Proposed resolution:

This wording is relative to N4981.

1. Modify [range.concat.iterator] as indicated:

   constexpr iterator(iterator<!Const> it)
     requires Const &&
              (convertible_to<iterator_t<Views>, iterator_t<const Views>> && ...);
   

   -?- Preconditions: it.it_.valueless_by_exception() is false.

   -8- Effects: Initializes parent_ with it.parent_, and let i be it.it_.index(), initializes it_ with base-iter(in_place_index<i>, std::get<i>(std::move(it.it_))).

[ 2024-05-08; Reflector poll ]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

This conversion constructor obtains the alternative iterator of the argument through std::get, which will throw when the variant is valueless. We seem to be missing a Preconditions element here.