[libc++] Updates the compilers used post LLVM-20 branching. (#126564)

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

Author: mordante

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

@@ -37,7 +37,7 @@ jobs:
   stage1:
     if: github.repository_owner == 'llvm'
     runs-on: libcxx-self-hosted-linux
-    container: ghcr.io/llvm/libcxx-linux-builder:d8a0709b1090350a7fe3604d8ab78c7d62f10698
+    container: ghcr.io/llvm/libcxx-linux-builder:b319dfef21f6c7b0bc6a356d6b9f41a3b3b98ae9
     continue-on-error: false
     strategy:
       fail-fast: false
@@ -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'
@@ -75,7 +75,7 @@ jobs:
   stage2:
     if: github.repository_owner == 'llvm'
     runs-on: libcxx-self-hosted-linux
-    container: ghcr.io/llvm/libcxx-linux-builder:d8a0709b1090350a7fe3604d8ab78c7d62f10698
+    container: ghcr.io/llvm/libcxx-linux-builder:b319dfef21f6c7b0bc6a356d6b9f41a3b3b98ae9
     needs: [ stage1 ]
     continue-on-error: false
     strategy:
@@ -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 }}
@@ -163,14 +167,14 @@ jobs:
         - config: 'generic-msan'
           machine: libcxx-self-hosted-linux
     runs-on: ${{ matrix.machine }}
-    container: ghcr.io/llvm/libcxx-linux-builder:d8a0709b1090350a7fe3604d8ab78c7d62f10698
+    container: ghcr.io/llvm/libcxx-linux-builder:b319dfef21f6c7b0bc6a356d6b9f41a3b3b98ae9
     steps:
       - uses: actions/checkout@v4
       - 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:
 

libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp

@@ -12,7 +12,7 @@
 // UNSUPPORTED: c++03
 
 // TODO: Investigate these failures which break the CI.
-// UNSUPPORTED: clang-18, clang-19, clang-20
+// UNSUPPORTED: clang-18, clang-19, clang-20, clang-21
 
 // The Android libc++ tests are run on a non-Android host, connected to an
 // Android device over adb. gdb needs special support to make this work (e.g.