https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119481
Bug ID: 119481
Summary: -fplan9-extensions violates the C standard (6.3.3.3p7)
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: alx at kernel dot org
Target Milestone: ---
<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3467.pdf#subsubsection.0.6.3.3.3>
```
alx@devuan:~/tmp$ cat cast.c | grep -Tn ^
1: #include <stdio.h>
2:
3: struct B {
4: int i;
5: };
6: struct A {
7: int j;
8: struct B;
9: };
10:
11: int
12: main(void)
13: {
14: struct A a;
15: struct B *b;
16: int x;
17:
18: b = (struct B *) &a;
19: x = (&a == (struct A *) b);
20:
21: printf("%d\n", x);
22:
23: b = &a;
24: x = (&a == (struct A *) b);
25:
26: printf("%d\n", x);
27: }
alx@devuan:~/tmp$ gcc -Wall -Wextra -fplan9-extensions cast.c
alx@devuan:~/tmp$ ./a.out
1
0
```
While some Plan9 extensions are interesting, there should be a diagnostic when
an implicit conversion adds an offset to a pointer.
The referenced paragraph from ISO C23 that is violated by this extension says:
```
A pointer to an object type can be converted to a pointer to a different object
type. [...] when converted back again, the result shall compare equal to the
original pointer.
```
