CustomZipIterator(F first, S second) : iter_(first, second) {
    }

// ^---- работает

CustomZipIterator(F first, S second) {
    iter_.first = first;
    iter_.second = second;
    }

// ^---- не работает

error: no matching function for call to ‘std::pair<__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> >, CustomRangeIterator>::pair()’
   61 |     CustomZipIterator(F first, S second) {
      |                                          ^

note: candidate: ‘template<class ... _Args1, long unsigned int ..._Indexes1, class ... _Args2, long unsigned int ..._Indexes2> constexpr std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {_Args1 ...}; long unsigned int ..._Indexes1 = {_Indexes1 ...}; _Args2 = {_Args2 ...}; long unsigned int ..._Indexes2 = {_Indexes2 ...}; _T1 = __gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> >; _T2 = CustomRangeIterator]’

  452 |         pair(tuple<_Args1...>&, tuple<_Args2...>&,
      |         ^~~~
/usr/include/c++/11/bits/stl_pair.h:452:9: note:   template argument deduction/substitution failed: