gcc (gcc-4.8.2-1.fc20.x86_64) seems to be not noticing a
return-without-value when it occurs inside the expansion of
glib's g_return_if_fail() macro:

    danw@laptop:tmp> cat foo.c
    #include <glib.h>

    int
    main (int argc, char **argv)
    {
      g_return_if_fail (argc == 1);

      return 0;
    }

    danw@laptop:tmp> gcc -Wall -c foo.c `pkg-config --cflags --libs glib-2.0`
    danw@laptop:tmp> echo $?
    0

If you force it to expand the macro first, it does notice:

    danw@laptop:tmp> gcc -E -o foo-expanded.c foo.c `pkg-config --cflags glib-2.0`

    danw@laptop:tmp> tail -9 foo-expanded.c
    # 2 "foo.c" 2
    
    int
    main (int argc, char **argv)
    {
      do{ if (argc == 1) { } else { g_return_if_fail_warning (((gchar*) 0), __PRETTY_FUNCTION__, "argc == 1"); return; }; }while (0);

      return 0;
    }

    danw@laptop:tmp> gcc -Wall -c foo-expanded.c `pkg-config --cflags --libs glib-2.0`
    foo.c: In function ‘main’:
    foo.c:6:108: error: ‘return’ with no value, in function returning non-void [-Werror=return-type]
       g_return_if_fail (argc == 1);
                                                                                                                ^