answer.code:16:5: error: ‘int index’ redeclared as different kind of entity
   16 | int index;
      |     ^~~~~
In file included from /usr/include/string.h:432,
                 from /usr/include/c++/11/cstring:42,
                 from /usr/include/x86_64-linux-gnu/c++/11/bits/stdc++.h:48,
                 from answer.code:1:
/usr/include/strings.h:61:1: note: previous declaration ‘const char* index(const char*, int)’
   61 | index (const char *__s, int __c) __THROW
      | ^~~~~
answer.code: In function ‘void sieve()’:
answer.code:19:11: error: overloaded function with no contextual type information
   19 |   index = 0;
      |           ^
answer.code:24:13: error: no post-increment operator for type
   24 |     ct[index++] = i;
      |             ^~