/in/foo.c:5:1: error: unknown type name 'bool'
 bool check(int x, int y)
 ^~~~
/in/foo.c: In function 'check':
/in/foo.c:8:10: error: 'true' undeclared (first use in this function)
   return true;
          ^~~~
/in/foo.c:8:10: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:10:10: error: 'false' undeclared (first use in this function)
   return false;
          ^~~~~
/in/foo.c:11:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^