foo.cc: In function 'int main()':
foo.cc:33:8: error: increment of member 'Digit::cnt' in read-only object
   33 |    it->cnt++;
      |    ~~~~^~~
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h: In instantiation of 'bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Digit]':
/usr/include/c++/9/bits/stl_tree.h:2564:8:   required from 'std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::find(const _Key&) [with _Key = Digit; _Val = Digit; _KeyOfValue = std::_Identity<Digit>; _Compare = std::less<Digit>; _Alloc = std::allocator<Digit>; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<Digit>]'
/usr/include/c++/9/bits/stl_set.h:795:29:   required from 'std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::find(const key_type&) [with _Key = Digit; _Compare = std::less<Digit>; _Alloc = std::allocator<Digit>; std::set<_Key, _Compare, _Alloc>::iterator = std::_Rb_tree_const_iterator<Digit>; std::set<_Key, _Compare, _Alloc>::key_type = Digit]'
foo.cc:28:35:   required from here
/usr/include/c++/9/bits/stl_function.h:386:20: error: no match for 'operator<' (operand types are 'const Digit' and 'const Digit')
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
foo.cc:10:7: note: candidate: 'bool Digit::operator<(Digit&)' <near match>
   10 |  bool operator<(Digit& d)
      |       ^~~~~~~~
foo.cc:10:7: note:   conversion of argument 1 would be ill-formed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: error: binding reference of type 'Digit&' to 'const Digit' discards qualifiers
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:64,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/ios:40,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_pair.h:454:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)'
  454 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/9/bits/stl_pair.h:454:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::pair<_T1, _T2>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:67,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/ios:40,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_iterator.h:331:5: note: candidate: 'template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)'
  331 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/9/bits/stl_iterator.h:331:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::reverse_iterator<_Iterator>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:67,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/ios:40,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_iterator.h:369:5: note: candidate: 'template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)'
  369 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/9/bits/stl_iterator.h:369:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::reverse_iterator<_Iterator>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:67,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/ios:40,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_iterator.h:1160:5: note: candidate: 'template<class _IteratorL, class _IteratorR> bool std::operator<(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)'
 1160 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/9/bits/stl_iterator.h:1160:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::move_iterator<_IteratorL>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:67,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/ios:40,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_iterator.h:1166:5: note: candidate: 'template<class _Iterator> bool std::operator<(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorL>&)'
 1166 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/9/bits/stl_iterator.h:1166:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::move_iterator<_IteratorL>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/basic_string.h:6229:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)'
 6229 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/9/bits/basic_string.h:6229:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/basic_string.h:6242:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)'
 6242 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/9/bits/basic_string.h:6242:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/basic_string.h:6254:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)'
 6254 |     operator<(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/9/bits/basic_string.h:6254:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   mismatched types 'const _CharT*' and 'Digit'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/bits/ios_base.h:46,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/system_error:208:3: note: candidate: 'bool std::operator<(const std::error_code&, const std::error_code&)'
  208 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |   ^~~~~~~~
/usr/include/c++/9/system_error:208:31: note:   no known conversion for argument 1 from 'const Digit' to 'const std::error_code&'
  208 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |             ~~~~~~~~~~~~~~~~~~^~~~~
/usr/include/c++/9/system_error:282:3: note: candidate: 'bool std::operator<(const std::error_condition&, const std::error_condition&)'
  282 |   operator<(const error_condition& __lhs,
      |   ^~~~~~~~
/usr/include/c++/9/system_error:282:36: note:   no known conversion for argument 1 from 'const Digit' to 'const std::error_condition&'
  282 |   operator<(const error_condition& __lhs,
      |             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~
In file included from /usr/include/c++/9/set:61,
                 from foo.cc:2:
/usr/include/c++/9/bits/stl_set.h:996:5: note: candidate: 'template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::set<_Key, _Compare, _Alloc>&, const std::set<_Key, _Compare, _Alloc>&)'
  996 |     operator<(const set<_Key, _Compare, _Alloc>& __x,
      |     ^~~~~~~~
/usr/include/c++/9/bits/stl_set.h:996:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::set<_Key, _Compare, _Alloc>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~
In file included from /usr/include/c++/9/set:62,
                 from foo.cc:2:
/usr/include/c++/9/bits/stl_multiset.h:981:5: note: candidate: 'template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::multiset<_Key, _Compare, _Alloc>&, const std::multiset<_Key, _Compare, _Alloc>&)'
  981 |     operator<(const multiset<_Key, _Compare, _Alloc>& __x,
      |     ^~~~~~~~
/usr/include/c++/9/bits/stl_multiset.h:981:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/9/string:48,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/stl_function.h:386:20: note:   'const Digit' is not derived from 'const std::multiset<_Key, _Compare, _Alloc>'
  386 |       { return __x < __y; }
      |                ~~~~^~~~~

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