Implement N4606 optional

Summary:
Adapt implementation of Library Fundamentals TS optional into an implementation of N4606 optional.

  - Update relational operators per http://wg21.link/P0307
  - Update to requirements of http://wg21.link/P0032
  - Extension: Implement trivial copy/move construction/assignment for `optional<T>` when `T` is trivially copyable.

Audit P/Rs for optional LWG issues:
  - 2756 "C++ WP optional<T> should 'forward' T's implicit conversions" Implemented, which also resolves 2753 "Optional's constructors and assignments need constraints" (modulo my refusal to explicitly delete the move operations, which is a design error that I'm working on correcting in the 2756 P/R).
  - 2736 "nullopt_t insufficiently constrained" Already conforming. I've added a test ensuring that `nullopt_t` is not copy-initializable from an empty braced-init-list, which I believe is the root intent of the issue, to avoid regression.
  - 2740 "constexpr optional<T>::operator->" Already conforming.
  - 2746 "Inconsistency between requirements for emplace between optional and variant" No P/R, but note that the author's '"suggested resolution" is already implemented.
  - 2748 "swappable traits for optionals" Already conforming.
  - 2753 "Optional's constructors and assignments need constraints" Implemented.

Most of the work for this patch was done by Casey Carter @ Microsoft. Thank you Casey!



Reviewers: mclow.lists, CaseyCarter, EricWF

Differential Revision: https://reviews.llvm.org/D22741

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@283980 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 1313 insertions, 0 deletions

diff --git a/include/optional b/include/optional
new file mode 100644
index 000000000..46252c7e0
--- /dev/null
+++ b/include/optional
@@ -0,0 +1,1313 @@
+// -*- C++ -*-
+//===-------------------------- optional ----------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_OPTIONAL
+#define _LIBCPP_OPTIONAL
+
+/*
+    optional synopsis
+
+// C++1z
+
+namespace std {
+  // 20.6.3, optional for object types
+  template <class T> class optional;
+
+  // 20.6.4, no-value state indicator
+  struct nullopt_t{see below };
+  constexpr nullopt_t nullopt(unspecified );
+
+  // 20.6.5, class bad_optional_access
+  class bad_optional_access;
+
+  // 20.6.6, relational operators
+  template <class T>
+  constexpr bool operator==(const optional<T>&, const optional<T>&);
+  template <class T>
+  constexpr bool operator!=(const optional<T>&, const optional<T>&);
+  template <class T>
+  constexpr bool operator<(const optional<T>&, const optional<T>&);
+  template <class T>
+  constexpr bool operator>(const optional<T>&, const optional<T>&);
+  template <class T>
+  constexpr bool operator<=(const optional<T>&, const optional<T>&);
+  template <class T>
+  constexpr bool operator>=(const optional<T>&, const optional<T>&);
+  template <class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
+  template <class T> constexpr bool operator==(nullopt_t, const optional<T>&) noexcept;
+  template <class T> constexpr bool operator!=(const optional<T>&, nullopt_t) noexcept;
+  template <class T> constexpr bool operator!=(nullopt_t, const optional<T>&) noexcept;
+  template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept;
+  template <class T> constexpr bool operator<(nullopt_t, const optional<T>&) noexcept;
+  template <class T> constexpr bool operator<=(const optional<T>&, nullopt_t) noexcept;
+  template <class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept;
+  template <class T> constexpr bool operator>(const optional<T>&, nullopt_t) noexcept;
+  template <class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept;
+  template <class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept;
+  template <class T> constexpr bool operator>=(nullopt_t, const optional<T>&) noexcept;
+
+  // 20.6.8, comparison with T
+  template <class T> constexpr bool operator==(const optional<T>&, const T&);
+  template <class T> constexpr bool operator==(const T&, const optional<T>&);
+  template <class T> constexpr bool operator!=(const optional<T>&, const T&);
+  template <class T> constexpr bool operator!=(const T&, const optional<T>&);
+  template <class T> constexpr bool operator<(const optional<T>&, const T&);
+  template <class T> constexpr bool operator<(const T&, const optional<T>&);
+  template <class T> constexpr bool operator<=(const optional<T>&, const T&);
+  template <class T> constexpr bool operator<=(const T&, const optional<T>&);
+  template <class T> constexpr bool operator>(const optional<T>&, const T&);
+  template <class T> constexpr bool operator>(const T&, const optional<T>&);
+  template <class T> constexpr bool operator>=(const optional<T>&, const T&);
+  template <class T> constexpr bool operator>=(const T&, const optional<T>&);
+
+  // 20.6.9, specialized algorithms
+  template <class T> void swap(optional<T>&, optional<T>&) noexcept(see below );
+  template <class T> constexpr optional<see below > make_optional(T&&);
+  template <class T, class... Args>
+    constexpr optional<T> make_optional(Args&&... args);
+  template <class T, class U, class... Args>
+    constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);
+
+  // 20.6.10, hash support
+  template <class T> struct hash;
+  template <class T> struct hash<optional<T>>;
+
+  template <class T> class optional {
+  public:
+    using value_type = T;
+
+    // 20.6.3.1, constructors
+    constexpr optional() noexcept;
+    constexpr optional(nullopt_t) noexcept;
+    optional(const optional &);
+    optional(optional &&) noexcept(see below );
+    template <class... Args> constexpr explicit optional(in_place_t, Args &&...);
+    template <class U, class... Args>
+      constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
+    template <class U = T>
+      constexpr EXPLICIT optional(U &&);
+    template <class U>
+      constexpr EXPLICIT optional(const optional<U> &);
+    template <class U>
+      constexpr EXPLICIT optional(optional<U> &&);
+
+    // 20.6.3.2, destructor
+    ~optional();
+
+    // 20.6.3.3, assignment
+    optional &operator=(nullopt_t) noexcept;
+    optional &operator=(const optional &);
+    optional &operator=(optional &&) noexcept(see below );
+    template <class U = T> optional &operator=(U &&);
+    template <class U> optional &operator=(const optional<U> &);
+    template <class U> optional &operator=(optional<U> &&);
+    template <class... Args> void emplace(Args &&...);
+    template <class U, class... Args>
+      void emplace(initializer_list<U>, Args &&...);
+
+    // 20.6.3.4, swap
+    void swap(optional &) noexcept(see below );
+
+    // 20.6.3.5, observers
+    constexpr T const *operator->() const;
+    constexpr T *operator->();
+    constexpr T const &operator*() const &;
+    constexpr T &operator*() &;
+    constexpr T &&operator*() &&;
+    constexpr const T &&operator*() const &&;
+    constexpr explicit operator bool() const noexcept;
+    constexpr bool has_value() const noexcept;
+    constexpr T const &value() const &;
+    constexpr T &value() &;
+    constexpr T &&value() &&;
+    constexpr const T &&value() const &&;
+    template <class U> constexpr T value_or(U &&) const &;
+    template <class U> constexpr T value_or(U &&) &&;
+
+    // 20.6.3.6, modifiers
+    void reset() noexcept;
+
+  private:
+    T *val; // exposition only
+  };
+} // namespace std
+
+*/
+
+#include <__config>
+#include <__debug>
+#include <__functional_base>
+#include <__undef_min_max>
+#include <functional>
+#include <initializer_list>
+#include <new>
+#include <stdexcept>
+#include <type_traits>
+#include <utility>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+namespace std  // purposefully not using versioning namespace
+{
+
+class _LIBCPP_EXCEPTION_ABI bad_optional_access
+    : public logic_error
+{
+public:
+    _LIBCPP_INLINE_VISIBILITY
+    bad_optional_access() : logic_error("bad optional access") {}
+
+    // Get the key function ~bad_optional_access() into the dylib
+    _LIBCPP_FUNC_VIS
+    virtual ~bad_optional_access() _NOEXCEPT;
+};
+
+}  // std
+
+#if _LIBCPP_STD_VER > 14
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+_LIBCPP_NORETURN
+inline _LIBCPP_INLINE_VISIBILITY
+void __throw_bad_optional_access() {
+#ifndef _LIBCPP_NO_EXCEPTIONS
+        throw bad_optional_access();
+#else
+        _VSTD::abort();
+#endif
+}
+
+struct nullopt_t
+{
+    struct __secret_tag { _LIBCPP_INLINE_VISIBILITY explicit __secret_tag() = default; };
+    _LIBCPP_INLINE_VISIBILITY constexpr explicit nullopt_t(__secret_tag, __secret_tag) noexcept {}
+};
+
+/* inline */ constexpr nullopt_t nullopt{nullopt_t::__secret_tag{}, nullopt_t::__secret_tag{}};
+
+template <class _Tp, bool = is_trivially_destructible<_Tp>::value>
+struct __optional_destruct_base;
+
+template <class _Tp>
+struct __optional_destruct_base<_Tp, false>
+{
+    typedef _Tp value_type;
+    static_assert(is_object_v<value_type>,
+        "instantiation of optional with a non-object type is undefined behavior");
+    union
+    {
+        char __null_state_;
+        value_type __val_;
+    };
+    bool __engaged_;
+
+    _LIBCPP_INLINE_VISIBILITY
+    ~__optional_destruct_base()
+    {
+        if (__engaged_)
+            __val_.~value_type();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr __optional_destruct_base() noexcept
+        :  __null_state_(),
+           __engaged_(false) {}
+
+    template <class... _Args>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit __optional_destruct_base(in_place_t, _Args&&... __args)
+        :  __val_(_VSTD::forward<_Args>(__args)...),
+           __engaged_(true) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    void reset() noexcept
+    {
+        if (__engaged_)
+        {
+            __val_.~value_type();
+            __engaged_ = false;
+        }
+    }
+};
+
+template <class _Tp>
+struct __optional_destruct_base<_Tp, true>
+{
+    typedef _Tp value_type;
+    static_assert(is_object_v<value_type>,
+        "instantiation of optional with a non-object type is undefined behavior");
+    union
+    {
+        char __null_state_;
+        value_type __val_;
+    };
+    bool __engaged_;
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr __optional_destruct_base() noexcept
+        :  __null_state_(),
+           __engaged_(false) {}
+
+    template <class... _Args>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit __optional_destruct_base(in_place_t, _Args&&... __args)
+        :  __val_(_VSTD::forward<_Args>(__args)...),
+           __engaged_(true) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    void reset() noexcept
+    {
+        if (__engaged_)
+        {
+            __engaged_ = false;
+        }
+    }
+};
+
+template <class _Tp, bool = is_reference<_Tp>::value>
+struct __optional_storage_base : __optional_destruct_base<_Tp>
+{
+    using __base = __optional_destruct_base<_Tp>;
+    using value_type = _Tp;
+    using __base::__base;
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr bool has_value() const noexcept
+    {
+        return this->__engaged_;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type& __get() & noexcept
+    {
+        return this->__val_;
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr const value_type& __get() const& noexcept
+    {
+        return this->__val_;
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type&& __get() && noexcept
+    {
+        return _VSTD::move(this->__val_);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr const value_type&& __get() const&& noexcept
+    {
+        return _VSTD::move(this->__val_);
+    }
+
+    template <class... _Args>
+    _LIBCPP_INLINE_VISIBILITY
+    void __construct(_Args&&... __args)
+    {
+        _LIBCPP_ASSERT(!has_value(), "__construct called for engaged __optional_storage");
+        ::new((void*)_VSTD::addressof(this->__val_)) value_type(_VSTD::forward<_Args>(__args)...);
+        this->__engaged_ = true;
+    }
+
+    template <class _That>
+    _LIBCPP_INLINE_VISIBILITY
+    void __construct_from(_That&& __opt)
+    {
+        if (__opt.has_value())
+            __construct(_VSTD::forward<_That>(__opt).__get());
+    }
+
+    template <class _That>
+    _LIBCPP_INLINE_VISIBILITY
+    void __assign_from(_That&& __opt)
+    {
+        if (this->__engaged_ == __opt.has_value())
+        {
+            if (this->__engaged_)
+                this->__val_ = _VSTD::forward<_That>(__opt).__get();
+        }
+        else
+        {
+            if (this->__engaged_)
+                this->reset();
+            else
+                __construct(_VSTD::forward<_That>(__opt).__get());
+        }
+    }
+};
+
+// optional<T&> is currently required ill-formed, however it may to be in the
+// future. For this reason it has already been implemented to ensure we can
+// make the change in an ABI compatible manner.
+template <class _Tp>
+struct __optional_storage_base<_Tp, true>
+{
+    using value_type = _Tp;
+    using __raw_type = remove_reference_t<_Tp>;
+    __raw_type* __value_;
+
+    template <class _Up>
+    static constexpr bool __can_bind_reference() {
+        using _RawUp = typename remove_reference<_Up>::type;
+        using _UpPtr = _RawUp*;
+        using _RawTp = typename remove_reference<_Tp>::type;
+        using _TpPtr = _RawTp*;
+        using _CheckLValueArg = integral_constant<bool,
+            (is_lvalue_reference<_Up>::value && is_convertible<_UpPtr, _TpPtr>::value)
+        ||  is_same<_RawUp, reference_wrapper<_RawTp>>::value
+        ||  is_same<_RawUp, reference_wrapper<typename remove_const<_RawTp>::type>>::value
+        >;
+        return (is_lvalue_reference<_Tp>::value && _CheckLValueArg::value)
+            || (is_rvalue_reference<_Tp>::value && !is_lvalue_reference<_Up>::value &&
+                is_convertible<_UpPtr, _TpPtr>::value);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr __optional_storage_base() noexcept
+        :  __value_(nullptr) {}
+
+    template <class _UArg>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit __optional_storage_base(in_place_t, _UArg&& __uarg)
+        :  __value_(_VSTD::addressof(__uarg))
+    {
+      static_assert(__can_bind_reference<_UArg>(),
+        "Attempted to construct a reference element in tuple from a "
+        "possible temporary");
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    void reset() noexcept { __value_ = nullptr; }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr bool has_value() const noexcept
+      { return __value_ != nullptr; }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type& __get() const& noexcept
+      { return *__value_; }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type&& __get() const&& noexcept
+      { return _VSTD::forward<value_type>(*__value_); }
+
+    template <class _UArg>
+    _LIBCPP_INLINE_VISIBILITY
+    void __construct(_UArg&& __val)
+    {
+        _LIBCPP_ASSERT(!has_value(), "__construct called for engaged __optional_storage");
+        static_assert(__can_bind_reference<_UArg>(),
+            "Attempted to construct a reference element in tuple from a "
+            "possible temporary");
+        __value_ = _VSTD::addressof(__val);
+    }
+
+    template <class _That>
+    _LIBCPP_INLINE_VISIBILITY
+    void __construct_from(_That&& __opt)
+    {
+        if (__opt.has_value())
+            __construct(_VSTD::forward<_That>(__opt).__get());
+    }
+
+    template <class _That>
+    _LIBCPP_INLINE_VISIBILITY
+    void __assign_from(_That&& __opt)
+    {
+        if (has_value() == __opt.has_value())
+        {
+            if (has_value())
+                *__value_ = _VSTD::forward<_That>(__opt).__get();
+        }
+        else
+        {
+            if (has_value())
+                reset();
+            else
+                __construct(_VSTD::forward<_That>(__opt).__get());
+        }
+    }
+};
+
+template <class _Tp, bool = is_trivially_copyable<_Tp>::value>
+struct __optional_storage;
+
+template <class _Tp>
+struct __optional_storage<_Tp, true> : __optional_storage_base<_Tp>
+{
+    using __optional_storage_base<_Tp>::__optional_storage_base;
+};
+
+template <class _Tp>
+struct __optional_storage<_Tp, false> : __optional_storage_base<_Tp>
+{
+    using value_type = _Tp;
+    using __optional_storage_base<_Tp>::__optional_storage_base;
+
+    _LIBCPP_INLINE_VISIBILITY
+    __optional_storage() = default;
+
+    _LIBCPP_INLINE_VISIBILITY
+    __optional_storage(const __optional_storage& __opt)
+    {
+        this->__construct_from(__opt);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __optional_storage(__optional_storage&& __opt)
+        noexcept(is_nothrow_move_constructible_v<value_type>)
+    {
+        this->__construct_from(_VSTD::move(__opt));
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __optional_storage& operator=(const __optional_storage& __opt)
+    {
+        this->__assign_from(__opt);
+        return *this;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __optional_storage& operator=(__optional_storage&& __opt)
+        noexcept(is_nothrow_move_assignable_v<value_type> &&
+                 is_nothrow_move_constructible_v<value_type>)
+    {
+        this->__assign_from(_VSTD::move(__opt));
+        return *this;
+    }
+};
+
+template <class _Tp>
+using __optional_sfinae_ctor_base_t = __sfinae_ctor_base<
+    is_copy_constructible<_Tp>::value,
+    is_move_constructible<_Tp>::value
+>;
+
+template <class _Tp>
+using __optional_sfinae_assign_base_t = __sfinae_assign_base<
+    (is_copy_constructible<_Tp>::value && is_copy_assignable<_Tp>::value),
+    (is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value)
+>;
+
+template <class _Tp>
+class optional
+    : private __optional_storage<_Tp>
+    , private __optional_sfinae_ctor_base_t<_Tp>
+    , private __optional_sfinae_assign_base_t<_Tp>
+{
+    using __base = __optional_storage<_Tp>;
+public:
+    using value_type = _Tp;
+
+private:
+     // Disable the reference extension using this static assert.
+    static_assert(!is_same_v<value_type, in_place_t>,
+        "instantiation of optional with in_place_t is ill-formed");
+    static_assert(!is_same_v<__uncvref_t<value_type>, nullopt_t>,
+        "instantiation of optional with nullopt_t is ill-formed");
+    static_assert(!is_reference_v<value_type>,
+        "instantiation of optional with a reference type is ill-formed");
+    static_assert(is_destructible_v<value_type>,
+        "instantiation of optional with a non-destructible type is ill-formed");
+
+    // LWG2756: conditionally explicit conversion from _Up
+    struct _CheckOptionalArgsConstructor {
+      template <class _Up>
+      static constexpr bool __enable_implicit() {
+          return is_constructible_v<_Tp, _Up&&> &&
+                 is_convertible_v<_Up&&, _Tp>;
+      }
+
+      template <class _Up>
+      static constexpr bool __enable_explicit() {
+          return is_constructible_v<_Tp, _Up&&> &&
+                 !is_convertible_v<_Up&&, _Tp>;
+      }
+    };
+    template <class _Up>
+    using _CheckOptionalArgsCtor = conditional_t<
+        !is_same_v<in_place_t, _Up> &&
+        !is_same_v<decay_t<_Up>, optional>,
+        _CheckOptionalArgsConstructor,
+        __check_tuple_constructor_fail
+    >;
+    template <class _QualUp>
+    struct _CheckOptionalLikeConstructor {
+      template <class _Up, class _Opt = optional<_Up>>
+      using __check_constructible_from_opt = __lazy_or<
+          is_constructible<_Tp, _Opt&>,
+          is_constructible<_Tp, _Opt const&>,
+          is_constructible<_Tp, _Opt&&>,
+          is_constructible<_Tp, _Opt const&&>,
+          is_convertible<_Opt&, _Tp>,
+          is_convertible<_Opt const&, _Tp>,
+          is_convertible<_Opt&&, _Tp>,
+          is_convertible<_Opt const&&, _Tp>
+      >;
+      template <class _Up, class _Opt = optional<_Up>>
+      using __check_assignable_from_opt = __lazy_or<
+          is_assignable<_Tp&, _Opt&>,
+          is_assignable<_Tp&, _Opt const&>,
+          is_assignable<_Tp&, _Opt&&>,
+          is_assignable<_Tp&, _Opt const&&>
+      >;
+      template <class _Up, class _QUp = _QualUp>
+      static constexpr bool __enable_implicit() {
+          return is_convertible<_QUp, _Tp>::value &&
+              !__check_constructible_from_opt<_Up>::value;
+      }
+      template <class _Up, class _QUp = _QualUp>
+      static constexpr bool __enable_explicit() {
+          return !is_convertible<_QUp, _Tp>::value &&
+              !__check_constructible_from_opt<_Up>::value;
+      }
+      template <class _Up, class _QUp = _QualUp>
+      static constexpr bool __enable_assign() {
+          // Construction and assignability of _Qup to _Tp has already been
+          // checked.
+          return !__check_constructible_from_opt<_Up>::value &&
+              !__check_assignable_from_opt<_Up>::value;
+      }
+    };
+
+    template <class _Up, class _QualUp>
+    using _CheckOptionalLikeCtor = conditional_t<
+      __lazy_and<
+          __lazy_not<is_same<_Up, _Tp>>,
+          is_constructible<_Tp, _QualUp>
+      >::value,
+      _CheckOptionalLikeConstructor<_QualUp>,
+      __check_tuple_constructor_fail
+    >;
+    template <class _Up, class _QualUp>
+    using _CheckOptionalLikeAssign = conditional_t<
+      __lazy_and<
+          __lazy_not<is_same<_Up, _Tp>>,
+          is_constructible<_Tp, _QualUp>,
+          is_assignable<_Tp&, _QualUp>
+      >::value,
+      _CheckOptionalLikeConstructor<_QualUp>,
+      __check_tuple_constructor_fail
+    >;
+public:
+
+    _LIBCPP_INLINE_VISIBILITY constexpr optional() noexcept {}
+    _LIBCPP_INLINE_VISIBILITY optional(const optional&) = default;
+    _LIBCPP_INLINE_VISIBILITY optional(optional&&) = default;
+    _LIBCPP_INLINE_VISIBILITY constexpr optional(nullopt_t) noexcept {}
+
+    template <class... _Args, class = enable_if_t<
+        is_constructible_v<value_type, _Args...>>
+    >
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit optional(in_place_t, _Args&&... __args)
+        : __base(in_place, _VSTD::forward<_Args>(__args)...) {}
+
+    template <class _Up, class... _Args, class = enable_if_t<
+        is_constructible_v<value_type, initializer_list<_Up>&, _Args...>>
+    >
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
+        : __base(in_place, __il, _VSTD::forward<_Args>(__args)...) {}
+
+    template <class _Up = value_type, enable_if_t<
+        _CheckOptionalArgsCtor<_Up>::template __enable_implicit<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr optional(_Up&& __v)
+        : __base(in_place, _VSTD::forward<_Up>(__v)) {}
+
+    template <class _Up, enable_if_t<
+        _CheckOptionalArgsCtor<_Up>::template __enable_explicit<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit optional(_Up&& __v)
+        : __base(in_place, _VSTD::forward<_Up>(__v)) {}
+
+    // LWG2756: conditionally explicit conversion from const optional<_Up>&
+    template <class _Up, enable_if_t<
+        _CheckOptionalLikeCtor<_Up, _Up const&>::template __enable_implicit<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    optional(const optional<_Up>& __v)
+    {
+        this->__construct_from(__v);
+    }
+    template <class _Up, enable_if_t<
+        _CheckOptionalLikeCtor<_Up, _Up const&>::template __enable_explicit<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    explicit optional(const optional<_Up>& __v)
+    {
+        this->__construct_from(__v);
+    }
+
+    // LWG2756: conditionally explicit conversion from optional<_Up>&&
+    template <class _Up, enable_if_t<
+        _CheckOptionalLikeCtor<_Up, _Up &&>::template __enable_implicit<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    optional(optional<_Up>&& __v)
+    {
+        this->__construct_from(_VSTD::move(__v));
+    }
+    template <class _Up, enable_if_t<
+        _CheckOptionalLikeCtor<_Up, _Up &&>::template __enable_explicit<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    explicit optional(optional<_Up>&& __v)
+    {
+        this->__construct_from(_VSTD::move(__v));
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    optional& operator=(nullopt_t) noexcept
+    {
+        reset();
+        return *this;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY optional& operator=(const optional&) = default;
+    _LIBCPP_INLINE_VISIBILITY optional& operator=(optional&&) = default;
+
+    // LWG2756
+    template <class _Up = value_type,
+              class = enable_if_t
+                      <
+                          !is_same_v<_Up, optional> &&
+                          !(is_same_v<_Up, value_type> && is_scalar_v<value_type>) &&
+                          is_constructible_v<value_type, _Up> &&
+                          is_assignable_v<value_type&, _Up>
+                      >
+             >
+    _LIBCPP_INLINE_VISIBILITY
+    optional&
+    operator=(_Up&& __v)
+    {
+        if (this->has_value())
+            this->__get() = _VSTD::forward<_Up>(__v);
+        else
+            this->__construct(_VSTD::forward<_Up>(__v));
+        return *this;
+    }
+
+    // LWG2756
+    template <class _Up, enable_if_t<
+        _CheckOptionalLikeAssign<_Up, _Up const&>::template __enable_assign<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    optional&
+    operator=(const optional<_Up>& __v)
+    {
+        this->__assign_from(__v);
+        return *this;
+    }
+
+    // LWG2756
+    template <class _Up, enable_if_t<
+        _CheckOptionalLikeCtor<_Up, _Up &&>::template __enable_assign<_Up>()
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    optional&
+    operator=(optional<_Up>&& __v)
+    {
+        this->__assign_from(_VSTD::move(__v));
+        return *this;
+    }
+
+    template <class... _Args,
+              class = enable_if_t
+                      <
+                          is_constructible_v<value_type, _Args...>
+                      >
+             >
+    _LIBCPP_INLINE_VISIBILITY
+    void
+    emplace(_Args&&... __args)
+    {
+        reset();
+        this->__construct(_VSTD::forward<_Args>(__args)...);
+    }
+
+    template <class _Up, class... _Args,
+              class = enable_if_t
+                      <
+                          is_constructible_v<value_type, initializer_list<_Up>&, _Args...>
+                      >
+             >
+    _LIBCPP_INLINE_VISIBILITY
+    void
+    emplace(initializer_list<_Up> __il, _Args&&... __args)
+    {
+        reset();
+        this->__construct(__il, _VSTD::forward<_Args>(__args)...);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    void swap(optional& __opt)
+        noexcept(is_nothrow_move_constructible_v<value_type> &&
+                 is_nothrow_swappable_v<value_type>)
+    {
+        if (this->has_value() == __opt.has_value())
+        {
+            using _VSTD::swap;
+            if (this->has_value())
+                swap(this->__get(), __opt.__get());
+        }
+        else
+        {
+            if (this->has_value())
+            {
+                __opt.__construct(_VSTD::move(this->__get()));
+                reset();
+            }
+            else
+            {
+                this->__construct(_VSTD::move(__opt.__get()));
+                __opt.reset();
+            }
+        }
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr
+    add_pointer_t<value_type const>
+    operator->() const
+    {
+        _LIBCPP_ASSERT(this->has_value(), "optional operator-> called for disengaged value");
+#ifndef _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
+        return _VSTD::addressof(this->__get());
+#else
+        return __operator_arrow(__has_operator_addressof<value_type>{}, this->__get());
+#endif
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr
+    add_pointer_t<value_type>
+    operator->()
+    {
+        _LIBCPP_ASSERT(this->has_value(), "optional operator-> called for disengaged value");
+#ifndef _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
+        return _VSTD::addressof(this->__get());
+#else
+        return __operator_arrow(__has_operator_addressof<value_type>{}, this->__get());
+#endif
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr
+    const value_type&
+    operator*() const&
+    {
+        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
+        return this->__get();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr
+    value_type&
+    operator*() &
+    {
+        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
+        return this->__get();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr
+    value_type&&
+    operator*() &&
+    {
+        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
+        return _VSTD::move(this->__get());
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr
+    const value_type&&
+    operator*() const&&
+    {
+        _LIBCPP_ASSERT(this->has_value(), "optional operator* called for disengaged value");
+        return _VSTD::move(this->__get());
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit operator bool() const noexcept { return has_value(); }
+
+    using __base::has_value;
+    using __base::__get;
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type const& value() const&
+    {
+        if (!this->has_value())
+            __throw_bad_optional_access();
+        return this->__get();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type& value() &
+    {
+        if (!this->has_value())
+            __throw_bad_optional_access();
+        return this->__get();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type&& value() &&
+    {
+        if (!this->has_value())
+            __throw_bad_optional_access();
+        return _VSTD::move(this->__get());
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type const&& value() const&&
+    {
+        if (!this->has_value())
+            __throw_bad_optional_access();
+        return _VSTD::move(this->__get());
+    }
+
+    template <class _Up>
+    _LIBCPP_INLINE_VISIBILITY
+    constexpr value_type value_or(_Up&& __v) const&
+    {
+        static_assert(is_copy_constructible_v<value_type>,
+                      "optional<T>::value_or: T must be copy constructible");
+        static_assert(is_convertible_v<_Up, value_type>,
+                      "optional<T>::value_or: U must be convertible to T");
+        return this->has_value() ? this->__get() :
+                                  static_cast<value_type>(_VSTD::forward<_Up>(__v));
+    }
+
+    template <class _Up>
+    _LIBCPP_INLINE_VISIBILITY
+    value_type value_or(_Up&& __v) &&
+    {
+        static_assert(is_move_constructible_v<value_type>,
+                      "optional<T>::value_or: T must be move constructible");
+        static_assert(is_convertible_v<_Up, value_type>,
+                      "optional<T>::value_or: U must be convertible to T");
+        return this->has_value() ? _VSTD::move(this->__get()) :
+                                  static_cast<value_type>(_VSTD::forward<_Up>(__v));
+    }
+
+    using __base::reset;
+
+private:
+    template <class _Up>
+    _LIBCPP_INLINE_VISIBILITY
+    static _Up*
+    __operator_arrow(true_type, _Up& __x)
+    {
+        return _VSTD::addressof(__x);
+    }
+
+    template <class _Up>
+    _LIBCPP_INLINE_VISIBILITY
+    static constexpr _Up*
+    __operator_arrow(false_type, _Up& __x)
+    {
+        return &__x;
+    }
+};
+
+// Comparisons between optionals
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() ==
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator==(const optional<_Tp>& __x, const optional<_Tp>& __y)
+{
+    if (static_cast<bool>(__x) != static_cast<bool>(__y))
+        return false;
+    if (!static_cast<bool>(__x))
+        return true;
+    return *__x == *__y;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() !=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator!=(const optional<_Tp>& __x, const optional<_Tp>& __y)
+{
+    if (static_cast<bool>(__x) != static_cast<bool>(__y))
+        return true;
+    if (!static_cast<bool>(__x))
+        return false;
+    return *__x != *__y;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator<(const optional<_Tp>& __x, const optional<_Tp>& __y)
+{
+    if (!static_cast<bool>(__y))
+        return false;
+    if (!static_cast<bool>(__x))
+        return true;
+    return *__x < *__y;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator>(const optional<_Tp>& __x, const optional<_Tp>& __y)
+{
+    if (!static_cast<bool>(__x))
+        return false;
+    if (!static_cast<bool>(__y))
+        return true;
+    return *__x > *__y;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator<=(const optional<_Tp>& __x, const optional<_Tp>& __y)
+{
+    if (!static_cast<bool>(__x))
+        return true;
+    if (!static_cast<bool>(__y))
+        return false;
+    return *__x <= *__y;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator>=(const optional<_Tp>& __x, const optional<_Tp>& __y)
+{
+    if (!static_cast<bool>(__y))
+        return true;
+    if (!static_cast<bool>(__x))
+        return false;
+    return *__x >= *__y;
+}
+
+// Comparisons with nullopt
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator==(const optional<_Tp>& __x, nullopt_t) noexcept
+{
+    return !static_cast<bool>(__x);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator==(nullopt_t, const optional<_Tp>& __x) noexcept
+{
+    return !static_cast<bool>(__x);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator!=(const optional<_Tp>& __x, nullopt_t) noexcept
+{
+    return static_cast<bool>(__x);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator!=(nullopt_t, const optional<_Tp>& __x) noexcept
+{
+    return static_cast<bool>(__x);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator<(const optional<_Tp>&, nullopt_t) noexcept
+{
+    return false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator<(nullopt_t, const optional<_Tp>& __x) noexcept
+{
+    return static_cast<bool>(__x);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator<=(const optional<_Tp>& __x, nullopt_t) noexcept
+{
+    return !static_cast<bool>(__x);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator<=(nullopt_t, const optional<_Tp>& __x) noexcept
+{
+    return true;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator>(const optional<_Tp>& __x, nullopt_t) noexcept
+{
+    return static_cast<bool>(__x);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator>(nullopt_t, const optional<_Tp>& __x) noexcept
+{
+    return false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator>=(const optional<_Tp>&, nullopt_t) noexcept
+{
+    return true;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+bool
+operator>=(nullopt_t, const optional<_Tp>& __x) noexcept
+{
+    return !static_cast<bool>(__x);
+}
+
+// Comparisons with T
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() ==
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator==(const optional<_Tp>& __x, const _Tp& __v)
+{
+    return static_cast<bool>(__x) ? *__x == __v : false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() ==
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator==(const _Tp& __v, const optional<_Tp>& __x)
+{
+    return static_cast<bool>(__x) ? __v == *__x : false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() !=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator!=(const optional<_Tp>& __x, const _Tp& __v)
+{
+    return static_cast<bool>(__x) ? *__x != __v : true;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() !=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator!=(const _Tp& __v, const optional<_Tp>& __x)
+{
+    return static_cast<bool>(__x) ? __v != *__x : true;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator<(const optional<_Tp>& __x, const _Tp& __v)
+{
+    return static_cast<bool>(__x) ? *__x < __v : true;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator<(const _Tp& __v, const optional<_Tp>& __x)
+{
+    return static_cast<bool>(__x) ? __v < *__x : false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator<=(const optional<_Tp>& __x, const _Tp& __v)
+{
+    return static_cast<bool>(__x) ? *__x <= __v : true;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() <=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator<=(const _Tp& __v, const optional<_Tp>& __x)
+{
+    return static_cast<bool>(__x) ? __v <= *__x : false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator>(const optional<_Tp>& __x, const _Tp& __v)
+{
+    return static_cast<bool>(__x) ? *__x > __v : false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator>(const _Tp& __v, const optional<_Tp>& __x)
+{
+    return static_cast<bool>(__x) ? __v > *__x : true;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator>=(const optional<_Tp>& __x, const _Tp& __v)
+{
+    return static_cast<bool>(__x) ? *__x >= __v : false;
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+enable_if_t<
+    is_convertible_v<decltype(_VSTD::declval<const _Tp&>() >=
+        _VSTD::declval<const _Tp&>()), bool>,
+    bool
+>
+operator>=(const _Tp& __v, const optional<_Tp>& __x)
+{
+    return static_cast<bool>(__x) ? __v >= *__x : true;
+}
+
+
+template <class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY
+enable_if_t<
+    is_move_constructible_v<_Tp> && is_swappable_v<_Tp>,
+    void
+>
+swap(optional<_Tp>& __x, optional<_Tp>& __y) noexcept(noexcept(__x.swap(__y)))
+{
+    __x.swap(__y);
+}
+
+template <class _Tp>
+_LIBCPP_INLINE_VISIBILITY constexpr
+optional<decay_t<_Tp>> make_optional(_Tp&& __v)
+{
+    return optional<decay_t<_Tp>>(_VSTD::forward<_Tp>(__v));
+}
+
+template <class _Tp, class... _Args>
+_LIBCPP_INLINE_VISIBILITY constexpr
+optional<_Tp> make_optional(_Args&&... __args)
+{
+    return optional<_Tp>(in_place, _VSTD::forward<_Args>(__args)...);
+}
+
+template <class _Tp, class _Up, class... _Args>
+_LIBCPP_INLINE_VISIBILITY constexpr
+optional<_Tp> make_optional(initializer_list<_Up> __il,  _Args&&... __args)
+{
+    return optional<_Tp>(in_place, __il, _VSTD::forward<_Args>(__args)...);
+}
+
+template <class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY hash<optional<_Tp> >
+{
+    typedef optional<_Tp> argument_type;
+    typedef size_t        result_type;
+
+    _LIBCPP_INLINE_VISIBILITY
+    result_type operator()(const argument_type& __opt) const _NOEXCEPT
+    {
+        return static_cast<bool>(__opt) ? hash<_Tp>()(*__opt) : 0;
+    }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif  // _LIBCPP_STD_VER > 14
+
+#endif  // _LIBCPP_OPTIONAL