[Bug c++/83431] New: -Wformat-truncation may incorrectly report truncation

[bugzilla@poradnik-webmastera.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83431

            Bug ID: 83431
           Summary: -Wformat-truncation may incorrectly report truncation
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugzilla@poradnik-webmastera.com
  Target Milestone: ---

This looks like another missing optimization - -Wformat-truncation does not
take into account that there is "if" which checks that truncation will not
happen.

[code]
#include <stdio.h>
#include <string.h>

struct S
{
    char str[20];
    char out[10];
};

void test(S* s)
{
    if (strlen(s->str) < sizeof(s->out) - 2)
        snprintf(s->out, sizeof(s->out), "[%s]", s->str);
}
[/code]

[out]
$ g++ -c -o test.o test.cc -O2 -Wall
test.cc: In function ‘void test(S*)’:
test.cc:10:6: warning: ‘%s’ directive output may be truncated writing up to 19
bytes into a region of size 9 [-Wformat-truncation=]
 void test(S* s)
      ^~~~
test.cc:13:17: note: ‘snprintf’ output between 3 and 22 bytes into a
destination of size 10
         snprintf(s->out, sizeof(s->out), "[%s]", s->str);
         ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[/out]

g++ --version
g++ (GCC) 8.0.0 20171210 (experimental)