[Bug c++/61421] Invalid -O2 optimization breaks program

["mimamer at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421

--- Comment #18 from mimamer at gmail dot com ---
Ah? I didn't figure that was allowed per strict aliasing rules. But it still
doesn't solve one problem:

inline T *dequeue() {
    node *head = anchor.next;
    anchor.next = head->next;
    head->next->prev = &anchor;
    return (T *)head;
};

The last typecast is invalid when the list is empty and thus returns the
anchor, a special element that so far I checked with equality to list2::end():
inline T* end() const { return (T *)&anchor; }

But it is a huge step forward as checking for an empty list in dequeue() is
just ok so as to return NULL instead of the anchor.