[Bug c/103292] [12 regression] xorg-server-1.20.13 -Werror=array-bounds false positive on unions

["slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103292

--- Comment #2 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #1)
> The warning is intended.  The program allocates an object of a size that's
> smaller than the size of the type used to access it:
> 
>        pPicture->pSourcePict = (union _SourcePict*) malloc(sizeof(struct
> _PictSolidFill));
>         pPicture->pSourcePict->type = 0;
> 
> It's not valid to access an object of one type using an lvalue of another. 
> In simple cases GCC diagnoses violations of this requirement by
> -Wstrict-aliasing.  -Warray-bounds doesn't detect aliasing violations but it
> does detect a subset of the problem that's apparent when the size of the
> lvalue's type is greater than the size of the object.  The object must be
> big enough for the whole lvalue, even if the accessed member is within the
> bounds of the smaller object.
> 
> The following is a smaller test case that should make the issue clearer. 
> See also pr102151 for a similar report.
> 
> $ cat a.c && gcc -O2 -S -Wall a.c
> struct A { char a[1]; };
> struct B { char a[2]; };
> union U { struct A a; struct B b; };
> 
> void* f (void)
> {
>   union U *p = __builtin_malloc (sizeof (struct A));
>   p->a.a[0] = 0;
>   return p;
> }
> a.c: In function ‘f’:
> a.c:8:4: warning: array subscript ‘union U[0]’ is partly outside array
> bounds of ‘unsigned char[1]’ [-Warray-bounds]
>     8 |   p->a.a[0] = 0;
>       |    ^~
> a.c:7:16: note: object of size 1 allocated by ‘__builtin_malloc’
>     7 |   union U *p = __builtin_malloc (sizeof (struct A));
>       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Aha, that makes sense. Filed upstream report as
https://gitlab.freedesktop.org/xorg/xserver/-/issues/1256

Related question:

It sounds like this diagnostic is somewhat related to -fstrict-aliasing. xorg
builds with -fno-strict-aliasing. Would it be fair to say the access in that
case is reasonable and -Warray-bounds is a false positive?