elements: view to extracts elements of tuple-like objects

[cor3ntin]

sequence_of_tuple | ranges::ext::view::elements<42>
static_array | ranges::ext::view::elements<0>

map | ranges::ext::view::keys;
map | ranges::ext::view::values;

[cor3ntin]

Hi @CaseyCarter, could tell me what you think about this?
The goal was to take a bit of work out of @cjdb for his range paper.

I currently have a weird issue if I try a static_assert(View<Rng>), any idea what I could have done wrong?
I suspect the issue is that I need to instantiate the view with an explicit type (given i have to pass 2 template parameter), and I'm probably screwing it up

/usr/include/c++/8/type_traits: In substitution of ‘template<class _Tp> decltype (__declval<_Tp>(0)) std::declval() [with _Tp = std::experimental::ranges::v1::ext::ref_view<std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int> > >, 0> >]’:
/home/cor3ntin/dev/cpp_proposals/move_only_io_iterator/cmcstl2/include/stl2/view/all.hpp:54:38:   recursively required by substitution of ‘template<class Rng>  requires  ViewableRange<Rng> using all_view = decltype (std::experimental::ranges::v1::view::all(declval<Rng>())) [with Rng = std::experimental::ranges::v1::ext::ref_view<std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int> > >, 0> >]’
/home/cor3ntin/dev/cpp_proposals/move_only_io_iterator/cmcstl2/include/stl2/view/all.hpp:54:38:   required from ‘struct std::is_constructible<std::experimental::ranges::v1::ext::ref_view<std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int> > >, 0> >, std::experimental::ranges::v1::ext::ref_view<std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int>, std::allocator<std::tuple<int, int, int> > > >, 0> > >’
/home/cor3ntin/dev/cpp_proposals/move_only_io_iterator/cmcstl2/include/stl2/detail/concepts/core.hpp:31:43:   required from ‘constexpr auto std::experimental::ranges::v1::view::__all_fn::operator()(Rng&&) const [with Rng = std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int> > >, 0>&]’
/home/cor3ntin/dev/cpp_proposals/move_only_io_iterator/cmcstl2/include/stl2/view/all.hpp:49:37:   required by substitution of ‘template<class Rng>  requires  ViewableRange<Rng> using all_view = decltype (std::experimental::ranges::v1::view::all(declval<Rng>())) [with Rng = std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int> > >, 0>&]’
/home/cor3ntin/dev/cpp_proposals/move_only_io_iterator/cmcstl2/include/stl2/view/all.hpp:54:38:   required from ‘struct std::is_constructible<std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int> > >, 0>, std::experimental::ranges::v1::ext::elements_view<std::experimental::ranges::v1::ext::ref_view<std::vector<std::tuple<int, int, int>, std::allocator<std::tuple<int, int, int> > > >, 0>&>’
/home/cor3ntin/dev/cpp_proposals/move_only_io_iterator/cmcstl2/include/stl2/detail/concepts/core.hpp:31:43:   required from here
/usr/include/c++/8/type_traits:756:55: fatal error: template instantiation depth exceeds maximum of 900 (use -ftemplate-depth= to increase the maximum)
     auto declval() noexcept -> decltype(__declval<_Tp>(0));

[cjdb]

This is what I've been using in my zip_with branch. Is there any reason we can't go with this instead? (I don't think it breaks any equality preservation).

namespace view {
	template<int X>
	requires (X == 0) || (X == 1)
	struct __get_fn {
		template<class P>
		requires _PairLike<remove_reference_t<P>> // from subrange.hpp
		constexpr decltype(auto) operator()(P&& p) const
		{
			using T = decltype(std::get<X>(std::forward<P>(p)));
			return std::forward<T>(std::get<X>(std::forward<P>(p)));
		}
	};

	inline constexpr auto keys = view::transform(__get_fn<0>{});
	inline constexpr auto values = view::transform(__get_fn<1>{});
} // namespace view