expected/expected.hpp

///
// expected - An implementation of std::expected 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_EXPECTED_HPP
#define TL_EXPECTED_HPP

#include <exception>
#include <functional>
#include <iostream>
#include <type_traits>
#include <utility>

#if (defined(_MSC_VER) && _MSC_VER == 1900)
#define TL_EXPECTED_MSVC2015
#endif

#if (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9)
#define TL_EXPECTED_GCC49
#endif

#if (defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 4)
#define TL_EXPECTED_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_EXPECTED_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_EXPECTED_CXX14
#endif

#if (__cplusplus == 201103L || defined(TL_EXPECTED_MSVC2015) ||                \
     defined(TL_EXPECTED_GCC49)) &&                                            \
    !defined(TL_EXPECTED_GCC54)
/// \exclude
#define TL_EXPECTED_11_CONSTEXPR
#else
/// \exclude
#define TL_EXPECTED_11_CONSTEXPR constexpr
#endif

namespace tl {
template <class T, class E> class expected;

/// \exclude
namespace detail {
template <bool E, class T = void>
using enable_if_t = typename std::enable_if<E, T>::type;
template <class T> using decay_t = typename std::decay<T>::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 {};

// Trait for checking if a type is a tl::expected
template <class T> struct is_expected_impl : std::false_type {};
template <class T, class E>
struct is_expected_impl<expected<T, E>> : std::true_type {};
template <class T> using is_expected = is_expected_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;

} // namespace detail

#ifndef TL_IN_PLACE_MONOSTATE_DEFINED
#define TL_IN_PLACE_MONOSTATE_DEFINED
/// \brief Used to represent an expected with no data
class monostate {};

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

/// Used as a wrapper to store the unexpected value
template <class E> class unexpected {
public:
  static_assert(!std::is_same<E, void>::value, "E must not be void");

  unexpected() = delete;
  constexpr explicit unexpected(const E &e) : m_val(e) {}

  constexpr explicit unexpected(E &&e) : m_val(std::move(e)) {}

  /// \returns the contained value
  /// \group unexpected_value
  constexpr const E &value() const & { return m_val; }
  /// \group unexpected_value
  constexpr E &value() & { return m_val; }
  /// \group unexpected_value
  constexpr E &&value() && { return std::move(m_val); }
  /// \exclude
  constexpr const E &&value() const && { return std::move(m_val); }

private:
  E m_val;
};

/// \brief Compares two unexpected objects
/// \details Simply compares lhs.value() to rhs.value()
/// \group unexpected_relop
template <class E>
constexpr bool operator==(const unexpected<E> &lhs, const unexpected<E> &rhs) {
  return lhs.value() == rhs.value();
}
/// \group unexpected_relop
template <class E>
constexpr bool operator!=(const unexpected<E> &lhs, const unexpected<E> &rhs) {
  return lhs.value() != rhs.value();
}
/// \group unexpected_relop
template <class E>
constexpr bool operator<(const unexpected<E> &lhs, const unexpected<E> &rhs) {
  return lhs.value() < rhs.value();
}
/// \group unexpected_relop
template <class E>
constexpr bool operator<=(const unexpected<E> &lhs, const unexpected<E> &rhs) {
  return lhs.value() <= rhs.value();
}
/// \group unexpected_relop
template <class E>
constexpr bool operator>(const unexpected<E> &lhs, const unexpected<E> &rhs) {
  return lhs.value() > rhs.value();
}
/// \group unexpected_relop
template <class E>
constexpr bool operator>=(const unexpected<E> &lhs, const unexpected<E> &rhs) {
  return lhs.value() >= rhs.value();
}

/// Create an `unexpected` from `e`, deducing the return type
///
/// *Example:*
/// auto e1 = tl::make_unexpected(42);
/// unexpected<int> e2 (42); //same semantics
template <class E> unexpected<E> make_unexpected(E &&e) {
  return unexpected<E>(std::forward<E>(e));
}

/// \brief A tag type to tell expected to construct the unexpected value
struct unexpect_t {
  unexpect_t() = default;
};
/// \brief A tag to tell expected to construct the unexpected value
static constexpr unexpect_t unexpect{};

/// \exclude
namespace detail {
// Implements the storage of the values, and ensures that the destructor is
// trivial if it can be.
//
// This specialization is for where neither `T` or `E` is trivially
// destructible, so the destructors must be called on destruction of the
// `expected`
template <class T, class E, bool = std::is_trivially_destructible<T>::value,
          bool = std::is_trivially_destructible<E>::value>
struct expected_storage_base {
  constexpr expected_storage_base() : m_val(T{}), m_has_val(true) {}

  template <class... Args,
            detail::enable_if_t<std::is_constructible<T, Args &&...>::value> * =
                nullptr>
  constexpr expected_storage_base(in_place_t, Args &&... args)
      : m_val(std::forward<Args>(args)...), m_has_val(true) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                T, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr expected_storage_base(in_place_t, std::initializer_list<U> il,
                                  Args &&... args)
      : m_val(il, std::forward<Args>(args)...), m_has_val(true) {}
  template <class... Args,
            detail::enable_if_t<std::is_constructible<E, Args &&...>::value> * =
                nullptr>
  constexpr explicit expected_storage_base(unexpect_t, Args &&... args)
      : m_unexpect(std::forward<Args>(args)...), m_has_val(false) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                E, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr explicit expected_storage_base(unexpect_t,
                                           std::initializer_list<U> il,
                                           Args &&... args)
      : m_unexpect(il, std::forward<Args>(args)...), m_has_val(false) {}

  ~expected_storage_base() {
    if (m_has_val) {
      m_val.~T();
    } else {
      m_unexpect.~unexpected<E>();
    }
  }
  bool m_has_val;
  union {
    T m_val;
    unexpected<E> m_unexpect;
  };
};

// This specialization is for when both `T` and `E` are trivially-destructible,
// so the destructor of the `expected` can be trivial.
template <class T, class E> struct expected_storage_base<T, E, true, true> {
  constexpr expected_storage_base() : m_val(T{}), m_has_val(true) {}

  template <class... Args,
            detail::enable_if_t<std::is_constructible<T, Args &&...>::value> * =
                nullptr>
  constexpr expected_storage_base(in_place_t, Args &&... args)
      : m_val(std::forward<Args>(args)...), m_has_val(true) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                T, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr expected_storage_base(in_place_t, std::initializer_list<U> il,
                                  Args &&... args)
      : m_val(il, std::forward<Args>(args)...), m_has_val(true) {}
  template <class... Args,
            detail::enable_if_t<std::is_constructible<E, Args &&...>::value> * =
                nullptr>
  constexpr explicit expected_storage_base(unexpect_t, Args &&... args)
      : m_unexpect(std::forward<Args>(args)...), m_has_val(false) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                E, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr explicit expected_storage_base(unexpect_t,
                                           std::initializer_list<U> il,
                                           Args &&... args)
      : m_unexpect(il, std::forward<Args>(args)...), m_has_val(false) {}

  ~expected_storage_base() = default;
  bool m_has_val;
  union {
    T m_val;
    unexpected<E> m_unexpect;
  };
};

// T is trivial, E is not.
template <class T, class E> struct expected_storage_base<T, E, true, false> {
  constexpr expected_storage_base() : m_val(T{}), m_has_val(true) {}

  template <class... Args,
            detail::enable_if_t<std::is_constructible<T, Args &&...>::value> * =
                nullptr>
  constexpr expected_storage_base(in_place_t, Args &&... args)
      : m_val(std::forward<Args>(args)...), m_has_val(true) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                T, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr expected_storage_base(in_place_t, std::initializer_list<U> il,
                                  Args &&... args)
      : m_val(il, std::forward<Args>(args)...), m_has_val(true) {}
  template <class... Args,
            detail::enable_if_t<std::is_constructible<E, Args &&...>::value> * =
                nullptr>
  constexpr explicit expected_storage_base(unexpect_t, Args &&... args)
      : m_unexpect(std::forward<Args>(args)...), m_has_val(false) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                E, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr explicit expected_storage_base(unexpect_t,
                                           std::initializer_list<U> il,
                                           Args &&... args)
      : m_unexpect(il, std::forward<Args>(args)...), m_has_val(false) {}

  ~expected_storage_base() {
    if (!m_has_val) {
      m_unexpect.~unexpected<E>();
    }
  }

  bool m_has_val;
  union {
    T m_val;
    unexpected<E> m_unexpect;
  };
};

// E is trivial, T is not.
template <class T, class E> struct expected_storage_base<T, E, false, true> {
  constexpr expected_storage_base() : m_val(T{}), m_has_val(true) {}

  template <class... Args,
            detail::enable_if_t<std::is_constructible<T, Args &&...>::value> * =
                nullptr>
  constexpr expected_storage_base(in_place_t, Args &&... args)
      : m_val(std::forward<Args>(args)...), m_has_val(true) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                T, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr expected_storage_base(in_place_t, std::initializer_list<U> il,
                                  Args &&... args)
      : m_val(il, std::forward<Args>(args)...), m_has_val(true) {}
  template <class... Args,
            detail::enable_if_t<std::is_constructible<E, Args &&...>::value> * =
                nullptr>
  constexpr explicit expected_storage_base(unexpect_t, Args &&... args)
      : m_unexpect(std::forward<Args>(args)...), m_has_val(false) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                E, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr explicit expected_storage_base(unexpect_t,
                                           std::initializer_list<U> il,
                                           Args &&... args)
      : m_unexpect(il, std::forward<Args>(args)...), m_has_val(false) {}

  ~expected_storage_base() {
    if (m_has_val) {
      m_val.~T();
    }
  }
  bool m_has_val;
  union {
    T m_val;
    unexpected<E> m_unexpect;
  };
};

// This base class provides some handy member functions which can be used in further derived classes
    template <class T, class E> struct expected_operations_base : expected_storage_base<T,E> {
        using expected_storage_base<T,E>::expected_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_val)) T(std::forward<Args>(args)...);
    this->m_has_value = true;
  }

  template <class... Args> void construct_error(Args &&... args) noexcept {
      new (std::addressof(this->m_unexpect)) unexpected<E>(std::forward<Args>(args)...);
    this->m_has_value = false;
  }

  // These assign overloads ensure that the most efficient assignment
  // implementation is used while maintaining the strong exception guarantee.
  // The problematic case is where rhs has a value, but *this does not.
  //
  // This overload handles the case where we can just copy-construct `T`
  // directly into place without throwing.
  template <class U = T,
            detail::enable_if_t<std::is_nothrow_copy_constructible<U>::value>
                * = nullptr>
  expected_operations_base &assign(const expected_operations_base &rhs) noexcept {
    if (!this->m_has_val && rhs.m_has_val) {
      geterr().~unexpected<E>();
      construct(rhs.get());
    }
    else {
        assign_common(rhs);
    }
  }

  // This overload handles the case where we can attempt to create a copy of
  // `T`, then no-throw move it into place if the copy was successful.
  template <class U = T,
            detail::enable_if_t<!std::is_nothrow_copy_constructible<U>::value &&
                                std::is_nothrow_move_constructible<U>::value>
                * = nullptr>
  expected_operations_base &assign(const expected_operations_base &rhs) noexcept {
    if (!this->m_has_val && rhs.m_has_val) {
        T tmp = rhs.get();
      geterr().~unexpected<E>();
        construct(std::move(tmp));
    }
    else {
        assign_common(rhs);
    }
  }

  // This overload is the worst-case, where we have to move-construct the
  // unexpected value into temporary storage, then try to copy the T into place.
  // If the construction succeeds, then everything is fine, but if it throws,
  // then we move the old unexpected value back into place before rethrowing the
  // exception.
  template <class U = T,
            detail::enable_if_t<!std::is_nothrow_copy_constructible<U>::value &&
                                !std::is_nothrow_move_constructible<U>::value>
                * = nullptr>
  expected_operations_base &assign(const expected_operations_base &rhs) {
    if (!this->m_has_val && rhs.m_has_val) {
      auto tmp = std::move(geterr());
      geterr().~unexpected<E>();

      try {
          construct(rhs.get());
      } catch (...) {
          geterr() = std::move(tmp);
        throw;
      }
    }
    else {
        assign_common(rhs);
    }
  }

  // These overloads do the same as above, but for rvalues
  template <class U = T,
            detail::enable_if_t<std::is_nothrow_move_constructible<U>::value>
                * = nullptr>
  expected_operations_base &assign(expected_operations_base &&rhs) noexcept {
    if (!this->m_has_val && rhs.m_has_val) {
        geterr().~unexpected<E>();
      ::new (valptr()) T(*std::move(rhs));
      this->m_has_val = true;
    }
    else {
        assign_common(rhs);
    }
  }

  template <class U = T,
            detail::enable_if_t<!std::is_nothrow_move_constructible<U>::value>
                * = nullptr>
  expected_operations_base &assign(expected_operations_base &&rhs) {
    if (!this->m_has_val && rhs.m_has_val) {
      auto tmp = std::move(geterr());
      geterr().~unexpected<E>();
      try {
          construct(std::move(rhs).get());
      } catch (...) {
          geterr() = std::move(tmp);
        throw;
      }
    }
    else {
        assign_common(rhs);
    }
  }

  // The common part of move/copy assigning
  template <class Rhs> void assign_common(Rhs &&rhs) {
    if (this->m_has_val) {
      if (rhs.m_has_val) {
          get() = std::forward<Rhs>(rhs).get();
      } else {
        get().~T();
        construct_err(std::forward<Rhs>(rhs).geterr());
      }
    } else {
      if (!rhs.m_has_val) {
          geterr() = std::forward<Rhs>(rhs).geterr();
      }
    }
  }

    bool has_value() const { return this->m_has_value; }

  TL_EXPECTED_11_CONSTEXPR T &get() & { return this->m_val; }
  TL_EXPECTED_11_CONSTEXPR const T &get() const & { return this->m_val; }
  TL_EXPECTED_11_CONSTEXPR T &&get() && { std::move(this->m_val); }
#ifndef TL_EXPECTED_NO_CONSTRR
  constexpr const T &&get() const && { return std::move(this->m_val); }
#endif

  TL_EXPECTED_11_CONSTEXPR T &geterr() & { return this->m_unexpect; }
  TL_EXPECTED_11_CONSTEXPR const T &geterr() const & { return this->m_unexpect; }
  TL_EXPECTED_11_CONSTEXPR T &&geterr() && { std::move(this->m_unexpect); }
#ifndef TL_EXPECTED_NO_CONSTRR
  constexpr const T &&geterr() const && { return std::move(this->m_unexpect); }
#endif
};

// This class manages conditionally having a trivial copy constructor
// This specialization is for when T is trivially copy constructible
 template <class T, class E, bool = IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)>
    struct expected_copy_base : expected_operations_base<T, E> {
     using expected_operations_base<T,E>::expected_operations_base;
};

// This specialization is for when T is not trivially copy constructible
    template <class T, class E>
    struct expected_copy_base<T, E, false> : expected_operations_base<T, E> {
        using expected_operations_base<T,E>::expected_operations_base;

  expected_copy_base() = default;
  expected_copy_base(const expected_copy_base &rhs) {
    if (rhs.has_value()) {
      this->construct(rhs.get());
    } else {
        this->construct_error(rhs.geterr());
    }
  }

  expected_copy_base(expected_copy_base &&rhs) = default;
  expected_copy_base &operator=(const expected_copy_base &rhs) = default;
  expected_copy_base &operator=(expected_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_EXPECTED_GCC49
    template <class T, class E, bool = std::is_trivially_move_constructible<T>::value>
    struct expected_move_base : expected_copy_base<T,E> {
        using expected_copy_base<T,E>::expected_copy_base;
};
#else
    template <class T, class E, bool = false>
    struct expected_move_base;
    #endif
    template <class T, class E> struct expected_move_base<T, E, false> : expected_copy_base<T,E> {
        using expected_copy_base<T,E>::expected_copy_base;

  expected_move_base() = default;
  expected_move_base(const expected_move_base &rhs) = default;

  expected_move_base(expected_move_base &&rhs) noexcept(
      std::is_nothrow_move_constructible<T>::value) {
    if (rhs.has_value()) {
      this->construct(std::move(rhs.get()));
    } else {
      this->construct_error(std::move(rhs.geterr()));
    }
  }
  expected_move_base &operator=(const expected_move_base &rhs) = default;
  expected_move_base &operator=(expected_move_base &&rhs) = default;
};

// This class manages conditionally having a trivial copy assignment operator
    template <class T, class E, bool = IS_TRIVIALLY_COPY_ASSIGNABLE(T) && IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) && IS_TRIVIALLY_DESTRUCTIBLE(T)>
        struct expected_copy_assign_base : expected_move_base<T,E> {
            using expected_move_base<T,E>::expected_move_base;
};

    template <class T, class E>
    struct expected_copy_assign_base<T, E, false> : expected_move_base<T,E> {
        using expected_move_base<T,E>::expected_move_base;

  expected_copy_assign_base() = default;
  expected_copy_assign_base(const expected_copy_assign_base &rhs) = default;

  expected_copy_assign_base(expected_copy_assign_base &&rhs) = default;
  expected_copy_assign_base &operator=(const expected_copy_assign_base &rhs) {
      this->assign(rhs);
  }
  expected_copy_assign_base &
  operator=(expected_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_EXPECTED_GCC49
    template <class T, class E, bool = std::is_trivially_destructible<T>::value && std::is_trivially_move_constructible<T>::value && std::is_trivially_move_assignable<T>::value>
        struct expected_move_assign_base : expected_copy_assign_base<T,E> {
            using expected_copy_assign_base<T,E>::expected_copy_assign_base;
};
#else
    template <class T, class E, bool = false>
    struct expected_move_assign_base;
#endif

    template <class T, class E>
    struct expected_move_assign_base<T, E, false> : expected_copy_assign_base<T,E> {
        using expected_copy_assign_base<T,E>::expected_copy_assign_base;

  expected_move_assign_base() = default;
  expected_move_assign_base(const expected_move_assign_base &rhs) = default;

  expected_move_assign_base(expected_move_assign_base &&rhs) = default;
  expected_move_assign_base &
  operator=(const expected_move_assign_base &rhs) noexcept(
      std::is_nothrow_move_constructible<T>::value
          &&std::is_nothrow_move_assignable<T>::value) {
      this->assign(std::move(rhs));
  }
  expected_move_assign_base &
  operator=(expected_move_assign_base &&rhs) = default;
};

// expected_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 && std::is_copy_constructible<E>::value),
              bool EnableMove = (std::is_move_constructible<T>::value && std::is_move_constructible<T>::value)>
struct expected_delete_ctor_base {
  expected_delete_ctor_base() = default;
  expected_delete_ctor_base(const expected_delete_ctor_base &) = default;
  expected_delete_ctor_base(expected_delete_ctor_base &&) noexcept = default;
  expected_delete_ctor_base &
  operator=(const expected_delete_ctor_base &) = default;
  expected_delete_ctor_base &
  operator=(expected_delete_ctor_base &&) noexcept = default;
};

template <class T> struct expected_delete_ctor_base<T, true, false> {
  expected_delete_ctor_base() = default;
  expected_delete_ctor_base(const expected_delete_ctor_base &) = default;
  expected_delete_ctor_base(expected_delete_ctor_base &&) noexcept = delete;
  expected_delete_ctor_base &
  operator=(const expected_delete_ctor_base &) = default;
  expected_delete_ctor_base &
  operator=(expected_delete_ctor_base &&) noexcept = default;
};

template <class T> struct expected_delete_ctor_base<T, false, true> {
  expected_delete_ctor_base() = default;
  expected_delete_ctor_base(const expected_delete_ctor_base &) = delete;
  expected_delete_ctor_base(expected_delete_ctor_base &&) noexcept = default;
  expected_delete_ctor_base &
  operator=(const expected_delete_ctor_base &) = default;
  expected_delete_ctor_base &
  operator=(expected_delete_ctor_base &&) noexcept = default;
};

template <class T> struct expected_delete_ctor_base<T, false, false> {
  expected_delete_ctor_base() = default;
  expected_delete_ctor_base(const expected_delete_ctor_base &) = delete;
  expected_delete_ctor_base(expected_delete_ctor_base &&) noexcept = delete;
  expected_delete_ctor_base &
  operator=(const expected_delete_ctor_base &) = default;
  expected_delete_ctor_base &
  operator=(expected_delete_ctor_base &&) noexcept = default;
};

// expected_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_constructible<E>::value &&
                             std::is_copy_assignable<T>::value &&
                             std::is_copy_assignable<E>::value),
          bool EnableMove = (std::is_move_constructible<T>::value &&
                             std::is_move_constructible<E>::value &&
                             std::is_move_assignable<T>::value &&
                             std::is_move_assignable<E>::value)>
struct expected_delete_assign_base {
  expected_delete_assign_base() = default;
  expected_delete_assign_base(const expected_delete_assign_base &) = default;
  expected_delete_assign_base(expected_delete_assign_base &&) noexcept =
      default;
  expected_delete_assign_base &
  operator=(const expected_delete_assign_base &) = default;
  expected_delete_assign_base &
  operator=(expected_delete_assign_base &&) noexcept = default;
};

template <class T> struct expected_delete_assign_base<T, true, false> {
  expected_delete_assign_base() = default;
  expected_delete_assign_base(const expected_delete_assign_base &) = default;
  expected_delete_assign_base(expected_delete_assign_base &&) noexcept =
      default;
  expected_delete_assign_base &
  operator=(const expected_delete_assign_base &) = default;
  expected_delete_assign_base &
  operator=(expected_delete_assign_base &&) noexcept = delete;
};

template <class T> struct expected_delete_assign_base<T, false, true> {
  expected_delete_assign_base() = default;
  expected_delete_assign_base(const expected_delete_assign_base &) = default;
  expected_delete_assign_base(expected_delete_assign_base &&) noexcept =
      default;
  expected_delete_assign_base &
  operator=(const expected_delete_assign_base &) = delete;
  expected_delete_assign_base &
  operator=(expected_delete_assign_base &&) noexcept = default;
};

template <class T> struct expected_delete_assign_base<T, false, false> {
  expected_delete_assign_base() = default;
  expected_delete_assign_base(const expected_delete_assign_base &) = default;
  expected_delete_assign_base(expected_delete_assign_base &&) noexcept =
      default;
  expected_delete_assign_base &
  operator=(const expected_delete_assign_base &) = delete;
  expected_delete_assign_base &
  operator=(expected_delete_assign_base &&) noexcept = delete;
};

// This is needed to be able to construct the expected_default_ctor_base which
// follows, while still conditionally deleting the default constructor.
struct default_constructor_tag {
  explicit constexpr default_constructor_tag() = default;
};

// expected_default_ctor_base will ensure that expected has a deleted default
// consturctor if T is not default constructible.
// This specialization is for when T is default constructible
template <class T, class E,
          bool Enable = std::is_default_constructible<T>::value>
struct expected_default_ctor_base {
  constexpr expected_default_ctor_base() noexcept = default;
  constexpr expected_default_ctor_base(
      expected_default_ctor_base const &) noexcept = default;
  constexpr expected_default_ctor_base(expected_default_ctor_base &&) noexcept =
      default;
  expected_default_ctor_base &
  operator=(expected_default_ctor_base const &) noexcept = default;
  expected_default_ctor_base &
  operator=(expected_default_ctor_base &&) noexcept = default;

  constexpr explicit expected_default_ctor_base(default_constructor_tag) {}
};

// This specialization is for when T is not default constructible
template <class T, class E> struct expected_default_ctor_base<T, E, false> {
  constexpr expected_default_ctor_base() noexcept = delete;
  constexpr expected_default_ctor_base(
      expected_default_ctor_base const &) noexcept = default;
  constexpr expected_default_ctor_base(expected_default_ctor_base &&) noexcept =
      default;
  expected_default_ctor_base &
  operator=(expected_default_ctor_base const &) noexcept = default;
  expected_default_ctor_base &
  operator=(expected_default_ctor_base &&) noexcept = default;

  constexpr explicit expected_default_ctor_base(default_constructor_tag) {}
};
} // namespace detail

template <class E> class bad_expected_access : public std::exception {
public:
  explicit bad_expected_access(E e) : m_val(std::move(e)) {}

  virtual const char *what() const noexcept override {
    return "Bad expected access";
  }

  const E &error() const & { return m_val; }
  E &error() & { return m_val; }
  const E &&error() const && { return std::move(m_val); }
  E &&error() && { return std::move(m_val); }

private:
  E m_val;
};

/// An `expected<T, E>` object is an object that contains the storage for
/// another object and manages the lifetime of this contained object `T`.
/// Alternatively it could contain the storage for another unexpected object
/// `E`. The contained object may not be initialized after the expected object
/// has been initialized, and may not be destroyed before the expected object
/// has been destroyed. The initialization state of the contained object is
/// tracked by the expected object.
template <class T, class E>
class expected : private detail::expected_move_assign<T, E>,
                 private detail::expected_delete_ctor_base<T, E>,
                 private detail::expected_delete_assign_base<T, E>,
                 private detail::expected_default_ctor_base<T, E> {
  static_assert(!std::is_reference<T>::value, "T must not be a reference");
  static_assert(!std::is_same<T, std::remove_cv<in_place_t>>::value,
                "T must not be in_place_t");
  static_assert(!std::is_same<T, std::remove_cv<unexpect_t>>::value,
                "T must not be unexpect_t");
  static_assert(!std::is_same<T, std::remove_cv<unexpected<E>>>::value,
                "T must not be unexpected<E>");
  static_assert(!std::is_reference<E>::value, "E must not be a reference");
  static_assert(!std::is_same<T, void>::value, "T must not be void");

  T *valptr() { return std::addressof(this->m_val); }
  unexpected<E> *errptr() { return std::addressof(this->m_unexpect); }
  T &val() { return this->m_val; }
  unexpected<E> &err() { return this->m_unexpect; }
  const T &val() const { return this->m_val; }
  const unexpected<E> &err() const { return this->m_unexpect; }

  using impl_base = detail::expected_impl<T, E>;
  using ctor_base = detail::expected_default_ctor_base<T, E>;

public:
  typedef T value_type;
  typedef E error_type;
  typedef unexpected<E> unexpected_type;

#if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
    !defined(TL_EXPECTED_GCC54)
  /// \group and_then
  /// Carries out some operation which returns an expected on the stored object
  /// if there is one. \requires `std::invoke(std::forward<F>(f), value())`
  /// returns a `std::expected<U>` for some `U`. \returns Let `U` be the result
  /// of `std::invoke(std::forward<F>(f), value())`. Returns a
  /// `std::expected<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_EXPECTED_11_CONSTEXPR auto and_then(F &&f) & {
    using result = detail::invoke_result_t<F, T &>;
    static_assert(detail::is_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), **this)
                       : result(unexpect, this->error());
  }

  /// \group and_then
  /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &&;
  template <class F> TL_EXPECTED_11_CONSTEXPR auto and_then(F &&f) && {
    using result = detail::invoke_result_t<F, T &&>;
    static_assert(detail::is_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
                       : result(unexpect, std::move(this->error()));
  }

  /// \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_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), **this)
                       : result(unexpect, this->error());
  }

#ifndef TL_EXPECTED_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_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
                       : result(unexpect, std::move(this->error()));
  }
#endif

#else
  /// \group and_then
  /// Carries out some operation which returns an expected on the stored object
  /// if there is one. \requires `std::invoke(std::forward<F>(f), value())`
  /// returns a `std::expected<U>` for some `U`. \returns Let `U` be the result
  /// of `std::invoke(std::forward<F>(f), value())`. Returns a
  /// `std::expected<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_EXPECTED_11_CONSTEXPR detail::invoke_result_t<F, T &> and_then(F &&f) & {
    using result = detail::invoke_result_t<F, T &>;
    static_assert(detail::is_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), **this)
                       : result(unexpect, this->error());
  }

  /// \group and_then
  /// \synopsis template <class F>\nconstexpr auto and_then(F &&f) &&;
  template <class F>
  TL_EXPECTED_11_CONSTEXPR detail::invoke_result_t<F, T &&> and_then(F &&f) && {
    using result = detail::invoke_result_t<F, T &&>;
    static_assert(detail::is_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), **this)
                       : result(unexpect, std::move(this->error()));
  }

  /// \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_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), **this)
                       : result(unexpect, this->error());
  }

#ifndef TL_EXPECTED_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_expected<result>::value,
                  "F must return an expected");

    return has_value() ? detail::invoke(std::forward<F>(f), **this)
                       : result(unexpect, std::move(this->error()));
  }
#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::expected<U,E>`. If `*this` is unexpected, the
  /// result is `*this`, otherwise an `expected<U,E>` 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_EXPECTED_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_EXPECTED_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::expected<U,E>`. If `*this` is unexpected, the
  /// result is `*this`, otherwise an `expected<U,E>` 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_EXPECTED_11_CONSTEXPR decltype(map_impl(std::declval<expected &>(),
                                             std::declval<F &&>()))
  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_EXPECTED_11_CONSTEXPR decltype(map_impl(std::declval<expected &>(),
                                             std::declval<F &&>()))
  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 decltype(map_impl(std::declval<const expected &>(),
                              std::declval<F &&>()))
  map(F &&f) const & {
    return map_impl(*this, std::forward<F>(f));
  }

#ifndef TL_EXPECTED_NO_CONSTRR
  /// \group map
  /// \synopsis template <class F> constexpr auto map(F &&f) const &&;
  template <class F>
  constexpr decltype(map_impl(std::declval<const expected &&>(),
                              std::declval<F &&>()))
  map(F &&f) const && {
    return map_impl(std::move(*this), std::forward<F>(f));
  }
#endif
#endif

#if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
    !defined(TL_EXPECTED_GCC54)
  /// \brief Carries out some operation on the stored unexpected object if there
  /// is one.
  /// \returns Let `U` be the result of `std::invoke(std::forward<F>(f),
  /// value())`. Returns a `std::expected<T,U>`. If `*this` has an expected
  /// value, the result is `*this`, otherwise an `expected<T,U>` is constructed
  /// from `make_unexpected(std::invoke(std::forward<F>(f), value()))` and is
  /// returned.
  ///
  /// \group map_error
  /// \synopsis template <class F> constexpr auto map_error(F &&f) &;
  template <class F> TL_EXPECTED_11_CONSTEXPR auto map_error(F &&f) & {
    return map_error_impl(*this, std::forward<F>(f));
  }

  /// \group map_error
  /// \synopsis template <class F> constexpr auto map_error(F &&f) &&;
  template <class F> TL_EXPECTED_11_CONSTEXPR auto map_error(F &&f) && {
    return map_error_impl(std::move(*this), std::forward<F>(f));
  }

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

  /// \group map_error
  /// \synopsis template <class F> constexpr auto map_error(F &&f) const &&;
  template <class F> constexpr auto map_error(F &&f) const && {
    return map_error_impl(std::move(*this), std::forward<F>(f));
  }
#else
  /// \brief Carries out some operation on the stored unexpected object if there
  /// is one.
  /// \returns Let `U` be the result of `std::invoke(std::forward<F>(f),
  /// value())`. Returns a `std::expected<T,U>`. If `*this` has an expected
  /// value, the result is `*this`, otherwise an `expected<T,U>` is constructed
  /// from `make_unexpected(std::invoke(std::forward<F>(f), value()))` and is
  /// returned.
  ///
  /// \group map_error
  /// \synopsis template <class F> constexpr auto map_error(F &&f) &;
  template <class F>
  TL_EXPECTED_11_CONSTEXPR decltype(map_error_impl(std::declval<expected &>(),
                                                   std::declval<F &&>()))
  map_error(F &&f) & {
    return map_error_impl(*this, std::forward<F>(f));
  }

  /// \group map_error
  /// \synopsis template <class F> constexpr auto map_error(F &&f) &&;
  template <class F>
  TL_EXPECTED_11_CONSTEXPR decltype(map_error_impl(std::declval<expected &&>(),
                                                   std::declval<F &&>()))
  map_error(F &&f) && {
    return map_error_impl(std::move(*this), std::forward<F>(f));
  }

  /// \group map_error
  /// \synopsis template <class F> constexpr auto map_error(F &&f) const &;
  template <class F>
  constexpr decltype(map_error_impl(std::declval<const expected &>(),
                                    std::declval<F &&>()))
  map_error(F &&f) const & {
    return map_error_impl(*this, std::forward<F>(f));
  }

#ifndef TL_EXPECTED_NO_CONSTRR
  /// \group map_error
  /// \synopsis template <class F> constexpr auto map_error(F &&f) const &&;
  template <class F>
  constexpr decltype(map_error_impl(std::declval<const expected &&>(),
                                    std::declval<F &&>()))
  map_error(F &&f) const && {
    return map_error_impl(std::move(*this), std::forward<F>(f));
  }
#endif
#endif

  constexpr expected() = default;
  constexpr expected(const expected &rhs) = default;
  constexpr expected(expected &&rhs) = default;
  constexpr expected &operator=(const expected &rhs) = default;
  constexpr expected &operator=(expected &&rhs) = default;

  template <class... Args,
            detail::enable_if_t<std::is_constructible<T, Args &&...>::value> * =
                nullptr>
  constexpr expected(in_place_t, Args &&... args)
      : impl_base(in_place, std::forward<Args>(args)...),
        ctor_base(detail::default_constructor_tag{}) {}

  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                T, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr expected(in_place_t, std::initializer_list<U> il, Args &&... args)
      : impl_base(in_place, il, std::forward<Args>(args)...),
        ctor_base(detail::default_constructor_tag{}) {}

  /// \group unexpected_ctor
  /// \synopsis EXPLICIT constexpr expected(const unexpected<G> &e);
  template <class G = E,
            detail::enable_if_t<std::is_constructible<E, const G &>::value> * =
                nullptr,
            detail::enable_if_t<!std::is_convertible<const G &, E>::value> * =
                nullptr>
  explicit constexpr expected(const unexpected<G> &e)
      : impl_base(unexpect, e.value()),
        ctor_base(detail::default_constructor_tag{}) {}

  /// \exclude
  template <
      class G = E,
      detail::enable_if_t<std::is_constructible<E, const G &>::value> * =
          nullptr,
      detail::enable_if_t<std::is_convertible<const G &, E>::value> * = nullptr>
  constexpr expected(unexpected<G> const &e)
      : impl_base(unexpect, e.value()),
        ctor_base(detail::default_constructor_tag{}) {}

  /// \group unexpected_ctor
  /// \synopsis EXPLICIT constexpr expected(unexpected<G> &&e);
  template <
      class G = E,
      detail::enable_if_t<std::is_constructible<E, G &&>::value> * = nullptr,
      detail::enable_if_t<!std::is_convertible<G &&, E>::value> * = nullptr>
  explicit constexpr expected(unexpected<G> &&e) noexcept(
      std::is_nothrow_constructible<E, G &&>::value)
      : impl_base(unexpect, std::move(e.value())),
        ctor_base(detail::default_constructor_tag{}) {}

  /// \exclude
  template <
      class G = E,
      detail::enable_if_t<std::is_constructible<E, G &&>::value> * = nullptr,
      detail::enable_if_t<std::is_convertible<G &&, E>::value> * = nullptr>
  constexpr expected(unexpected<G> &&e) noexcept(
      std::is_nothrow_constructible<E, G &&>::value)
      : impl_base(unexpect, std::move(e.value())),
        ctor_base(detail::default_constructor_tag{}) {}

  template <class... Args,
            detail::enable_if_t<std::is_constructible<E, Args &&...>::value> * =
                nullptr>
  constexpr explicit expected(unexpect_t, Args &&... args)
      : impl_base(unexpect, std::forward<Args>(args)...),
        ctor_base(detail::default_constructor_tag{}) {}

  /// \exclude
  template <class U, class... Args,
            detail::enable_if_t<std::is_constructible<
                E, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  constexpr explicit expected(unexpect_t, std::initializer_list<U> il,
                              Args &&... args)
      : impl_base(unexpect, il, std::forward<Args>(args)...),
        ctor_base(detail::default_constructor_tag{}) {}

  // TODO SFINAE
  template <class U, class G,
            detail::enable_if_t<!(std::is_convertible<U const &, T>::value &&
                                  std::is_convertible<G const &, E>::value)> * =
                nullptr>
  explicit constexpr expected(const expected<U, G> &rhs)
      : ctor_base(detail::default_constructor_tag{}) {
    if (rhs.has_value()) {
      ::new (valptr()) T(*rhs);
    } else {
      ::new (errptr()) unexpected_type(unexpected<E>(rhs.error()));
    }
  }

  // TODO SFINAE
  /// \exclude
  template <class U, class G,
            detail::enable_if_t<(std::is_convertible<U const &, T>::value &&
                                 std::is_convertible<G const &, E>::value)> * =
                nullptr>
  constexpr expected(const expected<U, G> &rhs)
      : ctor_base(detail::default_constructor_tag{}) {
    if (rhs.has_value()) {
      ::new (valptr()) T(*rhs);
    } else {
      ::new (errptr()) unexpected_type(unexpected<E>(rhs.error()));
    }
  }

  // TODO SFINAE
  template <
      class U, class G,
      detail::enable_if_t<!(std::is_convertible<U &&, T>::value &&
                            std::is_convertible<G &&, E>::value)> * = nullptr>
  explicit constexpr expected(expected<U, G> &&rhs)
      : ctor_base(detail::default_constructor_tag{}) {
    if (rhs.has_value()) {
      ::new (valptr()) T(std::move(*rhs));
    } else {
      ::new (errptr()) unexpected_type(unexpected<E>(std::move(rhs.error())));
    }
  }

  // TODO SFINAE
  /// \exclude
  template <
      class U, class G,
      detail::enable_if_t<(std::is_convertible<U &&, T>::value &&
                           std::is_convertible<G &&, E>::value)> * = nullptr>
  constexpr expected(expected<U, G> &&rhs)
      : ctor_base(detail::default_constructor_tag{}) {
    if (rhs.has_value()) {
      ::new (valptr()) T(std::move(*rhs));
    } else {
      ::new (errptr()) unexpected_type(unexpected<E>(std::move(rhs.error())));
    }
  }

  // TODO SFINAE
  template <class U = T, detail::enable_if_t<
                             !std::is_convertible<U &&, T>::value> * = nullptr>
  explicit constexpr expected(U &&v) : expected(in_place, std::forward<U>(v)) {}

  // TODO SFINAE
  /// \exclude
  template <class U = T, detail::enable_if_t<
                             std::is_convertible<U &&, T>::value> * = nullptr>
  constexpr expected(U &&v) : expected(in_place, std::forward<U>(v)) {}

  template <
      class U = T,
      detail::enable_if_t<
          (!std::is_same<expected<T, E>, 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 &&
           std::is_nothrow_move_constructible<E>::value)> * = nullptr,
      detail::enable_if_t<std::is_nothrow_constructible<T, U &&>::value> * =
          nullptr>
  expected &operator=(U &&v) {
    if (has_value()) {
      val() = std::forward<U>(v);
    } else {
      err().~unexpected<E>();
      ::new (valptr()) T(std::forward<U>(v));
      this->m_has_val = true;
    }

    return *this;
  }

  /// \exclude
  template <
      class U = T,
      detail::enable_if_t<
          (!std::is_same<expected<T, E>, 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 &&
           std::is_nothrow_move_constructible<E>::value)> * = nullptr,
      detail::enable_if_t<!std::is_nothrow_constructible<T, U &&>::value> * =
          nullptr>
  expected &operator=(U &&v) {
    if (has_value()) {
      val() = std::forward<U>(v);
    } else {
      auto tmp = std::move(err());
      err().~unexpected<E>();
      try {
        ::new (valptr()) T(std::move(v));
        this->m_has_val = true;
      } catch (...) {
        err() = std::move(tmp);
        throw;
      }
    }

    return *this;
  }

  template <class G = E,
            detail::enable_if_t<std::is_nothrow_copy_constructible<G>::value &&
                                std::is_assignable<G &, G>::value> * = nullptr>
  expected &operator=(const unexpected<G> &rhs) {
    if (!has_value()) {
      err() = rhs;
    } else {
      val().~T();
      ::new (errptr()) unexpected<E>(rhs);
      this->m_has_val = false;
    }

    return *this;
  }

  template <class G = E,
            detail::enable_if_t<std::is_nothrow_move_constructible<G>::value &&
                                std::is_move_assignable<G>::value> * = nullptr>
  expected &operator=(unexpected<G> &&rhs) noexcept {
    if (!has_value()) {
      err() = std::move(rhs);
    } else {
      val().~T();
      ::new (errptr()) unexpected<E>(std::move(rhs));
      this->m_has_val = false;
    }

    return *this;
  }

  template <class... Args, detail::enable_if_t<std::is_nothrow_constructible<
                               T, Args &&...>::value> * = nullptr>
  void emplace(Args &&... args) {
    if (has_value()) {
      val() = T(std::forward<Args>(args)...);
    } else {
      err().~unexpected<E>();
      ::new (valptr()) T(std::forward<Args>(args)...);
      this->m_has_val = true;
    }
  }

  /// \exclude
  template <class... Args, detail::enable_if_t<!std::is_nothrow_constructible<
                               T, Args &&...>::value> * = nullptr>
  void emplace(Args &&... args) {
    if (has_value()) {
      val() = T(std::forward<Args>(args)...);
    } else {
      auto tmp = std::move(err());
      err().~unexpected<E>();

      try {
        ::new (valptr()) T(std::forward<Args>(args)...);
        this->m_has_val = true;
      } catch (...) {
        err() = std::move(tmp);
        throw;
      }
    }
  }

  template <class U, class... Args,
            detail::enable_if_t<std::is_nothrow_constructible<
                T, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  void emplace(std::initializer_list<U> il, Args &&... args) {
    if (has_value()) {
      T t(il, std::forward<Args>(args)...);
      val() = std::move(t);
    } else {
      err().~unexpected<E>();
      ::new (valptr()) T(il, std::forward<Args>(args)...);
      this->m_has_val = true;
    }
  }

  /// \exclude
  template <class U, class... Args,
            detail::enable_if_t<!std::is_nothrow_constructible<
                T, std::initializer_list<U> &, Args &&...>::value> * = nullptr>
  void emplace(std::initializer_list<U> il, Args &&... args) {
    if (has_value()) {
      T t(il, std::forward<Args>(args)...);
      val() = std::move(t);
    } else {
      auto tmp = std::move(err());
      err().~unexpected<E>();

      try {
        ::new (valptr()) T(il, std::forward<Args>(args)...);
        this->m_has_val = true;
      } catch (...) {
        err() = std::move(tmp);
        throw;
      }
    }
  }

  // TODO SFINAE
  void swap(expected &rhs) noexcept(
      std::is_nothrow_move_constructible<T>::value &&noexcept(
          swap(std::declval<T &>(), std::declval<T &>())) &&
      std::is_nothrow_move_constructible<E>::value &&
      noexcept(swap(std::declval<E &>(), std::declval<E &>()))) {
    if (has_value() && rhs.has_value()) {
      using std::swap;
      swap(val(), rhs.val());
    } else if (!has_value() && rhs.has_value()) {
      using std::swap;
      swap(err(), rhs.err());
    } else if (has_value()) {
      auto temp = std::move(rhs.err());
      ::new (rhs.valptr()) T(val());
      ::new (errptr()) unexpected_type(std::move(temp));
      std::swap(this->m_has_val, rhs.m_has_val);
    } else {
      auto temp = std::move(this->err());
      ::new (valptr()) T(rhs.val());
      ::new (errptr()) unexpected_type(std::move(temp));
      std::swap(this->m_has_val, rhs.m_has_val);
    }
  }

  constexpr const T *operator->() const { return valptr(); }
  constexpr T *operator->() { return valptr(); }
  constexpr const T &operator*() const & { return val(); }
  constexpr T &operator*() & { return val(); }
  constexpr const T &&operator*() const && { return std::move(val()); }
  constexpr T &&operator*() && { return std::move(val()); }
  constexpr explicit operator bool() const noexcept { return this->m_has_val; }
  constexpr bool has_value() const noexcept { return this->m_has_val; }
  constexpr const T &value() const & {
    if (!has_value())
      throw bad_expected_access<E>(err());
    return val();
  }
  constexpr T &value() & {
    if (!has_value())
      throw bad_expected_access<E>(err());
    return val();
  }
  constexpr const T &&value() const && {
    if (!has_value())
      throw bad_expected_access<E>(err());
    return std::move(val());
  }
  constexpr T &&value() && {
    if (!has_value())
      throw bad_expected_access<E>(err());
    return std::move(val());
  }
  constexpr const E &error() const & { return err().value(); }
  constexpr E &error() & { return err().value(); }
  constexpr const E &&error() const && { return std::move(err().value()); }
  constexpr E &&error() && { return std::move(err().value()); }
  template <class U> constexpr T value_or(U &&v) const & {
    static_assert(std::is_copy_constructible<T>::value &&
                      std::is_convertible<U &&, T>::value,
                  "T must be copy-constructible and convertible to from U&&");
    return bool(*this) ? **this : static_cast<T>(std::forward<U>(v));
  }
  template <class U> T value_or(U &&v) && {
    static_assert(std::is_move_constructible<T>::value &&
                      std::is_convertible<U &&, T>::value,
                  "T must be move-constructible and convertible to from U&&");
    return bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(v));
  }
};

/// \exclude
namespace detail {
template <class Exp> using err_t = typename detail::decay_t<Exp>::error_type;
template <class Exp, class Ret> using ret_t = expected<Ret, err_t<Exp>>;

#ifdef TL_EXPECTED_CXX14
template <class Exp, class F,
          class Ret = decltype(detail::invoke(std::declval<F>(),
                                              *std::declval<Exp>())),
          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
constexpr auto map_impl(Exp &&exp, F &&f) {
  using result = ret_t<Exp, Ret>;
  return exp.has_value() ? result(detail::invoke(std::forward<F>(f),
                                                 *std::forward<Exp>(exp)))
                         : result(unexpect, std::forward<Exp>(exp).error());
}

template <class Exp, class F,
          class Ret = decltype(detail::invoke(std::declval<F>(),
                                              *std::declval<Exp>())),
          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
auto map_impl(Exp &&exp, F &&f) {
  using result = expected<monostate, err_t<Exp>>;
  if (exp.has_value()) {
    detail::invoke(std::forward<F>(f), *std::forward<Exp>(exp));
    return result(monostate{});
  }

  return result(unexpect, std::forward<Exp>(exp).error());
}
#else
template <class Exp, class F,
          class Ret = decltype(detail::invoke(std::declval<F>(),
                                              *std::declval<Exp>())),
          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>

constexpr auto map_impl(Exp &&exp, F &&f) -> ret_t<Exp, Ret> {
  using result = ret_t<Exp, Ret>;

  return exp.has_value() ? result(detail::invoke(std::forward<F>(f),
                                                 *std::forward<Exp>(exp)))
                         : result(unexpect, std::forward<Exp>(exp).error());
}

template <class Exp, class F,
          class Ret = decltype(detail::invoke(std::declval<F>(),
                                              *std::declval<Exp>())),
          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>

auto map_impl(Exp &&exp, F &&f) -> expected<monostate, err_t<Exp>> {
  if (exp.has_value()) {
    detail::invoke(std::forward<F>(f), *std::forward<Exp>(exp));
    return tl::monostate{};
  }

  return unexpected<err_t<Exp>>(std::forward<Exp>(exp).error());
}
#endif

#if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
    !defined(TL_EXPECTED_GCC54)
template <class Exp, class F,
          class Ret = decltype(detail::invoke(std::declval<F>(),
                                              *std::declval<Exp>()))>
constexpr auto map_error_impl(Exp &&exp, F &&f) {
  using result = ret_t<Exp, Ret>;
  return exp.has_value()
             ? result(*std::forward<Exp>(exp))
             : result(unexpect, detail::invoke(std::forward<F>(f),
                                               std::forward<Exp>(exp).error()));
}
#else
template <class Exp, class F,
          class Ret = decltype(detail::invoke(std::declval<F>(),
                                              *std::declval<Exp>()))>
constexpr auto map_error_impl(Exp &&exp, F &&f) -> ret_t<Exp, Ret> {
  using result = ret_t<Exp, Ret>;

  return exp.has_value()
             ? result(*std::forward<Exp>(exp))
             : result(unexpect, detail::invoke(std::forward<F>(f),
                                               std::forward<Exp>(exp).error()));
}
#endif

} // namespace detail

template <class T, class E, class U, class F>
constexpr bool operator==(const expected<T, E> &lhs,
                          const expected<U, F> &rhs) {
  return (lhs.has_value() != rhs.has_value())
             ? false
             : (!lhs.has_value() ? lhs.error() == rhs.error() : *lhs == *rhs);
}
template <class T, class E, class U, class F>
constexpr bool operator!=(const expected<T, E> &lhs,
                          const expected<U, F> &rhs) {
  return (lhs.has_value() != rhs.has_value())
             ? true
             : (!lhs.has_value() ? lhs.error() != rhs.error() : *lhs != *rhs);
}
template <class T, class E, class U, class F>
constexpr bool operator<(const expected<T, E> &lhs, const expected<U, F> &rhs) {
  return (lhs.has_value() != rhs.has_value())
             ? (!lhs.has_value() ? true : false)
             : (!lhs.has_value() ? lhs.error() < rhs.error() : *lhs < *rhs);
}
template <class T, class E, class U, class F>
constexpr bool operator>(const expected<T, E> &lhs, const expected<U, F> &rhs) {
  return (lhs.has_value() != rhs.has_value())
             ? (!lhs.has_value() ? false : true)
             : (!lhs.has_value() ? lhs.error() > rhs.error() : *lhs > *rhs);
}
template <class T, class E, class U, class F>
constexpr bool operator<=(const expected<T, E> &lhs,
                          const expected<U, F> &rhs) {
  return (lhs.has_value() != rhs.has_value())
             ? (!lhs.has_value() ? false : true)
             : (!lhs.has_value() ? lhs.error() <= rhs.error() : *lhs <= *rhs);
}
template <class T, class E, class U, class F>
constexpr bool operator>=(const expected<T, E> &lhs,
                          const expected<U, F> &rhs) {
  return (lhs.has_value() != rhs.has_value())
             ? (!lhs.has_value() ? true : false)
             : (!lhs.has_value() ? lhs.error() >= rhs.error() : *lhs >= *rhs);
}

template <class T, class E, class U>
constexpr bool operator==(const expected<T, E> &x, const U &v) {
  return x.has_value() ? *x == v : false;
}
template <class T, class E, class U>
constexpr bool operator==(const U &v, const expected<T, E> &x) {
  return x.has_value() ? *x == v : false;
}
template <class T, class E, class U>
constexpr bool operator!=(const expected<T, E> &x, const U &v) {
  return x.has_value() ? *x != v : true;
}
template <class T, class E, class U>
constexpr bool operator!=(const U &v, const expected<T, E> &x) {
  return x.has_value() ? *x != v : true;
}
template <class T, class E, class U>
constexpr bool operator<(const expected<T, E> &x, const U &v) {
  return x.has_value() ? *x < v : true;
}
template <class T, class E, class U>
constexpr bool operator<(const U &v, const expected<T, E> &x) {
  return x.has_value() ? v < *x : false;
}
template <class T, class E, class U>
constexpr bool operator<=(const expected<T, E> &x, const U &v) {
  return x.has_value() ? *x <= v : true;
}
template <class T, class E, class U>
constexpr bool operator<=(const U &v, const expected<T, E> &x) {
  return x.has_value() ? v <= *x : false;
}
template <class T, class E, class U>
constexpr bool operator>(const expected<T, E> &x, const U &v) {
  return x.has_value() ? *x > v : false;
}
template <class T, class E, class U>
constexpr bool operator>(const U &v, const expected<T, E> &x) {
  return x.has_value() ? v > *x : true;
}
template <class T, class E, class U>
constexpr bool operator>=(const expected<T, E> &x, const U &v) {
  return x.has_value() ? *x >= v : false;
}
template <class T, class E, class U>
constexpr bool operator>=(const U &v, const expected<T, E> &x) {
  return x.has_value() ? v >= *x : true;
}

template <class T, class E>
constexpr bool operator==(const expected<T, E> &x, const unexpected<E> &e) {
  return x.has_value() ? true : x.error() == e.value();
}
template <class T, class E>
constexpr bool operator==(const unexpected<E> &e, const expected<T, E> &x) {
  return x.has_value() ? true : x.error() == e.value();
}
template <class T, class E>
constexpr bool operator!=(const expected<T, E> &x, const unexpected<E> &e) {
  return x.has_value() ? false : x.error() != e.value();
}
template <class T, class E>
constexpr bool operator!=(const unexpected<E> &e, const expected<T, E> &x) {
  return x.has_value() ? false : x.error() != e.value();
}
template <class T, class E>
constexpr bool operator<(const expected<T, E> &x, const unexpected<E> &e) {
  return false;
}
template <class T, class E>
constexpr bool operator<(const unexpected<E> &e, const expected<T, E> &x) {
  return x.has_value();
}
template <class T, class E>
constexpr bool operator<=(const expected<T, E> &x, const unexpected<E> &e) {
  return !x.has_value();
}
template <class T, class E>
constexpr bool operator<=(const unexpected<E> &e, const expected<T, E> &x) {
  return true;
}
template <class T, class E>
constexpr bool operator>(const expected<T, E> &x, const unexpected<E> &e) {
  return x.has_value();
}
template <class T, class E>
constexpr bool operator>(const unexpected<E> &e, const expected<T, E> &x) {
  return false;
}
template <class T, class E>
constexpr bool operator>=(const expected<T, E> &x, const unexpected<E> &e) {
  return true;
}
template <class T, class E>
constexpr bool operator>=(const unexpected<E> &e, const expected<T, E> &x) {
  return !x.has_value();
}

// TODO is_swappable
template <class T, class E,
          detail::enable_if_t<std::is_move_constructible<T>::value &&
                              std::is_move_constructible<E>::value> * = nullptr>
void swap(expected<T, E> &lhs,
          expected<T, E> &rhs) noexcept(noexcept(lhs.swap(rhs))) {
  lhs.swap(rhs);
}
} // namespace tl

#endif