[libc++] Implement std::expected P0323R12

Implement `std::expected` https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0323r12.html

Added tests

Differential Revision: https://reviews.llvm.org/D124516

address comment

Author: huixie90

libcxx/docs/FeatureTestMacroTable.rst

@@ -318,6 +318,8 @@ Status
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_typeinfo``                  *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_expected``                            ``202202L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_forward_like``                        ``202207L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_invoke_r``                            *unimplemented*

libcxx/docs/Status/Cxx2bIssues.csv

@@ -167,11 +167,11 @@
 "`3671 <https://wg21.link/LWG3671>`__","``atomic_fetch_xor`` missing from ``stdatomic.h``","July 2022","",""
 "`3672 <https://wg21.link/LWG3672>`__","``common_iterator::operator->()`` should return by value","July 2022","","","|ranges|"
 "`3683 <https://wg21.link/LWG3683>`__","``operator==`` for ``polymorphic_allocator`` cannot deduce template argument in common cases","July 2022","",""
-"`3687 <https://wg21.link/LWG3687>`__","``expected<cv void, E>`` move constructor should move","July 2022","",""
+"`3687 <https://wg21.link/LWG3687>`__","``expected<cv void, E>`` move constructor should move","July 2022","Complete","16.0"
 "`3692 <https://wg21.link/LWG3692>`__","``zip_view::iterator``'s ``operator<=>`` is overconstrained","July 2022","","","|ranges| |spaceship|"
 "`3701 <https://wg21.link/LWG3701>`__","Make ``formatter<remove_cvref_t<const charT[N]>, charT>`` requirement explicit","July 2022","","","|format|"
 "`3702 <https://wg21.link/LWG3702>`__","Should ``zip_transform_view::iterator`` remove ``operator<``","July 2022","","","|ranges| |spaceship|"
-"`3703 <https://wg21.link/LWG3703>`__","Missing requirements for ``expected<T, E>`` requires ``is_void<T>``","July 2022","",""
+"`3703 <https://wg21.link/LWG3703>`__","Missing requirements for ``expected<T, E>`` requires ``is_void<T>``","July 2022","Complete","16.0"
 "`3704 <https://wg21.link/LWG3704>`__","LWG 2059 added overloads that might be ill-formed for sets","July 2022","",""
 "`3705 <https://wg21.link/LWG3705>`__","Hashability shouldn't depend on basic_string's allocator","July 2022","|Complete|","16.0"
 "`3707 <https://wg21.link/LWG3707>`__","chunk_view::outer-iterator::value_type::size should return unsigned type","July 2022","","","|ranges|"

libcxx/docs/Status/Cxx2bPapers.csv

@@ -38,7 +38,7 @@
 "`P2393R1 <https://wg21.link/P2393R1>`__","LWG","Cleaning up ``integer``-class types","October 2021","",""
 "`P2401R0 <https://wg21.link/P2401R0>`__","LWG","Add a conditional ``noexcept`` specification to ``std::exchange``","October 2021","|Complete|","14.0"
 "","","","","",""
-"`P0323R12 <https://wg21.link/P0323R12>`__","LWG","``std::expected``","February 2022","",""
+"`P0323R12 <https://wg21.link/P0323R12>`__","LWG","``std::expected``","February 2022","|Complete|","16.0"
 "`P0533R9 <https://wg21.link/P0533R9>`__","LWG","``constexpr`` for ``<cmath>`` and ``<cstdlib>``","February 2022","",""
 "`P0627R6 <https://wg21.link/P0627R6>`__","LWG","Function to mark unreachable code","February 2022","|Complete|","15.0"
 "`P1206R7 <https://wg21.link/P1206R7>`__","LWG","``ranges::to``: A function to convert any range to a container","February 2022","","","|ranges|"
@@ -86,7 +86,7 @@
 "`P2517R1 <https://wg21.link/P2517R1>`__","LWG","Add a conditional ``noexcept`` specification to ``std::apply``","July 2022","",""
 "`P2520R0 <https://wg21.link/P2520R0>`__","LWG","``move_iterator`` should be a random access iterator","July 2022","","","|ranges|"
 "`P2540R1 <https://wg21.link/P2540R1>`__","LWG","Empty Product for certain Views","July 2022","","","|ranges|"
-"`P2549R1 <https://wg21.link/P2549R1>`__","LWG","``std::unexpected`` should have ``error()`` as member accessor","July 2022","",""
+"`P2549R1 <https://wg21.link/P2549R1>`__","LWG","``std::unexpected`` should have ``error()`` as member accessor","July 2022","|Complete|","16.0"
 "`P2553R1 <https://wg21.link/P2553R1>`__","LWG","Make ``mdspan`` ``size_type`` controllable","July 2022","",""
 "`P2554R0 <https://wg21.link/P2554R0>`__","LWG","C-Array Interoperability of MDSpan","July 2022","",""
 "`P2585R0 <https://wg21.link/P2585R0>`__","LWG","Improving default container formatting","July 2022","",""

libcxx/include/CMakeLists.txt

@@ -269,6 +269,10 @@ set(files
   __debug
   __debug_utils/randomize_range.h
   __errc
+  __expected/bad_expected_access.h
+  __expected/expected.h
+  __expected/unexpect.h
+  __expected/unexpected.h
   __filesystem/copy_options.h
   __filesystem/directory_entry.h
   __filesystem/directory_iterator.h
@@ -740,6 +744,7 @@ set(files
   errno.h
   exception
   execution
+  expected
   experimental/__config
   experimental/__memory
   experimental/algorithm

libcxx/include/__expected/bad_expected_access.h

@@ -0,0 +1,64 @@
+// -*- 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 <__utility/move.h>
+
+#include <exception>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Err>
+class bad_expected_access;
+
+template <>
+class 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&)            = default;
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access(bad_expected_access&&)                 = default;
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access& operator=(const bad_expected_access&) = default;
+  _LIBCPP_HIDE_FROM_ABI bad_expected_access& operator=(bad_expected_access&&)      = default;
+  ~bad_expected_access() override                                                  = default;
+
+public:
+  // The way this has been designed (by using a class template below) means that we'll already
+  // have a profusion of these vtables in TUs, and the dynamic linker will already have a bunch
+  // of work to do. So it is not worth hiding the <void> specialization in the dylib, given that
+  // it adds deployment target restrictions.
+  const char* what() const noexcept override { return "bad access to std::expected"; }
+};
+
+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)) {}
+
+  _LIBCPP_HIDE_FROM_ABI _Err& error() & noexcept { return __unex_; }
+  _LIBCPP_HIDE_FROM_ABI const _Err& error() const& noexcept { return __unex_; }
+  _LIBCPP_HIDE_FROM_ABI _Err&& error() && noexcept { return std::move(__unex_); }
+  _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
+
+#endif // _LIBCPP___EXPECTED_BAD_EXPECTED_ACCESS_H