gccrs: optional: Import tl's optional type

This is directly adapted from https://github.com/TartanLlama/optional.
This class is tested for C++11 and on GCC 4.8, which is our target mimimum
GCC version.

This optional class provides multiple advantages over our existing
implementation of Optional<T>:

- Easier API (no static constructors)
- Better method chaining (and_then, map...)
- Harsher semantics (no assign-through on Optional<T&>)

Over the std::optional type present in C++'s standard library, this type offers:

- Optional references without going through std::reference_wrapper
- Better method chaining

For these reasons, I am in favor of importing it directly rather than
adapt C++14's std::optional type to work with C++11 and GCC 4.8.

gcc/rust/ChangeLog:

	* util/optional.h: New file.

1 files changed, 2062 insertions, 0 deletions

diff --git a/gcc/rust/util/optional.h b/gcc/rust/util/optional.h
new file mode 100644
index 0000000..2b48c45d
--- /dev/null
+++ b/gcc/rust/util/optional.h
@@ -0,0 +1,2062 @@
+
+///
+// optional - An implementation of std::optional with extensions
+// Written in 2017 by Sy Brand (tartanllama@gmail.com, @TartanLlama)
+//
+// Documentation available at https://tl.tartanllama.xyz/
+//
+// To the extent possible under law, the author(s) have dedicated all
+// copyright and related and neighboring rights to this software to the
+// public domain worldwide. This software is distributed without any warranty.
+//
+// You should have received a copy of the CC0 Public Domain Dedication
+// along with this software. If not, see
+// <http://creativecommons.org/publicdomain/zero/1.0/>.
+///
+
+#ifndef TL_OPTIONAL_HPP
+#define TL_OPTIONAL_HPP
+
+#define TL_OPTIONAL_VERSION_MAJOR 1
+#define TL_OPTIONAL_VERSION_MINOR 1
+#define TL_OPTIONAL_VERSION_PATCH 0
+
+#include <exception>
+#include <functional>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#if (defined(_MSC_VER) && _MSC_VER == 1900)
+#define TL_OPTIONAL_MSVC2015
+#endif
+
+#if (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 &&              \
+     !defined(__clang__))
+#define TL_OPTIONAL_GCC49
+#endif
+
+#if (defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 4 &&              \
+     !defined(__clang__))
+#define TL_OPTIONAL_GCC54
+#endif
+
+#if (defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 5 &&              \
+     !defined(__clang__))
+#define TL_OPTIONAL_GCC55
+#endif
+
+#if (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 &&              \
+     !defined(__clang__))
+// GCC < 5 doesn't support overloading on const&& for member functions
+#define TL_OPTIONAL_NO_CONSTRR
+
+// GCC < 5 doesn't support some standard C++11 type traits
+#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)                                     \
+  std::has_trivial_copy_constructor<T>::value
+#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) std::has_trivial_copy_assign<T>::value
+
+// This one will be different for GCC 5.7 if it's ever supported
+#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>::value
+
+// GCC 5 < v < 8 has a bug in is_trivially_copy_constructible which breaks std::vector
+// for non-copyable types
+#elif (defined(__GNUC__) && __GNUC__ < 8 &&                                                \
+     !defined(__clang__))
+#ifndef TL_GCC_LESS_8_TRIVIALLY_COPY_CONSTRUCTIBLE_MUTEX
+#define TL_GCC_LESS_8_TRIVIALLY_COPY_CONSTRUCTIBLE_MUTEX
+namespace tl {
+  namespace detail {
+      template<class T>
+      struct is_trivially_copy_constructible : std::is_trivially_copy_constructible<T>{};
+#ifdef _GLIBCXX_VECTOR
+      template<class T, class A>
+      struct is_trivially_copy_constructible<std::vector<T,A>>
+          : std::is_trivially_copy_constructible<T>{};
+#endif      
+  }
+}
+#endif
+
+#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)                                     \
+    tl::detail::is_trivially_copy_constructible<T>::value
+#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T)                                        \
+  std::is_trivially_copy_assignable<T>::value
+#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>::value
+#else
+#define TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)                                     \
+  std::is_trivially_copy_constructible<T>::value
+#define TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T)                                        \
+  std::is_trivially_copy_assignable<T>::value
+#define TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>::value
+#endif
+
+#if __cplusplus > 201103L
+#define TL_OPTIONAL_CXX14
+#endif
+
+// constexpr implies const in C++11, not C++14
+#if (__cplusplus == 201103L || defined(TL_OPTIONAL_MSVC2015) ||                \
+     defined(TL_OPTIONAL_GCC49))
+#define TL_OPTIONAL_11_CONSTEXPR
+#else
+#define TL_OPTIONAL_11_CONSTEXPR constexpr
+#endif
+
+namespace tl {
+#ifndef TL_MONOSTATE_INPLACE_MUTEX
+#define TL_MONOSTATE_INPLACE_MUTEX
+/// Used to represent an optional with no data; essentially a bool
+class monostate {};
+
+///  A tag type to tell optional to construct its value in-place
+struct in_place_t {
+  explicit in_place_t() = default;
+};
+/// A tag to tell optional to construct its value in-place
+static constexpr in_place_t in_place{};
+#endif
+
+template <class T> class optional;
+
+namespace detail {
+#ifndef TL_TRAITS_MUTEX
+#define TL_TRAITS_MUTEX
+// C++14-style aliases for brevity
+template <class T> using remove_const_t = typename std::remove_const<T>::type;
+template <class T>
+using remove_reference_t = typename std::remove_reference<T>::type;
+template <class T> using decay_t = typename std::decay<T>::type;
+template <bool E, class T = void>
+using enable_if_t = typename std::enable_if<E, T>::type;
+template <bool B, class T, class F>
+using conditional_t = typename std::conditional<B, T, F>::type;
+
+// std::conjunction from C++17
+template <class...> struct conjunction : std::true_type {};
+template <class B> struct conjunction<B> : B {};
+template <class B, class... Bs>
+struct conjunction<B, Bs...>
+    : std::conditional<bool(B::value), conjunction<Bs...>, B>::type {};
+
+#if defined(_LIBCPP_VERSION) && __cplusplus == 201103L
+#define TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
+#endif
+
+// In C++11 mode, there's an issue in libc++'s std::mem_fn
+// which results in a hard-error when using it in a noexcept expression
+// in some cases. This is a check to workaround the common failing case.
+#ifdef TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
+template <class T> struct is_pointer_to_non_const_member_func : std::false_type{};
+template <class T, class Ret, class... Args>
+struct is_pointer_to_non_const_member_func<Ret (T::*) (Args...)> : std::true_type{};
+template <class T, class Ret, class... Args>
+struct is_pointer_to_non_const_member_func<Ret (T::*) (Args...)&> : std::true_type{};
+template <class T, class Ret, class... Args>
+struct is_pointer_to_non_const_member_func<Ret (T::*) (Args...)&&> : std::true_type{};        
+template <class T, class Ret, class... Args>
+struct is_pointer_to_non_const_member_func<Ret (T::*) (Args...) volatile> : std::true_type{};
+template <class T, class Ret, class... Args>
+struct is_pointer_to_non_const_member_func<Ret (T::*) (Args...) volatile&> : std::true_type{};
+template <class T, class Ret, class... Args>
+struct is_pointer_to_non_const_member_func<Ret (T::*) (Args...) volatile&&> : std::true_type{};        
+
+template <class T> struct is_const_or_const_ref : std::false_type{};
+template <class T> struct is_const_or_const_ref<T const&> : std::true_type{};
+template <class T> struct is_const_or_const_ref<T const> : std::true_type{};    
+#endif
+
+// std::invoke from C++17
+// https://stackoverflow.com/questions/38288042/c11-14-invoke-workaround
+template <typename Fn, typename... Args,
+#ifdef TL_TRAITS_LIBCXX_MEM_FN_WORKAROUND
+          typename = enable_if_t<!(is_pointer_to_non_const_member_func<Fn>::value 
+                                 && is_const_or_const_ref<Args...>::value)>, 
+#endif
+          typename = enable_if_t<std::is_member_pointer<decay_t<Fn>>::value>,
+          int = 0>
+constexpr auto invoke(Fn &&f, Args &&... args) noexcept(
+    noexcept(std::mem_fn(f)(std::forward<Args>(args)...)))
+    -> decltype(std::mem_fn(f)(std::forward<Args>(args)...)) {
+  return std::mem_fn(f)(std::forward<Args>(args)...);
+}
+
+template <typename Fn, typename... Args,
+          typename = enable_if_t<!std::is_member_pointer<decay_t<Fn>>::value>>
+constexpr auto invoke(Fn &&f, Args &&... args) noexcept(
+    noexcept(std::forward<Fn>(f)(std::forward<Args>(args)...)))
+    -> decltype(std::forward<Fn>(f)(std::forward<Args>(args)...)) {
+  return std::forward<Fn>(f)(std::forward<Args>(args)...);
+}
+
+// std::invoke_result from C++17
+template <class F, class, class... Us> struct invoke_result_impl;
+
+template <class F, class... Us>
+struct invoke_result_impl<
+    F, decltype(detail::invoke(std::declval<F>(), std::declval<Us>()...), void()),
+    Us...> {
+  using type = decltype(detail::invoke(std::declval<F>(), std::declval<Us>()...));
+};
+
+template <class F, class... Us>
+using invoke_result = invoke_result_impl<F, void, Us...>;
+
+template <class F, class... Us>
+using invoke_result_t = typename invoke_result<F, Us...>::type;
+
+#if defined(_MSC_VER) && _MSC_VER <= 1900
+// TODO make a version which works with MSVC 2015
+template <class T, class U = T> struct is_swappable : std::true_type {};
+
+template <class T, class U = T> struct is_nothrow_swappable : std::true_type {};
+#else
+// https://stackoverflow.com/questions/26744589/what-is-a-proper-way-to-implement-is-swappable-to-test-for-the-swappable-concept
+namespace swap_adl_tests {
+// if swap ADL finds this then it would call std::swap otherwise (same
+// signature)
+struct tag {};
+
+template <class T> tag swap(T &, T &);
+template <class T, std::size_t N> tag swap(T (&a)[N], T (&b)[N]);
+
+// helper functions to test if an unqualified swap is possible, and if it
+// becomes std::swap
+template <class, class> std::false_type can_swap(...) noexcept(false);
+template <class T, class U,
+          class = decltype(swap(std::declval<T &>(), std::declval<U &>()))>
+std::true_type can_swap(int) noexcept(noexcept(swap(std::declval<T &>(),
+                                                    std::declval<U &>())));
+
+template <class, class> std::false_type uses_std(...);
+template <class T, class U>
+std::is_same<decltype(swap(std::declval<T &>(), std::declval<U &>())), tag>
+uses_std(int);
+
+template <class T>
+struct is_std_swap_noexcept
+    : std::integral_constant<bool,
+                             std::is_nothrow_move_constructible<T>::value &&
+                                 std::is_nothrow_move_assignable<T>::value> {};
+
+template <class T, std::size_t N>
+struct is_std_swap_noexcept<T[N]> : is_std_swap_noexcept<T> {};
+
+template <class T, class U>
+struct is_adl_swap_noexcept
+    : std::integral_constant<bool, noexcept(can_swap<T, U>(0))> {};
+} // namespace swap_adl_tests
+
+template <class T, class U = T>
+struct is_swappable
+    : std::integral_constant<
+          bool,
+          decltype(detail::swap_adl_tests::can_swap<T, U>(0))::value &&
+              (!decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value ||
+               (std::is_move_assignable<T>::value &&
+                std::is_move_constructible<T>::value))> {};
+
+template <class T, std::size_t N>
+struct is_swappable<T[N], T[N]>
+    : std::integral_constant<
+          bool,
+          decltype(detail::swap_adl_tests::can_swap<T[N], T[N]>(0))::value &&
+              (!decltype(
+                   detail::swap_adl_tests::uses_std<T[N], T[N]>(0))::value ||
+               is_swappable<T, T>::value)> {};
+
+template <class T, class U = T>
+struct is_nothrow_swappable
+    : std::integral_constant<
+          bool,
+          is_swappable<T, U>::value &&
+              ((decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value
+                    &&detail::swap_adl_tests::is_std_swap_noexcept<T>::value) ||
+               (!decltype(detail::swap_adl_tests::uses_std<T, U>(0))::value &&
+                    detail::swap_adl_tests::is_adl_swap_noexcept<T,
+                                                                 U>::value))> {
+};
+#endif
+#endif
+
+// std::void_t from C++17
+template <class...> struct voider { using type = void; };
+template <class... Ts> using void_t = typename voider<Ts...>::type;
+
+// Trait for checking if a type is a tl::optional
+template <class T> struct is_optional_impl : std::false_type {};
+template <class T> struct is_optional_impl<optional<T>> : std::true_type {};
+template <class T> using is_optional = is_optional_impl<decay_t<T>>;
+
+// Change void to tl::monostate
+template <class U>
+using fixup_void = conditional_t<std::is_void<U>::value, monostate, U>;
+
+template <class F, class U, class = invoke_result_t<F, U>>
+using get_map_return = optional<fixup_void<invoke_result_t<F, U>>>;
+
+// Check if invoking F for some Us returns void
+template <class F, class = void, class... U> struct returns_void_impl;
+template <class F, class... U>
+struct returns_void_impl<F, void_t<invoke_result_t<F, U...>>, U...>
+    : std::is_void<invoke_result_t<F, U...>> {};
+template <class F, class... U>
+using returns_void = returns_void_impl<F, void, U...>;
+
+template <class T, class... U>
+using enable_if_ret_void = enable_if_t<returns_void<T &&, U...>::value>;
+
+template <class T, class... U>
+using disable_if_ret_void = enable_if_t<!returns_void<T &&, U...>::value>;
+
+template <class T, class U>
+using enable_forward_value =
+    detail::enable_if_t<std::is_constructible<T, U &&>::value &&
+                        !std::is_same<detail::decay_t<U>, in_place_t>::value &&
+                        !std::is_same<optional<T>, detail::decay_t<U>>::value>;
+
+template <class T, class U, class Other>
+using enable_from_other = detail::enable_if_t<
+    std::is_constructible<T, Other>::value &&
+    !std::is_constructible<T, optional<U> &>::value &&
+    !std::is_constructible<T, optional<U> &&>::value &&
+    !std::is_constructible<T, const optional<U> &>::value &&
+    !std::is_constructible<T, const optional<U> &&>::value &&
+    !std::is_convertible<optional<U> &, T>::value &&
+    !std::is_convertible<optional<U> &&, T>::value &&
+    !std::is_convertible<const optional<U> &, T>::value &&
+    !std::is_convertible<const optional<U> &&, T>::value>;
+
+template <class T, class U>
+using enable_assign_forward = detail::enable_if_t<
+    !std::is_same<optional<T>, detail::decay_t<U>>::value &&
+    !detail::conjunction<std::is_scalar<T>,
+                         std::is_same<T, detail::decay_t<U>>>::value &&
+    std::is_constructible<T, U>::value && std::is_assignable<T &, U>::value>;
+
+template <class T, class U, class Other>
+using enable_assign_from_other = detail::enable_if_t<
+    std::is_constructible<T, Other>::value &&
+    std::is_assignable<T &, Other>::value &&
+    !std::is_constructible<T, optional<U> &>::value &&
+    !std::is_constructible<T, optional<U> &&>::value &&
+    !std::is_constructible<T, const optional<U> &>::value &&
+    !std::is_constructible<T, const optional<U> &&>::value &&
+    !std::is_convertible<optional<U> &, T>::value &&
+    !std::is_convertible<optional<U> &&, T>::value &&
+    !std::is_convertible<const optional<U> &, T>::value &&
+    !std::is_convertible<const optional<U> &&, T>::value &&
+    !std::is_assignable<T &, optional<U> &>::value &&
+    !std::is_assignable<T &, optional<U> &&>::value &&
+    !std::is_assignable<T &, const optional<U> &>::value &&
+    !std::is_assignable<T &, const optional<U> &&>::value>;
+
+// The storage base manages the actual storage, and correctly propagates
+// trivial destruction from T. This case is for when T is not trivially
+// destructible.
+template <class T, bool = ::std::is_trivially_destructible<T>::value>
+struct optional_storage_base {
+  TL_OPTIONAL_11_CONSTEXPR optional_storage_base() noexcept
+      : m_dummy(), m_has_value(false) {}
+
+  template <class... U>
+  TL_OPTIONAL_11_CONSTEXPR optional_storage_base(in_place_t, U &&... u)
+      : m_value(std::forward<U>(u)...), m_has_value(true) {}
+
+  ~optional_storage_base() {
+    if (m_has_value) {
+      m_value.~T();
+      m_has_value = false;
+    }
+  }
+
+  struct dummy {};
+  union {
+    dummy m_dummy;
+    T m_value;
+  };
+
+  bool m_has_value;
+};
+
+// This case is for when T is trivially destructible.
+template <class T> struct optional_storage_base<T, true> {
+  TL_OPTIONAL_11_CONSTEXPR optional_storage_base() noexcept
+      : m_dummy(), m_has_value(false) {}
+
+  template <class... U>
+  TL_OPTIONAL_11_CONSTEXPR optional_storage_base(in_place_t, U &&... u)
+      : m_value(std::forward<U>(u)...), m_has_value(true) {}
+
+  // No destructor, so this class is trivially destructible
+
+  struct dummy {};
+  union {
+    dummy m_dummy;
+    T m_value;
+  };
+
+  bool m_has_value = false;
+};
+
+// This base class provides some handy member functions which can be used in
+// further derived classes
+template <class T> struct optional_operations_base : optional_storage_base<T> {
+  using optional_storage_base<T>::optional_storage_base;
+
+  void hard_reset() noexcept {
+    get().~T();
+    this->m_has_value = false;
+  }
+
+  template <class... Args> void construct(Args &&... args) {
+    new (std::addressof(this->m_value)) T(std::forward<Args>(args)...);
+    this->m_has_value = true;
+  }
+
+  template <class Opt> void assign(Opt &&rhs) {
+    if (this->has_value()) {
+      if (rhs.has_value()) {
+        this->m_value = std::forward<Opt>(rhs).get();
+      } else {
+        this->m_value.~T();
+        this->m_has_value = false;
+      }
+    }
+
+    else if (rhs.has_value()) {
+      construct(std::forward<Opt>(rhs).get());
+    }
+  }
+
+  bool has_value() const { return this->m_has_value; }
+
+  TL_OPTIONAL_11_CONSTEXPR T &get() & { return this->m_value; }
+  TL_OPTIONAL_11_CONSTEXPR const T &get() const & { return this->m_value; }
+  TL_OPTIONAL_11_CONSTEXPR T &&get() && { return std::move(this->m_value); }
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  constexpr const T &&get() const && { return std::move(this->m_value); }
+#endif
+};
+
+// This class manages conditionally having a trivial copy constructor
+// This specialization is for when T is trivially copy constructible
+template <class T, bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)>
+struct optional_copy_base : optional_operations_base<T> {
+  using optional_operations_base<T>::optional_operations_base;
+};
+
+// This specialization is for when T is not trivially copy constructible
+template <class T>
+struct optional_copy_base<T, false> : optional_operations_base<T> {
+  using optional_operations_base<T>::optional_operations_base;
+
+  optional_copy_base() = default;
+  optional_copy_base(const optional_copy_base &rhs)
+  : optional_operations_base<T>() {
+    if (rhs.has_value()) {
+      this->construct(rhs.get());
+    } else {
+      this->m_has_value = false;
+    }
+  }
+
+  optional_copy_base(optional_copy_base &&rhs) = default;
+  optional_copy_base &operator=(const optional_copy_base &rhs) = default;
+  optional_copy_base &operator=(optional_copy_base &&rhs) = default;
+};
+
+// This class manages conditionally having a trivial move constructor
+// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
+// doesn't implement an analogue to std::is_trivially_move_constructible. We
+// have to make do with a non-trivial move constructor even if T is trivially
+// move constructible
+#ifndef TL_OPTIONAL_GCC49
+template <class T, bool = std::is_trivially_move_constructible<T>::value>
+struct optional_move_base : optional_copy_base<T> {
+  using optional_copy_base<T>::optional_copy_base;
+};
+#else
+template <class T, bool = false> struct optional_move_base;
+#endif
+template <class T> struct optional_move_base<T, false> : optional_copy_base<T> {
+  using optional_copy_base<T>::optional_copy_base;
+
+  optional_move_base() = default;
+  optional_move_base(const optional_move_base &rhs) = default;
+
+  optional_move_base(optional_move_base &&rhs) noexcept(
+      std::is_nothrow_move_constructible<T>::value) {
+    if (rhs.has_value()) {
+      this->construct(std::move(rhs.get()));
+    } else {
+      this->m_has_value = false;
+    }
+  }
+  optional_move_base &operator=(const optional_move_base &rhs) = default;
+  optional_move_base &operator=(optional_move_base &&rhs) = default;
+};
+
+// This class manages conditionally having a trivial copy assignment operator
+template <class T, bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) &&
+                          TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) &&
+                          TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T)>
+struct optional_copy_assign_base : optional_move_base<T> {
+  using optional_move_base<T>::optional_move_base;
+};
+
+template <class T>
+struct optional_copy_assign_base<T, false> : optional_move_base<T> {
+  using optional_move_base<T>::optional_move_base;
+
+  optional_copy_assign_base() = default;
+  optional_copy_assign_base(const optional_copy_assign_base &rhs) = default;
+
+  optional_copy_assign_base(optional_copy_assign_base &&rhs) = default;
+  optional_copy_assign_base &operator=(const optional_copy_assign_base &rhs) {
+    this->assign(rhs);
+    return *this;
+  }
+  optional_copy_assign_base &
+  operator=(optional_copy_assign_base &&rhs) = default;
+};
+
+// This class manages conditionally having a trivial move assignment operator
+// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
+// doesn't implement an analogue to std::is_trivially_move_assignable. We have
+// to make do with a non-trivial move assignment operator even if T is trivially
+// move assignable
+#ifndef TL_OPTIONAL_GCC49
+template <class T, bool = std::is_trivially_destructible<T>::value
+                       &&std::is_trivially_move_constructible<T>::value
+                           &&std::is_trivially_move_assignable<T>::value>
+struct optional_move_assign_base : optional_copy_assign_base<T> {
+  using optional_copy_assign_base<T>::optional_copy_assign_base;
+};
+#else
+template <class T, bool = false> struct optional_move_assign_base;
+#endif
+
+template <class T>
+struct optional_move_assign_base<T, false> : optional_copy_assign_base<T> {
+  using optional_copy_assign_base<T>::optional_copy_assign_base;
+
+  optional_move_assign_base() = default;
+  optional_move_assign_base(const optional_move_assign_base &rhs) = default;
+
+  optional_move_assign_base(optional_move_assign_base &&rhs) = default;
+
+  optional_move_assign_base &
+  operator=(const optional_move_assign_base &rhs) = default;
+
+  optional_move_assign_base &
+  operator=(optional_move_assign_base &&rhs) noexcept(
+      std::is_nothrow_move_constructible<T>::value
+          &&std::is_nothrow_move_assignable<T>::value) {
+    this->assign(std::move(rhs));
+    return *this;
+  }
+};
+
+// optional_delete_ctor_base will conditionally delete copy and move
+// constructors depending on whether T is copy/move constructible
+template <class T, bool EnableCopy = std::is_copy_constructible<T>::value,
+          bool EnableMove = std::is_move_constructible<T>::value>
+struct optional_delete_ctor_base {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = default;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_ctor_base<T, true, false> {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = delete;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_ctor_base<T, false, true> {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = delete;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = default;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_ctor_base<T, false, false> {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = delete;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = delete;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+// optional_delete_assign_base will conditionally delete copy and move
+// constructors depending on whether T is copy/move constructible + assignable
+template <class T,
+          bool EnableCopy = (std::is_copy_constructible<T>::value &&
+                             std::is_copy_assignable<T>::value),
+          bool EnableMove = (std::is_move_constructible<T>::value &&
+                             std::is_move_assignable<T>::value)>
+struct optional_delete_assign_base {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_assign_base<T, true, false> {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = delete;
+};
+
+template <class T> struct optional_delete_assign_base<T, false, true> {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = delete;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_assign_base<T, false, false> {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = delete;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = delete;
+};
+
+} // namespace detail
+
+/// 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 {}
+};
+/// Represents an empty optional
+static constexpr nullopt_t nullopt{nullopt_t::do_not_use{},
+                                   nullopt_t::do_not_use{}};
+
+class bad_optional_access : public std::exception {
+public:
+  bad_optional_access() = default;
+  const char *what() const noexcept { return "Optional has no value"; }
+};
+
+/// An optional object is an object that contains the storage for another
+/// object and manages the lifetime of this contained object, if any. The
+/// contained object may be initialized after the optional object has been
+/// initialized, and may be destroyed before the optional object has been
+/// destroyed. The initialization state of the contained object is tracked by
+/// the optional object.
+template <class T>
+class optional : private detail::optional_move_assign_base<T>,
+                 private detail::optional_delete_ctor_base<T>,
+                 private detail::optional_delete_assign_base<T> {
+  using base = detail::optional_move_assign_base<T>;
+
+  static_assert(!std::is_same<T, in_place_t>::value,
+                "instantiation of optional with in_place_t is ill-formed");
+  static_assert(!std::is_same<detail::decay_t<T>, nullopt_t>::value,
+                "instantiation of optional with nullopt_t is ill-formed");
+
+public:
+// The different versions for C++14 and 11 are needed because deduced return
+// types are not SFINAE-safe. This provides better support for things like
+// generic lambdas. C.f.
+// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0826r0.html
+#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) &&               \
+    !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
+  /// Carries out some operation which returns an optional on the stored
+  /// object if there is one.
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) & {
+    using result = detail::invoke_result_t<F, T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) && {
+    using result = detail::invoke_result_t<F, T &&>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : result(nullopt);
+  }
+
+  template <class F> constexpr auto and_then(F &&f) const & {
+    using result = detail::invoke_result_t<F, const T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F> constexpr auto and_then(F &&f) const && {
+    using result = detail::invoke_result_t<F, const T &&>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : result(nullopt);
+  }
+#endif
+#else
+  /// Carries out some operation which returns an optional on the stored
+  /// object if there is one.
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &> and_then(F &&f) & {
+    using result = detail::invoke_result_t<F, T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this) : result(nullopt);
+  }
+
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &&> and_then(F &&f) && {
+    using result = detail::invoke_result_t<F, T &&>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : result(nullopt);
+  }
+
+  template <class F>
+  constexpr detail::invoke_result_t<F, const T &> and_then(F &&f) const & {
+    using result = detail::invoke_result_t<F, const T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F>
+  constexpr detail::invoke_result_t<F, const T &&> and_then(F &&f) const && {
+    using result = detail::invoke_result_t<F, const T &&>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : result(nullopt);
+  }
+#endif
+#endif
+
+#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) &&               \
+    !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
+  /// Carries out some operation on the stored object if there is one.
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto map(F &&f) const & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto map(F &&f) const && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#else
+  /// Carries out some operation on the stored object if there is one.
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional &>(),
+                                             std::declval<F &&>()))
+  map(F &&f) & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional &&>(),
+                                             std::declval<F &&>()))
+  map(F &&f) && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F>
+  constexpr decltype(optional_map_impl(std::declval<const optional &>(),
+                              std::declval<F &&>()))
+  map(F &&f) const & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F>
+  constexpr decltype(optional_map_impl(std::declval<const optional &&>(),
+                              std::declval<F &&>()))
+  map(F &&f) const && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+#endif
+
+#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) &&               \
+    !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
+  /// Carries out some operation on the stored object if there is one.
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto transform(F&& f) const & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto transform(F&& f) const && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#else
+  /// Carries out some operation on the stored object if there is one.
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional&>(),
+    std::declval<F&&>()))
+    transform(F&& f) & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(optional_map_impl(std::declval<optional&&>(),
+    std::declval<F&&>()))
+    transform(F&& f) && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F>
+  constexpr decltype(optional_map_impl(std::declval<const optional&>(),
+    std::declval<F&&>()))
+    transform(F&& f) const & {
+    return optional_map_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F>
+  constexpr decltype(optional_map_impl(std::declval<const optional&&>(),
+    std::declval<F&&>()))
+    transform(F&& f) const && {
+    return optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+#endif
+
+  /// Calls `f` if the optional is empty
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) & {
+    if (has_value())
+      return *this;
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) & {
+    return has_value() ? *this : std::forward<F>(f)();
+  }
+
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) && {
+    if (has_value())
+      return std::move(*this);
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) && {
+    return has_value() ? std::move(*this) : std::forward<F>(f)();
+  }
+
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) const & {
+    if (has_value())
+      return *this;
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) const & {
+    return has_value() ? *this : std::forward<F>(f)();
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) const && {
+    if (has_value())
+      return std::move(*this);
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) const && {
+    return has_value() ? std::move(*this) : std::forward<F>(f)();
+  }
+#endif
+
+  /// Maps the stored value with `f` if there is one, otherwise returns `u`.
+  template <class F, class U> U map_or(F &&f, U &&u) & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u);
+  }
+
+  template <class F, class U> U map_or(F &&f, U &&u) && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u);
+  }
+
+  template <class F, class U> U map_or(F &&f, U &&u) const & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F, class U> U map_or(F &&f, U &&u) const && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u);
+  }
+#endif
+
+  /// Maps the stored value with `f` if there is one, otherwise calls
+  /// `u` and returns the result.
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u)();
+  }
+
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u)();
+  }
+
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u)();
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u)();
+  }
+#endif
+
+  /// 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.
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) & {
+    return has_value() ? *this : rhs;
+  }
+
+  constexpr optional disjunction(const optional &rhs) const & {
+    return has_value() ? *this : rhs;
+  }
+
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) && {
+    return has_value() ? std::move(*this) : rhs;
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  constexpr optional disjunction(const optional &rhs) const && {
+    return has_value() ? std::move(*this) : rhs;
+  }
+#endif
+
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) & {
+    return has_value() ? *this : std::move(rhs);
+  }
+
+  constexpr optional disjunction(optional &&rhs) const & {
+    return has_value() ? *this : std::move(rhs);
+  }
+
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) && {
+    return has_value() ? std::move(*this) : std::move(rhs);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  constexpr optional disjunction(optional &&rhs) const && {
+    return has_value() ? std::move(*this) : std::move(rhs);
+  }
+#endif
+
+  /// Takes the value out of the optional, leaving it empty
+  optional take() {
+    optional ret = std::move(*this);
+    reset();
+    return ret;
+  }
+
+  using value_type = T;
+
+  /// Constructs an optional that does not contain a value.
+  constexpr optional() noexcept = default;
+
+  constexpr optional(nullopt_t) noexcept {}
+
+  /// Copy constructor
+  ///
+  /// If `rhs` contains a value, the stored value is direct-initialized with
+  /// it. Otherwise, the constructed optional is empty.
+  TL_OPTIONAL_11_CONSTEXPR optional(const optional &rhs) = default;
+
+  /// Move constructor
+  ///
+  /// If `rhs` contains a value, the stored value is direct-initialized with
+  /// it. Otherwise, the constructed optional is empty.
+  TL_OPTIONAL_11_CONSTEXPR optional(optional &&rhs) = default;
+
+  /// Constructs the stored value in-place using the given arguments.
+ template <class... Args>
+  constexpr explicit optional(
+      detail::enable_if_t<std::is_constructible<T, Args...>::value, in_place_t>,
+      Args &&... args)
+      : base(in_place, std::forward<Args>(args)...) {}
+
+  template <class U, class... Args>
+  TL_OPTIONAL_11_CONSTEXPR explicit optional(
+      detail::enable_if_t<std::is_constructible<T, std::initializer_list<U> &,
+                                                Args &&...>::value,
+                          in_place_t>,
+      std::initializer_list<U> il, Args &&... args) {
+    this->construct(il, std::forward<Args>(args)...);
+  }
+
+  /// Constructs the stored value with `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)) {}
+
+  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.
+  template <
+      class U, detail::enable_from_other<T, U, const U &> * = nullptr,
+      detail::enable_if_t<std::is_convertible<const U &, T>::value> * = nullptr>
+  optional(const optional<U> &rhs) {
+    if (rhs.has_value()) {
+      this->construct(*rhs);
+    }
+  }
+
+  template <class U, detail::enable_from_other<T, U, const U &> * = nullptr,
+            detail::enable_if_t<!std::is_convertible<const U &, T>::value> * =
+                nullptr>
+  explicit optional(const optional<U> &rhs) {
+    if (rhs.has_value()) {
+      this->construct(*rhs);
+    }
+  }
+
+  /// Converting move constructor.
+  template <
+      class U, detail::enable_from_other<T, U, U &&> * = nullptr,
+      detail::enable_if_t<std::is_convertible<U &&, T>::value> * = nullptr>
+  optional(optional<U> &&rhs) {
+    if (rhs.has_value()) {
+      this->construct(std::move(*rhs));
+    }
+  }
+
+  template <
+      class U, detail::enable_from_other<T, U, U &&> * = nullptr,
+      detail::enable_if_t<!std::is_convertible<U &&, T>::value> * = nullptr>
+  explicit optional(optional<U> &&rhs) {
+    if (rhs.has_value()) {
+      this->construct(std::move(*rhs));
+    }
+  }
+
+  /// Destroys the stored value if there is one.
+  ~optional() = default;
+
+  /// Assignment to empty.
+  ///
+  /// Destroys the current value if there is one.
+  optional &operator=(nullopt_t) noexcept {
+    if (has_value()) {
+      this->m_value.~T();
+      this->m_has_value = false;
+    }
+
+    return *this;
+  }
+
+  /// Copy assignment.
+  ///
+  /// Copies the value from `rhs` if there is one. Otherwise resets the stored
+  /// value in `*this`.
+  optional &operator=(const optional &rhs) = default;
+
+  /// Move assignment.
+  ///
+  /// Moves the value from `rhs` if there is one. Otherwise resets the stored
+  /// value in `*this`.
+  optional &operator=(optional &&rhs) = default;
+
+  /// Assigns the stored value from `u`, destroying the old value if there was
+  /// one.
+  template <class U = T, detail::enable_assign_forward<T, U> * = nullptr>
+  optional &operator=(U &&u) {
+    if (has_value()) {
+      this->m_value = std::forward<U>(u);
+    } else {
+      this->construct(std::forward<U>(u));
+    }
+
+    return *this;
+  }
+
+  /// Converting copy assignment operator.
+  ///
+  /// Copies the value from `rhs` if there is one. Otherwise resets the stored
+  /// value in `*this`.
+  template <class U,
+            detail::enable_assign_from_other<T, U, const U &> * = nullptr>
+  optional &operator=(const optional<U> &rhs) {
+    if (has_value()) {
+      if (rhs.has_value()) {
+        this->m_value = *rhs;
+      } else {
+        this->hard_reset();
+      }
+    }
+
+    else if (rhs.has_value()) {
+      this->construct(*rhs);
+    }
+
+    return *this;
+  }
+
+  // TODO check exception guarantee
+  /// Converting move assignment operator.
+  ///
+  /// Moves the value from `rhs` if there is one. Otherwise resets the stored
+  /// value in `*this`.
+  template <class U, detail::enable_assign_from_other<T, U, U> * = nullptr>
+  optional &operator=(optional<U> &&rhs) {
+    if (has_value()) {
+      if (rhs.has_value()) {
+        this->m_value = std::move(*rhs);
+      } else {
+        this->hard_reset();
+      }
+    }
+
+    else if (rhs.has_value()) {
+      this->construct(std::move(*rhs));
+    }
+
+    return *this;
+  }
+
+  /// Constructs the value in-place, destroying the current one if there is
+  /// one.
+  template <class... Args> T &emplace(Args &&... args) {
+    static_assert(std::is_constructible<T, Args &&...>::value,
+                  "T must be constructible with Args");
+
+    *this = nullopt;
+    this->construct(std::forward<Args>(args)...);
+    return value();
+  }
+
+  template <class U, class... Args>
+  detail::enable_if_t<
+      std::is_constructible<T, std::initializer_list<U> &, Args &&...>::value,
+      T &>
+  emplace(std::initializer_list<U> il, Args &&... args) {
+    *this = nullopt;
+    this->construct(il, std::forward<Args>(args)...);
+    return value();    
+  }
+
+  /// 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) {
+    using std::swap;
+    if (has_value()) {
+      if (rhs.has_value()) {
+        swap(**this, *rhs);
+      } else {
+        new (std::addressof(rhs.m_value)) T(std::move(this->m_value));
+        this->m_value.T::~T();
+      }
+    } else if (rhs.has_value()) {
+      new (std::addressof(this->m_value)) T(std::move(rhs.m_value));
+      rhs.m_value.T::~T();
+    }
+    swap(this->m_has_value, rhs.m_has_value);
+  }
+
+  /// Returns a pointer to the stored value
+  constexpr const T *operator->() const {
+    return std::addressof(this->m_value);
+  }
+
+  TL_OPTIONAL_11_CONSTEXPR T *operator->() {
+    return std::addressof(this->m_value);
+  }
+
+  /// Returns the stored value
+  TL_OPTIONAL_11_CONSTEXPR T &operator*() & { return this->m_value; }
+
+  constexpr const T &operator*() const & { return this->m_value; }
+
+  TL_OPTIONAL_11_CONSTEXPR T &&operator*() && {
+    return std::move(this->m_value);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  constexpr const T &&operator*() const && { return std::move(this->m_value); }
+#endif
+
+  /// Returns whether or not the optional has a value
+  constexpr bool has_value() const noexcept { return this->m_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
+  TL_OPTIONAL_11_CONSTEXPR T &value() & {
+    if (has_value())
+      return this->m_value;
+    throw bad_optional_access();
+  }
+  TL_OPTIONAL_11_CONSTEXPR const T &value() const & {
+    if (has_value())
+      return this->m_value;
+    throw bad_optional_access();
+  }
+  TL_OPTIONAL_11_CONSTEXPR T &&value() && {
+    if (has_value())
+      return std::move(this->m_value);
+    throw bad_optional_access();
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  TL_OPTIONAL_11_CONSTEXPR const T &&value() const && {
+    if (has_value())
+      return std::move(this->m_value);
+    throw bad_optional_access();
+  }
+#endif
+
+  /// Returns the stored value if there is one, otherwise returns `u`
+  template <class U> constexpr T value_or(U &&u) const & {
+    static_assert(std::is_copy_constructible<T>::value &&
+                      std::is_convertible<U &&, T>::value,
+                  "T must be copy constructible and convertible from U");
+    return has_value() ? **this : static_cast<T>(std::forward<U>(u));
+  }
+
+  template <class U> TL_OPTIONAL_11_CONSTEXPR T value_or(U &&u) && {
+    static_assert(std::is_move_constructible<T>::value &&
+                      std::is_convertible<U &&, T>::value,
+                  "T must be move constructible and convertible from U");
+    return has_value() ? std::move(**this) : static_cast<T>(std::forward<U>(u));
+  }
+
+  /// 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
+
+/// Compares two optional objects
+template <class T, class U>
+inline constexpr bool operator==(const optional<T> &lhs,
+                                 const optional<U> &rhs) {
+  return lhs.has_value() == rhs.has_value() &&
+         (!lhs.has_value() || *lhs == *rhs);
+}
+template <class T, class U>
+inline constexpr bool operator!=(const optional<T> &lhs,
+                                 const optional<U> &rhs) {
+  return lhs.has_value() != rhs.has_value() ||
+         (lhs.has_value() && *lhs != *rhs);
+}
+template <class T, class U>
+inline constexpr bool operator<(const optional<T> &lhs,
+                                const optional<U> &rhs) {
+  return rhs.has_value() && (!lhs.has_value() || *lhs < *rhs);
+}
+template <class T, class U>
+inline constexpr bool operator>(const optional<T> &lhs,
+                                const optional<U> &rhs) {
+  return lhs.has_value() && (!rhs.has_value() || *lhs > *rhs);
+}
+template <class T, class U>
+inline constexpr bool operator<=(const optional<T> &lhs,
+                                 const optional<U> &rhs) {
+  return !lhs.has_value() || (rhs.has_value() && *lhs <= *rhs);
+}
+template <class T, class U>
+inline constexpr bool operator>=(const optional<T> &lhs,
+                                 const optional<U> &rhs) {
+  return !rhs.has_value() || (lhs.has_value() && *lhs >= *rhs);
+}
+
+/// Compares an optional to a `nullopt`
+template <class T>
+inline constexpr bool operator==(const optional<T> &lhs, nullopt_t) noexcept {
+  return !lhs.has_value();
+}
+template <class T>
+inline constexpr bool operator==(nullopt_t, const optional<T> &rhs) noexcept {
+  return !rhs.has_value();
+}
+template <class T>
+inline constexpr bool operator!=(const optional<T> &lhs, nullopt_t) noexcept {
+  return lhs.has_value();
+}
+template <class T>
+inline constexpr bool operator!=(nullopt_t, const optional<T> &rhs) noexcept {
+  return rhs.has_value();
+}
+template <class T>
+inline constexpr bool operator<(const optional<T> &, nullopt_t) noexcept {
+  return false;
+}
+template <class T>
+inline constexpr bool operator<(nullopt_t, const optional<T> &rhs) noexcept {
+  return rhs.has_value();
+}
+template <class T>
+inline constexpr bool operator<=(const optional<T> &lhs, nullopt_t) noexcept {
+  return !lhs.has_value();
+}
+template <class T>
+inline constexpr bool operator<=(nullopt_t, const optional<T> &) noexcept {
+  return true;
+}
+template <class T>
+inline constexpr bool operator>(const optional<T> &lhs, nullopt_t) noexcept {
+  return lhs.has_value();
+}
+template <class T>
+inline constexpr bool operator>(nullopt_t, const optional<T> &) noexcept {
+  return false;
+}
+template <class T>
+inline constexpr bool operator>=(const optional<T> &, nullopt_t) noexcept {
+  return true;
+}
+template <class T>
+inline constexpr bool operator>=(nullopt_t, const optional<T> &rhs) noexcept {
+  return !rhs.has_value();
+}
+
+/// Compares the optional with a value.
+template <class T, class U>
+inline constexpr bool operator==(const optional<T> &lhs, const U &rhs) {
+  return lhs.has_value() ? *lhs == rhs : false;
+}
+template <class T, class U>
+inline constexpr bool operator==(const U &lhs, const optional<T> &rhs) {
+  return rhs.has_value() ? lhs == *rhs : false;
+}
+template <class T, class U>
+inline constexpr bool operator!=(const optional<T> &lhs, const U &rhs) {
+  return lhs.has_value() ? *lhs != rhs : true;
+}
+template <class T, class U>
+inline constexpr bool operator!=(const U &lhs, const optional<T> &rhs) {
+  return rhs.has_value() ? lhs != *rhs : true;
+}
+template <class T, class U>
+inline constexpr bool operator<(const optional<T> &lhs, const U &rhs) {
+  return lhs.has_value() ? *lhs < rhs : true;
+}
+template <class T, class U>
+inline constexpr bool operator<(const U &lhs, const optional<T> &rhs) {
+  return rhs.has_value() ? lhs < *rhs : false;
+}
+template <class T, class U>
+inline constexpr bool operator<=(const optional<T> &lhs, const U &rhs) {
+  return lhs.has_value() ? *lhs <= rhs : true;
+}
+template <class T, class U>
+inline constexpr bool operator<=(const U &lhs, const optional<T> &rhs) {
+  return rhs.has_value() ? lhs <= *rhs : false;
+}
+template <class T, class U>
+inline constexpr bool operator>(const optional<T> &lhs, const U &rhs) {
+  return lhs.has_value() ? *lhs > rhs : false;
+}
+template <class T, class U>
+inline constexpr bool operator>(const U &lhs, const optional<T> &rhs) {
+  return rhs.has_value() ? lhs > *rhs : true;
+}
+template <class T, class U>
+inline constexpr bool operator>=(const optional<T> &lhs, const U &rhs) {
+  return lhs.has_value() ? *lhs >= rhs : false;
+}
+template <class T, class U>
+inline constexpr bool operator>=(const U &lhs, const optional<T> &rhs) {
+  return rhs.has_value() ? lhs >= *rhs : true;
+}
+
+template <class T,
+          detail::enable_if_t<std::is_move_constructible<T>::value> * = nullptr,
+          detail::enable_if_t<detail::is_swappable<T>::value> * = nullptr>
+void swap(optional<T> &lhs,
+          optional<T> &rhs) noexcept(noexcept(lhs.swap(rhs))) {
+  return lhs.swap(rhs);
+}
+
+namespace detail {
+struct i_am_secret {};
+} // namespace detail
+
+template <class T = detail::i_am_secret, class U,
+          class Ret =
+              detail::conditional_t<std::is_same<T, detail::i_am_secret>::value,
+                                    detail::decay_t<U>, T>>
+inline constexpr optional<Ret> make_optional(U &&v) {
+  return optional<Ret>(std::forward<U>(v));
+}
+
+template <class T, class... Args>
+inline constexpr optional<T> make_optional(Args &&... args) {
+  return optional<T>(in_place, std::forward<Args>(args)...);
+}
+template <class T, class U, class... Args>
+inline constexpr optional<T> make_optional(std::initializer_list<U> il,
+                                           Args &&... args) {
+  return optional<T>(in_place, il, std::forward<Args>(args)...);
+}
+
+#if __cplusplus >= 201703L
+template <class T> optional(T)->optional<T>;
+#endif
+
+/// \exclude
+namespace detail {
+#ifdef TL_OPTIONAL_CXX14
+template <class Opt, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Opt>())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto optional_map_impl(Opt &&opt, F &&f) {
+  return opt.has_value()
+             ? detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt))
+             : optional<Ret>(nullopt);
+}
+
+template <class Opt, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Opt>())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto optional_map_impl(Opt &&opt, F &&f) {
+  if (opt.has_value()) {
+    detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt));
+    return make_optional(monostate{});
+  }
+
+  return optional<monostate>(nullopt);
+}
+#else
+template <class Opt, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Opt>())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+
+constexpr auto optional_map_impl(Opt &&opt, F &&f) -> optional<Ret> {
+  return opt.has_value()
+             ? detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt))
+             : optional<Ret>(nullopt);
+}
+
+template <class Opt, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Opt>())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+
+auto optional_map_impl(Opt &&opt, F &&f) -> optional<monostate> {
+  if (opt.has_value()) {
+    detail::invoke(std::forward<F>(f), *std::forward<Opt>(opt));
+    return monostate{};
+  }
+
+  return nullopt;
+}
+#endif
+} // namespace detail
+
+/// Specialization for when `T` is a reference. `optional<T&>` acts similarly
+/// to a `T*`, but provides more operations and shows intent more clearly.
+template <class T> class optional<T &> {
+public:
+// The different versions for C++14 and 11 are needed because deduced return
+// types are not SFINAE-safe. This provides better support for things like
+// generic lambdas. C.f.
+// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0826r0.html
+#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) &&               \
+    !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
+
+  /// Carries out some operation which returns an optional on the stored
+  /// object if there is one.
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) & {
+    using result = detail::invoke_result_t<F, T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto and_then(F &&f) && {
+    using result = detail::invoke_result_t<F, T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+  template <class F> constexpr auto and_then(F &&f) const & {
+    using result = detail::invoke_result_t<F, const T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F> constexpr auto and_then(F &&f) const && {
+    using result = detail::invoke_result_t<F, const T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+#endif
+#else
+  /// Carries out some operation which returns an optional on the stored
+  /// object if there is one.
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &> and_then(F &&f) & {
+    using result = detail::invoke_result_t<F, T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR detail::invoke_result_t<F, T &> and_then(F &&f) && {
+    using result = detail::invoke_result_t<F, T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+  template <class F>
+  constexpr detail::invoke_result_t<F, const T &> and_then(F &&f) const & {
+    using result = detail::invoke_result_t<F, const T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : result(nullopt);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F>
+  constexpr detail::invoke_result_t<F, const T &> and_then(F &&f) const && {
+    using result = detail::invoke_result_t<F, const T &>;
+    static_assert(detail::is_optional<result>::value,
+                  "F must return an optional");
+
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : result(nullopt);
+  }
+#endif
+#endif
+
+#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) &&               \
+    !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
+  /// Carries out some operation on the stored object if there is one.
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto map(F &&f) && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto map(F &&f) const & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto map(F &&f) const && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#else
+  /// Carries out some operation on the stored object if there is one.
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional &>(),
+                                                     std::declval<F &&>()))
+  map(F &&f) & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional &&>(),
+                                                     std::declval<F &&>()))
+  map(F &&f) && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F>
+  constexpr decltype(detail::optional_map_impl(std::declval<const optional &>(),
+                                      std::declval<F &&>()))
+  map(F &&f) const & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F>
+  constexpr decltype(detail::optional_map_impl(std::declval<const optional &&>(),
+                                      std::declval<F &&>()))
+  map(F &&f) const && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+#endif
+
+#if defined(TL_OPTIONAL_CXX14) && !defined(TL_OPTIONAL_GCC49) &&               \
+    !defined(TL_OPTIONAL_GCC54) && !defined(TL_OPTIONAL_GCC55)
+  /// Carries out some operation on the stored object if there is one.
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> TL_OPTIONAL_11_CONSTEXPR auto transform(F&& f) && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto transform(F&& f) const & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> constexpr auto transform(F&& f) const && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#else
+  /// Carries out some operation on the stored object if there is one.
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional&>(),
+    std::declval<F&&>()))
+    transform(F&& f) & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+  /// \group map
+  /// \synopsis template <class F> auto transform(F &&f) &&;
+  template <class F>
+  TL_OPTIONAL_11_CONSTEXPR decltype(detail::optional_map_impl(std::declval<optional&&>(),
+    std::declval<F&&>()))
+    transform(F&& f) && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F>
+  constexpr decltype(detail::optional_map_impl(std::declval<const optional&>(),
+    std::declval<F&&>()))
+    transform(F&& f) const & {
+    return detail::optional_map_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F>
+  constexpr decltype(detail::optional_map_impl(std::declval<const optional&&>(),
+    std::declval<F&&>()))
+    transform(F&& f) const && {
+    return detail::optional_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+#endif
+
+  /// Calls `f` if the optional is empty
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) & {
+    if (has_value())
+      return *this;
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) & {
+    return has_value() ? *this : std::forward<F>(f)();
+  }
+
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) && {
+    if (has_value())
+      return std::move(*this);
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) && {
+    return has_value() ? std::move(*this) : std::forward<F>(f)();
+  }
+
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) const & {
+    if (has_value())
+      return *this;
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> TL_OPTIONAL_11_CONSTEXPR or_else(F &&f) const & {
+    return has_value() ? *this : std::forward<F>(f)();
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F, detail::enable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) const && {
+    if (has_value())
+      return std::move(*this);
+
+    std::forward<F>(f)();
+    return nullopt;
+  }
+
+  template <class F, detail::disable_if_ret_void<F> * = nullptr>
+  optional<T> or_else(F &&f) const && {
+    return has_value() ? std::move(*this) : std::forward<F>(f)();
+  }
+#endif
+
+  /// Maps the stored value with `f` if there is one, otherwise returns `u`
+  template <class F, class U> U map_or(F &&f, U &&u) & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u);
+  }
+
+  template <class F, class U> U map_or(F &&f, U &&u) && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u);
+  }
+
+  template <class F, class U> U map_or(F &&f, U &&u) const & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F, class U> U map_or(F &&f, U &&u) const && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u);
+  }
+#endif
+
+  /// Maps the stored value with `f` if there is one, otherwise calls
+  /// `u` and returns the result.
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u)();
+  }
+
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u)();
+  }
+
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const & {
+    return has_value() ? detail::invoke(std::forward<F>(f), **this)
+                       : std::forward<U>(u)();
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  template <class F, class U>
+  detail::invoke_result_t<U> map_or_else(F &&f, U &&u) const && {
+    return has_value() ? detail::invoke(std::forward<F>(f), std::move(**this))
+                       : std::forward<U>(u)();
+  }
+#endif
+
+  /// 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.
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) & {
+    return has_value() ? *this : rhs;
+  }
+
+  constexpr optional disjunction(const optional &rhs) const & {
+    return has_value() ? *this : rhs;
+  }
+
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(const optional &rhs) && {
+    return has_value() ? std::move(*this) : rhs;
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  constexpr optional disjunction(const optional &rhs) const && {
+    return has_value() ? std::move(*this) : rhs;
+  }
+#endif
+
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) & {
+    return has_value() ? *this : std::move(rhs);
+  }
+
+  constexpr optional disjunction(optional &&rhs) const & {
+    return has_value() ? *this : std::move(rhs);
+  }
+
+  TL_OPTIONAL_11_CONSTEXPR optional disjunction(optional &&rhs) && {
+    return has_value() ? std::move(*this) : std::move(rhs);
+  }
+
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  constexpr optional disjunction(optional &&rhs) const && {
+    return has_value() ? std::move(*this) : std::move(rhs);
+  }
+#endif
+
+  /// Takes the value out of the optional, leaving it empty
+  optional take() {
+    optional ret = std::move(*this);
+    reset();
+    return ret;
+  }
+
+  using value_type = T &;
+
+  /// Constructs an optional that does not contain a value.
+  constexpr optional() noexcept : m_value(nullptr) {}
+
+  constexpr optional(nullopt_t) noexcept : m_value(nullptr) {}
+
+  /// Copy constructor
+  ///
+  /// If `rhs` contains a value, the stored value is direct-initialized with
+  /// it. Otherwise, the constructed optional is empty.
+  TL_OPTIONAL_11_CONSTEXPR optional(const optional &rhs) noexcept = default;
+
+  /// Move constructor
+  ///
+  /// If `rhs` contains a value, the stored value is direct-initialized with
+  /// it. Otherwise, the constructed optional is empty.
+  TL_OPTIONAL_11_CONSTEXPR optional(optional &&rhs) = default;
+
+  /// Constructs the stored value with `u`.
+  template <class U = T,
+            detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value>
+                * = nullptr>
+  constexpr optional(U &&u)  noexcept : m_value(std::addressof(u)) {
+    static_assert(std::is_lvalue_reference<U>::value, "U must be an lvalue");
+  }
+
+  template <class U>
+  constexpr explicit optional(const optional<U> &rhs) noexcept : optional(*rhs) {}
+
+  /// No-op
+  ~optional() = default;
+
+  /// Assignment to empty.
+  ///
+  /// Destroys the current value if there is one.
+  optional &operator=(nullopt_t) noexcept {
+    m_value = nullptr;
+    return *this;
+  }
+
+  /// Copy assignment.
+  ///
+  /// Rebinds this optional to the referee of `rhs` if there is one. Otherwise
+  /// resets the stored value in `*this`.
+  optional &operator=(const optional &rhs) = default;
+
+  /// Rebinds this optional to `u`.
+  template <class U = T,
+            detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value>
+                * = nullptr>
+  optional &operator=(U &&u) {
+    static_assert(std::is_lvalue_reference<U>::value, "U must be an lvalue");
+    m_value = std::addressof(u);
+    return *this;
+  }
+
+  /// Converting copy assignment operator.
+  ///
+  /// Rebinds this optional to the referee of `rhs` if there is one. Otherwise
+  /// resets the stored value in `*this`.
+  template <class U> optional &operator=(const optional<U> &rhs) noexcept {
+    m_value = std::addressof(rhs.value());
+    return *this;
+  }
+
+  /// Rebinds this optional to `u`.
+  template <class U = T,
+            detail::enable_if_t<!detail::is_optional<detail::decay_t<U>>::value>
+                * = nullptr>
+  optional &emplace(U &&u) noexcept {
+    return *this = std::forward<U>(u);
+  }
+
+  void swap(optional &rhs) noexcept { std::swap(m_value, rhs.m_value); }
+
+  /// Returns a pointer to the stored value
+  constexpr const T *operator->() const noexcept { return m_value; }
+
+  TL_OPTIONAL_11_CONSTEXPR T *operator->() noexcept { return m_value; }
+
+  /// Returns the stored value
+  TL_OPTIONAL_11_CONSTEXPR T &operator*() noexcept { return *m_value; }
+
+  constexpr const T &operator*() const noexcept { return *m_value; }
+
+  constexpr bool has_value() const noexcept { return m_value != nullptr; }
+
+  constexpr explicit operator bool() const noexcept {
+    return m_value != nullptr;
+  }
+
+  /// Returns the contained value if there is one, otherwise throws bad_optional_access
+  TL_OPTIONAL_11_CONSTEXPR T &value() {
+    if (has_value())
+      return *m_value;
+    throw bad_optional_access();
+  }
+  TL_OPTIONAL_11_CONSTEXPR const T &value() const {
+    if (has_value())
+      return *m_value;
+    throw bad_optional_access();
+  }
+
+  /// Returns the stored value if there is one, otherwise returns `u`
+  template <class U> constexpr T value_or(U &&u) const & noexcept {
+    static_assert(std::is_copy_constructible<T>::value &&
+                      std::is_convertible<U &&, T>::value,
+                  "T must be copy constructible and convertible from U");
+    return has_value() ? **this : static_cast<T>(std::forward<U>(u));
+  }
+
+  /// \group value_or
+  template <class U> TL_OPTIONAL_11_CONSTEXPR T value_or(U &&u) && noexcept {
+    static_assert(std::is_move_constructible<T>::value &&
+                      std::is_convertible<U &&, T>::value,
+                  "T must be move constructible and convertible from U");
+    return has_value() ? **this : static_cast<T>(std::forward<U>(u));
+  }
+
+  /// Destroys the stored value if one exists, making the optional empty
+  void reset() noexcept { m_value = nullptr; }
+
+private:
+  T *m_value;
+}; // namespace tl
+
+
+
+} // namespace tl
+
+namespace std {
+// TODO SFINAE
+template <class T> struct hash<tl::optional<T>> {
+  ::std::size_t operator()(const tl::optional<T> &o) const {
+    if (!o.has_value())
+      return 0;
+
+    return std::hash<tl::detail::remove_const_t<T>>()(*o);
+  }
+};
+} // namespace std
+
+#endif
\ No newline at end of file