Language: C

n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}

Compiler output:
main.c:1:1: warning: data definition has no type or storage class
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      | ^
main.c:1:1: error: type defaults to 'int' in declaration of 'n' [-Wimplicit-int]
main.c:1:5: error: type defaults to 'int' in declaration of 'i' [-Wimplicit-int]
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |     ^
main.c:1:7: error: return type defaults to 'int' [-Wimplicit-int]
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |       ^~~~
main.c: In function 'main':
main.c:1:7: error: type of 'c' defaults to 'int' [-Wimplicit-int]
main.c:1:23: error: implicit declaration of function 'getchar' [-Wimplicit-function-declaration]
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |                       ^~~~~~~
main.c:1:1: note: 'getchar' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
  +++ |+#include <stdio.h>
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
main.c:1:22: error: lvalue required as left operand of assignment
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |                      ^
main.c:1:35: error: implicit declaration of function 'tolower' [-Wimplicit-function-declaration]
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |                                   ^~~~~~~
main.c:1:1: note: include '<ctype.h>' or provide a declaration of 'tolower'
  +++ |+#include <ctype.h>
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
main.c:1:69: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |                                                                     ^~~~~~
main.c:1:69: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:69: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:1:69: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:96: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |                                                                                                ^~~~
main.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
  +++ |+#include <stdlib.h>
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
main.c:1:96: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      |                                                                                                ^~~~
main.c:1:96: note: include '<stdlib.h>' or provide a declaration of 'exit'
main.c: At top level:
main.c:1:1: warning: array 'n' assumed to have one element
    1 | n[],i;main(c){for(;~c=getchar();n[tolower(c)]++);for(i=97;i<123;i++)printf("%c : %d\n",i,n[i]);exit(0);}
      | ^