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112 lines
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=========================================
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Libc++ 16.0.0 (In-Progress) Release Notes
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=========================================
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.. contents::
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:local:
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:depth: 2
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Written by the `Libc++ Team <https://libcxx.llvm.org>`_
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.. warning::
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These are in-progress notes for the upcoming libc++ 16 release.
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Release notes for previous releases can be found on
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`the Download Page <https://releases.llvm.org/download.html>`_.
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Introduction
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============
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This document contains the release notes for the libc++ C++ Standard Library,
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part of the LLVM Compiler Infrastructure, release 16.0.0. Here we describe the
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status of libc++ in some detail, including major improvements from the previous
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release and new feature work. For the general LLVM release notes, see `the LLVM
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documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
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be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.
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For more information about libc++, please see the `Libc++ Web Site
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<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.
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Note that if you are reading this file from a Git checkout or the
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main Libc++ web page, this document applies to the *next* release, not
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the current one. To see the release notes for a specific release, please
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see the `releases page <https://llvm.org/releases/>`_.
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What's New in Libc++ 16.0.0?
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============================
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Implemented Papers
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------------------
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- P2499R0 - ``string_view`` range constructor should be ``explicit``
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- P2417R2 - A more constexpr bitset
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- P2445R1 - ``std::forward_like``
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- P2273R3 - Making ``std::unique_ptr`` constexpr
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Improvements and New Features
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-----------------------------
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- Declarations of ``std::c8rtomb()`` and ``std::mbrtoc8()`` from P0482R6 are
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now provided when implementations in the global namespace are provided by
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the C library.
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Deprecations and Removals
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-------------------------
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- ``unary_function`` and ``binary_function`` are no longer provided in C++17 and newer Standard modes.
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They can be re-enabled with ``_LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION``.
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- Several incidental transitive includes have been removed from libc++. Those
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includes are removed based on the language version used. Incidental transitive
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inclusions of the following headers have been removed:
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- C++20: ``chrono``
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- C++2b: ``algorithm``, ``array``, ``atomic``, ``bit``, ``chrono``,
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``climits``, ``cmath``, ``compare``, ``concepts``, ``cstdarg`, ``cstddef``,
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``cstdint``, ``cstdlib``, ``cstring``, ``ctime``, ``exception``,
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``functional``, ``initializer_list``, ``iosfwd``, ``iterator``, ``limits``,
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``memory``, ``new``, ``numeric``, ``optional``, ``ratio``, ``stdexcept``,
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``string``, ``tuple``, ``type_traits``, ``typeinfo``, ``unordered_map``,
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``utility``, ``variant``, ``vector``.
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Users can also remove all incidental transitive includes by defining
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``_LIBCPP_REMOVE_TRANSITIVE_INCLUDES`` regardless of the language version
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in use. Note that in the future, libc++ reserves the right to remove
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incidental transitive includes more aggressively, in particular regardless
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of the language version in use.
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- The legacy testing system for libc++, libc++abi and libunwind has been removed.
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All known clients have been migrated to the new configuration system, but please
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reach out to the libc++ developers if you find something missing in the new
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configuration system.
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Upcoming Deprecations and Removals
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----------------------------------
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API Changes
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-----------
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- The comparison operators on ``thread::id`` are now defined as free-standing
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functions instead of as hidden friends, in conformance with the C++ standard.
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Also see `issue 56187 <https://github.com/llvm/llvm-project/issues/56187>`_.
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- ``_LIBCPP_ENABLE_NODISCARD`` and ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17`` are no longer respected.
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Any standards-required ``[[nodiscard]]`` applications in C++20 are now always enabled. Any extended applications
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are now enabled by default and can be disabled by defining ``_LIBCPP_DISABLE_NODISCARD_EXT``.
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- ``_LIBCPP_VERSION`` was previously defined to e.g. ``15001`` to represent LLVM 15.0.01, but this value had been
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left undocumented. Starting with LLVM 16, ``_LIBCPP_VERSION`` will contain the version of LLVM represented as
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``XXYYZZ``. In other words, ``_LIBCPP_VERSION`` is gaining a digit. This should not be an issue for existing
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code, since using e.g. ``_LIBCPP_VERSION > 15000`` will still give the right answer now that ``_LIBCPP_VERSION``
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is defined as e.g. ``160000`` (with one more digit).
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ABI Affecting Changes
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---------------------
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- In freestanding mode, ``atomic<small enum class>`` does not contain a lock byte anymore if the platform
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can implement lockfree atomics for that size. More specifically, in LLVM <= 11.0.1, an ``atomic<small enum class>``
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would not contain a lock byte. This was broken in LLVM >= 12.0.0, where it started including a lock byte despite
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the platform supporting lockfree atomics for that size. Starting in LLVM 15.0.1, the ABI for these types has been
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restored to what it used to be (no lock byte), which is the most efficient implementation.
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This ABI break only affects users that compile with ``-ffreestanding``, and only for ``atomic<T>`` where ``T``
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is a non-builtin type that could be lockfree on the platform. See https://llvm.org/D133377 for more details.
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Build System Changes
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--------------------
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