使用unordered_set of pairs时的令人惊讶的行为

#include <iostream>
#include <unordered_set>
#include <utility>

struct PairHash {
    template <class T1, class T2>
    std::size_t operator() (const std::pair<T1, T2>& p) const {
        auto h1 = std::hash<T1>{}(p.first);
        auto h2 = std::hash<T2>{}(p.second);
        return h1 ^ h2;
    }
};

struct PairEqual {
    template <class T1, class T2>
    bool operator() (const std::pair<T1, T2>& p1, const std::pair<T1, T2>& p2) const {
        return p1.first == p2.first && p1.second == p2.second;
    }
};

int main() {
    std::unordered_set<std::pair<int, int>, PairHash, PairEqual> mySet;
    mySet.insert(std::make_pair(1, 2));
    mySet.insert(std::make_pair(3, 4));

    for (const auto& pair : mySet) {
        std::cout << pair.first << ", " << pair.second << std::endl;
    }

    return 0;
}