Why isn't std::find() using my operator==?

In the following snippet of code, I've overloaded the operator== to compare my pair type with string. But for some reason, the compiler isn't finding my operator as a match for the find function. Why not?

Edit: Thanks for all the suggestions for alternatives, but I'd still like to understand why. The code looks like it should work; I'd like to know why it doesn't.

#include <vector>
#include <utility>
#include <string>
#include <algorithm>

typedef std::pair<std::string, int> RegPair;
typedef std::vector<RegPair> RegPairSeq;

bool operator== (const RegPair& lhs, const std::string& rhs)
{
    return lhs.first == rhs;
}

int main()
{
    RegPairSeq sequence;
    std::string foo("foo");
    // stuff that's not important
    std::find(sequence.begin(), sequence.end(), foo);
    // g++: error: no match for 'operator==' in '__first. __gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* [with _Iterator = std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int>*, _Container = std::vector<std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int>, std::allocator<std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> > >]() == __val'
    // clang++: error: invalid operands to binary expression ('std::pair<std::basic_string<char>, int>' and 'std::basic_string<char> const')
}