Language: C++

#include <bits/stdc++.h>
using namespace std;
 
int main() {
  char c;
  cin >> c;
  char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
  for(int i=0;i<26;i++)
    if(c==m[i]){
      cout << m[i+1] << endl;
    }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:7:15: error: 'a' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |               ^
a.cc:7:17: error: 'b' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                 ^
a.cc:7:21: error: 'd' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                     ^
a.cc:7:23: error: 'e' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                       ^
a.cc:7:25: error: 'f' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                         ^
a.cc:7:27: error: 'g' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                           ^
a.cc:7:29: error: 'h' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                             ^
a.cc:7:31: error: 'i' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                               ^
a.cc:7:33: error: 'j' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                 ^
a.cc:7:35: error: 'k' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                   ^
a.cc:7:37: error: 'l' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                     ^
a.cc:7:41: error: 'n' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                         ^
a.cc:7:43: error: 'o' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                           ^
a.cc:7:45: error: 'p' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                             ^
a.cc:7:47: error: 'q' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                               ^
a.cc:7:49: error: 'r' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                 ^
a.cc:7:51: error: 's' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                   ^
a.cc:7:53: error: 't' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                     ^
a.cc:7:55: error: 'u' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                       ^
a.cc:7:57: error: 'v' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                         ^
a.cc:7:59: error: 'w' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                           ^
a.cc:7:61: error: 'x' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                             ^
a.cc:7:63: error: 'y' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                               ^
a.cc:7:65: error: 'z' was not declared in this scope
    7 |   char m[26]={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                                 ^