Language: C++

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>

using namespace std;

int main()
{
    int k;
    cin >> k;
    vector v;
    bool flag = true;
    int c = 1;
    int a = 7;
    a %= k;
    while (flag)
    {
        if (a == 0)
        {
            cout << c << endl;
            break;
        }
        else
        {
            a = a * 10 + 7;
            a %= k;
            if (find(v.begin(), v.end(), a) != v.end())
            {
                v.push_back(a);
            }
            else
            {
                cout << -1 << endl;
                break;
            }
        }
    }
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:12:12: error: class template argument deduction failed:
   12 |     vector v;
      |            ^
a.cc:12:12: error: no matching function for call to 'vector()'
In file included from /usr/include/c++/14/vector:66,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_vector.h:707:9: note: candidate: 'template<class _Tp, class _Alloc, class _InputIterator, class> vector(_InputIterator, _InputIterator, const _Alloc&)-> std::vector<_Tp, _Alloc>'
  707 |         vector(_InputIterator __first, _InputIterator __last,
      |         ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:707:9: note:   candidate expects 2 arguments, 0 provided
/usr/include/c++/14/bits/stl_vector.h:678:7: note: candidate: 'template<class _Tp, class _Alloc> vector(std::initializer_list<_Tp>, const _Alloc&)-> std::vector<_Tp, _Alloc>'
  678 |       vector(initializer_list<value_type> __l,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:678:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/stl_vector.h:659:7: note: candidate: 'template<class _Tp, class _Alloc> vector(std::vector<_Tp, _Alloc>&&, std::__type_identity_t<_Alloc>&)-> std::vector<_Tp, _Alloc>'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:659:7: note:   candidate expects 2 arguments, 0 provided
/usr/include/c++/14/bits/stl_vector.h:640:7: note: candidate: 'template<class _Tp, class _Alloc> vector(std::vector<_Tp, _Alloc>&&, const _Alloc&, std::false_type)-> std::vector<_Tp, _Alloc>'
  640 |       vector(vector&& __rv, const allocator_type& __m, false_type)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note:   candidate expects 3 arguments, 0 provided
/usr/include/c++/14/bits/stl_vector.h:635:7: note: candidate: 'template<class _Tp, class _Alloc> vector(std::vector<_Tp, _Alloc>&&, const _Alloc&, std::true_type)-> std::vector<_Tp, _Alloc>'
  635 |       vector(vector&& __rv, const allocator_type& __m, true_type) noexcept
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:635:7: note:   candidate expects 3 arguments, 0 provided
/usr/include/c++/14/bits/stl_vector.h:624:7: note: candidate: 'template<class _Tp, class _Alloc> vector(const std::vector<_Tp, _Alloc>&, std::__type_identity_t<_Alloc>&)-> std::vector<_Tp, _Alloc>'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:624:7: note:   candidate expects 2 arguments, 0 provided
/usr/include/c++/14/bits/stl_vector.h:620:7: note: candidate: 'template<class _Tp, class _Alloc> vector(std::vector<_Tp, _Alloc>&&)-> std::vector<_Tp, _Alloc>'
  620 |       vector(vector&&) noexcept = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/stl_vector.h:601:7: note: candidate: 'template<class _Tp, class _Alloc> vector(const std::vector<_Tp, _Alloc>&)-> std::vector<_Tp, _Alloc>'
  601 |       vector(const vector& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:601:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/stl_vector.h:569:7: note: candidate: 'template<class _Tp, class _Alloc> vector(std::size_t, const _Tp&, const _Alloc&)-> std::vector<_Tp, _Alloc>'
  569 |       vector(size_type __n, const value_type& __value,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:569:7: note:   candidate expects 2 arguments, 0 provided
/usr/include/c++/14/bits/stl_vector.h:556:7: note: candidate: 'template<class _Tp, class _Alloc> vector(std::size_t, const _Alloc&)-> std::vector<_Tp, _Alloc>'
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/stl_vector.h:542:7: note: candidate: 'template<class _Tp, class _Alloc> vector(const _Alloc&)-> std::vector<_Tp, _Alloc>'
  542 |       vector(const allocator_type& __a) _GLIBCXX_NOEXCEPT
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/stl_vector.h:531:7: note: candidate: 'template<class _Tp, class _Alloc> vector()-> std::vector<_Tp, _Alloc>'
  531 |       vector() = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:531:7: note:   template argument deduction/substitution failed:
a.cc:12:12: note:   couldn't deduce template parameter '_Tp'
   12 |     vector v;
      |            ^
/usr/include/c++/14/bits/stl_vector.h:428:11: note: candidate: 'template<class _Tp, class _Alloc> vector(std::vector<_Tp, _Alloc>)-> std::vector<_Tp, _Alloc>'
  428 |     class vector : protected _Vector_base<_Tp, _Alloc>
      |           ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:428:11: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/stl_vector.h:2033:5: note: candidate: 'template<class _InputIterator, class _ValT, class _Allocator, class, class> std::vector(_InputIterator, _InputIterator, _Allocator)-> vector<_ValT, _Allocator>'
 2033 |     vector(_InputIterator, _InputIterator, _Allocator = _Allocator())
      |     ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:2033:5: note:   candidate expects 2 arguments, 0 provided