foo.cc: In function ‘int main()’:
foo.cc:54:18: error: no match for ‘operator<’ (operand types are ‘std::basic_ostream<char>’ and ‘<unresolved overloaded function type>’)
   54 |         cout<<ans<endl;
      |         ~~~~~~~~~^~~~~
In file included from /nix/gcc-13.2.0/include/c++/13.2.0/string:48,
                 from /nix/gcc-13.2.0/include/c++/13.2.0/bits/locale_classes.h:40,
                 from /nix/gcc-13.2.0/include/c++/13.2.0/bits/ios_base.h:41,
                 from /nix/gcc-13.2.0/include/c++/13.2.0/ios:44,
                 from /nix/gcc-13.2.0/include/c++/13.2.0/ostream:40,
                 from iostream:41:
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:455:5: note: candidate: ‘template<class _Iterator> bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)’
  455 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:455:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   ‘std::basic_ostream<char>’ is not derived from ‘const std::reverse_iterator<_Iterator>’
   54 |         cout<<ans<endl;
      |                   ^~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:500:5: note: candidate: ‘template<class _IteratorL, class _IteratorR> bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)’
  500 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:500:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   ‘std::basic_ostream<char>’ is not derived from ‘const std::reverse_iterator<_Iterator>’
   54 |         cout<<ans<endl;
      |                   ^~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:1705:5: note: candidate: ‘template<class _IteratorL, class _IteratorR> bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)’
 1705 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:1705:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   ‘std::basic_ostream<char>’ is not derived from ‘const std::move_iterator<_IteratorL>’
   54 |         cout<<ans<endl;
      |                   ^~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:1770:5: note: candidate: ‘template<class _Iterator> bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)’
 1770 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_iterator.h:1770:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   ‘std::basic_ostream<char>’ is not derived from ‘const std::move_iterator<_IteratorL>’
   54 |         cout<<ans<endl;
      |                   ^~~~
In file included from /nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_algobase.h:64,
                 from /nix/gcc-13.2.0/include/c++/13.2.0/string:51:
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_pair.h:835:5: note: candidate: ‘template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)’
  835 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/stl_pair.h:835:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   ‘std::basic_ostream<char>’ is not derived from ‘const std::pair<_T1, _T2>’
   54 |         cout<<ans<endl;
      |                   ^~~~
In file included from /nix/gcc-13.2.0/include/c++/13.2.0/string:54:
/nix/gcc-13.2.0/include/c++/13.2.0/bits/basic_string.h:3817:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Alloc>&, const __cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
 3817 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/basic_string.h:3817:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   ‘std::basic_ostream<char>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
   54 |         cout<<ans<endl;
      |                   ^~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/basic_string.h:3831:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)’
 3831 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/basic_string.h:3831:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   ‘std::basic_ostream<char>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
   54 |         cout<<ans<endl;
      |                   ^~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/basic_string.h:3844:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
 3844 |     operator<(const _CharT* __lhs,
      |     ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/bits/basic_string.h:3844:5: note:   template argument deduction/substitution failed:
foo.cc:54:19: note:   mismatched types ‘const _CharT*’ and ‘std::basic_ostream<char>’
   54 |         cout<<ans<endl;
      |                   ^~~~
In file included from /nix/gcc-13.2.0/include/c++/13.2.0/bits/ios_base.h:46:
/nix/gcc-13.2.0/include/c++/13.2.0/system_error:324:3: note: candidate: ‘bool std::operator<(const error_code&, const error_code&)’
  324 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |   ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/system_error:324:31: note:   no known conversion for argument 1 from ‘std::basic_ostream<char>’ to ‘const std::error_code&’
  324 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |             ~~~~~~~~~~~~~~~~~~^~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/system_error:507:3: note: candidate: ‘bool std::operator<(const error_condition&, const error_condition&)’
  507 |   operator<(const error_condition& __lhs,
      |   ^~~~~~~~
/nix/gcc-13.2.0/include/c++/13.2.0/system_error:507:36: note:   no known conversion for argument 1 from ‘std::basic_ostream<char>’ to ‘const std::error_condition&’
  507 |   operator<(const error_condition& __lhs,
      |             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~