Language: C++

#include<bits/stdc++.h>
using namespace std;
int main(){
  unordered_set<string>st;
  string ans,s="?a";
  st.insert("a");
  while(1){
    cout<<s<<endl;
    cout.flush();
    if(s[0]=='!')break;
    cin>>ans;
    if(st.count(ans)||s[s.size()-1]!=ans[0])s="!OUT";
    else st.insert(ans),s="?"+(string)ans[ans.size()-1]+ans+"a";
  }
  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:13:55: error: no matching function for call to 'std::__cxx11::basic_string<char>::basic_string(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&)'
   13 |     else st.insert(ans),s="?"+(string)ans[ans.size()-1]+ans+"a";
      |                                                       ^
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.h:800:9: note: candidate: 'template<class _Tp, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Tp&, const _Alloc&) [with <template-parameter-2-2> = _Tp; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  800 |         basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:800:9: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/14/bits/stl_pair.h:60,
                 from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/type_traits: In substitution of 'template<bool _Cond, class _Tp> using std::enable_if_t = typename std::enable_if::type [with bool _Cond = false; _Tp = void]':
/usr/include/c++/14/bits/basic_string.h:149:8:   required by substitution of 'template<class _CharT, class _Traits, class _Alloc> template<class _Tp, class _Res> using std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv = std::enable_if_t<((bool)std::__and_<std::is_convertible<const _Tp&, std::basic_string_view<_CharT, _Traits> >, std::__not_<std::is_convertible<const _Tp*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>*> >, std::__not_<std::is_convertible<const _Tp&, const _CharT*> > >::value), _Res> [with _Tp = char; _Res = void; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  149 |         using _If_sv = enable_if_t<
      |               ^~~~~~
/usr/include/c++/14/bits/basic_string.h:797:30:   required from here
  797 |       template<typename _Tp, typename = _If_sv<_Tp, void>>
      |                              ^~~~~~~~
/usr/include/c++/14/type_traits:2711:11: error: no type named 'type' in 'struct std::enable_if<false, void>'
 2711 |     using enable_if_t = typename enable_if<_Cond, _Tp>::type;
      |           ^~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:788:9: note: candidate: 'template<class _Tp, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Tp&, size_type, size_type, const _Alloc&) [with <template-parameter-2-2> = _Tp; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  788 |         basic_string(const _Tp& __t, size_type __pos, size_type __n,
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:788:9: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:765:9: note: candidate: 'template<class _InputIterator, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(_InputIterator, _InputIterator, const _Alloc&) [with <template-parameter-2-2> = _InputIterator; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  765 |         basic_string(_InputIterator __beg, _InputIterator __end,
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:765:9: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:669:7: note: candidate: 'template<class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(size_type, _CharT, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  669 |       basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:669:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:646:7: note: candidate: 'template<class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  646 |       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:646:7: note:   template argument deduction/substitution failed:
a.cc:13:55: note:   cannot convert 'ans.std::__cxx11::basic_string<char>::operator[]((ans.std::__cxx11::basic_string<char>::size() - 1))' (type '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'}) to type 'const char*'
   13 |     else st.insert(ans),s="?"+(string)ans[ans.size()-1]+ans+"a";
      |                                                       ^
/usr/include/c++/14/bits/basic_string.h:721:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  721 |       basic_string(basic_string&& __str, const _Alloc& __a)
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:721:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:716:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  716 |       basic_string(const basic_string& __str, const _Alloc& __a)
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:716:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:711:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::initializer_list<_Tp>, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  711 |       basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:711:45: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::initializer_list<char>'
  711 |       basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:682:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  682 |       basic_string(basic_string&& __str) noexcept
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:682:35: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::__cxx11::basic_string<char>&&'
  682 |       basic_string(basic_string&& __str) noexcept
      |                    ~~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:624:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  624 |       basic_string(const _CharT* __s, size_type __n,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:624:7: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:604:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, size_type, size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  604 |       basic_string(const basic_string& __str, size_type __pos,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:604:7: note:   candidate expects 4 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:586:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, size_type, size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  586 |       basic_string(const basic_string& __str, size_type __pos,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:586:7: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:569:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]'
  569 |       basic_string(const basic_string& __str, size_type __pos,
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:569:7: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:552:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  552 |       basic_string(const basic_string& __str)
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:552:40: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const std::__cxx11::basic_string<char>&'
  552 |       basic_string(const basic_string& __str)
      |                    ~~~~~~~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:540:7: note: candidate: 'std::__cxx11::basic_strin