https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116313
Bug ID: 116313
Summary: -Wsequence-point false positive with auto and/or
__auto_type
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: alx at kernel dot org
Target Milestone: ---
Originally found in
<https://software.codidact.com/comments/thread/9880#comment-24855>.
Originally reported in
<https://inbox.sourceware.org/gcc/577174b2608dc8e4ff7dd5f7ad57406ffe46da54.ca...@gwdg.de/T/>.
$ cat auto.c
#include <stdio.h>
int
main(void)
{
int i = 3;
int a[2 * i];
int (*p)[2 * i];
i = 1;
p = &a;
auto q = p + i++;
}
$ gcc -Wsequence-point -std=c23 auto.c
auto.c: In function ‘main’:
auto.c:12:23: warning: operation on ‘i’ may be undefined [-Wsequence-point]
12 | auto q = p + i++;
| ~^~
I suspect the warning is confusing auto with typeof(). If typeof()
was used, this would indeed cause double evaluation, once before the =
and once after the =, causing UB.
Maybe it's due to how auto is implemented? I suspect it's doing
internally:
typeof(p + i++) q = p + i++;
and that would be problematic. I can reproduce it with both gcc-13 and
gcc-14.
