1. const mismatch with
defaulted copy constructor

2. https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0641r2.html

3. C++ 20 이전
/usr/include/c++/7/tuple: In instantiation of ‘struct std::_Head_base<0, NonConstCopyable,
true>’:
/usr/include/c++/7/tuple:343:12: required from ‘struct std::_Tuple_impl<0,
NonConstCopyable>’
/usr/include/c++/7/tuple:556:11: required from ‘class std::tuple<NonConstCopyable>’
<span class="error_line" onclick="ide.gotoLine('main.cpp',19)">main.cpp:19:30</span>:
required from here
/usr/include/c++/7/tuple:82:17: error: ‘constexpr std::_Head_base<_Idx, _Head,
true>::_Head_base(const std::_Head_base<_Idx, _Head, true>&) [with long unsigned int _Idx =
0; _Head = NonConstCopyable]’ declared to take const reference, but implicit declaration
would take non-const
constexpr _Head_base(const _Head_base&) = default;

4. C++ 20 이후