[Bug c/78304] New: -Wformat doesn't warn anymore for inttypes.h format string argument type mismatches

[mark at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78304

            Bug ID: 78304
           Summary: -Wformat doesn't warn anymore for inttypes.h format
                    string argument type mismatches
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mark at gcc dot gnu.org
  Target Milestone: ---

For this program:

#include <inttypes.h>
#include <stdio.h>

int
main (int argc, char **argv)
{
  size_t size = argc;
  printf ("size: %" PRIu32 "\n", size);
  return 0;
}

gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11) warns when given -Wformat:

t.c: In function ‘main’:
t.c:8:3: warning: format ‘%u’ expects argument of type ‘unsigned int’, but
argument 2 has type ‘size_t’ [-Wformat=]
   printf ("size: %" PRIu32 "\n", size);
   ^

gcc (GCC) 7.0.0 20161110 (experimental) doesn't give any warning with -Wformat.

Note that both versions do warn when using "size: %u\n" as format string:

t.c: In function ‘main’:
t.c:8:19: warning: format ‘%u’ expects argument of type ‘unsigned int’, but
argument 2 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
   printf ("size: %u\n", size);
                  ~^
                  %lu

They also both warn when using normal string concatenation:

 t.c: In function ‘main’:
t.c:8:11: warning: format ‘%u’ expects argument of type ‘unsigned int’, but
argument 2 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
   printf ("size: %" "u" "\n", size);
           ^~~~~~~~~
t.c:8:22: note: format string is defined here
   printf ("size: %" "u" "\n", size);
                  ~~~~^
                  %" "lu

They even both warn when doing the format string define in the file itself:

#include <stdio.h>

#define UFORMAT "u"

int
main (int argc, char **argv)
{
  size_t size = argc;
  printf ("size: %" UFORMAT "\n", size);
  return 0;
}

But gcc 4.8.5 has a better warning message:

t.c: In function ‘main’:
t.c:9:3: warning: format ‘%u’ expects argument of type ‘unsigned int’, but
argument 2 has type ‘size_t’ [-Wformat=]
   printf ("size: %" UFORMAT "\n", size);
   ^

While 7.0.0 gives the somewhat cryptic:

t.c: In function ‘main’:
t.c:3:18: warning: format ‘%u’ expects argument of type ‘unsigned int’, but
argument 2 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
 #define UFORMAT "u"
                  ^