Language: C++

#include<bits/stdc++.h>
#define r(i,n) for(int i=0;i<n;i++)
using namespace std;
int main(){
  string s;
  cin>>s;
  r(i,s)if(i%2==0)cout<<s[i];
  cout<<endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:2:29: error: no match for 'operator<' (operand types are 'int' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                             ^
a.cc:7:3: note: in expansion of macro 'r'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |   ^
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:1:
/usr/include/c++/14/bits/regex.h:1143:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1143 |     operator<(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1143:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/regex.h:1224:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1224 |     operator<(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1224:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/regex.h:1317:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1317 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1317:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/regex.h:1391:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1391 |     operator<(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1391:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/regex.h:1485:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1485 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1485:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/regex.h:1560:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1560 |     operator<(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1560:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/regex.h:1660:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1660 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1660:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::pair<_T1, _T2>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #define r(i,n) for(int i=0;i<n;i++)
      |                              ^
/usr/include/c++/14/string_view:680:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  680 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:680:5: note:   template argument deduction/substitution failed:
a.cc:7:7: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
    7 |   r(i,s)if(i%2==0)cout<<s[i];
      |       ^
a.cc:2:30: note: in definition of macro 'r'
    2 | #