Language: C++

#include <iostream>
using namespace std;
int main() {
  char in;
  in >> cin;
  if (in == "A") {
    return 'T';
  } else if (in == "C") {
    return 'G';
  } else if (in == "G") {
    return 'C';
  } else if (in == "T") {    
    return 'A';
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:5:6: error: no match for 'operator>>' (operand types are 'char' and 'std::istream' {aka 'std::basic_istream<char>'})
    5 |   in >> cin;
      |   ~~ ^~ ~~~
      |   |     |
      |   char  std::istream {aka std::basic_istream<char>}
a.cc:5:6: note: candidate: 'operator>>(int, int)' (built-in)
    5 |   in >> cin;
      |   ~~~^~~~~~
a.cc:5:6: note:   no known conversion for argument 2 from 'std::istream' {aka 'std::basic_istream<char>'} to 'int'
In file included from /usr/include/c++/14/string:55,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.tcc:835:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
  835 |     operator>>(basic_istream<_CharT, _Traits>& __in,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.tcc:835:5: note:   template argument deduction/substitution failed:
a.cc:5:9: note:   mismatched types 'std::basic_istream<_CharT, _Traits>' and 'char'
    5 |   in >> cin;
      |         ^~~
In file included from /usr/include/c++/14/bits/memory_resource.h:38,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/cstddef:131:5: note: candidate: 'template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType> std::operator>>(byte, _IntegerType)'
  131 |     operator>>(byte __b, _IntegerType __shift) noexcept
      |     ^~~~~~~~
/usr/include/c++/14/cstddef:131:5: note:   template argument deduction/substitution failed:
a.cc:5:3: note:   cannot convert 'in' (type 'char') to type 'std::byte'
    5 |   in >> cin;
      |   ^~
In file included from /usr/include/c++/14/istream:1109,
                 from /usr/include/c++/14/iostream:42:
/usr/include/c++/14/bits/istream.tcc:978:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT&)'
  978 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT& __c)
      |     ^~~~~~~~
/usr/include/c++/14/bits/istream.tcc:978:5: note:   template argument deduction/substitution failed:
a.cc:5:9: note:   mismatched types 'std::basic_istream<_CharT, _Traits>' and 'char'
    5 |   in >> cin;
      |         ^~~
/usr/include/c++/14/istream:849:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char&)'
  849 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:849:5: note:   template argument deduction/substitution failed:
a.cc:5:9: note:   mismatched types 'std::basic_istream<char, _Traits>' and 'char'
    5 |   in >> cin;
      |         ^~~
/usr/include/c++/14/istream:854:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char&)'
  854 |     operator>>(basic_istream<char, _Traits>& __in, signed char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:854:5: note:   template argument deduction/substitution failed:
a.cc:5:9: note:   mismatched types 'std::basic_istream<char, _Traits>' and 'char'
    5 |   in >> cin;
      |         ^~~
/usr/include/c++/14/istream:896:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT*)'
  896 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:896:5: note:   template argument deduction/substitution failed:
a.cc:5:9: note:   mismatched types 'std::basic_istream<_CharT, _Traits>' and 'char'
    5 |   in >> cin;
      |         ^~~
/usr/include/c++/14/istream:939:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char*)'
  939 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:939:5: note:   template argument deduction/substitution failed:
a.cc:5:9: note:   mismatched types 'std::basic_istream<char, _Traits>' and 'char'
    5 |   in >> cin;
      |         ^~~
/usr/include/c++/14/istream:945:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char*)'
  945 |     operator>>(basic_istream<char, _Traits>& __in, signed char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:945:5: note:   template argument deduction/substitution failed:
a.cc:5:9: note:   mismatched types 'std::basic_istream<char, _Traits>' and 'char'
    5 |   in >> cin;
      |         ^~~
/usr/include/c++/14/istream:1099:5: note: candidate: 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&)'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~
/usr/include/c++/14/istream:1099:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/istream: In substitution of 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&) [with _Istream = char&; _Tp = std::basic_istream<char>&]':
a.cc:5:9:   required from here
    5 |   in >> cin;
      |         ^~~
/usr/include/c++/14/istream:1099:5: error: no type named 'type' in 'struct std::enable_if<false, void>'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~
a.cc:6:10: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
    6 |   if (in == "A") {
      |       ~~~^~~~~~
a.cc:8:17: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
    8 |   } else if (in == "C") {
      |              ~~~^~~~~~
a.cc:10:17: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   10 |   } else if (in == "G") {
      |              ~~~^~~~~~
a.cc:12:17: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   12 |   } else if (in == "T") {
      |              ~~~^~~~~~