return std::visit()

in nest/nest/nest.h [217:261]
45 lines of code
8 McCabe index (conditional complexity)

    return std::visit(
        overloaded{
            [&f](const T1 &t1, const T2 &t2) { return Nest<S>(f(t1, t2)); },
            [&f](const std::vector<Nest<T1>> &v1,
                 const std::vector<Nest<T2>> &v2) {
              auto size = v1.size();
              if (size != v2.size()) {
                throw std::invalid_argument(
                    "Expected vectors of same length but got " +
                    std::to_string(size) + " vs " + std::to_string(v2.size()));
              }
              std::vector<Nest<S>> result;
              result.reserve(size);
              auto it1 = v1.begin();
              auto it2 = v2.begin();
              for (; it1 != v1.end(); ++it1, ++it2) {
                result.emplace_back(map2(f, *it1, *it2));
              }
              return Nest<S>(result);
            },
            [&f](const std::map<std::string, Nest<T1>> &m1,
                 const std::map<std::string, Nest<T2>> &m2) {
              auto size = m1.size();
              if (size != m2.size()) {
                throw std::invalid_argument(
                    "Expected maps of same length but got " +
                    std::to_string(size) + " vs " + std::to_string(m2.size()));
              }
              std::map<std::string, Nest<S>> result;
              auto it1 = m1.begin();
              auto it2 = m2.begin();
              for (; it1 != m1.end(); ++it1, ++it2) {
                if ((*it1).first != (*it2).first) {
                  throw std::invalid_argument(
                      "Expected maps to have same keys but found '" +
                      (*it1).first + "' vs '" + (*it2).first + "'");
                }
                result.emplace_hint(result.end(), (*it1).first,
                                    map2(f, (*it1).second, (*it2).second));
              }
              return Nest<S>(result);
            },
            [](auto &&arg1, auto &&arg2) -> Nest<S> {
              throw std::invalid_argument("nests don't match");
            }},