[libc++][doc] Update the release notes for LLVM 20.

[mordante]

This is a preparation for the upcoming LLVM 20 release.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Mark de Wever (mordante)

Changes

This is a preparation for the upcoming LLVM 20 release.

---

Full diff: https://github.com/llvm/llvm-project/pull/124403.diff

1 Files Affected:

• (modified) libcxx/docs/ReleaseNotes/20.rst (+9-5)

diff --git a/libcxx/docs/ReleaseNotes/20.rst b/libcxx/docs/ReleaseNotes/20.rst
index 2736061544c531..dd5d7f40c1bacd 100644
--- a/libcxx/docs/ReleaseNotes/20.rst
+++ b/libcxx/docs/ReleaseNotes/20.rst
@@ -35,6 +35,8 @@ see the `releases page <https://llvm.org/releases/>`_.
 What's New in Libc++ 20.0.0?
 ==============================
 
+The main focus of the libc++ team has been to implement new C++20, C++23, and C++26 features.
+
 Implemented Papers
 ------------------
 
@@ -111,6 +113,13 @@ Improvements and New Features
   std::errc::not_a_directory``, or use ``err.default_error_condition()`` to map to an ``error_condition``, and then test
   its ``value()`` and ``category()``.
 
+- ``std::stable_sort`` uses radix sort for integral types now, which can improve the performance up to 10 times, depending
+  on type of sorted elements and the initial state of the sorted array.
+
+- Reduced the amount of debug information generated for internal typedefs. This reduces the size of debug builds.
+
+- Added :ref:`hardening mode <hardening>` support for ``forward_list`` and ``bitset``.
+
 Deprecations and Removals
 -------------------------
 
@@ -196,8 +205,3 @@ ABI Affecting Changes
 
 - 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

[mordante]

@winner245 since you did several patches recently feel free to suggest additions/changes.

[h-vetinari]

W.r.t.

llvm-project/libcxx/docs/ReleaseNotes/20.rst

Line 47 in 6805d7e

- P0429R9: A Standard ``flat_map`` is partially implemented and ``flat_map`` is provided (`Github <https://github.com/llvm/llvm-project/issues/105190>`__)

I believe the remaining bits were fully implemented by def50f7, including the FTM

[Zingam]

Thank you! LGTM! With the suggested change above: #124403 (comment)

P0429R9 - A Standard flat_map

It looks like line 55 in libcxx/docs/Status/Cxx23Papers.csv needs to be updated to include the LLVM version too:

[ldionne]

Thanks a lot!