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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>`_
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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
------------------
- 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>`__)
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_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception`` available in C++20 and later modes.
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
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