#pragma once

#include <optional>
#include <utility>
#include <concepts>

#include "pointer.h"

#include "exopt.h"

/// A better std::optional
namespace exopt::types { namespace optional [[gnu::visibility("internal")]] {
	template<typename T>
	struct null_optimise;

	template<typename T>
	struct null_optimise<ptr::NonNull<T> > { constexpr static inline bool value = true;
		using held_type = T*;
		using type = ptr::NonNull<T>;
		constexpr static decltype(auto) convert_to_held(std::add_rvalue_reference_t<type> t) noexcept { return t.get(); }
		constexpr static decltype(auto) convert_from_held(std::add_rvalue_reference_t<held_type> t) noexcept { return ptr::NonNull<T>::new_unchecked(t); }

		constexpr static inline auto sentinel = nullptr;
	};

	

	template<typename T>
	struct null_optimise<std::reference_wrapper<T> > { constexpr static inline bool value = true;
		using held_type = T*;
		using type = std::reference_wrapper<T>;
		constexpr static decltype(auto) convert_to_held(std::add_rvalue_reference_t<type> t) noexcept { return std::addressof(t.get()); }
		constexpr static decltype(auto) convert_from_held(std::add_rvalue_reference_t<held_type> t) noexcept { return std::ref<T>(*t); }

		constexpr static inline auto sentinel = nullptr;
	};

	template<typename T>
	class null_optimise<boxed::Box<T> > {
		using boxed::Box;
		using box_t = Box<T>;
		class /*XXX: Shouldn't be needed alignas(box_t)*/ invariant : box_t {
			//TODO: See phone notes.
			using box_t::UNSAFE;
		public:
			constexpr box_t& operator*() & noexcept { return *static_assert<box_t *>(this); }
			constexpr box_t const& operator*() const& noexcept { return *static_assert<box_t const*>(this); }
			constexpr box_t&& operator*() && noexcept { return std::move(*static_assert<box_t *>(this)); }
			constexpr box_t const&& operator*() const&& noexcept { return std::move(*static_assert<box_t const*>(this)); }

			constexpr static invariant&& back_conv(box_t&& b) noexcept { return static_cast<invariant&&>(b); } 
			constexpr static invariant const& back_conv(box_t const& b) noexcept { return static_cast<invariant const&>(b); }

			constexpr static invariant& back_conv(box_t& b) noexcept { return static_cast<invariant&>(b); } 
			constexpr static invariant const&& back_conv(box_t const&& b) noexcept { return static_cast<invariant const&&>(b); }

			constexpr invariant(box_t&& m) noexcept
				: box_t(UNSAFE, std::move(std::move(m).as_unique(UNSAFE))) {}
			constexpr invariant(box_t const& m) noexcept
				: box_t(UNSAFE, m) {}

			constexpr invariant(std::unique_ptr<T>&& m) noexcept
				: box_t(UNSAFE, std::move(m)) {}

			constexpr invariant(std::nullptr_t) noexcept
				: box_t(UNSAFE, std::unique_ptr<T> { nullptr } ) {}

			constexpr friend auto operator<=>(invariant const& a, invariant const& b) noexcept { return (*a).as_unsafe_ptr(UNSAFE) <=> (*b).as_unsafe_ptr(UNSAFE); }
			constexpr friend auto operator<=>(invariant const& a, box_t const& b) noexcept { return (*a).as_unsafe_ptr(UNSAFE) <=> b.as_unsafe_ptr(UNSAFE); }
			constexpr friend auto operator<=>(invariant const& a, T const* p) noexcept { return (*a).as_unsafe_ptr(UNSAFE) <=> p; }
			constexpr friend auto operator<=>(invariant const& a, std::nullptr_t) noexcept { return (*a).as_unsafe_ptr(UNSAFE) <=> nullptr; }
	
			constexpr invariant& operator=(box_t &&) noexcept = delete;
			constexpr invariant& operator=(box_t const&) = delete;

			constexpr invariant& operator=(invariant &&) noexcept = default;
			constexpr invariant& operator=(invariant const&) = default;

			
			constexpr invariant& operator=(std::nullptr_t)
			{ box_t::as_unique(UNSAFE).reset(); return *this; }

			constexpr invariant& operator=(T* const&& p)
			{ box_t::as_unique(UNSAFE).reset(p); return *this; }

			constexpr invariant(invariant&& m) noexcept
				: invariant( std::move((*std::move(m)).as_unique(UNSAFE)) ) {}
			constexpr invariant(invariant const& m) noexcept
				: box_t(UNSAFE, (*m).as_unique(UNSAFE)) {}

			constexpr ~invariant() = default;
		};
		static_assert(util::shares_layout<invariant, box_t>, "invariant (held_type) does not share layout with viewed type (Box<T>)");
	public:
		typedef invariant held_type;	// invariant ^: Held internal type.
		using type = box_t;		// Box<T>: API seen & operated on type

		constexpr static decltype(auto) convert_to_held(type ty) noexcept { return held_type{std::move(ty)}; }
		constexpr static decltype(auto) convert_to_held(type& ty) noexcept { return held_type::back_conv(ty); }
		constexpr static decltype(auto) convert_to_held(type&& ty) noexcept { return held_type::back_conv(std::move(ty)); }
		constexpr static decltype(auto) convert_to_held(type const& ty) noexcept { return held_type::back_conv(ty); }
		constexpr static decltype(auto) convert_to_held(type const&& ty) noexcept { return held_type::back_conv(std::move(ty)); }

		constexpr static type&		convert_from_held(held_type& ty) noexcept { return *ty; }
		constexpr static type const&&	convert_from_held(held_type const&& ty) noexcept { return std::move(*std::move(ty)); }
		constexpr static type&&		convert_from_held(held_type&& ty) noexcept { return std::move(*std::move(ty)); }
		constexpr static type const&	convert_from_held(held_type const& ty) noexcept { return *ty; }
		//TODO: See phone notes.

		constexpr static inline auto sentinel = nullptr;
	};
#ifdef DEBUG /* We don't want to instantiate `null_optimise<>` *at all* if we don't have to outside of actual real use in `Option<T>`. This check is for development only since Option<Box<T>> is a bit of an awkward developmental case.
		 Save compile time for release builds, the `static_assert` inside the null-opt partial spec for `..::invariant`'s layout will be checked if/when `Option<Box<T>>` is ever instantiated. This check is the same just assuring it's valid for *if* Option<Box<T>> is ever instantiated */
	static_assert(!requires(requires{ typename boxed::Box<int>; }) or // Skip check if `boxed.h` is not included.
		 requires(requires(null_optimise<boxed::Box<int>>::held_type const& held) { 
			{ static_cast<boxed::Box<int> const&>(held) };
		}), "Nullopt held_type for Box<T> either has a unneeded and unwanted generated vtable or has invalid alignment, check into that");
#endif

	template<typename T>
	struct null_optimise<ptr::Unique<T> > { constexpr static inline bool value = true;
		using held_type = T __restrict__*;
		using type = ptr::Unique<T>;
		constexpr static decltype(auto) convert_to_held(std::add_rvalue_reference_t<type> t) noexcept { return t.get(); }
		constexpr static decltype(auto) convert_from_held(std::add_rvalue_reference_t<held_type> t) noexcept { return ptr::Unique<T>::new_unchecked(t); }

		constexpr static inline auto sentinel = nullptr;
	};

	template<typename T>
	struct null_optimise<T&> { constexpr static inline bool value = true;
		using held_type = T*;
		using type = T&;

		constexpr static decltype(auto) convert_to_held(type t) noexcept { return std::addressof(t); }
		constexpr static auto& convert_from_held(held_type h) noexcept { return static_cast<type>(*h); }

		constexpr static inline auto sentinel = nullptr;
	};

	/* XXX: We can't optimise for this case with this model...
	template<typename T> requires(sizeof(T&&) <= sizeof(T*))
	struct null_optimise<T&&> { constexpr static inline bool value = true;	
		using held_type = T;
		using type = T&&;

		constexpr static decltype(auto) convert_to_held(type t) noexcept(std::is_nothrow_move_constructible_v<T>) { return std::move(t); }
		constexpr static auto&& convert_from_held(held_type h) noexcept { return std::forward<type>(h); }

		constexpr static inline auto sentinel = nullptr;
	};*/

	template<typename T>
	struct null_optimise<T&&> { constexpr static inline bool value = true;//sizeof(T&&) == sizeof(T*);
		using held_type = T*;
		using type = T&&;

		constexpr static decltype(auto) convert_to_held(type t) noexcept { return std::addressof(t); }
		constexpr static auto&& convert_from_held(held_type h) noexcept { return std::forward<type>(*h); }

		constexpr static inline auto sentinel = nullptr;
	};

	template<>
	struct null_optimise<void> { constexpr static inline bool value = true;
		using held_type = void;
		using type = void;
	};

	template<typename T>
	concept has_null_opt = std::is_void_v<T> 
		|| requires {
			typename null_optimise<T>::type;
			typename null_optimise<T>::held_type;

			requires(null_optimise<T>::value);

			/*requires(std::is_void_v<null_optimise<T>::held_type> || requires(std::add_rvalue_reference_t< null_optimise<T>::held_type> held) {
				{ null_optimise<T>::sentinel == std::as_const(held) } -> std::convertible_to<bool>;
			});*/
			/*requires(null_optimise<T>::type unheld
				, std::add_rvalue_reference_t<null_optimise<T>::held_type> held) {
				//, const std::remove_reference_t<null_optimise<T>::held_type>& held_const) {

				{ null_optimise<T>::sentinel } -> std::convertible_to<null_optmimise<T>::held_type>;

				{ null_optimise<T>::convert_to_held(unheld) } -> std::convertible_to<null_optimise<T>::held_type>;
				{ null_optimise<T>::convert_from_held(held) } -> std::convertible_to<null_optimise<T>::type>;

				{ null_optimise<T>::sentinel == std::as_const(held) } -> std::convertible_to<bool>;
				//{ null_optimise<T>::convert_from_held(held_const) } -> std::convertible_to<null_optimise<T>::type>;
			};
			requires(std::is_same_v<T, null_optimise<T>::type>);*/
		};

	namespace _null_opt_impl [[gnu::visibility("hidden")]] {
		template<typename, bool> struct types_for;
		template<typename T>
		struct types_for<T, true> {
			static_assert(has_null_opt<T>, "Type does not have nullptr optimisation enabled");
			using held = null_optimise<T>::held_type;
			using type = null_optimise<T>::type;
		};
		template<typename T>
		struct types_for<T, false> {
			using held=T;
			using type=T;
		};
		template<typename T>
		using held_type = typename types_for<T, has_null_opt<T>>::held;
		template<typename T>
		using used_type = typename types_for<T, has_null_opt<T>>::type;
	}
	
	namespace _details [[gnu::visibility("hidden")]] {

		template<typename T>
		struct not_void { using type = T; };
		template<>
		struct not_void<void> { struct type; };

		template<typename T> 
		using not_void_t = typename not_void<T>::type;

		template<typename T>
		struct rv_ref_or_void { using type = std::add_rvalue_reference_t<T>; };

		template<>
		struct rv_ref_or_void<void> { struct type; };

		template<typename T>
		using rv_ref_t = typename rv_ref_or_void<T>::type;

		template<typename T>
		struct cv_ref_or_void { using type = const std::remove_reference_t<T>&; };

		template<>
		struct cv_ref_or_void<void> { struct _type; using type = const _type&; };

		template<typename T>
		using cv_ref_t = typename cv_ref_or_void<T>::type;

		template<typename T> 
		struct const_ref_or_void { using type = const std::remove_const_t<T>&; };

		template<>
		struct const_ref_or_void<void> { struct type; };

		template<typename T>
		using const_ref_t = typename const_ref_or_void<T>::type;

		template<typename T, typename U, bool>
		struct if_then_else_type;

		template<typename If, typename U>
		struct if_then_else_type<If, U, true> { using type = If; using other = U; constexpr static inline bool result = true; };


		template<typename T, typename Else>
		struct if_then_else_type<T, Else, false> { using type = Else; using other = T; constexpr static inline bool result = false; };

		template<bool Predicate, typename Then, typename Else>
		using if_then_else_t = typename if_then_else_type<Then, Else, Predicate>::type;

		template<typename T> 
		class any_ref_or_void {
			template<typename _U = T, bool P = std::is_rvalue_reference_v<T>>
			struct is_rv_ref_type;

			template<typename U>
			struct is_rv_ref_type<U, true> { using type = U; static_assert(std::is_rvalue_reference_v<type>, "Invalid reference"); };
			template<typename U>
			struct is_rv_ref_type<U, false> { using type = U&; static_assert(!std::is_rvalue_reference_v<U>, "Invalid reference"); };
		public:
			using type = is_rv_ref_type<T, std::is_rvalue_reference_v<T>>::type;
		
		};

/* XXX: Having to account for all qualifiers is fucking dreck... we'd also need all variations of this with `__restrict__` too..
		template<typename T>
		struct any_ref_or_void<T&&> { using type = T&&; };

		template<typename T>
		struct any_ref_or_void<const T&&> { using type = const T&&; };

		template<typename T>
		struct any_ref_or_void<volatile T&&> { using type = volatile T&&; };

		template<typename T>
		struct any_ref_or_void<const volatile T&&> { using type = const volatile T&&; };
*/
		template<>
		struct any_ref_or_void<void> { struct type; };

		template<typename T>
		using any_ref_t = typename any_ref_or_void<T>::type;

		template<typename T> 
		struct mut_ref_or_void { using type = any_ref_or_void<std::remove_const_t<T>>::type; };

		template<>
		struct mut_ref_or_void<void> { struct type; };

		template<typename T>
		using mut_ref_t = typename mut_ref_or_void<T>::type;


		template<typename T> requires(requires(T v) { { v == nullptr } -> std::convertible_to<bool>; })
		[[gnu::always_inline]]
		constexpr decltype(auto) map_null(T&& value, auto&& with) noexcept(std::is_nothrow_invocable_v<decltype(with), decltype(std::forward<T>(value))>) { return (!bool(value == nullptr)) ? with(std::forward<T>(value)) : nullptr; }

	
		template<typename T, typename U> requires(requires(T v) { { v == nullptr } -> std::convertible_to<bool>; })
		[[gnu::always_inline]]
		constexpr decltype(auto) map_null(T&& value, auto&& with, std::add_rvalue_reference_t<U> otherwise) noexcept(std::is_nothrow_invocable_v<decltype(with), decltype(std::forward<T>(value))> && (!std::is_invocable_v<U> || std::is_nothrow_invocable_v<decltype(otherwise)>)) { 
			if (!bool(value == nullptr)) return with(std::forward<T>(value));
			else if constexpr(std::is_invocable_v<U>) return otherwise();
		 	else return std::forward<decltype(otherwise)>(otherwise);
		}

		template<typename T, typename... Types>
		constexpr inline bool is_same_any_v = (std::is_same_v<T, Types> || ...);

		constexpr inline auto identity = [] <typename T> (T v)  noexcept { return std::forward<T>(v); };
	}

	struct none_t {
		constexpr none_t() noexcept = default;
		constexpr ~none_t() noexcept = default;

		constexpr none_t(std::nullptr_t) noexcept : none_t() {}
		constexpr none_t(std::nullopt_t) noexcept : none_t() {}

		constexpr operator std::nullptr_t() const noexcept { return nullptr; }
	};
	constexpr inline none_t None;

	template<typename T>
	concept AsOption = (std::is_void_v<T> 
				|| requires{ requires(sizeof(T) == sizeof(T)); }
			)
			&& !_details::is_same_any_v<T, none_t, std::nullptr_t>;

	struct OptionUnwrapError /*TODO : public virtual error::Error, virtual std::exception*/
	{
		constexpr static inline const auto MESSAGE = "Unwrap operation on a None value";
		OptionUnwrapError() noexcept = default;
		inline virtual ~OptionUnwrapError() noexcept {}
		inline std::string_view message() const noexcept /*override*/ { return std::string_view{MESSAGE}; }
	};


	
	template<AsOption T>
	struct Option final {
		struct UnwrapError : public OptionUnwrapError
		{
			UnwrapError() noexcept =default;
			inline UnwrapError(const UnwrapError&) noexcept : UnwrapError() {}
			inline UnwrapError(UnwrapError&&) noexcept : UnwrapError() {}
			inline virtual ~UnwrapError() noexcept {}

			using OptionUnwrapError::message;
		};

		constexpr static inline bool is_null_optimised = has_null_opt<T>;

		constexpr friend void swap(Option& a, Option& b) noexcept {
			using std::swap;
			swap(a.inner_, b.inner_); //XXX: Swap the values if possible?
		}

		constexpr Option(std::optional<T>&& opt) noexcept requires(!std::is_void_v<T>)
			: inner_( bool(opt) ? std::move(*opt) : nullptr ) {}

		constexpr explicit Option(std::nullptr_t) noexcept : inner_(nullptr) {}
		constexpr Option(none_t) noexcept : Option(nullptr) {}
		constexpr ~Option() noexcept(std::is_nothrow_destructible_v<T>) {}

		constexpr Option(const Option& copy) noexcept(std::is_nothrow_copy_constructible_v<decltype(copy.inner_)>)
			: inner_(copy.inner_)
		{}
		constexpr Option(Option&& move) noexcept(std::is_nothrow_move_constructible_v<decltype(move.inner_)>)
			: inner_(std::move(move.inner_))
		{}

		constexpr explicit(std::convertible_to<std::add_rvalue_reference_t<T>, Option<T>>) Option(_details::rv_ref_t<T> value) noexcept(std::is_nothrow_invocable_v<convert_to_held_rv, std::add_rvalue_reference_t<T>>) requires(!std::is_void_v<T>)
			: inner_(convert_to_held_rv(std::forward<decltype(value)>(value)))
		{}

		//TODO: copy/move assign where appropriate
		constexpr Option& operator=(const Option& copy) noexcept(std::is_void_v<T> || std::is_nothrow_copy_constructible_v<decltype(copy.inner_)::held_type>)
		{
			if constexpr(std::is_void_v<T>) {
				inner_.has = copy.inner_.has;
			} else {
				if(this != std::addressof(copy)) {
					using inner_t = std::remove_reference_t<decltype(inner_)>;
					if(const auto* ptr = copy.try_get_value()) {
						using held_t = inner_t::held_type;
						inner_ = inner_t{held_t{*ptr}};
					} else  inner_ = inner_t{None};
				}
			} return *this;
		}
		constexpr Option& operator=(Option&& move) noexcept(std::is_void_v<T> || std::is_nothrow_move_constructible_v<decltype(move.inner_)::held_type>)
		{
			if constexpr(std::is_void_v<T>) std::exchange(inner_.has, move.inner_.has);
			else {
				if(this != std::addressof(move)) {
					using inner_t = std::remove_reference_t<decltype(inner_)>;
					if(auto* ptr = move.try_get_value()) {
						inner_ = inner_t{ std::move(*ptr) };
					} else	inner_ = inner_t{None};
				}
			} return *this;
		}

		constexpr Option& operator=(none_t) noexcept(std::is_nothrow_destructible_v<decltype(std::declval<Option<T>>().inner_)::held_type>)
		{
			using inner_t = std::remove_reference_t<decltype(inner_)>;
			inner_ = inner_t{ None };
			return *this;
		}

		//TODO: XXX: For this to work, we need *another* partial-spec "trait" that is (T == void ? void || alias std::add_rvalue_reference_t<T>)
		constexpr explicit(std::convertible_to<std::add_rvalue_reference_t<T>, Option<T>>) Option& operator=(_details::rv_ref_t<T> value) noexcept(std::is_void_v<T> || std::is_nothrow_constructible_v<decltype(std::declval<Option<T>>().inner_), std::add_rvalue_reference_t<std::invoke_result_t<decltype(convert_to_held_rv), std::add_rvalue_reference_t<T>>>>) requires(!std::is_void_v<T>)
		{
			using inner_t = std::remove_reference_t<decltype(inner_)>;
			if constexpr (!std::is_void_v<T>) 
				inner_ = inner_t { convert_to_held_rv(std::forward<decltype(value)>(value)) };
			else 	inner_ = inner_t { true };
			return *this;
		}


		//accessors
		constexpr bool has_value() const noexcept { return inner_.has_value(); }
		//XXX: We're returning held_type* here, which we don't want to expose...
		constexpr const auto* try_get_value() const noexcept { return _details::map_null(inner_.try_get_value(), [](const auto* ptr) { return std::addressof(convert_from_held_cv(*ptr)); }); }
		constexpr auto* try_get_value() noexcept { return _details::map_null(inner_.try_get_value(), [](auto* ptr) { return std::addressof(convert_from_held_ref(*ptr)); }); }

		//TODO: XXX: We need to `convert_from_held` here!
		constexpr const auto& value() const& { if (const auto* p = try_get_value()) return convert_from_held_cv(*p); throw _unwrap_error(); }
		constexpr auto& value() & { if(auto* p = try_get_value()) return convert_from_held_ref(*p); throw _unwrap_error(); }

		constexpr decltype(auto) value() && { if(auto* p = try_get_value()) return convert_from_held_rv(std::move(*p)); throw _unwrap_error(); }
		constexpr auto value() const&& { if(const auto* p = try_get_value()) return auto(convert_from_held_cv(*p)); throw _unwrap_error(); }

		constexpr operator bool() const noexcept { return has_value(); }
		constexpr const auto* operator->() const { return _details::map_null(try_get_value(), _details::identity, [] [[noreturn]] () -> std::nullptr_t { throw _unwrap_error(); }); }
		constexpr auto* operator->() { return _details::map_null(try_get_value(), _details::identity, [] [[noreturn]] () -> std::nullptr_t { throw _unwrap_error(); }); }

		constexpr const auto& operator*() const& { return value(); }
		constexpr auto& operator*() & { return value(); }

		constexpr decltype(auto) operator*() && { return value(); }
		constexpr auto operator*() const&& { return value(); }

	private:
		constexpr static decltype(auto) _unwrap_error() {
			if consteval {
				return UnwrapError{};
			} else {
				return std::runtime_error(UnwrapError{}.message());
			}
		}
		struct _impl_base {
			typedef _null_opt_impl::held_type<T> held_type;
			typedef _null_opt_impl::used_type<T> type;
			//struct none_type {};
			typedef none_t none_type;

			constexpr _impl_base() noexcept = default;
			constexpr ~_impl_base() noexcept = default;
		};
		template<typename _U = T, bool O = is_null_optimised> struct _impl;
		template<typename _U, bool O> struct _impl_type { using type = _impl<_U, O>; };

		template<bool O> struct _impl_type<void, O> {
			static_assert(O, "void should be `null_opt`");
		/// std::is_void_v<T>
			struct alignas(bool) type final : public _impl_base {
				using held_type = _impl_base::held_type;			
				using none_type = _impl_base::none_type;

				constexpr type(std::nullptr_t) noexcept : _impl_base(), has(false) {}
				constexpr explicit type(bool&& value) noexcept : _impl_base(), has(value) {}
				constexpr ~type() noexcept {}

				constexpr bool has_value() const noexcept { return has; }

				constexpr const bool* try_get_value() const noexcept { return &has; }
				constexpr bool* try_get_value() noexcept { return &has; }

				constexpr bool get_value() const noexcept { return has; }
				constexpr bool& get_value() noexcept { return has; }

				bool has;
			};
		};
		/// null_opt<T> == true
		template<typename _U> struct alignas(_U) _impl<_U, true> final : public _impl_base {
			using type_info = null_optimise<T>;
			using held_type = _impl_base::held_type;			
			using none_type = _impl_base::none_type;

			static_assert(has_null_opt<T>, "Invalid use of Option::_impl<true> where has_null_opt<T> == false");
			constexpr _impl(std::nullptr_t) noexcept : _impl_base(), value(type_info::sentinel) {}
			constexpr _impl(std::add_rvalue_reference_t<held_type> value) noexcept(std::is_nothrow_move_constructible_v<held_type>)
				: _impl_base()
				, value(std::move(value))
			{}

			/*//TODO: XXX: Make Option(T&&) ctor handle calling `convert_to_held` instead? Another template interface for this??
			constexpr explicit _impl(std::add_rvalue_reference_t<type_info::type> value) noexcept(std::is_nothrow_constructible_v<held_type, std::invoke_result_t<type_info::convert_to_held, decltype(value)>> && std::is_nothrow_invocable_v<type_info::convert_to_held, decltype(value)) requires(!std::is_same_v<type_info::type, held_type>)
				: _impl_base()
				, value(type_info::convert_to_held(std::forward<decltype(value)>(value)))
			{}*/

			constexpr _impl& operator=(std::add_rvalue_reference_t<held_type> value) noexcept(std::is_nothrow_move_constructible_v<held_type>)
			{
				std::exchange(value, std::forward<decltype(value)>(value));
				return *this;
			}
			constexpr _impl& operator=(none_t) noexcept(std::is_nothrow_destructible_v<held_type>)
			{
				if(has_value()) {
					using sen_t = decltype(type_info::sentinel);
					std::exchange(value, sen_t{type_info::sentinel});
				}
				return *this;
			}
			
			constexpr ~_impl() noexcept(std::is_nothrow_destructible_v<held_type>) {}

			constexpr bool has_value() const noexcept { return !bool(type_info::sentinel == value); }

			constexpr const held_type* try_get_value() const noexcept { return has_value() ? std::addressof(value) : nullptr; }
			constexpr held_type* try_get_value() noexcept { return has_value() ? std::addressof(value) : nullptr; }
	
			held_type value;
		};
		/// null_opt<T> == false
		template<typename _U> struct alignas(_U) _impl<_U, false> final : public _impl_base {
			using held_type = _impl_base::held_type;
			using none_type = _impl_base::none_type;

			constexpr _impl(std::nullptr_t) noexcept : _impl_base(), has(false), none(none_type{}) {}
			constexpr _impl(std::add_rvalue_reference_t<held_type> value) 
					noexcept(std::is_nothrow_move_constructible_v<held_type>)	
				: _impl_base()
				, has(true)
				, some(std::move(value))
			{}
			constexpr _impl& operator=(std::add_rvalue_reference_t<held_type> value) noexcept(std::is_nothrow_move_constructible_v<held_type> && std::is_nothrow_destructible_v<held_type>)
			{
				if(has) std::exchange(some, std::forward<decltype(value)>(value));
				else {
					none.~none_type();
					if constexpr(noexcept(std::is_nothrow_move_constructible_v<held_type>))
						some = std::move(value);
					else try { 
						some = std::move(value);
					} catch(...) {
						none = none_type{};
						throw;
					}
					
					has = true;
				}
				return *this;
			}
			constexpr _impl& operator=(none_t) noexcept(std::is_nothrow_destructible_v<held_type>)
			{
				if(has) {
					some.~held_type();
					none = none_type{};
					has = false;
				}
				return *this;
			}
			constexpr ~_impl() noexcept(std::is_nothrow_destructible_v<held_type>) {
				if(has) some.~held_type();
				else	none.~none_type();
			}

			constexpr bool has_value() const noexcept { return has; }

			constexpr const held_type* try_get_value() const noexcept {
				return has ? std::addressof(some) : nullptr;
			}
			constexpr held_type* try_get_value() noexcept {
				return has ? std::addressof(some) : nullptr;
			}

			bool has;
			union {
				held_type some;
				[[no_unique_address]] _impl_base::none_type none;
			};
		};

		_impl_type<T, is_null_optimised>::type inner_;

		// --- `held_type` convertions 

		constexpr static decltype(auto) convert_to_held_rv(_details::rv_ref_t<T> from) noexcept(!is_null_optimised || std::is_nothrow_invocable_v<null_optimise<T>::convert_to_held, _details::rv_ref_t<T>>)  { if constexpr(is_null_optimised) {
			return null_optimise<T>::convert_to_held(std::forward<T>(from));
		} else {
			return std::forward<decltype(from)>(from);
		} }
		constexpr static decltype(auto) convert_to_held_cv(_details::cv_ref_t<T> from) noexcept(!is_null_optimised || std::is_nothrow_invocable_v<null_optimise<T>::convert_to_held, _details::cv_ref_t<T>>) { if constexpr(is_null_optimised) {
			return null_optimise<T>::convert_to_held(from);
		} else {
			return std::forward<decltype(from)>(from);
		} }

		
		using this_held_type = typename std::remove_reference_t<
					decltype(std::declval<Option<T>>()
				.inner_)>
			::held_type;

		constexpr static decltype(auto) convert_from_held_cv(
			_details::cv_ref_t<this_held_type> held) noexcept(!is_null_optimised || std::is_nothrow_invocable_v<null_optimise<T>::convert_from_held, _details::cv_ref_t<this_held_type>>) { if constexpr(is_null_optimised) {
			return null_optimise<T>::convert_from_held(std::forward<decltype(held)>(held));
		} else {
			return std::forward<decltype(held)>(held);
		} }

		constexpr static decltype(auto) convert_from_held_ref(
			_details::mut_ref_t<this_held_type> held) noexcept(!is_null_optimised || std::is_nothrow_invocable_v<null_optimise<T>::convert_from_held, _details::mut_ref_t<this_held_type>>) { if constexpr(is_null_optimised) {
			return null_optimise<T>::convert_from_held(std::forward<decltype(held)>(held));
		} else {
			return std::forward<decltype(held)>(held);
		} }

		constexpr static decltype(auto) convert_from_held_rv(
			_details::rv_ref_t<this_held_type> held) noexcept(!is_null_optimised || std::is_nothrow_invocable_v<null_optimise<T>::convert_from_held, _details::rv_ref_t<this_held_type>>) { if constexpr(is_null_optimised) {
			return null_optimise<T>::convert_from_held(std::forward<decltype(held)>(held));
		} else {
			return std::forward<decltype(held)>(held);
		} }

		// ---
	};

	template<typename T, typename U> requires(requires(T&& p) { static_cast<U&&>(p); })
	constexpr Option<U> static_opt_cast(Option<T>&& p) noexcept 
	{
		if(bool(p))	return Option<U> { static_cast<U&&>(std::move(*p)) };
		return Option<U> { nullptr };
	}

	template<typename T, typename U> requires(requires(T&& p) { static_cast<U&&>(p); })
	constexpr Option<U> dynamic_opt_cast(Option<T>&& p)
	{
		if(T* ptr = p.try_get_value())
			if(U* pv = dynamic_cast<U*>(ptr))
				return Option<U> { std::move(*pv) };
		
		return Option<U> { nullptr };
	}
	

	static_assert(sizeof(Option<void*>) > sizeof(void*), "Option<T*>: Bad null_opt size");
	static_assert(alignof(Option<void*>) == alignof(void*), "Option<T*>: Bad null_opt align");
	static_assert(sizeof(Option<ptr::NonNull<int>>) == sizeof(int*), "Option<NonNull<T>>: Bad null_opt size");
	static_assert(alignof(Option<ptr::NonNull<int>>) == alignof(int*), "Option<NonNull<T>>: Bad null_opt align");
	static_assert(sizeof(Option<ptr::Unique<int>>) == sizeof(int*), "Option<Unique<T>>: Bad null_opt size");
	static_assert(alignof(Option<ptr::Unique<int>>) == alignof(int*), "Option<Unique<T>>: Bad null_opt align");
	static_assert(sizeof(Option<double&>) == sizeof(double&), "Option<T&>: Bad null_opt size");
	static_assert(alignof(Option<double&>) == alignof(double*), "Option<T&>: Bad null_opt align");
	static_assert(sizeof(Option<long long int&&>) == sizeof(char*), "Option<T&&>: Bad null_opt size");
	static_assert(alignof(Option<long long int&&>) == alignof(long*), "Option<T&&>: Bad null_opt align");
	static_assert(alignof(Option<int>) == alignof(int), "Option<int>: Bad non null_opt align");
	static_assert(sizeof(Option<void>) == sizeof(bool), "Option<void>: Bad size");
	static_assert(alignof(Option<void>) == alignof(bool), "Option<void>: Bad align");

}
using optional::Option;

}