Language: C++

#include <bits/stdc++.h>
using namespace std;

int main(){
  string S;
  int K;
  cin >> S>>K;
  if(S.at(K-1) == 'A'){
    swap(S.at(K-1),'a');
    cout <<S<< endl;
  }
  if(S.at(K-1) == 'B'){
    swap(S.at(K-1),'b');
    cout <<S<< endl;
  }
  if(S.at(K-1) == 'C'){
    swap(S.at(K-1),'c');
    cout <<S<< endl;
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:9:9: error: no matching function for call to 'swap(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&, char)'
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/bits/stl_pair.h:61,
                 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,
                 from a.cc:1:
/usr/include/c++/14/bits/move.h:226:5: note: candidate: 'std::_Require<std::__not_<std::__is_tuple_like<_Tp> >, std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp> > std::swap(_Tp&, _Tp&) [with _Tp = char; _Require<__not_<__is_tuple_like<_Tp> >, is_move_constructible<_Tp>, is_move_assignable<_Tp> > = void]' (near match)
  226 |     swap(_Tp& __a, _Tp& __b)
      |     ^~~~
/usr/include/c++/14/bits/move.h:226:5: note:   conversion of argument 2 would be ill-formed:
a.cc:9:20: error: cannot bind non-const lvalue reference of type 'char&' to an rvalue of type 'char'
    9 |     swap(S.at(K-1),'a');
      |                    ^~~
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:77:
/usr/include/c++/14/any:430:15: note: candidate: 'void std::swap(any&, any&)' (near match)
  430 |   inline void swap(any& __x, any& __y) noexcept { __x.swap(__y); }
      |               ^~~~
/usr/include/c++/14/any:430:15: note:   conversion of argument 2 would be ill-formed:
a.cc:9:20: error: cannot bind non-const lvalue reference of type 'std::any&' to an rvalue of type 'std::any'
    9 |     swap(S.at(K-1),'a');
      |                    ^~~
/usr/include/c++/14/any:191:7: note:   after user-defined conversion: 'std::any::any(_Tp&&) [with _Tp = char; _VTp = char; _Mgr = _Manager_internal<char>; typename std::enable_if<(is_copy_constructible_v<_VTp> && (! __is_in_place_type_v<_VTp>)), bool>::type <anonymous> = true]'
  191 |       any(_Tp&& __value)
      |       ^~~
In file included from /usr/include/c++/14/exception:166,
                 from /usr/include/c++/14/stdexcept:38,
                 from /usr/include/c++/14/system_error:43,
                 from /usr/include/c++/14/bits/ios_base.h:46,
                 from /usr/include/c++/14/streambuf:43,
                 from /usr/include/c++/14/bits/streambuf_iterator.h:35,
                 from /usr/include/c++/14/iterator:66,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:54:
/usr/include/c++/14/bits/exception_ptr.h:229:5: note: candidate: 'void std::__exception_ptr::swap(exception_ptr&, exception_ptr&)'
  229 |     swap(exception_ptr& __lhs, exception_ptr& __rhs)
      |     ^~~~
/usr/include/c++/14/bits/exception_ptr.h:229:25: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::__exception_ptr::exception_ptr&'
  229 |     swap(exception_ptr& __lhs, exception_ptr& __rhs)
      |          ~~~~~~~~~~~~~~~^~~~~
In file included from /usr/include/c++/14/future:51,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:176:
/usr/include/c++/14/bits/std_thread.h:328:3: note: candidate: 'void std::swap(thread&, thread&)'
  328 |   swap(thread& __x, thread& __y) noexcept
      |   ^~~~
/usr/include/c++/14/bits/std_thread.h:328:16: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::thread&'
  328 |   swap(thread& __x, thread& __y) noexcept
      |        ~~~~~~~~^~~
In file included from /usr/include/c++/14/complex:45,
                 from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/sstream:1204:5: note: candidate: 'template<class _CharT, class _Traits, class _Allocator> void std::__cxx11::swap(basic_stringbuf<_CharT, _Traits, _Alloc>&, basic_stringbuf<_CharT, _Traits, _Alloc>&)'
 1204 |     swap(basic_stringbuf<_CharT, _Traits, _Allocator>& __x,
      |     ^~~~
/usr/include/c++/14/sstream:1204:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::__cxx11::basic_stringbuf<_CharT, _Traits, _Alloc>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/sstream:1212:5: note: candidate: 'template<class _CharT, class _Traits, class _Allocator> void std::__cxx11::swap(basic_istringstream<_CharT, _Traits, _Allocator>&, basic_istringstream<_CharT, _Traits, _Allocator>&)'
 1212 |     swap(basic_istringstream<_CharT, _Traits, _Allocator>& __x,
      |     ^~~~
/usr/include/c++/14/sstream:1212:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::__cxx11::basic_istringstream<_CharT, _Traits, _Allocator>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/sstream:1219:5: note: candidate: 'template<class _CharT, class _Traits, class _Allocator> void std::__cxx11::swap(basic_ostringstream<_CharT, _Traits, _Allocator>&, basic_ostringstream<_CharT, _Traits, _Allocator>&)'
 1219 |     swap(basic_ostringstream<_CharT, _Traits, _Allocator>& __x,
      |     ^~~~
/usr/include/c++/14/sstream:1219:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::__cxx11::basic_ostringstream<_CharT, _Traits, _Allocator>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/sstream:1226:5: note: candidate: 'template<class _CharT, class _Traits, class _Allocator> void std::__cxx11::swap(basic_stringstream<_CharT, _Traits, _Allocator>&, basic_stringstream<_CharT, _Traits, _Allocator>&)'
 1226 |     swap(basic_stringstream<_CharT, _Traits, _Allocator>& __x,
      |     ^~~~
/usr/include/c++/14/sstream:1226:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::__cxx11::basic_stringstream<_CharT, _Traits, _Allocator>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181:
/usr/include/c++/14/bits/regex.h:896:5: note: candidate: 'template<class _Ch_type, class _Rx_traits> void std::__cxx11::swap(basic_regex<_Ch_type, _Rx_traits>&, basic_regex<_Ch_type, _Rx_traits>&)'
  896 |     swap(basic_regex<_Ch_type, _Rx_traits>& __lhs,
      |     ^~~~
/usr/include/c++/14/bits/regex.h:896:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::__cxx11::basic_regex<_Ch_type, _Rx_traits>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/bits/regex.h:2230:5: note: candidate: 'template<class _Bi_iter, class _Alloc> void std::__cxx11::swap(match_results<_BiIter, _Alloc>&, match_results<_BiIter, _Alloc>&)'
 2230 |     swap(match_results<_Bi_iter, _Alloc>& __lhs,
      |     ^~~~
/usr/include/c++/14/bits/regex.h:2230:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::__cxx11::match_results<_BiIter, _Alloc>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/bits/move.h:250:5: note: candidate: 'template<class _Tp, long unsigned int _Nm> std::__enable_if_t<((bool)std::__is_swappable<_Tp>::value)> std::swap(_Tp (&)[_Nm], _Tp (&)[_Nm])'
  250 |     swap(_Tp (&__a)[_Nm], _Tp (&__b)[_Nm])
      |     ^~~~
/usr/include/c++/14/bits/move.h:250:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types '_Tp [_Nm]' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1089:5: note: candidate: 'template<class _T1, class _T2> typename std::enable_if<std::__and_<std::__is_swappable<_T1>, std::__is_swappable<_T2> >::value>::type std::swap(pair<_T1, _T2>&, pair<_T1, _T2>&)'
 1089 |     swap(pair<_T1, _T2>& __x, pair<_T1, _T2>& __y)
      |     ^~~~
/usr/include/c++/14/bits/stl_pair.h:1089:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::pair<_T1, _T2>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1106:5: note: candidate: 'template<class _T1, class _T2> typename std::enable_if<(! std::__and_<std::__is_swappable<_T1>, std::__is_swappable<_T2> >::value)>::type std::swap(pair<_T1, _T2>&, pair<_T1, _T2>&)' (deleted)
 1106 |     swap(pair<_T1, _T2>&, pair<_T1, _T2>&) = delete;
      |     ^~~~
/usr/include/c++/14/bits/stl_pair.h:1106:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std::pair<_T1, _T2>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}
    9 |     swap(S.at(K-1),'a');
      |     ~~~~^~~~~~~~~~~~~~~
In file included 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/bits/basic_string.h:4039:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> void std::swap(__cxx11::basic_string<_CharT, _Traits, _Allocator>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 4039 |     swap(basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~
/usr/include/c++/14/bits/basic_string.h:4039:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   mismatched types 'std: