foo.cc:1:1: error: 'include' does not name a type; did you mean '__has_include'?
 include <iostream>
 ^~~~~~~
 __has_include
foo.cc: In function 'int main()':
foo.cc:5:5: error: 'cout' was not declared in this scope
     cout <<"Hello, world!" << endl;
     ^~~~
foo.cc:5:31: error: 'endl' was not declared in this scope
     cout <<"Hello, world!" << endl;
                               ^~~~
foo.cc:5:31: note: suggested alternative: 'enum'
     cout <<"Hello, world!" << endl;
                               ^~~~
                               enum

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。