foo.cc: In function 'int auto_gen(int, int, int)':
foo.cc:13:3: error: return-statement with no value, in function returning 'int' [-fpermissive]
   return;
   ^~~~~~
foo.cc: In function 'int main(int, char**)':
foo.cc:22:1: error: expected ';' before '}' token
 }
 ^

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