lokinet/include/tl/optional.hpp

///
// optional - An implementation of std::optional with extensions
// Written in 2017 by Simon Brand (@TartanLlama)
//
// 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/>.
///

#ifndef TL_OPTIONAL_HPP
#define TL_OPTIONAL_HPP

#define TL_OPTIONAL_VERSION_MAJOR 0
#define TL_OPTIONAL_VERSION_MINOR 5

#include <exception>
#include <functional>
#include <new>
#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 tl
{
  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 tl
#endif

#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) \
  tl::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))
/// \exclude
#define TL_OPTIONAL_11_CONSTEXPR
#else
   /// \exclude
#define TL_OPTIONAL_11_CONSTEXPR constexpr
#endif

namespace tl
{
#ifndef TL_MONOSTATE_INPLACE_MUTEX
#define TL_MONOSTATE_INPLACE_MUTEX
  /// \brief Used to represent an optional with no data; essentially a bool
  class monostate
  {
  };

  /// \brief A tag type to tell optional to construct its value in-place
  struct in_place_t
  {
    explicit in_place_t() = default;
  };
  /// \brief A tag to tell optional to construct its value in-place
  static constexpr in_place_t in_place{};
#endif

  template < class T >
  class optional;

  /// \exclude
  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_OPTIONAL_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_OPTIONAL_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_OPTIONAL_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;
#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 tl::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 tl::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 >;

#ifdef _MSC_VER
    // TODO make a version which works with MSVC
    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

    // 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) noexcept
      {
        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)
      {
        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

  /// \brief A tag type to represent an empty optional
  struct nullopt_t
  {
    struct do_not_use
    {
    };
    constexpr explicit nullopt_t(do_not_use, do_not_use) noexcept
    {
    }
  };
  /// \brief Represents an empty optional
  /// \synopsis static constexpr nullopt_t nullopt;
  ///
  /// *Examples*:
  /// ```
  /// tl::optional<int> a = tl::nullopt;
  /// void foo (tl::optional<int>);
  /// foo(tl::nullopt); //pass an empty optional
  /// ```
  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";
    }
  };

  /// An optional object is an object that contains the storage for another
  /// object and manages the lifetime of this contained object, if any. The
  /// contained object may be initialized after the optional object has been
  /// initialized, and may be destroyed before the optional object has been
  /// destroyed. The initialization state of the contained object is tracked by
  /// the optional object.
  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)
    /// \group and_then
    /// Carries out some operation which returns an optional on the stored
    /// object if there is one. \requires `std::invoke(std::forward<F>(f),
    /// value())` returns a `std::optional<U>` for some `U`. \returns Let `U` be
    /// the result of `std::invoke(std::forward<F>(f), value())`. Returns a
    /// `std::optional<U>`. The return value is empty if `*this` is empty,
    /// otherwise the return value of `std::invoke(std::forward<F>(f), value())`
    /// is returned.
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &&;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &;
    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
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &&;
    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
    /// \group and_then
    /// Carries out some operation which returns an optional on the stored
    /// object if there is one. \requires `std::invoke(std::forward<F>(f),
    /// value())` returns a `std::optional<U>` for some `U`.
    /// \returns Let `U` be the result of `std::invoke(std::forward<F>(f),
    /// value())`. Returns a `std::optional<U>`. The return value is empty if
    /// `*this` is empty, otherwise the return value of
    /// `std::invoke(std::forward<F>(f), value())` is returned.
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &&;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &;
    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
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &&;
    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)
    /// \brief Carries out some operation on the stored object if there is one.
    /// \returns Let `U` be the result of `std::invoke(std::forward<F>(f),
    /// value())`. Returns a `std::optional<U>`. The return value is empty if
    /// `*this` is empty, otherwise an `optional<U>` is constructed from the
    /// return value of `std::invoke(std::forward<F>(f), value())` and is
    /// returned.
    ///
    /// \group map
    /// \synopsis template <class F> constexpr auto map(F &&f) &;
    template < class F >
    TL_OPTIONAL_11_CONSTEXPR auto
    map(F &&f) &
    {
      return optional_map_impl(*this, std::forward< F >(f));
    }

    /// \group map
    /// \synopsis template <class F> constexpr auto map(F &&f) &&;
    template < class F >
    TL_OPTIONAL_11_CONSTEXPR auto
    map(F &&f) &&
    {
      return optional_map_impl(std::move(*this), std::forward< F >(f));
    }

    /// \group map
    /// \synopsis template <class F> constexpr auto map(F &&f) const&;
    template < class F >
    constexpr auto
    map(F &&f) const &
    {
      return optional_map_impl(*this, std::forward< F >(f));
    }

    /// \group map
    /// \synopsis template <class F> constexpr auto map(F &&f) const&&;
    template < class F >
    constexpr auto
    map(F &&f) const &&
    {
      return optional_map_impl(std::move(*this), std::forward< F >(f));
    }
#else
    /// \brief Carries out some operation on the stored object if there is one.
    /// \returns Let `U` be the result of `std::invoke(std::forward<F>(f),
    /// value())`. Returns a `std::optional<U>`. The return value is empty if
    /// `*this` is empty, otherwise an `optional<U>` is constructed from the
    /// return value of `std::invoke(std::forward<F>(f), value())` and is
    /// returned.
    ///
    /// \group map
    /// \synopsis template <class F> auto map(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(*this, std::forward< F >(f));
    }

    /// \group map
    /// \synopsis template <class F> auto map(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));
    }

    /// \group map
    /// \synopsis template <class F> auto map(F &&f) const&;
    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
    /// \group map
    /// \synopsis template <class F> auto map(F &&f) const&&;
    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

    /// \brief Calls `f` if the optional is empty
    /// \requires `std::invoke_result_t<F>` must be void or convertible to
    /// `optional<T>`.
    /// \effects If `*this` has a value, returns `*this`.
    /// Otherwise, if `f` returns `void`, calls `std::forward<F>(f)` and returns
    /// `std::nullopt`. Otherwise, returns `std::forward<F>(f)()`.
    ///
    /// \group or_else
    /// \synopsis template <class F> optional<T> or_else (F &&f) &;
    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;
    }

    /// \exclude
    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)();
    }

    /// \group or_else
    /// \synopsis template <class F> optional<T> or_else (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;
    }

    /// \exclude
    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)();
    }

    /// \group or_else
    /// \synopsis template <class F> optional<T> or_else (F &&f) const &;
    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;
    }

    /// \exclude
    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
    /// \exclude
    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;
    }

    /// \exclude
    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

    /// \brief Maps the stored value with `f` if there is one, otherwise returns
    /// `u`.
    ///
    /// \details If there is a value stored, then `f` is called with `**this`
    /// and the value is returned. Otherwise `u` is returned.
    ///
    /// \group map_or
    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);
    }

    /// \group map_or
    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);
    }

    /// \group map_or
    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
    /// \group map_or
    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

    /// \brief Maps the stored value with `f` if there is one, otherwise calls
    /// `u` and returns the result.
    ///
    /// \details If there is a value stored, then `f` is
    /// called with `**this` and the value is returned. Otherwise
    /// `std::forward<U>(u)()` is returned.
    ///
    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, 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), **this)
                         : std::forward< U >(u)();
    }

    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, 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)();
    }

    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, U &&u)
    /// const &;
    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
    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, U &&u)
    /// const &&;
    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

    /// \returns `u` if `*this` has a value, otherwise an empty optional.
    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};
    }

    /// \returns `rhs` if `*this` is empty, otherwise the current value.
    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(const optional &rhs) &
    {
      return has_value() ? *this : rhs;
    }

    /// \group disjunction
    constexpr optional
    disjunction(const optional &rhs) const &
    {
      return has_value() ? *this : rhs;
    }

    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(const optional &rhs) &&
    {
      return has_value() ? std::move(*this) : rhs;
    }

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

    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(optional &&rhs) &
    {
      return has_value() ? *this : std::move(rhs);
    }

    /// \group disjunction
    constexpr optional
    disjunction(optional &&rhs) const &
    {
      return has_value() ? *this : std::move(rhs);
    }

    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(optional &&rhs) &&
    {
      return has_value() ? std::move(*this) : std::move(rhs);
    }

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

    /// Takes the value out of the optional, leaving it empty
    /// \group take
    optional
    take() &
    {
      optional ret = *this;
      reset();
      return ret;
    }

    /// \group take
    optional
    take() const &
    {
      optional ret = *this;
      reset();
      return ret;
    }

    /// \group take
    optional
    take() &&
    {
      optional ret = std::move(*this);
      reset();
      return ret;
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    /// \group take
    optional
    take() const &&
    {
      optional ret = std::move(*this);
      reset();
      return ret;
    }
#endif

    using value_type = T;

    /// Constructs an optional that does not contain a value.
    /// \group ctor_empty
    constexpr optional() noexcept = default;

    /// \group ctor_empty
    constexpr optional(nullopt_t) noexcept
    {
    }

    /// Copy constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR
    optional(const optional &rhs) = default;

    /// Move constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR
    optional(optional &&rhs) = default;

    /// Constructs the stored value in-place using the given arguments.
    /// \group in_place
    /// \synopsis template <class... Args> constexpr explicit
    /// optional(in_place_t, Args&&... args);
    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)...)
    {
    }

    /// \group in_place
    /// \synopsis template <class U, class... Args>\nconstexpr explicit
    /// optional(in_place_t, std::initializer_list<U>&, 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)...);
    }

    /// Constructs the stored value with `u`.
    /// \synopsis template <class U=T> constexpr optional(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 optional(U &&u) : base(in_place, std::forward< U >(u))
    {
    }

    /// \exclude
    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))
    {
    }

    /// Converting copy constructor.
    /// \synopsis template <class U> optional(const optional<U> &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 >
    optional(const optional< U > &rhs)
    {
      this->construct(*rhs);
    }

    /// \exclude
    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)
    {
      this->construct(*rhs);
    }

    /// Converting move constructor.
    /// \synopsis template <class U> optional(optional<U> &&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)
    {
      this->construct(std::move(*rhs));
    }

    /// \exclude
    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)
    {
      this->construct(std::move(*rhs));
    }

    /// Destroys the stored value if there is one.
    ~optional() = default;

    /// Assignment to empty.
    ///
    /// Destroys the current value if there is one.
    optional &operator=(nullopt_t) noexcept
    {
      if(has_value())
      {
        this->m_value.~T();
        this->m_has_value = false;
      }

      return *this;
    }

    /// Copy assignment.
    ///
    /// Copies the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    optional &
    operator=(const optional &rhs) = default;

    /// Move assignment.
    ///
    /// Moves the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    optional &
    operator=(optional &&rhs) = default;

    /// Assigns the stored value from `u`, destroying the old value if there was
    /// one.
    /// \synopsis optional &operator=(U &&u);
    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;
    }

    /// Converting copy assignment operator.
    ///
    /// Copies the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    /// \synopsis optional &operator=(const optional<U> & rhs);
    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();
        }
      }

      if(rhs.has_value())
      {
        this->construct(*rhs);
      }

      return *this;
    }

    // TODO check exception guarantee
    /// Converting move assignment operator.
    ///
    /// Moves the value from `rhs` if there is one. Otherwise resets the stored
    /// value in `*this`.
    /// \synopsis optional &operator=(optional<U> && rhs);
    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();
        }
      }

      if(rhs.has_value())
      {
        this->construct(std::move(*rhs));
      }

      return *this;
    }

    /// Constructs the value in-place, destroying the current one if there is
    /// one.
    /// \group emplace
    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();
    }

    /// \group emplace
    /// \synopsis template <class U, class... Args>\nT&
    /// emplace(std::initializer_list<U> il, Args &&... args);
    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();
    }

    /// Swaps this optional with the other.
    ///
    /// If neither optionals have a value, nothing happens.
    /// If both have a value, the values are swapped.
    /// If one has a value, it is moved to the other and the movee is left
    /// valueless.
    void
    swap(optional &rhs) noexcept(std::is_nothrow_move_constructible< T >::value
                                     &&detail::is_nothrow_swappable< T >::value)
    {
      if(has_value())
      {
        if(rhs.has_value())
        {
          using std::swap;
          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();
      }
    }

    /// \returns a pointer to the stored value
    /// \requires a value is stored
    /// \group pointer
    /// \synopsis constexpr const T *operator->() const;
    constexpr const T *operator->() const
    {
      return std::addressof(this->m_value);
    }

    /// \group pointer
    /// \synopsis constexpr T *operator->();
    TL_OPTIONAL_11_CONSTEXPR T *operator->()
    {
      return std::addressof(this->m_value);
    }

    /// \returns the stored value
    /// \requires a value is stored
    /// \group deref
    /// \synopsis constexpr T &operator*();
    TL_OPTIONAL_11_CONSTEXPR T &operator*() &
    {
      return this->m_value;
    }

    /// \group deref
    /// \synopsis constexpr const T &operator*() const;
    constexpr const T &operator*() const &
    {
      return this->m_value;
    }

    /// \exclude
    TL_OPTIONAL_11_CONSTEXPR T &&operator*() &&
    {
      return std::move(this->m_value);
    }

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

    /// \returns whether or not the optional has a value
    /// \group has_value
    constexpr bool
    has_value() const noexcept
    {
      return this->m_has_value;
    }

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

    /// \returns the contained value if there is one, otherwise throws
    /// [bad_optional_access]
    /// \group value
    /// \synopsis constexpr T &value();
    TL_OPTIONAL_11_CONSTEXPR T &
    value() &
    {
      if(has_value())
        return this->m_value;
      throw bad_optional_access();
    }
    /// \group value
    /// \synopsis constexpr const T &value() const;
    TL_OPTIONAL_11_CONSTEXPR const T &
    value() const &
    {
      if(has_value())
        return this->m_value;
      throw bad_optional_access();
    }
    /// \exclude
    TL_OPTIONAL_11_CONSTEXPR T &&
    value() &&
    {
      if(has_value())
        return std::move(this->m_value);
      throw bad_optional_access();
    }

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

    /// \returns the stored value if there is one, otherwise returns `u`
    /// \group value_or
    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));
    }

    /// \group value_or
    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() ? **this : static_cast< T >(std::forward< U >(u));
    }

    /// Destroys the stored value if one exists, making the optional empty
    void
    reset() noexcept
    {
      if(has_value())
      {
        this->m_value.~T();
        this->m_has_value = false;
      }
    }
  };  // namespace tl

  /// \group relop
  /// \brief Compares two optional objects
  /// \details If both optionals contain a value, they are compared with `T`s
  /// relational operators. Otherwise `lhs` and `rhs` are equal only if they are
  /// both empty, and `lhs` is less than `rhs` only if `rhs` is empty and `lhs`
  /// is not.
  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);
  }
  /// \group relop
  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);
  }
  /// \group relop
  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);
  }
  /// \group relop
  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);
  }
  /// \group relop
  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);
  }
  /// \group relop
  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);
  }

  /// \group relop_nullopt
  /// \brief Compares an optional to a `nullopt`
  /// \details Equivalent to comparing the optional to an empty optional
  template < class T >
  inline constexpr bool
  operator==(const optional< T > &lhs, nullopt_t) noexcept
  {
    return !lhs.has_value();
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator==(nullopt_t, const optional< T > &rhs) noexcept
  {
    return !rhs.has_value();
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator!=(const optional< T > &lhs, nullopt_t) noexcept
  {
    return lhs.has_value();
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator!=(nullopt_t, const optional< T > &rhs) noexcept
  {
    return rhs.has_value();
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator<(const optional< T > &, nullopt_t) noexcept
  {
    return false;
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator<(nullopt_t, const optional< T > &rhs) noexcept
  {
    return rhs.has_value();
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator<=(const optional< T > &lhs, nullopt_t) noexcept
  {
    return !lhs.has_value();
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator<=(nullopt_t, const optional< T > &) noexcept
  {
    return true;
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator>(const optional< T > &lhs, nullopt_t) noexcept
  {
    return lhs.has_value();
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator>(nullopt_t, const optional< T > &) noexcept
  {
    return false;
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator>=(const optional< T > &, nullopt_t) noexcept
  {
    return true;
  }
  /// \group relop_nullopt
  template < class T >
  inline constexpr bool
  operator>=(nullopt_t, const optional< T > &rhs) noexcept
  {
    return !rhs.has_value();
  }

  /// \group relop_t
  /// \brief Compares the optional with a value.
  /// \details If the optional has a value, it is compared with the other value
  /// using `T`s relational operators. Otherwise, the optional is considered
  /// less than the value.
  template < class T, class U >
  inline constexpr bool
  operator==(const optional< T > &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs == rhs : false;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator==(const U &lhs, const optional< T > &rhs)
  {
    return rhs.has_value() ? lhs == *rhs : false;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator!=(const optional< T > &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs != rhs : true;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator!=(const U &lhs, const optional< T > &rhs)
  {
    return rhs.has_value() ? lhs != *rhs : true;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator<(const optional< T > &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs < rhs : true;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator<(const U &lhs, const optional< T > &rhs)
  {
    return rhs.has_value() ? lhs < *rhs : false;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator<=(const optional< T > &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs <= rhs : true;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator<=(const U &lhs, const optional< T > &rhs)
  {
    return rhs.has_value() ? lhs <= *rhs : false;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator>(const optional< T > &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs > rhs : false;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator>(const U &lhs, const optional< T > &rhs)
  {
    return rhs.has_value() ? lhs > *rhs : true;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator>=(const optional< T > &lhs, const U &rhs)
  {
    return lhs.has_value() ? *lhs >= rhs : false;
  }
  /// \group relop_t
  template < class T, class U >
  inline constexpr bool
  operator>=(const U &lhs, const optional< T > &rhs)
  {
    return rhs.has_value() ? lhs >= *rhs : true;
  }

  /// \synopsis template <class T>\nvoid swap(optional<T> &lhs, optional<T>
  /// &rhs);
  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

  /// \exclude
  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

  /// Specialization for when `T` is a reference. `optional<T&>` acts similarly
  /// to a `T*`, but provides more operations and shows intent more clearly.
  ///
  /// *Examples*:
  ///
  /// ```
  /// int i = 42;
  /// tl::optional<int&> o = i;
  /// *o == 42; //true
  /// i = 12;
  /// *o = 12; //true
  /// &*o == &i; //true
  /// ```
  ///
  /// Assignment has rebind semantics rather than assign-through semantics:
  ///
  /// ```
  /// int j = 8;
  /// o = j;
  ///
  /// &*o == &j; //true
  /// ```
  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)
    /// \group and_then
    /// Carries out some operation which returns an optional on the stored
    /// object if there is one. \requires `std::invoke(std::forward<F>(f),
    /// value())` returns a `std::optional<U>` for some `U`. \returns Let `U` be
    /// the result of `std::invoke(std::forward<F>(f), value())`. Returns a
    /// `std::optional<U>`. The return value is empty if `*this` is empty,
    /// otherwise the return value of `std::invoke(std::forward<F>(f), value())`
    /// is returned.
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &&;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &;
    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
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &&;
    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
    /// \group and_then
    /// Carries out some operation which returns an optional on the stored
    /// object if there is one. \requires `std::invoke(std::forward<F>(f),
    /// value())` returns a `std::optional<U>` for some `U`. \returns Let `U` be
    /// the result of `std::invoke(std::forward<F>(f), value())`. Returns a
    /// `std::optional<U>`. The return value is empty if `*this` is empty,
    /// otherwise the return value of `std::invoke(std::forward<F>(f), value())`
    /// is returned.
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &&;
    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);
    }

    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &;
    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
    /// \group and_then
    /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) const &&;
    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);
    }
#endif
#endif

#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) \
    && !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
    /// \brief Carries out some operation on the stored object if there is one.
    /// \returns Let `U` be the result of `std::invoke(std::forward<F>(f),
    /// value())`. Returns a `std::optional<U>`. The return value is empty if
    /// `*this` is empty, otherwise an `optional<U>` is constructed from the
    /// return value of `std::invoke(std::forward<F>(f), value())` and is
    /// returned.
    ///
    /// \group map
    /// \synopsis template <class F> constexpr auto map(F &&f) &;
    template < class F >
    TL_OPTIONAL_11_CONSTEXPR auto
    map(F &&f) &
    {
      return detail::optional_map_impl(*this, std::forward< F >(f));
    }

    /// \group map
    /// \synopsis template <class F> constexpr auto map(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));
    }

    /// \group map
    /// \synopsis template <class F> constexpr auto map(F &&f) const&;
    template < class F >
    constexpr auto
    map(F &&f) const &
    {
      return detail::optional_map_impl(*this, std::forward< F >(f));
    }

    /// \group map
    /// \synopsis template <class F> constexpr auto map(F &&f) const&&;
    template < class F >
    constexpr auto
    map(F &&f) const &&
    {
      return detail::optional_map_impl(std::move(*this), std::forward< F >(f));
    }
#else
    /// \brief Carries out some operation on the stored object if there is one.
    /// \returns Let `U` be the result of `std::invoke(std::forward<F>(f),
    /// value())`. Returns a `std::optional<U>`. The return value is empty if
    /// `*this` is empty, otherwise an `optional<U>` is constructed from the
    /// return value of `std::invoke(std::forward<F>(f), value())` and is
    /// returned.
    ///
    /// \group map
    /// \synopsis template <class F> auto map(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(*this, std::forward< F >(f));
    }

    /// \group map
    /// \synopsis template <class F> auto map(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));
    }

    /// \group map
    /// \synopsis template <class F> auto map(F &&f) const&;
    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
    /// \group map
    /// \synopsis template <class F> auto map(F &&f) const&&;
    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

    /// \brief Calls `f` if the optional is empty
    /// \requires `std::invoke_result_t<F>` must be void or convertible to
    /// `optional<T>`. \effects If `*this` has a value, returns `*this`.
    /// Otherwise, if `f` returns `void`, calls `std::forward<F>(f)` and returns
    /// `std::nullopt`. Otherwise, returns `std::forward<F>(f)()`.
    ///
    /// \group or_else
    /// \synopsis template <class F> optional<T> or_else (F &&f) &;
    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;
    }

    /// \exclude
    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)();
    }

    /// \group or_else
    /// \synopsis template <class F> optional<T> or_else (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;
    }

    /// \exclude
    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)();
    }

    /// \group or_else
    /// \synopsis template <class F> optional<T> or_else (F &&f) const &;
    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;
    }

    /// \exclude
    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
    /// \exclude
    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;
    }

    /// \exclude
    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

    /// \brief Maps the stored value with `f` if there is one, otherwise returns
    /// `u`.
    ///
    /// \details If there is a value stored, then `f` is called with `**this`
    /// and the value is returned. Otherwise `u` is returned.
    ///
    /// \group map_or
    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);
    }

    /// \group map_or
    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);
    }

    /// \group map_or
    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
    /// \group map_or
    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

    /// \brief Maps the stored value with `f` if there is one, otherwise calls
    /// `u` and returns the result.
    ///
    /// \details If there is a value stored, then `f` is
    /// called with `**this` and the value is returned. Otherwise
    /// `std::forward<U>(u)()` is returned.
    ///
    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, 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), **this)
                         : std::forward< U >(u)();
    }

    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, 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)();
    }

    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, U &&u)
    /// const &;
    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
    /// \group map_or_else
    /// \synopsis template <class F, class U>\nauto map_or_else(F &&f, U &&u)
    /// const &&;
    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

    /// \returns `u` if `*this` has a value, otherwise an empty optional.
    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};
    }

    /// \returns `rhs` if `*this` is empty, otherwise the current value.
    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(const optional &rhs) &
    {
      return has_value() ? *this : rhs;
    }

    /// \group disjunction
    constexpr optional
    disjunction(const optional &rhs) const &
    {
      return has_value() ? *this : rhs;
    }

    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(const optional &rhs) &&
    {
      return has_value() ? std::move(*this) : rhs;
    }

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

    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(optional &&rhs) &
    {
      return has_value() ? *this : std::move(rhs);
    }

    /// \group disjunction
    constexpr optional
    disjunction(optional &&rhs) const &
    {
      return has_value() ? *this : std::move(rhs);
    }

    /// \group disjunction
    TL_OPTIONAL_11_CONSTEXPR optional
    disjunction(optional &&rhs) &&
    {
      return has_value() ? std::move(*this) : std::move(rhs);
    }

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

    /// Takes the value out of the optional, leaving it empty
    /// \group take
    optional
    take() &
    {
      optional ret = *this;
      reset();
      return ret;
    }

    /// \group take
    optional
    take() const &
    {
      optional ret = *this;
      reset();
      return ret;
    }

    /// \group take
    optional
    take() &&
    {
      optional ret = std::move(*this);
      reset();
      return ret;
    }

#ifndef TL_OPTIONAL_NO_CONSTRR
    /// \group take
    optional
    take() const &&
    {
      optional ret = std::move(*this);
      reset();
      return ret;
    }
#endif

    using value_type = T &;

    /// Constructs an optional that does not contain a value.
    /// \group ctor_empty
    constexpr optional() noexcept : m_value(nullptr)
    {
    }

    /// \group ctor_empty
    constexpr optional(nullopt_t) noexcept : m_value(nullptr)
    {
    }

    /// Copy constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR
    optional(const optional &rhs) noexcept = default;

    /// Move constructor
    ///
    /// If `rhs` contains a value, the stored value is direct-initialized with
    /// it. Otherwise, the constructed optional is empty.
    TL_OPTIONAL_11_CONSTEXPR
    optional(optional &&rhs) = default;

    /// Constructs the stored value with `u`.
    /// \synopsis template <class U=T> constexpr optional(U &&u);
    template < class U                               = T,
               detail::enable_if_t< !detail::is_optional<
                   detail::decay_t< U > >::value > * = nullptr >
    constexpr optional(U &&u) : m_value(std::addressof(u))
    {
      static_assert(std::is_lvalue_reference< U >::value,
                    "U must be an lvalue");
    }

    /// \exclude
    template < class U >
    constexpr explicit optional(const optional< U > &rhs) : optional(*rhs)
    {
    }

    /// No-op
    ~optional() = default;

    /// Assignment to empty.
    ///
    /// Destroys the current value if there is one.
    optional &operator=(nullopt_t) noexcept
    {
      m_value = nullptr;
      return *this;
    }

    /// Copy assignment.
    ///
    /// Rebinds this optional to the referee of `rhs` if there is one. Otherwise
    /// resets the stored value in `*this`.
    optional &
    operator=(const optional &rhs) = default;

    /// Rebinds this optional to `u`.
    ///
    /// \requires `U` must be an lvalue reference.
    /// \synopsis optional &operator=(U &&u);
    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;
    }

    /// Converting copy assignment operator.
    ///
    /// Rebinds this optional to the referee of `rhs` if there is one. Otherwise
    /// resets the stored value in `*this`.
    template < class U >
    optional &
    operator=(const optional< U > &rhs)
    {
      m_value = std::addressof(rhs.value());
      return *this;
    }

    /// Constructs the value in-place, destroying the current one if there is
    /// one.
    ///
    /// \group emplace
    template < class... Args >
    T &
    emplace(Args &&... args) noexcept
    {
      static_assert(std::is_constructible< T, Args &&... >::value,
                    "T must be constructible with Args");

      *this = nullopt;
      this->construct(std::forward< Args >(args)...);
    }

    /// Swaps this optional with the other.
    ///
    /// If neither optionals have a value, nothing happens.
    /// If both have a value, the values are swapped.
    /// If one has a value, it is moved to the other and the movee is left
    /// valueless.
    void
    swap(optional &rhs) noexcept
    {
      std::swap(m_value, rhs.m_value);
    }

    /// \returns a pointer to the stored value
    /// \requires a value is stored
    /// \group pointer
    /// \synopsis constexpr const T *operator->() const;
    constexpr const T *operator->() const
    {
      return m_value;
    }

    /// \group pointer
    /// \synopsis constexpr T *operator->();
    TL_OPTIONAL_11_CONSTEXPR T *operator->()
    {
      return m_value;
    }

    /// \returns the stored value
    /// \requires a value is stored
    /// \group deref
    /// \synopsis constexpr T &operator*();
    TL_OPTIONAL_11_CONSTEXPR T &operator*()
    {
      return *m_value;
    }

    /// \group deref
    /// \synopsis constexpr const T &operator*() const;
    constexpr const T &operator*() const
    {
      return *m_value;
    }

    /// \returns whether or not the optional has a value
    /// \group has_value
    constexpr bool
    has_value() const noexcept
    {
      return m_value != nullptr;
    }

    /// \group has_value
    constexpr explicit operator bool() const noexcept
    {
      return m_value != nullptr;
    }

    /// \returns the contained value if there is one, otherwise throws
    /// [bad_optional_access]
    /// \group value
    /// synopsis constexpr T &value();
    TL_OPTIONAL_11_CONSTEXPR T &
    value()
    {
      if(has_value())
        return *m_value;
      throw bad_optional_access();
    }
    /// \group value
    /// \synopsis constexpr const T &value() const;
    TL_OPTIONAL_11_CONSTEXPR const T &
    value() const
    {
      if(has_value())
        return *m_value;
      throw bad_optional_access();
    }

    /// \returns the stored value if there is one, otherwise returns `u`
    /// \group value_or
    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));
    }

    /// \group value_or
    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() ? **this : static_cast< T >(std::forward< U >(u));
    }

    /// Destroys the stored value if one exists, making the optional empty
    void
    reset() noexcept
    {
      m_value = nullptr;
    }

   private:
    T *m_value;
  };  // namespace tl

}  // namespace tl

namespace std
{
  // TODO SFINAE
  template < class T >
  struct hash< tl::optional< T > >
  {
    ::std::size_t
    operator()(const tl::optional< T > &o) const
    {
      if(!o.has_value())
        return 0;

      return std::hash< tl::detail::remove_const_t< T > >()(*o);
    }
  };
}  // namespace std

#endif