STL/stl/inc/xutility

// xutility internal header

// Copyright (c) Microsoft Corporation.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#pragma once
#ifndef _XUTILITY_
#define _XUTILITY_
#include <yvals.h>
#if _STL_COMPILER_PREPROCESSOR

#include <climits>
#include <cstdlib>
#include <cstring>
#include <utility>

#pragma pack(push, _CRT_PACKING)
#pragma warning(push, _STL_WARNING_LEVEL)
#pragma warning(disable : _STL_DISABLED_WARNINGS)
_STL_DISABLE_CLANG_WARNINGS
#pragma push_macro("new")
#undef new

#if (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE)
_EXTERN_C
// The "noalias" attribute tells the compiler optimizer that pointers going into these hand-vectorized algorithms
// won't be stored beyond the lifetime of the function, and that the function will only reference arrays denoted by
// those pointers. The optimizer also assumes in that case that a pointer parameter is not returned to the caller via
// the return value, so functions using "noalias" must usually return void. This attribute is valuable because these
// functions are in native code objects that the compiler cannot analyze. In the absence of the noalias attribute, the
// compiler has to assume that the denoted arrays are "globally address taken", and that any later calls to
// unanalyzable routines may modify those arrays.
__declspec(noalias) void __cdecl __std_reverse_trivially_swappable_1(void* _First, void* _Last) noexcept;
__declspec(noalias) void __cdecl __std_reverse_trivially_swappable_2(void* _First, void* _Last) noexcept;
__declspec(noalias) void __cdecl __std_reverse_trivially_swappable_4(void* _First, void* _Last) noexcept;
__declspec(noalias) void __cdecl __std_reverse_trivially_swappable_8(void* _First, void* _Last) noexcept;
_END_EXTERN_C
#endif // (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE)

_STD_BEGIN

// STRUCT TEMPLATE _Get_first_parameter
template <class _Ty>
struct _Get_first_parameter;

template <template <class, class...> class _Ty, class _First, class... _Rest>
struct _Get_first_parameter<_Ty<_First, _Rest...>> { // given _Ty<_First, _Rest...>, extract _First
    using type = _First;
};

// STRUCT TEMPLATE _Replace_first_parameter
template <class _Newfirst, class _Ty>
struct _Replace_first_parameter;

template <class _Newfirst, template <class, class...> class _Ty, class _First, class... _Rest>
struct _Replace_first_parameter<_Newfirst, _Ty<_First, _Rest...>> { // given _Ty<_First, _Rest...>, replace _First
    using type = _Ty<_Newfirst, _Rest...>;
};

// STRUCT TEMPLATE _Get_element_type
template <class _Ty, class = void>
struct _Get_element_type {
    using type = typename _Get_first_parameter<_Ty>::type;
};

template <class _Ty>
struct _Get_element_type<_Ty, void_t<typename _Ty::element_type>> {
    using type = typename _Ty::element_type;
};

// STRUCT TEMPLATE _Get_ptr_difference_type
template <class _Ty, class = void>
struct _Get_ptr_difference_type {
    using type = ptrdiff_t;
};

template <class _Ty>
struct _Get_ptr_difference_type<_Ty, void_t<typename _Ty::difference_type>> {
    using type = typename _Ty::difference_type;
};

// STRUCT TEMPLATE _Get_rebind_alias
template <class _Ty, class _Other, class = void>
struct _Get_rebind_alias {
    using type = typename _Replace_first_parameter<_Other, _Ty>::type;
};

template <class _Ty, class _Other>
struct _Get_rebind_alias<_Ty, _Other, void_t<typename _Ty::template rebind<_Other>>> {
    using type = typename _Ty::template rebind<_Other>;
};

// STRUCT TEMPLATE pointer_traits
template <class _Ty>
struct pointer_traits {
    using pointer         = _Ty;
    using element_type    = typename _Get_element_type<_Ty>::type;
    using difference_type = typename _Get_ptr_difference_type<_Ty>::type;

    template <class _Other>
    using rebind = typename _Get_rebind_alias<_Ty, _Other>::type;

    using _Reftype = conditional_t<is_void_v<element_type>, char, element_type>&;

    _NODISCARD static pointer pointer_to(_Reftype _Val) noexcept(noexcept(_Ty::pointer_to(_Val))) /* strengthened */ {
        return _Ty::pointer_to(_Val);
    }
};

template <class _Ty>
struct pointer_traits<_Ty*> {
    using pointer         = _Ty*;
    using element_type    = _Ty;
    using difference_type = ptrdiff_t;

    template <class _Other>
    using rebind = _Other*;

    using _Reftype = conditional_t<is_void_v<_Ty>, char, _Ty>&;

    _NODISCARD static _CONSTEXPR20 pointer pointer_to(_Reftype _Val) noexcept {
        return _STD addressof(_Val);
    }
};

#if _HAS_CXX20
// FUNCTION TEMPLATE to_address
template <class _Ty, class = void>
inline constexpr bool _Has_to_address_v = false; // determines whether _Ptr has pointer_traits<_Ptr>::to_address(p)

template <class _Ty>
inline constexpr bool
    _Has_to_address_v<_Ty, void_t<decltype(pointer_traits<_Ty>::to_address(_STD declval<const _Ty&>()))>> = true;

template <class _Ty>
_NODISCARD constexpr _Ty* to_address(_Ty* const _Val) noexcept {
    static_assert(!is_function_v<_Ty>,
        "N4810 20.10.4 [pointer.conversion]/2: The program is ill-formed if T is a function type.");
    return _Val;
}

// constexpr per pending LWG issue, submitted 2020-01-14
template <class _Ptr>
_NODISCARD constexpr auto to_address(const _Ptr& _Val) noexcept {
    if constexpr (_Has_to_address_v<_Ptr>) {
        return pointer_traits<_Ptr>::to_address(_Val);
    } else {
        return _STD to_address(_Val.operator->()); // plain pointer overload must come first
    }
}
#endif // _HAS_CXX20

// FUNCTION TEMPLATE _Pass_fn
// TRANSITION, VSO-386225
template <class _Fx>
struct _Ref_fn { // pass function object by value as a reference
    template <class... _Args>
    constexpr decltype(auto) operator()(_Args&&... _Vals) { // forward function call operator
        return _Fn(_STD forward<_Args>(_Vals)...);
    }

    _Fx& _Fn;
};

template <class _Fn>
_INLINE_VAR constexpr bool
    _Pass_functor_by_value_v = sizeof(_Fn) <= sizeof(void*)
                               && conjunction_v<is_trivially_copy_constructible<_Fn>, is_trivially_destructible<_Fn>>;

template <class _Fn, enable_if_t<_Pass_functor_by_value_v<_Fn>, int> = 0> // TRANSITION, if constexpr
constexpr _Fn _Pass_fn(_Fn _Val) { // pass functor by value
    return _Val;
}

template <class _Fn, enable_if_t<!_Pass_functor_by_value_v<_Fn>, int> = 0>
constexpr _Ref_fn<_Fn> _Pass_fn(_Fn& _Val) { // pass functor by "reference"
    return {_Val};
}

// STRUCT _Unused_parameter
struct _Unused_parameter { // generic unused parameter struct
    constexpr _Unused_parameter() noexcept = default;
    template <class _Ty>
    constexpr _Unused_parameter(_Ty&&) noexcept {}
};

// ALIAS _Any_tag
using _Any_tag = _Unused_parameter; // generic fallback/default/"other" target for tag dispatch

// ALIAS TEMPLATE _Algorithm_int_t
template <class _Ty>
using _Algorithm_int_t = conditional_t<is_integral_v<_Ty>, _Ty, ptrdiff_t>;

// ITERATOR STUFF (from <iterator>)
// ITERATOR TAGS (from <iterator>)
struct input_iterator_tag {};

struct output_iterator_tag {};

struct forward_iterator_tag : input_iterator_tag {};

struct bidirectional_iterator_tag : forward_iterator_tag {};

struct random_access_iterator_tag : bidirectional_iterator_tag {};

#ifdef __cpp_lib_concepts
struct contiguous_iterator_tag : random_access_iterator_tag {};

// ALIAS TEMPLATE _With_reference
template <class _Ty>
using _With_reference = _Ty&;

// CONCEPT _Can_reference
// clang-format off
template <class _Ty>
concept _Can_reference = requires { typename _With_reference<_Ty>; };

// CONCEPT _Dereferenceable
template <class _Ty>
concept _Dereferenceable = requires(_Ty& __t) {
    { *__t } -> _Can_reference;
};
// clang-format on

// CONCEPT _Has_member_iterator_concept
template <class _Ty>
concept _Has_member_iterator_concept = requires {
    typename _Ty::iterator_concept;
};

// CONCEPT _Has_member_iterator_category
template <class _Ty>
concept _Has_member_iterator_category = requires {
    typename _Ty::iterator_category;
};

// CONCEPT _Has_member_value_type
template <class _Ty>
concept _Has_member_value_type = requires {
    typename _Ty::value_type;
};

// CONCEPT _Has_member_element_type
template <class _Ty>
concept _Has_member_element_type = requires {
    typename _Ty::element_type;
};

// CONCEPT _Has_member_difference_type
template <class _Ty>
concept _Has_member_difference_type = requires {
    typename _Ty::difference_type;
};

// CONCEPT _Has_member_pointer
template <class _Ty>
concept _Has_member_pointer = requires {
    typename _Ty::pointer;
};

// CONCEPT _Has_member_reference
template <class _Ty>
concept _Has_member_reference = requires {
    typename _Ty::reference;
};

// STRUCT TEMPLATE incrementable_traits
template <class>
struct incrementable_traits {};

// clang-format off
template <class _Ty>
    requires is_object_v<_Ty>
struct incrementable_traits<_Ty*> {
    using difference_type = ptrdiff_t;
};
// clang-format on

template <class _Ty>
struct incrementable_traits<const _Ty> : incrementable_traits<_Ty> {};

template <_Has_member_difference_type _Ty>
struct incrementable_traits<_Ty> {
    using difference_type = typename _Ty::difference_type;
};

// clang-format off
template <class _Ty>
concept _Can_difference = requires(const _Ty& __a, const _Ty& __b) {
    { __a - __b } -> integral;
};

template <class _Ty>
    requires (!_Has_member_difference_type<_Ty> && _Can_difference<_Ty>)
struct incrementable_traits<_Ty> {
    using difference_type = make_signed_t<decltype(_STD declval<_Ty>() - _STD declval<_Ty>())>;
};
// clang-format on

// ALIAS TEMPLATE iter_difference_t
template <class _Ty>
concept _Is_from_primary = _Same_impl<typename _Ty::_From_primary, _Ty>;

template <class>
struct iterator_traits;

template <class _Ty>
using iter_difference_t = typename conditional_t<_Is_from_primary<iterator_traits<_Ty>>, incrementable_traits<_Ty>,
    iterator_traits<_Ty>>::difference_type;

// STRUCT TEMPLATE readable_traits
template <class>
struct _Cond_value_type {};

// clang-format off
template <class _Ty>
    requires is_object_v<_Ty>
struct _Cond_value_type<_Ty> {
    using value_type = remove_cv_t<_Ty>;
};
// clang-format on

template <class>
struct readable_traits {};

template <class _Ty>
struct readable_traits<_Ty*> : _Cond_value_type<_Ty> {};

// clang-format off
template <class _Ty>
    requires is_array_v<_Ty>
struct readable_traits<_Ty> {
    using value_type = remove_cv_t<remove_extent_t<_Ty>>;
};
// clang-format on

template <class _Ty>
struct readable_traits<const _Ty> : readable_traits<_Ty> {};

template <_Has_member_value_type _Ty>
struct readable_traits<_Ty> : _Cond_value_type<typename _Ty::value_type> {};

template <_Has_member_element_type _Ty>
struct readable_traits<_Ty> : _Cond_value_type<typename _Ty::element_type> {};

// ALIAS TEMPLATE iter_value_t
template <class _Ty>
using iter_value_t = typename conditional_t<_Is_from_primary<iterator_traits<_Ty>>, readable_traits<_Ty>,
    iterator_traits<_Ty>>::value_type;

// ALIAS TEMPLATE iter_reference_t
template <_Dereferenceable _Ty>
using iter_reference_t = decltype(*_STD declval<_Ty&>());

// STRUCT TEMPLATE _Iterator_traits_base
template <class>
struct _Iterator_traits_base {};

// clang-format off
template <class _It>
concept _Has_iter_types = _Has_member_difference_type<_It> && _Has_member_value_type<_It> && _Has_member_reference<_It>
    && _Has_member_iterator_category<_It>;
// clang-format on

template <bool _Has_member_typedef>
struct _Old_iter_traits_pointer {
    template <class _It>
    using _Apply = typename _It::pointer;
};

template <>
struct _Old_iter_traits_pointer<false> {
    template <class>
    using _Apply = void;
};

template <_Has_iter_types _It>
struct _Iterator_traits_base<_It> {
    using iterator_category = typename _It::iterator_category;
    using difference_type   = typename _It::difference_type;
    using value_type        = typename _It::value_type;
    using pointer           = typename _Old_iter_traits_pointer<_Has_member_pointer<_It>>::template _Apply<_It>;
    using reference         = typename _It::reference;
};

template <bool _Has_member_typedef>
struct _Iter_traits_difference {
    template <class _It>
    using _Apply = typename incrementable_traits<_It>::difference_type;
};

template <>
struct _Iter_traits_difference<false> {
    template <class>
    using _Apply = void;
};

// clang-format off
template <class _It>
concept _Cpp17_iterator = copyable<_It> && requires(_It __i) {
    { *__i } -> _Can_reference;
    { ++__i } -> same_as<_It&>;
    { *__i++ } -> _Can_reference;
};

template <class _It>
concept _Cpp17_input_iterator = _Cpp17_iterator<_It> && equality_comparable<_It>
    && _Has_member_difference_type<incrementable_traits<_It>> && _Has_member_value_type<readable_traits<_It>>
    && requires(_It __i) {
        typename common_reference_t<iter_reference_t<_It>&&, typename readable_traits<_It>::value_type&>;
        typename common_reference_t<decltype(*__i++)&&, typename readable_traits<_It>::value_type&>;
        requires signed_integral<typename incrementable_traits<_It>::difference_type>;
    };

#if 1 // TRANSITION, VSO-1002863
template <class _Ty>
using _Member_iterator_category = typename _Ty::iterator_category;
#endif // TRANSITION, VSO-1002863

template <class _It>
    requires (!_Has_iter_types<_It> && _Cpp17_iterator<_It> && !_Cpp17_input_iterator<_It>
        // Implements the proposed resolution of LWG-3283:
#if 1 // TRANSITION, VSO-1002863
        && (!requires { typename _It::iterator_category; }
            || derived_from<_Member_iterator_category<_It>, output_iterator_tag>))
#else // ^^^ workaround / no workaround vvv
        && (!requires { typename _It::iterator_category; }
            || derived_from<typename _It::iterator_category, output_iterator_tag>))
#endif // TRANSITION, VSO-1002863
struct _Iterator_traits_base<_It> {
    using iterator_category = output_iterator_tag;
    using difference_type =
        typename _Iter_traits_difference<_Has_member_difference_type<incrementable_traits<_It>>>::template _Apply<_It>;
    using value_type = void;
    using pointer    = void;
    using reference  = void;
};
// clang-format on

enum class _Itraits_pointer_strategy { _Use_void, _Use_member, _Use_decltype };

template <_Itraits_pointer_strategy>
struct _Iter_traits_pointer;

template <>
struct _Iter_traits_pointer<_Itraits_pointer_strategy::_Use_void> {
    template <class>
    using _Apply = void;
};

template <>
struct _Iter_traits_pointer<_Itraits_pointer_strategy::_Use_member> {
    template <class _It>
    using _Apply = typename _It::pointer;
};

template <>
struct _Iter_traits_pointer<_Itraits_pointer_strategy::_Use_decltype> {
    template <class _It>
    using _Apply = decltype(_STD declval<_It&>().operator->());
};

template <class _Ty>
concept _Has_op_arrow = requires(_Ty&& __t) {
    static_cast<_Ty&&>(__t).operator->();
};

template <bool _Has_member_typedef>
struct _Iter_traits_reference {
    template <class _It>
    using _Apply = typename _It::reference;
};

template <>
struct _Iter_traits_reference<false> {
    template <class _It>
    using _Apply = iter_reference_t<_It>;
};

template <bool _Is_random>
struct _Iter_traits_category4 {
    using type = random_access_iterator_tag;
};

template <>
struct _Iter_traits_category4<false> {
    using type = bidirectional_iterator_tag;
};

// clang-format off
template <class _It>
concept _Cpp17_random_delta = totally_ordered<_It>
    && requires(_It __i, typename incrementable_traits<_It>::difference_type __n) {
        { __i += __n } -> same_as<_It&>;
        { __i -= __n } -> same_as<_It&>;
        { __i +  __n } -> same_as<_It>;
        { __n +  __i } -> same_as<_It>;
        { __i -  __n } -> same_as<_It>;
        { __i -  __i } -> same_as<decltype(__n)>;
        {  __i[__n]  } -> convertible_to<iter_reference_t<_It>>;
    };
// clang-format on

template <bool _Is_bidi>
struct _Iter_traits_category3 {
    template <class _It>
    using _Apply = typename _Iter_traits_category4<_Cpp17_random_delta<_It>>::type;
};

template <>
struct _Iter_traits_category3<false> {
    template <class>
    using _Apply = forward_iterator_tag;
};

// clang-format off
template <class _It>
concept _Cpp17_bidi_delta = requires(_It __i) {
    { --__i } -> same_as<_It&>;
    { __i-- } -> convertible_to<const _It&>;
    requires same_as<decltype(*__i--), iter_reference_t<_It>>;
};
// clang-format on

template <bool _Is_forward>
struct _Iter_traits_category2 {
    template <class _It>
    using _Apply = typename _Iter_traits_category3<_Cpp17_bidi_delta<_It>>::template _Apply<_It>;
};

template <>
struct _Iter_traits_category2<false> {
    template <class>
    using _Apply = input_iterator_tag;
};

// clang-format off
template <class _It>
concept _Cpp17_forward_delta = constructible_from<_It> && is_lvalue_reference_v<iter_reference_t<_It>>
    && same_as<remove_cvref_t<iter_reference_t<_It>>, typename readable_traits<_It>::value_type>
    && requires(_It __i) {
        { __i++ } -> convertible_to<const _It&>;
        requires same_as<decltype(*__i++), iter_reference_t<_It>>;
    };
// clang-format on

template <bool _Has_member_typedef>
struct _Iter_traits_category {
    template <class _It>
    using _Apply = typename _It::iterator_category;
};

template <>
struct _Iter_traits_category<false> {
    template <class _It>
    using _Apply = typename _Iter_traits_category2<_Cpp17_forward_delta<_It>>::template _Apply<_It>;
};

// clang-format off
template <class _It>
    requires (!_Has_iter_types<_It> && _Cpp17_input_iterator<_It>)
struct _Iterator_traits_base<_It> {
    using iterator_category = typename _Iter_traits_category<_Has_member_iterator_category<_It>>::template _Apply<_It>;
    using difference_type   = typename incrementable_traits<_It>::difference_type;
    using value_type        = typename readable_traits<_It>::value_type;
    using pointer           = typename _Iter_traits_pointer<(
        _Has_member_pointer<_It> ? _Itraits_pointer_strategy::_Use_member
                                 : _Has_op_arrow<_It&> ? _Itraits_pointer_strategy::_Use_decltype
                                                       : _Itraits_pointer_strategy::_Use_void)>::template _Apply<_It>;
    using reference         = typename _Iter_traits_reference<_Has_member_reference<_It>>::template _Apply<_It>;
};
// clang-format on

// STRUCT TEMPLATE iterator_traits
template <class _Ty>
struct iterator_traits : _Iterator_traits_base<_Ty> {
    using _From_primary = iterator_traits;
};

// clang-format off
template <class _Ty>
    requires is_object_v<_Ty>
struct iterator_traits<_Ty*> {
    using iterator_concept  = contiguous_iterator_tag;
    using iterator_category = random_access_iterator_tag;
    using value_type        = remove_cv_t<_Ty>;
    using difference_type   = ptrdiff_t;
    using pointer           = _Ty*;
    using reference         = _Ty&;
};
// clang-format on

// CUSTOMIZATION POINT OBJECT iter_move
namespace ranges {
    // STRUCT TEMPLATE _Choice_t
    template <class _Ty>
    struct _Choice_t {
        _Ty _Strategy  = _Ty{};
        bool _No_throw = false;
    };

    namespace _Iter_move {
        void iter_move(); // Block unqualified lookup per LWG-3247

        // clang-format off
        template <class _Ty>
        concept _Has_ADL = _Has_class_or_enum_type<_Ty> // Per LWG-3299
            && requires(_Ty&& __t) { iter_move(static_cast<_Ty&&>(__t)); };

        template <class _Ty>
        concept _Can_deref = requires(_Ty&& __t) {
            *static_cast<_Ty&&>(__t);
        };

        class _Cpo {
        private:
            enum class _St { _None, _Custom, _Fallback };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                if constexpr (_Has_ADL<_Ty>) {
                    return {_St::_Custom, noexcept(iter_move(_STD declval<_Ty>()))};
                } else if constexpr (_Can_deref<_Ty>) {
                    return {_St::_Fallback, noexcept(*_STD declval<_Ty>())};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            template <class _Ty>
                requires (_Choice<_Ty>._Strategy != _St::_None)
            _NODISCARD constexpr decltype(auto) operator()(_Ty&& _Val) const noexcept(_Choice<_Ty>._No_throw) {
                if constexpr (_Choice<_Ty>._Strategy == _St::_Custom) {
                    return iter_move(static_cast<_Ty&&>(_Val));
                } else if constexpr (_Choice<_Ty>._Strategy == _St::_Fallback) {
                    using _Ref = decltype(*static_cast<_Ty&&>(_Val));
                    if constexpr (is_lvalue_reference_v<_Ref>) {
                        return _STD move(*static_cast<_Ty&&>(_Val));
                    } else {
                        return *static_cast<_Ty&&>(_Val);
                    }
                } else {
                    static_assert(_Always_false<_Ty>, "should be unreachable");
                }
            }
        };
        // clang-format on
    }

    inline namespace _Cpos {
        inline constexpr _Iter_move::_Cpo iter_move;
    }
} // namespace ranges

// iter_swap defined below since it depends on indirectly_movable_storable

// ALIAS TEMPLATE iter_rvalue_reference_t
// clang-format off
template <class _Ty>
    requires _Dereferenceable<_Ty> && requires(_Ty& __t) {
        { _RANGES iter_move(__t) } -> _Can_reference;
    }
using iter_rvalue_reference_t = decltype(_RANGES iter_move(_STD declval<_Ty&>()));

// CONCEPT readable
template <class _It>
concept readable = requires {
    typename iter_value_t<_It>;
    typename iter_reference_t<_It>;
    typename iter_rvalue_reference_t<_It>;
} && common_reference_with<iter_reference_t<_It>&&, iter_value_t<_It>&>
  && common_reference_with<iter_reference_t<_It>&&, iter_rvalue_reference_t<_It>&&>
  && common_reference_with<iter_rvalue_reference_t<_It>&&, const iter_value_t<_It>&>;
// clang-format on

// ALIAS TEMPLATE iter_common_reference_t
template <readable _Ty>
using iter_common_reference_t = common_reference_t<iter_reference_t<_Ty>, iter_value_t<_Ty>&>;

// CONCEPT writable
template <class _It, class _Ty>
concept writable = requires(_It&& __i, _Ty&& __t) {
    *__i                                                                = static_cast<_Ty&&>(__t);
    *static_cast<_It&&>(__i)                                            = static_cast<_Ty&&>(__t);
    const_cast<const iter_reference_t<_It>&&>(*__i)                     = static_cast<_Ty&&>(__t);
    const_cast<const iter_reference_t<_It>&&>(*static_cast<_It&&>(__i)) = static_cast<_Ty&&>(__t);
};

// CONCEPT weakly_incrementable
// clang-format off
template <class _Ty>
concept weakly_incrementable = default_initializable<_Ty> && movable<_Ty> && requires(_Ty __i) {
    typename iter_difference_t<_Ty>;
    requires signed_integral<iter_difference_t<_Ty>>;
    { ++__i } -> same_as<_Ty&>;
    __i++;
};

// CONCEPT incrementable
template <class _Ty>
concept incrementable = regular<_Ty> && weakly_incrementable<_Ty> && requires(_Ty __t) {
    { __t++ } -> same_as<_Ty>;
};

// CONCEPT input_or_output_iterator
template <class _It>
concept input_or_output_iterator = requires(_It __i) {
    { *__i } -> _Can_reference;
    requires weakly_incrementable<_It>;
};

// CONCEPT sentinel_for
template <class _Se, class _It>
concept sentinel_for = semiregular<_Se> && input_or_output_iterator<_It> && _Weakly_equality_comparable_with<_Se, _It>;
// clang-format on

// VARIABLE TEMPLATE disable_sized_sentinel
template <class _Se, class _It>
inline constexpr bool disable_sized_sentinel = false;

// CONCEPT sized_sentinel_for
// clang-format off
template <class _Se, class _It>
concept sized_sentinel_for = sentinel_for<_Se, _It> && !disable_sized_sentinel<remove_cv_t<_Se>, remove_cv_t<_It>>
    && requires(const _It& __i, const _Se& __s) {
        { __s - __i } -> same_as<iter_difference_t<_It>>;
        { __i - __s } -> same_as<iter_difference_t<_It>>;
    };
// clang-format on

// ALIAS TEMPLATE _Iter_concept
template <class _It, bool _NotSpec = _Is_from_primary<iterator_traits<_It>>>
struct _Iter_concept_impl {};

// clang-format off
template <class _It>
    requires _Has_member_iterator_concept<iterator_traits<_It>>
struct _Iter_concept_impl<_It, false> {
    using type = typename iterator_traits<_It>::iterator_concept;
};

template <class _It>
    requires (!_Has_member_iterator_concept<iterator_traits<_It>>
        && _Has_member_iterator_category<iterator_traits<_It>>)
struct _Iter_concept_impl<_It, false> {
    using type = typename iterator_traits<_It>::iterator_category;
};
// clang-format on

template <class _It>
struct _Iter_concept_impl<_It, true> {
    using type = random_access_iterator_tag;
};

template <_Has_member_iterator_concept _It>
struct _Iter_concept_impl<_It, true> {
    using type = typename _It::iterator_concept;
};

// clang-format off
template <class _It>
    requires(!_Has_member_iterator_concept<_It> && _Has_member_iterator_category<_It>)
struct _Iter_concept_impl<_It, true> {
    using type = typename _It::iterator_category;
};
// clang-format on

template <class _It>
using _Iter_concept = typename _Iter_concept_impl<_It>::type;

// clang-format off
// CONCEPT input_iterator
template <class _It>
concept input_iterator = input_or_output_iterator<_It> && readable<_It> && requires { typename _Iter_concept<_It>; }
    && derived_from<_Iter_concept<_It>, input_iterator_tag>;

// CONCEPT output_iterator
template <class _It, class _Ty>
concept output_iterator = input_or_output_iterator<_It> && writable<_It, _Ty> && requires(_It __i, _Ty&& __t) {
    *__i++ = static_cast<_Ty&&>(__t);
};

// CONCEPT forward_iterator
template <class _It>
concept forward_iterator = input_iterator<_It> && derived_from<_Iter_concept<_It>, forward_iterator_tag>
    && incrementable<_It> && sentinel_for<_It, _It>;

// CONCEPT bidirectional_iterator
template <class _It>
concept bidirectional_iterator = forward_iterator<_It> && derived_from<_Iter_concept<_It>, bidirectional_iterator_tag>
    && requires(_It __i) {
        { --__i } -> same_as<_It&>;
        { __i-- } -> same_as<_It>;
    };

// CONCEPT random_access_iterator
template <class _It>
concept random_access_iterator = bidirectional_iterator<_It>
    && derived_from<_Iter_concept<_It>, random_access_iterator_tag> && totally_ordered<_It>
    && sized_sentinel_for<_It, _It> && requires(_It __i, const _It __j, const iter_difference_t<_It> __n) {
        { __i += __n } -> same_as<_It&>;
        { __j + __n } -> same_as<_It>;
        { __n + __j } -> same_as<_It>;
        { __i -= __n } -> same_as<_It&>;
        { __j - __n } -> same_as<_It>;
        { __j[__n] } -> same_as<iter_reference_t<_It>>;
    };

// CONCEPT contiguous_iterator
template <class _It>
concept contiguous_iterator = random_access_iterator<_It>
    && derived_from<_Iter_concept<_It>, contiguous_iterator_tag>
    && is_lvalue_reference_v<iter_reference_t<_It>>
    && same_as<iter_value_t<_It>, remove_cvref_t<iter_reference_t<_It>>>
    && requires(const _It& __i) {
        { _STD to_address(__i) } -> same_as<add_pointer_t<iter_reference_t<_It>>>;
    };

// CONCEPT indirectly_movable
template <class _In, class _Out>
concept indirectly_movable = readable<_In> && writable<_Out, iter_rvalue_reference_t<_In>>;

// CONCEPT indirectly_movable_storable
template <class _In, class _Out>
concept indirectly_movable_storable = indirectly_movable<_In, _Out> && writable<_Out, iter_value_t<_In>>
    && movable<iter_value_t<_In>> && constructible_from<iter_value_t<_In>, iter_rvalue_reference_t<_In>>
    && assignable_from<iter_value_t<_In>&, iter_rvalue_reference_t<_In>>;
// clang-format on

// CUSTOMIZATION POINT OBJECT iter_swap
namespace ranges {
    namespace _Iter_swap {
        template <class _Ty1, class _Ty2>
        void iter_swap(_Ty1, _Ty2) = delete;

        // clang-format off
        template <class _Ty1, class _Ty2>
        concept _Has_ADL = (_Has_class_or_enum_type<_Ty1> || _Has_class_or_enum_type<_Ty2>) // Per LWG-3299
            && requires(_Ty1&& __t1, _Ty2&& __t2) {
                iter_swap(static_cast<_Ty1&&>(__t1), static_cast<_Ty2&&>(__t2));
            };

        template <class _Ty1, class _Ty2>
        concept _Can_swap_references = readable<remove_reference_t<_Ty1>> && readable<remove_reference_t<_Ty2>>
            && swappable_with<iter_reference_t<_Ty1>, iter_reference_t<_Ty2>>;
        // clang-format on

        template <class _Xty, class _Yty>
        _NODISCARD constexpr iter_value_t<remove_reference_t<_Xty>> _Iter_exchange_move(
            _Xty&& _XVal, _Yty&& _YVal) noexcept(noexcept(iter_value_t<remove_reference_t<_Xty>>(iter_move(_XVal)))) {
            iter_value_t<remove_reference_t<_Xty>> _Tmp(iter_move(_XVal));
            *_XVal = iter_move(_YVal);
            return _Tmp;
        }

        class _Cpo {
        private:
            enum class _St { _None, _Custom, _Swap, _Exchange };

            // clang-format off
            template <class _Ty1, class _Ty2>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                if constexpr (_Has_ADL<_Ty1, _Ty2>) {
                    return {_St::_Custom, noexcept(iter_swap(_STD declval<_Ty1>(), _STD declval<_Ty2>()))};
                } else if constexpr (_Can_swap_references<_Ty1, _Ty2>) {
                    return {_St::_Swap, noexcept(swap(*_STD declval<_Ty1>(), *_STD declval<_Ty2>()))};
                } else if constexpr (indirectly_movable_storable<remove_reference_t<_Ty1>, remove_reference_t<_Ty2>>
                    && indirectly_movable_storable<remove_reference_t<_Ty2>, remove_reference_t<_Ty1>>) {
                    constexpr auto _Nothrow = noexcept(
                        *_STD declval<_Ty1>() = _Iter_exchange_move(_STD declval<_Ty1>(), _STD declval<_Ty2>()));
                    return {_St::_Exchange, _Nothrow};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty1, class _Ty2>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty1, _Ty2>();

        public:
            template <class _Ty1, class _Ty2>
                requires (_Choice<_Ty1, _Ty2>._Strategy != _St::_None)
            constexpr void operator()(_Ty1&& _Val1, _Ty2&& _Val2) const noexcept(_Choice<_Ty1, _Ty2>._No_throw) {
                if constexpr (_Choice<_Ty1, _Ty2>._Strategy == _St::_Custom) {
                    iter_swap(static_cast<_Ty1&&>(_Val1), static_cast<_Ty2&&>(_Val2));
                } else if constexpr (_Choice<_Ty1, _Ty2>._Strategy == _St::_Swap) {
                    swap(*static_cast<_Ty1&&>(_Val1), *static_cast<_Ty2&&>(_Val2));
                } else if constexpr (_Choice<_Ty1, _Ty2>._Strategy == _St::_Exchange) {
                    *static_cast<_Ty1&&>(_Val1) =
                        _Iter_exchange_move(static_cast<_Ty1&&>(_Val1), static_cast<_Ty2&&>(_Val2));
                } else {
                    static_assert(_Always_false<_Ty1>, "should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Iter_swap

    inline namespace _Cpos {
        inline constexpr _Iter_swap::_Cpo iter_swap;
    }
} // namespace ranges

// ALIAS TEMPLATE _Iter_ref_t
template <class _Iter>
using _Iter_ref_t = iter_reference_t<_Iter>;

// ALIAS TEMPLATE _Iter_value_t
template <class _Iter>
using _Iter_value_t = iter_value_t<_Iter>;

// ALIAS TEMPLATE _Iter_diff_t
template <class _Iter>
using _Iter_diff_t = iter_difference_t<_Iter>;

#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
// STRUCT TEMPLATE iterator_traits
template <class, class = void>
struct _Iterator_traits_base {}; // empty for non-iterators

template <class _Iter>
struct _Iterator_traits_base<_Iter,
    void_t<typename _Iter::iterator_category, typename _Iter::value_type, typename _Iter::difference_type,
        typename _Iter::pointer, typename _Iter::reference>> {
    // defined if _Iter::* types exist
    using iterator_category = typename _Iter::iterator_category;
    using value_type        = typename _Iter::value_type;
    using difference_type   = typename _Iter::difference_type;
    using pointer           = typename _Iter::pointer;
    using reference         = typename _Iter::reference;
};

template <class _Ty, bool = is_object_v<_Ty>>
struct _Iterator_traits_pointer_base { // iterator properties for pointers to object
    using iterator_category = random_access_iterator_tag;
    using value_type        = remove_cv_t<_Ty>;
    using difference_type   = ptrdiff_t;
    using pointer           = _Ty*;
    using reference         = _Ty&;
};

template <class _Ty>
struct _Iterator_traits_pointer_base<_Ty, false> {}; // iterator non-properties for pointers to non-object

template <class _Iter>
struct iterator_traits : _Iterator_traits_base<_Iter> {}; // get traits from iterator _Iter, if possible

template <class _Ty>
struct iterator_traits<_Ty*> : _Iterator_traits_pointer_base<_Ty> {}; // get traits from pointer, if possible

// ALIAS TEMPLATE _Iter_ref_t
template <class _Iter>
using _Iter_ref_t = typename iterator_traits<_Iter>::reference;

// ALIAS TEMPLATE _Iter_value_t
template <class _Iter>
using _Iter_value_t = typename iterator_traits<_Iter>::value_type;

// ALIAS TEMPLATE _Iter_diff_t
template <class _Iter>
using _Iter_diff_t = typename iterator_traits<_Iter>::difference_type;
#endif // __cpp_lib_concepts

// ALIAS TEMPLATE _Common_diff_t
template <class... _Iters>
using _Common_diff_t = common_type_t<_Iter_diff_t<_Iters>...>;

// ALIAS TEMPLATE _Iter_cat_t
template <class _Iter>
using _Iter_cat_t = typename iterator_traits<_Iter>::iterator_category;

// VARIABLE TEMPLATE _Is_iterator_v
template <class _Ty, class = void>
_INLINE_VAR constexpr bool _Is_iterator_v = false;

template <class _Ty>
_INLINE_VAR constexpr bool _Is_iterator_v<_Ty, void_t<_Iter_cat_t<_Ty>>> = true;

// TRAIT _Is_iterator
template <class _Ty>
struct _Is_iterator : bool_constant<_Is_iterator_v<_Ty>> {};

// VARIABLE TEMPLATE _Is_input_iter_v
template <class _Iter>
_INLINE_VAR constexpr bool _Is_input_iter_v = is_convertible_v<_Iter_cat_t<_Iter>, input_iterator_tag>;

// VARIABLE TEMPLATE _Is_fwd_iter_v
template <class _Iter>
_INLINE_VAR constexpr bool _Is_fwd_iter_v = is_convertible_v<_Iter_cat_t<_Iter>, forward_iterator_tag>;

// VARIABLE TEMPLATE _Is_bidi_iter_v
template <class _Iter>
_INLINE_VAR constexpr bool _Is_bidi_iter_v = is_convertible_v<_Iter_cat_t<_Iter>, bidirectional_iterator_tag>;

// VARIABLE TEMPLATE _Is_random_iter_v
template <class _Iter>
_INLINE_VAR constexpr bool _Is_random_iter_v = is_convertible_v<_Iter_cat_t<_Iter>, random_access_iterator_tag>;

// STRUCT TEMPLATE _Is_checked_helper
template <class, class = void>
struct _Is_checked_helper {}; // default definition, no longer used, retained due to pseudo-documentation

// FUNCTION TEMPLATE _Adl_verify_range
#if _ITERATOR_DEBUG_LEVEL != 0
template <class _Ty>
constexpr void _Verify_range(const _Ty* const _First, const _Ty* const _Last) noexcept {
    // special case range verification for pointers
    _STL_VERIFY(_First <= _Last, "transposed pointer range");
}
#endif // _ITERATOR_DEBUG_LEVEL != 0

template <class _Iter, class = void>
_INLINE_VAR constexpr bool _Allow_inheriting_unwrap_v = true;

template <class _Iter>
_INLINE_VAR constexpr bool _Allow_inheriting_unwrap_v<_Iter, void_t<typename _Iter::_Prevent_inheriting_unwrap>> =
    is_same_v<_Iter, typename _Iter::_Prevent_inheriting_unwrap>;

template <class _Iter, class _Sentinel = _Iter, class = void>
_INLINE_VAR constexpr bool _Range_verifiable_v = false;

template <class _Iter, class _Sentinel>
_INLINE_VAR constexpr bool _Range_verifiable_v<_Iter, _Sentinel,
    void_t<decltype(_Verify_range(_STD declval<const _Iter&>(), _STD declval<const _Sentinel&>()))>> =
    _Allow_inheriting_unwrap_v<_Iter>;

#if _HAS_IF_CONSTEXPR
template <class _Iter, class _Sentinel>
constexpr void _Adl_verify_range(const _Iter& _First, const _Sentinel& _Last) {
    // check that [_First, _Last) forms an iterator range
    if constexpr (_Range_verifiable_v<_Iter, _Sentinel>) {
        _Verify_range(_First, _Last);
    } else {
        (void) _First; // TRANSITION, VSO-486357
        (void) _Last; // TRANSITION, VSO-486357
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _Iter, class _Sentinel>
constexpr void _Adl_verify_range1(const _Iter& _First, const _Sentinel& _Last, true_type) {
    // check that [_First, _Last) forms an iterator range
    _Verify_range(_First, _Last);
}

template <class _Iter, class _Sentinel>
constexpr void _Adl_verify_range1(const _Iter&, const _Sentinel&, false_type) {
    // (don't) check that [_First, _Last) forms an iterator range
}

template <class _Iter, class _Sentinel>
constexpr void _Adl_verify_range(const _Iter& _First, const _Sentinel& _Last) {
    // check that [_First, _Last) forms an iterator range
    _Adl_verify_range1(_First, _Last, bool_constant<_Range_verifiable_v<_Iter, _Sentinel>>{});
}
#endif // _HAS_IF_CONSTEXPR


// FUNCTION TEMPLATE _Get_unwrapped
template <class _Iter, class = void>
_INLINE_VAR constexpr bool _Unwrappable_v = false;

template <class _Iter>
_INLINE_VAR constexpr bool _Unwrappable_v<_Iter,
    void_t<decltype(_STD declval<_Remove_cvref_t<_Iter>&>()._Seek_to(_STD declval<_Iter>()._Unwrapped()))>> =
    _Allow_inheriting_unwrap_v<_Remove_cvref_t<_Iter>>;

#if _HAS_IF_CONSTEXPR
template <class _Iter>
_NODISCARD constexpr decltype(auto) _Get_unwrapped(_Iter&& _It) {
    // unwrap an iterator previously subjected to _Adl_verify_range or otherwise validated
    if constexpr (is_pointer_v<decay_t<_Iter>>) { // special-case pointers and arrays
        return _It + 0;
    } else if constexpr (_Unwrappable_v<_Iter>) {
        return static_cast<_Iter&&>(_It)._Unwrapped();
    } else {
        return static_cast<_Iter&&>(_It);
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _Iter, enable_if_t<_Unwrappable_v<_Iter>, int> = 0>
_NODISCARD constexpr decltype(auto) _Get_unwrapped(_Iter&& _It) {
    // unwrap an iterator previously subjected to _Adl_verify_range or otherwise validated
    return static_cast<_Iter&&>(_It)._Unwrapped();
}

template <class _Iter, enable_if_t<!_Unwrappable_v<_Iter>, int> = 0>
_NODISCARD constexpr _Iter&& _Get_unwrapped(_Iter&& _It) {
    // (don't) unwrap an iterator previously subjected to _Adl_verify_range or otherwise validated
    return static_cast<_Iter&&>(_It);
}

template <class _Ty>
_NODISCARD constexpr _Ty* _Get_unwrapped(_Ty* const _Ptr) { // special case already-unwrapped pointers
    return _Ptr;
}
#endif // _HAS_IF_CONSTEXPR

template <class _Iter>
using _Unwrapped_t = _Remove_cvref_t<decltype(_Get_unwrapped(_STD declval<_Iter>()))>;

// FUNCTION TEMPLATE _Get_unwrapped_unverified
template <class _Iter, class = bool>
_INLINE_VAR constexpr bool _Do_unwrap_when_unverified_v = false;

template <class _Iter>
_INLINE_VAR constexpr bool
    _Do_unwrap_when_unverified_v<_Iter, decltype(static_cast<bool>(_Iter::_Unwrap_when_unverified))> =
        static_cast<bool>(_Iter::_Unwrap_when_unverified);

template <class _Iter>
_INLINE_VAR constexpr bool _Unwrappable_for_unverified_v =
    _Unwrappable_v<_Iter>&& _Do_unwrap_when_unverified_v<_Remove_cvref_t<_Iter>>;

#if _HAS_IF_CONSTEXPR
template <class _Iter>
_NODISCARD constexpr decltype(auto) _Get_unwrapped_unverified(_Iter&& _It) {
    // unwrap an iterator not previously subjected to _Adl_verify_range
    if constexpr (is_pointer_v<decay_t<_Iter>>) { // special-case pointers and arrays
        return _It + 0;
    } else if constexpr (_Unwrappable_for_unverified_v<_Iter>) {
        return static_cast<_Iter&&>(_It)._Unwrapped();
    } else {
        return static_cast<_Iter&&>(_It);
    }
}
#else // ^^^_HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _Iter, enable_if_t<_Unwrappable_for_unverified_v<_Iter>, int> = 0>
_NODISCARD constexpr decltype(auto) _Get_unwrapped_unverified(_Iter&& _It) {
    // unwrap an iterator not previously subjected to _Adl_verify_range
    return static_cast<_Iter&&>(_It)._Unwrapped();
}

template <class _Iter, enable_if_t<!_Unwrappable_for_unverified_v<_Iter>, int> = 0>
_NODISCARD constexpr _Iter&& _Get_unwrapped_unverified(_Iter&& _It) {
    // (don't) unwrap an iterator not previously subjected to _Adl_verify_range
    return static_cast<_Iter&&>(_It);
}

template <class _Ty>
_NODISCARD constexpr _Ty* _Get_unwrapped_unverified(_Ty* const _Ptr) { // special case already-unwrapped pointers
    return _Ptr;
}
#endif // _HAS_IF_CONSTEXPR

template <class _Iter>
using _Unwrapped_unverified_t = _Remove_cvref_t<decltype(_Get_unwrapped_unverified(_STD declval<_Iter>()))>;

// FUNCTION TEMPLATE _Get_unwrapped_n
struct _Distance_unknown {
    constexpr _Distance_unknown operator-() const noexcept {
        return {};
    }
};

template <class _Diff>
_INLINE_VAR constexpr auto _Max_possible_v = _Diff{static_cast<make_unsigned_t<_Diff>>(-1) >> 1};

template <class _Diff>
_INLINE_VAR constexpr auto _Min_possible_v = _Diff{-_Max_possible_v<_Diff> - 1};

template <class _Iter, class = void>
_INLINE_VAR constexpr bool _Offset_verifiable_v = false;

template <class _Iter>
_INLINE_VAR constexpr bool
    _Offset_verifiable_v<_Iter, void_t<decltype(_STD declval<const _Iter&>()._Verify_offset(_Iter_diff_t<_Iter>{}))>> =
        true;

template <class _Iter>
_INLINE_VAR constexpr bool _Unwrappable_for_offset_v =
    _Unwrappable_v<_Iter>&& _Offset_verifiable_v<_Remove_cvref_t<_Iter>>;

#if _HAS_IF_CONSTEXPR
template <class _Iter, class _Diff>
_NODISCARD constexpr decltype(auto) _Get_unwrapped_n(_Iter&& _It, const _Diff _Off) {
    (void) _Off;
    if constexpr (is_pointer_v<decay_t<_Iter>>) {
        return _It + 0;
    } else if constexpr (_Unwrappable_for_offset_v<_Iter> && is_integral_v<_Diff>) {
        // ask an iterator to assert that the iterator moved _Off positions is valid, and unwrap
        using _IDiff     = _Iter_diff_t<_Remove_cvref_t<_Iter>>;
        using _CDiff     = common_type_t<_Diff, _IDiff>;
        const auto _COff = static_cast<_CDiff>(_Off);

        _STL_ASSERT(_COff <= static_cast<_CDiff>(_Max_possible_v<_IDiff>)
                        && (is_unsigned_v<_Diff> || static_cast<_CDiff>(_Min_possible_v<_IDiff>) <= _COff),
            "integer overflow");
        (void) _COff;

        _It._Verify_offset(static_cast<_IDiff>(_Off));
        return static_cast<_Iter&&>(_It)._Unwrapped();
    } else if constexpr (_Unwrappable_for_unverified_v<_Iter>) {
        // iterator doesn't support offset-based asserts, or offset unknown; defer to unverified unwrap
        return static_cast<_Iter&&>(_It)._Unwrapped();
    } else {
        // pass through iterator that doesn't participate in checking
        return static_cast<_Iter&&>(_It);
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _Iter, class _Diff, enable_if_t<_Unwrappable_for_offset_v<_Iter> && is_integral_v<_Diff>, int> = 0>
_NODISCARD constexpr decltype(auto) _Get_unwrapped_n(_Iter&& _It, const _Diff _Off) {
    // ask an iterator to assert that the iterator moved _Off positions is valid, and unwrap
    using _IDiff     = _Iter_diff_t<_Remove_cvref_t<_Iter>>;
    using _CDiff     = common_type_t<_Diff, _IDiff>;
    const auto _COff = static_cast<_CDiff>(_Off);

    _STL_ASSERT(_COff <= static_cast<_CDiff>(_Max_possible_v<_IDiff>)
                    && (is_unsigned_v<_Diff> || static_cast<_CDiff>(_Min_possible_v<_IDiff>) <= _COff),
        "integer overflow");
    (void) _COff;

    _It._Verify_offset(static_cast<_IDiff>(_Off));
    return static_cast<_Iter&&>(_It)._Unwrapped();
}

template <class _Iter, class _Diff,
    enable_if_t<
        _Unwrappable_for_unverified_v<_Iter> //
            && ((!_Unwrappable_for_offset_v<_Iter> && is_integral_v<_Diff>) || is_same_v<_Diff, _Distance_unknown>),
        int> = 0>
_NODISCARD constexpr decltype(auto) _Get_unwrapped_n(_Iter&& _It, _Diff) {
    // iterator doesn't support offset-based asserts, or offset unknown; defer to unverified unwrap
    return static_cast<_Iter&&>(_It)._Unwrapped();
}

template <class _Iter, class _Diff,
    enable_if_t<
        !_Unwrappable_for_unverified_v<_Iter> //
            && ((!_Unwrappable_for_offset_v<_Iter> && is_integral_v<_Diff>) || is_same_v<_Diff, _Distance_unknown>),
        int> = 0>
_NODISCARD constexpr _Iter&& _Get_unwrapped_n(_Iter&& _It, _Diff) {
    // pass through iterator that doesn't participate in checking
    return static_cast<_Iter&&>(_It);
}

template <class _Ty, class _Diff, enable_if_t<is_same_v<_Diff, _Distance_unknown> || is_integral_v<_Diff>, int> = 0>
_NODISCARD constexpr _Ty* _Get_unwrapped_n(_Ty* const _Src, _Diff) {
    return _Src;
}
#endif // _HAS_IF_CONSTEXPR

template <class _Iter>
using _Unwrapped_n_t =
    _Remove_cvref_t<decltype(_Get_unwrapped_n(_STD declval<_Iter>(), _Iter_diff_t<_Remove_cvref_t<_Iter>>{}))>;

// FUNCTION TEMPLATE _Seek_wrapped
template <class _Iter, class _UIter, class = void>
_INLINE_VAR constexpr bool _Wrapped_seekable_v = false;

template <class _Iter, class _UIter>
_INLINE_VAR constexpr bool
    _Wrapped_seekable_v<_Iter, _UIter, void_t<decltype(_STD declval<_Iter&>()._Seek_to(_STD declval<_UIter>()))>> =
        true;

#if _HAS_IF_CONSTEXPR
template <class _Iter, class _UIter>
constexpr void _Seek_wrapped(_Iter& _It, _UIter&& _UIt) {
    if constexpr (_Wrapped_seekable_v<_Iter, _UIter>) {
        _It._Seek_to(static_cast<_UIter&&>(_UIt));
    } else {
        _It = static_cast<_UIter&&>(_UIt);
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _Iter, class _UIter, enable_if_t<_Wrapped_seekable_v<_Iter, _UIter>, int> = 0>
constexpr void _Seek_wrapped(_Iter& _It, _UIter&& _UIt) {
    _It._Seek_to(static_cast<_UIter&&>(_UIt));
}

template <class _Iter, class _UIter, enable_if_t<!_Wrapped_seekable_v<_Iter, _UIter>, int> = 0>
constexpr void _Seek_wrapped(_Iter& _It, _UIter&& _UIt) {
    _It = static_cast<_UIter&&>(_UIt);
}

template <class _Ty>
constexpr void _Seek_wrapped(_Ty*& _It, _Ty* const _UIt) {
    _It = _UIt;
}
#endif // _HAS_IF_CONSTEXPR

#if _HAS_CXX17
// STRUCT TEMPLATE _Is_allocator
template <class _Ty, class = void>
struct _Is_allocator : false_type { // selected when _Ty can't possibly be an allocator
};

template <class _Ty>
struct _Is_allocator<_Ty, void_t<typename _Ty::value_type, decltype(_STD declval<_Ty&>().deallocate(
                                                               _STD declval<_Ty&>().allocate(size_t{1}), size_t{1}))>>
    : true_type { // selected when _Ty resembles an allocator, N4687 26.2.1 [container.requirements.general]/17
};

// ALIAS TEMPLATES FOR DEDUCTION GUIDES, N4687 26.4.1 [associative.general]/2
template <class _Iter>
using _Guide_key_t = remove_const_t<typename iterator_traits<_Iter>::value_type::first_type>;

template <class _Iter>
using _Guide_val_t = typename iterator_traits<_Iter>::value_type::second_type;

template <class _Iter>
using _Guide_pair_t = pair<add_const_t<typename iterator_traits<_Iter>::value_type::first_type>,
    typename iterator_traits<_Iter>::value_type::second_type>;

// STRUCT TEMPLATE is_execution_policy
template <class _Ty>
struct is_execution_policy : false_type {};

template <class _Ty>
inline constexpr bool is_execution_policy_v = is_execution_policy<_Ty>::value;

// ALIAS TEMPLATE _Enable_if_execution_policy_t
// Note: The noexcept specifiers on all parallel algorithm overloads enforce termination as per
// N4713 23.19.4 [execpol.seq]/2, 23.19.5 [execpol.par]/2, and 23.19.6 [execpol.parunseq]/2
template <class _ExPo>
using _Enable_if_execution_policy_t = typename remove_reference_t<_ExPo>::_Standard_execution_policy;

#define _REQUIRE_PARALLEL_ITERATOR(_Iter) \
    static_assert(_Is_fwd_iter_v<_Iter>, "Parallel algorithms require forward iterators or stronger.")

#endif // _HAS_CXX17

// FUNCTION TEMPLATE _Idl_distance
#if _HAS_IF_CONSTEXPR
template <class _Checked, class _Iter>
_NODISCARD constexpr auto _Idl_distance(const _Iter& _First, const _Iter& _Last) {
    // tries to get the distance between _First and _Last if they are random-access iterators
    if constexpr (_Is_random_iter_v<_Iter>) {
        return static_cast<_Iter_diff_t<_Checked>>(_Last - _First);
    } else {
        (void) _First; // TRANSITION, VSO-486357
        (void) _Last; // TRANSITION, VSO-486357
        return _Distance_unknown{};
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _Checked, class _Iter>
_NODISCARD constexpr _Distance_unknown _Idl_distance1(const _Iter&, const _Iter&, input_iterator_tag) {
    // _Idl_distance for non-random-access iterators
    return {};
}

template <class _Checked, class _Iter>
_NODISCARD constexpr _Iter_diff_t<_Checked> _Idl_distance1(
    const _Iter& _First, const _Iter& _Last, random_access_iterator_tag) {
    // _Idl_distance for random-access iterators
    return static_cast<_Iter_diff_t<_Checked>>(_Last - _First);
}

template <class _Checked, class _Iter>
_NODISCARD constexpr auto _Idl_distance(const _Iter& _First, const _Iter& _Last) {
    // tries to get the distance between _First and _Last if they are random-access iterators
    return _Idl_distance1<_Checked>(_First, _Last, _Iter_cat_t<_Iter>());
}
#endif // _HAS_IF_CONSTEXPR


// STRUCT TEMPLATE _Unwrap_enum AND ALIAS
template <class _Elem, bool _Is_enum = is_enum_v<_Elem>>
struct _Unwrap_enum { // if _Elem is an enum, gets its underlying type; otherwise leaves _Elem unchanged
    using type = underlying_type_t<_Elem>;
};

template <class _Elem>
struct _Unwrap_enum<_Elem, false> { // passthrough non-enum type
    using type = _Elem;
};

template <class _Elem>
using _Unwrap_enum_t = typename _Unwrap_enum<_Elem>::type;

// DEBUG TESTING MACROS

#if _ITERATOR_DEBUG_LEVEL < 2
#define _DEBUG_LT_PRED(pred, x, y) static_cast<bool>(pred(x, y))
#define _DEBUG_ORDER_UNWRAPPED(first, last, pred)
#define _DEBUG_ORDER_SET_UNWRAPPED(otherIter, first, last, pred)

#else // _ITERATOR_DEBUG_LEVEL < 2
#define _DEBUG_LT_PRED(pred, x, y)                _Debug_lt_pred(pred, x, y)
#define _DEBUG_ORDER_UNWRAPPED(first, last, pred) _Debug_order_unchecked(first, last, pred)
#define _DEBUG_ORDER_SET_UNWRAPPED(otherIter, first, last, pred) \
    _Debug_order_set_unchecked<otherIter>(first, last, pred)

// FUNCTION TEMPLATE _Debug_lt_pred
template <class _Pr, class _Ty1, class _Ty2,
    enable_if_t<is_same_v<_Remove_cvref_t<_Ty1>, _Remove_cvref_t<_Ty2>>, int> = 0>
constexpr bool _Debug_lt_pred(_Pr&& _Pred, _Ty1&& _Left, _Ty2&& _Right) noexcept(
    noexcept(_Pred(_Left, _Right)) && noexcept(_Pred(_Right, _Left))) {
    // test if _Pred(_Left, _Right) and _Pred is strict weak ordering, when the arguments are the cv-same-type
    const auto _Result = static_cast<bool>(_Pred(_Left, _Right));
    if (_Result) {
        _STL_VERIFY(!_Pred(_Right, _Left), "invalid comparator");
    }

    return _Result;
}

template <class _Pr, class _Ty1, class _Ty2,
    enable_if_t<!is_same_v<_Remove_cvref_t<_Ty1>, _Remove_cvref_t<_Ty2>>, int> = 0>
constexpr bool _Debug_lt_pred(_Pr&& _Pred, _Ty1&& _Left, _Ty2&& _Right) noexcept(noexcept(_Pred(_Left, _Right))) {
    // test if _Pred(_Left, _Right); no debug checks as the types differ
    return static_cast<bool>(_Pred(_Left, _Right));
}

// FUNCTION TEMPLATE _Debug_order_unchecked
#if _HAS_IF_CONSTEXPR
template <class _InIt, class _Sentinel, class _Pr>
constexpr void _Debug_order_unchecked(_InIt _First, _Sentinel _Last, _Pr&& _Pred) {
    // test if range is ordered by predicate
    if constexpr (_Is_fwd_iter_v<_InIt>) {
        if (_First != _Last) {
            for (auto _Next = _First; ++_Next != _Last; _First = _Next) {
                _STL_VERIFY(!static_cast<bool>(_Pred(*_Next, *_First)), "sequence not ordered");
            }
        }
    } else {
        (void) _First; // TRANSITION, VSO-486357
        (void) _Last; // TRANSITION, VSO-486357
        (void) _Pred; // TRANSITION, VSO-486357
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt, class _Sentinel, class _Pr>
constexpr void _Debug_order_unchecked2(_InIt, _Sentinel, _Pr&, input_iterator_tag) {
    // (don't) test if range is ordered by predicate, input iterators
}

template <class _FwdIt, class _Sentinel, class _Pr>
constexpr void _Debug_order_unchecked2(_FwdIt _First, _Sentinel _Last, _Pr& _Pred, forward_iterator_tag) {
    // test if range is ordered by predicate, forward iterators
    if (_First != _Last) {
        for (_FwdIt _Next = _First; ++_Next != _Last; _First = _Next) {
            _STL_VERIFY(!static_cast<bool>(_Pred(*_Next, *_First)), "sequence not ordered");
        }
    }
}

template <class _InIt, class _Sentinel, class _Pr>
constexpr void _Debug_order_unchecked(_InIt _First, _Sentinel _Last, _Pr&& _Pred) {
    // test if range is ordered by predicate
    _Debug_order_unchecked2(_First, _Last, _Pred, _Iter_cat_t<_InIt>());
}
#endif // _HAS_IF_CONSTEXPR

// FUNCTION TEMPLATE _Debug_order_set_unchecked
#if _HAS_IF_CONSTEXPR
template <class _OtherIt, class _InIt, class _Pr>
constexpr void _Debug_order_set_unchecked(_InIt _First, _InIt _Last, _Pr&& _Pred) {
    // test if range is ordered by predicate
    if constexpr (is_same_v<_Iter_value_t<_OtherIt>, _Iter_value_t<_InIt>> && _Is_fwd_iter_v<_InIt>) {
        _Debug_order_unchecked(_First, _Last, _Pred);
    } else {
        (void) _First; // TRANSITION, VSO-486357
        (void) _Last; // TRANSITION, VSO-486357
        (void) _Pred; // TRANSITION, VSO-486357
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <size_t _Value>
struct _Priority_tag : _Priority_tag<_Value - 1> { // priority tag for tag dispatch
};

template <>
struct _Priority_tag<0> { // base case priority tag for tag dispatch
};

template <class _InIt, class _Pr>
void _Debug_order_set_unchecked2(_InIt, _InIt, _Pr&, input_iterator_tag,
    _Priority_tag<0>) { // (don't) test if range is ordered by predicate, input iterators or different types
}

template <class _FwdIt, class _Pr>
constexpr void _Debug_order_set_unchecked2(
    _FwdIt _First, _FwdIt _Last, _Pr& _Pred, forward_iterator_tag, _Priority_tag<1>) {
    // test if range is ordered by predicate, forward iterators and same types
    _Debug_order_unchecked2(_First, _Last, _Pred, forward_iterator_tag{});
}

template <class _OtherIt, class _InIt, class _Pr>
constexpr void _Debug_order_set_unchecked(_InIt _First, _InIt _Last, _Pr&& _Pred) {
    // test if range is ordered by predicate
    _Debug_order_set_unchecked2(_First, _Last, _Pred, _Iter_cat_t<_InIt>(),
        _Priority_tag<is_same_v<_Iter_value_t<_OtherIt>, _Iter_value_t<_InIt>>>());
}
#endif // _HAS_IF_CONSTEXPR
#endif // _ITERATOR_DEBUG_LEVEL < 2

// MORE ITERATOR STUFF (from <iterator>)
// FUNCTION TEMPLATE advance
#if _HAS_IF_CONSTEXPR
template <class _InIt, class _Diff>
_CONSTEXPR17 void advance(_InIt& _Where, _Diff _Off) { // increment iterator by offset
    if constexpr (_Is_random_iter_v<_InIt>) {
        _Where += _Off;
    } else {
        if constexpr (is_signed_v<_Diff> && !_Is_bidi_iter_v<_InIt>) {
            _STL_ASSERT(_Off >= 0, "negative advance of non-bidirectional iterator");
        }

        auto&& _UWhere              = _Get_unwrapped_n(_STD move(_Where), _Off);
        constexpr bool _Need_rewrap = !is_reference_v<decltype(_Get_unwrapped_n(_STD move(_Where), _Off))>;

        if constexpr (is_signed_v<_Diff> && _Is_bidi_iter_v<_InIt>) {
            for (; _Off < 0; ++_Off) {
                --_UWhere;
            }
        }

        for (; 0 < _Off; --_Off) {
            ++_UWhere;
        }

        if constexpr (_Need_rewrap) {
            _Seek_wrapped(_Where, _STD move(_UWhere));
        }
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt, class _Diff>
_CONSTEXPR17 void _Advance1(_InIt& _Where, _Diff _Off, input_iterator_tag) {
    // increment iterator by offset, input iterators
    _STL_ASSERT(_Off >= 0, "negative advance of non-bidirectional iterator");

    auto&& _UWhere              = _Get_unwrapped_n(_STD move(_Where), _Off);
    constexpr bool _Need_rewrap = !is_reference_v<decltype(_Get_unwrapped_n(_STD move(_Where), _Off))>;

    for (; 0 < _Off; --_Off) {
        ++_UWhere;
    }

    if (_Need_rewrap) {
        _Seek_wrapped(_Where, _STD move(_UWhere));
    }
}

template <class _BidIt, class _Diff>
_CONSTEXPR17 void _Advance1(_BidIt& _Where, _Diff _Off, bidirectional_iterator_tag) {
    // increment iterator by offset, bidirectional iterators
    auto&& _UWhere              = _Get_unwrapped_n(_STD move(_Where), _Off);
    constexpr bool _Need_rewrap = !is_reference_v<decltype(_Get_unwrapped_n(_STD move(_Where), _Off))>;

    for (; 0 < _Off; --_Off) {
        ++_UWhere;
    }

    // the following warning is triggered if _Diff is unsigned
#pragma warning(suppress : 6294) // Ill-defined for-loop: initial condition does not satisfy test.
                                 // Loop body not executed.
    for (; _Off < 0; ++_Off) {
        --_UWhere;
    }

    if (_Need_rewrap) {
        _Seek_wrapped(_Where, _STD move(_UWhere));
    }
}

template <class _RanIt, class _Diff>
_CONSTEXPR17 void _Advance1(_RanIt& _Where, _Diff _Off, random_access_iterator_tag) {
    // increment iterator by offset, random-access iterators
    _Where += _Off;
}

template <class _InIt, class _Diff>
_CONSTEXPR17 void advance(_InIt& _Where, _Diff _Off) {
    // increment iterator by offset, arbitrary iterators
    // we remove_const_t before _Iter_cat_t for better diagnostics if the user passes an iterator that is const
    _Advance1(_Where, _Off, _Iter_cat_t<remove_const_t<_InIt>>());
}
#endif // _HAS_IF_CONSTEXPR

// FUNCTION TEMPLATE distance
#if _HAS_IF_CONSTEXPR
template <class _InIt>
_NODISCARD _CONSTEXPR17 _Iter_diff_t<_InIt> distance(_InIt _First, _InIt _Last) {
    if constexpr (_Is_random_iter_v<_InIt>) {
        return _Last - _First; // assume the iterator will do debug checking
    } else {
        _Adl_verify_range(_First, _Last);
        auto _UFirst             = _Get_unwrapped(_First);
        const auto _ULast        = _Get_unwrapped(_Last);
        _Iter_diff_t<_InIt> _Off = 0;
        for (; _UFirst != _ULast; ++_UFirst) {
            ++_Off;
        }

        return _Off;
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt>
_CONSTEXPR17 _Iter_diff_t<_InIt> _Distance1(_InIt _First, _InIt _Last, input_iterator_tag) {
    // return distance between iterators; input
    _Adl_verify_range(_First, _Last);
    auto _UFirst             = _Get_unwrapped(_First);
    const auto _ULast        = _Get_unwrapped(_Last);
    _Iter_diff_t<_InIt> _Off = 0;
    for (; _UFirst != _ULast; ++_UFirst) {
        ++_Off;
    }

    return _Off;
}

template <class _RanIt>
_CONSTEXPR17 _Iter_diff_t<_RanIt> _Distance1(_RanIt _First, _RanIt _Last, random_access_iterator_tag) {
    // return distance between iterators; random-access
    return _Last - _First;
}

template <class _InIt>
_NODISCARD _CONSTEXPR17 _Iter_diff_t<_InIt> distance(_InIt _First, _InIt _Last) {
    return _Distance1(_First, _Last, _Iter_cat_t<_InIt>());
}
#endif // _HAS_IF_CONSTEXPR

// FUNCTION TEMPLATE _Next_iter
template <class _InIt>
constexpr _InIt _Next_iter(_InIt _First) { // increment iterator
    return ++_First;
}

// FUNCTION TEMPLATE next
template <class _InIt>
_NODISCARD _CONSTEXPR17 _InIt next(_InIt _First, _Iter_diff_t<_InIt> _Off = 1) { // increment iterator
    static_assert(_Is_input_iter_v<_InIt>, "next requires input iterator");

    _STD advance(_First, _Off);
    return _First;
}

// FUNCTION TEMPLATE _Prev_iter
template <class _BidIt>
constexpr _BidIt _Prev_iter(_BidIt _First) { // decrement iterator
    return --_First;
}

// FUNCTION TEMPLATE prev
template <class _BidIt>
_NODISCARD _CONSTEXPR17 _BidIt prev(_BidIt _First, _Iter_diff_t<_BidIt> _Off = 1) { // decrement iterator
    static_assert(_Is_bidi_iter_v<_BidIt>, "prev requires bidirectional iterator");

    _STD advance(_First, -_Off);
    return _First;
}

// CLASS TEMPLATE reverse_iterator
template <class _Iterator>
constexpr _Iterator _Operator_arrow(_Iterator _Target, true_type) { // return operator-> where _Iterator is a pointer
    return _Target;
}

template <class _Iterator>
constexpr decltype(auto) _Operator_arrow(_Iterator&& _Target, false_type) {
    // return operator-> where _Iterator is a class type
    return _STD forward<_Iterator>(_Target).operator->();
}

template <class _BidIt>
class reverse_iterator { // wrap iterator to run it backwards
public:
    using iterator_type     = _BidIt;
    using iterator_category = typename iterator_traits<_BidIt>::iterator_category;
    using value_type        = typename iterator_traits<_BidIt>::value_type;
    using difference_type   = typename iterator_traits<_BidIt>::difference_type;
    using pointer           = typename iterator_traits<_BidIt>::pointer;
    using reference         = typename iterator_traits<_BidIt>::reference;

    _CONSTEXPR17 reverse_iterator() = default;

    _CONSTEXPR17 explicit reverse_iterator(_BidIt _Right) noexcept(
        is_nothrow_move_constructible_v<_BidIt>) // strengthened
        : current(_STD move(_Right)) {}

    template <class _Other>
    _CONSTEXPR17 reverse_iterator(const reverse_iterator<_Other>& _Right) : current(_Right.base()) {}

    template <class _Other>
    _CONSTEXPR17 reverse_iterator& operator=(const reverse_iterator<_Other>& _Right) {

        current = _Right.base();
        return *this;
    }

    _NODISCARD _CONSTEXPR17 _BidIt base() const {
        return current;
    }

    _NODISCARD _CONSTEXPR17 reference operator*() const {
        _BidIt _Tmp = current;
        return *--_Tmp;
    }

    _NODISCARD _CONSTEXPR17 pointer operator->() const {
        _BidIt _Tmp = current;
        --_Tmp;
        return _Operator_arrow(_Tmp, is_pointer<_BidIt>());
    }

    _CONSTEXPR17 reverse_iterator& operator++() {
        --current;
        return *this;
    }

    _CONSTEXPR17 reverse_iterator operator++(int) {
        reverse_iterator _Tmp = *this;
        --current;
        return _Tmp;
    }

    _CONSTEXPR17 reverse_iterator& operator--() {
        ++current;
        return *this;
    }

    _CONSTEXPR17 reverse_iterator operator--(int) {
        reverse_iterator _Tmp = *this;
        ++current;
        return _Tmp;
    }

    // N.B. functions valid for random-access iterators only beyond this point

    _CONSTEXPR17 reverse_iterator& operator+=(const difference_type _Off) {
        current -= _Off;
        return *this;
    }

    _NODISCARD _CONSTEXPR17 reverse_iterator operator+(const difference_type _Off) const {
        return reverse_iterator(current - _Off);
    }

    _CONSTEXPR17 reverse_iterator& operator-=(const difference_type _Off) {
        current += _Off;
        return *this;
    }

    _NODISCARD _CONSTEXPR17 reverse_iterator operator-(const difference_type _Off) const {
        return reverse_iterator(current + _Off);
    }

    _NODISCARD _CONSTEXPR17 reference operator[](const difference_type _Off) const {
        return current[static_cast<difference_type>(-_Off - 1)];
    }

    using _Prevent_inheriting_unwrap = reverse_iterator;

    template <class _BidIt2 = _BidIt, enable_if_t<_Offset_verifiable_v<_BidIt2>, int> = 0>
    constexpr void _Verify_offset(const difference_type _Off) const {
        _STL_VERIFY(_Off != _Min_possible_v<difference_type>, "integer overflow");
        current._Verify_offset(-_Off);
    }

    template <class _BidIt2 = _BidIt, enable_if_t<_Unwrappable_v<const _BidIt2&>, int> = 0>
    _NODISCARD constexpr reverse_iterator<_Unwrapped_t<const _BidIt2&>> _Unwrapped() const {
        return static_cast<reverse_iterator<_Unwrapped_t<const _BidIt2&>>>(current._Unwrapped());
    }

    static constexpr bool _Unwrap_when_unverified = _Do_unwrap_when_unverified_v<_BidIt>;

    template <class _Src, enable_if_t<_Wrapped_seekable_v<_BidIt, _Src>, int> = 0>
    constexpr void _Seek_to(const reverse_iterator<_Src>& _It) {
        current._Seek_to(_It.base());
    }

protected:
    _BidIt current{}; // the wrapped iterator
};

template <class _BidIt, class _BidIt2, enable_if_t<_Range_verifiable_v<_BidIt, _BidIt2>, int> = 0>
constexpr void _Verify_range(const reverse_iterator<_BidIt>& _First, const reverse_iterator<_BidIt2>& _Last) {
    // TRANSITION, VSO-612785
    _Verify_range(_Last.base(), _First.base()); // note reversed parameters
}

template <class _BidIt>
_NODISCARD _CONSTEXPR17 reverse_iterator<_BidIt> operator+(
    typename reverse_iterator<_BidIt>::difference_type _Off, const reverse_iterator<_BidIt>& _Right) {
    return _Right + _Off;
}

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 auto operator-(const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right)
    -> decltype(_Right.base() - _Left.base()) {
    return _Right.base() - _Left.base();
}

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator==(
    const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right) {
    return _Left.base() == _Right.base();
}

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator!=(
    const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right) {
    return !(_Left == _Right);
}

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator<(
    const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right) {
    return _Right.base() < _Left.base();
}

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator>(
    const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right) {
    return _Right < _Left;
}

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator<=(
    const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right) {
    return !(_Right < _Left);
}

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator>=(
    const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right) {
    return !(_Left < _Right);
}

// FUNCTION TEMPLATE make_reverse_iterator
template <class _BidIt>
_NODISCARD _CONSTEXPR17 reverse_iterator<_BidIt> make_reverse_iterator(_BidIt _Iter) noexcept(
    is_nothrow_move_constructible_v<_BidIt>) /* strengthened */ {
    return reverse_iterator<_BidIt>(_STD move(_Iter));
}

// FUNCTION TEMPLATES begin AND end
template <class _Container>
_NODISCARD _CONSTEXPR17 auto begin(_Container& _Cont) -> decltype(_Cont.begin()) {
    return _Cont.begin();
}

template <class _Container>
_NODISCARD _CONSTEXPR17 auto begin(const _Container& _Cont) -> decltype(_Cont.begin()) {
    return _Cont.begin();
}

template <class _Container>
_NODISCARD _CONSTEXPR17 auto end(_Container& _Cont) -> decltype(_Cont.end()) {
    return _Cont.end();
}

template <class _Container>
_NODISCARD _CONSTEXPR17 auto end(const _Container& _Cont) -> decltype(_Cont.end()) {
    return _Cont.end();
}

template <class _Ty, size_t _Size>
_NODISCARD constexpr _Ty* begin(_Ty (&_Array)[_Size]) noexcept {
    return _Array;
}

template <class _Ty, size_t _Size>
_NODISCARD constexpr _Ty* end(_Ty (&_Array)[_Size]) noexcept {
    return _Array + _Size;
}

// FUNCTION TEMPLATES cbegin AND cend
template <class _Container>
_NODISCARD constexpr auto cbegin(const _Container& _Cont) noexcept(noexcept(_STD begin(_Cont)))
    -> decltype(_STD begin(_Cont)) {
    return _STD begin(_Cont);
}

template <class _Container>
_NODISCARD constexpr auto cend(const _Container& _Cont) noexcept(noexcept(_STD end(_Cont)))
    -> decltype(_STD end(_Cont)) {
    return _STD end(_Cont);
}

// FUNCTION TEMPLATES rbegin AND rend
template <class _Container>
_NODISCARD _CONSTEXPR17 auto rbegin(_Container& _Cont) -> decltype(_Cont.rbegin()) {
    return _Cont.rbegin();
}

template <class _Container>
_NODISCARD _CONSTEXPR17 auto rbegin(const _Container& _Cont) -> decltype(_Cont.rbegin()) {
    return _Cont.rbegin();
}

template <class _Container>
_NODISCARD _CONSTEXPR17 auto rend(_Container& _Cont) -> decltype(_Cont.rend()) {
    return _Cont.rend();
}

template <class _Container>
_NODISCARD _CONSTEXPR17 auto rend(const _Container& _Cont) -> decltype(_Cont.rend()) {
    return _Cont.rend();
}

template <class _Ty, size_t _Size>
_NODISCARD _CONSTEXPR17 reverse_iterator<_Ty*> rbegin(_Ty (&_Array)[_Size]) {
    return reverse_iterator<_Ty*>(_Array + _Size);
}

template <class _Ty, size_t _Size>
_NODISCARD _CONSTEXPR17 reverse_iterator<_Ty*> rend(_Ty (&_Array)[_Size]) {
    return reverse_iterator<_Ty*>(_Array);
}

template <class _Elem>
_NODISCARD _CONSTEXPR17 reverse_iterator<const _Elem*> rbegin(initializer_list<_Elem> _Ilist) {
    return reverse_iterator<const _Elem*>(_Ilist.end());
}

template <class _Elem>
_NODISCARD _CONSTEXPR17 reverse_iterator<const _Elem*> rend(initializer_list<_Elem> _Ilist) {
    return reverse_iterator<const _Elem*>(_Ilist.begin());
}

// FUNCTION TEMPLATES crbegin AND crend
template <class _Container>
_NODISCARD _CONSTEXPR17 auto crbegin(const _Container& _Cont) -> decltype(_STD rbegin(_Cont)) {
    return _STD rbegin(_Cont);
}

template <class _Container>
_NODISCARD _CONSTEXPR17 auto crend(const _Container& _Cont) -> decltype(_STD rend(_Cont)) {
    return _STD rend(_Cont);
}

template <class _Container>
_NODISCARD constexpr auto size(const _Container& _Cont) -> decltype(_Cont.size()) {
    return _Cont.size();
}

template <class _Ty, size_t _Size>
_NODISCARD constexpr size_t size(const _Ty (&)[_Size]) noexcept {
    return _Size;
}

#if _HAS_CXX20
// FUNCTION TEMPLATE ssize
template <class _Container>
_NODISCARD constexpr auto ssize(const _Container& _Cont)
    -> common_type_t<ptrdiff_t, make_signed_t<decltype(_Cont.size())>> {
    using _Common = common_type_t<ptrdiff_t, make_signed_t<decltype(_Cont.size())>>;
    return static_cast<_Common>(_Cont.size());
}

template <class _Ty, ptrdiff_t _Size>
_NODISCARD constexpr ptrdiff_t ssize(const _Ty (&)[_Size]) noexcept {
    return _Size;
}
#endif // _HAS_CXX20

template <class _Container>
_NODISCARD constexpr auto empty(const _Container& _Cont) -> decltype(_Cont.empty()) {
    return _Cont.empty();
}

template <class _Ty, size_t _Size>
_NODISCARD constexpr bool empty(const _Ty (&)[_Size]) noexcept {
    return false;
}

template <class _Elem>
_NODISCARD constexpr bool empty(initializer_list<_Elem> _Ilist) noexcept {
    return _Ilist.size() == 0;
}

template <class _Container>
_NODISCARD constexpr auto data(_Container& _Cont) -> decltype(_Cont.data()) {
    return _Cont.data();
}

template <class _Container>
_NODISCARD constexpr auto data(const _Container& _Cont) -> decltype(_Cont.data()) {
    return _Cont.data();
}

template <class _Ty, size_t _Size>
_NODISCARD constexpr _Ty* data(_Ty (&_Array)[_Size]) noexcept {
    return _Array;
}

template <class _Elem>
_NODISCARD constexpr const _Elem* data(initializer_list<_Elem> _Ilist) noexcept {
    return _Ilist.begin();
}

#ifdef __cpp_lib_concepts
// FUNCTION TEMPLATE _Fake_decay_copy
template <class _Ty>
_NODISCARD _Ty _Fake_decay_copy(_Ty) noexcept;
// _Fake_decay_copy<T>(E):
// (1) has type T [decay_t<decltype((E))> if T is deduced],
// (2) is well-formed if and only if E is implicitly convertible to T and T is destructible, and
// (3) is non-throwing if and only if both conversion from decltype((E)) to T and destruction of T are non-throwing.

namespace ranges {
    // VARIABLE TEMPLATE _Has_complete_elements
    template <class>
    inline constexpr bool _Has_complete_elements = false;

    // clang-format off
    template <class _Ty>
        requires requires(_Ty& __t) { sizeof(__t[0]); }
    inline constexpr bool _Has_complete_elements<_Ty> = true;
    // clang-format on

    // VARIABLE TEMPLATE ranges::enable_safe_range
    template <class>
    inline constexpr bool enable_safe_range = false;

    template <class _Rng>
    concept _Should_range_access = is_lvalue_reference_v<_Rng> || enable_safe_range<remove_cvref_t<_Rng>>;

    // CUSTOMIZATION POINT OBJECT ranges::begin (Implements D2091R0)
    namespace _Begin {
        template <class _Ty>
        void begin(_Ty&) = delete;
        template <class _Ty>
        void begin(const _Ty&) = delete;

        // clang-format off
        template <class _Ty>
        concept _Has_member = requires(_Ty __t) {
            { _Fake_decay_copy(__t.begin()) } -> input_or_output_iterator;
        };

        template <class _Ty>
        concept _Has_ADL = _Has_class_or_enum_type<_Ty> && requires(_Ty __t) {
            { _Fake_decay_copy(begin(__t)) } -> input_or_output_iterator;
        };
        // clang-format on

        class _Cpo {
        private:
            enum class _St { _None, _Array, _Member, _Non_member };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                _STL_INTERNAL_STATIC_ASSERT(is_lvalue_reference_v<_Ty>);
                if constexpr (is_array_v<remove_reference_t<_Ty>>) {
                    static_assert(_Has_complete_elements<_Ty>,
                        "The range access customization point objects "
                        "std::ranges::begin, std::ranges::end, std::ranges::rbegin, and std::ranges::rend "
                        "do not accept arrays with incomplete element types.");
                    return {_St::_Array, true};
                } else if constexpr (_Has_member<_Ty>) {
                    return {_St::_Member, noexcept(_Fake_decay_copy(_STD declval<_Ty>().begin()))};
                } else if constexpr (_Has_ADL<_Ty>) {
                    return {_St::_Non_member, noexcept(_Fake_decay_copy(begin(_STD declval<_Ty>())))};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            // clang-format off
            template <_Should_range_access _Ty>
                requires (_Choice<_Ty&>._Strategy != _St::_None)
            _NODISCARD constexpr auto operator()(_Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                if constexpr (_Choice<_Ty&>._Strategy == _St::_Array) {
                    return _Val;
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Member) {
                    return _Val.begin();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Non_member) {
                    return begin(_Val);
                } else {
                    static_assert(_Always_false<_Ty>, "Should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Begin

    inline namespace _Cpos {
        inline constexpr _Begin::_Cpo begin;
    }

    // ALIAS TEMPLATE ranges::iterator_t (Implements D2091R0)
    template <class _Ty>
    using iterator_t = decltype(_RANGES begin(_STD declval<_Ty&>()));

    // CUSTOMIZATION POINT OBJECT ranges::end (Implements D2091R0)
    namespace _End {
        template <class _Ty>
        void end(_Ty&) = delete;
        template <class _Ty>
        void end(const _Ty&) = delete;

        // clang-format off
        template <class _Ty>
        concept _Has_member = requires(_Ty __t) {
            { _Fake_decay_copy(__t.end()) } -> sentinel_for<iterator_t<_Ty>>;
        };

        template <class _Ty>
        concept _Has_ADL = _Has_class_or_enum_type<_Ty> && requires(_Ty __t) {
            { _Fake_decay_copy(end(__t)) } -> sentinel_for<iterator_t<_Ty>>;
        };
        // clang-format on

        class _Cpo {
        private:
            enum class _St { _None, _Array, _Member, _Non_member };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                _STL_INTERNAL_STATIC_ASSERT(is_lvalue_reference_v<_Ty>);
                using _UnRef = remove_reference_t<_Ty>;

                if constexpr (is_array_v<_UnRef>) {
                    static_assert(_Has_complete_elements<_UnRef>,
                        "The range access customization point objects "
                        "std::ranges::begin, std::ranges::end, std::ranges::rbegin, and std::ranges::rend "
                        "do not accept arrays with incomplete element types.");
                    if constexpr (extent_v<_UnRef> != 0) {
                        return {_St::_Array, true};
                    } else {
                        return {_St::_None};
                    }
                } else if constexpr (_Has_member<_Ty>) {
                    return {_St::_Member, noexcept(_Fake_decay_copy(_STD declval<_Ty>().end()))};
                } else if constexpr (_Has_ADL<_Ty>) {
                    return {_St::_Non_member, noexcept(_Fake_decay_copy(end(_STD declval<_Ty>())))};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            // clang-format off
            template <_Should_range_access _Ty>
                requires (_Choice<_Ty&>._Strategy != _St::_None)
            _NODISCARD constexpr auto operator()(_Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                if constexpr (_Choice<_Ty&>._Strategy == _St::_Array) {
                    // extent_v<remove_reference_t<_Ty&>> reuses specializations from _Choose
                    return _Val + extent_v<remove_reference_t<_Ty&>>;
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Member) {
                    return _Val.end();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Non_member) {
                    return end(_Val);
                } else {
                    static_assert(_Always_false<_Ty>, "should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _End

    inline namespace _Cpos {
        inline constexpr _End::_Cpo end;
    }

    // CONCEPT ranges::range
    template <class _Rng>
    concept range = requires(_Rng& __r) {
        _RANGES begin(__r);
        _RANGES end(__r);
    };

    // CONCEPT ranges::safe_range
    // clang-format off
    template <class _Rng>
    concept safe_range = range<_Rng> && _Should_range_access<_Rng>;
    // clang-format on

    // ALIAS TEMPLATE ranges::sentinel_t
    template <range _Rng>
    using sentinel_t = decltype(_RANGES end(_STD declval<_Rng&>()));

    // ALIAS TEMPLATE ranges::range_difference_t
    template <range _Rng>
    using range_difference_t = iter_difference_t<iterator_t<_Rng>>;

    // ALIAS TEMPLATE ranges::range_value_t
    template <range _Rng>
    using range_value_t = iter_value_t<iterator_t<_Rng>>;

    // ALIAS TEMPLATE ranges::range_reference_t
    template <range _Rng>
    using range_reference_t = iter_reference_t<iterator_t<_Rng>>;

    // ALIAS TEMPLATE ranges::range_rvalue_reference_t
    template <range _Rng>
    using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<_Rng>>;

    // CUSTOMIZATION POINT OBJECT ranges::cbegin
    struct _Cbegin_fn {
        // clang-format off
        template <class _Ty, class _CTy = _Const_thru_ref<_Ty>>
        _NODISCARD constexpr auto operator()(_Ty&& _Val) const
            noexcept(noexcept(_RANGES begin(static_cast<_CTy&&>(_Val))))
            requires requires { _RANGES begin(static_cast<_CTy&&>(_Val)); } {
            return _RANGES begin(static_cast<_CTy&&>(_Val));
        }
        // clang-format on
    };

    inline namespace _Cpos {
        inline constexpr _Cbegin_fn cbegin;
    }

    // CUSTOMIZATION POINT OBJECT ranges::cend
    struct _Cend_fn {
        // clang-format off
        template <class _Ty, class _CTy = _Const_thru_ref<_Ty>>
        _NODISCARD constexpr auto operator()(_Ty&& _Val) const
            noexcept(noexcept(_RANGES end(static_cast<_CTy&&>(_Val))))
            requires requires { _RANGES end(static_cast<_CTy&&>(_Val)); } {
            return _RANGES end(static_cast<_CTy&&>(_Val));
        }
        // clang-format on
    };

    inline namespace _Cpos {
        inline constexpr _Cend_fn cend;
    }

    // CUSTOMIZATION POINT OBJECT ranges::rbegin (Implements D2091R0)
    namespace _Rbegin {
        template <class _Ty>
        void rbegin(_Ty&) = delete;
        template <class _Ty>
        void rbegin(const _Ty&) = delete;

        // clang-format off
        template <class _Ty>
        concept _Has_member = requires(_Ty __t) {
            { _Fake_decay_copy(__t.rbegin()) } -> input_or_output_iterator;
        };

        template <class _Ty>
        concept _Has_ADL = _Has_class_or_enum_type<_Ty> && requires(_Ty __t) {
            { _Fake_decay_copy(rbegin(__t)) } -> input_or_output_iterator;
        };

        template <class _Ty>
        concept _Can_make_reverse = requires(_Ty __t) {
            { _RANGES begin(__t) } -> bidirectional_iterator;
            { _RANGES end(__t) } -> same_as<decltype(_RANGES begin(__t))>;
        };
        // clang-format on

        class _Cpo {
        private:
            enum class _St { _None, _Member, _Non_member, _Make_reverse };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                _STL_INTERNAL_STATIC_ASSERT(is_lvalue_reference_v<_Ty>);
                if constexpr (_Has_member<_Ty>) {
                    return {_St::_Member, noexcept(_Fake_decay_copy(_STD declval<_Ty>().rbegin()))};
                } else if constexpr (_Has_ADL<_Ty>) {
                    return {_St::_Non_member, noexcept(_Fake_decay_copy(rbegin(_STD declval<_Ty>())))};
                } else if constexpr (_Can_make_reverse<_Ty>) {
                    return {_St::_Make_reverse, noexcept(_STD make_reverse_iterator(_RANGES end(_STD declval<_Ty>())))};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            // clang-format off
            template <_Should_range_access _Ty>
                requires (_Choice<_Ty&>._Strategy != _St::_None)
            _NODISCARD constexpr auto operator()(_Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                if constexpr (_Choice<_Ty&>._Strategy == _St::_Member) {
                    return _Val.rbegin();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Non_member) {
                    return rbegin(_Val);
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Make_reverse) {
                    return _STD make_reverse_iterator(_RANGES end(_Val));
                } else {
                    static_assert(_Always_false<_Ty>, "should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Rbegin

    inline namespace _Cpos {
        inline constexpr _Rbegin::_Cpo rbegin;
    }

    // CUSTOMIZATION POINT OBJECT ranges::rend (Implements D2091R0)
    namespace _Rend {
        template <class _Ty>
        void rend(_Ty&) = delete;
        template <class _Ty>
        void rend(const _Ty&) = delete;

        // clang-format off
        template <class _Ty>
        concept _Has_member = requires(_Ty __t) {
            { _Fake_decay_copy(__t.rend()) } -> sentinel_for<decltype(_RANGES rbegin(__t))>;
        };

        template <class _Ty>
        concept _Has_ADL = _Has_class_or_enum_type<_Ty> && requires(_Ty __t) {
            { _Fake_decay_copy(rend(__t)) } -> sentinel_for<decltype(_RANGES rbegin(__t))>;
        };

        template <class _Ty>
        concept _Can_make_reverse = requires(_Ty __t) {
            { _RANGES begin(__t) } -> bidirectional_iterator;
            { _RANGES end(__t) } -> same_as<decltype(_RANGES begin(__t))>;
        };
        // clang-format on

        class _Cpo {
        private:
            enum class _St { _None, _Member, _Non_member, _Make_reverse };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                _STL_INTERNAL_STATIC_ASSERT(is_lvalue_reference_v<_Ty>);
                if constexpr (_Has_member<_Ty>) {
                    return {_St::_Member, noexcept(_Fake_decay_copy(_STD declval<_Ty>().rend()))};
                } else if constexpr (_Has_ADL<_Ty>) {
                    return {_St::_Non_member, noexcept(_Fake_decay_copy(rend(_STD declval<_Ty>())))};
                } else if constexpr (_Can_make_reverse<_Ty>) {
                    return {
                        _St::_Make_reverse, noexcept(_STD make_reverse_iterator(_RANGES begin(_STD declval<_Ty>())))};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            // clang-format off
            template <_Should_range_access _Ty>
                requires (_Choice<_Ty&>._Strategy != _St::_None)
            _NODISCARD constexpr auto operator()(_Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                if constexpr (_Choice<_Ty&>._Strategy == _St::_Member) {
                    return _Val.rend();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Non_member) {
                    return rend(_Val);
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Make_reverse) {
                    return _STD make_reverse_iterator(_RANGES begin(_Val));
                } else {
                    static_assert(_Always_false<_Ty>, "should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Rend

    inline namespace _Cpos {
        inline constexpr _Rend::_Cpo rend;
    }

    // CUSTOMIZATION POINT OBJECT ranges::crbegin
    struct _Crbegin_fn {
        // clang-format off
        template <class _Ty, class _CTy = _Const_thru_ref<_Ty>>
        _NODISCARD constexpr auto operator()(_Ty&& _Val) const
            noexcept(noexcept(_RANGES rbegin(static_cast<_CTy&&>(_Val))))
            requires requires { _RANGES rbegin(static_cast<_CTy&&>(_Val)); } {
            return _RANGES rbegin(static_cast<_CTy&&>(_Val));
        }
        // clang-format on
    };

    inline namespace _Cpos {
        inline constexpr _Crbegin_fn crbegin;
    }

    // CUSTOMIZATION POINT OBJECT ranges::crend
    struct _Crend_fn {
        // clang-format off
        template <class _Ty, class _CTy = _Const_thru_ref<_Ty>>
        _NODISCARD constexpr auto operator()(_Ty&& _Val) const
            noexcept(noexcept(_RANGES rend(static_cast<_CTy&&>(_Val))))
            requires requires { _RANGES rend(static_cast<_CTy&&>(_Val)); } {
            return _RANGES rend(static_cast<_CTy&&>(_Val));
        }
        // clang-format on
    };

    inline namespace _Cpos {
        inline constexpr _Crend_fn crend;
    }

    // VARIABLE TEMPLATE ranges::disable_sized_range
    template <class>
    inline constexpr bool disable_sized_range = false;

    // CUSTOMIZATION POINT OBJECT ranges::size (Implements D2091R0)
    namespace _Size {
        template <class _Ty>
        void size(_Ty&) = delete;
        template <class _Ty>
        void size(const _Ty&) = delete;

        // clang-format off
        template <class _Ty, class _UnCV>
        concept _Has_member = !disable_sized_range<_UnCV> && requires(_Ty __t) {
            { _Fake_decay_copy(__t.size()) } -> integral;
        };

        template <class _Ty, class _UnCV>
        concept _Has_ADL = _Has_class_or_enum_type<_Ty> && !disable_sized_range<_UnCV> && requires(_Ty __t) {
            { _Fake_decay_copy(size(__t)) } -> integral;
        };

        template <class _Ty>
        concept _Can_difference = requires(_Ty __t) {
            { _RANGES begin(__t) } -> forward_iterator;
            { _RANGES end(__t) } -> sized_sentinel_for<decltype(_RANGES begin(__t))>;
        };
        // clang-format on

        class _Cpo {
        private:
            enum class _St { _None, _Array, _Member, _Non_member, _Subtract };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                _STL_INTERNAL_STATIC_ASSERT(is_lvalue_reference_v<_Ty>);
                using _UnCV = remove_cvref_t<_Ty>;

                if constexpr (is_array_v<_UnCV>) {
                    if constexpr (extent_v<_UnCV> != 0) {
                        return {_St::_Array, true};
                    } else {
                        return {_St::_None};
                    }
                } else if constexpr (_Has_member<_Ty, _UnCV>) {
                    return {_St::_Member, noexcept(_Fake_decay_copy(_STD declval<_Ty>().size()))};
                } else if constexpr (_Has_ADL<_Ty, _UnCV>) {
                    return {_St::_Non_member, noexcept(_Fake_decay_copy(size(_STD declval<_Ty>())))};
                } else if constexpr (_Can_difference<_Ty>) {
                    return {_St::_Subtract,
                        noexcept(_RANGES end(_STD declval<_Ty>()) - _RANGES begin(_STD declval<_Ty>()))};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            // clang-format off
            template <class _Ty>
                requires (_Choice<_Ty&>._Strategy != _St::_None)
            _NODISCARD constexpr auto operator()([[maybe_unused]] _Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                if constexpr (_Choice<_Ty&>._Strategy == _St::_Array) {
                    // extent_v<remove_cvref_t<_Ty&>> reuses specializations from _Choose
                    return extent_v<remove_cvref_t<_Ty&>>;
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Member) {
                    return _Val.size();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Non_member) {
                    return size(_Val);
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Subtract) {
                    const auto _Delta = _RANGES end(_Val) - _RANGES begin(_Val);
                    return static_cast<make_unsigned_t<remove_cv_t<decltype(_Delta)>>>(_Delta);
                } else {
                    static_assert(_Always_false<_Ty>, "should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Size

    inline namespace _Cpos {
        inline constexpr _Size::_Cpo size;
    }

    // CUSTOMIZATION POINT OBJECT ranges::empty (Implements D2091R0)
    namespace _Empty {
        // clang-format off
        template <class _Ty>
        concept _Has_member = requires(_Ty __t) {
            static_cast<bool>(__t.empty());
        };

        template <class _Ty>
        concept _Has_size = requires(_Ty __t) {
            _RANGES size(__t);
        };

        template <class _Ty>
        concept _Can_begin_end = requires(_Ty __t) {
            { _RANGES begin(__t) } -> forward_iterator;
            _RANGES end(__t);
        };
        // clang-format on

        class _Cpo {
        private:
            enum class _St { _None, _Array, _Member, _Size, _Compare };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                _STL_INTERNAL_STATIC_ASSERT(is_lvalue_reference_v<_Ty>);
                if constexpr (is_array_v<remove_reference_t<_Ty>>) {
                    return {_St::_Array, true};
                } else if constexpr (_Has_member<_Ty>) {
                    return {_St::_Member, noexcept(static_cast<bool>(_STD declval<_Ty>().empty()))};
                } else if constexpr (_Has_size<_Ty>) {
                    return {_St::_Size, noexcept(_RANGES size(_STD declval<_Ty>()))};
                } else if constexpr (_Can_begin_end<_Ty>) {
                    constexpr auto _Nothrow = noexcept(
                        static_cast<bool>(_RANGES begin(_STD declval<_Ty>()) == _RANGES end(_STD declval<_Ty>())));
                    return {_St::_Compare, _Nothrow};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            // clang-format off
            template <class _Ty>
                requires (_Choice<_Ty&>._Strategy != _St::_None)
            _NODISCARD constexpr bool operator()([[maybe_unused]] _Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                if constexpr (_Choice<_Ty&>._Strategy == _St::_Array) {
                    return false;
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Member) {
                    return static_cast<bool>(_Val.empty());
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Size) {
                    return _RANGES size(_Val) == 0;
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Compare) {
                    return static_cast<bool>(_RANGES begin(_Val) == _RANGES end(_Val));
                } else {
                    static_assert(_Always_false<_Ty>, "should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Empty

    inline namespace _Cpos {
        inline constexpr _Empty::_Cpo empty;
    }

    // CUSTOMIZATION POINT OBJECT ranges::data (Implements D2091R0)
    namespace _Data {
        // clang-format off
        template <class _Ty>
        concept _Points_to_object = is_pointer_v<_Ty> && is_object_v<remove_pointer_t<_Ty>>;

        template <class _Ty>
        concept _Has_member = requires(_Ty __t) {
            { _Fake_decay_copy(__t.data()) } -> _Points_to_object;
        };

        template <class _Ty>
        concept _Has_contiguous_iterator = requires(_Ty __t) {
            { _RANGES begin(__t) } -> contiguous_iterator;
        };
        // clang-format on

        class _Cpo {
        private:
            enum class _St { _None, _Member, _Address };

            template <class _Ty>
            _NODISCARD static _CONSTEVAL _Choice_t<_St> _Choose() noexcept {
                _STL_INTERNAL_STATIC_ASSERT(is_lvalue_reference_v<_Ty>);
                if constexpr (_Has_member<_Ty>) {
                    return {_St::_Member, noexcept(_STD declval<_Ty>().data())};
                } else if constexpr (_Has_contiguous_iterator<_Ty>) {
                    return {_St::_Address, noexcept(_STD to_address(_RANGES begin(_STD declval<_Ty>())))};
                } else {
                    return {_St::_None};
                }
            }

            template <class _Ty>
            static constexpr _Choice_t<_St> _Choice = _Choose<_Ty>();

        public:
            // clang-format off
            template <_Should_range_access _Ty>
                requires (_Choice<_Ty&>._Strategy != _St::_None)
            _NODISCARD constexpr auto operator()(_Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                if constexpr (_Choice<_Ty&>._Strategy == _St::_Member) {
                    return _Val.data();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Address) {
                    return _STD to_address(_RANGES begin(_Val));
                } else {
                    static_assert(_Always_false<_Ty>, "should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Data

    inline namespace _Cpos {
        inline constexpr _Data::_Cpo data;
    }

    // CUSTOMIZATION POINT OBJECT ranges::cdata
    struct _Cdata_fn {
        // clang-format off
        template <class _Ty, class _CTy = _Const_thru_ref<_Ty>>
        _NODISCARD constexpr auto operator()(_Ty&& _Val) const
            noexcept(noexcept(_RANGES data(static_cast<_CTy&&>(_Val))))
            requires requires { _RANGES data(static_cast<_CTy&&>(_Val)); } {
            return _RANGES data(static_cast<_CTy&&>(_Val));
        }
        // clang-format on
    };

    inline namespace _Cpos {
        inline constexpr _Cdata_fn cdata;
    }

    // clang-format off
    // CONCEPT ranges::sized_range
    template <class _Rng>
    concept sized_range = range<_Rng>
#if 0 // TRANSITION, LWG-3264
        && !disable_sized_range<remove_cvref_t<_Rng>>
#endif // TRANSITION, LWG-3264
        && requires(_Rng& __r) { _RANGES size(__r); };
    // clang-format on

    // STRUCT ranges::view_base
    struct view_base {};

    // VARIABLE TEMPLATE ranges::enable_view
    // clang-format off
    template <class _Ty>
    concept _Enable_view_impl = derived_from<_Ty, view_base> || !range<_Ty> || !range<const _Ty>
        || same_as<range_reference_t<_Ty>, range_reference_t<const _Ty>>;
    // clang-format on

    template <class _Ty>
    inline constexpr bool enable_view = _Enable_view_impl<_Ty>;

    template <class _Ty>
    inline constexpr bool enable_view<initializer_list<_Ty>> = false;

    // enable_view specializations for (unordered_)?(multi)?set and match_results are in the corresponding headers.

    // clang-format off
    // CONCEPT ranges::view
    template <class _Ty>
    concept view = range<_Ty> && movable<_Ty> && default_initializable<_Ty> && enable_view<_Ty>;

    // CONCEPT ranges::output_range
    template <class _Rng, class _Ty>
    concept output_range = range<_Rng> && output_iterator<iterator_t<_Rng>, _Ty>;

    // CONCEPT ranges::input_range
    template <class _Rng>
    concept input_range = range<_Rng> && input_iterator<iterator_t<_Rng>>;

    // CONCEPT ranges::forward_range
    template <class _Rng>
    concept forward_range = range<_Rng> && forward_iterator<iterator_t<_Rng>>;

    // CONCEPT ranges::bidirectional_range
    template <class _Rng>
    concept bidirectional_range = range<_Rng> && bidirectional_iterator<iterator_t<_Rng>>;

    // CONCEPT ranges::random_access_range
    template <class _Rng>
    concept random_access_range = range<_Rng> && random_access_iterator<iterator_t<_Rng>>;

    // CONCEPT ranges::contiguous_range
    template <class _Rng>
    concept contiguous_range = range<_Rng> && contiguous_iterator<iterator_t<_Rng>> && requires(_Rng& __r) {
        { _RANGES data(__r) } -> same_as<add_pointer_t<range_reference_t<_Rng>>>;
    };
    // clang-format on

    // CLASS ranges::_Not_quite_object
    class _Not_quite_object {
    public:
        // Some overload sets in the library have the property that their constituent function templates are not visible
        // to argument-dependent name lookup (ADL) and that they inhibit ADL when found via unqualified name lookup.
        // This property allows these overload sets to be implemented as function objects. We derive such function
        // objects from this type to remove some typical object-ish behaviors which helps users avoid depending on their
        // non-specified object-ness.

        struct _Construct_tag {};

        _Not_quite_object() = delete;

        constexpr explicit _Not_quite_object(_Construct_tag) noexcept {}

        _Not_quite_object(const _Not_quite_object&) = delete;
        _Not_quite_object& operator=(const _Not_quite_object&) = delete;

        void operator&() const = delete;

    protected:
        ~_Not_quite_object() = default;
    };

    // CLASS ranges::_Advance_fn
    class _Advance_fn : private _Not_quite_object {
    public:
        using _Not_quite_object::_Not_quite_object;

        template <input_or_output_iterator _It>
        constexpr void operator()(_It& _Where, iter_difference_t<_It> _Off) const {
            if constexpr (random_access_iterator<_It>) {
                _Where += _Off;
            } else {
                if constexpr (!bidirectional_iterator<_It>) {
                    _STL_ASSERT(_Off >= 0, "negative advance of non-bidirectional iterator");
                }

                auto&& _UWhere              = _Get_unwrapped_n(_STD move(_Where), _Off);
                constexpr bool _Need_rewrap = !is_reference_v<decltype(_Get_unwrapped_n(_STD move(_Where), _Off))>;

                if constexpr (bidirectional_iterator<_It>) {
                    for (; _Off < 0; ++_Off) {
                        --_UWhere;
                    }
                }

                for (; _Off > 0; --_Off) {
                    ++_UWhere;
                }

                if constexpr (_Need_rewrap) {
                    _Seek_wrapped(_Where, _STD move(_UWhere));
                }
            }
        }

        template <input_or_output_iterator _It, sentinel_for<_It> _Se>
        constexpr void operator()(_It& _Where, _Se _Last) const {
            if constexpr (assignable_from<_It&, _Se>) {
                _Where = static_cast<_Se&&>(_Last);
            } else if constexpr (sized_sentinel_for<_Se, _It>) {
                (*this)(_Where, _Last - _Where);
            } else {
                _Adl_verify_range(_Where, _Last);

                auto&& _UWhere              = _Get_unwrapped(static_cast<_It&&>(_Where));
                constexpr bool _Need_rewrap = !is_reference_v<decltype(_Get_unwrapped(static_cast<_It&&>(_Where)))>;
                auto&& _ULast               = _Get_unwrapped(static_cast<_Se&&>(_Last));

                while (_UWhere != _ULast) {
                    ++_UWhere;
                }

                if constexpr (_Need_rewrap) {
                    _Seek_wrapped(_Where, static_cast<_It&&>(_UWhere));
                }
            }
        }

        template <input_or_output_iterator _It, sentinel_for<_It> _Se>
        constexpr iter_difference_t<_It> operator()(_It& _Where, iter_difference_t<_It> _Off, _Se _Last) const {
            if constexpr (sized_sentinel_for<_Se, _It>) {
                const iter_difference_t<_It> _Delta = _Last - _Where;
                if ((_Off < 0 && _Off <= _Delta) || (_Off > 0 && _Off >= _Delta)) {
                    if constexpr (assignable_from<_It&, _Se>) {
                        _Where = static_cast<_Se&&>(_Last);
                    } else {
                        (*this)(_Where, _Delta);
                    }
                    return _Off - _Delta;
                }

                (*this)(_Where, _Off);
                return 0;
            } else {
                // performance note: develop unwrapping technology for (i, n, s)?
                if constexpr (bidirectional_iterator<_It>) {
                    for (; _Off < 0 && _Where != _Last; ++_Off) {
                        --_Where;
                    }
                } else {
                    _STL_ASSERT(_Off >= 0, "negative advance of non-bidirectional iterator");
                }

                for (; _Off > 0 && _Where != _Last; --_Off) {
                    ++_Where;
                }

                return _Off;
            }
        }
    };

    // VARIABLE ranges::advance
    inline constexpr _Advance_fn advance{_Not_quite_object::_Construct_tag{}};

    // CLASS ranges::_Distance_fn
    class _Distance_fn : private _Not_quite_object {
    public:
        using _Not_quite_object::_Not_quite_object;

        template <input_or_output_iterator _It, sentinel_for<_It> _Se>
        _NODISCARD constexpr iter_difference_t<_It> operator()(_It _First, _Se _Last) const {
            if constexpr (sized_sentinel_for<_Se, _It>) {
                return _Last - _First;
            } else {
                _Adl_verify_range(_First, _Last);
                auto _UFirst                  = _Get_unwrapped(static_cast<_It&&>(_First));
                const auto _ULast             = _Get_unwrapped(static_cast<_Se&&>(_Last));
                iter_difference_t<_It> _Count = 0;
                for (; _UFirst != _ULast; ++_UFirst) {
                    ++_Count;
                }

                return _Count;
            }
        }

        template <range _Rng>
        _NODISCARD constexpr range_difference_t<_Rng> operator()(_Rng&& _Val) const {
            if constexpr (sized_range<_Rng>) {
                return static_cast<range_difference_t<_Rng>>(_RANGES size(_Val));
            } else {
                return (*this)(_RANGES begin(_Val), _RANGES end(_Val));
            }
        }
    };

    // VARIABLE ranges::distance
    inline constexpr _Distance_fn distance{_Not_quite_object::_Construct_tag{}};

    // CLASS ranges::_Next_fn
    class _Next_fn : private _Not_quite_object {
    public:
        using _Not_quite_object::_Not_quite_object;

        template <input_or_output_iterator _It>
        _NODISCARD constexpr _It operator()(_It _Where) const {
            ++_Where;
            return _Where;
        }

        template <input_or_output_iterator _It>
        _NODISCARD constexpr _It operator()(_It _Where, const iter_difference_t<_It> _Off) const {
            _RANGES advance(_Where, _Off);
            return _Where;
        }

        template <input_or_output_iterator _It, sentinel_for<_It> _Se>
        _NODISCARD constexpr _It operator()(_It _Where, _Se _Last) const {
            _RANGES advance(_Where, static_cast<_Se&&>(_Last));
            return _Where;
        }

        template <input_or_output_iterator _It, sentinel_for<_It> _Se>
        _NODISCARD constexpr _It operator()(_It _Where, const iter_difference_t<_It> _Off, _Se _Last) const {
            _RANGES advance(_Where, _Off, static_cast<_Se&&>(_Last));
            return _Where;
        }
    };

    // VARIABLE ranges::next
    inline constexpr _Next_fn next{_Not_quite_object::_Construct_tag{}};

    // CLASS ranges::_Prev_fn
    class _Prev_fn : private _Not_quite_object {
    public:
        using _Not_quite_object::_Not_quite_object;

        template <bidirectional_iterator _It>
        _NODISCARD constexpr _It operator()(_It _Where) const {
            --_Where;
            return _Where;
        }

        template <bidirectional_iterator _It>
        _NODISCARD constexpr _It operator()(_It _Where, const iter_difference_t<_It> _Off) const {
            _STL_ASSERT(_Off != _Min_possible_v<iter_difference_t<_It>>, "integer overflow");
            _RANGES advance(_Where, -_Off);
            return _Where;
        }

        template <bidirectional_iterator _It>
        _NODISCARD constexpr _It operator()(_It _Where, const iter_difference_t<_It> _Off, _It _Last) const {
            _STL_ASSERT(_Off != _Min_possible_v<iter_difference_t<_It>>, "integer overflow");
            _RANGES advance(_Where, -_Off, static_cast<_It&&>(_Last));
            return _Where;
        }
    };

    // VARIABLE ranges::prev
    inline constexpr _Prev_fn prev{_Not_quite_object::_Construct_tag{}};

    // STRUCT ranges::equal_to
    struct equal_to {
        // clang-format off
        template <class _Ty1, class _Ty2>
            requires equality_comparable_with<_Ty1, _Ty2> // TRANSITION, GH-489
        _NODISCARD constexpr bool operator()(_Ty1&& _Left, _Ty2&& _Right) const noexcept(noexcept(
            static_cast<bool>(static_cast<_Ty1&&>(_Left) == static_cast<_Ty2&&>(_Right)))) /* strengthened */ {
            return static_cast<bool>(static_cast<_Ty1&&>(_Left) == static_cast<_Ty2&&>(_Right));
        }
        // clang-format on

        using is_transparent = int;
    };

    // STRUCT ranges::less
    struct less {
        // clang-format off
        template <class _Ty1, class _Ty2>
            requires totally_ordered_with<_Ty1, _Ty2> // TRANSITION, GH-489
        _NODISCARD constexpr bool operator()(_Ty1&& _Left, _Ty2&& _Right) const noexcept(noexcept(
            static_cast<bool>(static_cast<_Ty1&&>(_Left) < static_cast<_Ty2&&>(_Right)))) /* strengthened */ {
            return static_cast<bool>(static_cast<_Ty1&&>(_Left) < static_cast<_Ty2&&>(_Right));
        }
        // clang-format on

        using is_transparent = int;
    };
} // namespace ranges
#endif // __cpp_lib_concepts

// MACRO _STL_VERIFY_ARRAY_SIZE
#define _STL_VERIFY_ARRAY_SIZE(_Array, _Desired) \
    _STL_VERIFY(                                 \
        (_STD size(_Array) >= static_cast<common_type_t<size_t, decltype(_Desired)>>(_Desired)), "array too small")

struct _Container_proxy;
struct _Iterator_base12;

// CLASS TEMPLATE _Array_const_iterator
#if _ITERATOR_DEBUG_LEVEL != 0
struct _Iterator_base12_compatible { // TRANSITION, for binary compatibility
    _Container_proxy* _Myproxy{};
    _Iterator_base12* _Mynextiter{};
};
#endif // _ITERATOR_DEBUG_LEVEL != 0

template <class _Ty, size_t _Size>
class _Array_const_iterator
#if _ITERATOR_DEBUG_LEVEL != 0
    : private _Iterator_base12_compatible
#endif // _ITERATOR_DEBUG_LEVEL != 0
{
public:
#ifdef __cpp_lib_concepts
    using iterator_concept = contiguous_iterator_tag;
#endif // __cpp_lib_concepts
    using iterator_category = random_access_iterator_tag;
    using value_type        = _Ty;
    using difference_type   = ptrdiff_t;
    using pointer           = const _Ty*;
    using reference         = const _Ty&;

    enum { _EEN_SIZE = _Size }; // helper for expression evaluator

#if _ITERATOR_DEBUG_LEVEL == 0
    _CONSTEXPR17 _Array_const_iterator() : _Ptr() {}

    _CONSTEXPR17 explicit _Array_const_iterator(pointer _Parg, size_t _Off = 0) : _Ptr(_Parg + _Off) {}

    _NODISCARD _CONSTEXPR17 reference operator*() const {
        return *_Ptr;
    }

    _NODISCARD _CONSTEXPR17 pointer operator->() const {
        return _Ptr;
    }

    _CONSTEXPR17 _Array_const_iterator& operator++() {
        ++_Ptr;
        return *this;
    }

    _CONSTEXPR17 _Array_const_iterator operator++(int) {
        _Array_const_iterator _Tmp = *this;
        ++_Ptr;
        return _Tmp;
    }

    _CONSTEXPR17 _Array_const_iterator& operator--() {
        --_Ptr;
        return *this;
    }

    _CONSTEXPR17 _Array_const_iterator operator--(int) {
        _Array_const_iterator _Tmp = *this;
        --_Ptr;
        return _Tmp;
    }

    _CONSTEXPR17 _Array_const_iterator& operator+=(const ptrdiff_t _Off) {
        _Ptr += _Off;
        return *this;
    }

    _NODISCARD _CONSTEXPR17 _Array_const_iterator operator+(const ptrdiff_t _Off) const {
        _Array_const_iterator _Tmp = *this;
        return _Tmp += _Off;
    }

    _CONSTEXPR17 _Array_const_iterator& operator-=(const ptrdiff_t _Off) {
        _Ptr -= _Off;
        return *this;
    }

    _NODISCARD _CONSTEXPR17 _Array_const_iterator operator-(const ptrdiff_t _Off) const {
        _Array_const_iterator _Tmp = *this;
        return _Tmp -= _Off;
    }

    _NODISCARD _CONSTEXPR17 ptrdiff_t operator-(const _Array_const_iterator& _Right) const {
        return _Ptr - _Right._Ptr;
    }

    _NODISCARD _CONSTEXPR17 reference operator[](const ptrdiff_t _Off) const {
        return _Ptr[_Off];
    }

    _NODISCARD _CONSTEXPR17 bool operator==(const _Array_const_iterator& _Right) const {
        return _Ptr == _Right._Ptr;
    }

    _NODISCARD _CONSTEXPR17 bool operator!=(const _Array_const_iterator& _Right) const {
        return !(*this == _Right);
    }

    _NODISCARD _CONSTEXPR17 bool operator<(const _Array_const_iterator& _Right) const {
        return _Ptr < _Right._Ptr;
    }

    _NODISCARD _CONSTEXPR17 bool operator>(const _Array_const_iterator& _Right) const {
        return _Right < *this;
    }

    _NODISCARD _CONSTEXPR17 bool operator<=(const _Array_const_iterator& _Right) const {
        return !(_Right < *this);
    }

    _NODISCARD _CONSTEXPR17 bool operator>=(const _Array_const_iterator& _Right) const {
        return !(*this < _Right);
    }

    using _Prevent_inheriting_unwrap = _Array_const_iterator;

    _NODISCARD constexpr pointer _Unwrapped() const {
        return _Ptr;
    }

    static constexpr bool _Unwrap_when_unverified = true;

    constexpr void _Seek_to(pointer _It) {
        _Ptr = _It;
    }

private:
    pointer _Ptr; // beginning of array

#else // ^^^ _ITERATOR_DEBUG_LEVEL == 0 / _ITERATOR_DEBUG_LEVEL != 0 vvv
    _CONSTEXPR17 _Array_const_iterator() : _Ptr(), _Idx(0) {}

    _CONSTEXPR17 explicit _Array_const_iterator(pointer _Parg, size_t _Off = 0) : _Ptr(_Parg), _Idx(_Off) {}

    _NODISCARD _CONSTEXPR17 reference operator*() const {
        return *operator->();
    }

    _NODISCARD _CONSTEXPR17 pointer operator->() const {
        _STL_VERIFY(_Ptr, "cannot dereference value-initialized array iterator");
        _STL_VERIFY(_Idx < _Size, "cannot dereference out of range array iterator");
        return _Ptr + _Idx;
    }

    _CONSTEXPR17 _Array_const_iterator& operator++() {
        _STL_VERIFY(_Ptr, "cannot increment value-initialized array iterator");
        _STL_VERIFY(_Idx < _Size, "cannot increment array iterator past end");
        ++_Idx;
        return *this;
    }

    _CONSTEXPR17 _Array_const_iterator operator++(int) {
        _Array_const_iterator _Tmp = *this;
        ++*this;
        return _Tmp;
    }

    _CONSTEXPR17 _Array_const_iterator& operator--() {
        _STL_VERIFY(_Ptr, "cannot decrement value-initialized array iterator");
        _STL_VERIFY(_Idx != 0, "cannot decrement array iterator before begin");
        --_Idx;
        return *this;
    }

    _CONSTEXPR17 _Array_const_iterator operator--(int) {
        _Array_const_iterator _Tmp = *this;
        --*this;
        return _Tmp;
    }

    constexpr void _Verify_offset(const ptrdiff_t _Off) const noexcept {
        if (_Off != 0) {
            _STL_VERIFY(_Ptr, "cannot seek value-initialized array iterator");
        }

        if (_Off < 0) {
#pragma warning(suppress : 4146) // unary minus operator applied to unsigned type, result still unsigned
            _STL_VERIFY(_Idx >= -static_cast<size_t>(_Off), "cannot seek array iterator before begin");
        }

        if (_Off > 0) {
            _STL_VERIFY(_Size - _Idx >= static_cast<size_t>(_Off), "cannot seek array iterator after end");
        }
    }

    _CONSTEXPR17 _Array_const_iterator& operator+=(const ptrdiff_t _Off) {
        _Verify_offset(_Off);
        _Idx += _Off;
        return *this;
    }

    _NODISCARD _CONSTEXPR17 _Array_const_iterator operator+(const ptrdiff_t _Off) const {
        _Array_const_iterator _Tmp = *this;
        return _Tmp += _Off;
    }

    _CONSTEXPR17 _Array_const_iterator& operator-=(const ptrdiff_t _Off) {
        return *this += -_Off;
    }

    _NODISCARD _CONSTEXPR17 _Array_const_iterator operator-(const ptrdiff_t _Off) const {
        _Array_const_iterator _Tmp = *this;
        return _Tmp -= _Off;
    }

    _NODISCARD _CONSTEXPR17 ptrdiff_t operator-(const _Array_const_iterator& _Right) const {
        _Compat(_Right);
        return static_cast<ptrdiff_t>(_Idx - _Right._Idx);
    }

    _NODISCARD _CONSTEXPR17 reference operator[](const ptrdiff_t _Off) const {
        return *(*this + _Off);
    }

    _NODISCARD _CONSTEXPR17 bool operator==(const _Array_const_iterator& _Right) const {
        _Compat(_Right);
        return _Idx == _Right._Idx;
    }

    _NODISCARD _CONSTEXPR17 bool operator!=(const _Array_const_iterator& _Right) const {
        return !(*this == _Right);
    }

    _NODISCARD _CONSTEXPR17 bool operator<(const _Array_const_iterator& _Right) const {
        _Compat(_Right);
        return _Idx < _Right._Idx;
    }

    _NODISCARD _CONSTEXPR17 bool operator>(const _Array_const_iterator& _Right) const {
        return _Right < *this;
    }

    _NODISCARD _CONSTEXPR17 bool operator<=(const _Array_const_iterator& _Right) const {
        return !(_Right < *this);
    }

    _NODISCARD _CONSTEXPR17 bool operator>=(const _Array_const_iterator& _Right) const {
        return !(*this < _Right);
    }

    _CONSTEXPR17 void _Compat(const _Array_const_iterator& _Right) const { // test for compatible iterator pair
        _STL_VERIFY(_Ptr == _Right._Ptr, "array iterators incompatible");
    }

    using _Prevent_inheriting_unwrap = _Array_const_iterator;

    _NODISCARD constexpr pointer _Unwrapped() const {
        return _Ptr + _Idx;
    }

    constexpr void _Verify_with(const _Array_const_iterator& _Last) const noexcept {
        // note _Compat check inside operator<=
        _STL_VERIFY(*this <= _Last, "array iterator range transposed");
    }

    constexpr void _Seek_to(pointer _It) {
        _Idx = static_cast<size_t>(_It - _Ptr);
    }

private:
    pointer _Ptr; // beginning of array
    size_t _Idx; // offset into array
#endif // _ITERATOR_DEBUG_LEVEL == 0
};

#if _ITERATOR_DEBUG_LEVEL != 0
template <class _Ty, size_t _Size>
constexpr void _Verify_range(
    const _Array_const_iterator<_Ty, _Size>& _First, const _Array_const_iterator<_Ty, _Size>& _Last) noexcept {
    // TRANSITION, VSO-612785
    _First._Verify_with(_Last);
}
#endif // _ITERATOR_DEBUG_LEVEL != 0

template <class _Ty, size_t _Size>
_NODISCARD _CONSTEXPR17 _Array_const_iterator<_Ty, _Size> operator+(
    ptrdiff_t _Off, _Array_const_iterator<_Ty, _Size> _Next) {
    return _Next += _Off;
}

#if _HAS_CXX20
template <class _Ty, size_t _Size>
struct pointer_traits<_Array_const_iterator<_Ty, _Size>> {
    using pointer         = _Array_const_iterator<_Ty, _Size>;
    using element_type    = const _Ty;
    using difference_type = ptrdiff_t;

    _NODISCARD static constexpr element_type* to_address(const pointer _Iter) noexcept {
        return _Iter._Unwrapped();
    }
};
#endif // _HAS_CXX20

// CLASS TEMPLATE _Array_iterator
template <class _Ty, size_t _Size>
class _Array_iterator : public _Array_const_iterator<_Ty, _Size> {
public:
    using _Mybase = _Array_const_iterator<_Ty, _Size>;

#ifdef __cpp_lib_concepts
    using iterator_concept = contiguous_iterator_tag;
#endif // __cpp_lib_concepts
    using iterator_category = random_access_iterator_tag;
    using value_type        = _Ty;
    using difference_type   = ptrdiff_t;
    using pointer           = _Ty*;
    using reference         = _Ty&;

    enum { _EEN_SIZE = _Size }; // helper for expression evaluator

    _CONSTEXPR17 _Array_iterator() noexcept {}

    _CONSTEXPR17 explicit _Array_iterator(pointer _Parg, size_t _Off = 0) : _Mybase(_Parg, _Off) {}

    _NODISCARD _CONSTEXPR17 reference operator*() const {
        return const_cast<reference>(_Mybase::operator*());
    }

    _NODISCARD _CONSTEXPR17 pointer operator->() const {
        return const_cast<pointer>(_Mybase::operator->());
    }

    _CONSTEXPR17 _Array_iterator& operator++() {
        _Mybase::operator++();
        return *this;
    }

    _CONSTEXPR17 _Array_iterator operator++(int) {
        _Array_iterator _Tmp = *this;
        _Mybase::operator++();
        return _Tmp;
    }

    _CONSTEXPR17 _Array_iterator& operator--() {
        _Mybase::operator--();
        return *this;
    }

    _CONSTEXPR17 _Array_iterator operator--(int) {
        _Array_iterator _Tmp = *this;
        _Mybase::operator--();
        return _Tmp;
    }

    _CONSTEXPR17 _Array_iterator& operator+=(const ptrdiff_t _Off) {
        _Mybase::operator+=(_Off);
        return *this;
    }

    _NODISCARD _CONSTEXPR17 _Array_iterator operator+(const ptrdiff_t _Off) const {
        _Array_iterator _Tmp = *this;
        return _Tmp += _Off;
    }

    _CONSTEXPR17 _Array_iterator& operator-=(const ptrdiff_t _Off) {
        _Mybase::operator-=(_Off);
        return *this;
    }

    using _Mybase::operator-;

    _NODISCARD _CONSTEXPR17 _Array_iterator operator-(const ptrdiff_t _Off) const {
        _Array_iterator _Tmp = *this;
        return _Tmp -= _Off;
    }

    _NODISCARD _CONSTEXPR17 reference operator[](const ptrdiff_t _Off) const {
        return const_cast<reference>(_Mybase::operator[](_Off));
    }

    using _Prevent_inheriting_unwrap = _Array_iterator;

    _NODISCARD constexpr pointer _Unwrapped() const {
        return const_cast<pointer>(_Mybase::_Unwrapped());
    }
};

template <class _Ty, size_t _Size>
_NODISCARD _CONSTEXPR17 _Array_iterator<_Ty, _Size> operator+(ptrdiff_t _Off, _Array_iterator<_Ty, _Size> _Next) {
    return _Next += _Off;
}

#if _HAS_CXX20
template <class _Ty, size_t _Size>
struct pointer_traits<_Array_iterator<_Ty, _Size>> {
    using pointer         = _Array_iterator<_Ty, _Size>;
    using element_type    = _Ty;
    using difference_type = ptrdiff_t;

    _NODISCARD static constexpr element_type* to_address(const pointer _Iter) noexcept {
        return _Iter._Unwrapped();
    }
};
#endif // _HAS_CXX20

// STRUCT _Default_sentinel
struct _Default_sentinel {}; // empty struct to serve as the end of a range

// CLASS TEMPLATE move_iterator
template <class _Iter>
class move_iterator { // wrap iterator to move rvalues
public:
    using iterator_category = typename iterator_traits<_Iter>::iterator_category;
    using value_type        = typename iterator_traits<_Iter>::value_type;
    using difference_type   = typename iterator_traits<_Iter>::difference_type;
    using pointer           = _Iter;
    using _Ref0             = typename iterator_traits<_Iter>::reference;
    using reference         = conditional_t<is_reference_v<_Ref0>, remove_reference_t<_Ref0>&&, _Ref0>;
    using iterator_type     = _Iter;

    _CONSTEXPR17 move_iterator() : current() {}

    _CONSTEXPR17 explicit move_iterator(iterator_type _Right) : current(_Right) {}

    template <class _Iter2>
    _CONSTEXPR17 move_iterator(const move_iterator<_Iter2>& _Right) : current(_Right.base()) {}

    template <class _Iter2>
    _CONSTEXPR17 move_iterator& operator=(const move_iterator<_Iter2>& _Right) {
        current = _Right.base();
        return *this;
    }

    _NODISCARD _CONSTEXPR17 _Iter base() const {
        return current;
    }

    _NODISCARD _CONSTEXPR17 reference operator*() const {
        return static_cast<reference>(*current);
    }

    _NODISCARD _CONSTEXPR17 pointer operator->() const {
        return current;
    }

    _CONSTEXPR17 move_iterator& operator++() {
        ++current;
        return *this;
    }

    _CONSTEXPR17 move_iterator operator++(int) {
        move_iterator _Tmp = *this;
        ++current;
        return _Tmp;
    }

    _CONSTEXPR17 move_iterator& operator--() {
        --current;
        return *this;
    }

    _CONSTEXPR17 move_iterator operator--(int) {
        move_iterator _Tmp = *this;
        --current;
        return _Tmp;
    }

    template <class _Iter2 = _Iter>
    _NODISCARD auto operator==(_Default_sentinel _Sentinel) const noexcept
        -> decltype(_STD declval<const _Iter2&>() == _Sentinel) {
        return current == _Sentinel;
    }

    template <class _Iter2 = _Iter>
    _NODISCARD auto operator!=(_Default_sentinel _Sentinel) const noexcept
        -> decltype(_STD declval<const _Iter2&>() != _Sentinel) {
        return current != _Sentinel;
    }

    // N.B. functions valid for random-access iterators only beyond this point

    _CONSTEXPR17 move_iterator& operator+=(const difference_type _Off) {
        current += _Off;
        return *this;
    }

    _NODISCARD _CONSTEXPR17 move_iterator operator+(const difference_type _Off) const {
        return move_iterator(current + _Off);
    }

    _CONSTEXPR17 move_iterator& operator-=(const difference_type _Off) {
        current -= _Off;
        return *this;
    }

    _NODISCARD _CONSTEXPR17 move_iterator operator-(const difference_type _Off) const {
        return move_iterator(current - _Off);
    }

    _NODISCARD _CONSTEXPR17 reference operator[](const difference_type _Off) const {
        return _STD move(current[_Off]);
    }

    template <class _Iter2, enable_if_t<_Range_verifiable_v<_Iter, _Iter2>, int> = 0>
    friend constexpr void _Verify_range(const move_iterator& _First, const move_iterator<_Iter2>& _Last) {
        _Verify_range(_First.base(), _Last.base());
    }

    using _Prevent_inheriting_unwrap = move_iterator;

    template <class _Iter2 = iterator_type, enable_if_t<_Offset_verifiable_v<_Iter2>, int> = 0>
    constexpr void _Verify_offset(const difference_type _Off) const {
        current._Verify_offset(_Off);
    }

    template <class _Iter2 = iterator_type, enable_if_t<_Unwrappable_v<const _Iter2&>, int> = 0>
    _NODISCARD constexpr move_iterator<_Unwrapped_t<const _Iter2&>> _Unwrapped() const {
        return static_cast<move_iterator<_Unwrapped_t<const _Iter2&>>>(current._Unwrapped());
    }

    static constexpr bool _Unwrap_when_unverified = _Do_unwrap_when_unverified_v<iterator_type>;

    template <class _Src, enable_if_t<_Wrapped_seekable_v<iterator_type, _Src>, int> = 0>
    constexpr void _Seek_to(const move_iterator<_Src>& _It) {
        current._Seek_to(_It.base());
    }

protected:
    iterator_type current; // the wrapped iterator
};

template <class _Iter>
_NODISCARD _CONSTEXPR17 move_iterator<_Iter> operator+(
    typename move_iterator<_Iter>::difference_type _Off, const move_iterator<_Iter>& _Right) {
    return _Right + _Off;
}

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 auto operator-(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right)
    -> decltype(_Left.base() - _Right.base()) {
    return _Left.base() - _Right.base();
}

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator==(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return _Left.base() == _Right.base();
}

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator!=(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return !(_Left == _Right);
}

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator<(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return _Left.base() < _Right.base();
}

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator>(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return _Right < _Left;
}

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator<=(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return !(_Right < _Left);
}

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator>=(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return !(_Left < _Right);
}

// FUNCTION TEMPLATE make_move_iterator
template <class _Iter>
_NODISCARD _CONSTEXPR17 move_iterator<_Iter> make_move_iterator(_Iter _It) { // make move_iterator from iterator
    return move_iterator<_Iter>(_It);
}

#ifdef __cpp_lib_concepts
// STRUCT default_sentinel_t
struct default_sentinel_t {};

// VARIABLE default_sentinel
inline constexpr default_sentinel_t default_sentinel{};

// STRUCT unreachable_sentinel_t
struct unreachable_sentinel_t;
namespace _Unreachable_sentinel_detail {
    struct _Base {
        template <weakly_incrementable _Winc>
        _NODISCARD friend constexpr bool operator==(const unreachable_sentinel_t&, const _Winc&) noexcept {
            return false;
        }
#if !defined(__cpp_impl_three_way_comparison) || __cpp_impl_three_way_comparison < 201902L
        template <weakly_incrementable _Winc>
        _NODISCARD friend constexpr bool operator==(const _Winc&, const unreachable_sentinel_t&) noexcept {
            return false;
        }

        template <weakly_incrementable _Winc>
        _NODISCARD friend constexpr bool operator!=(const unreachable_sentinel_t&, const _Winc&) noexcept {
            return true;
        }
        template <weakly_incrementable _Winc>
        _NODISCARD friend constexpr bool operator!=(const _Winc&, const unreachable_sentinel_t&) noexcept {
            return true;
        }
#endif // !defined(__cpp_impl_three_way_comparison) || __cpp_impl_three_way_comparison < 201902L
    };
} // namespace _Unreachable_sentinel_detail
struct unreachable_sentinel_t : _Unreachable_sentinel_detail::_Base {}; // TRANSITION, /permissive-

// VARIABLE unreachable_sentinel
inline constexpr unreachable_sentinel_t unreachable_sentinel{};
#endif // __cpp_lib_concepts

// FUNCTION TEMPLATE copy
template <class _Source, class _Dest>
struct _Ptr_cat_helper {
    using _USource                        = _Unwrap_enum_t<_Source>;
    using _UDest                          = _Unwrap_enum_t<_Dest>;
    static constexpr bool _Really_trivial = conjunction_v<
        bool_constant<sizeof(_USource) == sizeof(_UDest) && is_same_v<bool, _USource> == is_same_v<bool, _UDest>>,
        is_integral<_USource>, is_integral<_UDest>>;
    static constexpr bool _Trivially_copyable = _Really_trivial;
};

template <class _Elem>
struct _Ptr_cat_helper<_Elem, _Elem> { // determines _Ptr_cat's result when the types are the same
    static constexpr bool _Really_trivial     = is_trivial_v<_Elem>;
    static constexpr bool _Trivially_copyable = is_trivially_copyable_v<_Elem>;
};

template <class _Anything>
struct _Ptr_cat_helper<_Anything*, const _Anything*> {
    // determines _Ptr_cat's result when all we do is add const to a pointer
    static constexpr bool _Really_trivial     = true;
    static constexpr bool _Trivially_copyable = true;
};

template <class _Anything>
struct _Ptr_cat_helper<_Anything*, volatile _Anything*> {
    // determines _Ptr_cat's result when all we do is add volatile to a pointer
    static constexpr bool _Really_trivial     = true;
    static constexpr bool _Trivially_copyable = true;
};

template <class _Anything>
struct _Ptr_cat_helper<_Anything*, const volatile _Anything*> {
    // determines _Ptr_cat's result when all we do is add cv to a pointer
    static constexpr bool _Really_trivial     = true;
    static constexpr bool _Trivially_copyable = true;
};

struct _False_copy_cat {
    static constexpr bool _Really_trivial     = false;
    static constexpr bool _Trivially_copyable = false;
};

template <class _Source, class _Dest>
struct _Ptr_copy_cat : _False_copy_cat {}; // unwrap the pointer-like type and dispatch to _Ptr_cat_helper for copy

template <class _Source, class _Dest>
struct _Ptr_copy_cat<_Source*, _Dest*>
    : conditional_t<is_trivially_assignable_v<_Dest&, _Source&>,
          _Ptr_cat_helper<remove_cv_t<_Source>, remove_cv_t<_Dest>>, _False_copy_cat> {};

template <class _Source, class _Dest>
struct _Ptr_copy_cat<move_iterator<_Source*>, _Dest*> : _Ptr_copy_cat<_Source*, _Dest*> {};

template <class _Source, class _Dest>
struct _Ptr_move_cat : _False_copy_cat {}; // unwrap the pointer-like type and dispatch to _Ptr_cat_helper for move

template <class _Source, class _Dest>
struct _Ptr_move_cat<_Source*, _Dest*>
    : conditional_t<is_trivially_assignable_v<_Dest&, _Source>,
          _Ptr_cat_helper<remove_cv_t<_Source>, remove_cv_t<_Dest>>, _False_copy_cat> {};

template <class _Source, class _Dest>
struct _Ptr_move_cat<move_iterator<_Source*>, _Dest*> : _Ptr_move_cat<_Source*, _Dest*> {};

template <class _InIt, class _OutIt>
_OutIt _Copy_memmove(_InIt _First, _InIt _Last, _OutIt _Dest) {
    const char* const _First_ch = const_cast<const char*>(reinterpret_cast<const volatile char*>(_First));
    const char* const _Last_ch  = const_cast<const char*>(reinterpret_cast<const volatile char*>(_Last));
    char* const _Dest_ch        = const_cast<char*>(reinterpret_cast<volatile char*>(_Dest));
    const auto _Count           = static_cast<size_t>(_Last_ch - _First_ch);
    _CSTD memmove(_Dest_ch, _First_ch, _Count);
    return reinterpret_cast<_OutIt>(_Dest_ch + _Count);
}

template <class _InIt, class _OutIt>
_OutIt _Copy_memmove(move_iterator<_InIt> _First, move_iterator<_InIt> _Last, _OutIt _Dest) {
    return _Copy_memmove(_First.base(), _Last.base(), _Dest);
}

#if _HAS_IF_CONSTEXPR
template <class _InIt, class _OutIt>
_CONSTEXPR20 _OutIt _Copy_unchecked(_InIt _First, _InIt _Last, _OutIt _Dest) {
    // copy [_First, _Last) to [_Dest, ...)
    // note: _Copy_unchecked has callers other than the copy family
    if constexpr (_Ptr_copy_cat<_InIt, _OutIt>::_Trivially_copyable) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            return _Copy_memmove(_First, _Last, _Dest);
        }
    }

    for (; _First != _Last; ++_Dest, (void) ++_First) {
        *_Dest = *_First;
    }

    return _Dest;
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt, class _OutIt>
_OutIt _Copy_unchecked1(_InIt _First, _InIt _Last, _OutIt _Dest, false_type) {
    // copy [_First, _Last) to [_Dest, ...), arbitrary iterators
    for (; _First != _Last; ++_Dest, (void) ++_First) {
        *_Dest = *_First;
    }

    return _Dest;
}

template <class _InIt, class _OutIt>
_OutIt _Copy_unchecked1(_InIt _First, _InIt _Last, _OutIt _Dest, true_type) {
    // copy [_First, _Last) to [_Dest, ...), pointers to trivially copyable
    return _Copy_memmove(_First, _Last, _Dest);
}

template <class _InIt, class _OutIt>
_OutIt _Copy_unchecked(_InIt _First, _InIt _Last, _OutIt _Dest) {
    // copy [_First, _Last) to [_Dest, ...)
    // note: _Copy_unchecked has callers other than the copy family
    return _Copy_unchecked1(_First, _Last, _Dest, bool_constant<_Ptr_copy_cat<_InIt, _OutIt>::_Trivially_copyable>{});
}
#endif // _HAS_IF_CONSTEXPR

template <class _InIt, class _OutIt>
_CONSTEXPR20 _OutIt copy(_InIt _First, _InIt _Last, _OutIt _Dest) { // copy [_First, _Last) to [_Dest, ...)
    _Adl_verify_range(_First, _Last);
    const auto _UFirst = _Get_unwrapped(_First);
    const auto _ULast  = _Get_unwrapped(_Last);
    const auto _UDest  = _Get_unwrapped_n(_Dest, _Idl_distance<_InIt>(_UFirst, _ULast));
    _Seek_wrapped(_Dest, _Copy_unchecked(_UFirst, _ULast, _UDest));
    return _Dest;
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _InIt, class _DestTy, size_t _DestSize>
_CONSTEXPR20 _DestTy* copy(_InIt _First, _InIt _Last, _DestTy (&_Dest)[_DestSize]) {
    // copy [_First, _Last) to [_Dest, ...)
    return _STD copy(_First, _Last, _Array_iterator<_DestTy, _DestSize>(_Dest))._Unwrapped();
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _FwdIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_FwdIt2 copy(_ExPo&&, _FwdIt1 _First, _FwdIt1 _Last, _FwdIt2 _Dest) noexcept /* terminates */ {
    // copy [_First, _Last) to [_Dest, ...)
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt2);
    return _STD copy(_First, _Last, _Dest);
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _ExPo, class _FwdIt1, class _DestTy, size_t _DestSize, _Enable_if_execution_policy_t<_ExPo> = 0>
_DestTy* copy(_ExPo&&, _FwdIt1 _First, _FwdIt1 _Last, _DestTy (&_Dest)[_DestSize]) noexcept /* terminates */ {
    // copy [_First, _Last) to [_Dest, ...)
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    return _STD copy(_First, _Last, _Dest);
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS
#endif // _HAS_CXX17

// FUNCTION TEMPLATE copy_n
#if _HAS_IF_CONSTEXPR
template <class _InIt, class _Diff, class _OutIt>
_CONSTEXPR20 _OutIt copy_n(_InIt _First, _Diff _Count_raw, _OutIt _Dest) {
    // copy [_First, _First + _Count) to [_Dest, ...)
    _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count) {
        auto _UFirst = _Get_unwrapped_n(_First, _Count);
        auto _UDest  = _Get_unwrapped_n(_Dest, _Count);
        if constexpr (_Ptr_copy_cat<decltype(_UFirst), decltype(_UDest)>::_Trivially_copyable) {
#ifdef __cpp_lib_is_constant_evaluated
            if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
            {
                _UDest = _Copy_memmove(_UFirst, _UFirst + _Count, _UDest);
                _Seek_wrapped(_Dest, _UDest);
                return _Dest;
            }
        }

        for (;;) {
            *_UDest = *_UFirst;
            ++_UDest;
            --_Count;
            if (_Count == 0) { // note that we avoid an extra ++_First here to allow istream_iterator to work,
                               // see LWG-2471
                break;
            }

            ++_UFirst;
        }

        _Seek_wrapped(_Dest, _UDest);
    }

    return _Dest;
}

#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt, class _Diff, class _OutIt>
_OutIt _Copy_n_unchecked4(_InIt _First, _Diff _Count, _OutIt _Dest, false_type) {
    // copy [_First, _First + _Count) to [_Dest, ...), no special optimization
    // pre: 0 < _Count
    for (;;) {
        *_Dest = *_First;
        ++_Dest;
        --_Count;
        if (_Count == 0) { // note that we avoid an extra ++_First here to allow istream_iterator to work,
                           // see LWG-2471
            return _Dest;
        }

        ++_First;
    }
}

template <class _InIt, class _Diff, class _OutIt>
_OutIt _Copy_n_unchecked4(_InIt _First, _Diff _Count, _OutIt _Dest, true_type) {
    // copy [_First, _First + _Count) to [_Dest, ...), memmove optimization
    // pre: 0 < _Count
    return _Copy_memmove(_First, _First + _Count, _Dest);
}

template <class _InIt, class _Diff, class _OutIt>
_OutIt copy_n(_InIt _First, _Diff _Count_raw, _OutIt _Dest) { // copy [_First, _First + _Count) to [_Dest, ...)
    const _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count) {
        auto _UFirst = _Get_unwrapped_n(_First, _Count);
        auto _UDest  = _Get_unwrapped_n(_Dest, _Count);
        _Seek_wrapped(
            _Dest, _Copy_n_unchecked4(_UFirst, _Count, _UDest,
                       bool_constant<_Ptr_copy_cat<decltype(_UFirst), decltype(_UDest)>::_Trivially_copyable>{}));
    }

    return _Dest;
}
#endif // _HAS_IF_CONSTEXPR

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _SourceTy, size_t _SourceSize, class _Diff, class _OutIt>
_CONSTEXPR20 _OutIt copy_n(_SourceTy (&_First)[_SourceSize], _Diff _Count_raw, _OutIt _Dest) {
    // copy [_First, _First + _Count) to [_Dest, ...), array source
    const _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count) {
        _STL_VERIFY_ARRAY_SIZE(_First, _Count);
        _Seek_wrapped(_Dest, _STD copy_n(static_cast<_SourceTy*>(_First), _Count, _Get_unwrapped_n(_Dest, _Count)));
    }

    return _Dest;
}

template <class _InIt, class _Diff, class _DestTy, size_t _DestSize>
_CONSTEXPR20 _DestTy* copy_n(_InIt _First, _Diff _Count_raw, _DestTy (&_Dest)[_DestSize]) {
    // copy [_First, _First + _Count) to [_Dest, ...), array dest
    const _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count) {
        _STL_VERIFY_ARRAY_SIZE(_Dest, _Count);
        return _STD copy_n(_First, _Count, static_cast<_DestTy*>(_Dest));
    }

    return _Dest;
}

template <class _SourceTy, size_t _SourceSize, class _Diff, class _DestTy, size_t _DestSize>
_CONSTEXPR20 _DestTy* copy_n(_SourceTy (&_First)[_SourceSize], _Diff _Count_raw, _DestTy (&_Dest)[_DestSize]) {
    // copy [_First, _First + _Count) to [_Dest, ...), array source/dest
    const _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count) {
        _STL_VERIFY_ARRAY_SIZE(_First, _Count);
        _STL_VERIFY_ARRAY_SIZE(_Dest, _Count);
        return _STD copy_n(static_cast<_SourceTy*>(_First), _Count, static_cast<_DestTy*>(_Dest));
    }

    return _Dest;
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _Diff, class _FwdIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_FwdIt2 copy_n(_ExPo&&, _FwdIt1 _First, _Diff _Count_raw, _FwdIt2 _Dest) noexcept /* terminates */ {
    // copy [_First, _First + _Count) to [_Dest, ...)
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt2);
    return _STD copy_n(_First, _Count_raw, _Dest);
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _ExPo, class _SourceTy, size_t _SourceSize, class _Diff, class _FwdIt2,
    _Enable_if_execution_policy_t<_ExPo> = 0>
_FwdIt2 copy_n(_ExPo&&, _SourceTy (&_First)[_SourceSize], _Diff _Count_raw, _FwdIt2 _Dest) noexcept /* terminates */ {
    // copy [_First, _First + _Count) to [_Dest, ...), array source
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt2);
    return _STD copy_n(_First, _Count_raw, _Dest);
}

template <class _ExPo, class _FwdIt1, class _Diff, class _DestTy, size_t _DestSize,
    _Enable_if_execution_policy_t<_ExPo> = 0>
_DestTy* copy_n(_ExPo&&, _FwdIt1 _First, _Diff _Count_raw, _DestTy (&_Dest)[_DestSize]) noexcept /* terminates */ {
    // copy [_First, _First + _Count) to [_Dest, ...), array dest
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    return _STD copy_n(_First, _Count_raw, _Dest);
}

template <class _ExPo, class _SourceTy, size_t _SourceSize, class _Diff, class _DestTy, size_t _DestSize,
    _Enable_if_execution_policy_t<_ExPo> = 0>
_DestTy* copy_n(_ExPo&&, _SourceTy (&_First)[_SourceSize], _Diff _Count_raw,
    _DestTy (&_Dest)[_DestSize]) noexcept /* terminates */ {
    // copy [_First, _First + _Count) to [_Dest, ...), array source/dest
    // not parallelized as benchmarks show it isn't worth it
    return _STD copy_n(_First, _Count_raw, _Dest);
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS
#endif // _HAS_CXX17

// FUNCTION TEMPLATE copy_backward
template <class _BidIt1, class _BidIt2>
_BidIt2 _Copy_backward_memmove(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) {
    // implement copy_backward-like function as memmove
    const char* const _First_ch = const_cast<const char*>(reinterpret_cast<const volatile char*>(_First));
    const char* const _Last_ch  = const_cast<const char*>(reinterpret_cast<const volatile char*>(_Last));
    char* const _Dest_ch        = const_cast<char*>(reinterpret_cast<volatile char*>(_Dest));
    const auto _Count           = static_cast<size_t>(_Last_ch - _First_ch);
    return static_cast<_BidIt2>(_CSTD memmove(_Dest_ch - _Count, _First_ch, _Count));
}

template <class _BidIt1, class _BidIt2>
_BidIt2 _Copy_backward_memmove(move_iterator<_BidIt1> _First, move_iterator<_BidIt1> _Last, _BidIt2 _Dest) {
    return _Copy_backward_memmove(_First.base(), _Last.base(), _Dest);
}

#if _HAS_IF_CONSTEXPR
template <class _BidIt1, class _BidIt2>
_CONSTEXPR20 _BidIt2 copy_backward(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) {
    // copy [_First, _Last) backwards to [..., _Dest)
    _Adl_verify_range(_First, _Last);
    const auto _UFirst = _Get_unwrapped(_First);
    auto _ULast        = _Get_unwrapped(_Last);
    auto _UDest        = _Get_unwrapped_n(_Dest, -_Idl_distance<_BidIt1>(_UFirst, _ULast));
    if constexpr (_Ptr_copy_cat<decltype(_ULast), decltype(_UDest)>::_Trivially_copyable) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            _UDest = _Copy_backward_memmove(_UFirst, _ULast, _UDest);
            _Seek_wrapped(_Dest, _UDest);
            return _Dest;
        }
    }

    while (_UFirst != _ULast) {
        *--_UDest = *--_ULast;
    }
    _Seek_wrapped(_Dest, _UDest);
    return _Dest;
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _BidIt1, class _BidIt2>
_BidIt2 _Copy_backward_unchecked(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest, false_type) {
    // copy [_First, _Last) backwards to [..., _Dest), no special optimization
    while (_First != _Last) {
        *--_Dest = *--_Last;
    }

    return _Dest;
}

template <class _BidIt1, class _BidIt2>
_BidIt2 _Copy_backward_unchecked(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest, true_type) {
    // copy [_First, _Last) backwards to [..., _Dest), memmove optimization
    return _Copy_backward_memmove(_First, _Last, _Dest);
}

template <class _BidIt1, class _BidIt2>
_BidIt2 copy_backward(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) { // copy [_First, _Last) backwards to [..., _Dest)
    _Adl_verify_range(_First, _Last);
    auto _UFirst      = _Get_unwrapped(_First);
    const auto _ULast = _Get_unwrapped(_Last);
    auto _UDest       = _Get_unwrapped_n(_Dest, -_Idl_distance<_BidIt1>(_UFirst, _ULast));
    _Seek_wrapped(_Dest, _Copy_backward_unchecked(_UFirst, _ULast, _UDest,
                             bool_constant<_Ptr_copy_cat<decltype(_UFirst), decltype(_UDest)>::_Trivially_copyable>{}));
    return _Dest;
}
#endif // _HAS_IF_CONSTEXPR

#if _HAS_CXX17
template <class _ExPo, class _BidIt1, class _BidIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_BidIt2 copy_backward(_ExPo&&, _BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) noexcept /* terminates */ {
    // copy [_First, _Last) backwards to [..., _Dest)
    // not parallelized as benchmarks show it isn't worth it
    return _STD copy_backward(_First, _Last, _Dest);
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE move
#if _HAS_IF_CONSTEXPR
template <class _InIt, class _OutIt>
_CONSTEXPR20 _OutIt _Move_unchecked(_InIt _First, _InIt _Last, _OutIt _Dest) {
    // move [_First, _Last) to [_Dest, ...)
    // note: _Move_unchecked has callers other than the move family
    if constexpr (_Ptr_move_cat<_InIt, _OutIt>::_Trivially_copyable) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            return _Copy_memmove(_First, _Last, _Dest);
        }
    }

    for (; _First != _Last; ++_Dest, (void) ++_First) {
        *_Dest = _STD move(*_First);
    }

    return _Dest;
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt, class _OutIt>
_OutIt _Move_unchecked1(_InIt _First, _InIt _Last, _OutIt _Dest, false_type) {
    // move [_First, _Last) to [_Dest, ...), no special optimization
    for (; _First != _Last; ++_Dest, (void) ++_First) {
        *_Dest = _STD move(*_First);
    }

    return _Dest;
}

template <class _InIt, class _OutIt>
_OutIt _Move_unchecked1(_InIt _First, _InIt _Last, _OutIt _Dest, true_type) {
    // move [_First, _Last) to [_Dest, ...), memmove optimization
    return _Copy_memmove(_First, _Last, _Dest);
}

template <class _InIt, class _OutIt>
_OutIt _Move_unchecked(_InIt _First, _InIt _Last, _OutIt _Dest) {
    // move [_First, _Last) to [_Dest, ...), choose optimization
    // note: _Move_unchecked has callers other than the move family
    return _Move_unchecked1(_First, _Last, _Dest, bool_constant<_Ptr_move_cat<_InIt, _OutIt>::_Trivially_copyable>{});
}
#endif // _HAS_IF_CONSTEXPR

template <class _InIt, class _OutIt>
_CONSTEXPR20 _OutIt move(_InIt _First, _InIt _Last, _OutIt _Dest) {
    // move [_First, _Last) to [_Dest, ...)
    _Adl_verify_range(_First, _Last);
    const auto _UFirst = _Get_unwrapped(_First);
    const auto _ULast  = _Get_unwrapped(_Last);
    const auto _UDest  = _Get_unwrapped_n(_Dest, _Idl_distance<_InIt>(_UFirst, _ULast));
    _Seek_wrapped(_Dest, _Move_unchecked(_UFirst, _ULast, _UDest));
    return _Dest;
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _InIt, class _DestTy, size_t _DestSize>
_DestTy* move(_InIt _First, _InIt _Last, _DestTy (&_Dest)[_DestSize]) { // move [_First, _Last) to [_Dest, ...)
    return _STD move(_First, _Last, _Array_iterator<_DestTy, _DestSize>(_Dest))._Unwrapped();
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _FwdIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_FwdIt2 move(_ExPo&&, _FwdIt1 _First, _FwdIt1 _Last, _FwdIt2 _Dest) noexcept /* terminates */ {
    // move [_First, _Last) to [_Dest, ...)
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt2);
    return _STD move(_First, _Last, _Dest);
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _ExPo, class _FwdIt1, class _DestTy, size_t _DestSize, _Enable_if_execution_policy_t<_ExPo> = 0>
_DestTy* move(_ExPo&&, _FwdIt1 _First, _FwdIt1 _Last, _DestTy (&_Dest)[_DestSize]) noexcept /* terminates */ {
    // move [_First, _Last) to [_Dest, ...)
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    return _STD move(_First, _Last, _Dest);
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS
#endif // _HAS_CXX17

// FUNCTION TEMPLATE move_backward
#if _HAS_IF_CONSTEXPR
template <class _BidIt1, class _BidIt2>
_CONSTEXPR20 _BidIt2 _Move_backward_unchecked(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) {
    // move [_First, _Last) backwards to [..., _Dest)
    // note: _Move_backward_unchecked has callers other than the move_backward family
    if constexpr (_Ptr_move_cat<_BidIt1, _BidIt2>::_Trivially_copyable) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            return _Copy_backward_memmove(_First, _Last, _Dest);
        }
    }

    while (_First != _Last) {
        *--_Dest = _STD move(*--_Last);
    }

    return _Dest;
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _BidIt1, class _BidIt2>
_BidIt2 _Move_backward_unchecked1(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest, false_type) {
    // move [_First, _Last) backwards to [..., _Dest), no special optimization
    while (_First != _Last) {
        *--_Dest = _STD move(*--_Last);
    }

    return _Dest;
}

template <class _BidIt1, class _BidIt2>
_BidIt2 _Move_backward_unchecked1(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest, true_type) {
    // move [_First, _Last) backwards to [..., _Dest), memmove optimization
    return _Copy_backward_memmove(_First, _Last, _Dest);
}

template <class _BidIt1, class _BidIt2>
_BidIt2 _Move_backward_unchecked(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) {
    // move [_First, _Last) backwards to [..., _Dest), choose optimization
    // note: _Move_backward_unchecked has callers other than the move_backward family
    return _Move_backward_unchecked1(
        _First, _Last, _Dest, bool_constant<_Ptr_move_cat<_BidIt1, _BidIt2>::_Trivially_copyable>{});
}
#endif // _HAS_IF_CONSTEXPR

template <class _BidIt1, class _BidIt2>
_CONSTEXPR20 _BidIt2 move_backward(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) {
    // move [_First, _Last) backwards to [..., _Dest)
    _Adl_verify_range(_First, _Last);
    const auto _UFirst = _Get_unwrapped(_First);
    const auto _ULast  = _Get_unwrapped(_Last);
    const auto _UDest  = _Get_unwrapped_n(_Dest, -_Idl_distance<_BidIt1>(_UFirst, _ULast));
    _Seek_wrapped(_Dest, _Move_backward_unchecked(_UFirst, _ULast, _UDest));
    return _Dest;
}

#if _HAS_CXX17
template <class _ExPo, class _BidIt1, class _BidIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_BidIt2 move_backward(_ExPo&&, _BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) noexcept /* terminates */ {
    // move [_First, _Last) backwards to [..., _Dest)
    // not parallelized as benchmarks show it isn't worth it
    return _STD move_backward(_First, _Last, _Dest);
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE fill
template <class _Ty>
struct _Is_character : false_type {}; // by default, not a character type

template <>
struct _Is_character<char> : true_type {}; // chars are characters

template <>
struct _Is_character<signed char> : true_type {}; // signed chars are also characters

template <>
struct _Is_character<unsigned char> : true_type {}; // unsigned chars are also characters

#ifdef __cpp_char8_t
template <>
struct _Is_character<char8_t> : true_type {}; // UTF-8 code units are sort-of characters
#endif // __cpp_char8_t

template <class _Ty>
struct _Is_character_or_byte : _Is_character<_Ty>::type {};

#if _HAS_STD_BYTE
template <>
struct _Is_character_or_byte<byte> : true_type {};
#endif // _HAS_STD_BYTE

// _Fill_memset_is_safe determines if _FwdIt and _Ty are eligible for memset optimization in fill
template <class _FwdIt, class _Ty, bool = is_pointer_v<_FwdIt>>
_INLINE_VAR constexpr bool _Fill_memset_is_safe = conjunction_v<
    disjunction<conjunction<_Is_character_or_byte<_Unwrap_enum_t<_Ty>>,
                    _Is_character_or_byte<_Unwrap_enum_t<_Iter_value_t<_FwdIt>>>>,
        conjunction<is_same<bool, _Unwrap_enum_t<_Ty>>, is_same<bool, _Unwrap_enum_t<_Iter_value_t<_FwdIt>>>>>,
    is_assignable<_Iter_ref_t<_FwdIt>, const _Ty&>>;

template <class _FwdIt, class _Ty>
_INLINE_VAR constexpr bool _Fill_memset_is_safe<_FwdIt, _Ty, false> = false;

#if _HAS_IF_CONSTEXPR
template <class _FwdIt, class _Ty>
_CONSTEXPR20 void fill(const _FwdIt _First, const _FwdIt _Last, const _Ty& _Val) {
    // copy _Val through [_First, _Last)
    _Adl_verify_range(_First, _Last);
    auto _UFirst      = _Get_unwrapped(_First);
    const auto _ULast = _Get_unwrapped(_Last);
    if constexpr (_Fill_memset_is_safe<decltype(_UFirst), _Ty>) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            _CSTD memset(_UFirst, static_cast<unsigned char>(_Val), static_cast<size_t>(_ULast - _UFirst));
            return;
        }
    }

    for (; _UFirst != _ULast; ++_UFirst) {
        *_UFirst = _Val;
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR // !_HAS_IF_CONSTEXPR vvv
template <class _FwdIt, class _Ty>
void _Fill_unchecked1(_FwdIt _First, _FwdIt _Last, const _Ty& _Val, false_type) {
    // copy _Val through [_First, _Last), no special optimization
    for (; _First != _Last; ++_First) {
        *_First = _Val;
    }
}

template <class _FwdIt, class _Ty>
void _Fill_unchecked1(_FwdIt _First, _FwdIt _Last, const _Ty& _Val, true_type) {
    // copy _Val through [_First, _Last), memset optimization
    _CSTD memset(_First, static_cast<unsigned char>(_Val), static_cast<size_t>(_Last - _First));
}

template <class _FwdIt, class _Ty>
void fill(_FwdIt _First, _FwdIt _Last, const _Ty& _Val) { // copy _Val through [_First, _Last)
    _Adl_verify_range(_First, _Last);
    _Fill_unchecked1(_Get_unwrapped(_First), _Get_unwrapped(_Last), _Val,
        bool_constant<_Fill_memset_is_safe<_Unwrapped_t<const _FwdIt&>, _Ty>>{});
}
#endif // _HAS_IF_CONSTEXPR

#if _HAS_CXX17
template <class _ExPo, class _FwdIt, class _Ty, _Enable_if_execution_policy_t<_ExPo> = 0>
void fill(_ExPo&&, _FwdIt _First, _FwdIt _Last, const _Ty& _Val) noexcept /* terminates */ {
    // copy _Val through [_First, _Last)
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt);
    return _STD fill(_First, _Last, _Val);
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE fill_n
#if _HAS_IF_CONSTEXPR
template <class _OutIt, class _Diff, class _Ty>
_CONSTEXPR20 _OutIt fill_n(_OutIt _Dest, const _Diff _Count_raw, const _Ty& _Val) {
    // copy _Val _Count times through [_Dest, ...)
    _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count) {
        auto _UDest = _Get_unwrapped_n(_Dest, _Count);
        if constexpr (_Fill_memset_is_safe<decltype(_UDest), _Ty>) {
#ifdef __cpp_lib_is_constant_evaluated
            if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
            {
                _CSTD memset(_UDest, static_cast<unsigned char>(_Val), static_cast<size_t>(_Count));
                _UDest += _Count;
                _Seek_wrapped(_Dest, _UDest);
                return _Dest;
            }
        }

        for (; 0 < _Count; --_Count, (void) ++_UDest) {
            *_UDest = _Val;
        }

        _Seek_wrapped(_Dest, _UDest);
    }

    return _Dest;
}

#else // ^^^ _HAS_IF_CONSTEXPR // !_HAS_IF_CONSTEXPR vvv
template <class _OutIt, class _Diff, class _Ty>
_OutIt _Fill_n_unchecked2(_OutIt _Dest, _Diff _Count, const _Ty& _Val, false_type) {
    // copy _Val _Count times through [_Dest, ...), no special optimization
    for (; 0 < _Count; --_Count, (void) ++_Dest) {
        *_Dest = _Val;
    }

    return _Dest;
}

template <class _OutIt, class _Diff, class _Ty>
_OutIt _Fill_n_unchecked2(_OutIt _Dest, _Diff _Count, const _Ty& _Val, true_type) {
    // copy _Val _Count times through [_Dest, ...), memset optimization
    _CSTD memset(_Dest, static_cast<unsigned char>(_Val), static_cast<size_t>(_Count));
    return _Dest + _Count;
}

template <class _OutIt, class _Diff, class _Ty>
_OutIt fill_n(_OutIt _Dest, const _Diff _Count_raw, const _Ty& _Val) {
    // copy _Val _Count times through [_Dest, ...)
    const _Algorithm_int_t<_Diff> _Count = _Count_raw;
    if (0 < _Count) {
        auto _UDest = _Get_unwrapped_n(_Dest, _Count);
        _Seek_wrapped(_Dest,
            _Fill_n_unchecked2(_UDest, _Count, _Val, bool_constant<_Fill_memset_is_safe<decltype(_UDest), _Ty>>{}));
    }

    return _Dest;
}
#endif // _HAS_IF_CONSTEXPR

#if _HAS_CXX17
template <class _ExPo, class _FwdIt, class _Diff, class _Ty, _Enable_if_execution_policy_t<_ExPo> = 0>
_FwdIt fill_n(_ExPo&&, _FwdIt _Dest, _Diff _Count_raw, const _Ty& _Val) noexcept /* terminates */ {
    // copy _Val _Count times through [_Dest, ...)
    // not parallelized as benchmarks show it isn't worth it
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt);
    return _STD fill_n(_Dest, _Count_raw, _Val);
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE equal
template <class _Elem1, class _Elem2>
struct _Value_equality_is_bitwise_equality : bool_constant<static_cast<_Elem1>(-1) == static_cast<_Elem2>(-1)> {
    // Tests whether the usual arithmetic conversions will preserve the bit-pattern when promoting to int
    // e.g. short == unsigned short -> false
    //      int == unsigned int -> true
};

template <class _Elem1, class _Elem2, class _Pr>
struct _Equal_memcmp_is_safe_helper : false_type {
    // determines whether it is safe to call memcmp to compare things; defaults to false
};

template <class _Elem1, class _Elem2>
struct _Equal_memcmp_is_safe_helper<_Elem1, _Elem2, equal_to<>>
    : bool_constant<conjunction_v<bool_constant<sizeof(_Elem1) == sizeof(_Elem2)
                                                && _Is_nonbool_integral<_Elem1> && _Is_nonbool_integral<_Elem2>>,
          negation<is_volatile<_Elem1>>, negation<is_volatile<_Elem2>>,
          // order matters here: being integral is a precondition of _Value_equality_is_bitwise_equality
          _Value_equality_is_bitwise_equality<_Elem1, _Elem2>>> {
    // allow memcmping same-size integral non-bool non-volatile bitwise types using equal_to<>
};

template <class _Elem1, class _Elem2>
struct _Equal_memcmp_is_safe_helper<_Elem1*, _Elem2*, equal_to<>>
    : is_same<remove_cv_t<_Elem1>, remove_cv_t<_Elem2>>::type {}; // allow memcmping pointers-to-cv-T with equal_to<>

#if _HAS_STD_BYTE
template <>
struct _Equal_memcmp_is_safe_helper<byte, byte, equal_to<>> : true_type {}; // allow memcmping std::byte
#endif // _HAS_STD_BYTE

template <class _Elem>
struct _Equal_memcmp_is_safe_helper<_Elem, _Elem, equal_to<_Elem>>
    : _Equal_memcmp_is_safe_helper<_Elem, _Elem, equal_to<>>::type {
    // treat equal_to with exact T as equal_to<> this is safe because we only activate the optimization for builtin
    // _Elem (and std::byte)
};

template <class _Iter1, class _Iter2, class _Pr>
false_type _Equal_memcmp_is_safe(const _Iter1&, const _Iter2&, const _Pr&) {
    // return equal optimization category for arbitrary iterators
    return {};
}

template <class _Obj1, class _Obj2, class _Pr>
typename _Equal_memcmp_is_safe_helper<remove_const_t<_Obj1>, remove_const_t<_Obj2>, _Pr>::type _Equal_memcmp_is_safe(
    _Obj1* const&, _Obj2* const&, const _Pr&) { // return equal optimization category for pointers
    return {};
}

#if _HAS_IF_CONSTEXPR
template <class _InIt1, class _InIt2, class _Pr>
_NODISCARD _CONSTEXPR20 bool equal(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, ...) using _Pred
    _Adl_verify_range(_First1, _Last1);
    auto _UFirst1      = _Get_unwrapped(_First1);
    const auto _ULast1 = _Get_unwrapped(_Last1);
    auto _UFirst2      = _Get_unwrapped_n(_First2, _Idl_distance<_InIt1>(_UFirst1, _ULast1));
    if constexpr (decltype(_Equal_memcmp_is_safe(_UFirst1, _UFirst2, _Pred))::value) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            const auto _First1_ch = reinterpret_cast<const char*>(_UFirst1);
            const auto _First2_ch = reinterpret_cast<const char*>(_UFirst2);
            const auto _Count     = static_cast<size_t>(reinterpret_cast<const char*>(_ULast1) - _First1_ch);
            return _CSTD memcmp(_First1_ch, _First2_ch, _Count) == 0;
        }
    }

    for (; _UFirst1 != _ULast1; ++_UFirst1, (void) ++_UFirst2) {
        if (!_Pred(*_UFirst1, *_UFirst2)) {
            return false;
        }
    }

    return true;
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt1, class _InIt2, class _Pr>
bool _Equal_unchecked1(_InIt1 _First1, const _InIt1 _Last1, _InIt2 _First2, _Pr _Pred, false_type) {
    // compare [_First1, _Last1) to [_First2, ...) using _Pred, no special optimization
    for (; _First1 != _Last1; ++_First1, (void) ++_First2) {
        if (!_Pred(*_First1, *_First2)) {
            return false;
        }
    }

    return true;
}

template <class _InIt1, class _InIt2, class _Pr>
bool _Equal_unchecked1(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, _Pr, true_type) {
    // compare [_First1, _Last1) to [_First2, ...), memcmp optimization
    const auto _First1_ch = reinterpret_cast<const char*>(_First1);
    const auto _First2_ch = reinterpret_cast<const char*>(_First2);
    const auto _Count     = static_cast<size_t>(reinterpret_cast<const char*>(_Last1) - _First1_ch);
    return _CSTD memcmp(_First1_ch, _First2_ch, _Count) == 0;
}

template <class _InIt1, class _InIt2, class _Pr>
bool _Equal_unchecked(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, ...) using _Pred, choose optimization
    return _Equal_unchecked1(_First1, _Last1, _First2, _Pred, _Equal_memcmp_is_safe(_First1, _First2, _Pred));
}

template <class _InIt1, class _InIt2, class _Pr>
_NODISCARD bool equal(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, ...) using _Pred
    _Adl_verify_range(_First1, _Last1);
    const auto _UFirst1 = _Get_unwrapped(_First1);
    const auto _ULast1  = _Get_unwrapped(_Last1);
    const auto _UFirst2 = _Get_unwrapped_n(_First2, _Idl_distance<_InIt1>(_UFirst1, _ULast1));
    return _Equal_unchecked(_UFirst1, _ULast1, _UFirst2, _Pass_fn(_Pred));
}
#endif // _HAS_IF_CONSTEXPR

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _InIt1, class _RightTy, size_t _RightSize, class _Pr, enable_if_t<!is_same_v<_RightTy*, _Pr>, int> = 0>
_NODISCARD _CONSTEXPR20 bool equal(
    const _InIt1 _First1, const _InIt1 _Last1, _RightTy (&_First2)[_RightSize], _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, ...) using _Pred
    return _STD equal(_First1, _Last1, _Array_iterator<_RightTy, _RightSize>(_First2), _Pass_fn(_Pred));
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _FwdIt2, class _Pr, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD bool equal(_ExPo&& _Exec, const _FwdIt1 _First1, const _FwdIt1 _Last1, const _FwdIt2 _First2,
    _Pr _Pred) noexcept; // terminates

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _ExPo, class _FwdIt1, class _RightTy, size_t _RightSize, class _Pr,
    enable_if_t<is_execution_policy_v<decay_t<_ExPo>> && !is_same_v<_RightTy*, _Pr>, int> /* = 0 */>
_NODISCARD bool equal(_ExPo&& _Exec, _FwdIt1 _First1, _FwdIt1 _Last1, _RightTy (&_First2)[_RightSize],
    _Pr _Pred) noexcept /* terminates */ {
    // compare [_First1, _Last1) to [_First2, ...) using _Pred
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    _Adl_verify_range(_First1, _Last1);
    return _STD equal(_STD forward<_ExPo>(_Exec), _Get_unwrapped(_First1), _Get_unwrapped(_Last1),
        _Array_iterator<_RightTy, _RightSize>(_First2), _Pass_fn(_Pred));
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS
#endif // _HAS_CXX17

template <class _InIt1, class _InIt2>
_NODISCARD _CONSTEXPR20 bool equal(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2) {
    // compare [_First1, _Last1) to [_First2, ...)
    return _STD equal(_First1, _Last1, _First2, equal_to<>());
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _InIt1, class _RightTy, size_t _RightSize>
_NODISCARD _CONSTEXPR20 bool equal(const _InIt1 _First1, const _InIt1 _Last1, _RightTy (&_First2)[_RightSize]) {
    // compare [_First1, _Last1) to [_First2, ...)
    return _STD equal(_First1, _Last1, _First2, equal_to<>());
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _FwdIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD bool equal(
    _ExPo&& _Exec, const _FwdIt1 _First1, const _FwdIt1 _Last1, const _FwdIt2 _First2) noexcept /* terminates */ {
    // compare [_First1, _Last1) to [_First2, ...)
    return _STD equal(_STD forward<_ExPo>(_Exec), _First1, _Last1, _First2, equal_to<>());
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _ExPo, class _FwdIt1, class _RightTy, size_t _RightSize, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD bool equal(_ExPo&& _Exec, const _FwdIt1 _First1, const _FwdIt1 _Last1,
    _RightTy (&_First2)[_RightSize]) noexcept /* terminates */ {
    // compare [_First1, _Last1) to [_First2, ...)
    return _STD equal(_STD forward<_ExPo>(_Exec), _First1, _Last1, _First2, equal_to<>());
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS
#endif // _HAS_CXX17

#if _HAS_IF_CONSTEXPR
template <class _InIt1, class _InIt2, class _Pr>
_NODISCARD _CONSTEXPR20 bool equal(
    const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, const _InIt2 _Last2, _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, _Last2) using _Pred
    _Adl_verify_range(_First1, _Last1);
    _Adl_verify_range(_First2, _Last2);
    auto _UFirst1      = _Get_unwrapped(_First1);
    const auto _ULast1 = _Get_unwrapped(_Last1);
    auto _UFirst2      = _Get_unwrapped(_First2);
    const auto _ULast2 = _Get_unwrapped(_Last2);
    if constexpr (_Is_random_iter_v<_InIt1> && _Is_random_iter_v<_InIt2>) {
        if (_ULast1 - _UFirst1 != _ULast2 - _UFirst2) {
            return false;
        }

        return _STD equal(_UFirst1, _ULast1, _UFirst2, _Pass_fn(_Pred));
    } else {
        for (;;) {
            if (_UFirst1 == _ULast1) {
                return _UFirst2 == _ULast2;
            }

            if (_UFirst2 == _ULast2) {
                return false;
            }

            if (!_Pred(*_UFirst1, *_UFirst2)) {
                return false;
            }

            ++_UFirst1;
            ++_UFirst2;
        }
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _InIt1, class _InIt2, class _Pr>
bool _Equal_unchecked(_InIt1 _First1, const _InIt1 _Last1, _InIt2 _First2, const _InIt2 _Last2, _Pr _Pred,
    input_iterator_tag, input_iterator_tag) {
    // compare [_First1, _Last1) to [_First2, _Last2) using _Pred, arbitrary iterators
    for (;;) {
        if (_First1 == _Last1) {
            return _First2 == _Last2;
        }

        if (_First2 == _Last2) {
            return false;
        }

        if (!_Pred(*_First1, *_First2)) {
            return false;
        }

        ++_First1;
        ++_First2;
    }
}

template <class _InIt1, class _InIt2, class _Pr>
bool _Equal_unchecked(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, const _InIt2 _Last2, _Pr _Pred,
    random_access_iterator_tag, random_access_iterator_tag) {
    // compare [_First1, _Last1) to [_First2, _Last2) using _Pred, random-access iterators
    if (_Last1 - _First1 != _Last2 - _First2) {
        return false;
    }

    return _Equal_unchecked(_First1, _Last1, _First2, _Pred);
}

template <class _InIt1, class _InIt2, class _Pr>
_NODISCARD bool equal(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, const _InIt2 _Last2, _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, _Last2) using _Pred
    _Adl_verify_range(_First1, _Last1);
    _Adl_verify_range(_First2, _Last2);
    return _Equal_unchecked(_Get_unwrapped(_First1), _Get_unwrapped(_Last1), _Get_unwrapped(_First2),
        _Get_unwrapped(_Last2), _Pass_fn(_Pred), _Iter_cat_t<_InIt1>(), _Iter_cat_t<_InIt2>());
}
#endif // _HAS_IF_CONSTEXPR

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _FwdIt2, class _Pr, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD bool equal(_ExPo&& _Exec, const _FwdIt1 _First1, const _FwdIt1 _Last1, const _FwdIt2 _First2,
    const _FwdIt2 _Last2, _Pr _Pred) noexcept; // terminates
#endif // _HAS_CXX17

template <class _InIt1, class _InIt2>
_NODISCARD _CONSTEXPR20 bool equal(
    const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, const _InIt2 _Last2) {
    // compare [_First1, _Last1) to [_First2, _Last2)
    return _STD equal(_First1, _Last1, _First2, _Last2, equal_to<>());
}

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _FwdIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD bool equal(_ExPo&& _Exec, const _FwdIt1 _First1, const _FwdIt1 _Last1, const _FwdIt2 _First2,
    const _FwdIt2 _Last2) noexcept /* terminates */ {
    // compare [_First1, _Last1) to [_First2, _Last2)
    return _STD equal(_STD forward<_ExPo>(_Exec), _First1, _Last1, _First2, _Last2, equal_to<>());
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE lexicographical_compare
template <class _Elem1, class _Elem2, class _FTy>
struct _Lex_compare_check_element_types_helper
    : bool_constant<
          conjunction_v<_Is_character<_Elem1>, _Is_character<_Elem2>, _Is_character<_FTy>, is_unsigned<_FTy>>> {
    // checks the lex_compare element types for memcmp safety for builtin functors (e.g., less<unsigned char>)
};

template <class _Elem1, class _Elem2>
struct _Lex_compare_check_element_types_helper<_Elem1, _Elem2, void>
    : bool_constant<
          conjunction_v<_Is_character<_Elem1>, _Is_character<_Elem2>, is_unsigned<_Elem1>, is_unsigned<_Elem2>>> {
    // checks the lex_compare element types for memcmp safety for transparent functors (e.g. less<>)
};

#if _HAS_STD_BYTE
template <>
struct _Lex_compare_check_element_types_helper<byte, byte, byte> : true_type {
    // std::byte with builtin functors (e.g. less<byte>) is memcmp safe
};

template <>
struct _Lex_compare_check_element_types_helper<byte, byte, void> : true_type {
    // std::byte with transparent functors (e.g. less<>) is memcmp safe
};
#endif // _HAS_STD_BYTE

template <class _Memcmp_pr>
struct _Lex_compare_optimize {}; // optimization tag for lexicographical_compare

template <class _Memcmp_pr, class _Obj1, class _Obj2, class _FTy>
using _Lex_compare_check_element_types = _Lex_compare_optimize<conditional_t<
    _Lex_compare_check_element_types_helper<remove_const_t<_Obj1>, remove_const_t<_Obj2>, _FTy>::value, _Memcmp_pr,
    void>>; // checks the lex_compare element types for memcmp safety

template <class _InIt1, class _InIt2, class _Pr>
constexpr _Lex_compare_optimize<void> _Lex_compare_memcmp_classify(const _InIt1&, const _InIt2&, const _Pr&) {
    // return lex_compare optimization category for arbitrary iterators
    return {};
}

template <class _Obj1, class _Obj2, class _FTy>
constexpr _Lex_compare_check_element_types<less<int>, _Obj1, _Obj2, _FTy> _Lex_compare_memcmp_classify(
    _Obj1* const&, _Obj2* const&, const less<_FTy>&) {
    // return lex_compare optimization category for pointer iterators and less<_FTy>
    return {};
}

template <class _Obj1, class _Obj2, class _FTy>
constexpr _Lex_compare_check_element_types<greater<int>, _Obj1, _Obj2, _FTy> _Lex_compare_memcmp_classify(
    _Obj1* const&, _Obj2* const&, const greater<_FTy>&) {
    // return lex_compare optimization category for pointer iterators and greater<_FTy>
    return {};
}

template <class _InIt1, class _InIt2, class _Pr>
_NODISCARD constexpr bool _Lex_compare_unchecked(
    _InIt1 _First1, _InIt1 _Last1, _InIt2 _First2, _InIt2 _Last2, _Pr _Pred, _Lex_compare_optimize<void>) {
    // order [_First1, _Last1) vs. [_First2, _Last2) using _Pred, no special optimization
    for (; _First1 != _Last1 && _First2 != _Last2; ++_First1, (void) ++_First2) { // something to compare, do it
        if (_DEBUG_LT_PRED(_Pred, *_First1, *_First2)) {
            return true;
        } else if (_Pred(*_First2, *_First1)) {
            return false;
        }
    }

    return _First1 == _Last1 && _First2 != _Last2;
}

template <class _InIt1, class _InIt2, class _Pr, class _Memcmp_pr>
_NODISCARD _CONSTEXPR20 bool _Lex_compare_unchecked(
    _InIt1 _First1, _InIt1 _Last1, _InIt2 _First2, _InIt2 _Last2, _Pr _Pred, _Lex_compare_optimize<_Memcmp_pr>) {
    // order [_First1, _Last1) vs. [_First2, _Last2) memcmp optimization
#ifdef __cpp_lib_is_constant_evaluated
    if (_STD is_constant_evaluated()) {
        return _Lex_compare_unchecked(_First1, _Last1, _First2, _Last2, _Pred, _Lex_compare_optimize<void>{});
    }
#endif // __cpp_lib_is_constant_evaluated
    (void) _Pred;
    const auto _Num1 = static_cast<size_t>(_Last1 - _First1);
    const auto _Num2 = static_cast<size_t>(_Last2 - _First2);
    const int _Ans   = _CSTD memcmp(_First1, _First2, _Num1 < _Num2 ? _Num1 : _Num2);
    return _Memcmp_pr{}(_Ans, 0) || (_Ans == 0 && _Num1 < _Num2);
}

template <class _InIt1, class _InIt2, class _Pr>
_NODISCARD _CONSTEXPR20 bool lexicographical_compare(
    _InIt1 _First1, _InIt1 _Last1, _InIt2 _First2, _InIt2 _Last2, _Pr _Pred) {
    // order [_First1, _Last1) vs. [_First2, _Last2) using _Pred
    _Adl_verify_range(_First1, _Last1);
    _Adl_verify_range(_First2, _Last2);
    const auto _UFirst1 = _Get_unwrapped(_First1);
    const auto _ULast1  = _Get_unwrapped(_Last1);
    const auto _UFirst2 = _Get_unwrapped(_First2);
    const auto _ULast2  = _Get_unwrapped(_Last2);
    return _Lex_compare_unchecked(
        _UFirst1, _ULast1, _UFirst2, _ULast2, _Pass_fn(_Pred), _Lex_compare_memcmp_classify(_UFirst1, _UFirst2, _Pred));
}

template <class _InIt1, class _InIt2>
_NODISCARD _CONSTEXPR20 bool lexicographical_compare(_InIt1 _First1, _InIt1 _Last1, _InIt2 _First2, _InIt2 _Last2) {
    // order [_First1, _Last1) vs. [_First2, _Last2)
    return _STD lexicographical_compare(_First1, _Last1, _First2, _Last2, less<>());
}

#if _HAS_CXX17
template <class _ExPo, class _FwdIt1, class _FwdIt2, class _Pr, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD bool lexicographical_compare(
    _ExPo&&, _FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _FwdIt2 _Last2, _Pr _Pred) noexcept /* terminates */ {
    // order [_First1, _Last1) vs. [_First2, _Last2) using _Pred
    // not parallelized at present, parallelism expected to be feasible in a future release
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt2);
    return _STD lexicographical_compare(_First1, _Last1, _First2, _Last2, _Pass_fn(_Pred));
}

template <class _ExPo, class _FwdIt1, class _FwdIt2, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD bool lexicographical_compare(
    _ExPo&&, _FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _FwdIt2 _Last2) noexcept /* terminates */ {
    // order [_First1, _Last1) vs. [_First2, _Last2)
    // not parallelized at present, parallelism expected to be feasible in a future release
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt1);
    _REQUIRE_PARALLEL_ITERATOR(_FwdIt2);
    return _STD lexicographical_compare(_First1, _Last1, _First2, _Last2);
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE find
template <class _Ty>
_NODISCARD constexpr bool _Within_limits(const _Ty& _Val, true_type, true_type, _Any_tag) { // signed _Elem, signed _Ty
    return SCHAR_MIN <= _Val && _Val <= SCHAR_MAX;
}

template <class _Ty>
_NODISCARD constexpr bool _Within_limits(const _Ty& _Val, true_type, false_type, true_type) {
    // signed _Elem, unsigned _Ty, -1 == static_cast<_Ty>(-1)
    return _Val <= SCHAR_MAX || static_cast<_Ty>(SCHAR_MIN) <= _Val;
}

template <class _Ty>
_NODISCARD constexpr bool _Within_limits(const _Ty& _Val, true_type, false_type, false_type) {
    // signed _Elem, unsigned _Ty, -1 != static_cast<_Ty>(-1)
    return _Val <= SCHAR_MAX;
}

template <class _Ty>
_NODISCARD constexpr bool _Within_limits(const _Ty& _Val, false_type, true_type, _Any_tag) {
    // unsigned _Elem, signed _Ty
    return 0 <= _Val && _Val <= UCHAR_MAX;
}

template <class _Ty>
_NODISCARD constexpr bool _Within_limits(const _Ty& _Val, false_type, false_type, _Any_tag) {
    // unsigned _Elem, unsigned _Ty
    return _Val <= UCHAR_MAX;
}

template <class _InIt, class _Ty>
_NODISCARD constexpr bool _Within_limits(_InIt, const _Ty& _Val) { // check whether _Val is within the limits of _Elem
    using _Elem = remove_pointer_t<_InIt>;
    return _Within_limits(_Val, is_signed<_Elem>{}, is_signed<_Ty>{}, bool_constant<-1 == static_cast<_Ty>(-1)>{});
}

template <class _InIt>
_NODISCARD constexpr bool _Within_limits(_InIt, const bool&) { // bools are always within the limits of _Elem
    return true;
}

template <class _InIt, class _Ty>
_NODISCARD constexpr _InIt _Find_unchecked1(_InIt _First, const _InIt _Last, const _Ty& _Val, false_type) {
    // find first matching _Val
    for (; _First != _Last; ++_First) {
        if (*_First == _Val) {
            break;
        }
    }

    return _First;
}

template <class _InIt, class _Ty>
_NODISCARD _CONSTEXPR20 _InIt _Find_unchecked1(_InIt _First, const _InIt _Last, const _Ty& _Val, true_type) {
    // find first byte matching integral _Val
    if (!_Within_limits(_First, _Val)) {
        return _Last;
    }

#ifdef __cpp_lib_is_constant_evaluated
    if (_STD is_constant_evaluated()) {
        return _Find_unchecked1(_First, _Last, _Val, false_type{});
    }
#endif // __cpp_lib_is_constant_evaluated
    _First =
        static_cast<_InIt>(_CSTD memchr(_First, static_cast<unsigned char>(_Val), static_cast<size_t>(_Last - _First)));
    return _First ? _First : _Last;
}

template <class _InIt, class _Ty>
_NODISCARD _CONSTEXPR20 _InIt _Find_unchecked(const _InIt _First, const _InIt _Last, const _Ty& _Val) {
    // find first matching _Val; choose optimization
    // activate optimization for pointers to (const) bytes and integral values
    using _Memchr_opt = bool_constant<
        is_integral_v<_Ty> && _Is_any_of_v<_InIt, char*, signed char*, unsigned char*, //
            const char*, const signed char*, const unsigned char*>>;

    return _Find_unchecked1(_First, _Last, _Val, _Memchr_opt{});
}

template <class _InIt, class _Ty>
_NODISCARD _CONSTEXPR20 _InIt find(_InIt _First, const _InIt _Last, const _Ty& _Val) { // find first matching _Val
    _Adl_verify_range(_First, _Last);
    _Seek_wrapped(_First, _Find_unchecked(_Get_unwrapped(_First), _Get_unwrapped(_Last), _Val));
    return _First;
}

#if _HAS_CXX17
template <class _ExPo, class _FwdIt, class _Ty, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD _FwdIt find(_ExPo&& _Exec, _FwdIt _First, const _FwdIt _Last, const _Ty& _Val) noexcept; // terminates
#endif // _HAS_CXX17

// FUNCTION TEMPLATE count
template <class _InIt, class _Ty>
_NODISCARD _CONSTEXPR20 _Iter_diff_t<_InIt> count(const _InIt _First, const _InIt _Last, const _Ty& _Val) {
    // count elements that match _Val
    _Adl_verify_range(_First, _Last);
    auto _UFirst               = _Get_unwrapped(_First);
    const auto _ULast          = _Get_unwrapped(_Last);
    _Iter_diff_t<_InIt> _Count = 0;

    for (; _UFirst != _ULast; ++_UFirst) {
        if (*_UFirst == _Val) {
            ++_Count;
        }
    }

    return _Count;
}

#if _HAS_CXX17
template <class _ExPo, class _FwdIt, class _Ty, _Enable_if_execution_policy_t<_ExPo> = 0>
_NODISCARD _Iter_diff_t<_FwdIt> count(
    _ExPo&& _Exec, const _FwdIt _First, const _FwdIt _Last, const _Ty& _Val) noexcept; // terminates
#endif // _HAS_CXX17

// FUNCTION TEMPLATE is_permutation
template <class _InIt, class _Ty, class _Pr>
_NODISCARD constexpr _InIt _Find_pr(_InIt _First, const _InIt _Last, const _Ty& _Val, _Pr _Pred) {
    for (; _First != _Last; ++_First) {
        if (_Pred(*_First, _Val)) {
            break;
        }
    }

    return _First;
}

template <class _InIt, class _Ty, class _Pr>
_NODISCARD constexpr _Iter_diff_t<_InIt> _Count_pr(_InIt _First, const _InIt _Last, const _Ty& _Val, _Pr _Pred) {
    _Iter_diff_t<_InIt> _Count = 0;

    for (; _First != _Last; ++_First) {
        if (_Pred(*_First, _Val)) {
            ++_Count;
        }
    }

    return _Count;
}

#if !_HAS_IF_CONSTEXPR
template <class _FwdIt1, class _FwdIt2, class _Pr>
void _Trim_matching_suffixes(_FwdIt1&, _FwdIt2&, _Pr, forward_iterator_tag, forward_iterator_tag) {
    // trim matching suffixes, forward iterators (do nothing)
}

template <class _FwdIt1, class _FwdIt2, class _Pr>
void _Trim_matching_suffixes(
    _FwdIt1& _Last1, _FwdIt2& _Last2, _Pr _Pred, bidirectional_iterator_tag, bidirectional_iterator_tag) {
    // trim matching suffixes, bidirectional iterators
    // assumptions: same lengths, non-empty, !_Pred(*_First1, *_First2)
    do { // find last inequality
        --_Last1;
        --_Last2;
    } while (_Pred(*_Last1, *_Last2));
    ++_Last1;
    ++_Last2;
}
#endif // !_HAS_IF_CONSTEXPR

template <class _FwdIt1, class _FwdIt2, class _Pr>
_NODISCARD _CONSTEXPR20 bool _Check_match_counts(
    const _FwdIt1 _First1, _FwdIt1 _Last1, const _FwdIt2 _First2, _FwdIt2 _Last2, _Pr _Pred) {
    // test if [_First1, _Last1) == permuted [_First2, _Last2), after matching prefix removal
    _STL_INTERNAL_CHECK(!_Pred(*_First1, *_First2));
    _STL_INTERNAL_CHECK(_STD distance(_First1, _Last1) == _STD distance(_First2, _Last2));
#if _HAS_IF_CONSTEXPR
    if constexpr (_Is_bidi_iter_v<_FwdIt1> && _Is_bidi_iter_v<_FwdIt2>) {
        do { // find last inequality
            --_Last1;
            --_Last2;
        } while (_Pred(*_Last1, *_Last2));
        ++_Last1;
        ++_Last2;
    }
#else // ^^^ _HAS_IF_CONSTEXPR // !_HAS_IF_CONSTEXPR vvv
    _Trim_matching_suffixes(_Last1, _Last2, _Pred, _Iter_cat_t<_FwdIt1>(), _Iter_cat_t<_FwdIt2>());
#endif // _HAS_IF_CONSTEXPR
    for (_FwdIt1 _Next1 = _First1; _Next1 != _Last1; ++_Next1) {
        if (_Next1 == _Find_pr(_First1, _Next1, *_Next1, _Pred)) { // new value, compare match counts
            _Iter_diff_t<_FwdIt2> _Count2 = _Count_pr(_First2, _Last2, *_Next1, _Pred);
            if (_Count2 == 0) {
                return false; // second range lacks value, not a permutation
            }

            _FwdIt1 _Skip1                = _Next_iter(_Next1);
            _Iter_diff_t<_FwdIt1> _Count1 = _Count_pr(_Skip1, _Last1, *_Next1, _Pred) + 1;
            if (_Count2 != _Count1) {
                return false; // match counts differ, not a permutation
            }
        }
    }

    return true;
}

template <class _FwdIt1, class _FwdIt2, class _Pr>
_NODISCARD _CONSTEXPR20 bool _Is_permutation_unchecked(_FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _Pr _Pred) {
    // test if [_First1, _Last1) == permuted [_First2, ...), using _Pred
    for (; _First1 != _Last1; ++_First1, (void) ++_First2) { // trim matching prefix
        if (!_Pred(*_First1, *_First2)) {
            // found first inequality, check match counts in suffix
            //
            // narrowing _Iter_diff_t<_FwdIt1> to _Iter_diff_t<_FwdIt2> is OK because if the second range is shorter
            // than the first, the user already triggered UB
            auto _Last2 = _STD next(_First2, static_cast<_Iter_diff_t<_FwdIt2>>(_STD distance(_First1, _Last1)));
            return _Check_match_counts(_First1, _Last1, _First2, _Last2, _Pred);
        }
    }

    return true;
}

template <class _FwdIt1, class _FwdIt2, class _Pr>
_NODISCARD _CONSTEXPR20 bool is_permutation(_FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _Pr _Pred) {
    // test if [_First1, _Last1) == permuted [_First2, ...), using _Pred
    _Adl_verify_range(_First1, _Last1);
    const auto _UFirst1 = _Get_unwrapped(_First1);
    const auto _ULast1  = _Get_unwrapped(_Last1);
    const auto _UFirst2 = _Get_unwrapped_n(_First2, _Idl_distance<_FwdIt1>(_UFirst1, _ULast1));
    return _Is_permutation_unchecked(_UFirst1, _ULast1, _UFirst2, _Pass_fn(_Pred));
}

#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _FwdIt1, class _RightTy, size_t _RightSize, class _Pr, enable_if_t<!is_same_v<_RightTy*, _Pr>, int> = 0>
_NODISCARD _CONSTEXPR20 bool is_permutation(
    _FwdIt1 _First1, _FwdIt1 _Last1, _RightTy (&_First2)[_RightSize], _Pr _Pred) {
    // test if [_First1, _Last1) == permuted [_First2, ...), using _Pred
    return _STD is_permutation(_First1, _Last1, _Array_iterator<_RightTy, _RightSize>(_First2), _Pass_fn(_Pred));
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS

template <class _FwdIt1, class _FwdIt2>
_NODISCARD _CONSTEXPR20 bool is_permutation(_FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2) {
    // test if [_First1, _Last1) == permuted [_First2, ...)
    return _STD is_permutation(_First1, _Last1, _First2, equal_to<>());
}


#if _ITERATOR_DEBUG_ARRAY_OVERLOADS
template <class _FwdIt1, class _RightTy, size_t _RightSize>
_NODISCARD _CONSTEXPR20 bool is_permutation(_FwdIt1 _First1, _FwdIt1 _Last1, _RightTy (&_First2)[_RightSize]) {
    // test if [_First1, _Last1) == permuted [_First2, ...)
    return _STD is_permutation(_First1, _Last1, _First2, equal_to<>());
}
#endif // _ITERATOR_DEBUG_ARRAY_OVERLOADS

template <class _FwdIt1, class _FwdIt2, class _Pr>
_CONSTEXPR20 bool _Is_permutation_unchecked(_FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _FwdIt2 _Last2, _Pr _Pred,
    forward_iterator_tag, forward_iterator_tag) {
    // test if [_First1, _Last1) == permuted [_First2, _Last2), using _Pred, arbitrary iterators
    for (;;) { // trim matching prefix
        if (_First1 == _Last1) {
            return _First2 == _Last2;
        }

        if (_First2 == _Last2) {
            return false;
        }

        if (!_Pred(*_First1, *_First2)) { // found first inequality, check match counts in suffix
            break;
        }

        ++_First1;
        ++_First2;
    }

    auto _Next1 = _First1;
    auto _Next2 = _First2;
    for (;;) { // check for same lengths
        if (_Next1 == _Last1) {
            if (_Next2 == _Last2) {
                return _Check_match_counts(_First1, _Last1, _First2, _Last2, _Pred);
            }

            return false; // sequence 1 is shorter than sequence 2, not a permutation
        }

        if (_Next2 == _Last2) {
            return false; // sequence 1 is longer than sequence 2, not a permutation
        }

        ++_Next1;
        ++_Next2;
    }
}

template <class _FwdIt1, class _FwdIt2, class _Pr>
_CONSTEXPR20 bool _Is_permutation_unchecked(_FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _FwdIt2 _Last2, _Pr _Pred,
    random_access_iterator_tag, random_access_iterator_tag) {
    // test if [_First1, _Last1) == permuted [_First2, _Last2), using _Pred, random-access iterators
    if (_Last1 - _First1 != _Last2 - _First2) {
        return false;
    }

    for (; _First1 != _Last1; ++_First1, (void) ++_First2) { // trim matching prefix
        if (!_Pred(*_First1, *_First2)) {
            // found first inequality, check match counts in suffix
            return _Check_match_counts(_First1, _Last1, _First2, _Last2, _Pred);
        }
    }

    return true;
}

template <class _FwdIt1, class _FwdIt2, class _Pr>
_NODISCARD _CONSTEXPR20 bool is_permutation(
    _FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _FwdIt2 _Last2, _Pr _Pred) {
    // test if [_First1, _Last1) == permuted [_First2, _Last2), using _Pred
    _Adl_verify_range(_First1, _Last1);
    _Adl_verify_range(_First2, _Last2);
    return _Is_permutation_unchecked(_Get_unwrapped(_First1), _Get_unwrapped(_Last1), _Get_unwrapped(_First2),
        _Get_unwrapped(_Last2), _Pass_fn(_Pred), _Iter_cat_t<_FwdIt1>(), _Iter_cat_t<_FwdIt2>());
}

template <class _FwdIt1, class _FwdIt2>
_NODISCARD _CONSTEXPR20 bool is_permutation(_FwdIt1 _First1, _FwdIt1 _Last1, _FwdIt2 _First2, _FwdIt2 _Last2) {
    // test if [_First1, _Last1) == permuted [_First2, _Last2)
    return _STD is_permutation(_First1, _Last1, _First2, _Last2, equal_to<>());
}

// FUNCTION TEMPLATE reverse
#if _HAS_IF_CONSTEXPR
template <class _BidIt>
_CONSTEXPR20 void reverse(const _BidIt _First, const _BidIt _Last) { // reverse elements in [_First, _Last)
    _Adl_verify_range(_First, _Last);
    auto _UFirst = _Get_unwrapped(_First);
    auto _ULast  = _Get_unwrapped(_Last);
#if (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE) && !defined(_M_HYBRID)
    using _Elem = remove_pointer_t<decltype(_UFirst)>;
    constexpr bool _Allow_vectorization =
        conjunction_v<is_pointer<decltype(_UFirst)>, _Is_trivially_swappable<_Elem>, negation<is_volatile<_Elem>>>;

    if constexpr (_Allow_vectorization && sizeof(_Elem) == 1) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            __std_reverse_trivially_swappable_1(_UFirst, _ULast);
            return;
        }
    } else if constexpr (_Allow_vectorization && sizeof(_Elem) == 2) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            __std_reverse_trivially_swappable_2(_UFirst, _ULast);
            return;
        }
    } else if constexpr (_Allow_vectorization && sizeof(_Elem) == 4) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            __std_reverse_trivially_swappable_4(_UFirst, _ULast);
            return;
        }
    } else if constexpr (_Allow_vectorization && sizeof(_Elem) == 8) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            __std_reverse_trivially_swappable_8(_UFirst, _ULast);
            return;
        }
    }
#endif // (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE) && !defined(_M_HYBRID)

    for (; _UFirst != _ULast && _UFirst != --_ULast; ++_UFirst) {
        _STD iter_swap(_UFirst, _ULast);
    }
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _BidIt>
void _Reverse_unchecked1(_BidIt _First, _BidIt _Last, integral_constant<size_t, 0>) {
    // reverse elements in [_First, _Last), general bidirectional iterators
    for (; _First != _Last && _First != --_Last; ++_First) {
        _STD iter_swap(_First, _Last);
    }
}

#if (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE) && !defined(_M_HYBRID)
template <class _BidIt>
void _Reverse_unchecked1(const _BidIt _First, const _BidIt _Last, integral_constant<size_t, 1>) {
    // reverse elements in [_First, _Last), pointers to trivially swappable of size 1
    __std_reverse_trivially_swappable_1(_First, _Last);
}

template <class _BidIt>
void _Reverse_unchecked1(const _BidIt _First, const _BidIt _Last, integral_constant<size_t, 2>) {
    // reverse elements in [_First, _Last), pointers to trivially swappable of size 2
    __std_reverse_trivially_swappable_2(_First, _Last);
}

template <class _BidIt>
void _Reverse_unchecked1(const _BidIt _First, const _BidIt _Last, integral_constant<size_t, 4>) {
    // reverse elements in [_First, _Last), pointers to trivially swappable of size 4
    __std_reverse_trivially_swappable_4(_First, _Last);
}

template <class _BidIt>
void _Reverse_unchecked1(const _BidIt _First, const _BidIt _Last, integral_constant<size_t, 8>) {
    // reverse elements in [_First, _Last), pointers to trivially swappable of size 8
    __std_reverse_trivially_swappable_8(_First, _Last);
}
#endif // (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE) && !defined(_M_HYBRID)

template <class _BidIt>
void _Reverse_unchecked(const _BidIt _First, const _BidIt _Last) {
    // reverse elements in [_First, _Last), choose optimization
#if (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE) && !defined(_M_HYBRID)
    using _Elem = remove_pointer_t<_BidIt>;
    constexpr size_t _Opt =
        is_pointer_v<_BidIt> //
                && _Is_trivially_swappable_v<_Elem> //
                && !is_volatile_v<_Elem> //
                && (sizeof(_Elem) == 1 || sizeof(_Elem) == 2 || sizeof(_Elem) == 4 || sizeof(_Elem) == 8)
            ? sizeof(_Elem)
            : 0;
#else // ^^^ vectorize / no vectorize vvv
    constexpr size_t _Opt = 0;
#endif // (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE_PURE) && !defined(_M_HYBRID)
    _Reverse_unchecked1(_First, _Last, integral_constant<size_t, _Opt>{});
}

template <class _BidIt>
void reverse(const _BidIt _First, const _BidIt _Last) {
    // reverse elements in [_First, _Last)
    _Adl_verify_range(_First, _Last);
    _Reverse_unchecked(_Get_unwrapped(_First), _Get_unwrapped(_Last));
}
#endif // _HAS_IF_CONSTEXPR

#if _HAS_CXX17
template <class _ExPo, class _BidIt, _Enable_if_execution_policy_t<_ExPo> = 0>
void reverse(_ExPo&&, _BidIt _First, _BidIt _Last) noexcept /* terminates */ {
    // reverse elements in [_First, _Last)
    // not parallelized as benchmarks show it isn't worth it
    return _STD reverse(_First, _Last);
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE rotate
template <class _BidIt>
constexpr pair<_BidIt, _BidIt> _Reverse_until_sentinel_unchecked(_BidIt _First, _BidIt _Sentinel, _BidIt _Last) {
    // reverse until either _First or _Last hits _Sentinel
    while (_First != _Sentinel && _Last != _Sentinel) {
        _STD iter_swap(_First, --_Last);
        ++_First;
    }

    return pair<_BidIt, _BidIt>(_First, _Last);
}

#if _HAS_IF_CONSTEXPR
template <class _FwdIt>
_CONSTEXPR20 _FwdIt rotate(_FwdIt _First, _FwdIt _Mid, _FwdIt _Last) {
    // exchange the ranges [_First, _Mid) and [_Mid, _Last)
    // that is, rotates [_First, _Last) left by distance(_First, _Mid) positions
    // returns the iterator pointing at *_First's new home
    _Adl_verify_range(_First, _Mid);
    _Adl_verify_range(_Mid, _Last);
    auto _UFirst      = _Get_unwrapped(_First);
    auto _UMid        = _Get_unwrapped(_Mid);
    const auto _ULast = _Get_unwrapped(_Last);
    if (_UFirst == _UMid) {
        return _Last;
    }

    if (_UMid == _ULast) {
        return _First;
    }

    if constexpr (_Is_random_iter_v<_FwdIt>) {
        _STD reverse(_UFirst, _UMid);
        _STD reverse(_UMid, _ULast);
        _STD reverse(_UFirst, _ULast);
        _Seek_wrapped(_First, _UFirst + (_ULast - _UMid));
    } else if constexpr (_Is_bidi_iter_v<_FwdIt>) {
        _STD reverse(_UFirst, _UMid);
        _STD reverse(_UMid, _ULast);
        auto _Tmp = _Reverse_until_sentinel_unchecked(_UFirst, _UMid, _ULast);
        _STD reverse(_Tmp.first, _Tmp.second);
        _Seek_wrapped(_First, _UMid != _Tmp.first ? _Tmp.first : _Tmp.second);
    } else {
        auto _UNext = _UMid;
        do { // rotate the first cycle
            _STD iter_swap(_UFirst, _UNext);
            ++_UFirst;
            ++_UNext;
            if (_UFirst == _UMid) {
                _UMid = _UNext;
            }
        } while (_UNext != _ULast);
        _Seek_wrapped(_First, _UFirst);
        while (_UMid != _ULast) { // rotate subsequent cycles
            _UNext = _UMid;
            do {
                _STD iter_swap(_UFirst, _UNext);
                ++_UFirst;
                ++_UNext;
                if (_UFirst == _UMid) {
                    _UMid = _UNext;
                }
            } while (_UNext != _ULast);
        }
    }

    return _First;
}
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <class _FwdIt>
_FwdIt _Rotate_unchecked1(_FwdIt _First, _FwdIt _Mid, _FwdIt _Last, forward_iterator_tag) {
    // rotate [_First, _Last) left by distance(_First, _Mid) positions, forward iterators
    for (_FwdIt _Next = _Mid, _Res = _Last;;) { // swap [_First, ...) into place
        _STD iter_swap(_First, _Next);
        if (++_First == _Mid) { // quit if done, else define next interval
            if (++_Next == _Last) {
                return _Res == _Last ? _Mid : _Res;
            } else {
                _Mid = _Next; // mark end of next interval
            }
        } else if (++_Next == _Last) { // wrap to last end
            if (_Res == _Last) {
                _Res = _First;
            }

            _Next = _Mid;
        }
    }
}

template <class _BidIt>
_BidIt _Rotate_unchecked1(_BidIt _First, _BidIt _Mid, _BidIt _Last, bidirectional_iterator_tag) {
    // rotate [_First, _Last) left by distance(_First, _Mid) positions, bidirectional iterators
    _Reverse_unchecked(_First, _Mid);
    _Reverse_unchecked(_Mid, _Last);
    auto _Tmp = _Reverse_until_sentinel_unchecked(_First, _Mid, _Last);
    _Reverse_unchecked(_Tmp.first, _Tmp.second);
    return _Mid != _Tmp.first ? _Tmp.first : _Tmp.second;
}

template <class _RanIt>
_RanIt _Rotate_unchecked1(_RanIt _First, _RanIt _Mid, _RanIt _Last, random_access_iterator_tag) {
    // rotate [_First, _Last) left by distance(_First, _Mid) positions, random-access iterators
    _Reverse_unchecked(_First, _Mid);
    _Reverse_unchecked(_Mid, _Last);
    _Reverse_unchecked(_First, _Last);
    return _First + (_Last - _Mid);
}

template <class _FwdIt>
_FwdIt _Rotate_unchecked(_FwdIt _First, _FwdIt _Mid, _FwdIt _Last) {
    // rotate [_First, _Last) left by distance(_First, _Mid) positions
    if (_First == _Mid) {
        return _Last;
    }

    if (_Mid == _Last) {
        return _First;
    }

    return _Rotate_unchecked1(_First, _Mid, _Last, _Iter_cat_t<_FwdIt>());
}

template <class _FwdIt>
_FwdIt rotate(_FwdIt _First, _FwdIt _Mid, _FwdIt _Last) {
    // exchange the ranges [_First, _Mid) and [_Mid, _Last)
    // that is, rotates [_First, _Last) left by distance(_First, _Mid) positions
    // returns the iterator pointing at *_First's new home
    _Adl_verify_range(_First, _Mid);
    _Adl_verify_range(_Mid, _Last);
    _Seek_wrapped(_First, _Rotate_unchecked(_Get_unwrapped(_First), _Get_unwrapped(_Mid), _Get_unwrapped(_Last)));
    return _First;
}
#endif // _HAS_IF_CONSTEXPR

#if _HAS_CXX17
template <class _ExPo, class _FwdIt, _Enable_if_execution_policy_t<_ExPo> = 0>
_FwdIt rotate(_ExPo&&, _FwdIt _First, _FwdIt _Mid, _FwdIt _Last) noexcept /* terminates */ {
    // rotate [_First, _Last) left by distance(_First, _Mid) positions
    // not parallelized as benchmarks show it isn't worth it
    return _STD rotate(_First, _Mid, _Last);
}
#endif // _HAS_CXX17

// FUNCTION TEMPLATE find_if
template <class _InIt, class _Pr>
_NODISCARD _CONSTEXPR20 _InIt find_if(_InIt _First, const _InIt _Last, _Pr _Pred) { // find first satisfying _Pred
    _Adl_verify_range(_First, _Last);
    auto _UFirst      = _Get_unwrapped(_First);
    const auto _ULast = _Get_unwrapped(_Last);
    for (; _UFirst != _ULast; ++_UFirst) {
        if (_Pred(*_UFirst)) {
            break;
        }
    }

    _Seek_wrapped(_First, _UFirst);
    return _First;
}

// FUNCTION TEMPLATE lower_bound
template <class _FwdIt, class _Ty, class _Pr>
_NODISCARD _CONSTEXPR20 _FwdIt lower_bound(_FwdIt _First, const _FwdIt _Last, const _Ty& _Val, _Pr _Pred) {
    // find first element not before _Val, using _Pred
    _Adl_verify_range(_First, _Last);
    auto _UFirst                = _Get_unwrapped(_First);
    _Iter_diff_t<_FwdIt> _Count = _STD distance(_UFirst, _Get_unwrapped(_Last));

    while (0 < _Count) { // divide and conquer, find half that contains answer
        const _Iter_diff_t<_FwdIt> _Count2 = _Count / 2;
        const auto _UMid                   = _STD next(_UFirst, _Count2);
        if (_Pred(*_UMid, _Val)) { // try top half
            _UFirst = _Next_iter(_UMid);
            _Count -= _Count2 + 1;
        } else {
            _Count = _Count2;
        }
    }

    _Seek_wrapped(_First, _UFirst);
    return _First;
}

template <class _FwdIt, class _Ty>
_NODISCARD _CONSTEXPR20 _FwdIt lower_bound(_FwdIt _First, _FwdIt _Last, const _Ty& _Val) {
    // find first element not before _Val, using operator<
    return _STD lower_bound(_First, _Last, _Val, less<>());
}

// CLASS TEMPLATE _Rng_from_urng
template <class _Diff, class _Urng>
class _Rng_from_urng { // wrap a URNG as an RNG
public:
    using _Ty0 = make_unsigned_t<_Diff>;
    using _Ty1 = typename _Urng::result_type;

    using _Udiff = conditional_t<sizeof(_Ty1) < sizeof(_Ty0), _Ty0, _Ty1>;

    explicit _Rng_from_urng(_Urng& _Func) : _Ref(_Func), _Bits(CHAR_BIT * sizeof(_Udiff)), _Bmask(_Udiff(-1)) {
        for (; (_Urng::max)() - (_Urng::min)() < _Bmask; _Bmask >>= 1) {
            --_Bits;
        }
    }

    _Diff operator()(_Diff _Index) { // adapt _Urng closed range to [0, _Index)
        for (;;) { // try a sample random value
            _Udiff _Ret  = 0; // random bits
            _Udiff _Mask = 0; // 2^N - 1, _Ret is within [0, _Mask]

            while (_Mask < _Udiff(_Index - 1)) { // need more random bits
                _Ret <<= _Bits - 1; // avoid full shift
                _Ret <<= 1;
                _Ret |= _Get_bits();
                _Mask <<= _Bits - 1; // avoid full shift
                _Mask <<= 1;
                _Mask |= _Bmask;
            }

            // _Ret is [0, _Mask], _Index - 1 <= _Mask, return if unbiased
            if (_Ret / _Index < _Mask / _Index || _Mask % _Index == _Udiff(_Index - 1)) {
                return static_cast<_Diff>(_Ret % _Index);
            }
        }
    }

    _Udiff _Get_all_bits() {
        _Udiff _Ret = 0;

        for (size_t _Num = 0; _Num < CHAR_BIT * sizeof(_Udiff); _Num += _Bits) { // don't mask away any bits
            _Ret <<= _Bits - 1; // avoid full shift
            _Ret <<= 1;
            _Ret |= _Get_bits();
        }

        return _Ret;
    }

    _Rng_from_urng(const _Rng_from_urng&) = delete;
    _Rng_from_urng& operator=(const _Rng_from_urng&) = delete;

private:
    _Udiff _Get_bits() { // return a random value within [0, _Bmask]
        for (;;) { // repeat until random value is in range
            _Udiff _Val = _Ref() - (_Urng::min)();

            if (_Val <= _Bmask) {
                return _Val;
            }
        }
    }

    _Urng& _Ref; // reference to URNG
    size_t _Bits; // number of random bits generated by _Get_bits()
    _Udiff _Bmask; // 2^_Bits - 1
};

// CLASS TEMPLATE _Yarn
template <class _Elem>
class _CRTIMP2_PURE_IMPORT _Yarn { // wrap a NTCTS
public:
    __CLR_OR_THIS_CALL _Yarn() noexcept : _Myptr(nullptr), _Nul(0) {}

    __CLR_OR_THIS_CALL _Yarn(const _Yarn& _Right) noexcept : _Myptr(nullptr), _Nul(0) {
        *this = _Right;
    }

    __CLR_OR_THIS_CALL _Yarn(const _Elem* _Right) noexcept : _Myptr(nullptr), _Nul(0) {
        *this = _Right;
    }

    _Yarn& __CLR_OR_THIS_CALL operator=(const _Yarn& _Right) noexcept {
        return *this = _Right._Myptr;
    }

    _Yarn& __CLR_OR_THIS_CALL operator=(const _Elem* _Right) noexcept {
        if (_Myptr != _Right) { // new value, discard old and copy new
            _Tidy();

            if (_Right) { // new is not empty, copy it
                const _Elem* _Ptr = _Right;
                while (*_Ptr != _Elem{}) {
                    ++_Ptr;
                }

                const auto _Count = (++_Ptr - _Right) * sizeof(_Elem);

#ifdef _DEBUG
                _Myptr = static_cast<_Elem*>(_malloc_dbg(_Count, _CRT_BLOCK, __FILE__, __LINE__));
#else // _DEBUG
                _Myptr = static_cast<_Elem*>(_CSTD malloc(_Count));
#endif // _DEBUG

                if (_Myptr) {
                    _CSTD memcpy(_Myptr, _Right, _Count);
                }
            }
        }

        return *this;
    }

    __CLR_OR_THIS_CALL ~_Yarn() noexcept {
        _Tidy();
    }

    _NODISCARD bool __CLR_OR_THIS_CALL empty() const noexcept {
        return _Myptr == nullptr;
    }

    _Ret_z_ const _Elem* __CLR_OR_THIS_CALL c_str() const noexcept {
        return _Myptr ? _Myptr : &_Nul;
    }

    _NODISCARD bool __CLR_OR_THIS_CALL _Empty() const noexcept {
        return _Myptr == nullptr;
    }

    _Ret_z_ const _Elem* __CLR_OR_THIS_CALL _C_str() const noexcept {
        return _Myptr ? _Myptr : &_Nul;
    }

private:
    void __CLR_OR_THIS_CALL _Tidy() noexcept {
        if (_Myptr) {
#ifdef _DEBUG
            _free_dbg(_Myptr, _CRT_BLOCK);
#else // _DEBUG
            _CSTD free(_Myptr);
#endif // _DEBUG
        }

        _Myptr = nullptr;
    }

    _Elem* _Myptr; // pointer to allocated string
    _Elem _Nul; // nul terminator for unallocated string
};


// CLASS TEMPLATE back_insert_iterator
template <class _Container>
class back_insert_iterator { // wrap pushes to back of container as output iterator
public:
    using iterator_category = output_iterator_tag;
    using value_type        = void;
    using difference_type   = void;
    using pointer           = void;
    using reference         = void;

    using container_type = _Container;

    explicit back_insert_iterator(_Container& _Cont) noexcept /* strengthened */ : container(_STD addressof(_Cont)) {}

    back_insert_iterator& operator=(const typename _Container::value_type& _Val) {
        container->push_back(_Val);
        return *this;
    }

    back_insert_iterator& operator=(typename _Container::value_type&& _Val) {
        container->push_back(_STD move(_Val));
        return *this;
    }

    _NODISCARD back_insert_iterator& operator*() noexcept /* strengthened */ {
        return *this;
    }

    back_insert_iterator& operator++() noexcept /* strengthened */ {
        return *this;
    }

    back_insert_iterator operator++(int) noexcept /* strengthened */ {
        return *this;
    }

protected:
    _Container* container; // pointer to container
};

// FUNCTION TEMPLATE back_inserter
template <class _Container>
_NODISCARD back_insert_iterator<_Container> back_inserter(_Container& _Cont) noexcept /* strengthened */ {
    // return a back_insert_iterator
    return back_insert_iterator<_Container>(_Cont);
}


// STRUCT TEMPLATE _Has_allocator_type
template <class _Ty, class _Alloc, class = void>
struct _Has_allocator_type : false_type { // tests for suitable _Ty::allocator_type
};

template <class _Ty, class _Alloc>
struct _Has_allocator_type<_Ty, _Alloc, void_t<typename _Ty::allocator_type>>
    : is_convertible<_Alloc, typename _Ty::allocator_type>::type { // tests for suitable _Ty::allocator_type
};

// STRUCT allocator_arg_t
struct allocator_arg_t { // tag type for added allocator argument
    explicit allocator_arg_t() = default;
};

_INLINE_VAR constexpr allocator_arg_t allocator_arg{};

[[noreturn]] _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _Xbad_alloc();
[[noreturn]] _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _Xinvalid_argument(_In_z_ const char*);
[[noreturn]] _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _Xlength_error(_In_z_ const char*);
[[noreturn]] _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _Xout_of_range(_In_z_ const char*);
[[noreturn]] _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _Xoverflow_error(_In_z_ const char*);
[[noreturn]] _CRTIMP2_PURE void __CLRCALL_PURE_OR_CDECL _Xruntime_error(_In_z_ const char*);

// STRUCT TEMPLATE uses_allocator
template <class _Ty, class _Alloc>
struct uses_allocator
    : _Has_allocator_type<_Ty, _Alloc>::type { // determine whether _Ty has an allocator_type member type
};

template <class _Ty, class _Alloc>
_INLINE_VAR constexpr bool uses_allocator_v = uses_allocator<_Ty, _Alloc>::value;

// STRUCT TEMPLATE iterator
template <class _Category, class _Ty, class _Diff = ptrdiff_t, class _Pointer = _Ty*, class _Reference = _Ty&>
struct _CXX17_DEPRECATE_ITERATOR_BASE_CLASS iterator { // base type for iterator classes
    using iterator_category = _Category;
    using value_type        = _Ty;
    using difference_type   = _Diff;
    using pointer           = _Pointer;
    using reference         = _Reference;
};

// STRUCT TEMPLATE _Tidy_guard
template <class _Ty>
struct _Tidy_guard { // class with destructor that calls _Tidy
    _Ty* _Target;
    ~_Tidy_guard() {
        if (_Target) {
            _Target->_Tidy();
        }
    }
};

// STRUCT TEMPLATE _Tidy_deallocate_guard
template <class _Ty>
struct _Tidy_deallocate_guard { // class with destructor that calls _Tidy_deallocate
    _Ty* _Target;
    ~_Tidy_deallocate_guard() {
        if (_Target) {
            _Target->_Tidy_deallocate();
        }
    }
};

// VARIABLE TEMPLATE _Nothrow_compare
template <class _Keycmp, class _Lhs, class _Rhs>
_INLINE_VAR constexpr bool _Nothrow_compare = noexcept(
    static_cast<bool>(_STD declval<const _Keycmp&>()(_STD declval<const _Lhs&>(), _STD declval<const _Rhs&>())));
_STD_END
#pragma pop_macro("new")
_STL_RESTORE_CLANG_WARNINGS
#pragma warning(pop)
#pragma pack(pop)
#endif // _STL_COMPILER_PREPROCESSOR
#endif // _XUTILITY_