lib: establish Zircon libc++ and KTL foundation

4 files changed, 2141 insertions, 0 deletions

diff --git a/third_party/libcxx/include/c++/v1/__expected/bad_expected_access.h b/third_party/libcxx/include/c++/v1/__expected/bad_expected_access.h
new file mode 100644
index 0000000..b195810
--- /dev/null
+++ b/third_party/libcxx/include/c++/v1/__expected/bad_expected_access.h
@@ -0,0 +1,75 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef _LIBCPP___EXPECTED_BAD_EXPECTED_ACCESS_H
+#define _LIBCPP___EXPECTED_BAD_EXPECTED_ACCESS_H
+
+#include <__config>
+#include <__exception/exception.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Err>
+class bad_expected_access;
+
+_LIBCPP_DIAGNOSTIC_PUSH
+#  if !_LIBCPP_AVAILABILITY_HAS_BAD_EXPECTED_ACCESS_KEY_FUNCTION
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables")
+#  endif
+template <>
+class _LIBCPP_EXPORTED_FROM_ABI bad_expected_access<void> : public exception {
+protected:
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access() noexcept                                      = default;
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access(const bad_expected_access&) noexcept            = default;
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access(bad_expected_access&&) noexcept                 = default;
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access& operator=(const bad_expected_access&) noexcept = default;
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access& operator=(bad_expected_access&&) noexcept      = default;
+  _LIBCPP_HIDE_FROM_ABI_VIRTUAL ~bad_expected_access() override                             = default;
+
+public:
+#  if _LIBCPP_AVAILABILITY_HAS_BAD_EXPECTED_ACCESS_KEY_FUNCTION
+  [[nodiscard]] const char* what() const noexcept override;
+#  else
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI_VIRTUAL const char* what() const noexcept override {
+    return "bad access to std::expected";
+  }
+#  endif
+};
+_LIBCPP_DIAGNOSTIC_POP
+
+template <class _Err>
+class bad_expected_access : public bad_expected_access<void> {
+public:
+  _LIBCPP_HIDE_FROM_ABI explicit bad_expected_access(_Err __e) : __unex_(std::move(__e)) {}
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _Err& error() & noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const _Err& error() const& noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _Err&& error() && noexcept { return std::move(__unex_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const _Err&& error() const&& noexcept { return std::move(__unex_); }
+
+private:
+  _Err __unex_;
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___EXPECTED_BAD_EXPECTED_ACCESS_H
diff --git a/third_party/libcxx/include/c++/v1/__expected/expected.h b/third_party/libcxx/include/c++/v1/__expected/expected.h
new file mode 100644
index 0000000..24ae33d
--- /dev/null
+++ b/third_party/libcxx/include/c++/v1/__expected/expected.h
@@ -0,0 +1,1907 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef _LIBCPP___EXPECTED_EXPECTED_H
+#define _LIBCPP___EXPECTED_EXPECTED_H
+
+#include <__assert>
+#include <__config>
+#include <__expected/bad_expected_access.h>
+#include <__expected/unexpect.h>
+#include <__expected/unexpected.h>
+#include <__functional/invoke.h>
+#include <__memory/addressof.h>
+#include <__memory/construct_at.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/disjunction.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_assignable.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_convertible.h>
+#include <__type_traits/is_core_convertible.h>
+#include <__type_traits/is_function.h>
+#include <__type_traits/is_nothrow_assignable.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_swappable.h>
+#include <__type_traits/is_trivially_constructible.h>
+#include <__type_traits/is_trivially_destructible.h>
+#include <__type_traits/is_trivially_relocatable.h>
+#include <__type_traits/is_void.h>
+#include <__type_traits/lazy.h>
+#include <__type_traits/negation.h>
+#include <__type_traits/remove_cv.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/as_const.h>
+#include <__utility/exception_guard.h>
+#include <__utility/forward.h>
+#include <__utility/in_place.h>
+#include <__utility/move.h>
+#include <__utility/swap.h>
+#include <__verbose_abort>
+#include <initializer_list>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Tp, class _Err>
+class expected;
+
+template <class _Tp>
+struct __is_std_expected : false_type {};
+
+template <class _Tp, class _Err>
+struct __is_std_expected<expected<_Tp, _Err>> : true_type {};
+
+struct __expected_construct_in_place_from_invoke_tag {};
+struct __expected_construct_unexpected_from_invoke_tag {};
+
+template <class _Err, class _Arg>
+_LIBCPP_HIDE_FROM_ABI void __throw_bad_expected_access(_Arg&& __arg) {
+#  if _LIBCPP_HAS_EXCEPTIONS
+  throw bad_expected_access<_Err>(std::forward<_Arg>(__arg));
+#  else
+  (void)__arg;
+  _LIBCPP_VERBOSE_ABORT("bad_expected_access was thrown in -fno-exceptions mode");
+#  endif
+}
+
+// If parameter type `_Tp` of `__conditional_no_unique_address` is neither
+// copyable nor movable, a constructor with this tag is provided. For that
+// constructor, the user has to provide a function and arguments. The function
+// must return an object of type `_Tp`. When the function is invoked by the
+// constructor, guaranteed copy elision kicks in and the `_Tp` is constructed
+// in place.
+struct __conditional_no_unique_address_invoke_tag {};
+
+// This class implements an object with `[[no_unique_address]]` conditionally applied to it,
+// based on the value of `_NoUnique`.
+//
+// A member of this class must always have `[[no_unique_address]]` applied to
+// it. Otherwise, the `[[no_unique_address]]` in the "`_NoUnique == true`" case
+// would not have any effect. In the `false` case, the `__v` is not
+// `[[no_unique_address]]`, so nullifies the effects of the "outer"
+// `[[no_unique_address]]` regarding data layout.
+//
+// If we had a language feature, this class would basically be replaced by `[[no_unique_address(condition)]]`.
+template <bool _NoUnique, class _Tp>
+struct __conditional_no_unique_address;
+
+template <class _Tp>
+struct __conditional_no_unique_address<true, _Tp> {
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __conditional_no_unique_address(in_place_t, _Args&&... __args)
+      : __v(std::forward<_Args>(__args)...) {}
+
+  template <class _Func, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __conditional_no_unique_address(
+      __conditional_no_unique_address_invoke_tag, _Func&& __f, _Args&&... __args)
+      : __v(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+  _LIBCPP_NO_UNIQUE_ADDRESS _Tp __v;
+};
+
+template <class _Tp>
+struct __conditional_no_unique_address<false, _Tp> {
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __conditional_no_unique_address(in_place_t, _Args&&... __args)
+      : __v(std::forward<_Args>(__args)...) {}
+
+  template <class _Func, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __conditional_no_unique_address(
+      __conditional_no_unique_address_invoke_tag, _Func&& __f, _Args&&... __args)
+      : __v(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+  _Tp __v;
+};
+
+// This function returns whether the type `_Second` can be stuffed into the tail padding
+// of the `_First` type if both of them are given `[[no_unique_address]]`.
+template <class _First, class _Second>
+inline constexpr bool __fits_in_tail_padding = []() {
+  struct __x {
+    _LIBCPP_NO_UNIQUE_ADDRESS _First __first;
+    _LIBCPP_NO_UNIQUE_ADDRESS _Second __second;
+  };
+  return sizeof(__x) == sizeof(_First);
+}();
+
+// This class implements the storage used by `std::expected`. We have a few
+// goals for this storage:
+// 1. Whenever the underlying {_Tp | _Unex} combination has free bytes in its
+//    tail padding, we should reuse it to store the bool discriminator of the
+//    expected, so as to save space.
+// 2. Whenever the `expected<_Tp, _Unex>` as a whole has free bytes in its tail
+//    padding, we should allow an object following the expected to be stored in
+//    its tail padding.
+// 3. However, we never want a user object (say `X`) that would follow an
+//    `expected<_Tp, _Unex>` to be stored in the padding bytes of the
+//    underlying {_Tp | _Unex} union, if any. That is because we use
+//    `construct_at` on that union, which would end up overwriting the `X`
+//    member if it is stored in the tail padding of the union.
+//
+// To achieve this, `__expected_base`'s logic is implemented in an inner
+// `__repr` class. `__expected_base` holds one `__repr` member which is
+// conditionally `[[no_unique_address]]`. The `__repr` class holds the
+// underlying {_Tp | _Unex} union and a boolean "has value" flag.
+//
+// Which one of the `__repr_`/`__union_` members is `[[no_unique_address]]`
+// depends on whether the "has value" boolean fits into the tail padding of
+// the underlying {_Tp | _Unex} union:
+//
+// - In case the "has value" bool fits into the tail padding of the union, the
+//   whole `__repr_` member is _not_ `[[no_unique_address]]` as it needs to be
+//   transparently replaced on `emplace()`/`swap()` etc.
+// - In case the "has value" bool does not fit into the tail padding of the
+//   union, only the union member must be transparently replaced (therefore is
+//   _not_ `[[no_unique_address]]`) and the "has value" flag must be adjusted
+//   manually.
+//
+// This way, the member that is transparently replaced on mutating operations
+// is never `[[no_unique_address]]`, satisfying the requirements from
+// "[basic.life]" in the standard.
+//
+// Stripped away of all superfluous elements, the layout of `__expected_base`
+// then looks like this:
+//
+//     template <class Tp, class Err>
+//     class expected_base {
+//       union union_t {
+//         [[no_unique_address]] Tp val;
+//         [[no_unique_address]] Err unex;
+//       };
+//
+//       static constexpr bool put_flag_in_tail                    = fits_in_tail_padding<union_t, bool>;
+//       static constexpr bool allow_reusing_expected_tail_padding = !put_flag_in_tail;
+//
+//       struct repr {
+//       private:
+//         // If "has value" fits into the tail, this should be
+//         // `[[no_unique_address]]`, otherwise not.
+//         [[no_unique_address]] conditional_no_unique_address<
+//             put_flag_in_tail,
+//             union_t>::type union_;
+//         [[no_unique_address]] bool has_val_;
+//       };
+//
+//     protected:
+//       // If "has value" fits into the tail, this must _not_ be
+//       // `[[no_unique_address]]` so that we fill out the
+//       // complete `expected` object.
+//       [[no_unique_address]] conditional_no_unique_address<
+//           allow_reusing_expected_tail_padding,
+//           repr>::type repr_;
+//     };
+//
+template <class _Tp, class _Err>
+class __expected_base {
+  // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
+  // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
+  // it's not clear that it's implementable, given that the function is allowed to clobber
+  // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
+  union __union_t {
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&)
+      requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
+               is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(__union_t&&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(__union_t&&)
+      requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
+               is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(__union_t&&)      = delete;
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(in_place_t, _Args&&... __args)
+        : __val_(std::forward<_Args>(__args)...) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(unexpect_t, _Args&&... __args)
+        : __unex_(std::forward<_Args>(__args)...) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
+        std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __val_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
+        std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
+      requires(is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Err>)
+    = default;
+
+    // __repr's destructor handles this
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t() {}
+
+    _LIBCPP_NO_UNIQUE_ADDRESS _Tp __val_;
+    _LIBCPP_NO_UNIQUE_ADDRESS _Err __unex_;
+  };
+
+  static constexpr bool __put_flag_in_tail                    = __fits_in_tail_padding<__union_t, bool>;
+  static constexpr bool __allow_reusing_expected_tail_padding = !__put_flag_in_tail;
+
+  struct __repr {
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr() = delete;
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(in_place_t __tag, _Args&&... __args)
+        : __union_(in_place, __tag, std::forward<_Args>(__args)...), __has_val_(true) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(unexpect_t __tag, _Args&&... __args)
+        : __union_(in_place, __tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(std::__expected_construct_in_place_from_invoke_tag __tag,
+                                                    _Args&&... __args)
+        : __union_(in_place, __tag, std::forward<_Args>(__args)...), __has_val_(true) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(std::__expected_construct_unexpected_from_invoke_tag __tag,
+                                                    _Args&&... __args)
+        : __union_(in_place, __tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    // The return value of `__make_union` must be constructed in place in the
+    // `__v` member of `__union_`, relying on guaranteed copy elision. To do
+    // this, the `__conditional_no_unique_address_invoke_tag` constructor is
+    // called with a lambda that is immediately called inside
+    // `__conditional_no_unique_address`'s constructor.
+    template <class _OtherUnion>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(bool __has_val, _OtherUnion&& __other)
+      requires(__allow_reusing_expected_tail_padding)
+        : __union_(__conditional_no_unique_address_invoke_tag{},
+                   [&] { return __make_union(__has_val, std::forward<_OtherUnion>(__other)); }),
+          __has_val_(__has_val) {}
+
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(const __repr&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(const __repr&)
+      requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
+               is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(__repr&&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(__repr&&)
+      requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
+               is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>)
+    = default;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr& operator=(const __repr&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr& operator=(__repr&&)      = delete;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__repr()
+      requires(is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Err>)
+    = default;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__repr()
+      requires(!is_trivially_destructible_v<_Tp> || !is_trivially_destructible_v<_Err>)
+    {
+      __destroy_union_member();
+    }
+
+    _LIBCPP_HIDE_FROM_ABI constexpr void __destroy_union()
+      requires(__allow_reusing_expected_tail_padding &&
+               (is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Err>))
+    {
+      // Note: Since the destructor of the union is trivial, this does nothing
+      // except to end the lifetime of the union.
+      std::destroy_at(&__union_.__v);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI constexpr void __destroy_union()
+      requires(__allow_reusing_expected_tail_padding &&
+               (!is_trivially_destructible_v<_Tp> || !is_trivially_destructible_v<_Err>))
+    {
+      __destroy_union_member();
+      std::destroy_at(&__union_.__v);
+    }
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr void __construct_union(in_place_t, _Args&&... __args)
+      requires(__allow_reusing_expected_tail_padding)
+    {
+      std::construct_at(&__union_.__v, in_place, std::forward<_Args>(__args)...);
+      __has_val_ = true;
+    }
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr void __construct_union(unexpect_t, _Args&&... __args)
+      requires(__allow_reusing_expected_tail_padding)
+    {
+      std::construct_at(&__union_.__v, unexpect, std::forward<_Args>(__args)...);
+      __has_val_ = false;
+    }
+
+  private:
+    template <class, class>
+    friend class __expected_base;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr void __destroy_union_member()
+      requires(!is_trivially_destructible_v<_Tp> || !is_trivially_destructible_v<_Err>)
+    {
+      if (__has_val_) {
+        std::destroy_at(std::addressof(__union_.__v.__val_));
+      } else {
+        std::destroy_at(std::addressof(__union_.__v.__unex_));
+      }
+    }
+
+    template <class _OtherUnion>
+    _LIBCPP_HIDE_FROM_ABI static constexpr __union_t __make_union(bool __has_val, _OtherUnion&& __other)
+      requires(__allow_reusing_expected_tail_padding)
+    {
+      if (__has_val)
+        return __union_t(in_place, std::forward<_OtherUnion>(__other).__val_);
+      else
+        return __union_t(unexpect, std::forward<_OtherUnion>(__other).__unex_);
+    }
+
+    _LIBCPP_NO_UNIQUE_ADDRESS __conditional_no_unique_address<__put_flag_in_tail, __union_t> __union_;
+    _LIBCPP_NO_UNIQUE_ADDRESS bool __has_val_;
+  };
+
+  template <class _OtherUnion>
+  _LIBCPP_HIDE_FROM_ABI static constexpr __repr __make_repr(bool __has_val, _OtherUnion&& __other)
+    requires(__put_flag_in_tail)
+  {
+    if (__has_val)
+      return __repr(in_place, std::forward<_OtherUnion>(__other).__val_);
+    else
+      return __repr(unexpect, std::forward<_OtherUnion>(__other).__unex_);
+  }
+
+protected:
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_base(_Args&&... __args)
+      : __repr_(in_place, std::forward<_Args>(__args)...) {}
+
+  // In case we copy/move construct from another `expected` we need to create
+  // our `expected` so that it either has a value or not, depending on the "has
+  // value" flag of the other `expected`. To do this without falling back on
+  // `std::construct_at` we rely on guaranteed copy elision using two helper
+  // functions `__make_repr` and `__make_union`. There have to be two since
+  // there are two data layouts with different members being
+  // `[[no_unique_address]]`. GCC (as of version 13) does not do guaranteed
+  // copy elision when initializing `[[no_unique_address]]` members. The two
+  // cases are:
+  //
+  // - `__make_repr`: This is used when the "has value" flag lives in the tail
+  //   of the union. In this case, the `__repr` member is _not_
+  //   `[[no_unique_address]]`.
+  // - `__make_union`: When the "has value" flag does _not_ fit in the tail of
+  //   the union, the `__repr` member is `[[no_unique_address]]` and the union
+  //   is not.
+  //
+  // This constructor "catches" the first case and leaves the second case to
+  // `__union_t`, its constructors and `__make_union`.
+  template <class _OtherUnion>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_base(bool __has_val, _OtherUnion&& __other)
+    requires(__put_flag_in_tail)
+      : __repr_(__conditional_no_unique_address_invoke_tag{},
+                [&] { return __make_repr(__has_val, std::forward<_OtherUnion>(__other)); }) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void __destroy() {
+    if constexpr (__put_flag_in_tail)
+      std::destroy_at(&__repr_.__v);
+    else
+      __repr_.__v.__destroy_union();
+  }
+
+  template <class _Tag, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr void __construct(_Tag __tag, _Args&&... __args) {
+    if constexpr (__put_flag_in_tail)
+      std::construct_at(&__repr_.__v, __tag, std::forward<_Args>(__args)...);
+    else
+      __repr_.__v.__construct_union(__tag, std::forward<_Args>(__args)...);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __has_val() const { return __repr_.__v.__has_val_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr __union_t& __union() { return __repr_.__v.__union_.__v; }
+  _LIBCPP_HIDE_FROM_ABI constexpr const __union_t& __union() const { return __repr_.__v.__union_.__v; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& __val() { return __repr_.__v.__union_.__v.__val_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& __val() const { return __repr_.__v.__union_.__v.__val_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Err& __unex() { return __repr_.__v.__union_.__v.__unex_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr const _Err& __unex() const { return __repr_.__v.__union_.__v.__unex_; }
+
+private:
+  _LIBCPP_NO_UNIQUE_ADDRESS __conditional_no_unique_address<__allow_reusing_expected_tail_padding, __repr> __repr_;
+};
+
+template <class _Tp, class _Err>
+class expected : private __expected_base<_Tp, _Err> {
+  static_assert(!is_reference_v<_Tp> && !is_function_v<_Tp> && !is_same_v<remove_cv_t<_Tp>, in_place_t> &&
+                    !is_same_v<remove_cv_t<_Tp>, unexpect_t> && !__is_std_unexpected<remove_cv_t<_Tp>>::value &&
+                    __valid_std_unexpected<_Err>::value,
+                "[expected.object.general] A program that instantiates the definition of template expected<T, E> for a "
+                "reference type, a function type, or for possibly cv-qualified types in_place_t, unexpect_t, or a "
+                "specialization of unexpected for the T parameter is ill-formed. A program that instantiates the "
+                "definition of the template expected<T, E> with a type for the E parameter that is not a valid "
+                "template argument for unexpected is ill-formed.");
+
+  template <class _Up, class _OtherErr>
+  friend class expected;
+
+  using __base _LIBCPP_NODEBUG = __expected_base<_Tp, _Err>;
+
+public:
+  using value_type      = _Tp;
+  using error_type      = _Err;
+  using unexpected_type = unexpected<_Err>;
+
+  using __trivially_relocatable _LIBCPP_NODEBUG =
+      __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value && __libcpp_is_trivially_relocatable<_Err>::value,
+                      expected,
+                      void>;
+
+  template <class _Up>
+  using rebind = expected<_Up, error_type>;
+
+  // [expected.object.ctor], constructors
+  _LIBCPP_HIDE_FROM_ABI constexpr expected() noexcept(is_nothrow_default_constructible_v<_Tp>) // strengthened
+    requires is_default_constructible_v<_Tp>
+      : __base(in_place) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&)
+    requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> && is_trivially_copy_constructible_v<_Tp> &&
+             is_trivially_copy_constructible_v<_Err>)
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __other) noexcept(
+      is_nothrow_copy_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Err>) // strengthened
+    requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
+             !(is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>))
+      : __base(__other.__has_val(), __other.__union()) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
+    requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Tp> &&
+             is_trivially_move_constructible_v<_Err>)
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __other) noexcept(
+      is_nothrow_move_constructible_v<_Tp> && is_nothrow_move_constructible_v<_Err>)
+    requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
+             !(is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>))
+      : __base(__other.__has_val(), std::move(__other.__union())) {}
+
+private:
+  template <class _Up, class _OtherErr, class _UfQual, class _OtherErrQual>
+  using __can_convert _LIBCPP_NODEBUG = _And<
+      is_constructible<_Tp, _UfQual>,
+      is_constructible<_Err, _OtherErrQual>,
+      _If<_Not<is_same<remove_cv_t<_Tp>, bool>>::value,
+          _And< _Not<_And<is_same<_Tp, _Up>, is_same<_Err, _OtherErr>>>, // use the copy constructor instead, see #92676
+                _Not<is_constructible<_Tp, expected<_Up, _OtherErr>&>>,
+                _Not<is_constructible<_Tp, expected<_Up, _OtherErr>>>,
+                _Not<is_constructible<_Tp, const expected<_Up, _OtherErr>&>>,
+                _Not<is_constructible<_Tp, const expected<_Up, _OtherErr>>>,
+                _Not<is_convertible<expected<_Up, _OtherErr>&, _Tp>>,
+                _Not<is_convertible<expected<_Up, _OtherErr>&&, _Tp>>,
+                _Not<is_convertible<const expected<_Up, _OtherErr>&, _Tp>>,
+                _Not<is_convertible<const expected<_Up, _OtherErr>&&, _Tp>>>,
+          true_type>,
+      _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>&>>,
+      _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>>>,
+      _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>&>>,
+      _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>>> >;
+
+  template <class _Func, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
+      std::__expected_construct_in_place_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
+      : __base(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...) {}
+
+  template <class _Func, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
+      std::__expected_construct_unexpected_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
+      : __base(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...) {}
+
+public:
+  template <class _Up, class _OtherErr>
+    requires __can_convert<_Up, _OtherErr, const _Up&, const _OtherErr&>::value
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _Up&, _Tp> ||
+                                           !is_convertible_v<const _OtherErr&, _Err>)
+      expected(const expected<_Up, _OtherErr>& __other) noexcept(
+          is_nothrow_constructible_v<_Tp, const _Up&> &&
+          is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
+      : __base(__other.__has_val(), __other.__union()) {}
+
+  template <class _Up, class _OtherErr>
+    requires __can_convert<_Up, _OtherErr, _Up, _OtherErr>::value
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp> || !is_convertible_v<_OtherErr, _Err>)
+      expected(expected<_Up, _OtherErr>&& __other) noexcept(
+          is_nothrow_constructible_v<_Tp, _Up> && is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
+      : __base(__other.__has_val(), std::move(__other.__union())) {}
+
+  template <class _Up = remove_cv_t<_Tp>>
+    requires(!is_same_v<remove_cvref_t<_Up>, in_place_t> && !is_same_v<expected, remove_cvref_t<_Up>> &&
+             !is_same_v<remove_cvref_t<_Up>, unexpect_t> && is_constructible_v<_Tp, _Up> &&
+             !__is_std_unexpected<remove_cvref_t<_Up>>::value &&
+             (!is_same_v<remove_cv_t<_Tp>, bool> || !__is_std_expected<remove_cvref_t<_Up>>::value))
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp>)
+      expected(_Up&& __u) noexcept(is_nothrow_constructible_v<_Tp, _Up>) // strengthened
+      : __base(in_place, std::forward<_Up>(__u)) {}
+
+  template <class _OtherErr>
+    requires is_constructible_v<_Err, const _OtherErr&>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>) expected(
+      const unexpected<_OtherErr>& __unex) noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
+      : __base(unexpect, __unex.error()) {}
+
+  template <class _OtherErr>
+    requires is_constructible_v<_Err, _OtherErr>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
+      expected(unexpected<_OtherErr>&& __unex) noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
+      : __base(unexpect, std::move(__unex.error())) {}
+
+  template <class... _Args>
+    requires is_constructible_v<_Tp, _Args...>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t, _Args&&... __args) noexcept(
+      is_nothrow_constructible_v<_Tp, _Args...>) // strengthened
+      : __base(in_place, std::forward<_Args>(__args)...) {}
+
+  template <class _Up, class... _Args>
+    requires is_constructible_v< _Tp, initializer_list<_Up>&, _Args... >
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t, initializer_list<_Up> __il, _Args&&... __args) noexcept(
+      is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>) // strengthened
+      : __base(in_place, __il, std::forward<_Args>(__args)...) {}
+
+  template <class... _Args>
+    requires is_constructible_v<_Err, _Args...>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args) noexcept(
+      is_nothrow_constructible_v<_Err, _Args...>) // strengthened
+      : __base(unexpect, std::forward<_Args>(__args)...) {}
+
+  template <class _Up, class... _Args>
+    requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args) noexcept(
+      is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
+      : __base(unexpect, __il, std::forward<_Args>(__args)...) {}
+
+  // [expected.object.dtor], destructor
+
+  _LIBCPP_HIDE_FROM_ABI constexpr ~expected() = default;
+
+private:
+  template <class _Tag, class _OtherTag, class _T1, class _T2, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr void __reinit_expected(_T2& __oldval, _Args&&... __args) {
+    if constexpr (is_nothrow_constructible_v<_T1, _Args...>) {
+      this->__destroy();
+      this->__construct(_Tag{}, std::forward<_Args>(__args)...);
+    } else if constexpr (is_nothrow_move_constructible_v<_T1>) {
+      _T1 __tmp(std::forward<_Args>(__args)...);
+      this->__destroy();
+      this->__construct(_Tag{}, std::move(__tmp));
+    } else {
+      static_assert(
+          is_nothrow_move_constructible_v<_T2>,
+          "To provide strong exception guarantee, T2 has to satisfy `is_nothrow_move_constructible_v` so that it can "
+          "be reverted to the previous state in case an exception is thrown during the assignment.");
+      _T2 __tmp(std::move(__oldval));
+      this->__destroy();
+      auto __trans = std::__make_exception_guard([&] { this->__construct(_OtherTag{}, std::move(__tmp)); });
+      this->__construct(_Tag{}, std::forward<_Args>(__args)...);
+      __trans.__complete();
+    }
+  }
+
+public:
+  // [expected.object.assign], assignment
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected& __rhs) noexcept(
+      is_nothrow_copy_assignable_v<_Tp> && is_nothrow_copy_constructible_v<_Tp> && is_nothrow_copy_assignable_v<_Err> &&
+      is_nothrow_copy_constructible_v<_Err>) // strengthened
+    requires(is_copy_assignable_v<_Tp> && is_copy_constructible_v<_Tp> && is_copy_assignable_v<_Err> &&
+             is_copy_constructible_v<_Err> &&
+             (is_nothrow_move_constructible_v<_Tp> || is_nothrow_move_constructible_v<_Err>))
+  {
+    if (this->__has_val() && __rhs.__has_val()) {
+      this->__val() = __rhs.__val();
+    } else if (this->__has_val()) {
+      __reinit_expected<unexpect_t, in_place_t, _Err, _Tp>(this->__val(), __rhs.__unex());
+    } else if (__rhs.__has_val()) {
+      __reinit_expected<in_place_t, unexpect_t, _Tp, _Err>(this->__unex(), __rhs.__val());
+    } else {
+      this->__unex() = __rhs.__unex();
+    }
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected&
+  operator=(expected&& __rhs) noexcept(is_nothrow_move_assignable_v<_Tp> && is_nothrow_move_constructible_v<_Tp> &&
+                                       is_nothrow_move_assignable_v<_Err> && is_nothrow_move_constructible_v<_Err>)
+    requires(is_move_constructible_v<_Tp> && is_move_assignable_v<_Tp> && is_move_constructible_v<_Err> &&
+             is_move_assignable_v<_Err> &&
+             (is_nothrow_move_constructible_v<_Tp> || is_nothrow_move_constructible_v<_Err>))
+  {
+    if (this->__has_val() && __rhs.__has_val()) {
+      this->__val() = std::move(__rhs.__val());
+    } else if (this->__has_val()) {
+      __reinit_expected<unexpect_t, in_place_t, _Err, _Tp>(this->__val(), std::move(__rhs.__unex()));
+    } else if (__rhs.__has_val()) {
+      __reinit_expected<in_place_t, unexpect_t, _Tp, _Err>(this->__unex(), std::move(__rhs.__val()));
+    } else {
+      this->__unex() = std::move(__rhs.__unex());
+    }
+    return *this;
+  }
+
+  template <class _Up = remove_cv_t<_Tp>>
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(_Up&& __v)
+    requires(!is_same_v<expected, remove_cvref_t<_Up>> && !__is_std_unexpected<remove_cvref_t<_Up>>::value &&
+             is_constructible_v<_Tp, _Up> && is_assignable_v<_Tp&, _Up> &&
+             (is_nothrow_constructible_v<_Tp, _Up> || is_nothrow_move_constructible_v<_Tp> ||
+              is_nothrow_move_constructible_v<_Err>))
+  {
+    if (this->__has_val()) {
+      this->__val() = std::forward<_Up>(__v);
+    } else {
+      __reinit_expected<in_place_t, unexpect_t, _Tp, _Err>(this->__unex(), std::forward<_Up>(__v));
+    }
+    return *this;
+  }
+
+private:
+  template <class _OtherErrQual>
+  static constexpr bool __can_assign_from_unexpected =
+      _And< is_constructible<_Err, _OtherErrQual>,
+            is_assignable<_Err&, _OtherErrQual>,
+            _Lazy<_Or,
+                  is_nothrow_constructible<_Err, _OtherErrQual>,
+                  is_nothrow_move_constructible<_Tp>,
+                  is_nothrow_move_constructible<_Err>> >::value;
+
+public:
+  template <class _OtherErr>
+    requires(__can_assign_from_unexpected<const _OtherErr&>)
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const unexpected<_OtherErr>& __un) {
+    if (this->__has_val()) {
+      __reinit_expected<unexpect_t, in_place_t, _Err, _Tp>(this->__val(), __un.error());
+    } else {
+      this->__unex() = __un.error();
+    }
+    return *this;
+  }
+
+  template <class _OtherErr>
+    requires(__can_assign_from_unexpected<_OtherErr>)
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(unexpected<_OtherErr>&& __un) {
+    if (this->__has_val()) {
+      __reinit_expected<unexpect_t, in_place_t, _Err, _Tp>(this->__val(), std::move(__un.error()));
+    } else {
+      this->__unex() = std::move(__un.error());
+    }
+    return *this;
+  }
+
+  template <class... _Args>
+    requires is_nothrow_constructible_v<_Tp, _Args...>
+  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& emplace(_Args&&... __args) noexcept {
+    this->__destroy();
+    this->__construct(in_place, std::forward<_Args>(__args)...);
+    return this->__val();
+  }
+
+  template <class _Up, class... _Args>
+    requires is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>
+  _LIBCPP_HIDE_FROM_ABI constexpr _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) noexcept {
+    this->__destroy();
+    this->__construct(in_place, __il, std::forward<_Args>(__args)...);
+    return this->__val();
+  }
+
+public:
+  // [expected.object.swap], swap
+  _LIBCPP_HIDE_FROM_ABI constexpr void
+  swap(expected& __rhs) noexcept(is_nothrow_move_constructible_v<_Tp> && is_nothrow_swappable_v<_Tp> &&
+                                 is_nothrow_move_constructible_v<_Err> && is_nothrow_swappable_v<_Err>)
+    requires(is_swappable_v<_Tp> && is_swappable_v<_Err> && is_move_constructible_v<_Tp> &&
+             is_move_constructible_v<_Err> &&
+             (is_nothrow_move_constructible_v<_Tp> || is_nothrow_move_constructible_v<_Err>))
+  {
+    auto __swap_val_unex_impl = [](expected& __with_val, expected& __with_err) {
+      if constexpr (is_nothrow_move_constructible_v<_Err>) {
+        _Err __tmp(std::move(__with_err.__unex()));
+        __with_err.__destroy();
+        auto __trans = std::__make_exception_guard([&] { __with_err.__construct(unexpect, std::move(__tmp)); });
+        __with_err.__construct(in_place, std::move(__with_val.__val()));
+        __trans.__complete();
+        __with_val.__destroy();
+        __with_val.__construct(unexpect, std::move(__tmp));
+      } else {
+        static_assert(is_nothrow_move_constructible_v<_Tp>,
+                      "To provide strong exception guarantee, Tp has to satisfy `is_nothrow_move_constructible_v` so "
+                      "that it can be reverted to the previous state in case an exception is thrown during swap.");
+        _Tp __tmp(std::move(__with_val.__val()));
+        __with_val.__destroy();
+        auto __trans = std::__make_exception_guard([&] { __with_val.__construct(in_place, std::move(__tmp)); });
+        __with_val.__construct(unexpect, std::move(__with_err.__unex()));
+        __trans.__complete();
+        __with_err.__destroy();
+        __with_err.__construct(in_place, std::move(__tmp));
+      }
+    };
+
+    if (this->__has_val()) {
+      if (__rhs.__has_val()) {
+        using std::swap;
+        swap(this->__val(), __rhs.__val());
+      } else {
+        __swap_val_unex_impl(*this, __rhs);
+      }
+    } else {
+      if (__rhs.__has_val()) {
+        __swap_val_unex_impl(__rhs, *this);
+      } else {
+        using std::swap;
+        swap(this->__unex(), __rhs.__unex());
+      }
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(expected& __x, expected& __y) noexcept(noexcept(__x.swap(__y)))
+    requires requires { __x.swap(__y); }
+  {
+    __x.swap(__y);
+  }
+
+  // [expected.object.obs], observers
+  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* operator->() const noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        this->__has_val(), "expected::operator-> requires the expected to contain a value");
+    return std::addressof(this->__val());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr _Tp* operator->() noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        this->__has_val(), "expected::operator-> requires the expected to contain a value");
+    return std::addressof(this->__val());
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& operator*() const& noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        this->__has_val(), "expected::operator* requires the expected to contain a value");
+    return this->__val();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() & noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        this->__has_val(), "expected::operator* requires the expected to contain a value");
+    return this->__val();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& operator*() const&& noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        this->__has_val(), "expected::operator* requires the expected to contain a value");
+    return std::move(this->__val());
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator*() && noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        this->__has_val(), "expected::operator* requires the expected to contain a value");
+    return std::move(this->__val());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return this->__has_val(); }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& value() const& {
+    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
+    if (!this->__has_val()) {
+      std::__throw_bad_expected_access<_Err>(std::as_const(error()));
+    }
+    return this->__val();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp& value() & {
+    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
+    if (!this->__has_val()) {
+      std::__throw_bad_expected_access<_Err>(std::as_const(error()));
+    }
+    return this->__val();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& value() const&& {
+    static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
+                  "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
+    if (!this->__has_val()) {
+      std::__throw_bad_expected_access<_Err>(std::move(error()));
+    }
+    return std::move(this->__val());
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& value() && {
+    static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
+                  "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
+    if (!this->__has_val()) {
+      std::__throw_bad_expected_access<_Err>(std::move(error()));
+    }
+    return std::move(this->__val());
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return this->__unex();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return this->__unex();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return std::move(this->__unex());
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return std::move(this->__unex());
+  }
+
+  template <class _Up = remove_cv_t<_Tp>>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) const& {
+    static_assert(is_copy_constructible_v<_Tp>, "value_type has to be copy constructible");
+    static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
+    return this->__has_val() ? this->__val() : static_cast<_Tp>(std::forward<_Up>(__v));
+  }
+
+  template <class _Up = remove_cv_t<_Tp>>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) && {
+    static_assert(is_move_constructible_v<_Tp>, "value_type has to be move constructible");
+    static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
+    return this->__has_val() ? std::move(this->__val()) : static_cast<_Tp>(std::forward<_Up>(__v));
+  }
+
+  template <class _Up = _Err>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
+    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
+    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
+    if (has_value())
+      return std::forward<_Up>(__error);
+    return error();
+  }
+
+  template <class _Up = _Err>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
+    static_assert(is_move_constructible_v<_Err>, "error_type has to be move constructible");
+    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
+    if (has_value())
+      return std::forward<_Up>(__error);
+    return std::move(error());
+  }
+
+  // [expected.void.monadic], monadic
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
+    using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&>>;
+    static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Up::error_type, _Err>,
+                  "The result of f(value()) must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f), this->__val());
+    }
+    return _Up(unexpect, error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
+    using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&>>;
+    static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Up::error_type, _Err>,
+                  "The result of f(value()) must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f), this->__val());
+    }
+    return _Up(unexpect, error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
+    using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&&>>;
+    static_assert(
+        __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Up::error_type, _Err>,
+                  "The result of f(std::move(value())) must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f), std::move(this->__val()));
+    }
+    return _Up(unexpect, std::move(error()));
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
+    using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&&>>;
+    static_assert(
+        __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Up::error_type, _Err>,
+                  "The result of f(std::move(value())) must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f), std::move(this->__val()));
+    }
+    return _Up(unexpect, std::move(error()));
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, _Tp&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&>>;
+    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(error()) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp(in_place, this->__val());
+    }
+    return std::invoke(std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, const _Tp&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&>>;
+    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(error()) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp(in_place, this->__val());
+    }
+    return std::invoke(std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, _Tp&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&&>>;
+    static_assert(
+        __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(std::move(error())) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp(in_place, std::move(this->__val()));
+    }
+    return std::invoke(std::forward<_Func>(__f), std::move(error()));
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, const _Tp&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&&>>;
+    static_assert(
+        __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(std::move(error())) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp(in_place, std::move(this->__val()));
+    }
+    return std::invoke(std::forward<_Func>(__f), std::move(error()));
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
+    using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, error());
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(
+          __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), this->__val());
+    } else {
+      std::invoke(std::forward<_Func>(__f), this->__val());
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
+    using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, error());
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(
+          __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), this->__val());
+    } else {
+      std::invoke(std::forward<_Func>(__f), this->__val());
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
+    using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&&>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, std::move(error()));
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(
+          __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), std::move(this->__val()));
+    } else {
+      std::invoke(std::forward<_Func>(__f), std::move(this->__val()));
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
+    using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&&>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, std::move(error()));
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(
+          __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), std::move(this->__val()));
+    } else {
+      std::invoke(std::forward<_Func>(__f), std::move(this->__val()));
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, _Tp&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(error()) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>(in_place, this->__val());
+    }
+    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, const _Tp&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(error()) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>(in_place, this->__val());
+    }
+    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, _Tp&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>(in_place, std::move(this->__val()));
+    }
+    return expected<_Tp, _Gp>(
+        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Tp, const _Tp&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>(in_place, std::move(this->__val()));
+    }
+    return expected<_Tp, _Gp>(
+        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
+  }
+
+  // [expected.object.eq], equality operators
+  template <class _T2, class _E2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y)
+    requires(!is_void_v<_T2>)
+#  if _LIBCPP_STD_VER >= 26
+            && requires {
+                 { *__x == *__y } -> __core_convertible_to<bool>;
+                 { __x.error() == __y.error() } -> __core_convertible_to<bool>;
+               }
+#  endif
+  {
+    if (__x.__has_val() != __y.__has_val()) {
+      return false;
+    } else {
+      if (__x.__has_val()) {
+        return __x.__val() == __y.__val();
+      } else {
+        return __x.__unex() == __y.__unex();
+      }
+    }
+  }
+
+  template <class _T2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v)
+#  if _LIBCPP_STD_VER >= 26
+    requires(!__is_std_expected<_T2>::value) && requires {
+      { *__x == __v } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
+    return __x.__has_val() && static_cast<bool>(__x.__val() == __v);
+  }
+
+  template <class _E2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e)
+#  if _LIBCPP_STD_VER >= 26
+    requires requires {
+      { __x.error() == __e.error() } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
+    return !__x.__has_val() && static_cast<bool>(__x.__unex() == __e.error());
+  }
+};
+
+template <class _Err>
+class __expected_void_base {
+  struct __empty_t {};
+  // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
+  // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
+  // it's not clear that it's implementable, given that the function is allowed to clobber
+  // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
+  union __union_t {
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&)
+      requires(is_copy_constructible_v<_Err> && is_trivially_copy_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(__union_t&&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t(__union_t&&)
+      requires(is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(__union_t&&)      = delete;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(in_place_t) : __empty_() {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(unexpect_t, _Args&&... __args)
+        : __unex_(std::forward<_Args>(__args)...) {}
+
+    template <class _Func, class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
+        __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
+        : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
+      requires(is_trivially_destructible_v<_Err>)
+    = default;
+
+    // __repr's destructor handles this
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
+      requires(!is_trivially_destructible_v<_Err>)
+    {}
+
+    _LIBCPP_NO_UNIQUE_ADDRESS __empty_t __empty_;
+    _LIBCPP_NO_UNIQUE_ADDRESS _Err __unex_;
+  };
+
+  static constexpr bool __put_flag_in_tail                    = __fits_in_tail_padding<__union_t, bool>;
+  static constexpr bool __allow_reusing_expected_tail_padding = !__put_flag_in_tail;
+
+  struct __repr {
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr() = delete;
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(in_place_t __tag) : __union_(in_place, __tag), __has_val_(true) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(unexpect_t __tag, _Args&&... __args)
+        : __union_(in_place, __tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(std::__expected_construct_unexpected_from_invoke_tag __tag,
+                                                    _Args&&... __args)
+        : __union_(in_place, __tag, std::forward<_Args>(__args)...), __has_val_(false) {}
+
+    template <class _OtherUnion>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __repr(bool __has_val, _OtherUnion&& __other)
+      requires(__allow_reusing_expected_tail_padding)
+        : __union_(__conditional_no_unique_address_invoke_tag{},
+                   [&] { return __make_union(__has_val, std::forward<_OtherUnion>(__other)); }),
+          __has_val_(__has_val) {}
+
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(const __repr&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(const __repr&)
+      requires(is_copy_constructible_v<_Err> && is_trivially_copy_constructible_v<_Err>)
+    = default;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(__repr&&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr(__repr&&)
+      requires(is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Err>)
+    = default;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr& operator=(const __repr&) = delete;
+    _LIBCPP_HIDE_FROM_ABI constexpr __repr& operator=(__repr&&)      = delete;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__repr()
+      requires(is_trivially_destructible_v<_Err>)
+    = default;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr ~__repr()
+      requires(!is_trivially_destructible_v<_Err>)
+    {
+      __destroy_union_member();
+    }
+
+    _LIBCPP_HIDE_FROM_ABI constexpr void __destroy_union()
+      requires(__allow_reusing_expected_tail_padding && is_trivially_destructible_v<_Err>)
+    {
+      std::destroy_at(&__union_.__v);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI constexpr void __destroy_union()
+      requires(__allow_reusing_expected_tail_padding && !is_trivially_destructible_v<_Err>)
+    {
+      __destroy_union_member();
+      std::destroy_at(&__union_.__v);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI constexpr void __construct_union(in_place_t)
+      requires(__allow_reusing_expected_tail_padding)
+    {
+      std::construct_at(&__union_.__v, in_place);
+      __has_val_ = true;
+    }
+
+    template <class... _Args>
+    _LIBCPP_HIDE_FROM_ABI constexpr void __construct_union(unexpect_t, _Args&&... __args)
+      requires(__allow_reusing_expected_tail_padding)
+    {
+      std::construct_at(&__union_.__v, unexpect, std::forward<_Args>(__args)...);
+      __has_val_ = false;
+    }
+
+  private:
+    template <class>
+    friend class __expected_void_base;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr void __destroy_union_member()
+      requires(!is_trivially_destructible_v<_Err>)
+    {
+      if (!__has_val_)
+        std::destroy_at(std::addressof(__union_.__v.__unex_));
+    }
+
+    template <class _OtherUnion>
+    _LIBCPP_HIDE_FROM_ABI static constexpr __union_t __make_union(bool __has_val, _OtherUnion&& __other)
+      requires(__allow_reusing_expected_tail_padding)
+    {
+      if (__has_val)
+        return __union_t(in_place);
+      else
+        return __union_t(unexpect, std::forward<_OtherUnion>(__other).__unex_);
+    }
+
+    _LIBCPP_NO_UNIQUE_ADDRESS __conditional_no_unique_address<__put_flag_in_tail, __union_t> __union_;
+    _LIBCPP_NO_UNIQUE_ADDRESS bool __has_val_;
+  };
+
+  template <class _OtherUnion>
+  _LIBCPP_HIDE_FROM_ABI static constexpr __repr __make_repr(bool __has_val, _OtherUnion&& __other)
+    requires(__put_flag_in_tail)
+  {
+    if (__has_val)
+      return __repr(in_place);
+    else
+      return __repr(unexpect, std::forward<_OtherUnion>(__other).__unex_);
+  }
+
+protected:
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_void_base(_Args&&... __args)
+      : __repr_(in_place, std::forward<_Args>(__args)...) {}
+
+  template <class _OtherUnion>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __expected_void_base(bool __has_val, _OtherUnion&& __other)
+    requires(__put_flag_in_tail)
+      : __repr_(__conditional_no_unique_address_invoke_tag{},
+                [&] { return __make_repr(__has_val, std::forward<_OtherUnion>(__other)); }) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void __destroy() {
+    if constexpr (__put_flag_in_tail)
+      std::destroy_at(&__repr_.__v);
+    else
+      __repr_.__v.__destroy_union();
+  }
+
+  template <class _Tag, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr void __construct(_Tag __tag, _Args&&... __args) {
+    if constexpr (__put_flag_in_tail)
+      std::construct_at(&__repr_.__v, __tag, std::forward<_Args>(__args)...);
+    else
+      __repr_.__v.__construct_union(__tag, std::forward<_Args>(__args)...);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr bool __has_val() const { return __repr_.__v.__has_val_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr __union_t& __union() { return __repr_.__v.__union_.__v; }
+  _LIBCPP_HIDE_FROM_ABI constexpr const __union_t& __union() const { return __repr_.__v.__union_.__v; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Err& __unex() { return __repr_.__v.__union_.__v.__unex_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr const _Err& __unex() const { return __repr_.__v.__union_.__v.__unex_; }
+
+private:
+  _LIBCPP_NO_UNIQUE_ADDRESS __conditional_no_unique_address<__allow_reusing_expected_tail_padding, __repr> __repr_;
+};
+
+template <class _Tp, class _Err>
+  requires is_void_v<_Tp>
+class expected<_Tp, _Err> : private __expected_void_base<_Err> {
+  static_assert(__valid_std_unexpected<_Err>::value,
+                "[expected.void.general] A program that instantiates expected<T, E> with a E that is not a "
+                "valid argument for unexpected<E> is ill-formed");
+
+  template <class, class>
+  friend class expected;
+
+  template <class _Up, class _OtherErr, class _OtherErrQual>
+  using __can_convert _LIBCPP_NODEBUG =
+      _And< is_void<_Up>,
+            is_constructible<_Err, _OtherErrQual>,
+            _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>&>>,
+            _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>>>,
+            _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>&>>,
+            _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>>>>;
+
+  using __base _LIBCPP_NODEBUG = __expected_void_base<_Err>;
+
+public:
+  using value_type      = _Tp;
+  using error_type      = _Err;
+  using unexpected_type = unexpected<_Err>;
+
+  template <class _Up>
+  using rebind = expected<_Up, error_type>;
+
+  // [expected.void.ctor], constructors
+  _LIBCPP_HIDE_FROM_ABI constexpr expected() noexcept : __base(in_place) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&)
+    requires(is_copy_constructible_v<_Err> && is_trivially_copy_constructible_v<_Err>)
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __rhs) noexcept(
+      is_nothrow_copy_constructible_v<_Err>) // strengthened
+    requires(is_copy_constructible_v<_Err> && !is_trivially_copy_constructible_v<_Err>)
+      : __base(__rhs.__has_val(), __rhs.__union()) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
+    requires(is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Err>)
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __rhs) noexcept(is_nothrow_move_constructible_v<_Err>)
+    requires(is_move_constructible_v<_Err> && !is_trivially_move_constructible_v<_Err>)
+      : __base(__rhs.__has_val(), std::move(__rhs.__union())) {}
+
+  template <class _Up, class _OtherErr>
+    requires __can_convert<_Up, _OtherErr, const _OtherErr&>::value
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
+      expected(const expected<_Up, _OtherErr>& __rhs) noexcept(
+          is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
+      : __base(__rhs.__has_val(), __rhs.__union()) {}
+
+  template <class _Up, class _OtherErr>
+    requires __can_convert<_Up, _OtherErr, _OtherErr>::value
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
+      expected(expected<_Up, _OtherErr>&& __rhs) noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
+      : __base(__rhs.__has_val(), std::move(__rhs.__union())) {}
+
+  template <class _OtherErr>
+    requires is_constructible_v<_Err, const _OtherErr&>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>) expected(
+      const unexpected<_OtherErr>& __unex) noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
+      : __base(unexpect, __unex.error()) {}
+
+  template <class _OtherErr>
+    requires is_constructible_v<_Err, _OtherErr>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
+      expected(unexpected<_OtherErr>&& __unex) noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
+      : __base(unexpect, std::move(__unex.error())) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t) noexcept : __base(in_place) {}
+
+  template <class... _Args>
+    requires is_constructible_v<_Err, _Args...>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args) noexcept(
+      is_nothrow_constructible_v<_Err, _Args...>) // strengthened
+      : __base(unexpect, std::forward<_Args>(__args)...) {}
+
+  template <class _Up, class... _Args>
+    requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args) noexcept(
+      is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
+      : __base(unexpect, __il, std::forward<_Args>(__args)...) {}
+
+private:
+  template <class _Func, class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
+      __expected_construct_unexpected_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
+      : __base(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...) {}
+
+public:
+  // [expected.void.dtor], destructor
+
+  _LIBCPP_HIDE_FROM_ABI constexpr ~expected() = default;
+
+private:
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI constexpr void __reinit_expected(unexpect_t, _Args&&... __args) {
+    _LIBCPP_ASSERT_INTERNAL(this->__has_val(), "__reinit_expected(unexpect_t, ...) needs value to be set");
+
+    this->__destroy();
+    auto __trans = std::__make_exception_guard([&] { this->__construct(in_place); });
+    this->__construct(unexpect, std::forward<_Args>(__args)...);
+    __trans.__complete();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void __reinit_expected(in_place_t) {
+    _LIBCPP_ASSERT_INTERNAL(!this->__has_val(), "__reinit_expected(in_place_t, ...) needs value to be unset");
+
+    this->__destroy();
+    this->__construct(in_place);
+  }
+
+public:
+  // [expected.void.assign], assignment
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected& __rhs) noexcept(
+      is_nothrow_copy_assignable_v<_Err> && is_nothrow_copy_constructible_v<_Err>) // strengthened
+    requires(is_copy_assignable_v<_Err> && is_copy_constructible_v<_Err>)
+  {
+    if (this->__has_val()) {
+      if (!__rhs.__has_val()) {
+        __reinit_expected(unexpect, __rhs.__unex());
+      }
+    } else {
+      if (__rhs.__has_val()) {
+        __reinit_expected(in_place);
+      } else {
+        this->__unex() = __rhs.__unex();
+      }
+    }
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr expected&
+  operator=(expected&& __rhs) noexcept(is_nothrow_move_assignable_v<_Err> && is_nothrow_move_constructible_v<_Err>)
+    requires(is_move_assignable_v<_Err> && is_move_constructible_v<_Err>)
+  {
+    if (this->__has_val()) {
+      if (!__rhs.__has_val()) {
+        __reinit_expected(unexpect, std::move(__rhs.__unex()));
+      }
+    } else {
+      if (__rhs.__has_val()) {
+        __reinit_expected(in_place);
+      } else {
+        this->__unex() = std::move(__rhs.__unex());
+      }
+    }
+    return *this;
+  }
+
+  template <class _OtherErr>
+    requires(is_constructible_v<_Err, const _OtherErr&> && is_assignable_v<_Err&, const _OtherErr&>)
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const unexpected<_OtherErr>& __un) {
+    if (this->__has_val()) {
+      __reinit_expected(unexpect, __un.error());
+    } else {
+      this->__unex() = __un.error();
+    }
+    return *this;
+  }
+
+  template <class _OtherErr>
+    requires(is_constructible_v<_Err, _OtherErr> && is_assignable_v<_Err&, _OtherErr>)
+  _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(unexpected<_OtherErr>&& __un) {
+    if (this->__has_val()) {
+      __reinit_expected(unexpect, std::move(__un.error()));
+    } else {
+      this->__unex() = std::move(__un.error());
+    }
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void emplace() noexcept {
+    if (!this->__has_val()) {
+      __reinit_expected(in_place);
+    }
+  }
+
+  // [expected.void.swap], swap
+  _LIBCPP_HIDE_FROM_ABI constexpr void
+  swap(expected& __rhs) noexcept(is_nothrow_move_constructible_v<_Err> && is_nothrow_swappable_v<_Err>)
+    requires(is_swappable_v<_Err> && is_move_constructible_v<_Err>)
+  {
+    auto __swap_val_unex_impl = [](expected& __with_val, expected& __with_err) {
+      // May throw, but will re-engage `__with_val` in that case.
+      __with_val.__reinit_expected(unexpect, std::move(__with_err.__unex()));
+      // Will not throw.
+      __with_err.__reinit_expected(in_place);
+    };
+
+    if (this->__has_val()) {
+      if (!__rhs.__has_val()) {
+        __swap_val_unex_impl(*this, __rhs);
+      }
+    } else {
+      if (__rhs.__has_val()) {
+        __swap_val_unex_impl(__rhs, *this);
+      } else {
+        using std::swap;
+        swap(this->__unex(), __rhs.__unex());
+      }
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(expected& __x, expected& __y) noexcept(noexcept(__x.swap(__y)))
+    requires requires { __x.swap(__y); }
+  {
+    __x.swap(__y);
+  }
+
+  // [expected.void.obs], observers
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return this->__has_val(); }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return this->__has_val(); }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void operator*() const noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        this->__has_val(), "expected::operator* requires the expected to contain a value");
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void value() const& {
+    static_assert(is_copy_constructible_v<_Err>);
+    if (!this->__has_val()) {
+      std::__throw_bad_expected_access<_Err>(this->__unex());
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void value() && {
+    static_assert(is_copy_constructible_v<_Err> && is_move_constructible_v<_Err>);
+    if (!this->__has_val()) {
+      std::__throw_bad_expected_access<_Err>(std::move(this->__unex()));
+    }
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return this->__unex();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return this->__unex();
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return std::move(this->__unex());
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        !this->__has_val(), "expected::error requires the expected to contain an error");
+    return std::move(this->__unex());
+  }
+
+  template <class _Up = _Err>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
+    static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
+    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
+    if (has_value()) {
+      return std::forward<_Up>(__error);
+    }
+    return error();
+  }
+
+  template <class _Up = _Err>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
+    static_assert(is_move_constructible_v<_Err>, "error_type has to be move constructible");
+    static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
+    if (has_value()) {
+      return std::forward<_Up>(__error);
+    }
+    return std::move(error());
+  }
+
+  // [expected.void.monadic], monadic
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
+    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
+    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
+    static_assert(
+        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f));
+    }
+    return _Up(unexpect, error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
+    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
+    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
+    static_assert(
+        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f));
+    }
+    return _Up(unexpect, error());
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
+    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
+    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
+    static_assert(
+        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f));
+    }
+    return _Up(unexpect, std::move(error()));
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
+    using _Up = remove_cvref_t<invoke_result_t<_Func>>;
+    static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
+    static_assert(
+        is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
+    if (has_value()) {
+      return std::invoke(std::forward<_Func>(__f));
+    }
+    return _Up(unexpect, std::move(error()));
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&>>;
+    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(error()) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp();
+    }
+    return std::invoke(std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&>>;
+    static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(error()) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp();
+    }
+    return std::invoke(std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&&>>;
+    static_assert(
+        __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(std::move(error())) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp();
+    }
+    return std::invoke(std::forward<_Func>(__f), std::move(error()));
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
+    using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&&>>;
+    static_assert(
+        __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
+    static_assert(is_same_v<typename _Gp::value_type, _Tp>,
+                  "The result of f(std::move(error())) must have the same value_type as this expected");
+    if (has_value()) {
+      return _Gp();
+    }
+    return std::invoke(std::forward<_Func>(__f), std::move(error()));
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
+    using _Up = remove_cv_t<invoke_result_t<_Func>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, error());
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
+    } else {
+      std::invoke(std::forward<_Func>(__f));
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
+    using _Up = remove_cv_t<invoke_result_t<_Func>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, error());
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
+    } else {
+      std::invoke(std::forward<_Func>(__f));
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
+    using _Up = remove_cv_t<invoke_result_t<_Func>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, std::move(error()));
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
+    } else {
+      std::invoke(std::forward<_Func>(__f));
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+    requires is_constructible_v<_Err, const _Err&&>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
+    using _Up = remove_cv_t<invoke_result_t<_Func>>;
+    if (!has_value()) {
+      return expected<_Up, _Err>(unexpect, std::move(error()));
+    }
+    if constexpr (!is_void_v<_Up>) {
+      return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
+    } else {
+      std::invoke(std::forward<_Func>(__f));
+      return expected<_Up, _Err>();
+    }
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(error()) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>();
+    }
+    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(error()) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>();
+    }
+    return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>();
+    }
+    return expected<_Tp, _Gp>(
+        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
+  }
+
+  template <class _Func>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
+    using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&&>>;
+    static_assert(__valid_std_unexpected<_Gp>::value,
+                  "The result of f(std::move(error())) must be a valid template argument for unexpected");
+    if (has_value()) {
+      return expected<_Tp, _Gp>();
+    }
+    return expected<_Tp, _Gp>(
+        __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
+  }
+
+  // [expected.void.eq], equality operators
+  template <class _T2, class _E2>
+    requires is_void_v<_T2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y)
+#  if _LIBCPP_STD_VER >= 26
+    requires requires {
+      { __x.error() == __y.error() } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
+    if (__x.__has_val() != __y.__has_val()) {
+      return false;
+    } else {
+      return __x.__has_val() || static_cast<bool>(__x.__unex() == __y.__unex());
+    }
+  }
+
+  template <class _E2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __y)
+#  if _LIBCPP_STD_VER >= 26
+    requires requires {
+      { __x.error() == __y.error() } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
+    return !__x.__has_val() && static_cast<bool>(__x.__unex() == __y.error());
+  }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___EXPECTED_EXPECTED_H
diff --git a/third_party/libcxx/include/c++/v1/__expected/unexpect.h b/third_party/libcxx/include/c++/v1/__expected/unexpect.h
new file mode 100644
index 0000000..df52787
--- /dev/null
+++ b/third_party/libcxx/include/c++/v1/__expected/unexpect.h
@@ -0,0 +1,32 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef _LIBCPP___EXPECTED_UNEXPECT_H
+#define _LIBCPP___EXPECTED_UNEXPECT_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct unexpect_t {
+  explicit unexpect_t() = default;
+};
+
+inline constexpr unexpect_t unexpect{};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 23
+
+#endif // _LIBCPP___EXPECTED_UNEXPECT_H
diff --git a/third_party/libcxx/include/c++/v1/__expected/unexpected.h b/third_party/libcxx/include/c++/v1/__expected/unexpected.h
new file mode 100644
index 0000000..fc4f52c
--- /dev/null
+++ b/third_party/libcxx/include/c++/v1/__expected/unexpected.h
@@ -0,0 +1,127 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef _LIBCPP___EXPECTED_UNEXPECTED_H
+#define _LIBCPP___EXPECTED_UNEXPECTED_H
+
+#include <__config>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/is_array.h>
+#include <__type_traits/is_const.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_object.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_swappable.h>
+#include <__type_traits/is_volatile.h>
+#include <__type_traits/negation.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/forward.h>
+#include <__utility/in_place.h>
+#include <__utility/move.h>
+#include <__utility/swap.h>
+#include <initializer_list>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Err>
+class unexpected;
+
+template <class _Tp>
+struct __is_std_unexpected : false_type {};
+
+template <class _Err>
+struct __is_std_unexpected<unexpected<_Err>> : true_type {};
+
+template <class _Tp>
+using __valid_std_unexpected _LIBCPP_NODEBUG = _BoolConstant< //
+    is_object_v<_Tp> &&                                       //
+    !is_array_v<_Tp> &&                                       //
+    !__is_std_unexpected<_Tp>::value &&                       //
+    !is_const_v<_Tp> &&                                       //
+    !is_volatile_v<_Tp>                                       //
+    >;
+
+template <class _Err>
+class unexpected {
+  static_assert(__valid_std_unexpected<_Err>::value,
+                "[expected.un.general] states a program that instantiates std::unexpected for a non-object type, an "
+                "array type, a specialization of unexpected, or a cv-qualified type is ill-formed.");
+
+public:
+  _LIBCPP_HIDE_FROM_ABI constexpr unexpected(const unexpected&) = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr unexpected(unexpected&&)      = default;
+
+  template <class _Error = _Err>
+    requires(!is_same_v<remove_cvref_t<_Error>, unexpected> && //
+             !is_same_v<remove_cvref_t<_Error>, in_place_t> && //
+             is_constructible_v<_Err, _Error>)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit unexpected(_Error&& __error) //
+      noexcept(is_nothrow_constructible_v<_Err, _Error>)                // strengthened
+      : __unex_(std::forward<_Error>(__error)) {}
+
+  template <class... _Args>
+    requires is_constructible_v<_Err, _Args...>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit unexpected(in_place_t, _Args&&... __args) //
+      noexcept(is_nothrow_constructible_v<_Err, _Args...>)                           // strengthened
+      : __unex_(std::forward<_Args>(__args)...) {}
+
+  template <class _Up, class... _Args>
+    requires is_constructible_v<_Err, initializer_list<_Up>&, _Args...>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit unexpected(in_place_t, initializer_list<_Up> __il, _Args&&... __args) //
+      noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
+      : __unex_(__il, std::forward<_Args>(__args)...) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr unexpected& operator=(const unexpected&) = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr unexpected& operator=(unexpected&&)      = default;
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept { return __unex_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept { return std::move(__unex_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept { return std::move(__unex_); }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr void swap(unexpected& __other) noexcept(is_nothrow_swappable_v<_Err>) {
+    static_assert(is_swappable_v<_Err>, "unexpected::swap requires is_swappable_v<E> to be true");
+    using std::swap;
+    swap(__unex_, __other.__unex_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(unexpected& __x, unexpected& __y) noexcept(noexcept(__x.swap(__y)))
+    requires is_swappable_v<_Err>
+  {
+    __x.swap(__y);
+  }
+
+  template <class _Err2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const unexpected& __x, const unexpected<_Err2>& __y) {
+    return __x.__unex_ == __y.error();
+  }
+
+private:
+  _Err __unex_;
+};
+
+template <class _Err>
+unexpected(_Err) -> unexpected<_Err>;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___EXPECTED_UNEXPECTED_H