[Bug c/100395] New: Bogus -Wstringop-overflow warning

["lavr at ncbi dot nlm.nih.gov" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100395

            Bug ID: 100395
           Summary: Bogus -Wstringop-overflow warning
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lavr at ncbi dot nlm.nih.gov
  Target Milestone: ---

$ gcc --version
gcc (GCC) 10.2.0

$ cat test.c
#include <stdio.h>
#include <string.h>

int main(int argc, const char* argv[])
{
    size_t len0 = strlen(argv[0]), len1 = strlen(argv[1]);
    char buf[444];
    char* s;

    s = buf + sizeof(buf) - len0;
    memcpy(s, argv[0], len0);
    s -= len1;
    memcpy(--s, argv[1], len1);
    s[len1++] = ' ';
    printf("%.*s\n", (int)(len0 + len1), s);
    return 0;
}

$ gcc -O2 -Wall test.c
test.c: In function ‘main’:
test.c:14:15: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
   14 |     s[len1++] = ' ';
      |     ~~~~~~~~~~^~~~~
test.c:7:10: note: at offset -1 to object ‘buf’ with size 444 declared here
    7 |     char buf[444];
      |          ^~~

The offset shown in the message is wrong, which most likely was the reason to
emit the warning altogether.

The test code above mocks up a situation where the lengths of both strings,
which are being copied in the reverse order into "buf", are known and small,
i.e. both fit (a few times, actually) together into the buffer, including the
separator and terminator.