The `noexcept` specification of concat_view::iterator's `iter_swap` has noexcept(ranges::swap(*x, *y)), where `x` and `y` is concat_view::iterator. However, concat_view::iterator's `operator*` does not have a `noexcept` specification, therefore noexcept(ranges::swap(*x, *y)) is always `false` and then the `noexcept` specification of concat_view::iterator's `iter_swap` is always `noexcept(false)`.