The issue of the `concat_view::iterator`'s conversion constructor

[hewillk]

Hi Hui,

I was reading your wonderful paper recently and found a potential issue here:

cpp_papers/impl/concat/concat.hpp

Lines 355 to 359 in c97f7c5

	constexpr iterator(iterator<!Const> i)
	requires Const && (convertible_to<iterator_t<Views>, iterator_t<const Views>> && ...)
	// [TODO] noexcept specs?
	: parent_{i.parent_}
	, it_{std::move(i.it_)} {}

Using variant<Bs...> to initialize variant<As...> seems not work here, because std::variant does not have a constructor that accepts the heterogeneous sibling like range-v3 variant. The following seems can fix it:

constexpr iterator(iterator<!Const> i)
  requires Const && (convertible_to<iterator_t<Views>, iterator_t<const Views>> && ...)
  // [TODO] noexcept specs?
  : parent_{i.parent_}
  , it_{
      xo::visit_i(
        std::move(i.it_),
        [](auto I, auto it) { 
          return decltype(it_)(in_place_index<I>, std::move(it)); 
        })
    } {}

Thanks again for your contributions, I really like this view.

[huixie90]

Thank you for your comments. we will address it!