optional.hpp

// optional - An implementation of std::optional with extensions
// Written in 2017 by Sy Brand (tartanllama@gmail.com, @TartanLlama)
//
// Documentation available at https://tl.tartanllama.xyz/
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to the
// public domain worldwide. This software is distributed without any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software. If not, see
// <http://creativecommons.org/publicdomain/zero/1.0/>.
 
#pragma once
 
#if __cplusplus > 201402L
#include <optional>
namespace nonstd {
using std::optional;
}
#else
#ifndef TL_OPTIONAL_HPP
#define TL_OPTIONAL_HPP
 
#define TL_OPTIONAL_VERSION_MAJOR 1
#define TL_OPTIONAL_VERSION_MINOR 1
#define TL_OPTIONAL_VERSION_PATCH 0
 
#include <exception>
#include <functional>
#include <type_traits>
#include <utility>
 
#if(defined(_MSC_VER) && _MSC_VER == 1900)
#define TL_OPTIONAL_MSVC2015
#endif
 
#if(defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 && !defined(__clang__))
#define TL_OPTIONAL_GCC49
#endif
 
#if(defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 4 && !defined(__clang__))
#define TL_OPTIONAL_GCC54
#endif
 
#if(defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 5 && !defined(__clang__))
#define TL_OPTIONAL_GCC55
#endif
 
#if(defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 && !defined(__clang__))
// GCC < 5 doesn't support overloading on const&& for member functions
#define TL_OPTIONAL_NO_CONSTRR
 
// GCC < 5 doesn't support some standard C++11 type traits
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) std::has_trivial_copy_constructor<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) std::has_trivial_copy_assign<T>::value
 
// This one will be different for GCC 5.7 if it's ever supported
#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>::value
 
// GCC 5 < v < 8 has a bug in is_trivially_copy_constructible which breaks std::vector
// for non-copyable types
#elif(defined(__GNUC__) && __GNUC__ < 8 && !defined(__clang__))
#ifndef TL_GCC_LESS_8_TRIVIALLY_COPY_CONSTRUCTIBLE_MUTEX
#define TL_GCC_LESS_8_TRIVIALLY_COPY_CONSTRUCTIBLE_MUTEX
namespace nonstd {
namespace detail {
template <class T> struct is_trivially_copy_constructible : std::is_trivially_copy_constructible<T> {};
#ifdef _GLIBCXX_VECTOR
template <class T, class A> struct is_trivially_copy_constructible<std::vector<T, A>> : std::is_trivially_copy_constructible<T> {};
#endif
} // namespace detail
} // namespace nonstd
#endif
 
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) nonstd::detail::is_trivially_copy_constructible<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) std::is_trivially_copy_assignable<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>::value
#else
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) std::is_trivially_copy_constructible<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) std::is_trivially_copy_assignable<T>::value
#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>::value
#endif
 
#if __cplusplus > 201103L
#define TL_OPTIONAL_CXX14
#endif
 
// constexpr implies const in C++11, not C++14
#if(__cplusplus == 201103L || defined(TL_OPTIONAL_MSVC2015) || defined(TL_OPTIONAL_GCC49))
#define TL_OPTIONAL_11_CONSTEXPR
#else
#define TL_OPTIONAL_11_CONSTEXPR constexpr
#endif
 
namespace nonstd {
#ifndef TL_MONOSTATE_INPLACE_MUTEX
#define TL_MONOSTATE_INPLACE_MUTEX
class monostate {};

struct in_place_t {
    explicit in_place_t() = default;
};

static constexpr in_place_t in_place{};
#endif
 
template <class T> class optional;
 
namespace detail {
#ifndef TL_TRAITS_MUTEX
#define TL_TRAITS_MUTEX
// C++14-style aliases for brevity
template <class T> using remove_const_t = typename std::remove_const<T>::type;
template <class T> using remove_reference_t = typename std::remove_reference<T>::type;
template <class T> using decay_t = typename std::decay<T>::type;
template <bool E, class T = void> using enable_if_t = typename std::enable_if<E, T>::type;
template <bool B, class T, class F> using conditional_t = typename std::conditional<B, T, F>::type;
 
// std::conjunction from C++17
template <class...> struct conjunction : std::true_type {};
template <class B> struct conjunction<B> : B {};
template <class B, class... Bs> struct conjunction<B, Bs...> : std::conditional<bool(B::value), conjunction<Bs...>, B>::type {};
 
#if defined(_LIBCPP_VERSION) && __cplusplus == 201103L
#define TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
#endif
 
// In C++11 mode, there's an issue in libc++'s std::mem_fn
// which results in a hard-error when using it in a noexcept expression
// in some cases. This is a check to workaround the common failing case.
#ifdef TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
template <class T> struct is_pointer_to_non_const_member_func : std::false_type {};
template <class T, class Ret, class... Args> struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...)> : std::true_type {};
template <class T, class Ret, class... Args> struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...)&> : std::true_type {};
template <class T, class Ret, class... Args> struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) &&> : std::true_type {};
template <class T, class Ret, class... Args> struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) volatile> : std::true_type {};
template <class T, class Ret, class... Args> struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) volatile&> : std::true_type {};
template <class T, class Ret, class... Args> struct is_pointer_to_non_const_member_func<Ret (T::*)(Args...) volatile&&> : std::true_type {};
 
template <class T> struct is_const_or_const_ref : std::false_type {};
template <class T> struct is_const_or_const_ref<T const&> : std::true_type {};
template <class T> struct is_const_or_const_ref<T const> : std::true_type {};
#endif
 
// std::invoke from C++17
// https://stackoverflow.com/questions/38288042/c11-14-invoke-workaround
template <typename Fn, typename... Args,
#ifdef TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
          typename = enable_if_t<!(is_pointer_to_non_const_member_func<Fn>::value && is_const_or_const_ref<Args...>::value)>,
#endif
          typename = enable_if_t<std::is_member_pointer<decay_t<Fn>>::value>, int = 0>
constexpr auto invoke(Fn&& f, Args&&... args) noexcept(noexcept(std::mem_fn(f)(std::forward<Args>(args)...)))
    -> decltype(std::mem_fn(f)(std::forward<Args>(args)...)) {
    return std::mem_fn(f)(std::forward<Args>(args)...);
}
 
template <typename Fn, typename... Args, typename = enable_if_t<!std::is_member_pointer<decay_t<Fn>>::value>>
constexpr auto invoke(Fn&& f, Args&&... args) noexcept(noexcept(std::forward<Fn>(f)(std::forward<Args>(args)...)))
    -> decltype(std::forward<Fn>(f)(std::forward<Args>(args)...)) {
    return std::forward<Fn>(f)(std::forward<Args>(args)...);
}
 
// std::invoke_result from C++17
template <class F, class, class... Us> struct invoke_result_impl;
 
template <class F, class... Us>
struct invoke_result_impl<F, decltype(detail::invoke(std::declval<F>(), std::declval<Us>()...), void()), Us...> {
    using type = decltype(detail::invoke(std::declval<F>(), std::declval<Us>()...));
};
 
template <class F, class... Us> using invoke_result = invoke_result_impl<F, void, Us...>;
 
template <class F, class... Us> using invoke_result_t = typename invoke_result<F, Us...>::type;
 
#if defined(_MSC_VER) && _MSC_VER <= 1900
// TODO make a version which works with MSVC 2015
template <class T, class U = T> struct is_swappable : std::true_type {};
 
template <class T, class U = T> struct is_nothrow_swappable : std::true_type {};
#else
// https://stackoverflow.com/questions/26744589/what-is-a-proper-way-to-implement-is-swappable-to-test-for-the-swappable-concept
namespace swap_adl_tests {
// if swap ADL finds this then it would call std::swap otherwise (same
// signature)
struct tag {};
 
template <class T> tag swap(T&, T&);
template <class T, std::size_t N> tag swap(T (&a)[N], T (&b)[N]);
 
// helper functions to test if an unqualified swap is possible, and if it
// becomes std::swap
template <class, class> std::false_type can_swap(...) noexcept(false);
template <class T, class U, class = decltype(swap(std::declval<T&>(), std::declval<U&>()))>
std::true_type can_swap(int) noexcept(noexcept(swap(std::declval<T&>(), std::declval<U&>())));
 
template <class, class> std::false_type uses_std(...);
template <class T, class U> std::is_same<decltype(swap(std::declval<T&>(), std::declval<U&>())), tag> uses_std(int);
 
template <class T>
struct is_std_swap_noexcept
: std::integral_constant<bool, std::is_nothrow_move_constructible<T>::value && std::is_nothrow_move_assignable<T>::value> {};
 
template <class T, std::size_t N> struct is_std_swap_noexcept<T[N]> : is_std_swap_noexcept<T> {};
 
template <class T, class U> struct is_adl_swap_noexcept : std::integral_constant<bool, noexcept(can_swap<T, U>(0))> {};
} // namespace swap_adl_tests
 
template <class T, class U = T>
struct is_swappable : std::integral_constant<bool, decltype(detail::swap_adl_tests::can_swap<T, U>(0))::value &&
                                                       (!decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value ||
                                                        (std::is_move_assignable<T>::value && std::is_move_constructible<T>::value))> {};
 
template <class T, std::size_t N>
struct is_swappable<T[N], T[N]>
: std::integral_constant<bool, decltype(detail::swap_adl_tests::can_swap<T[N], T[N]>(0))::value &&
                                   (!decltype(detail::swap_adl_tests::uses_std<T[N], T[N]>(0))::value || is_swappable<T, T>::value)> {};
 
template <class T, class U = T>
struct is_nothrow_swappable
: std::integral_constant<bool, is_swappable<T, U>::value && ((decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value &&
                                                              detail::swap_adl_tests::is_std_swap_noexcept<T>::value) ||
                                                             (!decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value &&
                                                              detail::swap_adl_tests::is_adl_swap_noexcept<T, U>::value))> {};
#endif
#endif
 
// std::void_t from C++17
template <class...> struct voider { using type = void; };
template <class... Ts> using void_t = typename voider<Ts...>::type;
 
// Trait for checking if a type is a nonstd::optional
template <class T> struct is_optional_impl : std::false_type {};
template <class T> struct is_optional_impl<optional<T>> : std::true_type {};
template <class T> using is_optional = is_optional_impl<decay_t<T>>;
 
// Change void to nonstd::monostate
template <class U> using fixup_void = conditional_t<std::is_void<U>::value, monostate, U>;
 
template <class F, class U, class = invoke_result_t<F, U>> using get_map_return = optional<fixup_void<invoke_result_t<F, U>>>;
 
// Check if invoking F for some Us returns void
template <class F, class = void, class... U> struct returns_void_impl;
template <class F, class... U>
struct returns_void_impl<F, void_t<invoke_result_t<F, U...>>, U...> : std::is_void<invoke_result_t<F, U...>> {};
template <class F, class... U> using returns_void = returns_void_impl<F, void, U...>;
 
template <class T, class... U> using enable_if_ret_void = enable_if_t<returns_void<T&&, U...>::value>;
 
template <class T, class... U> using disable_if_ret_void = enable_if_t<!returns_void<T&&, U...>::value>;
 
template <class T, class U>
using enable_forward_value =
    detail::enable_if_t<std::is_constructible<T, U&&>::value && !std::is_same<detail::decay_t<U>, in_place_t>::value &&
                        !std::is_same<optional<T>, detail::decay_t<U>>::value>;
 
template <class T, class U, class Other>
using enable_from_other =
    detail::enable_if_t<std::is_constructible<T, Other>::value && !std::is_constructible<T, optional<U>&>::value &&
                        !std::is_constructible<T, optional<U>&&>::value && !std::is_constructible<T, const optional<U>&>::value &&
                        !std::is_constructible<T, const optional<U>&&>::value && !std::is_convertible<optional<U>&, T>::value &&
                        !std::is_convertible<optional<U>&&, T>::value && !std::is_convertible<const optional<U>&, T>::value &&
                        !std::is_convertible<const optional<U>&&, T>::value>;
 
template <class T, class U>
using enable_assign_forward = detail::enable_if_t<!std::is_same<optional<T>, detail::decay_t<U>>::value &&
                                                  !detail::conjunction<std::is_scalar<T>, std::is_same<T, detail::decay_t<U>>>::value &&
                                                  std::is_constructible<T, U>::value && std::is_assignable<T&, U>::value>;
 
template <class T, class U, class Other>
using enable_assign_from_other =
    detail::enable_if_t<std::is_constructible<T, Other>::value && std::is_assignable<T&, Other>::value &&
                        !std::is_constructible<T, optional<U>&>::value && !std::is_constructible<T, optional<U>&&>::value &&
                        !std::is_constructible<T, const optional<U>&>::value && !std::is_constructible<T, const optional<U>&&>::value &&
                        !std::is_convertible<optional<U>&, T>::value && !std::is_convertible<optional<U>&&, T>::value &&
                        !std::is_convertible<const optional<U>&, T>::value && !std::is_convertible<const optional<U>&&, T>::value &&
                        !std::is_assignable<T&, optional<U>&>::value && !std::is_assignable<T&, optional<U>&&>::value &&
                        !std::is_assignable<T&, const optional<U>&>::value && !std::is_assignable<T&, const optional<U>&&>::value>;
 
// The storage base manages the actual storage, and correctly propagates
// trivial destruction from T. This case is for when T is not trivially
// destructible.
template <class T, bool = ::std::is_trivially_destructible<T>::value> struct optional_storage_base {
    TL_OPTIONAL_11_CONSTEXPR optional_storage_base() noexcept
    : m_dummy()
    , m_has_value(false) {}
 
    template <class... U>
    TL_OPTIONAL_11_CONSTEXPR optional_storage_base(in_place_t, U&&... u)
    : m_value(std::forward<U>(u)...)
    , m_has_value(true) {}
 
    ~optional_storage_base() {
        if(m_has_value) {
            m_value.~T();
            m_has_value = false;
        }
    }
 
    struct dummy {};
    union {
        dummy m_dummy;
        T m_value;
    };
 
    bool m_has_value;
};
 
// This case is for when T is trivially destructible.
template <class T> struct optional_storage_base<T, true> {
    TL_OPTIONAL_11_CONSTEXPR optional_storage_base() noexcept
    : m_dummy()
    , m_has_value(false) {}
 
    template <class... U>
    TL_OPTIONAL_11_CONSTEXPR optional_storage_base(in_place_t, U&&... u)
    : m_value(std::forward<U>(u)...)
    , m_has_value(true) {}
 
    // No destructor, so this class is trivially destructible
 
    struct dummy {};
    union {
        dummy m_dummy;
        T m_value;
    };
 
    bool m_has_value = false;
};
 
// This base class provides some handy member functions which can be used in
// further derived classes
template <class T> struct optional_operations_base : optional_storage_base<T> {
    using optional_storage_base<T>::optional_storage_base;
 
    void hard_reset() noexcept {
        get().~T();
        this->m_has_value = false;
    }
 
    template <class... Args> void construct(Args&&... args) {
        new(std::addressof(this->m_value)) T(std::forward<Args>(args)...);
        this->m_has_value = true;
    }
 
    template <class Opt> void assign(Opt&& rhs) {
        if(this->has_value()) {
            if(rhs.has_value()) {
                this->m_value = std::forward<Opt>(rhs).get();
            } else {
                this->m_value.~T();
                this->m_has_value = false;
            }
        }
 
        else if(rhs.has_value()) {
            construct(std::forward<Opt>(rhs).get());
        }
    }
 
    bool has_value() const { return this->m_has_value; }
 
    TL_OPTIONAL_11_CONSTEXPR T& get() & { return this->m_value; }
    TL_OPTIONAL_11_CONSTEXPR const T& get() const& { return this->m_value; }
    TL_OPTIONAL_11_CONSTEXPR T&& get() && { return std::move(this->m_value); }
#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr const T&& get() const&& { return std::move(this->m_value); }
#endif
};
 
// This class manages conditionally having a trivial copy constructor
// This specialization is for when T is trivially copy constructible
template <class T, bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)> struct optional_copy_base : optional_operations_base<T> {
    using optional_operations_base<T>::optional_operations_base;
};
 
// This specialization is for when T is not trivially copy constructible
template <class T> struct optional_copy_base<T, false> : optional_operations_base<T> {
    using optional_operations_base<T>::optional_operations_base;
 
    optional_copy_base() = default;
    optional_copy_base(const optional_copy_base& rhs)
    : optional_operations_base<T>() {
        if(rhs.has_value()) {
            this->construct(rhs.get());
        } else {
            this->m_has_value = false;
        }
    }
 
    optional_copy_base(optional_copy_base&& rhs) = default;
    optional_copy_base& operator=(const optional_copy_base& rhs) = default;
    optional_copy_base& operator=(optional_copy_base&& rhs) = default;
};
 
// This class manages conditionally having a trivial move constructor
// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
// doesn't implement an analogue to std::is_trivially_move_constructible. We
// have to make do with a non-trivial move constructor even if T is trivially
// move constructible
#ifndef TL_OPTIONAL_GCC49
template <class T, bool = std::is_trivially_move_constructible<T>::value> struct optional_move_base : optional_copy_base<T> {
    using optional_copy_base<T>::optional_copy_base;
};
#else
template <class T, bool = false> struct optional_move_base;
#endif
template <class T> struct optional_move_base<T, false> : optional_copy_base<T> {
    using optional_copy_base<T>::optional_copy_base;
 
    optional_move_base() = default;
    optional_move_base(const optional_move_base& rhs) = default;
 
    optional_move_base(optional_move_base&& rhs) noexcept(std::is_nothrow_move_constructible<T>::value) {
        if(rhs.has_value()) {
            this->construct(std::move(rhs.get()));
        } else {
            this->m_has_value = false;
        }
    }
    optional_move_base& operator=(const optional_move_base& rhs) = default;
    optional_move_base& operator=(optional_move_base&& rhs) = default;
};
 
// This class manages conditionally having a trivial copy assignment operator
template <class T, bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) && TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) &&
                          TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T)>
struct optional_copy_assign_base : optional_move_base<T> {
    using optional_move_base<T>::optional_move_base;
};
 
template <class T> struct optional_copy_assign_base<T, false> : optional_move_base<T> {
    using optional_move_base<T>::optional_move_base;
 
    optional_copy_assign_base() = default;
    optional_copy_assign_base(const optional_copy_assign_base& rhs) = default;
 
    optional_copy_assign_base(optional_copy_assign_base&& rhs) = default;
    optional_copy_assign_base& operator=(const optional_copy_assign_base& rhs) {
        this->assign(rhs);
        return *this;
    }
    optional_copy_assign_base& operator=(optional_copy_assign_base&& rhs) = default;
};
 
// This class manages conditionally having a trivial move assignment operator
// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
// doesn't implement an analogue to std::is_trivially_move_assignable. We have
// to make do with a non-trivial move assignment operator even if T is trivially
// move assignable
#ifndef TL_OPTIONAL_GCC49
template <class T, bool = std::is_trivially_destructible<T>::value&& std::is_trivially_move_constructible<T>::value&&
                       std::is_trivially_move_assignable<T>::value>
struct optional_move_assign_base : optional_copy_assign_base<T> {
    using optional_copy_assign_base<T>::optional_copy_assign_base;
};
#else
template <class T, bool = false> struct optional_move_assign_base;
#endif
 
template <class T> struct optional_move_assign_base<T, false> : optional_copy_assign_base<T> {
    using optional_copy_assign_base<T>::optional_copy_assign_base;
 
    optional_move_assign_base() = default;
    optional_move_assign_base(const optional_move_assign_base& rhs) = default;
 
    optional_move_assign_base(optional_move_assign_base&& rhs) = default;
 
    optional_move_assign_base& operator=(const optional_move_assign_base& rhs) = default;
 
    optional_move_assign_base& operator=(optional_move_assign_base&& rhs) noexcept(
        std::is_nothrow_move_constructible<T>::value&& std::is_nothrow_move_assignable<T>::value) {
        this->assign(std::move(rhs));
        return *this;
    }
};
 
// optional_delete_ctor_base will conditionally delete copy and move
// constructors depending on whether T is copy/move constructible
template <class T, bool EnableCopy = std::is_copy_constructible<T>::value, bool EnableMove = std::is_move_constructible<T>::value>
struct optional_delete_ctor_base {
    optional_delete_ctor_base() = default;
    optional_delete_ctor_base(const optional_delete_ctor_base&) = default;
    optional_delete_ctor_base(optional_delete_ctor_base&&) noexcept = default;
    optional_delete_ctor_base& operator=(const optional_delete_ctor_base&) = default;
    optional_delete_ctor_base& operator=(optional_delete_ctor_base&&) noexcept = default;
};
 
template <class T> struct optional_delete_ctor_base<T, true, false> {
    optional_delete_ctor_base() = default;
    optional_delete_ctor_base(const optional_delete_ctor_base&) = default;
    optional_delete_ctor_base(optional_delete_ctor_base&&) noexcept = delete;
    optional_delete_ctor_base& operator=(const optional_delete_ctor_base&) = default;
    optional_delete_ctor_base& operator=(optional_delete_ctor_base&&) noexcept = default;
};
 
template <class T> struct optional_delete_ctor_base<T, false, true> {
    optional_delete_ctor_base() = default;
    optional_delete_ctor_base(const optional_delete_ctor_base&) = delete;
    optional_delete_ctor_base(optional_delete_ctor_base&&) noexcept = default;
    optional_delete_ctor_base& operator=(const optional_delete_ctor_base&) = default;
    optional_delete_ctor_base& operator=(optional_delete_ctor_base&&) noexcept = default;
};
 
template <class T> struct optional_delete_ctor_base<T, false, false> {
    optional_delete_ctor_base() = default;
    optional_delete_ctor_base(const optional_delete_ctor_base&) = delete;
    optional_delete_ctor_base(optional_delete_ctor_base&&) noexcept = delete;
    optional_delete_ctor_base& operator=(const optional_delete_ctor_base&) = default;
    optional_delete_ctor_base& operator=(optional_delete_ctor_base&&) noexcept = default;
};
 
// optional_delete_assign_base will conditionally delete copy and move
// constructors depending on whether T is copy/move constructible + assignable
template <class T, bool EnableCopy = (std::is_copy_constructible<T>::value && std::is_copy_assignable<T>::value),
          bool EnableMove = (std::is_move_constructible<T>::value && std::is_move_assignable<T>::value)>
struct optional_delete_assign_base {
    optional_delete_assign_base() = default;
    optional_delete_assign_base(const optional_delete_assign_base&) = default;
    optional_delete_assign_base(optional_delete_assign_base&&) noexcept = default;
    optional_delete_assign_base& operator=(const optional_delete_assign_base&) = default;
    optional_delete_assign_base& operator=(optional_delete_assign_base&&) noexcept = default;
};
 
template <class T> struct optional_delete_assign_base<T, true, false> {
    optional_delete_assign_base() = default;
    optional_delete_assign_base(const optional_delete_assign_base&) = default;
    optional_delete_assign_base(optional_delete_assign_base&&) noexcept = default;
    optional_delete_assign_base& operator=(const optional_delete_assign_base&) = default;
    optional_delete_assign_base& operator=(optional_delete_assign_base&&) noexcept = delete;
};
 
template <class T> struct optional_delete_assign_base<T, false, true> {
    optional_delete_assign_base() = default;
    optional_delete_assign_base(const optional_delete_assign_base&) = default;
    optional_delete_assign_base(optional_delete_assign_base&&) noexcept = default;
    optional_delete_assign_base& operator=(const optional_delete_assign_base&) = delete;
    optional_delete_assign_base& operator=(optional_delete_assign_base&&) noexcept = default;
};
 
template <class T> struct optional_delete_assign_base<T, false, false> {
    optional_delete_assign_base() = default;
    optional_delete_assign_base(const optional_delete_assign_base&) = default;
    optional_delete_assign_base(optional_delete_assign_base&&) noexcept = default;
    optional_delete_assign_base& operator=(const optional_delete_assign_base&) = delete;
    optional_delete_assign_base& operator=(optional_delete_assign_base&&) noexcept = delete;
};
 
} // namespace detail

struct nullopt_t {
    struct do_not_use {};
    constexpr explicit nullopt_t(do_not_use, do_not_use) noexcept {}
};

static constexpr nullopt_t nullopt{nullopt_t::do_not_use{}, nullopt_t::do_not_use{}};
 
class bad_optional_access : public std::exception {
public:
    bad_optional_access() = default;
    const char* what() const noexcept { return "Optional has no value"; }
};

template <class T>
class optional : private detail::optional_move_assign_base<T>,
                 private detail::optional_delete_ctor_base<T>,
                 private detail::optional_delete_assign_base<T> {
    using base = detail::optional_move_assign_base<T>;
 
    static_assert(!std::is_same<T, in_place_t>::value, "instantiation of optional with in_place_t is ill-formed");
    static_assert(!std::is_same<detail::decay_t<T>, nullopt_t>::value, "instantiation of optional with nullopt_t is ill-formed");
 
public:
// The different versions for C++14 and 11 are needed because deduced return
// types are not SFINAE-safe. This provides better support for things like
// generic lambdas. C.f.
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0826r0.html
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F&& f) & {
        using result = detail::invoke_result_t<F, T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F&& f) && {
        using result = detail::invoke_result_t<F, T&&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
 
    template <class F> constexpr auto and_then(F&& f) const& {
        using result = detail::invoke_result_t<F, const T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr auto and_then(F&& f) const&& {
        using result = detail::invoke_result_t<F, const T&&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
#endif
#else
    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T&> and_then(F&& f) & {
        using result = detail::invoke_result_t<F, T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T&&> and_then(F&& f) && {
        using result = detail::invoke_result_t<F, T&&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
 
    template <class F> constexpr detail::invoke_result_t<F, const T&> and_then(F&& f) const& {
        using result = detail::invoke_result_t<F, const T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr detail::invoke_result_t<F, const T&&> and_then(F&& f) const&& {
        using result = detail::invoke_result_t<F, const T&&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
#endif
#endif
 
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F&& f) & { return optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F&& f) && { return optional_map_impl(std::move(*this), std::forward<F>(f)); }
 
    template <class F> constexpr auto map(F&& f) const& { return optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> constexpr auto map(F&& f) const&& { return optional_map_impl(std::move(*this), std::forward<F>(f)); }
#else
    template <class F> TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional&>(), std::declval<F&&>())) map(F&& f) & {
        return optional_map_impl(*this, std::forward<F>(f));
    }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional&&>(), std::declval<F&&>())) map(F&& f) && {
        return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
 
    template <class F> constexpr decltype(optional_map_impl(std::declval<const optional&>(), std::declval<F&&>())) map(F&& f) const& {
        return optional_map_impl(*this, std::forward<F>(f));
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr decltype(optional_map_impl(std::declval<const optional&&>(), std::declval<F&&>())) map(F&& f) const&& {
        return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif
 
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) & { return optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) && { return optional_map_impl(std::move(*this), std::forward<F>(f)); }
 
    template <class F> constexpr auto transform(F&& f) const& { return optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> constexpr auto transform(F&& f) const&& { return optional_map_impl(std::move(*this), std::forward<F>(f)); }
#else
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional&>(), std::declval<F&&>())) transform(F&& f) & {
        return optional_map_impl(*this, std::forward<F>(f));
    }
 
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional&&>(), std::declval<F&&>())) transform(F&& f) && {
        return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
 
    template <class F> constexpr decltype(optional_map_impl(std::declval<const optional&>(), std::declval<F&&>())) transform(F&& f) const& {
        return optional_map_impl(*this, std::forward<F>(f));
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F>
    constexpr decltype(optional_map_impl(std::declval<const optional&&>(), std::declval<F&&>())) transform(F&& f) const&& {
        return optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif

    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) & {
        if(has_value())
            return *this;
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) & {
        return has_value() ? *this : std::forward<F>(f)();
    }
 
    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) && {
        if(has_value())
            return std::move(*this);
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) && {
        return has_value() ? std::move(*this) : std::forward<F>(f)();
    }
 
    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) const& {
        if(has_value())
            return *this;
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) const& {
        return has_value() ? *this : std::forward<F>(f)();
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) const&& {
        if(has_value())
            return std::move(*this);
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) const&& {
        return has_value() ? std::move(*this) : std::forward<F>(f)();
    }
#endif

    template <class F, class U> U map_or(F&& f, U&& u) & {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }
 
    template <class F, class U> U map_or(F&& f, U&& u) && {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }
 
    template <class F, class U> U map_or(F&& f, U&& u) const& {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> U map_or(F&& f, U&& u) const&& {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }
#endif

    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) & {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }
 
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) && {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }
 
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) const& {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) const&& {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }
#endif

    template <class U> constexpr optional<typename std::decay<U>::type> conjunction(U&& u) const {
        using result = optional<detail::decay_t<U>>;
        return has_value() ? result{u} : result{nullopt};
    }

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional& rhs) & { return has_value() ? *this : rhs; }
 
    constexpr optional disjunction(const optional& rhs) const& { return has_value() ? *this : rhs; }
 
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional& rhs) && { return has_value() ? std::move(*this) : rhs; }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(const optional& rhs) const&& { return has_value() ? std::move(*this) : rhs; }
#endif
 
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional&& rhs) & { return has_value() ? *this : std::move(rhs); }
 
    constexpr optional disjunction(optional&& rhs) const& { return has_value() ? *this : std::move(rhs); }
 
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional&& rhs) && { return has_value() ? std::move(*this) : std::move(rhs); }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(optional&& rhs) const&& { return has_value() ? std::move(*this) : std::move(rhs); }
#endif

    optional take() {
        optional ret = std::move(*this);
        reset();
        return ret;
    }
 
    using value_type = T;

    constexpr optional() noexcept = default;
 
    constexpr optional(nullopt_t) noexcept {}

    TL_OPTIONAL_11_CONSTEXPR optional(const optional& rhs) = default;

    TL_OPTIONAL_11_CONSTEXPR optional(optional&& rhs) = default;

    template <class... Args>
    constexpr explicit optional(detail::enable_if_t<std::is_constructible<T, Args...>::value, in_place_t>, Args&&... args)
    : base(in_place, std::forward<Args>(args)...) {}
 
    template <class U, class... Args>
    TL_OPTIONAL_11_CONSTEXPR explicit optional(
        detail::enable_if_t<std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value, in_place_t>, std::initializer_list<U> il,
        Args&&... args) {
        this->construct(il, std::forward<Args>(args)...);
    }

    template <class U = T, detail::enable_if_t<std::is_convertible<U&&, T>::value>* = nullptr,
              detail::enable_forward_value<T, U>* = nullptr>
    constexpr optional(U&& u)
    : base(in_place, std::forward<U>(u)) {}
 
    template <class U = T, detail::enable_if_t<!std::is_convertible<U&&, T>::value>* = nullptr,
              detail::enable_forward_value<T, U>* = nullptr>
    constexpr explicit optional(U&& u)
    : base(in_place, std::forward<U>(u)) {}

    template <class U, detail::enable_from_other<T, U, const U&>* = nullptr,
              detail::enable_if_t<std::is_convertible<const U&, T>::value>* = nullptr>
    optional(const optional<U>& rhs) {
        if(rhs.has_value()) {
            this->construct(*rhs);
        }
    }
 
    template <class U, detail::enable_from_other<T, U, const U&>* = nullptr,
              detail::enable_if_t<!std::is_convertible<const U&, T>::value>* = nullptr>
    explicit optional(const optional<U>& rhs) {
        if(rhs.has_value()) {
            this->construct(*rhs);
        }
    }

    template <class U, detail::enable_from_other<T, U, U&&>* = nullptr, detail::enable_if_t<std::is_convertible<U&&, T>::value>* = nullptr>
    optional(optional<U>&& rhs) {
        if(rhs.has_value()) {
            this->construct(std::move(*rhs));
        }
    }
 
    template <class U, detail::enable_from_other<T, U, U&&>* = nullptr, detail::enable_if_t<!std::is_convertible<U&&, T>::value>* = nullptr>
    explicit optional(optional<U>&& rhs) {
        if(rhs.has_value()) {
            this->construct(std::move(*rhs));
        }
    }

    ~optional() = default;

    optional& operator=(nullopt_t) noexcept {
        if(has_value()) {
            this->m_value.~T();
            this->m_has_value = false;
        }
 
        return *this;
    }

    optional& operator=(const optional& rhs) = default;

    optional& operator=(optional&& rhs) = default;

    template <class U = T, detail::enable_assign_forward<T, U>* = nullptr> optional& operator=(U&& u) {
        if(has_value()) {
            this->m_value = std::forward<U>(u);
        } else {
            this->construct(std::forward<U>(u));
        }
 
        return *this;
    }

    template <class U, detail::enable_assign_from_other<T, U, const U&>* = nullptr> optional& operator=(const optional<U>& rhs) {
        if(has_value()) {
            if(rhs.has_value()) {
                this->m_value = *rhs;
            } else {
                this->hard_reset();
            }
        }
 
        else if(rhs.has_value()) {
            this->construct(*rhs);
        }
 
        return *this;
    }
 
    // TODO check exception guarantee
    template <class U, detail::enable_assign_from_other<T, U, U>* = nullptr> optional& operator=(optional<U>&& rhs) {
        if(has_value()) {
            if(rhs.has_value()) {
                this->m_value = std::move(*rhs);
            } else {
                this->hard_reset();
            }
        }
 
        else if(rhs.has_value()) {
            this->construct(std::move(*rhs));
        }
 
        return *this;
    }

    template <class... Args> T& emplace(Args&&... args) {
        static_assert(std::is_constructible<T, Args&&...>::value, "T must be constructible with Args");
 
        *this = nullopt;
        this->construct(std::forward<Args>(args)...);
        return value();
    }
 
    template <class U, class... Args>
    detail::enable_if_t<std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value, T&> emplace(std::initializer_list<U> il,
                                                                                                           Args&&... args) {
        *this = nullopt;
        this->construct(il, std::forward<Args>(args)...);
        return value();
    }

    void swap(optional& rhs) noexcept(std::is_nothrow_move_constructible<T>::value&& detail::is_nothrow_swappable<T>::value) {
        using std::swap;
        if(has_value()) {
            if(rhs.has_value()) {
                swap(**this, *rhs);
            } else {
                new(std::addressof(rhs.m_value)) T(std::move(this->m_value));
                this->m_value.T::~T();
            }
        } else if(rhs.has_value()) {
            new(std::addressof(this->m_value)) T(std::move(rhs.m_value));
            rhs.m_value.T::~T();
        }
        swap(this->m_has_value, rhs.m_has_value);
    }

    constexpr const T* operator->() const { return std::addressof(this->m_value); }
 
    TL_OPTIONAL_11_CONSTEXPR T* operator->() { return std::addressof(this->m_value); }

    TL_OPTIONAL_11_CONSTEXPR T& operator*() & { return this->m_value; }
 
    constexpr const T& operator*() const& { return this->m_value; }
 
    TL_OPTIONAL_11_CONSTEXPR T&& operator*() && { return std::move(this->m_value); }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr const T&& operator*() const&& { return std::move(this->m_value); }
#endif

    constexpr bool has_value() const noexcept { return this->m_has_value; }
 
    constexpr explicit operator bool() const noexcept { return this->m_has_value; }

    TL_OPTIONAL_11_CONSTEXPR T& value() & {
        if(has_value())
            return this->m_value;
        throw bad_optional_access();
    }
    TL_OPTIONAL_11_CONSTEXPR const T& value() const& {
        if(has_value())
            return this->m_value;
        throw bad_optional_access();
    }
    TL_OPTIONAL_11_CONSTEXPR T&& value() && {
        if(has_value())
            return std::move(this->m_value);
        throw bad_optional_access();
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    TL_OPTIONAL_11_CONSTEXPR const T&& value() const&& {
        if(has_value())
            return std::move(this->m_value);
        throw bad_optional_access();
    }
#endif

    template <class U> constexpr T value_or(U&& u) const& {
        static_assert(std::is_copy_constructible<T>::value && std::is_convertible<U&&, T>::value,
                      "T must be copy constructible and convertible from U");
        return has_value() ? **this : static_cast<T>(std::forward<U>(u));
    }
 
    template <class U> TL_OPTIONAL_11_CONSTEXPR T value_or(U&& u) && {
        static_assert(std::is_move_constructible<T>::value && std::is_convertible<U&&, T>::value,
                      "T must be move constructible and convertible from U");
        return has_value() ? std::move(**this) : static_cast<T>(std::forward<U>(u));
    }

    void reset() noexcept {
        if(has_value()) {
            this->m_value.~T();
            this->m_has_value = false;
        }
    }
}; // namespace tl

template <class T, class U> inline constexpr bool operator==(const optional<T>& lhs, const optional<U>& rhs) {
    return lhs.has_value() == rhs.has_value() && (!lhs.has_value() || *lhs == *rhs);
}
template <class T, class U> inline constexpr bool operator!=(const optional<T>& lhs, const optional<U>& rhs) {
    return lhs.has_value() != rhs.has_value() || (lhs.has_value() && *lhs != *rhs);
}
template <class T, class U> inline constexpr bool operator<(const optional<T>& lhs, const optional<U>& rhs) {
    return rhs.has_value() && (!lhs.has_value() || *lhs < *rhs);
}
template <class T, class U> inline constexpr bool operator>(const optional<T>& lhs, const optional<U>& rhs) {
    return lhs.has_value() && (!rhs.has_value() || *lhs > *rhs);
}
template <class T, class U> inline constexpr bool operator<=(const optional<T>& lhs, const optional<U>& rhs) {
    return !lhs.has_value() || (rhs.has_value() && *lhs <= *rhs);
}
template <class T, class U> inline constexpr bool operator>=(const optional<T>& lhs, const optional<U>& rhs) {
    return !rhs.has_value() || (lhs.has_value() && *lhs >= *rhs);
}

template <class T> inline constexpr bool operator==(const optional<T>& lhs, nullopt_t) noexcept { return !lhs.has_value(); }
template <class T> inline constexpr bool operator==(nullopt_t, const optional<T>& rhs) noexcept { return !rhs.has_value(); }
template <class T> inline constexpr bool operator!=(const optional<T>& lhs, nullopt_t) noexcept { return lhs.has_value(); }
template <class T> inline constexpr bool operator!=(nullopt_t, const optional<T>& rhs) noexcept { return rhs.has_value(); }
template <class T> inline constexpr bool operator<(const optional<T>&, nullopt_t) noexcept { return false; }
template <class T> inline constexpr bool operator<(nullopt_t, const optional<T>& rhs) noexcept { return rhs.has_value(); }
template <class T> inline constexpr bool operator<=(const optional<T>& lhs, nullopt_t) noexcept { return !lhs.has_value(); }
template <class T> inline constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept { return true; }
template <class T> inline constexpr bool operator>(const optional<T>& lhs, nullopt_t) noexcept { return lhs.has_value(); }
template <class T> inline constexpr bool operator>(nullopt_t, const optional<T>&) noexcept { return false; }
template <class T> inline constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept { return true; }
template <class T> inline constexpr bool operator>=(nullopt_t, const optional<T>& rhs) noexcept { return !rhs.has_value(); }

template <class T, class U> inline constexpr bool operator==(const optional<T>& lhs, const U& rhs) {
    return lhs.has_value() ? *lhs == rhs : false;
}
template <class T, class U> inline constexpr bool operator==(const U& lhs, const optional<T>& rhs) {
    return rhs.has_value() ? lhs == *rhs : false;
}
template <class T, class U> inline constexpr bool operator!=(const optional<T>& lhs, const U& rhs) {
    return lhs.has_value() ? *lhs != rhs : true;
}
template <class T, class U> inline constexpr bool operator!=(const U& lhs, const optional<T>& rhs) {
    return rhs.has_value() ? lhs != *rhs : true;
}
template <class T, class U> inline constexpr bool operator<(const optional<T>& lhs, const U& rhs) {
    return lhs.has_value() ? *lhs < rhs : true;
}
template <class T, class U> inline constexpr bool operator<(const U& lhs, const optional<T>& rhs) {
    return rhs.has_value() ? lhs < *rhs : false;
}
template <class T, class U> inline constexpr bool operator<=(const optional<T>& lhs, const U& rhs) {
    return lhs.has_value() ? *lhs <= rhs : true;
}
template <class T, class U> inline constexpr bool operator<=(const U& lhs, const optional<T>& rhs) {
    return rhs.has_value() ? lhs <= *rhs : false;
}
template <class T, class U> inline constexpr bool operator>(const optional<T>& lhs, const U& rhs) {
    return lhs.has_value() ? *lhs > rhs : false;
}
template <class T, class U> inline constexpr bool operator>(const U& lhs, const optional<T>& rhs) {
    return rhs.has_value() ? lhs > *rhs : true;
}
template <class T, class U> inline constexpr bool operator>=(const optional<T>& lhs, const U& rhs) {
    return lhs.has_value() ? *lhs >= rhs : false;
}
template <class T, class U> inline constexpr bool operator>=(const U& lhs, const optional<T>& rhs) {
    return rhs.has_value() ? lhs >= *rhs : true;
}
 
template <class T, detail::enable_if_t<std::is_move_constructible<T>::value>* = nullptr,
          detail::enable_if_t<detail::is_swappable<T>::value>* = nullptr>
void swap(optional<T>& lhs, optional<T>& rhs) noexcept(noexcept(lhs.swap(rhs))) {
    return lhs.swap(rhs);
}
 
namespace detail {
struct i_am_secret {};
} // namespace detail
 
template <class T = detail::i_am_secret, class U,
          class Ret = detail::conditional_t<std::is_same<T, detail::i_am_secret>::value, detail::decay_t<U>, T>>
inline constexpr optional<Ret> make_optional(U&& v) {
    return optional<Ret>(std::forward<U>(v));
}
 
template <class T, class... Args> inline constexpr optional<T> make_optional(Args&&... args) {
    return optional<T>(in_place, std::forward<Args>(args)...);
}
template <class T, class U, class... Args> inline constexpr optional<T> make_optional(std::initializer_list<U> il, Args&&... args) {
    return optional<T>(in_place, il, std::forward<Args>(args)...);
}
 
#if __cplusplus >= 201703L
template <class T> optional(T) -> optional<T>;
#endif

namespace detail {
#ifdef TL_OPTIONAL_CXX14
template <class Opt, class F, class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
          detail::enable_if_t<!std::is_void<Ret>::value>* = nullptr>
constexpr auto optional_map_impl(Opt&& opt, F&& f) {
    return opt.has_value() ? detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt)) : optional<Ret>(nullopt);
}
 
template <class Opt, class F, class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
          detail::enable_if_t<std::is_void<Ret>::value>* = nullptr>
auto optional_map_impl(Opt&& opt, F&& f) {
    if(opt.has_value()) {
        detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt));
        return make_optional(monostate{});
    }
 
    return optional<monostate>(nullopt);
}
#else
template <class Opt, class F, class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
          detail::enable_if_t<!std::is_void<Ret>::value>* = nullptr>
 
constexpr auto optional_map_impl(Opt&& opt, F&& f) -> optional<Ret> {
    return opt.has_value() ? detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt)) : optional<Ret>(nullopt);
}
 
template <class Opt, class F, class Ret = decltype(detail::invoke(std::declval<F>(), *std::declval<Opt>())),
          detail::enable_if_t<std::is_void<Ret>::value>* = nullptr>
 
auto optional_map_impl(Opt&& opt, F&& f) -> optional<monostate> {
    if(opt.has_value()) {
        detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt));
        return monostate{};
    }
 
    return nullopt;
}
#endif
} // namespace detail

template <class T> class optional<T&> {
public:
// The different versions for C++14 and 11 are needed because deduced return
// types are not SFINAE-safe. This provides better support for things like
// generic lambdas. C.f.
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0826r0.html
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)

    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F&& f) & {
        using result = detail::invoke_result_t<F, T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F&& f) && {
        using result = detail::invoke_result_t<F, T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
    template <class F> constexpr auto and_then(F&& f) const& {
        using result = detail::invoke_result_t<F, const T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr auto and_then(F&& f) const&& {
        using result = detail::invoke_result_t<F, const T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
#endif
#else
    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T&> and_then(F&& f) & {
        using result = detail::invoke_result_t<F, T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T&> and_then(F&& f) && {
        using result = detail::invoke_result_t<F, T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
    template <class F> constexpr detail::invoke_result_t<F, const T&> and_then(F&& f) const& {
        using result = detail::invoke_result_t<F, const T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F> constexpr detail::invoke_result_t<F, const T&> and_then(F&& f) const&& {
        using result = detail::invoke_result_t<F, const T&>;
        static_assert(detail::is_optional<result>::value, "F must return an optional");
 
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : result(nullopt);
    }
#endif
#endif
 
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F&& f) & { return detail::optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F&& f) && {
        return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
 
    template <class F> constexpr auto map(F&& f) const& { return detail::optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> constexpr auto map(F&& f) const&& { return detail::optional_map_impl(std::move(*this), std::forward<F>(f)); }
#else
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional&>(), std::declval<F&&>())) map(F&& f) & {
        return detail::optional_map_impl(*this, std::forward<F>(f));
    }
 
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional&&>(), std::declval<F&&>())) map(F&& f) && {
        return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
 
    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional&>(), std::declval<F&&>())) map(F&& f) const& {
        return detail::optional_map_impl(*this, std::forward<F>(f));
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional&&>(), std::declval<F&&>())) map(F&& f) const&& {
        return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif
 
#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) && !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) & { return detail::optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) && {
        return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
 
    template <class F> constexpr auto transform(F&& f) const& { return detail::optional_map_impl(*this, std::forward<F>(f)); }
 
    template <class F> constexpr auto transform(F&& f) const&& { return detail::optional_map_impl(std::move(*this), std::forward<F>(f)); }
#else
    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional&>(), std::declval<F&&>())) transform(F&& f) & {
        return detail::optional_map_impl(*this, std::forward<F>(f));
    }

    template <class F>
    TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional&&>(), std::declval<F&&>())) transform(F&& f) && {
        return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
 
    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional&>(), std::declval<F&&>())) transform(F&& f) const& {
        return detail::optional_map_impl(*this, std::forward<F>(f));
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F>
    constexpr decltype(detail::optional_map_impl(std::declval<const optional&&>(), std::declval<F&&>())) transform(F&& f) const&& {
        return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
    }
#endif
#endif

    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) & {
        if(has_value())
            return *this;
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) & {
        return has_value() ? *this : std::forward<F>(f)();
    }
 
    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) && {
        if(has_value())
            return std::move(*this);
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) && {
        return has_value() ? std::move(*this) : std::forward<F>(f)();
    }
 
    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) const& {
        if(has_value())
            return *this;
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F&& f) const& {
        return has_value() ? *this : std::forward<F>(f)();
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, detail::enable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) const&& {
        if(has_value())
            return std::move(*this);
 
        std::forward<F>(f)();
        return nullopt;
    }
 
    template <class F, detail::disable_if_ret_void<F>* = nullptr> optional<T> or_else(F&& f) const&& {
        return has_value() ? std::move(*this) : std::forward<F>(f)();
    }
#endif

    template <class F, class U> U map_or(F&& f, U&& u) & {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }
 
    template <class F, class U> U map_or(F&& f, U&& u) && {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }
 
    template <class F, class U> U map_or(F&& f, U&& u) const& {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> U map_or(F&& f, U&& u) const&& {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u);
    }
#endif

    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) & {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }
 
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) && {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }
 
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) const& {
        return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u)();
    }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    template <class F, class U> detail::invoke_result_t<U> map_or_else(F&& f, U&& u) const&& {
        return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this)) : std::forward<U>(u)();
    }
#endif

    template <class U> constexpr optional<typename std::decay<U>::type> conjunction(U&& u) const {
        using result = optional<detail::decay_t<U>>;
        return has_value() ? result{u} : result{nullopt};
    }

    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional& rhs) & { return has_value() ? *this : rhs; }
 
    constexpr optional disjunction(const optional& rhs) const& { return has_value() ? *this : rhs; }
 
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional& rhs) && { return has_value() ? std::move(*this) : rhs; }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(const optional& rhs) const&& { return has_value() ? std::move(*this) : rhs; }
#endif
 
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional&& rhs) & { return has_value() ? *this : std::move(rhs); }
 
    constexpr optional disjunction(optional&& rhs) const& { return has_value() ? *this : std::move(rhs); }
 
    TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional&& rhs) && { return has_value() ? std::move(*this) : std::move(rhs); }
 
#ifndef TL_OPTIONAL_NO_CONSTRR
    constexpr optional disjunction(optional&& rhs) const&& { return has_value() ? std::move(*this) : std::move(rhs); }
#endif

    optional take() {
        optional ret = std::move(*this);
        reset();
        return ret;
    }
 
    using value_type = T&;

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

    TL_OPTIONAL_11_CONSTEXPR optional(const optional& rhs) noexcept = default;

    TL_OPTIONAL_11_CONSTEXPR optional(optional&& rhs) = default;

    template <class U = T, detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value>* = nullptr>
    constexpr optional(U&& u) noexcept
    : m_value(std::addressof(u)) {
        static_assert(std::is_lvalue_reference<U>::value, "U must be an lvalue");
    }
 
    template <class U>
    constexpr explicit optional(const optional<U>& rhs) noexcept
    : optional(*rhs) {}

    ~optional() = default;

    optional& operator=(nullopt_t) noexcept {
        m_value = nullptr;
        return *this;
    }

    optional& operator=(const optional& rhs) = default;

    template <class U = T, detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value>* = nullptr> optional& operator=(U&& u) {
        static_assert(std::is_lvalue_reference<U>::value, "U must be an lvalue");
        m_value = std::addressof(u);
        return *this;
    }

    template <class U> optional& operator=(const optional<U>& rhs) noexcept {
        m_value = std::addressof(rhs.value());
        return *this;
    }

    template <class U = T, detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value>* = nullptr>
    optional& emplace(U&& u) noexcept {
        return *this = std::forward<U>(u);
    }
 
    void swap(optional& rhs) noexcept { std::swap(m_value, rhs.m_value); }

    constexpr const T* operator->() const noexcept { return m_value; }
 
    TL_OPTIONAL_11_CONSTEXPR T* operator->() noexcept { return m_value; }

    TL_OPTIONAL_11_CONSTEXPR T& operator*() noexcept { return *m_value; }
 
    constexpr const T& operator*() const noexcept { return *m_value; }
 
    constexpr bool has_value() const noexcept { return m_value != nullptr; }
 
    constexpr explicit operator bool() const noexcept { return m_value != nullptr; }

    TL_OPTIONAL_11_CONSTEXPR T& value() {
        if(has_value())
            return *m_value;
        throw bad_optional_access();
    }
    TL_OPTIONAL_11_CONSTEXPR const T& value() const {
        if(has_value())
            return *m_value;
        throw bad_optional_access();
    }

    template <class U> constexpr T value_or(U&& u) const& noexcept {
        static_assert(std::is_copy_constructible<T>::value && std::is_convertible<U&&, T>::value,
                      "T must be copy constructible and convertible from U");
        return has_value() ? **this : static_cast<T>(std::forward<U>(u));
    }

    template <class U> TL_OPTIONAL_11_CONSTEXPR T value_or(U&& u) && noexcept {
        static_assert(std::is_move_constructible<T>::value && std::is_convertible<U&&, T>::value,
                      "T must be move constructible and convertible from U");
        return has_value() ? **this : static_cast<T>(std::forward<U>(u));
    }

    void reset() noexcept { m_value = nullptr; }
 
private:
    T* m_value;
}; // namespace tl
 
} // namespace nonstd
 
namespace std {
// TODO SFINAE
template <class T> struct hash<nonstd::optional<T>> {
    ::std::size_t operator()(const nonstd::optional<T>& o) const {
        if(!o.has_value())
            return 0;
 
        return std::hash<nonstd::detail::remove_const_t<T>>()(*o);
    }
};
} // namespace std
 
#endif
#endif