GCC 4.1: too strict aliasing?

[Igor Bukanov]

Consider the following code that starting with GCC 4.1.0 generates
'dereferencing type-punned pointer will break strict-aliasing rules'
warning:


~> cat test.c
struct inner {
   struct inner *next;
};

struct outer {
   struct inner base;
   int value;
};

/* List of outer elements where all outer.base.next point to struct outer */
struct outer_list {
   struct outer *head;
};

struct outer *search_list(struct outer_list *list, int value)
{
   struct outer *elem, **pelem;

   pelem = &list->head;
   while ((elem = *pelem)) {
       if (elem->base.value == value) {
           /* Hit, move atom's element to the front of the list. */
           *pelem = (struct outer*)elem->base.next;
           elem->base.next = &list->head->base;
           list->head = elem;
           return elem;
       }

       /*** LINE GENERATING WARNING */
       pelem = (struct outer **)&elem->base.next;
   }
   return 0;
}

~> gcc -c -Wall -O2 test.c
test.c: In function 'search_list':
test.c:29: warning: dereferencing type-punned pointer will break
strict-aliasing rules

But why the warning is generated? Doesn't it guaranteed that
offsetof(struct outer, base) == 0 and one can always safely cast
struct inner* to struct outer* if struct inner is a part struct outer
so struct* outer can alias struct* inner?