OK, the patch should now be complete. There is condition whether to use
all-duplicate code path:

        if (pre_assignment_length != post_assignment_length ||
        v->pm->node.flags & (PM_SPECIAL|PM_UNIQUE)) {

This covers special arrays and uniq arrays, that should be always
calling setfn() to trigger side-effects. Other arrays use this code:

            /* v->start is 0-based */
            p = old + v->start;
            for (r = val; *r;) {
                /* Free previous string */
                zsfree(*p);
                /* Give away ownership of the string */
                *p++ = *r++;
            }

And it should be OK, zsfree() for non-special array should be always
successful.

-- 
  Sebastian Gniazdowski
  psprint@fastmail.com