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Libc++ 20.0.0 (In-Progress) Release Notes
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Written by the `Libc++ Team <https://libcxx.llvm.org>`_

.. warning::

   These are in-progress notes for the upcoming libc++ 20.0.0 release.
   Release notes for previous releases can be found on
   `the Download Page <https://releases.llvm.org/download.html>`_.

Introduction
============

This document contains the release notes for the libc++ C++ Standard Library,
part of the LLVM Compiler Infrastructure, release 20.0.0. Here we describe the
status of libc++ in some detail, including major improvements from the previous
release and new feature work. For the general LLVM release notes, see `the LLVM
documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.

For more information about libc++, please see the `Libc++ Web Site
<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.

Note that if you are reading this file from a Git checkout or the
main Libc++ web page, this document applies to the *next* release, not
the current one. To see the release notes for a specific release, please
see the `releases page <https://llvm.org/releases/>`_.

What's New in Libc++ 20.0.0?
==============================

Implemented Papers
------------------

- P0619R4: Reviewing Deprecated Facilities of C++17 for C++20 (`Github <https://github.com/llvm/llvm-project/issues/99985>`__)
- P2747R2: ``constexpr`` placement new (`Github <https://github.com/llvm/llvm-project/issues/105427>`__)
- P2609R3: Relaxing Ranges Just A Smidge (`Github <https://github.com/llvm/llvm-project/issues/105253>`__)
- P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
- ``std::jthread`` and ``<stop_token>`` are not guarded behind ``-fexperimental-library`` anymore
- P2674R1: A trait for implicit lifetime types (`Github <https://github.com/llvm/llvm-project/issues/105259>`__)
- P0429R9: A Standard ``flat_map`` is partially implemented and ``flat_map`` is provided (`Github <https://github.com/llvm/llvm-project/issues/105190>`__)

Improvements and New Features
-----------------------------

- The ``lexicographical_compare`` and ``ranges::lexicographical_compare`` algorithms have been optimized for trivially
  equality comparable types, resulting in a performance improvement of up to 40x.

- The ``_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER`` macro has been added to make ``std::get_temporary_buffer`` and
  ``std::return_temporary_buffer`` available.

- The ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception``
  available in C++20 and later modes.

- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``, resulting in reduced
  compile times and smaller debug information as well as better code generation if optimizations are disabled.
  The Chromium project measured a 5% reduction in object file and debug information size.

- The ``_LIBCPP_ABI_BOUNDED_UNIQUE_PTR`` ABI configuration was added, which allows ``std::unique_ptr<T[]>`` to
  detect out-of-bounds accesses in certain circumstances. ``std::unique_ptr<T[]>`` can now also detect out-of-bounds
  accesses for a limited set of types (non-trivially destructible types) when the ABI configuration is disabled.

- The ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY`` ABI configuration was added, which allows storing valid bounds
  in ``std::array::iterator`` and detecting OOB accesses when the appropriate hardening mode is enabled.

- The input iterator overload of `assign(_InputIterator, _InputIterator)` in `std::vector<_Tp, _Allocator>` has been
  optimized, resulting in a performance improvement of up to 2x for trivial element types (e.g., `std::vector<int>`),
  and up to 3.4x for non-trivial element types (e.g., `std::vector<std::vector<int>>`).

- On Windows, ``<system_error>``'s ``std::system_category`` is now distinct from ``std::generic_category``. The behavior
  on other operating systems is unchanged.

  On Windows -- unlike on Unix systems -- the libc and system APIs use distinct error codes. The libc functions return
  ``errno.h`` error codes via the ``errno`` global, while Win32 API functions return ``winerror.h`` error codes via
  ``GetLastError()``.

  The C++ standard's ``std::error_code`` and ``std::error_category`` functionality was designed to support multiple
  error domains, precisely in order to handle situations such as this. However, libc++ formerly treated
  ``generic_category()`` and ``system_category()`` as equivalent, even on Windows. It now implements the intended split,
  where ``system_category`` represents native ``winerror.h`` error codes, and ``generic_category`` represents libc error
  codes (and, equivalently, ``std::errc::*`` errors).

  This change enables code like ``std::error_code(GetLastError(), std::system_category()) ==
  std::errc::invalid_argument`` to function as desired: constructing an ``error_code`` with the Windows error number in
  the "system" category, and then mapping it to a generic code with ``error_condition``, for comparison with the
  ``std::errc`` constant.

  This is an incompatible change: ``std::error_code(ENOSYS, std::system_category()) ==
  std::errc::function_not_supported`` would formerly have returned true, but now returns false on Windows. Code
  providing a number from the ``errno.h`` domain should be migrated to construct a ``generic_category`` error_code,
  instead. (E.g., use ``std::error_code(ENOSYS, std::generic_category())``). The new behavior matches MSVC.

- On Windows, the ``std::filesystem`` library now returns the Win32 ``system_category`` error codes, where it's feasible
  to do so. This allows interrogation and reporting of the original error code, which is useful if multiple Windows
  errors map to a single generic error (such as with ``std::errc::no_such_file_or_directory``).

  This is also a slightly-incompatible API change: code inspecting the raw integer value from the returned error_code
  expecting an integer from ``generic_category`` (e.g. ``err.value() == ENOTDIR``) will not work as desired. Instead,
  such code should use the comparison operators which implicitly handle eror mappings, ``err ==
  std::errc::not_a_directory``, or use ``err.default_error_condition()`` to map to an ``error_condition``, and then test
  its ``value()`` and ``category()``.

Deprecations and Removals
-------------------------

- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to
  enable the safe mode have been removed in LLVM 20. Please use :ref:`support for hardening <using-hardening-modes>`
  instead.

- Support for the C++20 synchronization library (``<barrier>``, ``<latch>``, ``atomic::wait``, etc.) has been
  removed in language modes prior to C++20. If you are using these features prior to C++20, you will need to
  update to ``-std=c++20``.

- TODO: The relational operators for ``std::chrono::weekday`` will be removed entirely, and the
  ``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro that was used to re-enable this extension will be
  ignored in LLVM 20.

- The ``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro no longer has any effect. ``std::allocator<const T>`` is not
  supported as an extension anymore, please migrate any code that uses e.g. ``std::vector<const T>`` to be
  standards conforming.

- Non-conforming member typedefs ``base``, ``iterator`` and ``const_iterator`` of ``std::bitset``, and member typedef
  ``base`` of ``std::forward_list`` and ``std::list`` are removed. Previously, they were private but could cause
  ambiguity in name lookup. Code that expects such ambiguity will possibly not compile in LLVM 20.

- The function ``__libcpp_verbose_abort()`` is now ``noexcept``, to match ``std::terminate()``. (The combination of
  ``noexcept`` and ``[[noreturn]]`` has special significance for function effects analysis.) For backwards compatibility,
  the ``_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT`` macro can be defined to make the function non-``noexcept``. That macro
  will be removed in LLVM 21.

- ``<ccomplex>``, ``<cstdalign>`` (previously missing), ``<cstdbool>``, and ``<ctgmath>`` are deprecated since C++17 as
  specified by the standard. They, together with ``<ciso646>``, are removed in C++20, but libc++ still provides these
  headers as an extension and only deprecates them. The ``_LIBCPP_DISABLE_DEPRECATION_WARNINGS`` macro can be defined to
  suppress deprecation for these headers.

- The ``_LIBCPP_DISABLE_AVAILABILITY`` macro that was used to force-disable availability markup has now been removed.
  Whether availability markup is used by the library is now solely controlled at configuration-time.

- The pointer safety functions ``declare_reachable``, ``declare_no_pointers``, ``undeclare_no_pointers`` and
  ``__undeclare_reachable`` have been removed from the library. These functions were never implemented in a non-trivial
  way, making it very unlikely that any binary depends on them.

Upcoming Deprecations and Removals
----------------------------------

LLVM 21
~~~~~~~

- The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22,
  non-critical bug fixes may not be back-ported to C++03, including LWG issues. C++03 is a legacy platform, where most
  projects are no longer actively maintained. To reduce the amount of fixes required to keep such legacy projects
  compiling with up-to-date toolchains, libc++ will aim to freeze the status of the headers in C++03 mode to avoid
  unintended breaking changes. See https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc for more details.

  If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most
  out of libc++.

- The ``_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT`` macro will be removed in LLVM 21, making ``std::__libcpp_verbose_abort``
  unconditionally ``noexcept``.


ABI Affecting Changes
---------------------

- The ABI breaks for removing undefined behaviour in ``std::forward_list``, ``std::list``, ``std::map``, ``std::set``,
  ``std::multimap``, ``std::multiset``, ``std::unordered_map``, ``std::unordered_set``, ``std::unordered_multimap`` and
  ``std::unordered_multiset`` are now applied unconditionally. This only affects fancy pointers which have a different
  value representation when pointing at the base of an internal node type instead of the type itself. A size or
  alignment difference is diagnosed, but more subtle ABI breaks may result in unexpected behaviour.

- The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``. The ABI impact is:

  - When using the Itanium ABI (most non-MSVC platforms), empty types are now placed at the beginning of the enclosing
    object instead of where the beginning of the ``__compressed_pair`` subobject was. This is only observable by
    checking the address of the empty allocator, equality comparator or hasher.
  - Additionally, using an overaligned empty type as an allocator, comparator or hasher in the associative containers
    (and only those containers) may result in the container's object object size and data layout changing beyond only
    the address of the empty member.
  - When using the MSVC ABI, this change results in some classes having a completely different memory layout, so this is
    a genuine ABI break. However, the library does not currently guarantee ABI stability on MSVC platforms.

- The localization support base API has been reimplemented, leading to different functions being exported from the
  libc++ built library on Windows and Windows-like platforms.

Build System Changes
--------------------

- TODO