include <libswscale/swscale.h>
}