Language: C++

#include <iostream>
#include <string>
#include <utility>

#define co(a) cout << a << endl

int main(){
    using namespace std;
    char n;
    cin>>n;
    pair<char> a('A','T');
    pair<char> b('G','C');
    if (n==a.first)
        co(a.second);
    else if (n==a.second)
        co(a.first);
    else if (n==b.first)
        co(b.second);
    else if (n==b.second)
        co(a.first);
}

Compiler output:
a.cc: In function 'int main()':
a.cc:11:14: error: wrong number of template arguments (1, should be 2)
   11 |     pair<char> a('A','T');
      |              ^
In file included from /usr/include/c++/14/string:48,
                 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/stl_iterator.h:3013:12: note: provided for 'template<class _T1, class _T2> struct std::pair'
 3013 |     struct pair;
      |            ^~~~
a.cc:11:25: error: expression list treated as compound expression in initializer [-fpermissive]
   11 |     pair<char> a('A','T');
      |                         ^
a.cc:12:14: error: wrong number of template arguments (1, should be 2)
   12 |     pair<char> b('G','C');
      |              ^
/usr/include/c++/14/bits/stl_iterator.h:3013:12: note: provided for 'template<class _T1, class _T2> struct std::pair'
 3013 |     struct pair;
      |            ^~~~
a.cc:12:25: error: expression list treated as compound expression in initializer [-fpermissive]
   12 |     pair<char> b('G','C');
      |                         ^
a.cc:13:14: error: request for member 'first' in 'a', which is of non-class type 'int'
   13 |     if (n==a.first)
      |              ^~~~~
a.cc:14:14: error: request for member 'second' in 'a', which is of non-class type 'int'
   14 |         co(a.second);
      |              ^~~~~~
a.cc:5:23: note: in definition of macro 'co'
    5 | #define co(a) cout << a << endl
      |                       ^
a.cc:15:19: error: request for member 'second' in 'a', which is of non-class type 'int'
   15 |     else if (n==a.second)
      |                   ^~~~~~
a.cc:16:14: error: request for member 'first' in 'a', which is of non-class type 'int'
   16 |         co(a.first);
      |              ^~~~~
a.cc:5:23: note: in definition of macro 'co'
    5 | #define co(a) cout << a << endl
      |                       ^
a.cc:17:19: error: request for member 'first' in 'b', which is of non-class type 'int'
   17 |     else if (n==b.first)
      |                   ^~~~~
a.cc:18:14: error: request for member 'second' in 'b', which is of non-class type 'int'
   18 |         co(b.second);
      |              ^~~~~~
a.cc:5:23: note: in definition of macro 'co'
    5 | #define co(a) cout << a << endl
      |                       ^
a.cc:19:19: error: request for member 'second' in 'b', which is of non-class type 'int'
   19 |     else if (n==b.second)
      |                   ^~~~~~
a.cc:20:14: error: request for member 'first' in 'a', which is of non-class type 'int'
   20 |         co(a.first);
      |              ^~~~~
a.cc:5:23: note: in definition of macro 'co'
    5 | #define co(a) cout << a << endl
      |                       ^