foo.cc:11:8: warning: missing terminating ' character
   case '
        ^
foo.cc:11:8: error: missing terminating ' character
foo.cc: In function 'int ys(int, int, char)':
foo.cc:11:3: error: expected primary-expression at end of input
   case '
   ^~~~
foo.cc:11:3: error: expected ':' at end of input
foo.cc:11:3: error: expected statement at end of input
foo.cc:11:3: error: expected '}' at end of input
foo.cc:11:3: error: expected '}' at end of input
foo.cc:11:3: warning: control reaches end of non-void function [-Wreturn-type]
   case '
   ^~~~

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