Language: C

i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}

Compiler output:
main.c:1:1: warning: data definition has no type or storage class
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
      | ^
main.c:1:1: error: type defaults to 'int' in declaration of 'i' [-Wimplicit-int]
main.c:1:3: error: type defaults to 'int' in declaration of 'z' [-Wimplicit-int]
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
      |   ^
main.c:1:5: error: type defaults to 'int' in declaration of 'a' [-Wimplicit-int]
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
      |     ^
main.c:1:7: error: return type defaults to 'int' [-Wimplicit-int]
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
      |       ^~~~
main.c: In function 'main':
main.c:1:7: error: type of 'f' defaults to 'int' [-Wimplicit-int]
main.c:1:19: error: implicit declaration of function 'puts' [-Wimplicit-function-declaration]
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
      |                   ^~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'puts'
  +++ |+#include <stdio.h>
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
main.c:1:31: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
      |                               ^~~~~
main.c:1:31: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:31: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
main.c:1:31: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:112: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    1 | i,z,a;main(f){for(puts("?aa");scanf("%s",&i);){a=i>>8;if(a){if(i&31^1|f&1<<a-97)return!puts("!OUT");f|=1<<a-97;printf("?%ca\n",a);}else{if(z++)return!puts("!OUT");puts("?aaa");}}}
      |                                                                                                                ^~~~~~
main.c:1:112: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:112: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:1:112: note: include '<stdio.h>' or provide a declaration of 'printf'