#include <boost/concept/requires.hpp>
#include <boost/concept_check.hpp>
#include <vector>

// (1) Forward declaration with concepts.
template<class Iter, class T>
BOOST_CONCEPT_REQUIRES(
	((boost::ForwardIterator<Iter>)) ((boost::EqualityComparable<T>))
	,
	(bool) ) all_equals(Iter first, Iter last, const T& val);

// (2) Actual definition.
template<class Iter, class T>
BOOST_CONCEPT_REQUIRES(
	((boost::ForwardIterator<Iter>)) ((boost::EqualityComparable<T>))
	,
	(bool) ) all_equals(Iter first, Iter last, const T& val) {
		for (Iter i = first; i < last; ++i) {
			if (*i != val) return false;
		}
		return true;
}

int main() {
	std::vector<double> v;
	// MSVC error: Cannot resolved ambiguous call between (1) and (2).
	all_equals(v.begin(), v.end(), double());
	return 0;
}