deque的insert_dispatch，对输入迭代器进行了--操作：auto cur = --last;

[Peas-Li]

// insert_dispatch 函数
template
template
void deque::
insert_dispatch(iterator position, IIter first, IIter last, input_iterator_tag)
{
if (last <= first) return;
const size_type n = mystl::distance(first, last);
const size_type elems_before = position - begin_;
if (elems_before < (size() / 2))
{
require_capacity(n, true);
}
else
{
require_capacity(n, false);
}
position = begin_ + elems_before;
auto cur = --last;
for (size_type i = 0; i < n; ++i, --cur)
{
insert(position, *cur);
}
}

[frederick-vs-ja]

last <= first 这里开始就不对了，只有随机访问迭代才保证能这样比较。