[Bug c++/64332] New: gcc/g++ handles system_header differently

[a3at.mail at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64332

            Bug ID: 64332
           Summary: gcc/g++ handles system_header differently
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: a3at.mail at gmail dot com

Created attachment 34292
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34292&action=edit
warning-constructor-attribute-ignored.tgz

Digging through one or compilation errors, after doing this:
#define __constructor __attribute__((constructor))

I turned out that gcc/g++ have different behaviours for system_header pragma.

Here is a simple example (also archived in attach):
$ head *.[ch]
==> c.h <==
#define __constructor __attribute__((constructor))

==> c-impl.h <==
#pragma GCC system_header

typedef void (*__cb_type)(void *);
int foo(__cb_type __constructor);

==> main.c <==
#include "c.h"
#include "c-impl.h"

$ gcc -c main.c
$ gcc -Wall -Wextra -Wpedantic -Wattributes -c main.c
$ g++ -c main.c
In file included from main.c:1:0:
c.h:1:50: warning: ‘constructor’ attribute ignored [-Wattributes]
 #define __constructor __attribute__((constructor))
$ clang++-3.5 -c main.c
clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is
deprecated
$ clang-3.5 -c main.c

$ g++ --version
g++ (Debian 4.9.1-16) 4.9.1
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Is this desired behavior?