输入迭代器可以用在需要前向迭代器的地方吗?

I always thought there was a rule, that if an algorithm expects a particular type of iterator, you can provide iterators of categories up the chain, but not down. So I was reading this answer, where ildjarn suggests suggested (then later corrected himself) using std::ifstream with std::istream_iterator and std::search to find data in a in a file. I was about to comment that you can't do that, because search expects Forward iterators, and istream_iterator is an Input iterator. But just to make sure, I tried this:

std::istringstream iss("Elephant hats for sale.");
std::istream_iterator<char> begin(iss), end;

std::string sub("hat");
auto i = std::search(begin, end, sub.begin(), sub.end());

我没想到它会编译，但它确实编译了.但是，结果似乎毫无用处，因为如果我按照以下方式进行操作:

while(i != end)
{
    std::cout << *i;
    ++i;
}