stability

stability.cpp

std::vector<std::pair<int, int>> first_elements_equal{{1, 1}, {1, 2}};
std::sort(first_elements_equal.begin(),
          first_elements_equal.end(),
          [](const auto& lhs, const auto& rhs) {
            // Compare only by a part of sorted data.
            return lhs.first < rhs.first;
          });
// Serialize or make assumptions about all data.
// Wrong, might be either 1 or 2.
assert(first_elements_equal[0].second == 1);