Given the following test case:

  #define f(x, y) "x y"
  
  extern void abort (void);
  
  int main ()
  {
    const char *str1 = f("a", "\"a\"");
    const char *str2 = f( \t, " \t");
  
    if (strcmp (str1, "\"a\" \"\\\"a\\\"\""))
      abort ();
    if (strcmp (str2, "\t \" \\t\""))
      abort ();
    return 0;
  }

Gcc 2.95.3 will accept it and do the right thing:

  $ gcc -v
  Reading specs from /opt/local/fsf/lib/gcc-lib/i686-pc-linux-gnu/2.95.3/specs
  gcc version 2.95.3 20010315 (release)
  $ gcc -traditional-cpp -o macroargs macroargs.c
  $ ./macroargs
  $

Current gcc 4.X fails to accept this code.  For example, using the Fedora Core 6 compiler:

  $ /usr/bin/gcc -v
  Using built-in specs.
  Target: i386-redhat-linux
  Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --enable-plugin --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic --host=i386-redhat-linux
  Thread model: posix
  gcc version 4.1.1 20061011 (Red Hat 4.1.1-30)
  $ /usr/bin/gcc -traditional-cpp -o macroargs macroargs.c
  macroargs.c: In function ‘main’:
  macroargs.c:7: error: expected ‘,’ or ‘;’ before ‘a’
  macroargs.c:7: error: stray ‘\’ in program
  macroargs.c:7: error: missing terminating " character
  macroargs.c:8: error: stray ‘\’ in program
  macroargs.c:12: error: ‘str2’ undeclared (first use in this function)
  macroargs.c:12: error: (Each undeclared identifier is reported only once
  macroargs.c:12: error: for each function it appears in.)

The attached patch (also posted to gcc-patches) will fix it, although
it's handling of quoted arguments is not exactly identical to gcc
2.95.3.  Notice the difference between the test case above, and the
test case included with the patch.

Here is an example of a patched gcc's behavior:

  $ /opt/specifix/experimental/bin/gcc -v
  Using built-in specs.
  Target: i686-pc-linux-gnu
  Configured with: /src/specifix/experimental/src/gcc/configure --enable-languages=c,c++,fortran --prefix=/opt/specifix/experimental --disable-werror --disable-bootstrap --with-mpfr=/opt/specifix/experimental --with-gmp=/opt/specifix/experimental --cache-file=/dev/null --srcdir=/src/specifix/experimental/src/gcc
  Thread model: posix
  gcc version 4.3.0 20061231 (experimental)
  $ /opt/specifix/experimental/bin/gcc -traditional-cpp -o macroargs macroargs.c
  $

Note however that the above test case WILL abort if run, while the
test case included with the patch will not, due to the handling of
leading and trailing whitespace in macro args.  See the code inside
"#if 0 ... #endif" in the patch and the associated comments.  Either the
current gcc testsuite is wrong in how it tests for this whitespace, or
gcc 2.95.3 is wrong.  It's not clear to me which behavior is more correct.