clang++: error: call to 'partition' is ambiguous

Question

#include <algorithm>
#include <vector>

template <class BidirectionalIterator, class UnaryPredicate>
BidirectionalIterator partition(BidirectionalIterator first,
    BidirectionalIterator last, UnaryPredicate pred)
{
    while (first != last) {
        while (pred(*first)) {
            ++first;
            if (first == last) return first;
        }
        do {
            --last;
            if (first == last) return first;
        } while (!pred(*last));
        std::swap(*first, *last);
        ++first;
    }
    return first;
}

int main() {
    std::vector<int> v = { 1, 55, 17, 65, 40, 18, 77, 37, 77, 37 };
    partition(v.begin(), v.end(), [](const int &i) {
        return i < 40;
    });
    return 0;
}

The code won't compile. Both clang++(3.5.2/cygwin) and Visual Studio(2013) complain about ambiguous call. Since no using directive is used, I don't understand what's wrong. To compile successfully, using :: prefix helps.

Answer 1

Your partition has a name collision with std::partition

The reason it is doing so, even without the std:: prefix is because it is using argument dependent lookup (ADL) on the arguments, which are std::vector<int>::iterator, which carry the std:: namespace. Therefore, the compiler is able to "see" the std::partition function as well as your partition function.

From cppreference (emphasis mine)

... for every argument in a function call expression and for every template argument of a template function, its type is examined to determine the associated set of namespaces and classes that it will add to the lookup