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) && !defined(_M_HYBRID)
#define _USE_STD_VECTOR_ALGORITHMS 1
#else
#define _USE_STD_VECTOR_ALGORITHMS 0
#endif

#ifdef __CUDACC__
#define _CONSTEXPR_BIT_CAST inline
#else // ^^^ workaround ^^^ / vvv no workaround vvv
#define _CONSTEXPR_BIT_CAST constexpr
#endif // ^^^ no workaround ^^^

#if _USE_STD_VECTOR_ALGORITHMS
_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;
__declspec(noalias) void __cdecl __std_swap_ranges_trivially_swappable_noalias(
    void* _First1, void* _Last1, void* _First2) noexcept;
_END_EXTERN_C
#endif // _USE_STD_VECTOR_ALGORITHMS

_STD_BEGIN

// FUNCTION TEMPLATE _Bit_cast
template <class _To, class _From,
    enable_if_t<conjunction_v<bool_constant<sizeof(_To) == sizeof(_From)>, is_trivially_copyable<_To>,
                    is_trivially_copyable<_From>>,
        int> = 0>
_NODISCARD _CONSTEXPR_BIT_CAST _To _Bit_cast(const _From& _Val) noexcept {
#ifdef __CUDACC__
    _To _To_obj; // assumes default-init
    _CSTD memcpy(_STD addressof(_To_obj), _STD addressof(_Val), sizeof(_To));
    return _To_obj;
#else // ^^^ workaround ^^^ / vvv no workaround vvv
    return __builtin_bit_cast(_To, _Val);
#endif // ^^^ no workaround ^^^
}

// 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;
}

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
    }
}

// STRUCT identity
struct identity {
    template <class _Ty>
    _NODISCARD constexpr _Ty&& operator()(_Ty&& _Left) const noexcept {
        return _STD forward<_Ty>(_Left);
    }

    using is_transparent = int;
};
#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
#if _HAS_IF_CONSTEXPR
        if constexpr (is_member_pointer_v<_Fx>) {
            return _STD invoke(_Fn, _STD forward<_Args>(_Vals)...);
        } else
#endif // _HAS_IF_CONSTEXPR
        {
            return _Fn(_STD forward<_Args>(_Vals)...);
        }
    }

    _Fx& _Fn;
};

template <class _Fn>
_INLINE_VAR constexpr bool _Pass_functor_by_value_v = conjunction_v<bool_constant<sizeof(_Fn) <= sizeof(void*)>,
    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<remove_cvref_t<_Ty>>>,
    incrementable_traits<remove_cvref_t<_Ty>>, iterator_traits<remove_cvref_t<_Ty>>>::difference_type;

// STRUCT TEMPLATE indirectly_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 indirectly_readable_traits {};

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

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

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

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

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

// clang-format off
template <_Has_member_value_type _Ty> // Per LWG-3446
    requires _Has_member_element_type<_Ty>
        && same_as<remove_cv_t<typename _Ty::value_type>, remove_cv_t<typename _Ty::element_type>>
struct indirectly_readable_traits<_Ty> : _Cond_value_type<typename _Ty::value_type> {};
// clang-format on

// ALIAS TEMPLATE iter_value_t
template <class _Ty>
using iter_value_t = typename conditional_t<_Is_from_primary<iterator_traits<remove_cvref_t<_Ty>>>,
    indirectly_readable_traits<remove_cvref_t<_Ty>>, iterator_traits<remove_cvref_t<_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 = // per LWG-3420
    requires(_It __i) {
        { *__i } -> _Can_reference;
        { ++__i } -> same_as<_It&>;
        { *__i++ } -> _Can_reference;
    }
    && copyable<_It>;

template <class _It>
concept _Cpp17_input_iterator = _Cpp17_iterator<_It>
    && equality_comparable<_It>
    && _Has_member_difference_type<incrementable_traits<_It>>
    && _Has_member_value_type<indirectly_readable_traits<_It>>
    && requires(_It __i) {
        typename common_reference_t<iter_reference_t<_It>&&, typename indirectly_readable_traits<_It>::value_type&>;
        typename common_reference_t<decltype(*__i++)&&, typename indirectly_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 indirectly_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 indirectly_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 name lookup

        // clang-format off
        template <class _Ty>
        concept _Has_ADL = _Has_class_or_enum_type<_Ty> && 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
    } // namespace _Iter_move

    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 indirectly_readable
template <class _It>
concept _Indirectly_readable_impl = requires(const _It __i) {
    typename iter_value_t<_It>;
    typename iter_reference_t<_It>;
    typename iter_rvalue_reference_t<_It>;
    { *__i } -> same_as<iter_reference_t<_It>>;
    { _RANGES iter_move(__i) } -> same_as<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>&>;

template <class _It>
concept indirectly_readable = _Indirectly_readable_impl<remove_cvref_t<_It>>;
// clang-format on

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

// CONCEPT indirectly_writable
template <class _It, class _Ty>
concept indirectly_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 _Integer_like
// clang-format off
template <class _Ty>
concept _Integer_like = _Is_nonbool_integral<_Ty>; // per the proposed resolution of LWG-3467

// CONCEPT _Signed_integer_like
template <class _Ty>
concept _Signed_integer_like = _Integer_like<_Ty> && static_cast<_Ty>(-1) < static_cast<_Ty>(0);
// clang-format on

// ALIAS TEMPLATE _Make_unsigned_like_t
template <class _Ty>
using _Make_unsigned_like_t = make_unsigned_t<_Ty>;

// ALIAS TEMPLATE _Make_signed_like_t
template <class _Ty>
using _Make_signed_like_t = make_signed_t<_Ty>; // per the proposed resolution of LWG-3403

// 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_integer_like<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_for
template <class _Se, class _It>
inline constexpr bool disable_sized_sentinel_for = false;

// CONCEPT sized_sentinel_for
// clang-format off
template <class _Se, class _It>
concept sized_sentinel_for = sentinel_for<_Se, _It>
    && !disable_sized_sentinel_for<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 <bool _Iterator_category_present>
struct _Iter_concept_impl2 {
    template <class _It, class _Traits>
    using _Apply = typename _Traits::iterator_category;
};
template <>
struct _Iter_concept_impl2<false> {
    // clang-format off
    template <class _It, class _Traits>
        requires _Is_from_primary<iterator_traits<_It>>
    using _Apply = random_access_iterator_tag;
    // clang-format on
};

template <bool _Iterator_concept_present>
struct _Iter_concept_impl1 {
    template <class _It, class _Traits>
    using _Apply = typename _Traits::iterator_concept;
};
template <>
struct _Iter_concept_impl1<false> {
    template <class _It, class _Traits>
    using _Apply = typename _Iter_concept_impl2<_Has_member_iterator_category<_Traits>>::template _Apply<_It, _Traits>;
};

template <class _It, class _Traits = conditional_t<_Is_from_primary<iterator_traits<_It>>, _It, iterator_traits<_It>>>
using _Iter_concept =
    typename _Iter_concept_impl1<_Has_member_iterator_concept<_Traits>>::template _Apply<_It, _Traits>;

// clang-format off
// CONCEPT input_iterator
template <class _It>
concept input_iterator = input_or_output_iterator<_It> && indirectly_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> && indirectly_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_unary_invocable
template <class _Fn, class _It>
concept indirectly_unary_invocable = indirectly_readable<_It>
    && copy_constructible<_Fn>
    && invocable<_Fn&, iter_value_t<_It>&>
    && invocable<_Fn&, iter_reference_t<_It>>
    && invocable<_Fn&, iter_common_reference_t<_It>>
    && common_reference_with<
        invoke_result_t<_Fn&, iter_value_t<_It>&>,
        invoke_result_t<_Fn&, iter_reference_t<_It>>>;

// CONCEPT indirectly_regular_unary_invocable
template <class _Fn, class _It>
concept indirectly_regular_unary_invocable = indirectly_readable<_It>
    && copy_constructible<_Fn>
    && regular_invocable<_Fn&, iter_value_t<_It>&>
    && regular_invocable<_Fn&, iter_reference_t<_It>>
    && regular_invocable<_Fn&, iter_common_reference_t<_It>>
    && common_reference_with<
        invoke_result_t<_Fn&, iter_value_t<_It>&>,
        invoke_result_t<_Fn&, iter_reference_t<_It>>>;

// CONCEPT indirect_unary_predicate
template <class _Fn, class _It>
concept indirect_unary_predicate = indirectly_readable<_It>
    && copy_constructible<_Fn>
    && predicate<_Fn&, iter_value_t<_It>&>
    && predicate<_Fn&, iter_reference_t<_It>>
    && predicate<_Fn&, iter_common_reference_t<_It>>;

// CONCEPT indirect_binary_predicate
template <class _Fn, class _It1, class _It2>
concept indirect_binary_predicate = indirectly_readable<_It1>
    && indirectly_readable<_It2>
    && copy_constructible<_Fn>
    && predicate<_Fn&, iter_value_t<_It1>&, iter_value_t<_It2>&>
    && predicate<_Fn&, iter_value_t<_It1>&, iter_reference_t<_It2>>
    && predicate<_Fn&, iter_reference_t<_It1>, iter_value_t<_It2>&>
    && predicate<_Fn&, iter_reference_t<_It1>, iter_reference_t<_It2>>
    && predicate<_Fn&, iter_common_reference_t<_It1>, iter_common_reference_t<_It2>>;

// CONCEPT indirect_equivalence_relation
template <class _Fn, class _It1, class _It2 = _It1>
concept indirect_equivalence_relation = indirectly_readable<_It1>
    && indirectly_readable<_It2>
    && copy_constructible<_Fn>
    && equivalence_relation<_Fn&, iter_value_t<_It1>&, iter_value_t<_It2>&>
    && equivalence_relation<_Fn&, iter_value_t<_It1>&, iter_reference_t<_It2>>
    && equivalence_relation<_Fn&, iter_reference_t<_It1>, iter_value_t<_It2>&>
    && equivalence_relation<_Fn&, iter_reference_t<_It1>, iter_reference_t<_It2>>
    && equivalence_relation<_Fn&, iter_common_reference_t<_It1>, iter_common_reference_t<_It2>>;

// CONCEPT indirect_strict_weak_order
template <class _Fn, class _It1, class _It2 = _It1>
concept indirect_strict_weak_order = indirectly_readable<_It1>
    && indirectly_readable<_It2>
    && copy_constructible<_Fn>
    && strict_weak_order<_Fn&, iter_value_t<_It1>&, iter_value_t<_It2>&>
    && strict_weak_order<_Fn&, iter_value_t<_It1>&, iter_reference_t<_It2>>
    && strict_weak_order<_Fn&, iter_reference_t<_It1>, iter_value_t<_It2>&>
    && strict_weak_order<_Fn&, iter_reference_t<_It1>, iter_reference_t<_It2>>
    && strict_weak_order<_Fn&, iter_common_reference_t<_It1>, iter_common_reference_t<_It2>>;

// ALIAS TEMPLATE indirect_result_t
template <class _Fn, class... _Its>
    requires (indirectly_readable<_Its> && ...)
        && invocable<_Fn, iter_reference_t<_Its>...>
using indirect_result_t = invoke_result_t<_Fn, iter_reference_t<_Its>...>;
// clang-format on

// STRUCT TEMPLATE projected
#pragma warning(push)
#pragma warning(disable : 5046) // '%s': Symbol involving type with internal linkage not defined
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wundefined-internal" // function '%s' has internal linkage but is not defined
#endif // __clang__

template <indirectly_readable _It, indirectly_regular_unary_invocable<_It> _Proj>
struct projected {
    using value_type = remove_cvref_t<indirect_result_t<_Proj&, _It>>;
    indirect_result_t<_Proj&, _It> operator*() const;
};

#ifdef __clang__
#pragma clang diagnostic pop
#endif // __clang__
#pragma warning(pop)

template <weakly_incrementable _It, class _Proj>
struct incrementable_traits<projected<_It, _Proj>> {
    using difference_type = iter_difference_t<_It>;
};

// clang-format off
// CONCEPT indirectly_movable
template <class _In, class _Out>
concept indirectly_movable = indirectly_readable<_In> && indirectly_writable<_Out, iter_rvalue_reference_t<_In>>;

// CONCEPT indirectly_movable_storable
template <class _In, class _Out>
concept indirectly_movable_storable = indirectly_movable<_In, _Out>
    && indirectly_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>>;

// CONCEPT indirectly_copyable
template <class _In, class _Out>
concept indirectly_copyable = indirectly_readable<_In> && indirectly_writable<_Out, iter_reference_t<_In>>;

// CONCEPT indirectly_copyable_storable
template <class _In, class _Out>
concept indirectly_copyable_storable = indirectly_copyable<_In, _Out>
    && indirectly_writable<_Out, iter_value_t<_In>&>
    && indirectly_writable<_Out, const iter_value_t<_In>&>
    && indirectly_writable<_Out, iter_value_t<_In>&&>
    && indirectly_writable<_Out, const iter_value_t<_In>&&>
    && copyable<iter_value_t<_In>>
    && constructible_from<iter_value_t<_In>, iter_reference_t<_In>>
    && assignable_from<iter_value_t<_In>&, iter_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>)
            && requires(_Ty1&& __t1, _Ty2&& __t2) {
                iter_swap(static_cast<_Ty1&&>(__t1), static_cast<_Ty2&&>(__t2));
            };

        template <class _Ty1, class _Ty2>
        concept _Can_swap_references = indirectly_readable<remove_reference_t<_Ty1>>
            && indirectly_readable<remove_reference_t<_Ty2>>
            && swappable_with<iter_reference_t<_Ty1>, iter_reference_t<_Ty2>>;

        template <class _Ty1, class _Ty2>
        concept _Symmetric_indirectly_movable_storable =
               indirectly_movable_storable<remove_reference_t<_Ty1>, remove_reference_t<_Ty2>>
            && indirectly_movable_storable<remove_reference_t<_Ty2>, remove_reference_t<_Ty1>>;
        // 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>>(_RANGES iter_move(_XVal)))) {
            iter_value_t<remove_reference_t<_Xty>> _Tmp(_RANGES iter_move(_XVal));
            *_XVal = _RANGES iter_move(_YVal);
            return _Tmp;
        }

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

            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(_RANGES swap(*_STD declval<_Ty1>(), *_STD declval<_Ty2>()))};
                } else if constexpr (_Symmetric_indirectly_movable_storable<_Ty1, _Ty2>) {
                    constexpr auto _Nothrow = noexcept(
                        *_STD declval<_Ty1>() = _Iter_exchange_move(_STD declval<_Ty2>(), _STD declval<_Ty1>()));
                    return {_St::_Exchange, _Nothrow};
                } else {
                    return {_St::_None};
                }
            }

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

        public:
            // clang-format off
            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) {
                    _RANGES 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<_Ty2&&>(_Val2), static_cast<_Ty1&&>(_Val1));
                } 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

// clang-format off
// CONCEPT indirectly_swappable
template <class _It1, class _It2 = _It1>
concept indirectly_swappable = indirectly_readable<_It1> && indirectly_readable<_It2>
    && requires(const _It1 __i1, const _It2 __i2) {
        _RANGES iter_swap(__i1, __i1);
        _RANGES iter_swap(__i2, __i2);
        _RANGES iter_swap(__i1, __i2);
        _RANGES iter_swap(__i2, __i1);
    };

// CONCEPT indirectly_comparable
template <class _It1, class _It2, class _Rel, class _Proj1 = identity, class _Proj2 = identity>
concept indirectly_comparable =
    indirect_binary_predicate<_Rel,
        projected<_It1, _Proj1>,
        projected<_It2, _Proj2>>;

// CONCEPT permutable
template <class _It>
concept permutable = forward_iterator<_It>
    && indirectly_movable_storable<_It, _It>
    && indirectly_swappable<_It, _It>;

// CONCEPT mergeable
namespace ranges { struct less; }
template <class _It1, class _It2, class _Out, class _Pr = ranges::less, class _Pj1 = identity, class _Pj2 = identity>
concept mergeable = input_iterator<_It1> && input_iterator<_It2>
    && weakly_incrementable<_Out>
    && indirectly_copyable<_It1, _Out>
    && indirectly_copyable<_It2, _Out>
    && indirect_strict_weak_order<_Pr, projected<_It1, _Pj1>, projected<_It2, _Pj2>>;

// CONCEPT sortable
template <class _It, class _Pr = ranges::less, class _Proj = identity>
concept sortable = permutable<_It> && indirect_strict_weak_order<_Pr, projected<_It, _Proj>>;
// clang-format on

// 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 // ^^^ _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) {
    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 {
        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 // ^^^ _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 // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
template <size_t _Value>
struct _Priority_tag : _Priority_tag<_Value - 1> { // priority tag for tag dispatch
    explicit _Priority_tag() = default;
};

template <>
struct _Priority_tag<0> { // base case priority tag for tag dispatch
    explicit _Priority_tag() = default;
};

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");
        }

        decltype(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");

    decltype(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
    decltype(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;
    }

    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
#if !_HAS_IF_CONSTEXPR
template <class _Iter>
_NODISCARD constexpr _Iter _Operator_arrow(true_type, _Iter _Target) {
    return _Target;
}

template <class _Iter>
_NODISCARD constexpr decltype(auto) _Operator_arrow(false_type, _Iter& _Target) {
    return _Target.operator->();
}
#endif // !_HAS_IF_CONSTEXPR

template <class _BidIt>
class reverse_iterator {
public:
    using iterator_type = _BidIt;

#ifdef __cpp_lib_concepts
    using iterator_concept =
        conditional_t<random_access_iterator<_BidIt>, random_access_iterator_tag, bidirectional_iterator_tag>;
    using iterator_category = conditional_t<derived_from<_Iter_cat_t<_BidIt>, random_access_iterator_tag>,
        random_access_iterator_tag, _Iter_cat_t<_BidIt>>;
#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
    using iterator_category = _Iter_cat_t<_BidIt>;
#endif // __cpp_lib_concepts
    using value_type      = _Iter_value_t<_BidIt>;
    using difference_type = _Iter_diff_t<_BidIt>;
    using pointer         = typename iterator_traits<_BidIt>::pointer;
    using reference       = _Iter_ref_t<_BidIt>;

    template <class>
    friend class reverse_iterator;

    _CONSTEXPR17 reverse_iterator() = default;

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

    // clang-format off
    template <class _Other>
#ifdef __cpp_lib_concepts
        // Per LWG-3435
        requires (!is_same_v<_Other, _BidIt>) && convertible_to<const _Other&, _BidIt>
#endif // __cpp_lib_concepts
    _CONSTEXPR17 reverse_iterator(const reverse_iterator<_Other>& _Right) noexcept(
        is_nothrow_constructible_v<_BidIt, const _Other&>) // strengthened
        : current(_Right.current) {}

    template <class _Other>
#ifdef __cpp_lib_concepts
        // Per LWG-3435
        requires (!is_same_v<_Other, _BidIt>) && convertible_to<const _Other&, _BidIt>
            && assignable_from<_BidIt&, const _Other&>
#endif // __cpp_lib_concepts
    _CONSTEXPR17 reverse_iterator& operator=(const reverse_iterator<_Other>& _Right) {
        current = _Right.current;
        return *this;
    }
    // clang-format on

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

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

#ifdef __cpp_lib_concepts
    // clang-format off
    _NODISCARD constexpr pointer operator->() const
        requires (is_pointer_v<_BidIt> || requires(const _BidIt __i) { __i.operator->(); }) {
        _BidIt _Tmp = current;
        --_Tmp;
        if constexpr (is_pointer_v<_BidIt>) {
            return _Tmp;
        } else {
            return _Tmp.operator->();
        }
    }
    // clang-format on
#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
    _NODISCARD _CONSTEXPR17 pointer operator->() const {
        _BidIt _Tmp = current;
        --_Tmp;
#if _HAS_IF_CONSTEXPR
        if constexpr (is_pointer_v<_BidIt>) {
            return _Tmp;
        } else {
            return _Tmp.operator->();
        }
#else // ^^^ _HAS_IF_CONSTEXPR / !_HAS_IF_CONSTEXPR vvv
        return _Operator_arrow(is_pointer<_BidIt>{}, _Tmp);
#endif // _HAS_IF_CONSTEXPR
    }
#endif // __cpp_lib_concepts

    _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;
    }

    _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);
    }

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

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

#ifdef __cpp_lib_concepts
    _NODISCARD friend constexpr iter_rvalue_reference_t<_BidIt> iter_move(const reverse_iterator& _It) noexcept(
        is_nothrow_copy_constructible_v<_BidIt>&& noexcept(_RANGES iter_move(--_STD declval<_BidIt&>()))) {
        auto _Tmp = _It.current;
        --_Tmp;
        return _RANGES iter_move(_Tmp);
    }

    template <indirectly_swappable<_BidIt> _BidIt2>
    friend constexpr void iter_swap(const reverse_iterator& _Left, const reverse_iterator<_BidIt2>& _Right) noexcept(
        is_nothrow_copy_constructible_v<_BidIt>&& is_nothrow_copy_constructible_v<_BidIt2>&& noexcept(
            _RANGES iter_swap(--_STD declval<_BidIt&>(), --_STD declval<_BidIt2&>()))) {
        auto _LTmp = _Left.current;
        auto _RTmp = _Right.base();
        --_LTmp;
        --_RTmp;
        _RANGES iter_swap(_LTmp, _RTmp);
    }
#endif // __cpp_lib_concepts

    using _Prevent_inheriting_unwrap = reverse_iterator;

    template <class _BidIt2, enable_if_t<_Range_verifiable_v<_BidIt, _BidIt2>, int> = 0>
    friend constexpr void _Verify_range(const reverse_iterator& _First, const reverse_iterator<_BidIt2>& _Last) {
        _Verify_range(_Last._Get_current(), _First.current); // note reversed parameters
    }

    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, const _Src&>, int> = 0>
    constexpr void _Seek_to(const reverse_iterator<_Src>& _It) {
        current._Seek_to(_It.current);
    }

    _NODISCARD constexpr const _BidIt& _Get_current() const noexcept {
        return current;
    }

protected:
    _BidIt current{};
};

#ifdef __cpp_lib_concepts
template <class _From, class _To>
concept _Implicitly_convertible_to = is_convertible_v<_From, _To>;
#endif // __cpp_lib_concepts

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator==(const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left._Get_current() == _Right._Get_current() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left._Get_current() == _Right._Get_current(); }

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator!=(const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left._Get_current() != _Right._Get_current() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left._Get_current() != _Right._Get_current(); }

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator<(const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left._Get_current() > _Right._Get_current() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left._Get_current() > _Right._Get_current(); }

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator>(const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left._Get_current() < _Right._Get_current() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left._Get_current() < _Right._Get_current(); }

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator<=(const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left._Get_current() >= _Right._Get_current() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left._Get_current() >= _Right._Get_current(); }

template <class _BidIt1, class _BidIt2>
_NODISCARD _CONSTEXPR17 bool operator>=(const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left._Get_current() <= _Right._Get_current() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left._Get_current() <= _Right._Get_current(); }

#ifdef __cpp_lib_concepts
template <class _BidIt1, three_way_comparable_with<_BidIt1> _BidIt2>
_NODISCARD constexpr compare_three_way_result_t<_BidIt1, _BidIt2> operator<=>(
    const reverse_iterator<_BidIt1>& _Left, const reverse_iterator<_BidIt2>& _Right) {
    return _Right._Get_current() <=> _Left._Get_current();
}
#endif // __cpp_lib_concepts

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

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

// 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));
}

#ifdef __cpp_lib_concepts
// clang-format off
template <class _BidIt1, class _BidIt2>
    requires (!sized_sentinel_for<_BidIt1, _BidIt2>)
inline constexpr bool disable_sized_sentinel_for<reverse_iterator<_BidIt1>, reverse_iterator<_BidIt2>> = true;
// clang-format on
#endif // __cpp_lib_concepts

// 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_borrowed_range
    template <class>
    inline constexpr bool enable_borrowed_range = false;

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

    // CUSTOMIZATION POINT OBJECT ranges::begin
    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, std::ranges::rend, "
                        "and std::ranges::data 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
    template <class _Ty>
    using iterator_t = decltype(_RANGES begin(_STD declval<_Ty&>()));

    // CUSTOMIZATION POINT OBJECT ranges::_Ubegin
    namespace _Unchecked_begin {
        // clang-format off
        template <class _Ty>
        concept _Has_member = requires(_Ty& __t) {
            { __t._Unchecked_begin() } -> input_or_output_iterator;
        };

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

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

            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>) {
                    _STL_INTERNAL_STATIC_ASSERT(
                        same_as<decltype(_STD declval<_Ty>()._Unchecked_begin()), _Unwrapped_t<iterator_t<_Ty>>>);
                    return {_St::_Member, noexcept(_STD declval<_Ty>()._Unchecked_begin())};
                } else if constexpr (_Can_begin<_Ty>) {
                    return {_St::_Unwrap, noexcept(_Get_unwrapped(_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._Unchecked_begin();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Unwrap) {
                    return _Get_unwrapped(_RANGES begin(_Val));
                } else {
                    static_assert(_Always_false<_Ty>, "Should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Unchecked_begin

    inline namespace _Cpos {
        inline constexpr _Unchecked_begin::_Cpo _Ubegin;
    }

    // CUSTOMIZATION POINT OBJECT ranges::end
    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, std::ranges::rend, "
                        "and std::ranges::data 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;
    }

    // CUSTOMIZATION POINT OBJECT ranges::_Uend
    namespace _Unchecked_end {
        // clang-format off
        template <class _Ty>
        concept _Has_member = _Unchecked_begin::_Has_member<_Ty> && requires(_Ty& __t) {
            __t._Unchecked_begin(); // required explicitly for better diagnostics
            { __t._Unchecked_end() } -> sentinel_for<decltype(__t._Unchecked_begin())>;
        };

        template <class _Ty>
        concept _Can_end = requires(_Ty& __t) {
            _Get_unwrapped(_RANGES end(__t));
        };
        // clang-format on

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

            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>) {
                    _STL_INTERNAL_STATIC_ASSERT(same_as<decltype(_STD declval<_Ty>()._Unchecked_end()),
                        decltype(_Get_unwrapped(_RANGES end(_STD declval<_Ty>())))>);
                    return {_St::_Member, noexcept(_STD declval<_Ty>()._Unchecked_end())};
                } else if constexpr (_Can_end<_Ty>) {
                    return {_St::_Unwrap, noexcept(_Get_unwrapped(_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._Unchecked_end();
                } else if constexpr (_Choice<_Ty&>._Strategy == _St::_Unwrap) {
                    return _Get_unwrapped(_RANGES end(_Val));
                } else {
                    static_assert(_Always_false<_Ty>, "Should be unreachable");
                }
            }
            // clang-format on
        };
    } // namespace _Unchecked_end

    inline namespace _Cpos {
        inline constexpr _Unchecked_end::_Cpo _Uend;
    }

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

    // CONCEPT ranges::borrowed_range
    // clang-format off
    template <class _Rng>
    concept borrowed_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
    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
    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
    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()) } -> _Integer_like;
        };

        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)) } -> _Integer_like;
        };

        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()(_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_like_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
    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, _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_unbounded_array_v<remove_reference_t<_Ty>>) {
                    return {_St::_None};
                } 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()(_Ty&& _Val) const noexcept(_Choice<_Ty&>._No_throw) {
                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
    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> && requires(_Rng& __r) {
        _RANGES size(__r);
    };
    // clang-format on

    // ALIAS TEMPLATE ranges::range_size_t
    template <sized_range _Rng>
    using range_size_t = decltype(_RANGES size(_STD declval<_Rng&>()));

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

    // VARIABLE TEMPLATE ranges::enable_view
    template <class _Ty>
    inline constexpr bool enable_view = derived_from<_Ty, view_base>;

    // 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 {
            explicit _Construct_tag() = default;
        };

        _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;
    };

    // VARIABLE ranges::advance
    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");
                }

                decltype(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);

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

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

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

    inline constexpr _Advance_fn advance{_Not_quite_object::_Construct_tag{}};

    // VARIABLE ranges::distance
    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);
                return _Distance_unchecked(_Get_unwrapped(_STD move(_First)), _Get_unwrapped(_STD move(_Last)));
            }
        }

        template <range _Rng>
        _NODISCARD constexpr range_difference_t<_Rng> operator()(_Rng&& _Range) const {
            if constexpr (sized_range<_Rng>) {
                return static_cast<range_difference_t<_Rng>>(_RANGES size(_Range));
            } else {
                return _Distance_unchecked(_Ubegin(_Range), _Uend(_Range));
            }
        }

    private:
        template <class _It, class _Se>
        _NODISCARD static constexpr iter_difference_t<_It> _Distance_unchecked(_It _First, const _Se _Last) {
            _STL_INTERNAL_STATIC_ASSERT(input_or_output_iterator<_It>);
            _STL_INTERNAL_STATIC_ASSERT(sentinel_for<_Se, _It>);

            iter_difference_t<_It> _Count = 0;
            for (; _First != _Last; ++_First) {
                ++_Count;
            }

            return _Count;
        }
    };

    inline constexpr _Distance_fn distance{_Not_quite_object::_Construct_tag{}};

    // VARIABLE ranges::ssize
    class _Ssize_fn { // Per the proposed resolution of LWG-3403
    public:
        // clang-format off
        template <class _Rng>
        _NODISCARD constexpr auto operator()(_Rng&& _Range) const requires requires { _RANGES size(_Range); } {
            using _Sty = _Make_signed_like_t<decltype(_RANGES size(_Range))>;
            using _Ty  = common_type_t<conditional_t<is_integral_v<_Sty>, ptrdiff_t, _Sty>, _Sty>;
            return static_cast<_Ty>(_RANGES size(_Range));
        }
        // clang-format on
    };

    inline namespace _Cpos {
        inline constexpr _Ssize_fn ssize;
    }

    // VARIABLE ranges::next
    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;
        }
    };

    inline constexpr _Next_fn next{_Not_quite_object::_Construct_tag{}};

    // VARIABLE ranges::prev
    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;
        }
    };

    inline constexpr _Prev_fn prev{_Not_quite_object::_Construct_tag{}};

    // FUNCTION TEMPLATE _Find_last_iterator
    template <forward_iterator _It, sentinel_for<_It> _Se>
    _NODISCARD constexpr _It _Find_last_iterator(
        const _It& _First, const _Se& _Last, const iter_difference_t<_It> _Count) {
        // Find the iterator in [_First, _Last) (of length _Count) which equals _Last
        _STL_INTERNAL_CHECK(_RANGES distance(_First, _Last) == _Count);
        if constexpr (is_same_v<_It, _Se>) {
            return _Last;
        } else {
            return _RANGES next(_First, _Count);
        }
    }

    // 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;
    };

    // CONCEPT _Not_same_as
    template <class _Ty1, class _Ty2>
    concept _Not_same_as = !same_as<remove_cvref_t<_Ty1>, remove_cvref_t<_Ty2>>;

    // CONCEPT ranges::common_range
    // clang-format off
    template <class _Rng>
    concept common_range = range<_Rng>
        && same_as<iterator_t<_Rng>, sentinel_t<_Rng>>;

    // CLASS TEMPLATE ranges::view_interface
    template <class _Ty>
    concept _Can_empty = requires(_Ty __t) { _RANGES empty(__t); };

    template <class _Derived>
        requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
    class view_interface : public view_base {
    private:
        _NODISCARD constexpr _Derived& _Cast() noexcept {
            static_assert(derived_from<_Derived, view_interface>,
                "view_interface's template argument D must derive from view_interface<D> (N4849 [view.interface]/2).");
            static_assert(view<_Derived>,
                "view_interface's template argument must model the view concept (N4849 [view.interface]/2).");
            return static_cast<_Derived&>(*this);
        }

        _NODISCARD constexpr const _Derived& _Cast() const noexcept {
            static_assert(derived_from<_Derived, view_interface>,
                "view_interface's template argument D must derive from view_interface<D> (N4849 [view.interface]/2).");
            static_assert(view<_Derived>,
                "view_interface's template argument must model the view concept (N4849 [view.interface]/2).");
            return static_cast<const _Derived&>(*this);
        }

    public:
#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr bool empty() requires forward_range<_Dx>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr bool empty() requires forward_range<_Derived>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
            return _RANGES begin(_Self) == _RANGES end(_Self);
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr bool empty() const requires forward_range<const _Dx>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr bool empty() const requires forward_range<const _Derived>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
            return _RANGES begin(_Self) == _RANGES end(_Self);
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        constexpr explicit operator bool() requires _Can_empty<_Dx>
#else // ^^^ workaround / no workaround vvv
        constexpr explicit operator bool() requires _Can_empty<_Derived>
#endif // TRANSITION, LLVM-44833
        {
            return !_RANGES empty(_Cast());
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        constexpr explicit operator bool() const requires _Can_empty<const _Dx>
#else // ^^^ workaround / no workaround vvv
        constexpr explicit operator bool() const requires _Can_empty<const _Derived>
#endif // TRANSITION, LLVM-44833
        {
            return !_RANGES empty(_Cast());
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr auto data() requires contiguous_iterator<iterator_t<_Dx>>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr auto data() requires contiguous_iterator<iterator_t<_Derived>>
#endif // TRANSITION, LLVM-44833
        {
            return _STD to_address(_RANGES begin(_Cast()));
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr auto data() const
            requires range<const _Dx> && contiguous_iterator<iterator_t<const _Dx>>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr auto data() const
            requires range<const _Derived> && contiguous_iterator<iterator_t<const _Derived>>
#endif // TRANSITION, LLVM-44833
        {
            return _STD to_address(_RANGES begin(_Cast()));
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr auto size()
            requires forward_range<_Dx> && sized_sentinel_for<sentinel_t<_Dx>, iterator_t<_Dx>>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr auto size()
            requires forward_range<_Derived> && sized_sentinel_for<sentinel_t<_Derived>, iterator_t<_Derived>>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
            return _RANGES end(_Self) - _RANGES begin(_Self);
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr auto size() const requires forward_range<const _Dx>
            && sized_sentinel_for<sentinel_t<const _Dx>, iterator_t<const _Dx>>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr auto size() const requires forward_range<const _Derived>
            && sized_sentinel_for<sentinel_t<const _Derived>, iterator_t<const _Derived>>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
            return _RANGES end(_Self) - _RANGES begin(_Self);
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr decltype(auto) front() requires forward_range<_Dx>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr decltype(auto) front() requires forward_range<_Derived>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
#if _CONTAINER_DEBUG_LEVEL > 0
            _STL_VERIFY(!_RANGES empty(_Self), "front called on empty view_interface");
#endif // _CONTAINER_DEBUG_LEVEL > 0
            return *_RANGES begin(_Self);
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr decltype(auto) front() const requires forward_range<const _Dx>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr decltype(auto) front() const requires forward_range<const _Derived>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
#if _CONTAINER_DEBUG_LEVEL > 0
            _STL_VERIFY(!_RANGES empty(_Self), "front called on empty view_interface");
#endif // _CONTAINER_DEBUG_LEVEL > 0
            return *_RANGES begin(_Self);
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr decltype(auto) back() requires bidirectional_range<_Dx> && common_range<_Dx>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr decltype(auto) back() requires bidirectional_range<_Derived> && common_range<_Derived>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
#if _CONTAINER_DEBUG_LEVEL > 0
            _STL_VERIFY(!_RANGES empty(_Self), "back called on empty view_interface");
#endif // _CONTAINER_DEBUG_LEVEL > 0
            auto _Last = _RANGES end(_Self);
            return *--_Last;
        }

#ifdef __clang__ // TRANSITION, LLVM-44833
        template <class _Dx = _Derived>
        _NODISCARD constexpr decltype(auto) back() const
            requires bidirectional_range<const _Dx> && common_range<const _Dx>
#else // ^^^ workaround / no workaround vvv
        _NODISCARD constexpr decltype(auto) back() const
            requires bidirectional_range<const _Derived> && common_range<const _Derived>
#endif // TRANSITION, LLVM-44833
        {
            auto& _Self = _Cast();
#if _CONTAINER_DEBUG_LEVEL > 0
            _STL_VERIFY(!_RANGES empty(_Self), "back called on empty view_interface");
#endif // _CONTAINER_DEBUG_LEVEL > 0
            auto _Last = _RANGES end(_Self);
            return *--_Last;
        }

        template <random_access_range _Rng = _Derived>
        _NODISCARD constexpr decltype(auto) operator[](const range_difference_t<_Rng> _Idx) {
            auto& _Self = _Cast();
#if _CONTAINER_DEBUG_LEVEL > 0
            if constexpr (sized_range<_Derived>) {
                _STL_VERIFY(_Idx < _RANGES size(_Self), "index out of range for view_interface");
            }
#endif // _CONTAINER_DEBUG_LEVEL > 0
            return _RANGES begin(_Self)[_Idx];
        }

        template <random_access_range _Rng = const _Derived>
        _NODISCARD constexpr decltype(auto) operator[](const range_difference_t<_Rng> _Idx) const {
            auto& _Self = _Cast();
#if _CONTAINER_DEBUG_LEVEL > 0
            if constexpr (sized_range<_Derived>) {
                _STL_VERIFY(_Idx < _RANGES size(_Self), "index out of range for view_interface");
            }
#endif // _CONTAINER_DEBUG_LEVEL > 0
            return _RANGES begin(_Self)[_Idx];
        }
    };

    // ENUM CLASS ranges::subrange_kind
    enum class subrange_kind : bool { unsized, sized };
    // clang-format on
} // namespace ranges

// These declarations must be visible to qualified name lookup for _STD get in _Pair_like below, even if <tuple> hasn't
// yet been included.
template <size_t _Index, class... _Types>
_NODISCARD constexpr tuple_element_t<_Index, tuple<_Types...>>& get(tuple<_Types...>& _Tuple) noexcept;
template <size_t _Index, class... _Types>
_NODISCARD constexpr const tuple_element_t<_Index, tuple<_Types...>>& get(const tuple<_Types...>& _Tuple) noexcept;
template <size_t _Index, class... _Types>
_NODISCARD constexpr tuple_element_t<_Index, tuple<_Types...>>&& get(tuple<_Types...>&& _Tuple) noexcept;
template <size_t _Index, class... _Types>
_NODISCARD constexpr const tuple_element_t<_Index, tuple<_Types...>>&& get(const tuple<_Types...>&& _Tuple) noexcept;

namespace ranges {
    // clang-format off
    // CLASS TEMPLATE ranges::subrange
    template <class _From, class _To>
    concept _Convertible_to_non_slicing = convertible_to<_From, _To>
        && !(is_pointer_v<decay_t<_From>>
            && is_pointer_v<decay_t<_To>>
            && _Not_same_as<remove_pointer_t<decay_t<_From>>, remove_pointer_t<decay_t<_To>>>);

    template <class _Ty>
    concept _Pair_like = !is_reference_v<_Ty> && requires(_Ty __t) {
        typename tuple_size<_Ty>::type;
        requires derived_from<tuple_size<_Ty>, integral_constant<size_t, 2>>;
        typename tuple_element_t<0, remove_const_t<_Ty>>;
        typename tuple_element_t<1, remove_const_t<_Ty>>;
        { _STD get<0>(__t) } -> convertible_to<const tuple_element_t<0, _Ty>&>;
        { _STD get<1>(__t) } -> convertible_to<const tuple_element_t<1, _Ty>&>;
    };

    template <class _Ty, class _First, class _Second>
    concept _Pair_like_convertible_from = !range<_Ty> && _Pair_like<_Ty>
        && constructible_from<_Ty, _First, _Second>
        && _Convertible_to_non_slicing<_First, tuple_element_t<0, _Ty>>
        && convertible_to<_Second, tuple_element_t<1, _Ty>>;

    template <input_or_output_iterator _It, sentinel_for<_It> _Se = _It,
        subrange_kind _Ki = sized_sentinel_for<_Se, _It> ? subrange_kind::sized : subrange_kind::unsized>
        requires (_Ki == subrange_kind::sized || !sized_sentinel_for<_Se, _It>)
    class subrange;
    // clang-format on

    template <class _It, class _Se, subrange_kind _Ki,
        bool _Store = _Ki == subrange_kind::sized && !sized_sentinel_for<_Se, _It>>
    class _Subrange_base : public view_interface<subrange<_It, _Se, _Ki>> { // TRANSITION, [[no_unique_address]]
    protected:
        using _Size_type                  = _Make_unsigned_like_t<iter_difference_t<_It>>;
        static constexpr bool _Store_size = true;

        _Size_type _Size = 0;

    public:
        _Subrange_base() = default;
        constexpr explicit _Subrange_base(const _Size_type& _Size_) noexcept : _Size(_Size_) {}
    };

    template <class _It, class _Se, subrange_kind _Ki>
    class _Subrange_base<_It, _Se, _Ki, false> : public view_interface<subrange<_It, _Se, _Ki>> {
    protected:
        using _Size_type                  = _Make_unsigned_like_t<iter_difference_t<_It>>;
        static constexpr bool _Store_size = false;

    public:
        _Subrange_base() = default;
        constexpr explicit _Subrange_base(const _Size_type&) noexcept {}
    };

    // clang-format off
    template <input_or_output_iterator _It, sentinel_for<_It> _Se, subrange_kind _Ki>
        requires (_Ki == subrange_kind::sized || !sized_sentinel_for<_Se, _It>)
    class subrange : public _Subrange_base<_It, _Se, _Ki> {
    private:
        using _Mybase = _Subrange_base<_It, _Se, _Ki>;
        using typename _Mybase::_Size_type;
        using _Mybase::_Store_size;

        // TRANSITION, [[no_unique_address]]:
        /* [[no_unique_address]] */ _It _First{};
        /* [[no_unique_address]] */ _Se _Last{};
        // [[no_unique_address]] conditional_t<_Store_size, _Size_type, _Nil> _Size{};

        template <class _Rng>
        constexpr subrange(true_type, _Rng&& _Val) : subrange{_STD forward<_Rng>(_Val), _RANGES size(_Val)} {
            // delegation target for subrange(_Rng&&) when we must store the range size
            _STL_INTERNAL_STATIC_ASSERT(_Store_size);
        }

        template <class _Rng>
        constexpr subrange(false_type, _Rng&& _Val) : subrange{_RANGES begin(_Val), _RANGES end(_Val)} {
            // delegation target for subrange(_Rng&&) when we need not store the range size
            _STL_INTERNAL_STATIC_ASSERT(!_Store_size);
        }

    public:
        subrange() = default;

        template <_Convertible_to_non_slicing<_It> _It2>
        constexpr subrange(_It2 _First_, _Se _Last_) requires (!_Store_size)
            : _First(_STD move(_First_)), _Last(_STD move(_Last_)) {}

        template <_Convertible_to_non_slicing<_It> _It2>
        constexpr subrange(_It2 _First_, _Se _Last_, const _Size_type _Size_) requires (_Ki == subrange_kind::sized)
            : _Mybase(_Size_), _First(_STD move(_First_)), _Last(_STD move(_Last_)) {
            if constexpr (sized_sentinel_for<_Se, _It>) {
                _STL_ASSERT(_Size_ == static_cast<_Size_type>(_Last - _First),
                    "This constructor's third argument should be equal to the distance "
                    "between the first and second arguments (N4849 [range.subrange.ctor]/3).");
            }
        }

        template <_Not_same_as<subrange> _Rng>
            requires borrowed_range<_Rng>
                && _Convertible_to_non_slicing<iterator_t<_Rng>, _It>
                && convertible_to<sentinel_t<_Rng>, _Se>
        constexpr subrange(_Rng&& _Val) requires (!_Store_size || sized_range<_Rng>)
            : subrange{bool_constant<_Store_size>{}, _STD forward<_Rng>(_Val)} {}

        template <borrowed_range _Rng>
            requires _Convertible_to_non_slicing<iterator_t<_Rng>, _It> && convertible_to<sentinel_t<_Rng>, _Se>
        constexpr subrange(_Rng&& _Val, const _Size_type _Count) requires (_Ki == subrange_kind::sized)
            : subrange{_RANGES begin(_Val), _RANGES end(_Val), _Count} {}

        template <_Not_same_as<subrange> _Pair_like>
            requires _Pair_like_convertible_from<_Pair_like, const _It&, const _Se&>
        constexpr operator _Pair_like() const {
            return _Pair_like(_First, _Last);
        }

        _NODISCARD constexpr _It begin() const requires copyable<_It> {
            return _First;
        }
        _NODISCARD constexpr _It begin() requires (!copyable<_It>) {
            return _STD move(_First);
        }

        _NODISCARD constexpr _Se end() const {
            return _Last;
        }

        _NODISCARD constexpr bool empty() const {
            return _First == _Last;
        }

        _NODISCARD constexpr _Size_type size() const requires (_Ki == subrange_kind::sized) {
            if constexpr (_Store_size) {
                return this->_Size;
            } else {
                return static_cast<_Size_type>(_Last - _First);
            }
        }

        _NODISCARD constexpr subrange next() const & requires forward_iterator<_It> {
            auto _Tmp = *this;
            if (_Tmp._First != _Tmp._Last) {
                ++_Tmp._First;
                if constexpr (_Store_size) {
                    --_Tmp._Size;
                }
            }
            return _Tmp;
        }
        _NODISCARD constexpr subrange next(const iter_difference_t<_It> _Count) const & requires forward_iterator<_It> {
            auto _Tmp = *this;
            _Tmp.advance(_Count);
            return _Tmp;
        }

        _NODISCARD constexpr subrange next() && {
            if (_First != _Last) {
                ++_First;
                if constexpr (_Store_size) {
                    --this->_Size;
                }
            }
            return _STD move(*this);
        }
        _NODISCARD constexpr subrange next(const iter_difference_t<_It> _Count) && {
            advance(_Count);
            return _STD move(*this);
        }

        _NODISCARD constexpr subrange prev() const requires bidirectional_iterator<_It> {
            auto _Tmp = *this;
            --_Tmp._First;
            if constexpr (_Store_size) {
                ++_Tmp._Size;
            }
            return _Tmp;
        }
        _NODISCARD constexpr subrange prev(const iter_difference_t<_It> _Count) const
            requires bidirectional_iterator<_It> {
            auto _Tmp = *this;
            _Tmp.advance(-_Count);
            return _Tmp;
        }

        constexpr subrange& advance(const iter_difference_t<_It> _Count) {
            // Per LWG-3413, this has defined behavior when _Count < 0.
            if constexpr (bidirectional_iterator<_It>) {
                if (_Count < 0) {
                    _RANGES advance(_First, _Count);
                    if constexpr (_Store_size) {
                        this->_Size += static_cast<_Size_type>(-_Count);
                    }
                    return *this;
                }
            }

            const auto _Remainder = _RANGES advance(_First, _Count, _Last);
            if constexpr (_Store_size) {
                this->_Size -= static_cast<_Size_type>(_Count - _Remainder);
            }
            return *this;
        }
    };
    // clang-format on

    template <input_or_output_iterator _It, sentinel_for<_It> _Se>
    subrange(_It, _Se) -> subrange<_It, _Se>;

    template <input_or_output_iterator _It, sentinel_for<_It> _Se>
    subrange(_It, _Se, _Make_unsigned_like_t<iter_difference_t<_It>>) -> subrange<_It, _Se, subrange_kind::sized>;

    template <borrowed_range _Rng>
    subrange(_Rng &&) -> subrange<iterator_t<_Rng>, sentinel_t<_Rng>,
        (sized_range<_Rng> || sized_sentinel_for<sentinel_t<_Rng>, iterator_t<_Rng>>) ? subrange_kind::sized
                                                                                      : subrange_kind::unsized>;

    template <borrowed_range _Rng>
    subrange(_Rng&&, _Make_unsigned_like_t<range_difference_t<_Rng>>)
        -> subrange<iterator_t<_Rng>, sentinel_t<_Rng>, subrange_kind::sized>;

    template <class _It, class _Se, subrange_kind _Ki>
    inline constexpr bool enable_borrowed_range<subrange<_It, _Se, _Ki>> = true;

    // clang-format off
    template <size_t _Idx, class _It, class _Se, subrange_kind _Ki>
        requires (_Idx < 2)
    _NODISCARD constexpr auto get(const subrange<_It, _Se, _Ki>& _Val) {
        if constexpr (_Idx == 0) {
            return _Val.begin();
        } else {
            return _Val.end();
        }
    }

    template <size_t _Idx, class _It, class _Se, subrange_kind _Ki>
        requires (_Idx < 2)
    _NODISCARD constexpr auto get(subrange<_It, _Se, _Ki>&& _Val) {
        if constexpr (_Idx == 0) {
            return _Val.begin();
        } else {
            return _Val.end();
        }
    }
    // clang-format on
} // namespace ranges

using ranges::get;

template <class _It, class _Se, ranges::subrange_kind _Ki>
struct tuple_size<ranges::subrange<_It, _Se, _Ki>> : integral_constant<size_t, 2> {};

template <class _It, class _Se, ranges::subrange_kind _Ki>
struct tuple_element<0, ranges::subrange<_It, _Se, _Ki>> {
    using type = _It;
};

template <class _It, class _Se, ranges::subrange_kind _Ki>
struct tuple_element<1, ranges::subrange<_It, _Se, _Ki>> {
    using type = _Se;
};

template <class _It, class _Se, ranges::subrange_kind _Ki>
struct tuple_element<0, const ranges::subrange<_It, _Se, _Ki>> {
    using type = _It;
};

template <class _It, class _Se, ranges::subrange_kind _Ki>
struct tuple_element<1, const ranges::subrange<_It, _Se, _Ki>> {
    using type = _Se;
};

namespace ranges {
    // STRUCT ranges::dangling
    struct dangling {
        constexpr dangling() noexcept = default;
        template <class... _Args>
        constexpr dangling(_Args&&...) noexcept {}
    };

    // ALIAS TEMPLATE ranges::borrowed_iterator_t
    template <range _Rng>
    using borrowed_iterator_t = conditional_t<borrowed_range<_Rng>, iterator_t<_Rng>, dangling>;

    // ALIAS TEMPLATE ranges::borrowed_subrange_t
    template <range _Rng>
    using borrowed_subrange_t = conditional_t<borrowed_range<_Rng>, subrange<iterator_t<_Rng>>, dangling>;
} // namespace ranges
#endif // __cpp_lib_concepts

struct _Container_proxy;
struct _Iterator_base12;

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

// CLASS TEMPLATE move_iterator
#ifdef __cpp_lib_concepts
template <semiregular>
class move_sentinel;
#endif // __cpp_lib_concepts

template <class _Iter>
class move_iterator {
public:
    using iterator_type = _Iter;
#ifdef __cpp_lib_concepts
    using iterator_concept  = input_iterator_tag;
    using iterator_category = conditional_t<derived_from<_Iter_cat_t<_Iter>, random_access_iterator_tag>,
        random_access_iterator_tag, _Iter_cat_t<_Iter>>;
#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
    using iterator_category = _Iter_cat_t<_Iter>;
#endif // __cpp_lib_concepts
    using value_type      = _Iter_value_t<_Iter>;
    using difference_type = _Iter_diff_t<_Iter>;
    using pointer         = _Iter;
#ifdef __cpp_lib_concepts
    using reference = iter_rvalue_reference_t<_Iter>;
#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
    using reference =
        conditional_t<is_reference_v<_Iter_ref_t<_Iter>>, remove_reference_t<_Iter_ref_t<_Iter>>&&, _Iter_ref_t<_Iter>>;
#endif // __cpp_lib_concepts

    _CONSTEXPR17 move_iterator() = default;

    _CONSTEXPR17 explicit move_iterator(_Iter _Right) noexcept(is_nothrow_move_constructible_v<_Iter>) // strengthened
        : _Current(_STD move(_Right)) {}

    // clang-format off
    template <class _Other>
#ifdef __cpp_lib_concepts
        // per LWG-3435
        requires (!is_same_v<_Other, _Iter>) && convertible_to<const _Other&, _Iter>
#endif // __cpp_lib_concepts
    _CONSTEXPR17 move_iterator(const move_iterator<_Other>& _Right) noexcept(
        is_nothrow_constructible_v<_Iter, const _Other&>) // strengthened
        : _Current(_Right.base()) {}

    template <class _Other>
#ifdef __cpp_lib_concepts
        // per LWG-3435
        requires (!is_same_v<_Other, _Iter>) && convertible_to<const _Other&, _Iter>
            && assignable_from<_Iter&, const _Other&>
#endif // __cpp_lib_concepts
    _CONSTEXPR17 move_iterator& operator=(const move_iterator<_Other>& _Right) noexcept(
        is_nothrow_assignable_v<_Iter&, const _Other&>) /* strengthened */ {
        _Current = _Right.base();
        return *this;
    }
    // clang-format on

#ifdef __cpp_lib_concepts
    _NODISCARD constexpr const iterator_type& base() const& { // Per LWG-3391
        return _Current;
    }
    _NODISCARD constexpr iterator_type base() && {
        return _STD move(_Current);
    }
#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
    _NODISCARD _CONSTEXPR17 iterator_type base() const {
        return _Current;
    }
#endif // __cpp_lib_concepts

    _NODISCARD _CONSTEXPR17 reference operator*() const {
#ifdef __cpp_lib_concepts
        return _RANGES iter_move(_Current);
#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
        return static_cast<reference>(*_Current);
#endif // __cpp_lib_concepts
    }

    _NODISCARD _CXX20_DEPRECATE_MOVE_ITERATOR_ARROW _CONSTEXPR17 pointer operator->() const {
        return _Current;
    }

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

    _CONSTEXPR17 auto operator++(int) {
#ifdef __cpp_lib_concepts
        if constexpr (forward_iterator<_Iter>) {
#endif // __cpp_lib_concepts
            move_iterator _Tmp = *this;
            ++_Current;
            return _Tmp;
#ifdef __cpp_lib_concepts
        } else {
            ++_Current;
        }
#endif // __cpp_lib_concepts
    }

    _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;
    }

    _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);
    }

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

    _NODISCARD _CONSTEXPR17 reference operator[](const difference_type _Off) const {
#ifdef __cpp_lib_concepts
        return _RANGES iter_move(_Current + _Off);
#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
        return _STD move(_Current[_Off]);
#endif // __cpp_lib_concepts
    }

#ifdef __cpp_lib_concepts
    template <sentinel_for<_Iter> _Sent>
    _NODISCARD friend constexpr bool operator==(const move_iterator& _Left, const move_sentinel<_Sent>& _Right) {
        return _Left._Current == _Right._Get_last();
    }

    template <sized_sentinel_for<_Iter> _Sent>
    _NODISCARD friend constexpr difference_type operator-(
        const move_sentinel<_Sent>& _Left, const move_iterator& _Right) {
        return _Left._Get_last() - _Right._Current;
    }

    template <sized_sentinel_for<_Iter> _Sent>
    _NODISCARD friend constexpr difference_type operator-(
        const move_iterator& _Left, const move_sentinel<_Sent>& _Right) {
        return _Left._Current - _Right._Get_last();
    }

    _NODISCARD friend constexpr reference iter_move(const move_iterator& _It)
#ifdef __EDG__ // TRANSITION, VSO-1132105
        noexcept(noexcept(_RANGES iter_move(_STD declval<const _Iter&>())))
#else // ^^^ workaround / no workaround vvv
        noexcept(noexcept(_RANGES iter_move(_It._Current)))
#endif // TRANSITION, VSO-1132105
    {
        return _RANGES iter_move(_It._Current);
    }

    template <indirectly_swappable<_Iter> _Iter2>
    friend constexpr void iter_swap(const move_iterator& _Left, const move_iterator<_Iter2>& _Right)
#ifdef __EDG__ // TRANSITION, VSO-1132105
        noexcept(noexcept(_RANGES iter_swap(_STD declval<const _Iter&>(), _STD declval<const _Iter2&>())))
#else // ^^^ workaround / no workaround vvv
        noexcept(noexcept(_RANGES iter_swap(_Left._Current, _Right.base())))
#endif // TRANSITION, VSO-1132105
    {
        _RANGES iter_swap(_Left._Current, _Right.base());
    }
#endif // __cpp_lib_concepts

    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._Current, _Last.base());
    }
#ifdef __cpp_lib_concepts
    template <sentinel_for<_Iter> _Sent, enable_if_t<_Range_verifiable_v<_Iter, _Sent>, int> = 0>
    friend constexpr void _Verify_range(const move_iterator& _First, const move_sentinel<_Sent>& _Last) {
        _Verify_range(_First._Current, _Last._Get_last());
    }
#endif // __cpp_lib_concepts

    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());
    }
    template <class _Iter2 = iterator_type, enable_if_t<_Unwrappable_v<_Iter2>, int> = 0>
    _NODISCARD constexpr move_iterator<_Unwrapped_t<_Iter2>> _Unwrapped() && {
        return static_cast<move_iterator<_Unwrapped_t<_Iter2>>>(_STD move(_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, const _Src&>, int> = 0>
    constexpr void _Seek_to(const move_iterator<_Src>& _It) {
        _Current._Seek_to(_It.base());
    }
    template <class _Src, enable_if_t<_Wrapped_seekable_v<iterator_type, _Src>, int> = 0>
    constexpr void _Seek_to(move_iterator<_Src>&& _It) {
        _Current._Seek_to(_STD move(_It).base());
    }

private:
    iterator_type _Current{};
};

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator==(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left.base() == _Right.base() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left.base() == _Right.base(); }

#if !_HAS_CXX20
template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator!=(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return !(_Left == _Right);
}
#endif // !_HAS_CXX20

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator<(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Left.base() < _Right.base() } -> _Implicitly_convertible_to<bool>;
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Left.base() < _Right.base(); }

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator>(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires { _Right < _Left; }
// clang-format on
#endif // __cpp_lib_concepts
{ return _Right < _Left; }

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator<=(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires { _Right < _Left; }
// clang-format on
#endif // __cpp_lib_concepts
{ return !(_Right < _Left); }

template <class _Iter1, class _Iter2>
_NODISCARD _CONSTEXPR17 bool operator>=(const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires { _Left < _Right; }
// clang-format on
#endif // __cpp_lib_concepts
{ return !(_Left < _Right); }

#ifdef __cpp_lib_concepts
template <class _Iter1, three_way_comparable_with<_Iter1> _Iter2>
_NODISCARD constexpr compare_three_way_result_t<_Iter1, _Iter2> operator<=>(
    const move_iterator<_Iter1>& _Left, const move_iterator<_Iter2>& _Right) {
    return _Left.base() <=> _Right.base();
}
#endif // __cpp_lib_concepts

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 _Iter>
_NODISCARD _CONSTEXPR17 move_iterator<_Iter> operator+(
    typename move_iterator<_Iter>::difference_type _Off, const move_iterator<_Iter>& _Right)
#ifdef __cpp_lib_concepts
    // clang-format off
    requires requires {
        { _Right.base() + _Off } -> same_as<_Iter>; // LWG-3293
    }
// clang-format on
#endif // __cpp_lib_concepts
{ return move_iterator<_Iter>{_Right.base() + _Off}; }

// 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>(_STD move(_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;
        }
    };
} // 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
// VARIABLE TEMPLATE _Is_vb_iterator
template <class _It, bool _RequiresMutable = false>
_INLINE_VAR constexpr bool _Is_vb_iterator = false;

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 _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);
}
#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 _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);
}
#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>
_NODISCARD _CONSTEXPR20 _BidIt2 _Copy_backward_unchecked(_BidIt1 _First, _BidIt1 _Last, _BidIt2 _Dest) {
    // copy [_First, _Last) backwards to [..., _Dest)
    if constexpr (_Ptr_copy_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 = *--_Last;
    }

    return _Dest;
}

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);
    const auto _ULast  = _Get_unwrapped(_Last);
    const auto _UDest  = _Get_unwrapped_n(_Dest, -_Idl_distance<_BidIt1>(_UFirst, _ULast));
    _Seek_wrapped(_Dest, _Copy_backward_unchecked(_UFirst, _ULast, _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 _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);
}
#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 {};

#ifdef __cpp_lib_byte
template <>
struct _Is_character_or_byte<byte> : true_type {};
#endif // __cpp_lib_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);
    if constexpr (_Is_vb_iterator<_FwdIt, true>) {
        _Fill_vbool(_First, _Last, _Val);
    } else {
        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) {
        if constexpr (_Is_vb_iterator<_OutIt, true>) {
            const auto _Last = _Dest + static_cast<typename _OutIt::difference_type>(_Count);
            _Fill_vbool(_Dest, _Last, _Val);
            return _Last;
        } else {
            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

// _Can_memcmp_elements<_Elem1, _Elem2> reports whether `_Elem1 == _Elem2` can be optimized to memcmp.
// Here, _Elem1 and _Elem2 aren't top-level const, because we remove_const_t before using _Can_memcmp_elements.

// Integral types are eligible for memcmp in very specific cases.
// * They must be the same size. (`int == long` is eligible; `int == long long` isn't.)
// * They can't be bool. (Not even `bool == bool`; we're concerned about representations other than 0 and 1.)
// * They can't be volatile.
// * Finally, the usual arithmetic conversions must preserve bit patterns. (This is true for `int == unsigned int`,
//   but false for `short == unsigned short`.)
template <class _Elem1, class _Elem2,
    bool = sizeof(_Elem1) == sizeof(_Elem2) //
           && _Is_nonbool_integral<_Elem1> && !is_volatile_v<_Elem1> //
           && _Is_nonbool_integral<_Elem2> && !is_volatile_v<_Elem2>>
_INLINE_VAR constexpr bool _Can_memcmp_elements = static_cast<_Elem1>(-1) == static_cast<_Elem2>(-1);

#ifdef __cpp_lib_byte
// Allow memcmping std::byte.
// inline is required here as explicit specializations of variable templates are problematic in C++14.
// However, std::byte is C++17 and above so we are safe.
template <>
inline constexpr bool _Can_memcmp_elements<byte, byte, false> = true;
#endif // __cpp_lib_byte

// Pointer elements are eligible for memcmp when they point to the same type, ignoring cv-qualification.
// This handles pointers to object types, pointers to void, and pointers to function types.
// Performance note: This doesn't attempt to handle `object* == void*`.
template <class _Ty1, class _Ty2>
_INLINE_VAR constexpr bool _Can_memcmp_elements<_Ty1*, _Ty2*, false> = is_same_v<remove_cv_t<_Ty1>, remove_cv_t<_Ty2>>;

template <class _Elem1, class _Elem2>
_INLINE_VAR constexpr bool _Can_memcmp_elements<_Elem1, _Elem2, false> = false;

// _Pred_is_consistent_with_memcmp<_Elem1, _Elem2, _Pr> reports whether `_Pr(_Elem1, _Elem2)` returns
// `_Elem1 == _Elem2` without performing any type conversions that could affect the memcmp optimization.
// (It isn't concerned with whether the elements are actually eligible.)
// Again, _Elem1 and _Elem2 aren't top-level const here.
template <class _Elem1, class _Elem2, class _Pr>
_INLINE_VAR constexpr bool _Pred_is_consistent_with_memcmp = false;

// When all of the types are the same, equal_to<_Elem> is effectively transparent.
// Performance note: This doesn't attempt to handle equal_to<long>(int, long) or equal_to<const T*>(T*, const T*).
template <class _Elem>
_INLINE_VAR constexpr bool _Pred_is_consistent_with_memcmp<_Elem, _Elem, equal_to<_Elem>> = true;

// equal_to<> is transparent.
template <class _Elem1, class _Elem2>
_INLINE_VAR constexpr bool _Pred_is_consistent_with_memcmp<_Elem1, _Elem2, equal_to<>> = true;

#ifdef __cpp_lib_concepts
// ranges::equal_to is also transparent.
template <class _Elem1, class _Elem2>
_INLINE_VAR constexpr bool _Pred_is_consistent_with_memcmp<_Elem1, _Elem2, _RANGES equal_to> = true;
#endif // __cpp_lib_concepts

// _Can_memcmp_elements_with_pred<_Elem1, _Elem2, _Pr> reports whether the memcmp optimization is applicable,
// given contiguously stored elements. (This avoids having to repeat the metaprogramming that finds the element types.)
// _Elem1 and _Elem2 aren't top-level const here.
template <class _Elem1, class _Elem2, class _Pr>
_INLINE_VAR constexpr bool _Can_memcmp_elements_with_pred = _Can_memcmp_elements<_Elem1, _Elem2> //
    && _Pred_is_consistent_with_memcmp<_Elem1, _Elem2, _Pr>;

// _Iterators_are_contiguous<_Iter1, _Iter2> reports whether both iterators are known to be contiguous.
// (Without concepts, this detection is limited, which will limit when we can activate the memcmp optimization.)

#ifdef __cpp_lib_concepts
// When concepts are available, we can detect arbitrary contiguous iterators.
template <class _Iter1, class _Iter2>
_INLINE_VAR constexpr bool _Iterators_are_contiguous = contiguous_iterator<_Iter1> //
    && contiguous_iterator<_Iter2>;
#else // ^^^ defined(__cpp_lib_concepts) ^^^ / vvv !defined(__cpp_lib_concepts) vvv
// When concepts aren't available, we can detect pointers. (Iterators should be unwrapped before using this.)
template <class _Iter1, class _Iter2>
_INLINE_VAR constexpr bool _Iterators_are_contiguous = conjunction_v<is_pointer<_Iter1>, is_pointer<_Iter2>>;
#endif // ^^^ !defined(__cpp_lib_concepts) ^^^

// _Equal_memcmp_is_safe<_Iter1, _Iter2, _Pr> reports whether we can activate the memcmp optimization
// for arbitrary iterators and predicates.
// It ignores top-level constness on the iterators and on the elements.
template <class _Iter1, class _Iter2, class _Pr>
_INLINE_VAR constexpr bool _Equal_memcmp_is_safe_helper = _Iterators_are_contiguous<_Iter1, _Iter2> //
    && _Can_memcmp_elements_with_pred<remove_const_t<remove_reference_t<_Iter_ref_t<_Iter1>>>,
        remove_const_t<remove_reference_t<_Iter_ref_t<_Iter2>>>, _Pr>;

template <class _Iter1, class _Iter2, class _Pr>
_INLINE_VAR constexpr bool _Equal_memcmp_is_safe =
    _Equal_memcmp_is_safe_helper<remove_const_t<_Iter1>, remove_const_t<_Iter2>, _Pr>;

#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, ...)
    _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 (_Equal_memcmp_is_safe<decltype(_UFirst1), decltype(_UFirst2), _Pr>) {
#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, enable_if_t<!_Equal_memcmp_is_safe<_InIt1, _InIt2, _Pr>, int> = 0>
bool _Equal_unchecked(_InIt1 _First1, const _InIt1 _Last1, _InIt2 _First2, _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, ...), no special optimization
    for (; _First1 != _Last1; ++_First1, (void) ++_First2) {
        if (!_Pred(*_First1, *_First2)) {
            return false;
        }
    }

    return true;
}

template <class _InIt1, class _InIt2, class _Pr, enable_if_t<_Equal_memcmp_is_safe<_InIt1, _InIt2, _Pr>, int> = 0>
bool _Equal_unchecked(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, _Pr) {
    // 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>
_NODISCARD bool equal(const _InIt1 _First1, const _InIt1 _Last1, const _InIt2 _First2, _Pr _Pred) {
    // compare [_First1, _Last1) to [_First2, ...)
    _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 _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
#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 _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{});
}
#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)
    _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), 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), 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)
    _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<>)
};

#ifdef __cpp_lib_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 // __cpp_lib_byte

template <class _Memcmp_pr>
struct _Lex_compare_optimize {
    explicit _Lex_compare_optimize() = default;
}; // 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 auto _Lex_compare_memcmp_classify(const _InIt1&, const _InIt2&, const _Pr&) {
    // return lex_compare optimization category for arbitrary iterators
    return _Lex_compare_optimize<void>{};
}

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

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

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), 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)
    _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)
    // 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

#ifdef __cpp_lib_concepts
// FUNCTION TEMPLATE lexicographical_compare_three_way
template <class _InIt1, class _InIt2, class _Cmp>
_NODISCARD constexpr auto lexicographical_compare_three_way(
    _InIt1 _First1, _InIt1 _Last1, _InIt2 _First2, _InIt2 _Last2, _Cmp _Comp) -> decltype(_Comp(*_First1, *_First2)) {
    _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);
    using _UIt1        = decltype(_UFirst1);
    using _UIt2        = decltype(_UFirst2);
    using _Ty1         = remove_const_t<remove_pointer_t<_UIt1>>;
    using _Ty2         = remove_const_t<remove_pointer_t<_UIt2>>;

    if constexpr (conjunction_v<is_same<_Cmp, compare_three_way>, is_pointer<_UIt1>, is_pointer<_UIt2>,
                      disjunction<
#ifdef __cpp_lib_byte
                          conjunction<is_same<_Ty1, byte>, is_same<_Ty2, byte>>,
#endif // __cpp_lib_byte
                          conjunction<_Is_character<_Ty1>, is_unsigned<_Ty1>, _Is_character<_Ty2>,
                              is_unsigned<_Ty2>>>>) {
        if (!_STD is_constant_evaluated()) {
            const auto _Num1 = static_cast<size_t>(_ULast1 - _UFirst1);
            const auto _Num2 = static_cast<size_t>(_ULast2 - _UFirst2);
            const int _Ans   = _CSTD memcmp(_UFirst1, _UFirst2, (_STD min)(_Num1, _Num2));
            if (_Ans == 0) {
                return _Num1 <=> _Num2;
            } else {
                return _Ans <=> 0;
            }
        }
    }

    for (;;) {
        if (_UFirst1 == _ULast1) {
            return _UFirst2 == _ULast2 ? strong_ordering::equal : strong_ordering::less;
        }

        if (_UFirst2 == _ULast2) {
            return strong_ordering::greater;
        }

        if (const auto _CmpResult = _Comp(*_UFirst1, *_UFirst2); _CmpResult != 0) {
            return _CmpResult;
        }

        ++_UFirst1;
        ++_UFirst2;
    }
}

template <class _InIt1, class _InIt2>
_NODISCARD constexpr auto lexicographical_compare_three_way(
    _InIt1 _First1, _InIt1 _Last1, _InIt2 _First2, _InIt2 _Last2) {
    return _STD lexicographical_compare_three_way(_First1, _Last1, _First2, _Last2, compare_three_way{});
}
#endif // __cpp_lib_concepts

// 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()) {
        using _Elem = remove_pointer_t<_InIt>;
        return _Find_unchecked1(_First, _Last, static_cast<_Elem>(_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 _Check_match_counts
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;
}

// FUNCTION TEMPLATE reverse
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 _HAS_IF_CONSTEXPR && _USE_STD_VECTOR_ALGORITHMS
    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>>>;
    constexpr size_t _Nx = sizeof(_Elem);

#pragma warning(suppress : 6326) // Potential comparison of a constant with another constant
    if constexpr (_Allow_vectorization && _Nx <= 8 && (_Nx & (_Nx - 1)) == 0) {
#ifdef __cpp_lib_is_constant_evaluated
        if (!_STD is_constant_evaluated())
#endif // __cpp_lib_is_constant_evaluated
        {
            if constexpr (_Nx == 1) {
                __std_reverse_trivially_swappable_1(_UFirst, _ULast);
            } else if constexpr (_Nx == 2) {
                __std_reverse_trivially_swappable_2(_UFirst, _ULast);
            } else if constexpr (_Nx == 4) {
                __std_reverse_trivially_swappable_4(_UFirst, _ULast);
            } else {
                __std_reverse_trivially_swappable_8(_UFirst, _ULast);
            }

            return;
        }
    }
#endif // _HAS_IF_CONSTEXPR && _USE_STD_VECTOR_ALGORITHMS

    for (; _UFirst != _ULast && _UFirst != --_ULast; ++_UFirst) {
        _STD iter_swap(_UFirst, _ULast);
    }
}

#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
    _STD reverse(_First, _Mid);
    _STD reverse(_Mid, _Last);
    auto _Tmp = _Reverse_until_sentinel_unchecked(_First, _Mid, _Last);
    _STD reverse(_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
    _STD reverse(_First, _Mid);
    _STD reverse(_Mid, _Last);
    _STD reverse(_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
    _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
    return _STD lower_bound(_First, _Last, _Val, less<>{});
}

// FUNCTION TEMPLATE _Swap_ranges_unchecked
template <class _FwdIt1, class _FwdIt2>
_CONSTEXPR20 _FwdIt2 _Swap_ranges_unchecked(_FwdIt1 _First1, const _FwdIt1 _Last1, _FwdIt2 _First2) {
    // swap [_First1, _Last1) with [_First2, ...), no special optimization
    for (; _First1 != _Last1; ++_First1, (void) ++_First2) {
        _STD iter_swap(_First1, _First2);
    }

    return _First2;
}

#if _USE_STD_VECTOR_ALGORITHMS
template <class _Ty, enable_if_t<_Is_trivially_swappable_v<_Ty>, int> = 0>
_CONSTEXPR20 _Ty* _Swap_ranges_unchecked(_Ty* _First1, _Ty* const _Last1, _Ty* _First2) {
    // swap [_First1, _Last1) with [_First2, ...), trivially swappable optimization
#ifdef __cpp_lib_is_constant_evaluated
    if (_STD is_constant_evaluated()) {
        for (; _First1 != _Last1; ++_First1, (void) ++_First2) {
            _STD iter_swap(_First1, _First2);
        }

        return _First2;
    }
#endif // __cpp_lib_is_constant_evaluated

    __std_swap_ranges_trivially_swappable_noalias(_First1, _Last1, _First2);
    return _First2 + (_Last1 - _First1);
}
#endif // _USE_STD_VECTOR_ALGORITHMS

// 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
};

// 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 _Float_traits
template <class _Ty>
struct _Float_traits {
    static_assert(is_floating_point_v<_Ty>, "_Float_traits<NonFloatingPoint> is invalid");

    // traits for double and long double:
    using type = unsigned long long;

    static constexpr type _Magnitude_mask = 0x7fff'ffff'ffff'ffffULL;
    static constexpr type _Exponent_mask  = 0x7ff0'0000'0000'0000ULL;
};

template <>
struct _Float_traits<float> {
    using type = unsigned int;

    static constexpr type _Magnitude_mask = 0x7fff'ffffU;
    static constexpr type _Exponent_mask  = 0x7f80'0000U;
};

// FUNCTION TEMPLATE _Float_abs_bits
template <class _Ty, enable_if_t<is_floating_point_v<_Ty>, int> = 0>
_NODISCARD _CONSTEXPR_BIT_CAST auto _Float_abs_bits(const _Ty& _Xx) {
    const auto _Bits = _Bit_cast<typename _Float_traits<_Ty>::type>(_Xx);
    return _Bits & _Float_traits<_Ty>::_Magnitude_mask;
}

// FUNCTION TEMPLATE _Float_abs
template <class _Ty, enable_if_t<is_floating_point_v<_Ty>, int> = 0>
_NODISCARD _CONSTEXPR_BIT_CAST _Ty _Float_abs(const _Ty _Xx) { // constexpr floating point abs()
    return _Bit_cast<_Ty>(_Float_abs_bits(_Xx));
}

// FUNCTION TEMPLATE _Is_nan
template <class _Ty, enable_if_t<is_floating_point_v<_Ty>, int> = 0>
_NODISCARD _CONSTEXPR_BIT_CAST bool _Is_nan(const _Ty _Xx) { // constexpr isnan()
    return _Float_abs_bits(_Xx) > _Float_traits<_Ty>::_Exponent_mask;
}

// FUNCTION TEMPLATE _Is_finite
template <class _Ty, enable_if_t<is_floating_point_v<_Ty>, int> = 0>
_NODISCARD _CONSTEXPR_BIT_CAST bool _Is_finite(const _Ty _Xx) { // constexpr isfinite()
    return _Float_abs_bits(_Xx) < _Float_traits<_Ty>::_Exponent_mask;
}
_STD_END
#undef _CONSTEXPR_BIT_CAST
#pragma pop_macro("new")
_STL_RESTORE_CLANG_WARNINGS
#pragma warning(pop)
#pragma pack(pop)
#endif // _STL_COMPILER_PREPROCESSOR
#endif // _XUTILITY_