Exception handling inside tag_invoke customizations #2858

template <concepts::appendable_containers T, typename ValT>
error_code tag_invoke(deserialize_tag, ValT &val, T &out) noexcept(false) {
  using value_type = typename std::remove_cvref_t<T>::value_type;
  static_assert(
      deserializable<value_type, ValT>,
      "The specified type inside the container must itself be deserializable");
  static_assert(
      std::is_default_constructible_v<value_type>,
      "The specified type inside the container must default constructible.");
  arm64::ondemand::array arr;
  if constexpr (std::is_same_v<std::remove_cvref_t<ValT>, arm64::ondemand::array>) {
    arr = val;
  } else {
    SIMDJSON_TRY(val.get_array().get(arr));
  }

  for (auto v : arr) {
    if constexpr (concepts::returns_reference<T>) {
      if (auto const err = v.get<value_type>().get(concepts::emplace_one(out));
          err) {
        // If an error occurs, the empty element that we just inserted gets
        // removed. We're not using a temp variable because if T is a heavy
        // type, we want the valid path to be the fast path and the slow path be
        // the path that has errors in it.
        if constexpr (requires { out.pop_back(); }) {
          static_cast<void>(out.pop_back());
        }
        return err;
      }
    } else {
      value_type temp;
      if (auto const err = v.get<value_type>().get(temp); err) {
        return err;
      }
      concepts::emplace_one(out, std::move(temp));
    }
  }
  return SUCCESS;
}