[libc++] Updates the compilers used post LLVM-20 branching.

Once LLVM 20 is released the clang-18 will no longer be supported.

Author: mordante

.github/workflows/libcxx-build-and-test.yaml

@@ -48,8 +48,8 @@ jobs:
           'generic-cxx26',
           'generic-modules'
         ]
-        cc: [  'clang-20' ]
-        cxx: [ 'clang++-20' ]
+        cc: [  'clang-21' ]
+        cxx: [ 'clang++-21' ]
         include:
           - config: 'generic-gcc'
             cc: 'gcc-14'
@@ -88,18 +88,22 @@ jobs:
           'generic-cxx20',
           'generic-cxx23'
         ]
-        cc: [ 'clang-20' ]
-        cxx: [ 'clang++-20' ]
+        cc: [ 'clang-21' ]
+        cxx: [ 'clang++-21' ]
         include:
           - config: 'generic-gcc-cxx11'
             cc: 'gcc-14'
             cxx: 'g++-14'
-          - config: 'generic-cxx23'
-            cc: 'clang-18'
-            cxx: 'clang++-18'
+          - config: 'generic-cxx26'
+            cc: 'clang-20'
+            cxx: 'clang++-20'
           - config: 'generic-cxx26'
             cc: 'clang-19'
             cxx: 'clang++-19'
+          # Release transition
+          - config: 'generic-cxx23'
+            cc: 'clang-18'
+            cxx: 'clang++-18'
     steps:
       - uses: actions/checkout@v4
       - name: ${{ matrix.config }}
@@ -169,8 +173,8 @@ jobs:
       - name: ${{ matrix.config }}
         run: libcxx/utils/ci/run-buildbot ${{ matrix.config }}
         env:
-          CC: clang-20
-          CXX: clang++-20
+          CC: clang-21
+          CXX: clang++-21
       - uses: actions/upload-artifact@26f96dfa697d77e81fd5907df203aa23a56210a8 # v4.3.0
         if: always()
         with:

libcxx/docs/index.rst

@@ -128,14 +128,14 @@ Libc++ aims to support common compilers that implement the C++11 Standard. In or
 good balance between stability for users and maintenance cost, testing coverage and development
 velocity, libc++ drops support for older compilers as newer ones are released.
 
-============ =============== ========================== =====================
-Compiler     Versions        Restrictions               Support policy
-============ =============== ========================== =====================
-Clang        17, 18, 19-git                             latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
-AppleClang   15                                         latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
-Open XL      17.1 (AIX)                                 latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
-GCC          14              In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
-============ =============== ========================== =====================
+============ =================== ========================== =====================
+Compiler     Versions            Restrictions               Support policy
+============ =================== ========================== =====================
+Clang        18, 19, 20, 21-git                             latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
+AppleClang   15                                             latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
+Open XL      17.1 (AIX)                                     latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
+GCC          14                  In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
+============ =================== ========================== =====================
 
 Libc++ also supports common platforms and architectures: