Language: C++

#include<iostream>

int main(){
  char b;
  cin << b;
  
  if (b = 'A') cout >> 'T' >> endl;
  else if (b = 'T') cout >> 'A' >> endl;
  else if (b = 'G') cout >> 'C' >> endl;
  else cout >> 'G' >> endl;
  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:5:3: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    5 |   cin << b;
      |   ^~~
      |   std::cin
In file included from a.cc:1:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:7:16: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    7 |   if (b = 'A') cout >> 'T' >> endl;
      |                ^~~~
      |                std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:7:31: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    7 |   if (b = 'A') cout >> 'T' >> endl;
      |                               ^~~~
      |                               std::endl
In file included from /usr/include/c++/14/iostream:41:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:8:21: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    8 |   else if (b = 'T') cout >> 'A' >> endl;
      |                     ^~~~
      |                     std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:8:36: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    8 |   else if (b = 'T') cout >> 'A' >> endl;
      |                                    ^~~~
      |                                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:9:21: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    9 |   else if (b = 'G') cout >> 'C' >> endl;
      |                     ^~~~
      |                     std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:9:36: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    9 |   else if (b = 'G') cout >> 'C' >> endl;
      |                                    ^~~~
      |                                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:10:8: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   10 |   else cout >> 'G' >> endl;
      |        ^~~~
      |        std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:10:23: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   10 |   else cout >> 'G' >> endl;
      |                       ^~~~
      |                       std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~