utils: add Result / Option types

Add some Rust-style Result/Option types, based on TartanLlama's
expected, optional classes.

There's std::optional of course, but we can't depend on C++17 yet.

6 files changed, 4465 insertions, 6 deletions

diff --git a/lib/utils/Makefile.am b/lib/utils/Makefile.am
index 7bfb699..d8cc0d5 100644
--- a/lib/utils/Makefile.am
+++ b/lib/utils/Makefile.am
@@ -45,17 +45,24 @@ AM_LDFLAGS=							\
 noinst_LTLIBRARIES=						\
 	libmu-utils.la
 
+
+third_party=							\
+	optional.hpp						\
+	expected.hpp
+
 libmu_utils_la_SOURCES=						\
 	mu-async-queue.hh					\
+	mu-command-parser.cc					\
+	mu-command-parser.hh					\
 	mu-date.c						\
 	mu-date.h						\
 	mu-error.hh						\
 	mu-logger.cc						\
 	mu-logger.hh						\
-	mu-command-parser.cc					\
-	mu-command-parser.hh					\
+	mu-option.hh						\
 	mu-readline.cc						\
 	mu-readline.hh						\
+	mu-result.hh						\
 	mu-sexp.cc						\
 	mu-sexp.hh						\
 	mu-str.c						\
@@ -63,7 +70,8 @@ libmu_utils_la_SOURCES=						\
 	mu-util.c						\
 	mu-util.h						\
 	mu-utils.cc						\
-	mu-utils.hh
+	mu-utils.hh						\
+	${third_party}
 
 libmu_utils_la_LIBADD=						\
 	$(GLIB_LIBS)						\
diff --git a/lib/utils/expected.hpp b/lib/utils/expected.hpp
new file mode 100644
index 0000000..31a5193
--- /dev/null
+++ b/lib/utils/expected.hpp
@@ -0,0 +1,2326 @@
+///
+// expected - An implementation of std::expected with extensions
+// Written in 2017 by Simon Brand (simonrbrand@gmail.com, @TartanLlama)
+//
+// Documentation available at http://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_EXPECTED_HPP
+#define TL_EXPECTED_HPP
+
+#define TL_EXPECTED_VERSION_MAJOR 1
+#define TL_EXPECTED_VERSION_MINOR 0
+#define TL_EXPECTED_VERSION_PATCH 1
+
+#include <exception>
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#if defined(__EXCEPTIONS) || defined(_CPPUNWIND)
+#define TL_EXPECTED_EXCEPTIONS_ENABLED
+#endif
+
+#if (defined(_MSC_VER) && _MSC_VER == 1900)
+#define TL_EXPECTED_MSVC2015
+#define TL_EXPECTED_MSVC2015_CONSTEXPR
+#else
+#define TL_EXPECTED_MSVC2015_CONSTEXPR constexpr
+#endif
+
+#if (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 &&              \
+     !defined(__clang__))
+#define TL_EXPECTED_GCC49
+#endif
+
+#if (defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 4 &&              \
+     !defined(__clang__))
+#define TL_EXPECTED_GCC54
+#endif
+
+#if (defined(__GNUC__) && __GNUC__ == 5 && __GNUC_MINOR__ <= 5 &&              \
+     !defined(__clang__))
+#define TL_EXPECTED_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_EXPECTED_NO_CONSTRR
+// GCC < 5 doesn't support some standard C++11 type traits
+#define TL_EXPECTED_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)                         \
+  std::has_trivial_copy_constructor<T>
+#define TL_EXPECTED_IS_TRIVIALLY_COPY_ASSIGNABLE(T)                            \
+  std::has_trivial_copy_assign<T>
+
+// This one will be different for GCC 5.7 if it's ever supported
+#define TL_EXPECTED_IS_TRIVIALLY_DESTRUCTIBLE(T)                               \
+  std::is_trivially_destructible<T>
+
+// 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::false_type{};
+#endif
+  }
+}
+#endif
+
+#define TL_EXPECTED_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)                                     \
+  tl::detail::is_trivially_copy_constructible<T>
+#define TL_EXPECTED_IS_TRIVIALLY_COPY_ASSIGNABLE(T)                                        \
+  std::is_trivially_copy_assignable<T>
+#define TL_EXPECTED_IS_TRIVIALLY_DESTRUCTIBLE(T) std::is_trivially_destructible<T>
+#else
+#define TL_EXPECTED_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)                         \
+  std::is_trivially_copy_constructible<T>
+#define TL_EXPECTED_IS_TRIVIALLY_COPY_ASSIGNABLE(T)                            \
+  std::is_trivially_copy_assignable<T>
+#define TL_EXPECTED_IS_TRIVIALLY_DESTRUCTIBLE(T)                               \
+  std::is_trivially_destructible<T>
+#endif
+
+#if __cplusplus > 201103L
+#define TL_EXPECTED_CXX14
+#endif
+
+#ifdef TL_EXPECTED_GCC49
+#define TL_EXPECTED_GCC49_CONSTEXPR
+#else
+#define TL_EXPECTED_GCC49_CONSTEXPR constexpr
+#endif
+
+#if (__cplusplus == 201103L || defined(TL_EXPECTED_MSVC2015) ||                \
+     defined(TL_EXPECTED_GCC49))
+#define TL_EXPECTED_11_CONSTEXPR
+#else
+#define TL_EXPECTED_11_CONSTEXPR constexpr
+#endif
+
+namespace tl {
+template <class T, class E> class expected;
+
+#ifndef TL_MONOSTATE_INPLACE_MUTEX
+#define TL_MONOSTATE_INPLACE_MUTEX
+class monostate {};
+
+struct in_place_t {
+  explicit in_place_t() = default;
+};
+static constexpr in_place_t in_place{};
+#endif
+
+template <class 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)) {}
+
+  constexpr const E &value() const & { return m_val; }
+  TL_EXPECTED_11_CONSTEXPR E &value() & { return m_val; }
+  TL_EXPECTED_11_CONSTEXPR E &&value() && { return std::move(m_val); }
+  constexpr const E &&value() const && { return std::move(m_val); }
+
+private:
+  E m_val;
+};
+
+template <class E>
+constexpr bool operator==(const unexpected<E> &lhs, const unexpected<E> &rhs) {
+  return lhs.value() == rhs.value();
+}
+template <class E>
+constexpr bool operator!=(const unexpected<E> &lhs, const unexpected<E> &rhs) {
+  return lhs.value() != rhs.value();
+}
+template <class E>
+constexpr bool operator<(const unexpected<E> &lhs, const unexpected<E> &rhs) {
+  return lhs.value() < rhs.value();
+}
+template <class E>
+constexpr bool operator<=(const unexpected<E> &lhs, const unexpected<E> &rhs) {
+  return lhs.value() <= rhs.value();
+}
+template <class E>
+constexpr bool operator>(const unexpected<E> &lhs, const unexpected<E> &rhs) {
+  return lhs.value() > rhs.value();
+}
+template <class E>
+constexpr bool operator>=(const unexpected<E> &lhs, const unexpected<E> &rhs) {
+  return lhs.value() >= rhs.value();
+}
+
+template <class E>
+unexpected<typename std::decay<E>::type> make_unexpected(E &&e) {
+  return unexpected<typename std::decay<E>::type>(std::forward<E>(e));
+}
+
+struct unexpect_t {
+  unexpect_t() = default;
+};
+static constexpr unexpect_t unexpect{};
+
+namespace detail {
+template<typename E>
+[[noreturn]] TL_EXPECTED_11_CONSTEXPR void throw_exception(E &&e) {
+#ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+    throw std::forward<E>(e);
+#else
+  #ifdef _MSC_VER
+    __assume(0);
+  #else
+    __builtin_unreachable();
+  #endif
+#endif
+}
+
+#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
+
+// 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>>;
+
+template <class T, class E, class U>
+using expected_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<expected<T, E>, detail::decay_t<U>>::value &&
+    !std::is_same<unexpected<E>, detail::decay_t<U>>::value>;
+
+template <class T, class E, class U, class G, class UR, class GR>
+using expected_enable_from_other = detail::enable_if_t<
+    std::is_constructible<T, UR>::value &&
+    std::is_constructible<E, GR>::value &&
+    !std::is_constructible<T, expected<U, G> &>::value &&
+    !std::is_constructible<T, expected<U, G> &&>::value &&
+    !std::is_constructible<T, const expected<U, G> &>::value &&
+    !std::is_constructible<T, const expected<U, G> &&>::value &&
+    !std::is_convertible<expected<U, G> &, T>::value &&
+    !std::is_convertible<expected<U, G> &&, T>::value &&
+    !std::is_convertible<const expected<U, G> &, T>::value &&
+    !std::is_convertible<const expected<U, G> &&, T>::value>;
+
+template <class T, class U>
+using is_void_or = conditional_t<std::is_void<T>::value, std::true_type, U>;
+
+template <class T>
+using is_copy_constructible_or_void =
+    is_void_or<T, std::is_copy_constructible<T>>;
+
+template <class T>
+using is_move_constructible_or_void =
+    is_void_or<T, std::is_move_constructible<T>>;
+
+template <class T>
+using is_copy_assignable_or_void =
+    is_void_or<T, std::is_copy_assignable<T>>;
+
+
+template <class T>
+using is_move_assignable_or_void =
+    is_void_or<T, std::is_move_assignable<T>>;
+    
+
+} // namespace detail
+
+namespace detail {
+struct no_init_t {};
+static constexpr no_init_t no_init{};
+
+// 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) {}
+  constexpr expected_storage_base(no_init_t) : m_no_init(), m_has_val(false) {}
+
+  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>();
+    }
+  }
+  union {
+    T m_val;
+    unexpected<E> m_unexpect;
+    char m_no_init;
+  };
+  bool m_has_val;
+};
+
+// 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) {}
+  constexpr expected_storage_base(no_init_t) : m_no_init(), m_has_val(false) {}
+
+  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;
+  union {
+    T m_val;
+    unexpected<E> m_unexpect;
+    char m_no_init;
+  };
+  bool m_has_val;
+};
+
+// 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) {}
+  TL_EXPECTED_MSVC2015_CONSTEXPR expected_storage_base(no_init_t)
+      : m_no_init(), m_has_val(false) {}
+
+  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>();
+    }
+  }
+
+  union {
+    T m_val;
+    unexpected<E> m_unexpect;
+    char m_no_init;
+  };
+  bool m_has_val;
+};
+
+// 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) {}
+  constexpr expected_storage_base(no_init_t) : m_no_init(), m_has_val(false) {}
+
+  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();
+    }
+  }
+  union {
+    T m_val;
+    unexpected<E> m_unexpect;
+    char m_no_init;
+  };
+  bool m_has_val;
+};
+
+// `T` is `void`, `E` is trivially-destructible
+template <class E> struct expected_storage_base<void, E, false, true> {
+  TL_EXPECTED_MSVC2015_CONSTEXPR expected_storage_base() : m_has_val(true) {}
+  constexpr expected_storage_base(no_init_t) : m_val(), m_has_val(false) {}
+
+  constexpr expected_storage_base(in_place_t) : 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;
+  struct dummy {};
+  union {
+    unexpected<E> m_unexpect;
+    dummy m_val;
+  };
+  bool m_has_val;
+};
+
+// `T` is `void`, `E` is not trivially-destructible
+template <class E> struct expected_storage_base<void, E, false, false> {
+  constexpr expected_storage_base() : m_dummy(), m_has_val(true) {}
+  constexpr expected_storage_base(no_init_t) : m_dummy(), m_has_val(false) {}
+
+  constexpr expected_storage_base(in_place_t) : m_dummy(), 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>();
+    }
+  }
+
+  union {
+    unexpected<E> m_unexpect;
+    char m_dummy;
+  };
+  bool m_has_val;
+};
+
+// 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;
+
+  template <class... Args> void construct(Args &&... args) noexcept {
+    new (std::addressof(this->m_val)) T(std::forward<Args>(args)...);
+    this->m_has_val = true;
+  }
+
+  template <class Rhs> void construct_with(Rhs &&rhs) noexcept {
+    new (std::addressof(this->m_val)) T(std::forward<Rhs>(rhs).get());
+    this->m_has_val = 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_val = false;
+  }
+
+  #ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+
+  // 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>
+  void 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>
+  void 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>
+  void assign(const expected_operations_base &rhs) {
+    if (!this->m_has_val && rhs.m_has_val) {
+      auto tmp = std::move(geterr());
+      geterr().~unexpected<E>();
+
+#ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+      try {
+        construct(rhs.get());
+      } catch (...) {
+        geterr() = std::move(tmp);
+        throw;
+      }
+#else
+      construct(rhs.get());
+#endif
+    } 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>
+  void assign(expected_operations_base &&rhs) noexcept {
+    if (!this->m_has_val && rhs.m_has_val) {
+      geterr().~unexpected<E>();
+      construct(std::move(rhs).get());
+    } else {
+      assign_common(std::move(rhs));
+    }
+  }
+
+  template <class U = T,
+            detail::enable_if_t<!std::is_nothrow_move_constructible<U>::value>
+                * = nullptr>
+  void assign(expected_operations_base &&rhs) {
+    if (!this->m_has_val && rhs.m_has_val) {
+      auto tmp = std::move(geterr());
+      geterr().~unexpected<E>();
+#ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+      try {
+        construct(std::move(rhs).get());
+      } catch (...) {
+        geterr() = std::move(tmp);
+        throw;
+      }
+#else
+      construct(std::move(rhs).get());
+#endif
+    } else {
+      assign_common(std::move(rhs));
+    }
+  }
+
+  #else
+
+  // If exceptions are disabled then we can just copy-construct
+  void 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);
+    }
+  }
+
+  void assign(expected_operations_base &&rhs) noexcept {
+    if (!this->m_has_val && rhs.m_has_val) {
+      geterr().~unexpected<E>();
+      construct(std::move(rhs).get());
+    } else {
+      assign_common(rhs);
+    }
+  }
+
+  #endif
+
+  // 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 {
+		destroy_val();
+        construct_error(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_val; }
+
+  TL_EXPECTED_11_CONSTEXPR T &get() & { return this->m_val; }
+  constexpr const T &get() const & { return this->m_val; }
+  TL_EXPECTED_11_CONSTEXPR T &&get() && { return 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 unexpected<E> &geterr() & {
+    return this->m_unexpect;
+  }
+  constexpr const unexpected<E> &geterr() const & { return this->m_unexpect; }
+  TL_EXPECTED_11_CONSTEXPR unexpected<E> &&geterr() && {
+    return std::move(this->m_unexpect);
+  }
+#ifndef TL_EXPECTED_NO_CONSTRR
+  constexpr const unexpected<E> &&geterr() const && {
+    return std::move(this->m_unexpect);
+  }
+#endif
+
+  TL_EXPECTED_11_CONSTEXPR void destroy_val() {
+	get().~T();
+  }
+};
+
+// This base class provides some handy member functions which can be used in
+// further derived classes
+template <class E>
+struct expected_operations_base<void, E> : expected_storage_base<void, E> {
+  using expected_storage_base<void, E>::expected_storage_base;
+
+  template <class... Args> void construct() noexcept { this->m_has_val = true; }
+
+  // This function doesn't use its argument, but needs it so that code in
+  // levels above this can work independently of whether T is void
+  template <class Rhs> void construct_with(Rhs &&) noexcept {
+    this->m_has_val = 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_val = false;
+  }
+
+  template <class Rhs> void assign(Rhs &&rhs) noexcept {
+    if (!this->m_has_val) {
+      if (rhs.m_has_val) {
+        geterr().~unexpected<E>();
+        construct();
+      } else {
+        geterr() = std::forward<Rhs>(rhs).geterr();
+      }
+    } else {
+      if (!rhs.m_has_val) {
+        construct_error(std::forward<Rhs>(rhs).geterr());
+      }
+    }
+  }
+
+  bool has_value() const { return this->m_has_val; }
+
+  TL_EXPECTED_11_CONSTEXPR unexpected<E> &geterr() & {
+    return this->m_unexpect;
+  }
+  constexpr const unexpected<E> &geterr() const & { return this->m_unexpect; }
+  TL_EXPECTED_11_CONSTEXPR unexpected<E> &&geterr() && {
+    return std::move(this->m_unexpect);
+  }
+#ifndef TL_EXPECTED_NO_CONSTRR
+  constexpr const unexpected<E> &&geterr() const && {
+    return std::move(this->m_unexpect);
+  }
+#endif
+
+  TL_EXPECTED_11_CONSTEXPR void destroy_val() {
+	  //no-op
+  }
+};
+
+// This class manages conditionally having a trivial copy constructor
+// This specialization is for when T and E are trivially copy constructible
+template <class T, class E,
+          bool = is_void_or<T, TL_EXPECTED_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)>::
+              value &&TL_EXPECTED_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(E)::value>
+struct expected_copy_base : expected_operations_base<T, E> {
+  using expected_operations_base<T, E>::expected_operations_base;
+};
+
+// This specialization is for when T or E are 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)
+      : expected_operations_base<T, E>(no_init) {
+    if (rhs.has_value()) {
+      this->construct_with(rhs);
+    } 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 = is_void_or<T, std::is_trivially_move_constructible<T>>::value
+              &&std::is_trivially_move_constructible<E>::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)
+      : expected_copy_base<T, E>(no_init) {
+    if (rhs.has_value()) {
+      this->construct_with(std::move(rhs));
+    } 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_void_or<
+              T, conjunction<TL_EXPECTED_IS_TRIVIALLY_COPY_ASSIGNABLE(T),
+                             TL_EXPECTED_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T),
+                             TL_EXPECTED_IS_TRIVIALLY_DESTRUCTIBLE(T)>>::value
+              &&TL_EXPECTED_IS_TRIVIALLY_COPY_ASSIGNABLE(E)::value
+                  &&TL_EXPECTED_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(E)::value
+                      &&TL_EXPECTED_IS_TRIVIALLY_DESTRUCTIBLE(E)::value>
+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);
+    return *this;
+  }
+  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 =
+              is_void_or<T, conjunction<std::is_trivially_destructible<T>,
+                                        std::is_trivially_move_constructible<T>,
+                                        std::is_trivially_move_assignable<T>>>::
+                  value &&std::is_trivially_destructible<E>::value
+                      &&std::is_trivially_move_constructible<E>::value
+                          &&std::is_trivially_move_assignable<E>::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) = default;
+
+  expected_move_assign_base &
+  operator=(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));
+    return *this;
+  }
+};
+
+// expected_delete_ctor_base will conditionally delete copy and move
+// constructors depending on whether T is copy/move constructible
+template <class T, class E,
+          bool EnableCopy = (is_copy_constructible_or_void<T>::value &&
+                             std::is_copy_constructible<E>::value),
+          bool EnableMove = (is_move_constructible_or_void<T>::value &&
+                             std::is_move_constructible<E>::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, class E>
+struct expected_delete_ctor_base<T, E, 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, class E>
+struct expected_delete_ctor_base<T, E, 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, class E>
+struct expected_delete_ctor_base<T, E, 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 and E are copy/move constructible +
+// assignable
+template <class T, class E,
+          bool EnableCopy = (is_copy_constructible_or_void<T>::value &&
+                             std::is_copy_constructible<E>::value &&
+                             is_copy_assignable_or_void<T>::value &&
+                             std::is_copy_assignable<E>::value),
+          bool EnableMove = (is_move_constructible_or_void<T>::value &&
+                             std::is_move_constructible<E>::value &&
+                             is_move_assignable_or_void<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, class E>
+struct expected_delete_assign_base<T, E, 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, class E>
+struct expected_delete_assign_base<T, E, 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, class E>
+struct expected_delete_assign_base<T, E, 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 || std::is_void<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_base<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");
+
+  T *valptr() { return std::addressof(this->m_val); }
+  const T *valptr() const { return std::addressof(this->m_val); }    
+  unexpected<E> *errptr() { return std::addressof(this->m_unexpect); }
+  const unexpected<E> *errptr() const { return std::addressof(this->m_unexpect); }    
+
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR U &val() {
+    return this->m_val;
+  }
+  TL_EXPECTED_11_CONSTEXPR unexpected<E> &err() { return this->m_unexpect; }
+
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  constexpr const U &val() const {
+    return this->m_val;
+  }
+  constexpr const unexpected<E> &err() const { return this->m_unexpect; }
+
+  using impl_base = detail::expected_move_assign_base<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) && !defined(TL_EXPECTED_GCC55)
+  template <class F> TL_EXPECTED_11_CONSTEXPR auto and_then(F &&f) & {
+    return and_then_impl(*this, std::forward<F>(f));
+  }
+  template <class F> TL_EXPECTED_11_CONSTEXPR auto and_then(F &&f) && {
+    return and_then_impl(std::move(*this), std::forward<F>(f));
+  }
+  template <class F> constexpr auto and_then(F &&f) const & {
+    return and_then_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_EXPECTED_NO_CONSTRR
+  template <class F> constexpr auto and_then(F &&f) const && {
+    return and_then_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+
+#else
+  template <class F>
+  TL_EXPECTED_11_CONSTEXPR auto
+  and_then(F &&f) & -> decltype(and_then_impl(std::declval<expected&>(), std::forward<F>(f))) {
+    return and_then_impl(*this, std::forward<F>(f));
+  }
+  template <class F>
+  TL_EXPECTED_11_CONSTEXPR auto and_then(F &&f) && -> decltype(
+      and_then_impl(std::declval<expected&&>(), std::forward<F>(f))) {
+    return and_then_impl(std::move(*this), std::forward<F>(f));
+  }
+  template <class F>
+  constexpr auto and_then(F &&f) const & -> decltype(
+      and_then_impl(std::declval<expected const&>(), std::forward<F>(f))) {
+    return and_then_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_EXPECTED_NO_CONSTRR
+  template <class F>
+  constexpr auto and_then(F &&f) const && -> decltype(
+      and_then_impl(std::declval<expected const&&>(), std::forward<F>(f))) {
+    return and_then_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+#endif
+
+#if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
+    !defined(TL_EXPECTED_GCC54) && !defined(TL_EXPECTED_GCC55)
+    template <class F> TL_EXPECTED_11_CONSTEXPR auto map(F &&f) & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+  template <class F> TL_EXPECTED_11_CONSTEXPR auto map(F &&f) && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+  template <class F> constexpr auto map(F &&f) const & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+  template <class F> constexpr auto map(F &&f) const && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#else
+  template <class F>
+  TL_EXPECTED_11_CONSTEXPR decltype(
+      expected_map_impl(std::declval<expected &>(), std::declval<F &&>()))
+  map(F &&f) & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+  template <class F>
+  TL_EXPECTED_11_CONSTEXPR decltype(
+      expected_map_impl(std::declval<expected>(), std::declval<F &&>()))
+  map(F &&f) && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+  template <class F>
+  constexpr decltype(expected_map_impl(std::declval<const expected &>(),
+                                       std::declval<F &&>()))
+  map(F &&f) const & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_EXPECTED_NO_CONSTRR
+  template <class F>
+  constexpr decltype(expected_map_impl(std::declval<const expected &&>(),
+                                       std::declval<F &&>()))
+  map(F &&f) const && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+#endif
+
+#if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
+    !defined(TL_EXPECTED_GCC54) && !defined(TL_EXPECTED_GCC55)
+    template <class F> TL_EXPECTED_11_CONSTEXPR auto transform(F &&f) & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+  template <class F> TL_EXPECTED_11_CONSTEXPR auto transform(F &&f) && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+  template <class F> constexpr auto transform(F &&f) const & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+  template <class F> constexpr auto transform(F &&f) const && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#else
+    template <class F>
+  TL_EXPECTED_11_CONSTEXPR decltype(
+      expected_map_impl(std::declval<expected &>(), std::declval<F &&>()))
+  transform(F &&f) & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+  template <class F>
+  TL_EXPECTED_11_CONSTEXPR decltype(
+      expected_map_impl(std::declval<expected>(), std::declval<F &&>()))
+  transform(F &&f) && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+  template <class F>
+  constexpr decltype(expected_map_impl(std::declval<const expected &>(),
+                                       std::declval<F &&>()))
+  transform(F &&f) const & {
+    return expected_map_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_EXPECTED_NO_CONSTRR
+  template <class F>
+  constexpr decltype(expected_map_impl(std::declval<const expected &&>(),
+                                       std::declval<F &&>()))
+  transform(F &&f) const && {
+    return expected_map_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+#endif
+
+#if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
+    !defined(TL_EXPECTED_GCC54) && !defined(TL_EXPECTED_GCC55)
+  template <class F> TL_EXPECTED_11_CONSTEXPR auto map_error(F &&f) & {
+    return map_error_impl(*this, std::forward<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));
+  }
+  template <class F> constexpr auto map_error(F &&f) const & {
+    return map_error_impl(*this, std::forward<F>(f));
+  }
+  template <class F> constexpr auto map_error(F &&f) const && {
+    return map_error_impl(std::move(*this), std::forward<F>(f));
+  }
+#else
+  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));
+  }
+  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));
+  }
+  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
+  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
+  template <class F> expected TL_EXPECTED_11_CONSTEXPR or_else(F &&f) & {
+    return or_else_impl(*this, std::forward<F>(f));
+  }
+
+  template <class F> expected TL_EXPECTED_11_CONSTEXPR or_else(F &&f) && {
+    return or_else_impl(std::move(*this), std::forward<F>(f));
+  }
+
+  template <class F> expected constexpr or_else(F &&f) const & {
+    return or_else_impl(*this, std::forward<F>(f));
+  }
+
+#ifndef TL_EXPECTED_NO_CONSTRR
+  template <class F> expected constexpr or_else(F &&f) const && {
+    return or_else_impl(std::move(*this), std::forward<F>(f));
+  }
+#endif
+  constexpr expected() = default;
+  constexpr expected(const expected &rhs) = default;
+  constexpr expected(expected &&rhs) = default;
+  expected &operator=(const expected &rhs) = default;
+  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{}) {}
+
+  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{}) {}
+
+    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{}) {}
+
+  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{}) {}
+
+    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{}) {}
+
+    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{}) {}
+
+  template <class U, class G,
+            detail::enable_if_t<!(std::is_convertible<U const &, T>::value &&
+                                  std::is_convertible<G const &, E>::value)> * =
+                nullptr,
+            detail::expected_enable_from_other<T, E, U, G, const U &, const G &>
+                * = nullptr>
+  explicit TL_EXPECTED_11_CONSTEXPR expected(const expected<U, G> &rhs)
+      : ctor_base(detail::default_constructor_tag{}) {
+    if (rhs.has_value()) {
+      this->construct(*rhs);
+    } else {
+      this->construct_error(rhs.error());        
+    }
+  }
+
+    template <class U, class G,
+            detail::enable_if_t<(std::is_convertible<U const &, T>::value &&
+                                 std::is_convertible<G const &, E>::value)> * =
+                nullptr,
+            detail::expected_enable_from_other<T, E, U, G, const U &, const G &>
+                * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR expected(const expected<U, G> &rhs)
+      : ctor_base(detail::default_constructor_tag{}) {
+    if (rhs.has_value()) {
+      this->construct(*rhs);
+    } else {
+      this->construct_error(rhs.error());        
+    }      
+  }
+
+  template <
+      class U, class G,
+      detail::enable_if_t<!(std::is_convertible<U &&, T>::value &&
+                            std::is_convertible<G &&, E>::value)> * = nullptr,
+      detail::expected_enable_from_other<T, E, U, G, U &&, G &&> * = nullptr>
+  explicit TL_EXPECTED_11_CONSTEXPR expected(expected<U, G> &&rhs)
+      : ctor_base(detail::default_constructor_tag{}) {
+    if (rhs.has_value()) {
+      this->construct(std::move(*rhs));
+    } else {
+      this->construct_error(std::move(rhs.error()));        
+    }            
+  }
+
+    template <
+      class U, class G,
+      detail::enable_if_t<(std::is_convertible<U &&, T>::value &&
+                           std::is_convertible<G &&, E>::value)> * = nullptr,
+      detail::expected_enable_from_other<T, E, U, G, U &&, G &&> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR expected(expected<U, G> &&rhs)
+      : ctor_base(detail::default_constructor_tag{}) {
+    if (rhs.has_value()) {
+      this->construct(std::move(*rhs));
+    } else {
+      this->construct_error(std::move(rhs.error()));        
+    }                  
+  }
+
+  template <
+      class U = T,
+      detail::enable_if_t<!std::is_convertible<U &&, T>::value> * = nullptr,
+      detail::expected_enable_forward_value<T, E, U> * = nullptr>
+  explicit TL_EXPECTED_MSVC2015_CONSTEXPR expected(U &&v)
+      : expected(in_place, std::forward<U>(v)) {}
+
+    template <
+      class U = T,
+      detail::enable_if_t<std::is_convertible<U &&, T>::value> * = nullptr,
+      detail::expected_enable_forward_value<T, E, U> * = nullptr>
+  TL_EXPECTED_MSVC2015_CONSTEXPR expected(U &&v)
+      : expected(in_place, std::forward<U>(v)) {}
+
+  template <
+      class U = T, class G = T,
+      detail::enable_if_t<std::is_nothrow_constructible<T, U &&>::value> * =
+          nullptr,
+      detail::enable_if_t<!std::is_void<G>::value> * = nullptr,
+      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<G &, U>::value &&
+           std::is_nothrow_move_constructible<E>::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;
+  }
+
+    template <
+      class U = T, class G = T,
+      detail::enable_if_t<!std::is_nothrow_constructible<T, U &&>::value> * =
+          nullptr,
+      detail::enable_if_t<!std::is_void<U>::value> * = nullptr,
+      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<G &, U>::value &&
+           std::is_nothrow_move_constructible<E>::value)> * = nullptr>
+  expected &operator=(U &&v) {
+    if (has_value()) {
+      val() = std::forward<U>(v);
+    } else {
+      auto tmp = std::move(err());
+      err().~unexpected<E>();
+
+      #ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+      try {
+        ::new (valptr()) T(std::forward<U>(v));
+        this->m_has_val = true;
+      } catch (...) {
+        err() = std::move(tmp);
+        throw;
+      }
+      #else
+        ::new (valptr()) T(std::forward<U>(v));
+        this->m_has_val = true;
+      #endif
+    }
+
+    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 {
+      this->destroy_val();
+      ::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 {
+      this->destroy_val();
+      ::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;
+    }
+  }
+
+    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>();
+
+      #ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+      try {
+        ::new (valptr()) T(std::forward<Args>(args)...);
+        this->m_has_val = true;
+      } catch (...) {
+        err() = std::move(tmp);
+        throw;
+      }
+      #else
+      ::new (valptr()) T(std::forward<Args>(args)...);
+      this->m_has_val = true;
+      #endif
+    }
+  }
+
+  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;
+    }
+  }
+
+    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>();
+
+      #ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+      try {
+        ::new (valptr()) T(il, std::forward<Args>(args)...);
+        this->m_has_val = true;
+      } catch (...) {
+        err() = std::move(tmp);
+        throw;
+      }
+      #else
+      ::new (valptr()) T(il, std::forward<Args>(args)...);
+      this->m_has_val = true;
+      #endif
+    }
+  }
+
+private:
+  using t_is_void = std::true_type;
+  using t_is_not_void = std::false_type;
+  using t_is_nothrow_move_constructible = std::true_type;
+  using move_constructing_t_can_throw = std::false_type;
+  using e_is_nothrow_move_constructible = std::true_type;
+  using move_constructing_e_can_throw = std::false_type;
+
+  void swap_where_both_have_value(expected &/*rhs*/ , t_is_void) noexcept {
+    // swapping void is a no-op
+  }
+
+  void swap_where_both_have_value(expected &rhs, t_is_not_void) {
+    using std::swap;
+    swap(val(), rhs.val());
+  }
+
+  void swap_where_only_one_has_value(expected &rhs, t_is_void) noexcept(
+      std::is_nothrow_move_constructible<E>::value) {
+    ::new (errptr()) unexpected_type(std::move(rhs.err()));
+    rhs.err().~unexpected_type();
+    std::swap(this->m_has_val, rhs.m_has_val);
+  }
+
+  void swap_where_only_one_has_value(expected &rhs, t_is_not_void) {
+    swap_where_only_one_has_value_and_t_is_not_void(
+        rhs, typename std::is_nothrow_move_constructible<T>::type{},
+        typename std::is_nothrow_move_constructible<E>::type{});
+  }
+
+  void swap_where_only_one_has_value_and_t_is_not_void(
+      expected &rhs, t_is_nothrow_move_constructible,
+      e_is_nothrow_move_constructible) noexcept {
+    auto temp = std::move(val());
+    val().~T();
+    ::new (errptr()) unexpected_type(std::move(rhs.err()));
+    rhs.err().~unexpected_type();
+    ::new (rhs.valptr()) T(std::move(temp));
+    std::swap(this->m_has_val, rhs.m_has_val);
+  }
+
+  void swap_where_only_one_has_value_and_t_is_not_void(
+      expected &rhs, t_is_nothrow_move_constructible,
+      move_constructing_e_can_throw) {
+    auto temp = std::move(val());
+    val().~T();
+#ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+    try {
+      ::new (errptr()) unexpected_type(std::move(rhs.err()));
+      rhs.err().~unexpected_type();
+      ::new (rhs.valptr()) T(std::move(temp));
+      std::swap(this->m_has_val, rhs.m_has_val);
+    } catch (...) {
+      val() = std::move(temp);
+      throw;
+    }
+#else
+    ::new (errptr()) unexpected_type(std::move(rhs.err()));
+    rhs.err().~unexpected_type();
+    ::new (rhs.valptr()) T(std::move(temp));
+    std::swap(this->m_has_val, rhs.m_has_val);
+#endif
+  }
+
+  void swap_where_only_one_has_value_and_t_is_not_void(
+      expected &rhs, move_constructing_t_can_throw,
+      t_is_nothrow_move_constructible) {
+    auto temp = std::move(rhs.err());
+    rhs.err().~unexpected_type();
+#ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+    try {
+      ::new (rhs.valptr()) T(val());
+      val().~T();
+      ::new (errptr()) unexpected_type(std::move(temp));
+      std::swap(this->m_has_val, rhs.m_has_val);
+    } catch (...) {
+      rhs.err() = std::move(temp);
+      throw;
+    }
+#else
+    ::new (rhs.valptr()) T(val());
+    val().~T();
+    ::new (errptr()) unexpected_type(std::move(temp));
+    std::swap(this->m_has_val, rhs.m_has_val);
+#endif
+  }
+
+public:
+  template <class OT = T, class OE = E>
+  detail::enable_if_t<detail::is_swappable<OT>::value &&
+                      detail::is_swappable<OE>::value &&
+                      (std::is_nothrow_move_constructible<OT>::value ||
+                       std::is_nothrow_move_constructible<OE>::value)>
+  swap(expected &rhs) noexcept(
+      std::is_nothrow_move_constructible<T>::value
+          &&detail::is_nothrow_swappable<T>::value
+              &&std::is_nothrow_move_constructible<E>::value
+                  &&detail::is_nothrow_swappable<E>::value) {
+    if (has_value() && rhs.has_value()) {
+      swap_where_both_have_value(rhs, typename std::is_void<T>::type{});
+    } else if (!has_value() && rhs.has_value()) {
+      rhs.swap(*this);
+    } else if (has_value()) {
+      swap_where_only_one_has_value(rhs, typename std::is_void<T>::type{});
+    } else {
+      using std::swap;
+      swap(err(), rhs.err());
+    }
+  }
+
+  constexpr const T *operator->() const { return valptr(); }
+  TL_EXPECTED_11_CONSTEXPR T *operator->() { return valptr(); }
+
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  constexpr const U &operator*() const & {
+    return val();
+  }
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR U &operator*() & {
+    return val();
+  }
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  constexpr const U &&operator*() const && {
+    return std::move(val());
+  }
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR U &&operator*() && {
+    return std::move(val());
+  }
+
+  constexpr bool has_value() const noexcept { return this->m_has_val; }
+  constexpr explicit operator bool() const noexcept { return this->m_has_val; }
+
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR const U &value() const & {
+    if (!has_value())
+      detail::throw_exception(bad_expected_access<E>(err().value()));
+    return val();
+  }
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR U &value() & {
+    if (!has_value())
+      detail::throw_exception(bad_expected_access<E>(err().value()));
+    return val();
+  }
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR const U &&value() const && {
+    if (!has_value())
+      detail::throw_exception(bad_expected_access<E>(std::move(err()).value()));
+    return std::move(val());
+  }
+  template <class U = T,
+            detail::enable_if_t<!std::is_void<U>::value> * = nullptr>
+  TL_EXPECTED_11_CONSTEXPR U &&value() && {
+    if (!has_value())
+      detail::throw_exception(bad_expected_access<E>(std::move(err()).value()));
+    return std::move(val());
+  }
+
+  constexpr const E &error() const & { return err().value(); }
+  TL_EXPECTED_11_CONSTEXPR E &error() & { return err().value(); }
+  constexpr const E &&error() const && { return std::move(err().value()); }
+  TL_EXPECTED_11_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> TL_EXPECTED_11_CONSTEXPR 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));
+  }
+};
+
+namespace detail {
+template <class Exp> using exp_t = typename detail::decay_t<Exp>::value_type;
+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,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Exp>()))>
+constexpr auto and_then_impl(Exp &&exp, F &&f) {
+  static_assert(detail::is_expected<Ret>::value, "F must return an expected");
+
+  return exp.has_value()
+             ? detail::invoke(std::forward<F>(f), *std::forward<Exp>(exp))
+             : Ret(unexpect, std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,
+          class Ret = decltype(detail::invoke(std::declval<F>()))>
+constexpr auto and_then_impl(Exp &&exp, F &&f) {
+  static_assert(detail::is_expected<Ret>::value, "F must return an expected");
+
+  return exp.has_value() ? detail::invoke(std::forward<F>(f))
+                         : Ret(unexpect, std::forward<Exp>(exp).error());
+}
+#else
+template <class> struct TC;
+template <class Exp, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Exp>())),
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr>
+auto and_then_impl(Exp &&exp, F &&f) -> Ret {
+  static_assert(detail::is_expected<Ret>::value, "F must return an expected");
+
+  return exp.has_value()
+             ? detail::invoke(std::forward<F>(f), *std::forward<Exp>(exp))
+             : Ret(unexpect, std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>())),
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr>
+constexpr auto and_then_impl(Exp &&exp, F &&f) -> Ret {
+  static_assert(detail::is_expected<Ret>::value, "F must return an expected");
+
+  return exp.has_value() ? detail::invoke(std::forward<F>(f))
+                         : Ret(unexpect, std::forward<Exp>(exp).error());
+}
+#endif
+
+#ifdef TL_EXPECTED_CXX14
+template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,          
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Exp>())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto expected_map_impl(Exp &&exp, F &&f) {
+  using result = ret_t<Exp, detail::decay_t<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,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,          
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Exp>())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto expected_map_impl(Exp &&exp, F &&f) {
+  using result = expected<void, err_t<Exp>>;
+  if (exp.has_value()) {
+    detail::invoke(std::forward<F>(f), *std::forward<Exp>(exp));
+    return result();
+  }
+
+  return result(unexpect, std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,
+          class Ret = decltype(detail::invoke(std::declval<F>())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto expected_map_impl(Exp &&exp, F &&f) {
+  using result = ret_t<Exp, detail::decay_t<Ret>>;
+  return exp.has_value() ? result(detail::invoke(std::forward<F>(f)))
+                         : result(unexpect, std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,          
+          class Ret = decltype(detail::invoke(std::declval<F>())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto expected_map_impl(Exp &&exp, F &&f) {
+  using result = expected<void, err_t<Exp>>;
+  if (exp.has_value()) {
+    detail::invoke(std::forward<F>(f));
+    return result();
+  }
+
+  return result(unexpect, std::forward<Exp>(exp).error());
+}    
+#else
+template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,          
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Exp>())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+
+constexpr auto expected_map_impl(Exp &&exp, F &&f)
+    -> ret_t<Exp, detail::decay_t<Ret>> {
+  using result = ret_t<Exp, detail::decay_t<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,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,                    
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              *std::declval<Exp>())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+
+auto expected_map_impl(Exp &&exp, F &&f) -> expected<void, err_t<Exp>> {
+  if (exp.has_value()) {
+    detail::invoke(std::forward<F>(f), *std::forward<Exp>(exp));
+    return {};
+  }
+
+  return unexpected<err_t<Exp>>(std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                              
+          class Ret = decltype(detail::invoke(std::declval<F>())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+
+constexpr auto expected_map_impl(Exp &&exp, F &&f)
+    -> ret_t<Exp, detail::decay_t<Ret>> {
+  using result = ret_t<Exp, detail::decay_t<Ret>>;
+
+  return exp.has_value() ? result(detail::invoke(std::forward<F>(f)))
+                         : result(unexpect, std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                                        
+          class Ret = decltype(detail::invoke(std::declval<F>())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+
+auto expected_map_impl(Exp &&exp, F &&f) -> expected<void, err_t<Exp>> {
+  if (exp.has_value()) {
+    detail::invoke(std::forward<F>(f));
+    return {};
+  }
+
+  return unexpected<err_t<Exp>>(std::forward<Exp>(exp).error());
+}    
+#endif
+
+#if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
+    !defined(TL_EXPECTED_GCC54) && !defined(TL_EXPECTED_GCC55)
+template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,          
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto map_error_impl(Exp &&exp, F &&f) {
+  using result = expected<exp_t<Exp>, detail::decay_t<Ret>>;
+  return exp.has_value()
+             ? result(*std::forward<Exp>(exp))
+             : result(unexpect, detail::invoke(std::forward<F>(f),
+                                               std::forward<Exp>(exp).error()));
+}
+template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,                    
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto map_error_impl(Exp &&exp, F &&f) {
+  using result = expected<exp_t<Exp>, monostate>;
+  if (exp.has_value()) {
+    return result(*std::forward<Exp>(exp));
+  }
+
+  detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+  return result(unexpect, monostate{});
+}
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,          
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto map_error_impl(Exp &&exp, F &&f) {
+  using result = expected<exp_t<Exp>, detail::decay_t<Ret>>;
+  return exp.has_value()
+             ? result()
+             : result(unexpect, detail::invoke(std::forward<F>(f),
+                                               std::forward<Exp>(exp).error()));
+}
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                    
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto map_error_impl(Exp &&exp, F &&f) {
+  using result = expected<exp_t<Exp>, monostate>;
+  if (exp.has_value()) {
+    return result();
+  }
+
+  detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+  return result(unexpect, monostate{});
+}    
+#else
+template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,                              
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto map_error_impl(Exp &&exp, F &&f)
+    -> expected<exp_t<Exp>, detail::decay_t<Ret>> {
+  using result = expected<exp_t<Exp>, detail::decay_t<Ret>>;
+
+  return exp.has_value()
+             ? result(*std::forward<Exp>(exp))
+             : result(unexpect, detail::invoke(std::forward<F>(f),
+                                               std::forward<Exp>(exp).error()));
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,                                        
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto map_error_impl(Exp &&exp, F &&f) -> expected<exp_t<Exp>, monostate> {
+  using result = expected<exp_t<Exp>, monostate>;
+  if (exp.has_value()) {
+    return result(*std::forward<Exp>(exp));
+  }
+
+  detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+  return result(unexpect, monostate{});
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                              
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto map_error_impl(Exp &&exp, F &&f)
+    -> expected<exp_t<Exp>, detail::decay_t<Ret>> {
+  using result = expected<exp_t<Exp>, detail::decay_t<Ret>>;
+
+  return exp.has_value()
+             ? result()
+             : result(unexpect, detail::invoke(std::forward<F>(f),
+                                               std::forward<Exp>(exp).error()));
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                                        
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto map_error_impl(Exp &&exp, F &&f) -> expected<exp_t<Exp>, monostate> {
+  using result = expected<exp_t<Exp>, monostate>;
+  if (exp.has_value()) {
+    return result();
+  }
+
+  detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+  return result(unexpect, monostate{});
+}    
+#endif
+
+#ifdef TL_EXPECTED_CXX14
+template <class Exp, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto or_else_impl(Exp &&exp, F &&f) {
+  static_assert(detail::is_expected<Ret>::value, "F must return an expected");
+  return exp.has_value()
+  ? std::forward<Exp>(exp)
+  : detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+detail::decay_t<Exp> or_else_impl(Exp &&exp, F &&f) {
+  return exp.has_value()
+  ? std::forward<Exp>(exp)
+  : (detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error()),
+    std::forward<Exp>(exp));
+}
+#else
+template <class Exp, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+  detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+auto or_else_impl(Exp &&exp, F &&f) -> Ret {
+  static_assert(detail::is_expected<Ret>::value, "F must return an expected");
+  return exp.has_value()
+         ? std::forward<Exp>(exp)
+         : detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+}
+
+template <class Exp, class F,
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+  detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+detail::decay_t<Exp> or_else_impl(Exp &&exp, F &&f) {
+  return exp.has_value()
+         ? std::forward<Exp>(exp)
+         : (detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error()),
+            std::forward<Exp>(exp));
+}
+#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>
+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>
+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 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,
+          detail::enable_if_t<(std::is_void<T>::value ||
+                               std::is_move_constructible<T>::value) &&
+                              detail::is_swappable<T>::value &&
+                              std::is_move_constructible<E>::value &&
+                              detail::is_swappable<E>::value> * = nullptr>
+void swap(expected<T, E> &lhs,
+          expected<T, E> &rhs) noexcept(noexcept(lhs.swap(rhs))) {
+  lhs.swap(rhs);
+}
+} // namespace tl
+
+#endif
diff --git a/lib/utils/mu-error.hh b/lib/utils/mu-error.hh
index f911977..7c39587 100644
--- a/lib/utils/mu-error.hh
+++ b/lib/utils/mu-error.hh
@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2019 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2019-2020 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -129,8 +129,6 @@ private:
         std::string  what_;
 };
 
-
 } // namespace Mu
 
-
 #endif /* MU_ERROR_HH__ */
diff --git a/lib/utils/mu-option.hh b/lib/utils/mu-option.hh
new file mode 100644
index 0000000..06bb3a3
--- /dev/null
+++ b/lib/utils/mu-option.hh
@@ -0,0 +1,26 @@
+/*
+ * Created on 2020-11-08 by Dirk-Jan C. Binnema <dbinnema@logitech.com>
+ *
+ * Copyright (c) 2020 Logitech, Inc.  All Rights Reserved
+ * This program is a trade secret of LOGITECH, and it is not to be reproduced,
+ * published, disclosed to others, copied, adapted, distributed or displayed
+ * without the prior authorization of LOGITECH.
+ *
+ * Licensee agrees to attach or embed this notice on all copies of the program,
+ * including partial copies or modified versions thereof.
+ *
+ */
+
+#ifndef MU_OPTION__
+#define MU_OPTION__
+
+#include "optional.hpp"
+
+
+namespace Mu {
+
+/// Either a value of type T, or nothing.
+template <typename T> using Option=tl::optional<T>;
+
+}
+#endif /*MU_OPTION__*/
diff --git a/lib/utils/mu-result.hh b/lib/utils/mu-result.hh
new file mode 100644
index 0000000..b871353
--- /dev/null
+++ b/lib/utils/mu-result.hh
@@ -0,0 +1,39 @@
+/*
+** Copyright (C) 2019-2020 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 3, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+
+#ifndef MU_RESULT_HH__
+#define MU_RESULT_HH__
+
+
+#include "expected.hpp"
+#include "utils/mu-error.hh"
+
+
+namespace Mu {
+/**
+ * A Result is _either_ some value of type T, _or_ an error.
+ *
+ */
+template <typename T> using Result = tl::expected<T, Error>;
+
+} // namespace Mu
+
+
+#endif /* MU_ERROR_HH__ */
diff --git a/lib/utils/optional.hpp b/lib/utils/optional.hpp
new file mode 100644
index 0000000..ad8ae2f
--- /dev/null
+++ b/lib/utils/optional.hpp
@@ -0,0 +1,2062 @@
+
+///
+// optional - An implementation of std::optional with extensions
+// Written in 2017 by Simon Brand (simonrbrand@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 0
+#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) noexcept {
+    new (std::addressof(this->m_value)) T(std::forward<Args>(args)...);
+    this->m_has_value = true;
+  }
+
+  template <class Opt> void assign(Opt &&rhs) {
+    if (this->has_value()) {
+      if (rhs.has_value()) {
+        this->m_value = std::forward<Opt>(rhs).get();
+      } else {
+        this->m_value.~T();
+        this->m_has_value = false;
+      }
+    }
+
+    else if (rhs.has_value()) {
+      construct(std::forward<Opt>(rhs).get());
+    }
+  }
+
+  bool has_value() const { return this->m_has_value; }
+
+  TL_OPTIONAL_11_CONSTEXPR T &get() & { return this->m_value; }
+  TL_OPTIONAL_11_CONSTEXPR const T &get() const & { return this->m_value; }
+  TL_OPTIONAL_11_CONSTEXPR T &&get() && { return std::move(this->m_value); }
+#ifndef TL_OPTIONAL_NO_CONSTRR
+  constexpr const T &&get() const && { return std::move(this->m_value); }
+#endif
+};
+
+// This class manages conditionally having a trivial copy constructor
+// This specialization is for when T is trivially copy constructible
+template <class T, bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T)>
+struct optional_copy_base : optional_operations_base<T> {
+  using optional_operations_base<T>::optional_operations_base;
+};
+
+// This specialization is for when T is not trivially copy constructible
+template <class T>
+struct optional_copy_base<T, false> : optional_operations_base<T> {
+  using optional_operations_base<T>::optional_operations_base;
+
+  optional_copy_base() = default;
+  optional_copy_base(const optional_copy_base &rhs) {
+    if (rhs.has_value()) {
+      this->construct(rhs.get());
+    } else {
+      this->m_has_value = false;
+    }
+  }
+
+  optional_copy_base(optional_copy_base &&rhs) = default;
+  optional_copy_base &operator=(const optional_copy_base &rhs) = default;
+  optional_copy_base &operator=(optional_copy_base &&rhs) = default;
+};
+
+// This class manages conditionally having a trivial move constructor
+// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
+// doesn't implement an analogue to std::is_trivially_move_constructible. We
+// have to make do with a non-trivial move constructor even if T is trivially
+// move constructible
+#ifndef TL_OPTIONAL_GCC49
+template <class T, bool = std::is_trivially_move_constructible<T>::value>
+struct optional_move_base : optional_copy_base<T> {
+  using optional_copy_base<T>::optional_copy_base;
+};
+#else
+template <class T, bool = false> struct optional_move_base;
+#endif
+template <class T> struct optional_move_base<T, false> : optional_copy_base<T> {
+  using optional_copy_base<T>::optional_copy_base;
+
+  optional_move_base() = default;
+  optional_move_base(const optional_move_base &rhs) = default;
+
+  optional_move_base(optional_move_base &&rhs) noexcept(
+      std::is_nothrow_move_constructible<T>::value) {
+    if (rhs.has_value()) {
+      this->construct(std::move(rhs.get()));
+    } else {
+      this->m_has_value = false;
+    }
+  }
+  optional_move_base &operator=(const optional_move_base &rhs) = default;
+  optional_move_base &operator=(optional_move_base &&rhs) = default;
+};
+
+// This class manages conditionally having a trivial copy assignment operator
+template <class T, bool = TL_OPTIONAL_IS_TRIVIALLY_COPY_ASSIGNABLE(T) &&
+                          TL_OPTIONAL_IS_TRIVIALLY_COPY_CONSTRUCTIBLE(T) &&
+                          TL_OPTIONAL_IS_TRIVIALLY_DESTRUCTIBLE(T)>
+struct optional_copy_assign_base : optional_move_base<T> {
+  using optional_move_base<T>::optional_move_base;
+};
+
+template <class T>
+struct optional_copy_assign_base<T, false> : optional_move_base<T> {
+  using optional_move_base<T>::optional_move_base;
+
+  optional_copy_assign_base() = default;
+  optional_copy_assign_base(const optional_copy_assign_base &rhs) = default;
+
+  optional_copy_assign_base(optional_copy_assign_base &&rhs) = default;
+  optional_copy_assign_base &operator=(const optional_copy_assign_base &rhs) {
+    this->assign(rhs);
+    return *this;
+  }
+  optional_copy_assign_base &
+  operator=(optional_copy_assign_base &&rhs) = default;
+};
+
+// This class manages conditionally having a trivial move assignment operator
+// Unfortunately there's no way to achieve this in GCC < 5 AFAIK, since it
+// doesn't implement an analogue to std::is_trivially_move_assignable. We have
+// to make do with a non-trivial move assignment operator even if T is trivially
+// move assignable
+#ifndef TL_OPTIONAL_GCC49
+template <class T, bool = std::is_trivially_destructible<T>::value
+                       &&std::is_trivially_move_constructible<T>::value
+                           &&std::is_trivially_move_assignable<T>::value>
+struct optional_move_assign_base : optional_copy_assign_base<T> {
+  using optional_copy_assign_base<T>::optional_copy_assign_base;
+};
+#else
+template <class T, bool = false> struct optional_move_assign_base;
+#endif
+
+template <class T>
+struct optional_move_assign_base<T, false> : optional_copy_assign_base<T> {
+  using optional_copy_assign_base<T>::optional_copy_assign_base;
+
+  optional_move_assign_base() = default;
+  optional_move_assign_base(const optional_move_assign_base &rhs) = default;
+
+  optional_move_assign_base(optional_move_assign_base &&rhs) = default;
+
+  optional_move_assign_base &
+  operator=(const optional_move_assign_base &rhs) = default;
+
+  optional_move_assign_base &
+  operator=(optional_move_assign_base &&rhs) noexcept(
+      std::is_nothrow_move_constructible<T>::value
+          &&std::is_nothrow_move_assignable<T>::value) {
+    this->assign(std::move(rhs));
+    return *this;
+  }
+};
+
+// optional_delete_ctor_base will conditionally delete copy and move
+// constructors depending on whether T is copy/move constructible
+template <class T, bool EnableCopy = std::is_copy_constructible<T>::value,
+          bool EnableMove = std::is_move_constructible<T>::value>
+struct optional_delete_ctor_base {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = default;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_ctor_base<T, true, false> {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = delete;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_ctor_base<T, false, true> {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = delete;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = default;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_ctor_base<T, false, false> {
+  optional_delete_ctor_base() = default;
+  optional_delete_ctor_base(const optional_delete_ctor_base &) = delete;
+  optional_delete_ctor_base(optional_delete_ctor_base &&) noexcept = delete;
+  optional_delete_ctor_base &
+  operator=(const optional_delete_ctor_base &) = default;
+  optional_delete_ctor_base &
+  operator=(optional_delete_ctor_base &&) noexcept = default;
+};
+
+// optional_delete_assign_base will conditionally delete copy and move
+// constructors depending on whether T is copy/move constructible + assignable
+template <class T,
+          bool EnableCopy = (std::is_copy_constructible<T>::value &&
+                             std::is_copy_assignable<T>::value),
+          bool EnableMove = (std::is_move_constructible<T>::value &&
+                             std::is_move_assignable<T>::value)>
+struct optional_delete_assign_base {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_assign_base<T, true, false> {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = delete;
+};
+
+template <class T> struct optional_delete_assign_base<T, false, true> {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = delete;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = default;
+};
+
+template <class T> struct optional_delete_assign_base<T, false, false> {
+  optional_delete_assign_base() = default;
+  optional_delete_assign_base(const optional_delete_assign_base &) = default;
+  optional_delete_assign_base(optional_delete_assign_base &&) noexcept =
+      default;
+  optional_delete_assign_base &
+  operator=(const optional_delete_assign_base &) = delete;
+  optional_delete_assign_base &
+  operator=(optional_delete_assign_base &&) noexcept = delete;
+};
+
+} // namespace detail
+
+/// 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();
+      }
+    }
+
+    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();
+      }
+    }
+
+    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() ? **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), **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