[Bug preprocessor/81794] New: "would be stringified in traditional C" warning should be controlled by -Wtraditional

["egallager at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81794

            Bug ID: 81794
           Summary: "would be stringified in traditional C" warning should
                    be controlled by -Wtraditional
           Product: gcc
           Version: 8.0
               URL: https://gcc.gnu.org/ml/gcc-patches/2017-03/msg01325.ht
                    ml
            Status: UNCONFIRMED
          Keywords: diagnostic, easyhack, patch
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: egallager at gcc dot gnu.org
                CC: dmalcolm at gcc dot gnu.org
  Target Milestone: ---
              Host: i386-apple-darwin9.8.0
            Target: i386-apple-darwin9.8.0
             Build: i386-apple-darwin9.8.0

Opening a bug to track the underlying issue that caused me to submit this
patch:
https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00481.html
https://gcc.gnu.org/ml/gcc-patches/2017-03/msg01325.html

When compiling my fork of binutils-gdb, I get a lot of warnings like this one
on one file:

unwind-ia64.c:183:30: warning: macro argument "code" would be stringified in
traditional C
 #define UNW_DEC_BAD_CODE(code)   \
                              ^

To reduce it to a testcase, we can modify gcc.dg/pragma-diag-7.c to look like
this:

$ cat pragma-diag-7.c
/* { dg-do compile } */

unsigned long ok = 0UL;
#pragma GCC diagnostic push
#pragma GCC diagnostic warning "-Wtraditional"
unsigned long bad = 1UL; /* { dg-warning "suffix" } */
/* Note the extra space before the pragma on this next line: */
 #pragma GCC diagnostic pop
unsigned long ok_again = 2UL; /* { dg-bogus "suffix" } */

/* Redundant with the previous pop, but just shows that it fails to stop the
 * following warning with an unpatched GCC: */
#pragma GCC diagnostic ignored "-Wtraditional"

/* { dg-bogus "would be stringified" .+1 } */
#define UNW_DEC_PROLOGUE(fmt, body, rlen, arg) \
  do {                                                                  \
      unw_rlen = rlen;                                                  \
      *(int *)arg = body;                                               \
      printf("    %s:%s(rlen=%lu)\n",                                   \
             fmt, (body ? "body" : "prologue"), (unsigned long)rlen);   \
  } while (0)
$ /usr/local/bin/gcc -c pragma-diag-7.c
pragma-diag-7.c:6:21: warning: traditional C rejects the "UL" suffix
[-Wtraditional]
 unsigned long bad = 1UL; /* { dg-warning "suffix" } */
                     ^~~
pragma-diag-7.c:16:46: warning: macro argument "rlen" would be stringified in
traditional C
 #define UNW_DEC_PROLOGUE(fmt, body, rlen, arg) \
                                              ^
pragma-diag-7.c:16:46: warning: macro argument "body" would be stringified in
traditional C
$ 

To get rid of the unwanted warnings, all that is necessary is a 1-line patch to
libcpp/macro.c to change a call to cpp_error with CPP_DL_WARNING into a call to
cpp_warning with CPP_W_TRADITIONAL instead. The patch is already linked above.