optional/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 2

#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__ == 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 IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) std::has_trivial_copy_constructor<T>::value
#define 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 IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>::value
#else
#define IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) std::is_trivially_copy_constructible<T>::value
#define IS_TRIVIALLY_COPY_ASSIGNABLE(T) std::is_trivially_copy_assignable<T>::value
#define 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)) &&                                            \
    !defined(TL_OPTIONAL_GCC54)
/// \exclude
#define TL_OPTIONAL_11_CONSTEXPR
#else
/// \exclude
#define TL_OPTIONAL_11_CONSTEXPR constexpr
#endif

namespace tl {
/// \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{};

template <class T> class optional;

/// \exclude
namespace detail {

// 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 {};

// 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>>;

// std::invoke from C++17
// https://stackoverflow.com/questions/38288042/c11-14-invoke-workaround
template <typename Fn, typename... Args,
          typename = enable_if_t<std::is_member_pointer<decay_t<Fn>>{}>,
          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>>{}>>
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(invoke(std::declval<F>(), std::declval<Us>()...), void()),
    Us...> {
  using type = decltype(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;

// 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 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 not 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;
      }
    }

    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() && { 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 = 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 = IS_TRIVIALLY_COPY_ASSIGNABLE(T) && IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) && 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);
  }
  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) noexcept(
      std::is_nothrow_move_constructible<T>::value
          &&std::is_nothrow_move_assignable<T>::value) {
      this->assign(std::move(rhs));

  }
  optional_move_assign_base &
  operator=(optional_move_assign_base &&rhs) = default;
};


// 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>;

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_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) && !defined(TL_EXPECTED_GCC54)
  /// \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_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) && !defined(TL_EXPECTED_GCC54)
  /// \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 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 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 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 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(map_impl(std::declval<optional &>(),
                                             std::declval<F &&>()))
  map(F &&f) & {
    return 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(map_impl(std::declval<optional &&>(),
                                             std::declval<F &&>()))
  map(F &&f) && {
    return 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(map_impl(std::declval<const optional &>(),
                              std::declval<F &&>()))
  map(F &&f) const & {
    return 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(map_impl(std::declval<const optional &&>(),
                              std::declval<F &&>()))
  map(F &&f) const && {
    return 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;

  // TODO conditionally disable
  /// 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;

  // TODO conditionally delete, check exception guarantee
  /// 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;
  }

  // TODO check exception guarantee
  /// 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)...);
  }

  /// \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)...);
  }

  /// 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
  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> 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);
}

template <class T>
inline constexpr optional<detail::decay_t<T>> make_optional(T &&v) {
  return optional<detail::decay_t<T>>(std::forward<T>(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_CX14
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 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 map_impl(Opt &&opt, F &&f) {
  if (opt.has_value()) {
    detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt));
    return monostate{};
  }

  return optional<Ret>(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 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 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

} // 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