sol2/sol/optional_implementation.hpp

// The MIT License (MIT) 

// Copyright (c) 2013-2016 Rapptz, ThePhD and contributors

// Permission is hereby granted, free of charge, to any person obtaining a copy of
// this software and associated documentation files (the "Software"), to deal in
// the Software without restriction, including without limitation the rights to
// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
// the Software, and to permit persons to whom the Software is furnished to do so,
// subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
// FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
// COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
// IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

// Copyright (C) 2011 - 2012 Andrzej Krzemienski.
// Use, modification, and distribution is subject to the Boost Software
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
//
// The idea and interface is based on Boost.Optional library
// authored by Fernando Luis Cacciola Carballal

# ifndef SOL_OPTIONAL_IMPLEMENTATION_HPP
# define SOL_OPTIONAL_IMPLEMENTATION_HPP

# include <utility>
# include <type_traits>
# include <initializer_list>
# include <cassert>
# include <functional>
# include <string>
# include <stdexcept>
#ifdef SOL_NO_EXCEPTIONS
#include <cstdlib>
#endif // Exceptions

# define TR2_OPTIONAL_REQUIRES(...) typename ::std::enable_if<__VA_ARGS__::value, bool>::type = false

# if defined __GNUC__ // NOTE: GNUC is also defined for Clang
#   if (__GNUC__ >= 5)
#     define TR2_OPTIONAL_GCC_5_0_AND_HIGHER___
#     define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
#   elif (__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)
#     define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
#   elif (__GNUC__ > 4)
#     define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
#   endif
#
#   if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)
#     define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
#   elif (__GNUC__ > 4)
#     define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___
#   endif
#
#   if (__GNUC__ == 4) && (__GNUC_MINOR__ == 8) && (__GNUC_PATCHLEVEL__ >= 1)
#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#   elif (__GNUC__ == 4) && (__GNUC_MINOR__ >= 9)
#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#   elif (__GNUC__ > 4)
#     define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#   endif
# endif
#
# if defined __clang_major__
#   if (__clang_major__ == 3 && __clang_minor__ >= 5)
#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#   elif (__clang_major__ > 3)
#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#   endif
#   if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
#   elif (__clang_major__ == 3 && __clang_minor__ == 4 && __clang_patchlevel__ >= 2)
#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
#   endif
# endif
#
# if defined _MSC_VER
#   if (_MSC_VER >= 1900)
#     define TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
#   endif
# endif

# if defined __clang__
#   if (__clang_major__ > 2) || (__clang_major__ == 2) && (__clang_minor__ >= 9)
#     define OPTIONAL_HAS_THIS_RVALUE_REFS 1
#   else
#     define OPTIONAL_HAS_THIS_RVALUE_REFS 0
#   endif
# elif defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#   define OPTIONAL_HAS_THIS_RVALUE_REFS 1
# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
#   define OPTIONAL_HAS_THIS_RVALUE_REFS 1
# else
#   define OPTIONAL_HAS_THIS_RVALUE_REFS 0
# endif


# if defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___
#   define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 1
#   define OPTIONAL_CONSTEXPR_INIT_LIST constexpr
# else
#   define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 0
#   define OPTIONAL_CONSTEXPR_INIT_LIST
# endif

# if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ && (defined __cplusplus) && (__cplusplus != 201103L)
#   define OPTIONAL_HAS_MOVE_ACCESSORS 1
# else
#   define OPTIONAL_HAS_MOVE_ACCESSORS 0
# endif

# // In C++11 constexpr implies const, so we need to make non-const members also non-constexpr
# if (defined __cplusplus) && (__cplusplus == 201103L)
#   define OPTIONAL_MUTABLE_CONSTEXPR
# else
#   define OPTIONAL_MUTABLE_CONSTEXPR constexpr
# endif

namespace sol {

	// BEGIN workaround for missing is_trivially_destructible
# if defined TR2_OPTIONAL_GCC_4_8_AND_HIGHER___
	// leave it: it is already there
# elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
	// leave it: it is already there
# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
	// leave it: it is already there
# elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
	// leave it: the user doesn't want it
# else
	template <typename T>
	using is_trivially_destructible = ::std::has_trivial_destructor<T>;
# endif
	// END workaround for missing is_trivially_destructible

# if (defined TR2_OPTIONAL_GCC_4_7_AND_HIGHER___)
	// leave it; our metafunctions are already defined.
# elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
	// leave it; our metafunctions are already defined.
# elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___
	// leave it: it is already there
# elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS
	// leave it: the user doesn't want it
# else


// workaround for missing traits in GCC and CLANG
	template <class T>
	struct is_nothrow_move_constructible
	{
		constexpr static bool value = ::std::is_nothrow_constructible<T, T&&>::value;
	};


	template <class T, class U>
	struct is_assignable
	{
		template <class X, class Y>
		constexpr static bool has_assign(...) { return false; }

		template <class X, class Y, size_t S = sizeof((::std::declval<X>() = ::std::declval<Y>(), true)) >
		// the comma operator is necessary for the cases where operator= returns void
		constexpr static bool has_assign(bool) { return true; }

		constexpr static bool value = has_assign<T, U>(true);
	};


	template <class T>
	struct is_nothrow_move_assignable
	{
		template <class X, bool has_any_move_assign>
		struct has_nothrow_move_assign {
			constexpr static bool value = false;
		};

		template <class X>
		struct has_nothrow_move_assign<X, true> {
			constexpr static bool value = noexcept(::std::declval<X&>() = ::std::declval<X&&>());
		};

		constexpr static bool value = has_nothrow_move_assign<T, is_assignable<T&, T&&>::value>::value;
	};
	// end workaround


# endif



// 20.5.4, optional for object types
	template <class T> class optional;

	// 20.5.5, optional for lvalue reference types
	template <class T> class optional<T&>;


	// workaround: std utility functions aren't constexpr yet
	template <class T> inline constexpr T&& constexpr_forward(typename ::std::remove_reference<T>::type& t) noexcept
	{
		return static_cast<T&&>(t);
	}

	template <class T> inline constexpr T&& constexpr_forward(typename ::std::remove_reference<T>::type&& t) noexcept
	{
		static_assert(!::std::is_lvalue_reference<T>::value, "!!");
		return static_cast<T&&>(t);
	}

	template <class T> inline constexpr typename ::std::remove_reference<T>::type&& constexpr_move(T&& t) noexcept
	{
		return static_cast<typename ::std::remove_reference<T>::type&&>(t);
	}


#if defined NDEBUG
# define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR)
#else
# define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([]{assert(!#CHECK);}(), (EXPR)))
#endif


	namespace detail_
	{

		// static_addressof: a constexpr version of addressof
		template <typename T>
		struct has_overloaded_addressof
		{
			template <class X>
			constexpr static bool has_overload(...) { return false; }

			template <class X, size_t S = sizeof(::std::declval<X&>().operator&()) >
			constexpr static bool has_overload(bool) { return true; }

			constexpr static bool value = has_overload<T>(true);
		};

		template <typename T, TR2_OPTIONAL_REQUIRES(!has_overloaded_addressof<T>)>
		constexpr T* static_addressof(T& ref)
		{
			return &ref;
		}

		template <typename T, TR2_OPTIONAL_REQUIRES(has_overloaded_addressof<T>)>
		T* static_addressof(T& ref)
		{
			return ::std::addressof(ref);
		}


		// the call to convert<A>(b) has return type A and converts b to type A iff b decltype(b) is implicitly convertible to A  
		template <class U>
		constexpr U convert(U v) { return v; }

	} // namespace detail_

	constexpr struct trivial_init_t {} trivial_init{};

	// 20.5.7, Disengaged state indicator
	struct nullopt_t
	{
		struct init {};
		constexpr explicit nullopt_t(init) {}
	};
	constexpr nullopt_t nullopt{ nullopt_t::init() };


	// 20.5.8, class bad_optional_access
	class bad_optional_access : public ::std::logic_error {
	public:
		explicit bad_optional_access(const ::std::string& what_arg) : ::std::logic_error{ what_arg } {}
		explicit bad_optional_access(const char* what_arg) : ::std::logic_error{ what_arg } {}
	};


	template <class T>
	struct optional_base
	{
		bool init_;
		char storage_[sizeof(T)];

		constexpr optional_base() noexcept : init_(false), storage_() {};

		explicit optional_base(const T& v) : init_(true), storage_() {
			new (&storage())T(v);
		}

		explicit optional_base(T&& v) : init_(true), storage_() {
			new (&storage())T(constexpr_move(v));
		}

		template <class... Args> explicit optional_base(in_place_t, Args&&... args)
			: init_(true), storage_() {
			new (&storage())T(constexpr_forward<Args>(args)...);
		}

		template <class U, class... Args, TR2_OPTIONAL_REQUIRES(::std::is_constructible<T, ::std::initializer_list<U>>)>
		explicit optional_base(in_place_t, ::std::initializer_list<U> il, Args&&... args)
			: init_(true), storage_() {
			new (&storage())T(il, constexpr_forward<Args>(args)...);
		}
#if defined __GNUC__ 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstrict-aliasing"
#endif
		T& storage() {
			return *reinterpret_cast<T*>(&storage_[0]);
		}

		constexpr const T& storage() const {
			return *reinterpret_cast<T const*>(&storage_[0]);
		}
#if defined __GNUC__
#pragma GCC diagnostic pop
#endif

		~optional_base() { if (init_) { storage().T::~T(); } }
	};

#if defined __GNUC__ && !defined TR2_OPTIONAL_GCC_5_0_AND_HIGHER___
	// Sorry, GCC 4.x; you're just a piece of shit
	template <typename T>
	using constexpr_optional_base = optional_base<T>;
#else
	template <class T>
	struct constexpr_optional_base
	{
		bool init_;
		char storage_[sizeof(T)];
		constexpr constexpr_optional_base() noexcept : init_(false), storage_() {}

		explicit constexpr constexpr_optional_base(const T& v) : init_(true), storage_() {
			new (&storage())T(v);
		}

		explicit constexpr constexpr_optional_base(T&& v) : init_(true), storage_() {
			new (&storage())T(constexpr_move(v));
		}

		template <class... Args> explicit constexpr constexpr_optional_base(in_place_t, Args&&... args)
			: init_(true), storage_() {
			new (&storage())T(constexpr_forward<Args>(args)...);
		}

		template <class U, class... Args, TR2_OPTIONAL_REQUIRES(::std::is_constructible<T, ::std::initializer_list<U>>)>
		OPTIONAL_CONSTEXPR_INIT_LIST explicit constexpr_optional_base(in_place_t, ::std::initializer_list<U> il, Args&&... args)
			: init_(true), storage_() {
			new (&storage())T(il, constexpr_forward<Args>(args)...);
		}

#if defined __GNUC__ 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstrict-aliasing"
#endif
		T& storage() {
			return (*reinterpret_cast<T*>(&storage_[0]));
		}

		constexpr const T& storage() const {
			return (*reinterpret_cast<T const*>(&storage_[0]));
		}
#if defined __GNUC__
#pragma GCC diagnostic pop
#endif

		~constexpr_optional_base() = default;
	};
#endif

	template <class T>
	using OptionalBase = typename ::std::conditional<
		::std::is_trivially_destructible<T>::value,
		constexpr_optional_base<typename ::std::remove_const<T>::type>,
		optional_base<typename ::std::remove_const<T>::type>
	>::type;



	template <class T>
	class optional : private OptionalBase<T>
	{
		static_assert(!::std::is_same<typename ::std::decay<T>::type, nullopt_t>::value, "bad T");
		static_assert(!::std::is_same<typename ::std::decay<T>::type, in_place_t>::value, "bad T");


		constexpr bool initialized() const noexcept { return OptionalBase<T>::init_; }
		typename ::std::remove_const<T>::type* dataptr() { return ::std::addressof(OptionalBase<T>::storage()); }
		constexpr const T* dataptr() const { return detail_::static_addressof(OptionalBase<T>::storage()); }

# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1
		constexpr const T& contained_val() const& { return OptionalBase<T>::storage(); }
#   if OPTIONAL_HAS_MOVE_ACCESSORS == 1
		OPTIONAL_MUTABLE_CONSTEXPR T&& contained_val() && { return ::std::move(OptionalBase<T>::storage()); }
		OPTIONAL_MUTABLE_CONSTEXPR T& contained_val() & { return OptionalBase<T>::storage(); }
#   else
		T& contained_val() & { return OptionalBase<T>::storage(); }
		T&& contained_val() && { return ::std::move(OptionalBase<T>::storage()); }
#   endif
# else
		constexpr const T& contained_val() const { return OptionalBase<T>::storage(); }
		T& contained_val() { return OptionalBase<T>::storage(); }
# endif

		void clear() noexcept {
			if (initialized()) dataptr()->T::~T();
			OptionalBase<T>::init_ = false;
		}

		template <class... Args>
		void initialize(Args&&... args) noexcept(noexcept(T(::std::forward<Args>(args)...)))
		{
			assert(!OptionalBase<T>::init_);
			::new (static_cast<void*>(dataptr())) T(::std::forward<Args>(args)...);
			OptionalBase<T>::init_ = true;
		}

		template <class U, class... Args>
		void initialize(::std::initializer_list<U> il, Args&&... args) noexcept(noexcept(T(il, ::std::forward<Args>(args)...)))
		{
			assert(!OptionalBase<T>::init_);
			::new (static_cast<void*>(dataptr())) T(il, ::std::forward<Args>(args)...);
			OptionalBase<T>::init_ = true;
		}

	public:
		typedef T value_type;

		// 20.5.5.1, constructors
		constexpr optional() noexcept : OptionalBase<T>() {};
		constexpr optional(nullopt_t) noexcept : OptionalBase<T>() {};

		optional(const optional& rhs)
			: OptionalBase<T>()
		{
			if (rhs.initialized()) {
				::new (static_cast<void*>(dataptr())) T(*rhs);
				OptionalBase<T>::init_ = true;
			}
		}

		optional(const optional<T&>& rhs) : optional()
		{
			if (rhs) {
				::new (static_cast<void*>(dataptr())) T(*rhs);
				OptionalBase<T>::init_ = true;
			}
		}


		optional(optional&& rhs) noexcept(::std::is_nothrow_move_constructible<T>::value)
			: OptionalBase<T>()
		{
			if (rhs.initialized()) {
				::new (static_cast<void*>(dataptr())) T(::std::move(*rhs));
				OptionalBase<T>::init_ = true;
			}
		}

		constexpr optional(const T& v) : OptionalBase<T>(v) {}

		constexpr optional(T&& v) : OptionalBase<T>(constexpr_move(v)) {}

		template <class... Args>
		explicit constexpr optional(in_place_t, Args&&... args)
			: OptionalBase<T>(in_place, constexpr_forward<Args>(args)...) {}

		template <class U, class... Args, TR2_OPTIONAL_REQUIRES(::std::is_constructible<T, ::std::initializer_list<U>>)>
		OPTIONAL_CONSTEXPR_INIT_LIST explicit optional(in_place_t, ::std::initializer_list<U> il, Args&&... args)
			: OptionalBase<T>(in_place, il, constexpr_forward<Args>(args)...) {}

		// 20.5.4.2, Destructor
		~optional() = default;

		// 20.5.4.3, assignment
		optional& operator=(nullopt_t) noexcept
		{
			clear();
			return *this;
		}

		optional& operator=(const optional& rhs)
		{
			if (initialized() == true && rhs.initialized() == false) clear();
			else if (initialized() == false && rhs.initialized() == true)  initialize(*rhs);
			else if (initialized() == true && rhs.initialized() == true)  contained_val() = *rhs;
			return *this;
		}

		optional& operator=(optional&& rhs)
			noexcept(::std::is_nothrow_move_assignable<T>::value && ::std::is_nothrow_move_constructible<T>::value)
		{
			if (initialized() == true && rhs.initialized() == false) clear();
			else if (initialized() == false && rhs.initialized() == true)  initialize(::std::move(*rhs));
			else if (initialized() == true && rhs.initialized() == true)  contained_val() = ::std::move(*rhs);
			return *this;
		}

		template <class U>
		auto operator=(U&& v)
			-> typename ::std::enable_if
			<
			::std::is_same<typename ::std::decay<U>::type, T>::value,
			optional&
			>::type
		{
			if (initialized()) { contained_val() = ::std::forward<U>(v); }
			else { initialize(::std::forward<U>(v)); }
			return *this;
		}


		template <class... Args>
		void emplace(Args&&... args)
		{
			clear();
			initialize(::std::forward<Args>(args)...);
		}

		template <class U, class... Args>
		void emplace(::std::initializer_list<U> il, Args&&... args)
		{
			clear();
			initialize<U, Args...>(il, ::std::forward<Args>(args)...);
		}

		// 20.5.4.4, Swap
		void swap(optional<T>& rhs) noexcept(::std::is_nothrow_move_constructible<T>::value && noexcept(swap(::std::declval<T&>(), ::std::declval<T&>())))
		{
			if (initialized() == true && rhs.initialized() == false) { rhs.initialize(::std::move(**this)); clear(); }
			else if (initialized() == false && rhs.initialized() == true) { initialize(::std::move(*rhs)); rhs.clear(); }
			else if (initialized() == true && rhs.initialized() == true) { using ::std::swap; swap(**this, *rhs); }
		}

		// 20.5.4.5, Observers

		explicit constexpr operator bool() const noexcept { return initialized(); }

		constexpr T const* operator ->() const {
			return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), dataptr());
		}

# if OPTIONAL_HAS_MOVE_ACCESSORS == 1

		OPTIONAL_MUTABLE_CONSTEXPR T* operator ->() {
			assert(initialized());
			return dataptr();
		}

		constexpr T const& operator *() const& {
			return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
		}

		OPTIONAL_MUTABLE_CONSTEXPR T& operator *() & {
			assert(initialized());
			return contained_val();
		}

		OPTIONAL_MUTABLE_CONSTEXPR T&& operator *() && {
			assert(initialized());
			return constexpr_move(contained_val());
		}

		constexpr T const& value() const& {
			return initialized() ?
				contained_val()
#ifdef SOL_NO_EXCEPTIONS
				// we can't abort here
				// because there's no constexpr abort
				: *(T*)nullptr;
#else
				: (throw bad_optional_access("bad optional access"), contained_val());
#endif
		}

		OPTIONAL_MUTABLE_CONSTEXPR T& value() & {
			return initialized() ?
				contained_val()
#ifdef SOL_NO_EXCEPTIONS
				: *(T*)nullptr;
#else
				: (throw bad_optional_access("bad optional access"), contained_val());
#endif
		}

		OPTIONAL_MUTABLE_CONSTEXPR T&& value() && {
			return initialized() ?
				contained_val()
#ifdef SOL_NO_EXCEPTIONS
				// we can't abort here
				// because there's no constexpr abort
				: std::move(*(T*)nullptr);
#else
				: (throw bad_optional_access("bad optional access"), contained_val());
#endif
		}

# else

		T* operator ->() {
			assert(initialized());
			return dataptr();
		}

		constexpr T const& operator *() const {
			return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val());
		}

		T& operator *() {
			assert(initialized());
			return contained_val();
		}

		constexpr T const& value() const {
			return initialized() ? 
				contained_val() 
#ifdef SOL_NO_EXCEPTIONS
				// we can't abort here
				// because there's no constexpr abort
				: *(T*)nullptr;
#else
				: (throw bad_optional_access("bad optional access"), contained_val());
#endif
		}

		T& value() {
			return initialized() ?
				contained_val()
#ifdef SOL_NO_EXCEPTIONS
				// we can abort here
				// but the others are constexpr, so we can't...
				: (std::abort(), *(T*)nullptr)
#else
				: (throw bad_optional_access("bad optional access"), contained_val());
#endif
		}

# endif

# if OPTIONAL_HAS_THIS_RVALUE_REFS == 1

		template <class V>
		constexpr T value_or(V&& v) const&
		{
			return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
		}

#   if OPTIONAL_HAS_MOVE_ACCESSORS == 1

		template <class V>
		OPTIONAL_MUTABLE_CONSTEXPR T value_or(V&& v) &&
		{
			return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
		}

#   else

		template <class V>
		T value_or(V&& v) &&
		{
			return *this ? constexpr_move(const_cast<optional<T>&>(*this).contained_val()) : detail_::convert<T>(constexpr_forward<V>(v));
		}

#   endif

# else

		template <class V>
		constexpr T value_or(V&& v) const
		{
			return *this ? **this : detail_::convert<T>(constexpr_forward<V>(v));
		}

# endif

	};


	template <class T>
	class optional<T&>
	{
		static_assert(!::std::is_same<T, nullopt_t>::value, "bad T");
		static_assert(!::std::is_same<T, in_place_t>::value, "bad T");
		T* ref;

	public:

		// 20.5.5.1, construction/destruction
		constexpr optional() noexcept : ref(nullptr) {}

		constexpr optional(nullopt_t) noexcept : ref(nullptr) {}

		constexpr optional(T& v) noexcept : ref(detail_::static_addressof(v)) {}

		optional(T&&) = delete;

		constexpr optional(const optional& rhs) noexcept : ref(rhs.ref) {}

		explicit constexpr optional(in_place_t, T& v) noexcept : ref(detail_::static_addressof(v)) {}

		explicit optional(in_place_t, T&&) = delete;

		~optional() = default;

		// 20.5.5.2, mutation
		optional& operator=(nullopt_t) noexcept {
			ref = nullptr;
			return *this;
		}

		// optional& operator=(const optional& rhs) noexcept {
		  // ref = rhs.ref;
		  // return *this;
		// }

		// optional& operator=(optional&& rhs) noexcept {
		  // ref = rhs.ref;
		  // return *this;
		// }

		template <typename U>
		auto operator=(U&& rhs) noexcept
			-> typename ::std::enable_if
			<
			::std::is_same<typename ::std::decay<U>::type, optional<T&>>::value,
			optional&
			>::type
		{
			ref = rhs.ref;
			return *this;
		}

		template <typename U>
		auto operator=(U&& rhs) noexcept
			-> typename ::std::enable_if
			<
			!::std::is_same<typename ::std::decay<U>::type, optional<T&>>::value,
			optional&
			>::type
			= delete;

		void emplace(T& v) noexcept {
			ref = detail_::static_addressof(v);
		}

		void emplace(T&&) = delete;


		void swap(optional<T&>& rhs) noexcept
		{
			::std::swap(ref, rhs.ref);
		}

		// 20.5.5.3, observers
		constexpr T* operator->() const {
			return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, ref);
		}

		constexpr T& operator*() const {
			return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, *ref);
		}

		constexpr T& value() const {
			return ref ?
				*ref
#ifdef SOL_NO_EXCEPTIONS
				// we can't abort here
				// because there's no constexpr abort
				: *(T*)nullptr;
#else
				: throw bad_optional_access("bad optional access");
#endif
		}

		explicit constexpr operator bool() const noexcept {
			return ref != nullptr;
		}

		template <typename V>
		constexpr T& value_or(V&& v) const
		{
			return *this ? **this : detail_::convert<T&>(constexpr_forward<V>(v));
		}
	};


	template <class T>
	class optional<T&&>
	{
		static_assert(sizeof(T) == 0, "optional rvalue references disallowed");
	};


	// 20.5.8, Relational operators
	template <class T> constexpr bool operator==(const optional<T>& x, const optional<T>& y)
	{
		return bool(x) != bool(y) ? false : bool(x) == false ? true : *x == *y;
	}

	template <class T> constexpr bool operator!=(const optional<T>& x, const optional<T>& y)
	{
		return !(x == y);
	}

	template <class T> constexpr bool operator<(const optional<T>& x, const optional<T>& y)
	{
		return (!y) ? false : (!x) ? true : *x < *y;
	}

	template <class T> constexpr bool operator>(const optional<T>& x, const optional<T>& y)
	{
		return (y < x);
	}

	template <class T> constexpr bool operator<=(const optional<T>& x, const optional<T>& y)
	{
		return !(y < x);
	}

	template <class T> constexpr bool operator>=(const optional<T>& x, const optional<T>& y)
	{
		return !(x < y);
	}


	// 20.5.9, Comparison with nullopt
	template <class T> constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept
	{
		return (!x);
	}

	template <class T> constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept
	{
		return (!x);
	}

	template <class T> constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept
	{
		return bool(x);
	}

	template <class T> constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept
	{
		return bool(x);
	}

	template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept
	{
		return false;
	}

	template <class T> constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept
	{
		return bool(x);
	}

	template <class T> constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept
	{
		return (!x);
	}

	template <class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept
	{
		return true;
	}

	template <class T> constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept
	{
		return bool(x);
	}

	template <class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept
	{
		return false;
	}

	template <class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept
	{
		return true;
	}

	template <class T> constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept
	{
		return (!x);
	}



	// 20.5.10, Comparison with T
	template <class T> constexpr bool operator==(const optional<T>& x, const T& v)
	{
		return bool(x) ? *x == v : false;
	}

	template <class T> constexpr bool operator==(const T& v, const optional<T>& x)
	{
		return bool(x) ? v == *x : false;
	}

	template <class T> constexpr bool operator!=(const optional<T>& x, const T& v)
	{
		return bool(x) ? *x != v : true;
	}

	template <class T> constexpr bool operator!=(const T& v, const optional<T>& x)
	{
		return bool(x) ? v != *x : true;
	}

	template <class T> constexpr bool operator<(const optional<T>& x, const T& v)
	{
		return bool(x) ? *x < v : true;
	}

	template <class T> constexpr bool operator>(const T& v, const optional<T>& x)
	{
		return bool(x) ? v > *x : true;
	}

	template <class T> constexpr bool operator>(const optional<T>& x, const T& v)
	{
		return bool(x) ? *x > v : false;
	}

	template <class T> constexpr bool operator<(const T& v, const optional<T>& x)
	{
		return bool(x) ? v < *x : false;
	}

	template <class T> constexpr bool operator>=(const optional<T>& x, const T& v)
	{
		return bool(x) ? *x >= v : false;
	}

	template <class T> constexpr bool operator<=(const T& v, const optional<T>& x)
	{
		return bool(x) ? v <= *x : false;
	}

	template <class T> constexpr bool operator<=(const optional<T>& x, const T& v)
	{
		return bool(x) ? *x <= v : true;
	}

	template <class T> constexpr bool operator>=(const T& v, const optional<T>& x)
	{
		return bool(x) ? v >= *x : true;
	}


	// Comparison of optional<T&> with T
	template <class T> constexpr bool operator==(const optional<T&>& x, const T& v)
	{
		return bool(x) ? *x == v : false;
	}

	template <class T> constexpr bool operator==(const T& v, const optional<T&>& x)
	{
		return bool(x) ? v == *x : false;
	}

	template <class T> constexpr bool operator!=(const optional<T&>& x, const T& v)
	{
		return bool(x) ? *x != v : true;
	}

	template <class T> constexpr bool operator!=(const T& v, const optional<T&>& x)
	{
		return bool(x) ? v != *x : true;
	}

	template <class T> constexpr bool operator<(const optional<T&>& x, const T& v)
	{
		return bool(x) ? *x < v : true;
	}

	template <class T> constexpr bool operator>(const T& v, const optional<T&>& x)
	{
		return bool(x) ? v > *x : true;
	}

	template <class T> constexpr bool operator>(const optional<T&>& x, const T& v)
	{
		return bool(x) ? *x > v : false;
	}

	template <class T> constexpr bool operator<(const T& v, const optional<T&>& x)
	{
		return bool(x) ? v < *x : false;
	}

	template <class T> constexpr bool operator>=(const optional<T&>& x, const T& v)
	{
		return bool(x) ? *x >= v : false;
	}

	template <class T> constexpr bool operator<=(const T& v, const optional<T&>& x)
	{
		return bool(x) ? v <= *x : false;
	}

	template <class T> constexpr bool operator<=(const optional<T&>& x, const T& v)
	{
		return bool(x) ? *x <= v : true;
	}

	template <class T> constexpr bool operator>=(const T& v, const optional<T&>& x)
	{
		return bool(x) ? v >= *x : true;
	}

	// Comparison of optional<T const&> with T
	template <class T> constexpr bool operator==(const optional<const T&>& x, const T& v)
	{
		return bool(x) ? *x == v : false;
	}

	template <class T> constexpr bool operator==(const T& v, const optional<const T&>& x)
	{
		return bool(x) ? v == *x : false;
	}

	template <class T> constexpr bool operator!=(const optional<const T&>& x, const T& v)
	{
		return bool(x) ? *x != v : true;
	}

	template <class T> constexpr bool operator!=(const T& v, const optional<const T&>& x)
	{
		return bool(x) ? v != *x : true;
	}

	template <class T> constexpr bool operator<(const optional<const T&>& x, const T& v)
	{
		return bool(x) ? *x < v : true;
	}

	template <class T> constexpr bool operator>(const T& v, const optional<const T&>& x)
	{
		return bool(x) ? v > *x : true;
	}

	template <class T> constexpr bool operator>(const optional<const T&>& x, const T& v)
	{
		return bool(x) ? *x > v : false;
	}

	template <class T> constexpr bool operator<(const T& v, const optional<const T&>& x)
	{
		return bool(x) ? v < *x : false;
	}

	template <class T> constexpr bool operator>=(const optional<const T&>& x, const T& v)
	{
		return bool(x) ? *x >= v : false;
	}

	template <class T> constexpr bool operator<=(const T& v, const optional<const T&>& x)
	{
		return bool(x) ? v <= *x : false;
	}

	template <class T> constexpr bool operator<=(const optional<const T&>& x, const T& v)
	{
		return bool(x) ? *x <= v : true;
	}

	template <class T> constexpr bool operator>=(const T& v, const optional<const T&>& x)
	{
		return bool(x) ? v >= *x : true;
	}


	// 20.5.12, Specialized algorithms
	template <class T>
	void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y))) {
		x.swap(y);
	}


	template <class T>
	constexpr optional<typename ::std::decay<T>::type> make_optional(T&& v) {
		return optional<typename ::std::decay<T>::type>(constexpr_forward<T>(v));
	}

	template <class X>
	constexpr optional<X&> make_optional(::std::reference_wrapper<X> v) {
		return optional<X&>(v.get());
	}


} // namespace 

namespace std
{
	template <typename T>
	struct hash<sol::optional<T>> {
		typedef typename hash<T>::result_type result_type;
		typedef sol::optional<T> argument_type;

		constexpr result_type operator()(argument_type const& arg) const {
			return arg ? ::std::hash<T>{}(*arg) : result_type{};
		}
	};

	template <typename T>
	struct hash<sol::optional<T&>> {
		typedef typename hash<T>::result_type result_type;
		typedef sol::optional<T&> argument_type;

		constexpr result_type operator()(argument_type const& arg) const {
			return arg ? ::std::hash<T>{}(*arg) : result_type{};
		}
	};
}

# undef TR2_OPTIONAL_REQUIRES
# undef TR2_OPTIONAL_ASSERTED_EXPRESSION

# endif // SOL_OPTIONAL_IMPLEMENTATION_HPP