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From: "megahallon at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/60014] New: Bad warning suppression
Date: Fri, 31 Jan 2014 21:27:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60014

            Bug ID: 60014
           Summary: Bad warning suppression
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: megahallon at gmail dot com

Using gcc 4.8.2:

/tmp/foo/foo.h:
-------------------------------
#define FOO(a, b) __LINE__;
-------------------------------

bug.c:
-------------------------------
#include "foo.h"

int main()
{
  FOO(1, 
      0);
  char* a = 1;
}
-------------------------------

$ /usr/bin/gcc bug.c -isystem /tmp/foo -no-integrated-cpp

should give a warning on line 7 but it not seen.

These works:

$ /usr/bin/gcc bug.c -isystem /tmp/foo -no-integrated-cpp
-ftrack-macro-expansion=0
$ /usr/bin/gcc bug.c -I /tmp/foo -no-integrated-cpp
$ /usr/bin/gcc bug.c -isystem /tmp/foo

-save-temps gives the same results as -no-integrated-cpp.

Output from -save-temps:

bug.i
-----------------------------
void main()
{

 6
# 5 "bug.c" 3 4
  ;
        ;
  char *a = 1;
}
-----------------------------

The suppress flag in the line directive apparently affects line 7 which is not
as expected.

In practice this means that ccache and similar systems that compiles
preprocessed output may fail to show warnings that are seen when compiling
normally.