foo.cc: In function ‘int main()’:
foo.cc:5:5: error: ‘cin’ was not declared in this scope; did you mean ‘std::cin’?
    5 |     cin >> a >> b;
      |     ^~~
      |     std::cin
In file included from stdc++.h:146:
/nix/gcc-13.2.0/include/c++/13.2.0/iostream:62:18: note: ‘std::cin’ declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
foo.cc:6:9: error: ‘cout’ was not declared in this scope; did you mean ‘std::cout’?
    6 |         cout << a + b;
      |         ^~~~
      |         std::cout
/nix/gcc-13.2.0/include/c++/13.2.0/iostream:63:18: note: ‘std::cout’ declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~