libcxx/docs/ReleaseNotes/20.rst

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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
------------------

- TODO


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.


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

- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to enable
  the safe mode will be removed in LLVM 20.

- 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.

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

LLVM 20
~~~~~~~

- TODO


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++.


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

- TODO


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

- TODO