/in/foo.cc:125:43: warning: 'nodiscard' attribute directive ignored [-Wattributes]
     [[nodiscard]] std::string to_string() const {
                                           ^~~~~
/in/foo.cc:151:10: error: 'strong_ordering' in namespace 'std' does not name a type
     std::strong_ordering operator<=>(const Bigint &other) const {
          ^~~~~~~~~~~~~~~
/in/foo.cc:174:50: error: 'bool Bigint::operator==(const Bigint&) const' cannot be defaulted
     bool operator==(const Bigint &other) const = default;
                                                  ^~~~~~~
/in/foo.cc: In member function 'Bigint Bigint::div_mod(const Bigint&, Bigint&) const':
/in/foo.cc:55:21: error: no match for 'operator<' (operand types are 'Bigint' and 'Bigint')
         if (current < dividend) {
             ~~~~~~~~^~~~~~~~~~
/in/foo.cc:55:21: note: candidate: operator<(int, int) <built-in>
/in/foo.cc:55:21: note:   no known conversion for argument 2 from 'Bigint' to 'int'
/in/foo.cc:55:21: note: candidate: operator<(int, long int) <built-in>
/in/foo.cc:55:21: note:   no known conversion for argument 2 from 'Bigint' to 'long int'
/in/foo.cc:55:21: note: candidate: operator<(long int, int) <built-in>
/in/foo.cc:55:21: note:   no known conversion for argument 2 from 'Bigint' to 'int'
/in/foo.cc:55:21: note: candidate: operator<(long int, long int) <built-in>
/in/foo.cc:55:21: note:   no known conversion for argument 2 from 'Bigint' to 'long int'
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/regex.h:962:5: note: candidate: template<class _BiIter> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, const std::__cxx11::sub_match<_BiIter>&)
     operator<(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:962:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/regex.h:1041:5: note: candidate: template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const std::__cxx11::sub_match<_BiIter>&)
     operator<(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1041:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/regex.h:1121:5: note: candidate: template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)
     operator<(const sub_match<_Bi_iter>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1121:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/regex.h:1198:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type*, const std::__cxx11::sub_match<_BiIter>&)
     operator<(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1198:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/regex.h:1272:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)
     operator<(const sub_match<_Bi_iter>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1272:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/regex.h:1349:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type&, const std::__cxx11::sub_match<_BiIter>&)
     operator<(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1349:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/regex.h:1429:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)
     operator<(const sub_match<_Bi_iter>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1429:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_pair.h:441:5: note: candidate: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
     ^~~~~~~~
/usr/include/c++/6/bits/stl_pair.h:441:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::pair<_T1, _T2>'
         if (current < dividend) {
                       ^~~~~~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:298:5: note: candidate: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator<(const reverse_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:298:5: note:   template argument deduction/substitution failed:
/in/foo.cc:55:23: note:   'Bigint' is not derived from 'const std::reverse_iterator<_Iterator>'
         if (current < divide