/in/foo.c:2:1: error: unknown type name 'using'
 using namespace std;
 ^~~~~
/in/foo.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^~~
/in/foo.c:6:1: error: unknown type name 'bool'
 bool check(int x, int y)
 ^~~~
/in/foo.c: In function 'check':
/in/foo.c:9:10: error: 'true' undeclared (first use in this function)
   return true;
          ^~~~
/in/foo.c:9:10: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:11:10: error: 'false' undeclared (first use in this function)
   return false;
          ^~~~~
/in/foo.c:12:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^