Language: C++

#include <bits/stdc++.h>
using namespace std;
 
int main() {
    char a ; cin >> a;
    if (a=='A'or'B'or'C'or'D'or'E'or'F'or'G'or'H'or'I'or'J'or'K'or'
    L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
    'or'W'or'X'or'Y'or'Z')
    {
        cout << "A" << endl;
    }
    else
    {
        cout << "B" << endl;
    }
    
    
}

Compiler output:
a.cc:6:67: warning: missing terminating ' character
    6 |     if (a=='A'or'B'or'C'or'D'or'E'or'F'or'G'or'H'or'I'or'J'or'K'or'
      |                                                                   ^
a.cc:6:67: error: missing terminating ' character
a.cc:7:5: warning: multi-character literal cannot have an encoding prefix
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |     ^~~~~~
a.cc:7:11: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |           ^~~~~
a.cc:7:16: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                ^~~~~
a.cc:7:21: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                     ^~~~~
a.cc:7:26: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                          ^~~~~
a.cc:7:31: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                               ^~~~~
a.cc:7:36: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                                    ^~~~~
a.cc:7:41: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                                         ^~~~~
a.cc:7:46: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                                              ^~~~~
a.cc:7:51: warning: multi-character character constant [-Wmultichar]
    7 |     L'or'M'or'N'or'O'or'P'or'Q'or'R'or'S'or'T'or'U'or'V
      |                                                   ^~~~~
a.cc:8:5: warning: multi-character character constant [-Wmultichar]
    8 |     'or'W'or'X'or'Y'or'Z')
      |     ^~~~~
a.cc:8:10: warning: multi-character character constant [-Wmultichar]
    8 |     'or'W'or'X'or'Y'or'Z')
      |          ^~~~~
a.cc:8:15: warning: multi-character character constant [-Wmultichar]
    8 |     'or'W'or'X'or'Y'or'Z')
      |               ^~~~~
a.cc:8:20: warning: multi-character character constant [-Wmultichar]
    8 |     'or'W'or'X'or'Y'or'Z')
      |                    ^~~~~
a.cc:8:25: warning: missing terminating ' character
    8 |     'or'W'or'X'or'Y'or'Z')
      |                         ^
a.cc:8:25: error: missing terminating ' character
    8 |     'or'W'or'X'or'Y'or'Z')
      |                         ^~
a.cc: In function 'int main()':
a.cc:6:12: error: unable to find character literal operator 'operator""or' with 'char' argument
    6 |     if (a=='A'or'B'or'C'or'D'or'E'or'F'or'G'or'H'or'I'or'J'or'K'or'
      |            ^~~~~
a.cc:6:17: error: expected ')' before user-defined character literal
    6 |     if (a=='A'or'B'or'C'or'D'or'E'or'F'or'G'or'H'or'I'or'J'or'K'or'
      |        ~        ^~~~~
      |                 )
a.cc:18:1: error: expected primary-expression before '}' token
   18 | }
      | ^