[Bug c/100842] New: Invalid -Wstringop-truncation with strncat and -O2

[cerrigno at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100842

            Bug ID: 100842
           Summary: Invalid -Wstringop-truncation with strncat and -O2
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cerrigno at gmail dot com
  Target Milestone: ---

Created attachment 50899
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50899&action=edit
Test source code

I have the following code

////

#include <string.h>

char last_str[4];

void copy_strncat(const char *str) {
    last_str[0] = '\0';
    strncat(last_str, str, sizeof(last_str) - 1);
}

void copy_strncpy(const char *str) {
    strncpy(last_str, str, sizeof(last_str));
    last_str[sizeof(last_str) - 1] = '\0';
}

void test() {
    char str[] = "test";
    copy_strncat(str);
    copy_strncpy(str);
}

////

If compiled with "-O2 -Wall" on GCC 11 (tested since GCC 8.1) I get an invalid
warning only in the strncat version the copy function, even if both are safe
and perform (almost) the same thing:

warning: 'strncat' output truncated copying 3 bytes from a string of length 4
[-Wstringop-truncation]
     strncat(last_str, str, sizeof(last_str) - 1);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In the strncpy version, the -Wstringop-truncation warning is disarmed by the
null terminator assignment after strncpy. I would expect the same from the
assignment made before strncat.

Here you may find my test in godbolt: https://godbolt.org/z/zEaczYzja