The naive implementation of std::copy_n() incorrectly increments InputIterator one more time than needed, which fails with single-pass input iterators such as istream_iterator

Test program:

#include <algorithm>
#include <vector>
#include <sstream>
#include <iterator>
#include <iostream>
int main()
{
    std::istringstream s("1 2 3 4 5");
    std::vector<int> v;
    copy_n(std::istream_iterator<int>(s), 2, back_inserter(v));
    copy_n(std::istream_iterator<int>(s), 2, back_inserter(v));

    copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

Output with GCC 4.5.3 and 4.7.0-alpha20110716
1 2 4 5
expected output:
1 2 3 4

Similar bug was fixed in LLVM's libc++ in february 2011, the fix is straightforward: http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110221/039404.html