Language: C++

#include <bits/stdc++.h>
#include <string.h>
using namespace std;

int main(){
    char SUN,MON,TUE,WED,THU,FRI,SAT;
    char S;

    cin >>("&c",S);

    switch(S){
        case SUN:
            cout << "7" << endl;
            break;
        case MON:
            cout << "6" << endl;
            break;
        case TUE:
            cout << "5" << endl;
            break;
        case WED:
            cout << "4" << endl;
            break;
        case THU:
            cout << "3" << endl;
            break;
        case FRI:
            cout << "2" << endl;
            break;
        case SAT:
            cout << "1" << endl; 
            break;
    }
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:12:14: error: the value of 'SUN' is not usable in a constant expression
   12 |         case SUN:
      |              ^~~
a.cc:6:10: note: 'char SUN' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |          ^~~
a.cc:12:14: error: the value of 'SUN' is not usable in a constant expression
   12 |         case SUN:
      |              ^~~
a.cc:6:10: note: 'char SUN' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |          ^~~
a.cc:15:14: error: the value of 'MON' is not usable in a constant expression
   15 |         case MON:
      |              ^~~
a.cc:6:14: note: 'char MON' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |              ^~~
a.cc:15:14: error: the value of 'MON' is not usable in a constant expression
   15 |         case MON:
      |              ^~~
a.cc:6:14: note: 'char MON' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |              ^~~
a.cc:18:14: error: the value of 'TUE' is not usable in a constant expression
   18 |         case TUE:
      |              ^~~
a.cc:6:18: note: 'char TUE' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                  ^~~
a.cc:18:14: error: the value of 'TUE' is not usable in a constant expression
   18 |         case TUE:
      |              ^~~
a.cc:6:18: note: 'char TUE' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                  ^~~
a.cc:21:14: error: the value of 'WED' is not usable in a constant expression
   21 |         case WED:
      |              ^~~
a.cc:6:22: note: 'char WED' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                      ^~~
a.cc:21:14: error: the value of 'WED' is not usable in a constant expression
   21 |         case WED:
      |              ^~~
a.cc:6:22: note: 'char WED' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                      ^~~
a.cc:24:14: error: the value of 'THU' is not usable in a constant expression
   24 |         case THU:
      |              ^~~
a.cc:6:26: note: 'char THU' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                          ^~~
a.cc:24:14: error: the value of 'THU' is not usable in a constant expression
   24 |         case THU:
      |              ^~~
a.cc:6:26: note: 'char THU' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                          ^~~
a.cc:27:14: error: the value of 'FRI' is not usable in a constant expression
   27 |         case FRI:
      |              ^~~
a.cc:6:30: note: 'char FRI' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                              ^~~
a.cc:27:14: error: the value of 'FRI' is not usable in a constant expression
   27 |         case FRI:
      |              ^~~
a.cc:6:30: note: 'char FRI' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                              ^~~
a.cc:30:14: error: the value of 'SAT' is not usable in a constant expression
   30 |         case SAT:
      |              ^~~
a.cc:6:34: note: 'char SAT' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                                  ^~~
a.cc:30:14: error: the value of 'SAT' is not usable in a constant expression
   30 |         case SAT:
      |              ^~~
a.cc:6:34: note: 'char SAT' is not const
    6 |     char SUN,MON,TUE,WED,THU,FRI,SAT;
      |                                  ^~~