$ g++ -c -std=c++17 show.cpp
show.cpp: In instantiation of ‘maybe_either<T>::maybe_either(U&&) [with U = either<var>; T = var]’:
show.cpp:24:54:   required from here
show.cpp:15:68: error: no matching function for call to ‘std::variant<var, either<maybe_either<var> > >::variant(either<var>)’
         std::variant<T, either<maybe_either<T>>>(std::forward<U>(v)) {
                                                                    ^
In file included from show.cpp:2:0:
/usr/include/c++/7/variant:990:2: note: candidate: template<long unsigned int _Np, class _Up, class ... _Args, class> constexpr std::variant<_Types>::variant(std::in_place_index_t<_Np>, std::initializer_list<_Up>, _Args&& ...)
  variant(in_place_index_t<_Np>, initializer_list<_Up> __il,
  ^~~~~~~
/usr/include/c++/7/variant:990:2: note:   template argument deduction/substitution failed:
show.cpp:15:68: note:   ‘either<var>’ is not derived from ‘std::in_place_index_t<_Idx>’
         std::variant<T, either<maybe_either<T>>>(std::forward<U>(v)) {
                                                                    ^
In file included from show.cpp:2:0:
/usr/include/c++/7/variant:981:2: note: candidate: template<long unsigned int _Np, class ... _Args, class> constexpr std::variant<_Types>::variant(std::in_place_index_t<_Np>, _Args&& ...)
  variant(in_place_index_t<_Np>, _Args&&... __args)
  ^~~~~~~
/usr/include/c++/7/variant:981:2: note:   template argument deduction/substitution failed:
show.cpp:15:68: note:   ‘either<var>’ is not derived from ‘std::in_place_index_t<_Idx>’
         std::variant<T, either<maybe_either<T>>>(std::forward<U>(v)) {
                                                                    ^
In file included from show.cpp:2:0:
/usr/include/c++/7/variant:971:2: note: candidate: template<class _Tp, class _Up, class ... _Args, class> constexpr std::variant<_Types>::variant(std::in_place_type_t<_Tp>, std::initializer_list<_Up>, _Args&& ...)
  variant(in_place_type_t<_Tp>, initializer_list<_Up> __il,
  ^~~~~~~
/usr/include/c++/7/variant:971:2: note:   template argument deduction/substitution failed:
show.cpp:15:68: note:   ‘either<var>’ is not derived from ‘std::in_place_type_t<_Tp>’
         std::variant<T, either<maybe_either<T>>>(std::forward<U>(v)) {
                                                                    ^
In file included from show.cpp:2:0:
/usr/include/c++/7/variant:962:2: note: candidate: template<class _Tp, class ... _Args, class> constexpr std::variant<_Types>::variant(std::in_place_type_t<_Tp>, _Args&& ...)
  variant(in_place_type_t<_Tp>, _Args&&... __args)
  ^~~~~~~
/usr/include/c++/7/variant:962:2: note:   template argument deduction/substitution failed:
show.cpp:15:68: note:   ‘either<var>’ is not derived from ‘std::in_place_type_t<_Tp>’
         std::variant<T, either<maybe_either<T>>>(std::forward<U>(v)) {
                                                                    ^
In file included from show.cpp:2:0:
/usr/include/c++/7/variant:953:2: note: candidate: template<class _Tp, class, class, class> constexpr std::variant<_Types>::variant(_Tp&&)
  variant(_Tp&& __t)
  ^~~~~~~
/usr/include/c++/7/variant:953:2: note:   template argument deduction/substitution failed:
/usr/include/c++/7/variant:951:6: error: invalid use of incomplete type ‘struct std::variant<var, either<maybe_either<var> > >::__to_type_impl<18446744073709551615, false>’
         typename = enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>>>
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/7/variant:924:9: note: declaration of ‘struct std::variant<var, either<maybe_either<var> > >::__to_type_impl<18446744073709551615, false>’
  struct __to_type_impl;
         ^~~~~~~~~~~~~~
/usr/include/c++/7/variant:951:6: error: invalid use of incomplete type ‘struct std::variant<var, either<maybe_either<var> > >::__to_type_impl<18446744073709551615, false>’
         typename = enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>>>
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/7/variant:924:9: note: declaration of ‘struct std::variant<var, either<maybe_either<var> > >::__to_type_impl<18446744073709551615, false>’
  struct __to_type_impl;
         ^~~~~~~~~~~~~~
/usr/include/c++/7/variant:944:7: note: candidate: std::variant<_Types>::variant(std::variant<_Types>&&) [with _Types = {var, either<maybe_either<var> >}]
       variant(variant&&)
       ^~~~~~~
/usr/include/c++/7/variant:944:7: note:   no known conversion for argument 1 from ‘either<var>’ to ‘std::variant<var, either<maybe_either<var> > >&&’
/usr/include/c++/7/variant:943:7: note: candidate: std::variant<_Types>::variant(const std::variant<_Types>&) [with _Types = {var, either<maybe_either<var> >}]
       variant(const variant&) = default;
       ^~~~~~~
/usr/include/c++/7/variant:943:7: note:   no known conversion for argument 1 from ‘either<var>’ to ‘const std::variant<var, either<maybe_either<var> > >&’
/usr/include/c++/7/variant:941:17: note: candidate: constexpr std::variant<_Types>::variant() [with _Types = {var, either<maybe_either<var> >}]
       constexpr variant()
                 ^~~~~~~
/usr/include/c++/7/variant:941:17: note:   candidate expects 0 arguments, 1 provided