type-safety through obfuscation: enforcing an output iterator

output_iterator.cpp

#include <iterator>
#include <type_traits>
#include <iostream>

template <class Iter, class Tag>
using has_tag =
    std::enable_if<
        std::is_convertible<
            typename std::iterator_traits<Iter>::iterator_category,
            Tag>::value,
        Iter>;

template <typename Iter>
using is_output_iterator = has_tag<Iter, std::output_iterator_tag>;

template<class Iter, class Value = typename Iter::value_type,
         class = typename is_output_iterator<Iter>::type>
void do_something(std::initializer_list<Value> xs, Iter& iter)
{
    for (const Value& x : xs) {
        *iter = x;
        ++iter;
    }
}

int main()
{

    // fails as expected
    // auto iiter = std::istream_iterator<int>();
    // do_something({1, 2, 3, 4}, iter);

    // passes
    auto oiter = std::ostream_iterator<int>(std::cout, " ");
    do_something({1, 2, 3, 4}, oiter);
}