include/experimental/filesystem

// -*- C++ -*-
//===--------------------------- filesystem -------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP_EXPERIMENTAL_FILESYSTEM
#define _LIBCPP_EXPERIMENTAL_FILESYSTEM
/*
    filesystem synopsis

    namespace std { namespace experimental { namespace filesystem { inline namespace v1 {

    class path;

    void swap(path& lhs, path& rhs) _NOEXCEPT;
    size_t hash_value(const path& p) _NOEXCEPT;

    bool operator==(const path& lhs, const path& rhs) _NOEXCEPT;
    bool operator!=(const path& lhs, const path& rhs) _NOEXCEPT;
    bool operator< (const path& lhs, const path& rhs) _NOEXCEPT;
    bool operator<=(const path& lhs, const path& rhs) _NOEXCEPT;
    bool operator> (const path& lhs, const path& rhs) _NOEXCEPT;
    bool operator>=(const path& lhs, const path& rhs) _NOEXCEPT;

    path operator/ (const path& lhs, const path& rhs);

    template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const path& p);

    template <class charT, class traits>
    basic_istream<charT, traits>&
    operator>>(basic_istream<charT, traits>& is, path& p);

    template <class Source>
      path u8path(const Source& source);
    template <class InputIterator>
      path u8path(InputIterator first, InputIterator last);

    class filesystem_error;
    class directory_entry;

    class directory_iterator;

    // enable directory_iterator range-based for statements
    directory_iterator begin(directory_iterator iter) noexcept;
    directory_iterator end(const directory_iterator&) noexcept;

    class recursive_directory_iterator;

    // enable recursive_directory_iterator range-based for statements
    recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;
    recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;

    class file_status;

    struct space_info
    {
      uintmax_t capacity;
      uintmax_t free;
      uintmax_t available;
    };

    enum class file_type;
    enum class perms;
    enum class copy_options;
    enum class directory_options;

    typedef chrono::time_point<trivial-clock>  file_time_type;

    // operational functions

    path absolute(const path& p, const path& base=current_path());

    path canonical(const path& p, const path& base = current_path());
    path canonical(const path& p, error_code& ec);
    path canonical(const path& p, const path& base, error_code& ec);

    void copy(const path& from, const path& to);
    void copy(const path& from, const path& to, error_code& ec) _NOEXCEPT;
    void copy(const path& from, const path& to, copy_options options);
    void copy(const path& from, const path& to, copy_options options,
                   error_code& ec) _NOEXCEPT;

    bool copy_file(const path& from, const path& to);
    bool copy_file(const path& from, const path& to, error_code& ec) _NOEXCEPT;
    bool copy_file(const path& from, const path& to, copy_options option);
    bool copy_file(const path& from, const path& to, copy_options option,
                           error_code& ec) _NOEXCEPT;

    void copy_symlink(const path& existing_symlink, const path& new_symlink);
    void copy_symlink(const path& existing_symlink, const path& new_symlink,
                              error_code& ec) _NOEXCEPT;

    bool create_directories(const path& p);
    bool create_directories(const path& p, error_code& ec) _NOEXCEPT;

    bool create_directory(const path& p);
    bool create_directory(const path& p, error_code& ec) _NOEXCEPT;

    bool create_directory(const path& p, const path& attributes);
    bool create_directory(const path& p, const path& attributes,
                                  error_code& ec) _NOEXCEPT;

    void create_directory_symlink(const path& to, const path& new_symlink);
    void create_directory_symlink(const path& to, const path& new_symlink,
                                          error_code& ec) _NOEXCEPT;

    void create_hard_link(const path& to, const path& new_hard_link);
    void create_hard_link(const path& to, const path& new_hard_link,
                                  error_code& ec) _NOEXCEPT;

    void create_symlink(const path& to, const path& new_symlink);
    void create_symlink(const path& to, const path& new_symlink,
                                error_code& ec) _NOEXCEPT;

    path current_path();
    path current_path(error_code& ec);
    void current_path(const path& p);
    void current_path(const path& p, error_code& ec) _NOEXCEPT;

    bool exists(file_status s) _NOEXCEPT;
    bool exists(const path& p);
    bool exists(const path& p, error_code& ec) _NOEXCEPT;

    bool equivalent(const path& p1, const path& p2);
    bool equivalent(const path& p1, const path& p2, error_code& ec) _NOEXCEPT;

    uintmax_t    file_size(const path& p);
    uintmax_t    file_size(const path& p, error_code& ec) _NOEXCEPT;

    uintmax_t    hard_link_count(const path& p);
    uintmax_t    hard_link_count(const path& p, error_code& ec) _NOEXCEPT;

    bool is_block_file(file_status s) _NOEXCEPT;
    bool is_block_file(const path& p);
    bool is_block_file(const path& p, error_code& ec) _NOEXCEPT;

    bool is_character_file(file_status s) _NOEXCEPT;
    bool is_character_file(const path& p);
    bool is_character_file(const path& p, error_code& ec) _NOEXCEPT;

    bool is_directory(file_status s) _NOEXCEPT;
    bool is_directory(const path& p);
    bool is_directory(const path& p, error_code& ec) _NOEXCEPT;

    bool is_empty(const path& p);
    bool is_empty(const path& p, error_code& ec) _NOEXCEPT;

    bool is_fifo(file_status s) _NOEXCEPT;
    bool is_fifo(const path& p);
    bool is_fifo(const path& p, error_code& ec) _NOEXCEPT;

    bool is_other(file_status s) _NOEXCEPT;
    bool is_other(const path& p);
    bool is_other(const path& p, error_code& ec) _NOEXCEPT;

    bool is_regular_file(file_status s) _NOEXCEPT;
    bool is_regular_file(const path& p);
    bool is_regular_file(const path& p, error_code& ec) _NOEXCEPT;

    bool is_socket(file_status s) _NOEXCEPT;
    bool is_socket(const path& p);
    bool is_socket(const path& p, error_code& ec) _NOEXCEPT;

    bool is_symlink(file_status s) _NOEXCEPT;
    bool is_symlink(const path& p);
    bool is_symlink(const path& p, error_code& ec) _NOEXCEPT;

    file_time_type  last_write_time(const path& p);
    file_time_type  last_write_time(const path& p, error_code& ec) _NOEXCEPT;
    void last_write_time(const path& p, file_time_type new_time);
    void last_write_time(const path& p, file_time_type new_time,
                                 error_code& ec) _NOEXCEPT;

    void permissions(const path& p, perms prms);
    void permissions(const path& p, perms prms, error_code& ec) _NOEXCEPT;

    path read_symlink(const path& p);
    path read_symlink(const path& p, error_code& ec);

    bool remove(const path& p);
    bool remove(const path& p, error_code& ec) _NOEXCEPT;

    uintmax_t    remove_all(const path& p);
    uintmax_t    remove_all(const path& p, error_code& ec) _NOEXCEPT;

    void rename(const path& from, const path& to);
    void rename(const path& from, const path& to, error_code& ec) _NOEXCEPT;

    void resize_file(const path& p, uintmax_t size);
    void resize_file(const path& p, uintmax_t size, error_code& ec) _NOEXCEPT;

    space_info   space(const path& p);
    space_info   space(const path& p, error_code& ec) _NOEXCEPT;

    file_status  status(const path& p);
    file_status  status(const path& p, error_code& ec) _NOEXCEPT;

    bool status_known(file_status s) _NOEXCEPT;

    file_status  symlink_status(const path& p);
    file_status  symlink_status(const path& p, error_code& ec) _NOEXCEPT;

    path system_complete(const path& p);
    path system_complete(const path& p, error_code& ec);

    path temp_directory_path();
    path temp_directory_path(error_code& ec);

} } } }  // namespaces std::experimental::filesystem::v1

*/

#include <experimental/__config>
#include <cstddef>
#include <chrono>
#include <iterator>
#include <iosfwd>
#include <locale>
#include <memory>
#include <stack>
#include <string>
#include <system_error>
#include <utility>
#include <iomanip> // for quoted
#include <string_view>

#include <__debug>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

#define __cpp_lib_experimental_filesystem 201406

_LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_FILESYSTEM

typedef chrono::time_point<std::chrono::system_clock>  file_time_type;

struct _LIBCPP_TYPE_VIS space_info
{
  uintmax_t capacity;
  uintmax_t free;
  uintmax_t available;
};

enum class _LIBCPP_ENUM_VIS file_type : signed char
{
    none       = 0,
    not_found  = -1,
    regular    = 1,
    directory  = 2,
    symlink    = 3,
    block      = 4,
    character  = 5,
    fifo       = 6,
    socket     = 7,
    unknown    = 8
};

enum class _LIBCPP_ENUM_VIS perms : unsigned
{
    none         = 0,

    owner_read   = 0400,
    owner_write  = 0200,
    owner_exec   = 0100,
    owner_all    = 0700,

    group_read   = 040,
    group_write  = 020,
    group_exec   = 010,
    group_all    = 070,

    others_read  = 04,
    others_write = 02,
    others_exec  = 01,
    others_all   = 07,

    all          = 0777,

    set_uid      = 04000,
    set_gid      = 02000,
    sticky_bit   = 01000,
    mask         = 07777,
    unknown      = 0xFFFF,

    add_perms        = 0x10000,
    remove_perms     = 0x20000,
    symlink_nofollow = 0x40000
};

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR perms operator&(perms _LHS, perms _RHS)
{ return static_cast<perms>(static_cast<unsigned>(_LHS) & static_cast<unsigned>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR perms operator|(perms _LHS, perms _RHS)
{ return static_cast<perms>(static_cast<unsigned>(_LHS) | static_cast<unsigned>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR perms operator^(perms _LHS, perms _RHS)
{ return static_cast<perms>(static_cast<unsigned>(_LHS) ^ static_cast<unsigned>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR perms operator~(perms _LHS)
{ return static_cast<perms>(~static_cast<unsigned>(_LHS)); }

_LIBCPP_INLINE_VISIBILITY
inline perms& operator&=(perms& _LHS, perms _RHS)
{ return _LHS = _LHS & _RHS; }

_LIBCPP_INLINE_VISIBILITY
inline perms& operator|=(perms& _LHS, perms _RHS)
{ return _LHS = _LHS | _RHS; }

_LIBCPP_INLINE_VISIBILITY
inline perms& operator^=(perms& _LHS, perms _RHS)
{ return _LHS = _LHS ^ _RHS; }

enum class _LIBCPP_ENUM_VIS copy_options : unsigned short
{
    none               = 0,
    skip_existing      = 1,
    overwrite_existing = 2,
    update_existing    = 4,
    recursive          = 8,
    copy_symlinks      = 16,
    skip_symlinks      = 32,
    directories_only   = 64,
    create_symlinks    = 128,
    create_hard_links  = 256,
    __in_recursive_copy = 512,
};

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR copy_options operator&(copy_options _LHS, copy_options _RHS)
{ return static_cast<copy_options>(static_cast<unsigned short>(_LHS) & static_cast<unsigned short>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR copy_options operator|(copy_options _LHS, copy_options _RHS)
{ return static_cast<copy_options>(static_cast<unsigned short>(_LHS) | static_cast<unsigned short>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR copy_options operator^(copy_options _LHS, copy_options _RHS)
{ return static_cast<copy_options>(static_cast<unsigned short>(_LHS) ^ static_cast<unsigned short>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR copy_options operator~(copy_options _LHS)
{ return static_cast<copy_options>(~static_cast<unsigned short>(_LHS)); }

_LIBCPP_INLINE_VISIBILITY
inline copy_options& operator&=(copy_options& _LHS, copy_options _RHS)
{ return _LHS = _LHS & _RHS; }

_LIBCPP_INLINE_VISIBILITY
inline copy_options& operator|=(copy_options& _LHS, copy_options _RHS)
{ return _LHS = _LHS | _RHS; }

_LIBCPP_INLINE_VISIBILITY
inline copy_options& operator^=(copy_options& _LHS, copy_options _RHS)
{ return _LHS = _LHS ^ _RHS; }


enum class _LIBCPP_ENUM_VIS directory_options : unsigned char
{
    none                     = 0,
    follow_directory_symlink = 1,
    skip_permission_denied   = 2
};

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR directory_options operator&(directory_options _LHS, directory_options _RHS)
{ return static_cast<directory_options>(static_cast<unsigned char>(_LHS) & static_cast<unsigned char>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR directory_options operator|(directory_options _LHS, directory_options _RHS)
{ return static_cast<directory_options>(static_cast<unsigned char>(_LHS) | static_cast<unsigned char>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR directory_options operator^(directory_options _LHS, directory_options _RHS)
{ return static_cast<directory_options>(static_cast<unsigned char>(_LHS) ^ static_cast<unsigned char>(_RHS)); }

_LIBCPP_INLINE_VISIBILITY
inline _LIBCPP_CONSTEXPR directory_options operator~(directory_options _LHS)
{ return static_cast<directory_options>(~static_cast<unsigned char>(_LHS)); }

_LIBCPP_INLINE_VISIBILITY
inline directory_options& operator&=(directory_options& _LHS, directory_options _RHS)
{ return _LHS = _LHS & _RHS; }

_LIBCPP_INLINE_VISIBILITY
inline directory_options& operator|=(directory_options& _LHS, directory_options _RHS)
{ return _LHS = _LHS | _RHS; }

_LIBCPP_INLINE_VISIBILITY
inline directory_options& operator^=(directory_options& _LHS, directory_options _RHS)
{ return _LHS = _LHS ^ _RHS; }


class _LIBCPP_TYPE_VIS file_status
{
public:
    // constructors
    _LIBCPP_INLINE_VISIBILITY
    explicit file_status(file_type __ft = file_type::none,
                         perms __prms   = perms::unknown) _NOEXCEPT
      : __ft_(__ft), __prms_(__prms)
    {}

    file_status(const file_status&) _NOEXCEPT = default;
    file_status(file_status&&) _NOEXCEPT = default;

    _LIBCPP_INLINE_VISIBILITY
    ~file_status() {}

    file_status& operator=(const file_status&) _NOEXCEPT = default;
    file_status& operator=(file_status&&) _NOEXCEPT = default;

    // observers
    _LIBCPP_ALWAYS_INLINE
    file_type type() const _NOEXCEPT {
        return __ft_;
    }

    _LIBCPP_ALWAYS_INLINE
    perms permissions() const _NOEXCEPT {
        return __prms_;
    }

    // modifiers
    _LIBCPP_ALWAYS_INLINE
    void type(file_type __ft) _NOEXCEPT {
        __ft_ = __ft;
    }

    _LIBCPP_ALWAYS_INLINE
    void permissions(perms __p) _NOEXCEPT {
        __prms_ = __p;
    }
private:
    file_type __ft_;
    perms __prms_;
};

class _LIBCPP_TYPE_VIS directory_entry;

template <class _Tp> struct __can_convert_char {
  static const bool value = false;
};
template <class _Tp> struct __can_convert_char<const _Tp>
    : public __can_convert_char<_Tp> {
};
template <> struct __can_convert_char<char> {
    static const bool value = true;
    using __char_type = char;
};
template <> struct __can_convert_char<wchar_t>  {
    static const bool value = true;
    using __char_type = wchar_t;
};
template <> struct __can_convert_char<char16_t> {
    static const bool value = true;
    using __char_type = char16_t;
};
template <> struct __can_convert_char<char32_t> {
    static const bool value = true;
    using __char_type = char32_t;
};

template <class _ECharT>
typename enable_if<__can_convert_char<_ECharT>::value, bool>::type
__is_separator(_ECharT __e) {
    return __e == _ECharT('/');
};

struct _NullSentinal {};

template <class _Tp>
using _Void = void;

template <class _Tp, class = void>
struct __is_pathable_string : public false_type {};

template <class _ECharT, class _Traits, class _Alloc>
struct __is_pathable_string<basic_string<_ECharT, _Traits, _Alloc>,
                            _Void<typename __can_convert_char<_ECharT>::__char_type>>
: public __can_convert_char<_ECharT>
{
    using _Str = basic_string<_ECharT, _Traits, _Alloc>;
    using _Base = __can_convert_char<_ECharT>;
    static _ECharT const* __range_begin(_Str const& __s) { return __s.data(); }
    static _ECharT const* __range_end(_Str const& __s) { return __s.data() + __s.length(); }
    static _ECharT __first_or_null(_Str const& __s) {
        return __s.empty() ? _ECharT{} : __s[0];
    }
};


template <class _ECharT, class _Traits>
struct __is_pathable_string<basic_string_view<_ECharT, _Traits>,
                            _Void<typename __can_convert_char<_ECharT>::__char_type>>
: public __can_convert_char<_ECharT>
{
    using _Str = basic_string_view<_ECharT, _Traits>;
    using _Base = __can_convert_char<_ECharT>;
    static _ECharT const* __range_begin(_Str const& __s) { return __s.data(); }
    static _ECharT const* __range_end(_Str const& __s) { return __s.data() + __s.length(); }
    static _ECharT __first_or_null(_Str const& __s) {
        return __s.empty() ? _ECharT{} : __s[0];
    }
};

template <class _Source,
          class _DS = typename decay<_Source>::type,
          class _UnqualPtrType = typename remove_const<
              typename remove_pointer<_DS>::type>::type,
          bool _IsCharPtr = is_pointer<_DS>::value &&
                            __can_convert_char<_UnqualPtrType>::value
        >
struct __is_pathable_char_array : false_type {};

template <class _Source, class _ECharT, class _UPtr>
struct __is_pathable_char_array<_Source, _ECharT*, _UPtr, true>
        : __can_convert_char<typename remove_const<_ECharT>::type>
{
  using _Base = __can_convert_char<typename remove_const<_ECharT>::type>;

  static _ECharT const* __range_begin(const _ECharT* __b) { return __b; }
  static _ECharT const* __range_end(const _ECharT* __b)
  {
      using _Iter = const _ECharT*;
      const _ECharT __sentinal = _ECharT{};
      _Iter __e = __b;
      for (; *__e != __sentinal; ++__e)
          ;
      return __e;
  }

  static _ECharT __first_or_null(const _ECharT* __b) { return *__b; }
};

template <class _Iter, bool _IsIt = __is_input_iterator<_Iter>::value, class = void>
struct __is_pathable_iter : false_type {};

template <class _Iter>
struct __is_pathable_iter<_Iter, true,
        _Void<typename __can_convert_char<typename iterator_traits<_Iter>::value_type>::__char_type>>
        : __can_convert_char<typename iterator_traits<_Iter>::value_type>
{
  using _ECharT = typename iterator_traits<_Iter>::value_type;
  using _Base = __can_convert_char<_ECharT>;

  static _Iter         __range_begin(_Iter __b) { return __b; }
  static _NullSentinal __range_end(_Iter)       { return _NullSentinal{}; }

  static _ECharT __first_or_null(_Iter __b) { return *__b; }
};

template <class _Tp, bool _IsStringT =  __is_pathable_string<_Tp>::value,
                     bool _IsCharIterT = __is_pathable_char_array<_Tp>::value,
                     bool _IsIterT = !_IsCharIterT && __is_pathable_iter<_Tp>::value
         >
struct __is_pathable : false_type {
  static_assert(!_IsStringT && !_IsCharIterT && !_IsIterT, "Must all be false");
};

template <class _Tp>
struct __is_pathable<_Tp, true, false, false> : __is_pathable_string<_Tp> {};


template <class _Tp>
struct __is_pathable<_Tp, false, true, false> : __is_pathable_char_array<_Tp> {};


template <class _Tp>
struct __is_pathable<_Tp, false, false, true> : __is_pathable_iter<_Tp> {};


template <class _ECharT>
struct _PathCVT {
    static_assert(__can_convert_char<_ECharT>::value, "Char type not convertible");

    typedef __narrow_to_utf8<sizeof(_ECharT)*__CHAR_BIT__> _Narrower;

    static void __append_range(string& __dest, _ECharT const* __b, _ECharT const* __e)  {
        _Narrower()(back_inserter(__dest), __b, __e);
    }

    template <class _Iter>
    static void __append_range(string& __dest, _Iter __b, _Iter __e) {
        static_assert(!is_same<_Iter, _ECharT*>::value, "Call const overload");
        if (__b == __e) return;
        basic_string<_ECharT> __tmp(__b, __e);
        _Narrower()(back_inserter(__dest), __tmp.data(),
                    __tmp.data() + __tmp.length());
    }

    template <class _Iter>
    static void __append_range(string& __dest, _Iter __b, _NullSentinal) {
        static_assert(!is_same<_Iter, _ECharT*>::value, "Call const overload");
        const _ECharT __sentinal = _ECharT{};
        if (*__b == __sentinal) return;
        basic_string<_ECharT> __tmp;
        for (; *__b != __sentinal; ++__b)
            __tmp.push_back(*__b);
        _Narrower()(back_inserter(__dest), __tmp.data(),
                    __tmp.data() + __tmp.length());
    }

    template <class _Source>
    static void __append_source(string& __dest, _Source const& __s)
    {
        using _Traits = __is_pathable<_Source>;
        __append_range(__dest, _Traits::__range_begin(__s), _Traits::__range_end(__s));
    }
};

template <>
struct _PathCVT<char> {

    template <class _Iter>
    static typename enable_if<
        __is_exactly_input_iterator<_Iter>::value
    >::type __append_range(string& __dest, _Iter __b, _Iter __e) {
        for (; __b != __e; ++__b)
            __dest.push_back(*__b);
    }

    template <class _Iter>
    static typename enable_if<
        __is_forward_iterator<_Iter>::value
    >::type __append_range(string& __dest, _Iter __b, _Iter __e) {
        __dest.__append_forward_unsafe(__b, __e);
    }

    template <class _Iter>
    static void __append_range(string& __dest, _Iter __b, _NullSentinal) {
        const char __sentinal = char{};
        for (; *__b != __sentinal; ++__b)
            __dest.push_back(*__b);
    }

    template <class _Source>
    static void __append_source(string& __dest, _Source const& __s)
    {
        using _Traits = __is_pathable<_Source>;
        __append_range(__dest, _Traits::__range_begin(__s),
                               _Traits::__range_end(__s));
    }
};


class _LIBCPP_TYPE_VIS path
{
    template <class _SourceOrIter, class _Tp = path&>
    using _EnableIfPathable = typename
        enable_if<__is_pathable<_SourceOrIter>::value, _Tp>::type;

    template <class _Tp>
    using _SourceChar = typename __is_pathable<_Tp>::__char_type;

    template <class _Tp>
    using _SourceCVT = _PathCVT<_SourceChar<_Tp>>;

public:
    typedef char value_type;
    typedef basic_string<value_type> string_type;
    typedef _VSTD::string_view __string_view;
    static _LIBCPP_CONSTEXPR value_type preferred_separator = '/';

    // constructors and destructor
    _LIBCPP_INLINE_VISIBILITY path() _NOEXCEPT {}
    _LIBCPP_INLINE_VISIBILITY path(const path& __p)      : __pn_(__p.__pn_) {}
    _LIBCPP_INLINE_VISIBILITY path(path&& __p) _NOEXCEPT : __pn_(_VSTD::move(__p.__pn_)) {}

    _LIBCPP_INLINE_VISIBILITY
    path(string_type&& __s) _NOEXCEPT : __pn_(_VSTD::move(__s)) {}

    template <
        class _Source,
        class = _EnableIfPathable<_Source, void>
        >
    path(const _Source& __src) {
        _SourceCVT<_Source>::__append_source(__pn_, __src);
    }

    template <class _InputIt>
    path(_InputIt __first, _InputIt __last) {
        typedef typename iterator_traits<_InputIt>::value_type _ItVal;
        _PathCVT<_ItVal>::__append_range(__pn_, __first, __last);
    }

    // TODO Implement locale conversions.
    template <class _Source,
              class = _EnableIfPathable<_Source, void>
             >
        path(const _Source& __src, const locale& __loc);
    template <class _InputIt>
        path(_InputIt __first, _InputIt _last, const locale& __loc);

    _LIBCPP_INLINE_VISIBILITY
    ~path() = default;

    // assignments
    _LIBCPP_INLINE_VISIBILITY
    path& operator=(const path& __p) {
        __pn_ = __p.__pn_;
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    path& operator=(path&& __p) _NOEXCEPT {
        __pn_ = _VSTD::move(__p.__pn_);
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    path& operator=(string_type&& __s) _NOEXCEPT {
        __pn_ = _VSTD::move(__s);
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    path& assign(string_type&& __s) _NOEXCEPT {
        __pn_ = _VSTD::move(__s);
        return *this;
    }

    template <class _Source>
    _LIBCPP_INLINE_VISIBILITY
    _EnableIfPathable<_Source>
    operator=(const _Source& __src)
    { return this->assign(__src); }


    template <class _Source>
    _EnableIfPathable<_Source>
    assign(const _Source& __src) {
        __pn_.clear();
        _SourceCVT<_Source>::__append_source(__pn_, __src);
        return *this;
    }

    template <class _InputIt>
    path& assign(_InputIt __first, _InputIt __last) {
        typedef typename iterator_traits<_InputIt>::value_type _ItVal;
        __pn_.clear();
        _PathCVT<_ItVal>::__append_range(__pn_, __first, __last);
        return *this;
    }

private:
    template <class _ECharT>
    void __append_sep_if_needed(_ECharT __first_or_null) {
        const _ECharT __null_val = {};
        bool __append_sep = !empty()                       &&
                            !__is_separator(__pn_.back())  &&
                            __first_or_null != __null_val  && // non-empty
                            !__is_separator(__first_or_null);
        if (__append_sep)
            __pn_ += preferred_separator;
    }

public:
    // appends
    path& operator/=(const path& __p) {
        _LIBCPP_ASSERT(!__p.has_root_name(),
                      "cannot append to a path with a root name");
        __append_sep_if_needed(__p.empty() ? char{} : __p.__pn_[0]);
        __pn_ += __p.native();
        return *this;
    }

    template <class _Source>
    _LIBCPP_INLINE_VISIBILITY
    _EnableIfPathable<_Source>
    operator/=(const _Source& __src) {
        return this->append(__src);
    }

    template <class _Source>
    _EnableIfPathable<_Source>
    append(const _Source& __src) {
        using _Traits = __is_pathable<_Source>;
        using _CVT = _PathCVT<_SourceChar<_Source>>;
        __append_sep_if_needed(_Traits::__first_or_null(__src));
        _CVT::__append_source(__pn_, __src);
        return *this;
    }

    template <class _InputIt>
    path& append(_InputIt __first, _InputIt __last) {
        typedef typename iterator_traits<_InputIt>::value_type _ItVal;
        static_assert(__can_convert_char<_ItVal>::value, "Must convertible");
        using _CVT = _PathCVT<_ItVal>;
        if (__first != __last) {
            __append_sep_if_needed(*__first);
            _CVT::__append_range(__pn_, __first, __last);
        }
        return *this;
    }

    // concatenation
    _LIBCPP_INLINE_VISIBILITY
    path& operator+=(const path& __x) {
        __pn_ += __x.__pn_;
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    path& operator+=(const string_type& __x) {
        __pn_ += __x;
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    path& operator+=(__string_view __x) {
        __pn_ += __x;
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    path& operator+=(const value_type* __x) {
        __pn_ += __x;
        return *this;
    }

    _LIBCPP_INLINE_VISIBILITY
    path& operator+=(value_type __x) {
        __pn_ += __x;
        return *this;
    }

    template <class _ECharT>
    typename enable_if<__can_convert_char<_ECharT>::value, path&>::type
    operator+=(_ECharT __x)
    {
        basic_string<_ECharT> __tmp;
        __tmp += __x;
        _PathCVT<_ECharT>::__append_source(__pn_, __tmp);
        return *this;
    }

    template <class _Source>
    _EnableIfPathable<_Source>
    operator+=(const _Source& __x) {
       return this->concat(__x);
    }

    template <class _Source>
    _EnableIfPathable<_Source>
    concat(const _Source& __x) {
         _SourceCVT<_Source>::__append_source(__pn_, __x);
        return *this;
    }

    template <class _InputIt>
    path& concat(_InputIt __first, _InputIt __last) {
        typedef typename iterator_traits<_InputIt>::value_type _ItVal;
        _PathCVT<_ItVal>::__append_range(__pn_, __first, __last);
        return *this;
    }

    // modifiers
    _LIBCPP_INLINE_VISIBILITY
    void clear() _NOEXCEPT {
        __pn_.clear();
    }

    path& make_preferred() { return *this; }

    _LIBCPP_INLINE_VISIBILITY
    path& remove_filename() {
      if (__pn_.size() == __root_path_raw().size())
        clear();
      else
        __pn_ = __parent_path();
      return *this;
    }

    path& replace_filename(const path& __replacement) {
        remove_filename();
        return (*this /= __replacement);
    }

    path& replace_extension(const path& __replacement = path());

    _LIBCPP_INLINE_VISIBILITY
    void  swap(path& __rhs) _NOEXCEPT {
        __pn_.swap(__rhs.__pn_);
    }

    // native format observers
    _LIBCPP_INLINE_VISIBILITY
    const string_type& native() const _NOEXCEPT {
        return __pn_;
    }

    _LIBCPP_INLINE_VISIBILITY
    const value_type* c_str() const _NOEXCEPT { return __pn_.c_str(); }

    _LIBCPP_INLINE_VISIBILITY operator string_type() const { return __pn_;  }

    template <class _ECharT, class _Traits = char_traits<_ECharT>,
              class _Allocator = allocator<_ECharT> >
    basic_string<_ECharT, _Traits, _Allocator>
    string(const _Allocator& __a = _Allocator()) const {
        using _CVT = __widen_from_utf8<sizeof(_ECharT)*__CHAR_BIT__>;
        using _Str = basic_string<_ECharT, _Traits, _Allocator>;
        _Str __s(__a);
        __s.reserve(__pn_.size());
        _CVT()(back_inserter(__s), __pn_.data(), __pn_.data() + __pn_.size());
        return __s;
    }

    _LIBCPP_INLINE_VISIBILITY std::string    string()    const { return __pn_; }
    _LIBCPP_INLINE_VISIBILITY std::wstring   wstring()   const { return string<wchar_t>(); }
    _LIBCPP_INLINE_VISIBILITY std::string    u8string()  const { return __pn_; }
    _LIBCPP_INLINE_VISIBILITY std::u16string u16string() const { return string<char16_t>(); }
    _LIBCPP_INLINE_VISIBILITY std::u32string u32string() const { return string<char32_t>(); }

    // generic format observers
    template <class _ECharT, class _Traits = char_traits<_ECharT>,
              class _Allocator = allocator<_ECharT>
    >
    basic_string<_ECharT, _Traits, _Allocator>
    generic_string(const _Allocator& __a = _Allocator()) const {
        return string<_ECharT, _Traits, _Allocator>(__a);
    }

    std::string    generic_string()    const { return __pn_; }
    std::wstring   generic_wstring()   const { return string<wchar_t>(); }
    std::string    generic_u8string()  const { return __pn_; }
    std::u16string generic_u16string() const { return string<char16_t>(); }
    std::u32string generic_u32string() const { return string<char32_t>(); }

private:
    _LIBCPP_FUNC_VIS int __compare(__string_view) const;
    _LIBCPP_FUNC_VIS __string_view __root_name() const;
    _LIBCPP_FUNC_VIS __string_view __root_directory() const;
    _LIBCPP_FUNC_VIS __string_view __root_path_raw() const;
    _LIBCPP_FUNC_VIS __string_view __relative_path() const;
    _LIBCPP_FUNC_VIS __string_view __parent_path() const;
    _LIBCPP_FUNC_VIS __string_view __filename() const;
    _LIBCPP_FUNC_VIS __string_view __stem() const;
    _LIBCPP_FUNC_VIS __string_view __extension() const;

public:
    // compare
    _LIBCPP_INLINE_VISIBILITY int compare(const path& __p) const _NOEXCEPT { return __compare(__p.__pn_);}
    _LIBCPP_INLINE_VISIBILITY int compare(const string_type& __s) const { return __compare(__s); }
    _LIBCPP_INLINE_VISIBILITY int compare(__string_view __s) const { return __compare(__s); }
    _LIBCPP_INLINE_VISIBILITY int compare(const value_type* __s) const  { return __compare(__s); }

    // decomposition
    _LIBCPP_INLINE_VISIBILITY path root_name()      const { return  string_type(__root_name()); }
    _LIBCPP_INLINE_VISIBILITY path root_directory() const { return  string_type(__root_directory()); }
    _LIBCPP_INLINE_VISIBILITY path root_path()      const { return root_name().append(string_type(__root_directory())); }
    _LIBCPP_INLINE_VISIBILITY path relative_path()  const { return string_type(__relative_path()); }
    _LIBCPP_INLINE_VISIBILITY path parent_path()    const { return string_type(__parent_path()); }
    _LIBCPP_INLINE_VISIBILITY path filename()       const { return string_type(__filename()); }
    _LIBCPP_INLINE_VISIBILITY path stem()           const { return string_type(__stem());}
    _LIBCPP_INLINE_VISIBILITY path extension()      const { return string_type(__extension()); }

    // query
    _LIBCPP_INLINE_VISIBILITY bool empty() const _NOEXCEPT { return __pn_.empty(); }

    _LIBCPP_INLINE_VISIBILITY bool has_root_name()      const { return !__root_name().empty(); }
    _LIBCPP_INLINE_VISIBILITY bool has_root_directory() const { return !__root_directory().empty(); }
    _LIBCPP_INLINE_VISIBILITY bool has_root_path()      const { return !__root_path_raw().empty(); }
    _LIBCPP_INLINE_VISIBILITY bool has_relative_path()  const { return !__relative_path().empty(); }
    _LIBCPP_INLINE_VISIBILITY bool has_parent_path()    const { return !__parent_path().empty(); }
    _LIBCPP_INLINE_VISIBILITY bool has_filename()       const { return !__filename().empty(); }
    _LIBCPP_INLINE_VISIBILITY bool has_stem()           const { return !__stem().empty(); }
    _LIBCPP_INLINE_VISIBILITY bool has_extension()      const { return !__extension().empty(); }

    _LIBCPP_INLINE_VISIBILITY bool is_absolute()        const { return has_root_directory(); }
    _LIBCPP_INLINE_VISIBILITY bool is_relative()        const { return !is_absolute(); }

    // iterators
    class _LIBCPP_TYPE_VIS iterator;
    typedef iterator const_iterator;

    _LIBCPP_FUNC_VIS iterator begin() const;
    _LIBCPP_FUNC_VIS iterator end() const;

private:
    inline _LIBCPP_INLINE_VISIBILITY
    path& __assign_view(__string_view const& __s) noexcept { __pn_ = string_type(__s); return *this; }
    string_type __pn_;
};

inline _LIBCPP_ALWAYS_INLINE
void swap(path& __lhs, path& __rhs) _NOEXCEPT {
    __lhs.swap(__rhs);
}

_LIBCPP_FUNC_VIS
size_t hash_value(const path& __p) _NOEXCEPT;

inline _LIBCPP_INLINE_VISIBILITY
bool operator==(const path& __lhs, const path& __rhs) _NOEXCEPT
{ return __lhs.compare(__rhs) == 0; }

inline _LIBCPP_INLINE_VISIBILITY
bool operator!=(const path& __lhs, const path& __rhs) _NOEXCEPT
{ return __lhs.compare(__rhs) != 0; }

inline _LIBCPP_INLINE_VISIBILITY
bool operator<(const path& __lhs, const path& __rhs) _NOEXCEPT
{ return __lhs.compare(__rhs) < 0; }

inline _LIBCPP_INLINE_VISIBILITY
bool operator<=(const path& __lhs, const path& __rhs) _NOEXCEPT
{ return __lhs.compare(__rhs) <= 0; }

inline _LIBCPP_INLINE_VISIBILITY
bool operator>(const path& __lhs, const path& __rhs) _NOEXCEPT
{ return __lhs.compare(__rhs) > 0; }

inline _LIBCPP_INLINE_VISIBILITY
bool operator>=(const path& __lhs, const path& __rhs) _NOEXCEPT
{ return __lhs.compare(__rhs) >= 0; }

inline _LIBCPP_INLINE_VISIBILITY
path operator/(const path& __lhs, const path& __rhs) {
    return path(__lhs) /= __rhs;
}

template <class _CharT, class _Traits>
_LIBCPP_INLINE_VISIBILITY
typename enable_if<is_same<_CharT, char>::value &&
                   is_same<_Traits, char_traits<char>>::value,
                   basic_ostream<_CharT, _Traits>&
>::type
operator<<(basic_ostream<_CharT, _Traits>& __os, const path& __p) {
    __os << std::__quoted(__p.native());
    return __os;
}

template <class _CharT, class _Traits>
_LIBCPP_INLINE_VISIBILITY
typename enable_if<!is_same<_CharT, char>::value ||
                   !is_same<_Traits, char_traits<char>>::value,
                   basic_ostream<_CharT, _Traits>&
>::type
operator<<(basic_ostream<_CharT, _Traits>& __os, const path& __p) {
    __os << std::__quoted(__p.string<_CharT, _Traits>());
    return __os;
}

template <class _CharT, class _Traits>
_LIBCPP_INLINE_VISIBILITY
basic_istream<_CharT, _Traits>&
operator>>(basic_istream<_CharT, _Traits>& __is, path& __p)
{
    basic_string<_CharT, _Traits> __tmp;
    __is >> __quoted(__tmp);
    __p = __tmp;
    return __is;
}

template <class _Source>
_LIBCPP_INLINE_VISIBILITY
typename enable_if<__is_pathable<_Source>::value, path>::type
u8path(const _Source& __s){
    static_assert(is_same<typename __is_pathable<_Source>::__char_type, char>::value,
        "u8path(Source const&) requires Source have a character type of type 'char'");
    return path(__s);
}

template <class _InputIt>
_LIBCPP_INLINE_VISIBILITY
typename enable_if<__is_pathable<_InputIt>::value, path>::type
u8path(_InputIt __f, _InputIt __l) {
    static_assert(is_same<typename __is_pathable<_InputIt>::__char_type, char>::value,
        "u8path(Iter, Iter) requires Iter have a value_type of type 'char'");
    return path(__f, __l);
}

class _LIBCPP_TYPE_VIS path::iterator
{
public:
    typedef bidirectional_iterator_tag iterator_category;
    typedef path                       value_type;
    typedef std::ptrdiff_t             difference_type;
    typedef const path*                pointer;
    typedef const path&                reference;
public:
    _LIBCPP_INLINE_VISIBILITY
    iterator() : __stashed_elem_(), __path_ptr_(nullptr),
                 __entry_(), __state_(__singular) {}

    iterator(const iterator&) = default;
    ~iterator() = default;

    iterator& operator=(const iterator&) = default;

    _LIBCPP_INLINE_VISIBILITY
    reference operator*() const {
        return __stashed_elem_;
    }

    _LIBCPP_INLINE_VISIBILITY
    pointer operator->() const {
        return &__stashed_elem_;
    }

    _LIBCPP_INLINE_VISIBILITY
    iterator& operator++() {
        _LIBCPP_ASSERT(__state_ != __singular,
                       "attempting to increment a singular iterator");
        _LIBCPP_ASSERT(__state_ != __at_end,
                      "attempting to increment the end iterator");
        return __increment();
    }

    _LIBCPP_INLINE_VISIBILITY
    iterator operator++(int) {
        iterator __it(*this);
        this->operator++();
        return __it;
    }

    _LIBCPP_INLINE_VISIBILITY
    iterator& operator--() {
        _LIBCPP_ASSERT(__state_ != __singular,
                       "attempting to decrement a singular iterator");
        _LIBCPP_ASSERT(__entry_.data() != __path_ptr_->native().data(),
                       "attempting to decrement the begin iterator");
        return __decrement();
    }

    _LIBCPP_INLINE_VISIBILITY
    iterator operator--(int) {
        iterator __it(*this);
        this->operator--();
        return __it;
    }

private:
    friend class path;

    static constexpr unsigned char __singular = 0;
    static constexpr unsigned char __at_end = 6;

    inline _LIBCPP_INLINE_VISIBILITY
    friend bool operator==(const iterator&, const iterator&);

    _LIBCPP_FUNC_VIS iterator& __increment();
    _LIBCPP_FUNC_VIS iterator& __decrement();

    path __stashed_elem_;
    const path* __path_ptr_;
    path::__string_view __entry_;
    unsigned char __state_;
};

inline _LIBCPP_INLINE_VISIBILITY
bool operator==(const path::iterator& __lhs, const path::iterator& __rhs) {
    return __lhs.__path_ptr_ == __rhs.__path_ptr_ &&
           __lhs.__entry_.data() == __rhs.__entry_.data();
}

inline _LIBCPP_INLINE_VISIBILITY
bool operator!=(const path::iterator& __lhs, const path::iterator& __rhs) {
    return !(__lhs == __rhs);
}

class _LIBCPP_EXCEPTION_ABI filesystem_error : public system_error
{
public:
    _LIBCPP_INLINE_VISIBILITY
    filesystem_error(const string& __what, error_code __ec)
        : system_error(__ec, __what),
          __paths_(make_shared<_Storage>(path(), path()))
    {}

    _LIBCPP_INLINE_VISIBILITY
    filesystem_error(const string& __what, const path& __p1, error_code __ec)
        : system_error(__ec, __what),
        __paths_(make_shared<_Storage>(__p1, path()))
    {}

    _LIBCPP_INLINE_VISIBILITY
    filesystem_error(const string& __what, const path& __p1, const path& __p2,
                     error_code __ec)
        : system_error(__ec, __what),
          __paths_(make_shared<_Storage>(__p1, __p2))
    {}

    _LIBCPP_INLINE_VISIBILITY
    const path& path1() const _NOEXCEPT {
        return __paths_->first;
    }

    _LIBCPP_INLINE_VISIBILITY
    const path& path2() const _NOEXCEPT {
        return __paths_->second;
    }

    _LIBCPP_FUNC_VIS
    ~filesystem_error() override; // key function

    // TODO(ericwf): Create a custom error message.
    //const char* what() const _NOEXCEPT;

private:
    typedef pair<path, path> _Storage;
    shared_ptr<_Storage> __paths_;
};

template <class... _Args>
_LIBCPP_NORETURN inline _LIBCPP_ALWAYS_INLINE
void __throw_filesystem_error(_Args && ...__args)
{
#ifndef _LIBCPP_NO_EXCEPTIONS
    throw filesystem_error(std::forward<_Args>(__args)...);
#else
    _VSTD::abort();
#endif
}

// operational functions

_LIBCPP_FUNC_VIS
path __canonical(const path&, const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
void __copy(const path& __from, const path& __to, copy_options __opt,
        error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
bool __copy_file(const path& __from, const path& __to, copy_options __opt,
        error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
void __copy_symlink(const path& __existing_symlink, const path& __new_symlink,
        error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
bool __create_directories(const path& p, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
bool __create_directory(const path& p, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
bool __create_directory(const path& p, const path & attributes,
        error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
void __create_directory_symlink(const path& __to, const path& __new_symlink,
        error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
void __create_hard_link(const path& __to, const path& __new_hard_link,
        error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
void __create_symlink(const path& __to, const path& __new_symlink,
        error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
path __current_path(error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
void __current_path(const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
bool __equivalent(const path&, const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
uintmax_t __file_size(const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
uintmax_t __hard_link_count(const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
bool __fs_is_empty(const path& p, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
file_time_type __last_write_time(const path& p, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
void __last_write_time(const path& p, file_time_type new_time,
        error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
void __permissions(const path& p, perms prms, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
path __read_symlink(const path& p, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
bool __remove(const path& p, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
uintmax_t __remove_all(const path& p, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
void __rename(const path& from, const path& to, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
void __resize_file(const path& p, uintmax_t size, error_code *ec=nullptr);
_LIBCPP_FUNC_VIS
space_info __space(const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
file_status __status(const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
file_status __symlink_status(const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
path __system_complete(const path&, error_code *__ec=nullptr);
_LIBCPP_FUNC_VIS
path __temp_directory_path(error_code *__ec=nullptr);

inline _LIBCPP_INLINE_VISIBILITY
path current_path() {
    return __current_path();
}

inline _LIBCPP_INLINE_VISIBILITY
path current_path(error_code& __ec) {
    return __current_path(&__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void current_path(const path& __p) {
    __current_path(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
void current_path(const path& __p, error_code& __ec) _NOEXCEPT {
    __current_path(__p, &__ec);
}

_LIBCPP_FUNC_VIS
path absolute(const path&, const path& __p2 = current_path());

inline _LIBCPP_INLINE_VISIBILITY
path canonical(const path& __p, const path& __base = current_path()) {
    return __canonical(__p, __base);
}

inline _LIBCPP_INLINE_VISIBILITY
path canonical(const path& __p, error_code& __ec) {
    path __base = __current_path(&__ec);
    if (__ec) return {};
    return __canonical(__p, __base, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
path canonical(const path& __p, const path& __base, error_code& __ec) {
    return __canonical(__p, __base, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void copy(const path& __from, const path& __to) {
    __copy(__from, __to, copy_options::none);
}

inline _LIBCPP_INLINE_VISIBILITY
void copy(const path& __from, const path& __to, error_code& __ec) _NOEXCEPT {
    __copy(__from, __to, copy_options::none, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void copy(const path& __from, const path& __to, copy_options __opt) {
    __copy(__from, __to, __opt);
}

inline _LIBCPP_INLINE_VISIBILITY
void copy(const path& __from, const path& __to,
          copy_options __opt, error_code& __ec) _NOEXCEPT {
    __copy(__from, __to, __opt, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool copy_file(const path& __from, const path& __to) {
    return __copy_file(__from, __to, copy_options::none);
}

inline _LIBCPP_INLINE_VISIBILITY
bool copy_file(const path& __from, const path& __to, error_code& __ec) _NOEXCEPT {
    return __copy_file(__from, __to, copy_options::none, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool copy_file(const path& __from, const path& __to, copy_options __opt) {
    return __copy_file(__from, __to, __opt);
}

inline _LIBCPP_INLINE_VISIBILITY
bool copy_file(const path& __from, const path& __to,
               copy_options __opt, error_code& __ec) _NOEXCEPT {
    return __copy_file(__from, __to, __opt, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void copy_symlink(const path& __existing, const path& __new) {
    __copy_symlink(__existing, __new);
}

inline _LIBCPP_INLINE_VISIBILITY
void copy_symlink(const path& __ext, const path& __new, error_code& __ec) _NOEXCEPT {
    __copy_symlink(__ext, __new, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool create_directories(const path& __p) {
    return __create_directories(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
bool create_directories(const path& __p, error_code& __ec) _NOEXCEPT {
    return __create_directories(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool create_directory(const path& __p) {
    return __create_directory(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
bool create_directory(const path& __p, error_code& __ec) _NOEXCEPT {
    return __create_directory(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool create_directory(const path& __p, const path& __attrs) {
    return __create_directory(__p, __attrs);
}

inline _LIBCPP_INLINE_VISIBILITY
bool create_directory(const path& __p, const path& __attrs, error_code& __ec) _NOEXCEPT {
    return __create_directory(__p, __attrs, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void create_directory_symlink(const path& __to, const path& __new) {
    __create_directory_symlink(__to, __new);
}

inline _LIBCPP_INLINE_VISIBILITY
void create_directory_symlink(const path& __to, const path& __new,
                              error_code& __ec) _NOEXCEPT {
    __create_directory_symlink(__to, __new, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void create_hard_link(const path& __to, const path& __new) {
    __create_hard_link(__to, __new);
}

inline _LIBCPP_INLINE_VISIBILITY
void create_hard_link(const path& __to, const path& __new, error_code& __ec) _NOEXCEPT {
    __create_hard_link(__to, __new, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void create_symlink(const path& __to, const path& __new) {
    __create_symlink(__to, __new);
}

inline _LIBCPP_INLINE_VISIBILITY
void create_symlink(const path& __to, const path& __new, error_code& __ec) _NOEXCEPT {
    return __create_symlink(__to, __new, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool status_known(file_status __s) _NOEXCEPT {
    return __s.type() != file_type::none;
}

inline _LIBCPP_INLINE_VISIBILITY
bool exists(file_status __s) _NOEXCEPT {
    return status_known(__s) && __s.type() != file_type::not_found;
}

inline _LIBCPP_INLINE_VISIBILITY
bool exists(const path& __p) {
    return exists(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool exists(const path& __p, error_code& __ec) _NOEXCEPT {
    auto __s = __status(__p, &__ec);
    if (status_known(__s)) __ec.clear();
    return exists(__s);
}

inline _LIBCPP_INLINE_VISIBILITY
bool equivalent(const path& __p1, const path& __p2) {
    return __equivalent(__p1, __p2);
}

inline _LIBCPP_INLINE_VISIBILITY
bool equivalent(const path& __p1, const path& __p2, error_code& __ec) _NOEXCEPT {
    return __equivalent(__p1, __p2, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
uintmax_t file_size(const path& __p) {
    return __file_size(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
uintmax_t file_size(const path& __p, error_code& __ec) _NOEXCEPT {
    return __file_size(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
uintmax_t hard_link_count(const path& __p) {
    return __hard_link_count(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
uintmax_t hard_link_count(const path& __p, error_code& __ec) _NOEXCEPT {
    return __hard_link_count(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_block_file(file_status __s) _NOEXCEPT {
    return __s.type() == file_type::block;
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_block_file(const path& __p) {
    return is_block_file(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_block_file(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_block_file(__status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_character_file(file_status __s) _NOEXCEPT {
    return __s.type() == file_type::character;
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_character_file(const path& __p) {
    return is_character_file(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_character_file(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_character_file(__status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_directory(file_status __s) _NOEXCEPT {
    return __s.type() == file_type::directory;
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_directory(const path& __p) {
    return is_directory(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_directory(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_directory(__status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_empty(const path& __p) {
    return __fs_is_empty(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_empty(const path& __p, error_code& __ec) _NOEXCEPT {
    return __fs_is_empty(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_fifo(file_status __s) _NOEXCEPT {
    return __s.type() == file_type::fifo;
}
inline _LIBCPP_INLINE_VISIBILITY
bool is_fifo(const path& __p) {
    return is_fifo(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_fifo(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_fifo(__status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_regular_file(file_status __s) _NOEXCEPT {
    return __s.type() == file_type::regular;
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_regular_file(const path& __p) {
    return is_regular_file(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_regular_file(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_regular_file(__status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_socket(file_status __s) _NOEXCEPT {
    return __s.type() == file_type::socket;
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_socket(const path& __p) {
    return is_socket(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_socket(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_socket(__status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_symlink(file_status __s) _NOEXCEPT {
    return __s.type() == file_type::symlink;
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_symlink(const path& __p) {
    return is_symlink(__symlink_status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_symlink(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_symlink(__symlink_status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_other(file_status __s) _NOEXCEPT {
    return exists(__s)
        && !is_regular_file(__s) && !is_directory(__s) && !is_symlink(__s);
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_other(const path& __p) {
    return is_other(__status(__p));
}

inline _LIBCPP_INLINE_VISIBILITY
bool is_other(const path& __p, error_code& __ec) _NOEXCEPT {
    return is_other(__status(__p, &__ec));
}

inline _LIBCPP_INLINE_VISIBILITY
file_time_type last_write_time(const path& __p) {
    return __last_write_time(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
file_time_type last_write_time(const path& __p, error_code& __ec) _NOEXCEPT {
    return __last_write_time(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void last_write_time(const path& __p, file_time_type __t) {
    __last_write_time(__p, __t);
}

inline _LIBCPP_INLINE_VISIBILITY
void last_write_time(const path& __p, file_time_type __t, error_code& __ec) _NOEXCEPT {
    __last_write_time(__p, __t, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void permissions(const path& __p, perms __prms) {
    __permissions(__p, __prms);
}

inline _LIBCPP_INLINE_VISIBILITY
void permissions(const path& __p, perms __prms, error_code& __ec)  {
    __permissions(__p, __prms, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
path read_symlink(const path& __p) {
    return __read_symlink(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
path read_symlink(const path& __p, error_code& __ec) {
    return __read_symlink(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
bool remove(const path& __p) {
    return __remove(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
bool remove(const path& __p, error_code& __ec) _NOEXCEPT {
    return __remove(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
uintmax_t remove_all(const path& __p) {
    return __remove_all(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
uintmax_t remove_all(const path& __p, error_code& __ec) _NOEXCEPT {
    return __remove_all(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void rename(const path& __from, const path& __to) {
    return __rename(__from, __to);
}

inline _LIBCPP_INLINE_VISIBILITY
void rename(const path& __from, const path& __to, error_code& __ec) _NOEXCEPT {
    return __rename(__from, __to, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
void resize_file(const path& __p, uintmax_t __ns) {
    return __resize_file(__p, __ns);
}

inline _LIBCPP_INLINE_VISIBILITY
void resize_file(const path& __p, uintmax_t __ns, error_code& __ec) _NOEXCEPT {
    return __resize_file(__p, __ns, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
space_info space(const path& __p) {
    return __space(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
space_info space(const path& __p, error_code& __ec) _NOEXCEPT {
    return __space(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
file_status status(const path& __p) {
    return __status(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
file_status status(const path& __p, error_code& __ec) _NOEXCEPT {
    return __status(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
file_status symlink_status(const path& __p) {
    return __symlink_status(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
file_status symlink_status(const path& __p, error_code& __ec) _NOEXCEPT {
    return __symlink_status(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
path system_complete(const path& __p) {
    return __system_complete(__p);
}

inline _LIBCPP_INLINE_VISIBILITY
path system_complete(const path& __p, error_code& __ec) {
    return __system_complete(__p, &__ec);
}

inline _LIBCPP_INLINE_VISIBILITY
path temp_directory_path() {
    return __temp_directory_path();
}

inline _LIBCPP_INLINE_VISIBILITY
path temp_directory_path(error_code& __ec) {
    return __temp_directory_path(&__ec);
}


class directory_entry
{
    typedef _VSTD_FS::path _Path;

public:
    // constructors and destructors
    directory_entry() _NOEXCEPT = default;
    directory_entry(directory_entry const&) = default;
    directory_entry(directory_entry&&) _NOEXCEPT = default;

    _LIBCPP_INLINE_VISIBILITY
    explicit directory_entry(_Path const& __p) : __p_(__p) {}

    ~directory_entry() {}

    directory_entry& operator=(directory_entry const&) = default;
    directory_entry& operator=(directory_entry&&) _NOEXCEPT = default;

    _LIBCPP_INLINE_VISIBILITY
    void assign(_Path const& __p) {
        __p_ = __p;
    }

    _LIBCPP_INLINE_VISIBILITY
    void replace_filename(_Path const& __p) {
        __p_ = __p_.parent_path() / __p;
    }

    _LIBCPP_INLINE_VISIBILITY
    _Path const& path() const _NOEXCEPT {
        return __p_;
    }

    _LIBCPP_INLINE_VISIBILITY
    operator const _Path&() const _NOEXCEPT {
        return __p_;
    }

    _LIBCPP_INLINE_VISIBILITY
    file_status status() const {
        return _VSTD_FS::status(__p_);
    }

    _LIBCPP_INLINE_VISIBILITY
    file_status status(error_code& __ec) const _NOEXCEPT {
        return _VSTD_FS::status(__p_, __ec);
    }

    _LIBCPP_INLINE_VISIBILITY
    file_status symlink_status() const {
        return _VSTD_FS::symlink_status(__p_);
    }

    _LIBCPP_INLINE_VISIBILITY
    file_status symlink_status(error_code& __ec) const _NOEXCEPT {
        return _VSTD_FS::symlink_status(__p_, __ec);
    }

    _LIBCPP_INLINE_VISIBILITY
    bool operator< (directory_entry const& __rhs) const _NOEXCEPT {
        return __p_ < __rhs.__p_;
    }

    _LIBCPP_INLINE_VISIBILITY
    bool operator==(directory_entry const& __rhs) const _NOEXCEPT {
        return __p_ == __rhs.__p_;
    }

    _LIBCPP_INLINE_VISIBILITY
    bool operator!=(directory_entry const& __rhs) const _NOEXCEPT {
        return __p_ != __rhs.__p_;
    }

    _LIBCPP_INLINE_VISIBILITY
    bool operator<=(directory_entry const& __rhs) const _NOEXCEPT {
        return __p_ <= __rhs.__p_;
    }

    _LIBCPP_INLINE_VISIBILITY
    bool operator> (directory_entry const& __rhs) const _NOEXCEPT {
        return __p_ > __rhs.__p_;
    }

    _LIBCPP_INLINE_VISIBILITY
    bool operator>=(directory_entry const& __rhs) const _NOEXCEPT {
        return __p_ >= __rhs.__p_;
    }
private:
    _Path __p_;
};


class directory_iterator;
class recursive_directory_iterator;
class __dir_stream;

class __dir_element_proxy {
public:

    inline _LIBCPP_INLINE_VISIBILITY
    directory_entry operator*() { return _VSTD::move(__elem_); }

private:
    friend class directory_iterator;
    friend class recursive_directory_iterator;
    explicit __dir_element_proxy(directory_entry const& __e) : __elem_(__e) {}
    __dir_element_proxy(__dir_element_proxy&& __o) : __elem_(_VSTD::move(__o.__elem_)) {}
    directory_entry __elem_;
};

class directory_iterator
{
public:
    typedef directory_entry value_type;
    typedef ptrdiff_t difference_type;
    typedef value_type const* pointer;
    typedef value_type const& reference;
    typedef input_iterator_tag iterator_category;

public:
    //ctor & dtor
    directory_iterator() _NOEXCEPT
    { }

    explicit directory_iterator(const path& __p)
        : directory_iterator(__p, nullptr)
    { }

    directory_iterator(const path& __p, directory_options __opts)
        : directory_iterator(__p, nullptr, __opts)
    { }

    directory_iterator(const path& __p, error_code& __ec) _NOEXCEPT
        : directory_iterator(__p, &__ec)
    { }

    directory_iterator(const path& __p, directory_options __opts,
                       error_code& __ec) _NOEXCEPT
        : directory_iterator(__p, &__ec, __opts)
    { }

    directory_iterator(const directory_iterator&) = default;
    directory_iterator(directory_iterator&&) = default;
    directory_iterator& operator=(const directory_iterator&) = default;

    directory_iterator& operator=(directory_iterator&& __o) _NOEXCEPT {
        // non-default implementation provided to support self-move assign.
        if (this != &__o) {
            __imp_ = _VSTD::move(__o.__imp_);
        }
        return *this;
    }

    ~directory_iterator() = default;

    const directory_entry& operator*() const {
        _LIBCPP_ASSERT(__imp_, "The end iterator cannot be dereferenced");
        return __deref();
    }

    const directory_entry* operator->() const
    { return &**this; }

    directory_iterator& operator++()
    { return __increment(); }

    __dir_element_proxy operator++(int) {
        __dir_element_proxy __p(**this);
        __increment();
        return __p;
    }

    directory_iterator& increment(error_code& __ec) _NOEXCEPT
    { return __increment(&__ec); }

private:
    inline _LIBCPP_INLINE_VISIBILITY
    friend bool operator==(const directory_iterator& __lhs,
                           const directory_iterator& __rhs) _NOEXCEPT;

    // construct the dir_stream
    _LIBCPP_FUNC_VIS
    directory_iterator(const path&, error_code *, directory_options = directory_options::none);
    _LIBCPP_FUNC_VIS
    directory_iterator& __increment(error_code * __ec = nullptr);
    _LIBCPP_FUNC_VIS
    const directory_entry& __deref() const;

private:
    shared_ptr<__dir_stream> __imp_;
};


inline _LIBCPP_INLINE_VISIBILITY
bool operator==(const directory_iterator& __lhs,
                const directory_iterator& __rhs) _NOEXCEPT {
    return __lhs.__imp_ == __rhs.__imp_;
}

inline _LIBCPP_INLINE_VISIBILITY
bool operator!=(const directory_iterator& __lhs,
                const directory_iterator& __rhs) _NOEXCEPT {
    return !(__lhs == __rhs);
}

// enable directory_iterator range-based for statements
inline _LIBCPP_INLINE_VISIBILITY
directory_iterator begin(directory_iterator __iter) _NOEXCEPT {
    return __iter;
}

inline _LIBCPP_INLINE_VISIBILITY
directory_iterator end(const directory_iterator&) _NOEXCEPT {
    return directory_iterator();
}

class recursive_directory_iterator {
public:
    using value_type = directory_entry;
    using difference_type = std::ptrdiff_t;
    using pointer = directory_entry const *;
    using reference = directory_entry const &;
    using iterator_category = std::input_iterator_tag;

public:
    // constructors and destructor
    _LIBCPP_INLINE_VISIBILITY
    recursive_directory_iterator()  _NOEXCEPT
        : __rec_(false)
    {}

    _LIBCPP_INLINE_VISIBILITY
    explicit recursive_directory_iterator(const path& __p,
                directory_options __xoptions = directory_options::none)
        : recursive_directory_iterator(__p, __xoptions,  nullptr)
    { }

    _LIBCPP_INLINE_VISIBILITY
    recursive_directory_iterator(const path& __p,
        directory_options __xoptions, error_code& __ec) _NOEXCEPT
        : recursive_directory_iterator(__p, __xoptions, &__ec)
    { }

    _LIBCPP_INLINE_VISIBILITY
    recursive_directory_iterator(const path& __p, error_code& __ec) _NOEXCEPT
        : recursive_directory_iterator(__p, directory_options::none,  &__ec)
    { }

    recursive_directory_iterator(const recursive_directory_iterator&) = default;
    recursive_directory_iterator(recursive_directory_iterator&&) = default;

    recursive_directory_iterator &
    operator=(const recursive_directory_iterator&) = default;

    _LIBCPP_INLINE_VISIBILITY
    recursive_directory_iterator &
    operator=(recursive_directory_iterator&& __o) noexcept {
        // non-default implementation provided to support self-move assign.
        if (this != &__o) {
            __imp_ = _VSTD::move(__o.__imp_);
            __rec_ = __o.__rec_;
        }
        return *this;
    }

    ~recursive_directory_iterator() = default;

    _LIBCPP_INLINE_VISIBILITY
    const directory_entry& operator*() const
    { return __deref(); }

    _LIBCPP_INLINE_VISIBILITY
    const directory_entry* operator->() const
    { return &__deref(); }

    recursive_directory_iterator& operator++()
    { return __increment(); }

    _LIBCPP_INLINE_VISIBILITY
    __dir_element_proxy operator++(int) {
        __dir_element_proxy __p(**this);
        __increment();
        return __p;
    }

    _LIBCPP_INLINE_VISIBILITY
    recursive_directory_iterator& increment(error_code& __ec) _NOEXCEPT
    { return __increment(&__ec); }

    _LIBCPP_FUNC_VIS directory_options options() const;
    _LIBCPP_FUNC_VIS int  depth() const;

    _LIBCPP_INLINE_VISIBILITY
    void pop() { __pop(); }

    _LIBCPP_INLINE_VISIBILITY
    void pop(error_code& __ec)
    { __pop(&__ec); }

    _LIBCPP_INLINE_VISIBILITY
    bool recursion_pending() const
    { return __rec_; }

    _LIBCPP_INLINE_VISIBILITY
    void disable_recursion_pending()
    { __rec_ = false; }

private:
    recursive_directory_iterator(const path& __p, directory_options __opt,
                                 error_code *__ec);

    _LIBCPP_FUNC_VIS
    const directory_entry& __deref() const;

    _LIBCPP_FUNC_VIS
    bool __try_recursion(error_code* __ec);

    _LIBCPP_FUNC_VIS
    void __advance(error_code* __ec=nullptr);

    _LIBCPP_FUNC_VIS
    recursive_directory_iterator& __increment(error_code *__ec=nullptr);

    _LIBCPP_FUNC_VIS
    void __pop(error_code* __ec=nullptr);

    inline _LIBCPP_INLINE_VISIBILITY
    friend bool operator==(const recursive_directory_iterator&,
                           const recursive_directory_iterator&) _NOEXCEPT;

    struct __shared_imp;
    shared_ptr<__shared_imp> __imp_;
    bool __rec_;
};                                     // class recursive_directory_iterator


inline _LIBCPP_INLINE_VISIBILITY
bool operator==(const recursive_directory_iterator& __lhs,
                const recursive_directory_iterator& __rhs) _NOEXCEPT
{
    return __lhs.__imp_ == __rhs.__imp_;
}

_LIBCPP_INLINE_VISIBILITY
inline bool operator!=(const recursive_directory_iterator& __lhs,
                        const recursive_directory_iterator& __rhs) _NOEXCEPT
{
    return !(__lhs == __rhs);
}
// enable recursive_directory_iterator range-based for statements
inline _LIBCPP_INLINE_VISIBILITY
recursive_directory_iterator begin(recursive_directory_iterator __iter) _NOEXCEPT {
    return __iter;
}

inline _LIBCPP_INLINE_VISIBILITY
recursive_directory_iterator end(const recursive_directory_iterator&) _NOEXCEPT {
    return recursive_directory_iterator();
}

_LIBCPP_END_NAMESPACE_EXPERIMENTAL_FILESYSTEM

#endif // _LIBCPP_EXPERIMENTAL_FILESYSTEM