mkc_check_decl prototype does not longer work in GCC 16

[carlosrodfern]

The check for prototype does not longer work in GCC 16.

How to Reproduce:

podman run -it fedora:44 /bin/bash
# inside container
dnf install -y mk-configure

mkc_check_decl prototype 'char* strstr(const char*, const char* )' string.h
# it returns 0

The same steps but for GCC 15 in fedora:43 returns 1.

It comes down to the generated code for prototype check does no longer compiled:

Generated code:

#include <string.h>
void func2 (void);
void func (void);
void func2 (void)
{
   if (strstr) return;
}
char* strstr(const char*, const char* );
void func (void)
{
   if (strstr) return;
}

Error message:

In file included from /usr/include/features.h:540,
                 from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/string.h:26,
                 from ./_mkc_prototype_char8~strstr_const~char8,~const~char8~__string_h.c:1:
./_mkc_prototype_char8~strstr_const~char8,~const~char8~__string_h.c:8:7: error: expected identifier or '(' before '_Generic'
    8 | char* strstr(const char*, const char* );
      |       ^~~~~~

[carlosrodfern]

<string.h> from glibc has some changes with GCC 16 in Fedora:

extern char *strstr (const char *__haystack, const char *__needle)
     __THROW __attribute_pure__ __nonnull ((1, 2));
# if __GLIBC_USE (ISOC23) && defined __glibc_const_generic && !defined _LIBC
#  define strstr(HAYSTACK, NEEDLE)                      \
  __glibc_const_generic (HAYSTACK, const char *,        \
                         strstr (HAYSTACK, NEEDLE))
# endif

So, disabling isoc23 generic macros for these kinds prototype with -std=gnu17 makes it compile.

[carlosrodfern]

How about this way of checking for prototypes?

#include <string.h>

void func(void) {
    char *(*p)(const char *, const char *) = strstr;
}

gcc -c test.c -Werror
# compiles fine

clang -c test.c -Werror
# compiles fine

But,

#include <string.h>

void func(void) {
    char *(*p)(const char *, const char *, int foo) = strstr;
}

gcc -c test.c -Werror

test.c: In function ‘func’:
test.c:4:55: error: initialization of ‘char * (*)(const char *, const char *, int)’ from incompatible pointer type ‘char * (*)(const char *, const char *)’ [-Wincompatible-pointer-types]
    4 |     char *(*p)(const char *, const char *, int foo) = strstr;
      |                                                       ^~~~~~
In file included from test.c:1:
/usr/include/string.h:377:14: note: ‘strstr’ declared here
  377 | extern char *strstr (const char *__haystack, const char *__needle)
      |              ^~~~~~

lang -c test.c -Werror
test.c:4:13: error: incompatible function pointer types initializing 'char *(*)(const char *, const char *, int)' with an
      expression of type 'char *(const char *, const char *)' [-Wincompatible-function-pointer-types]
    4 |     char *(*p)(const char *, const char *, int foo) = strstr;
      |             ^                                         ~~~~~~
1 error generated.