[libc++][CI] Upgrade compiler HEAD version to Clang-20

[robincaloudis]

Why

The release branch for LLVM 19.x is created (release/19.x) as anounced here. As a result, the version on main is now 20.0.0git. Therefore, we no longer officially support Clang-17, but instead Clang-{18,19,20}.

What

Upgrade HEAD version to Clang-20 in CI configs. Adjustments for libc++ tests and implementations will be done in follow-up patches.

[github-actions]

✅ With the latest revision this PR passed the Python code formatter.

[llvmbot]

@llvm/pr-subscribers-github-workflow

@llvm/pr-subscribers-libcxx

Author: Robin Caloudis (robincaloudis)

Changes

Why

The release branch for LLVM 19.x is created (release/19.x) as anounced here. As a result, the version on main is now 20.0.0git. Therefore, we no longer officially support Clang-17, but instead Clang-{18,19,20}.

What

Upgrade HEAD version to Clang-20 in CI configs. Adjustments for libc++ tests and implementations will be done in follow-up patches.

---

Full diff: https://github.com/llvm/llvm-project/pull/108761.diff

4 Files Affected:

• (modified) .github/workflows/libcxx-build-and-test.yaml (+13-13)
• (modified) libcxx/docs/index.rst (+1-1)
• (modified) libcxx/include/__configuration/compiler.h (+2-2)
• (modified) libcxx/test/libcxx/clang_tidy.gen.py (+4-2)

diff --git a/.github/workflows/libcxx-build-and-test.yaml b/.github/workflows/libcxx-build-and-test.yaml
index b5e60781e00064..426c40cc2f248c 100644
--- a/.github/workflows/libcxx-build-and-test.yaml
+++ b/.github/workflows/libcxx-build-and-test.yaml
@@ -38,11 +38,11 @@ env:
   # LLVM POST-BRANCH bump version
   # LLVM POST-BRANCH add compiler test for ToT - 1, e.g. "Clang 17"
   # LLVM RELEASE bump remove compiler ToT - 3, e.g. "Clang 15"
-  LLVM_HEAD_VERSION: "19"   # Used compiler, update POST-BRANCH.
-  LLVM_PREVIOUS_VERSION: "18"
-  LLVM_OLDEST_VERSION: "17"
+  LLVM_HEAD_VERSION: "20"   # Used compiler, update POST-BRANCH.
+  LLVM_PREVIOUS_VERSION: "19"
+  LLVM_OLDEST_VERSION: "18"
   GCC_STABLE_VERSION: "13"
-  LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-19"
+  LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-20"
   CLANG_CRASH_DIAGNOSTICS_DIR: "crash_diagnostics"
 
 
@@ -59,8 +59,8 @@ jobs:
           'generic-cxx26',
           'generic-modules'
         ]
-        cc: [  'clang-19' ]
-        cxx: [ 'clang++-19' ]
+        cc: [  'clang-20' ]
+        cxx: [ 'clang++-20' ]
         include:
           - config: 'generic-gcc'
             cc: 'gcc-14'
@@ -97,18 +97,18 @@ jobs:
           'generic-cxx20',
           'generic-cxx23'
         ]
-        cc: [ 'clang-19' ]
-        cxx: [ 'clang++-19' ]
+        cc: [ 'clang-20' ]
+        cxx: [ 'clang++-20' ]
         include:
           - config: 'generic-gcc-cxx11'
             cc: 'gcc-14'
             cxx: 'g++-14'
-          - config: 'generic-cxx23'
-            cc: 'clang-17'
-            cxx: 'clang++-17'
           - config: 'generic-cxx26'
             cc: 'clang-18'
             cxx: 'clang++-18'
+          - config: 'generic-cxx26'
+            cc: 'clang-19'
+            cxx: 'clang++-19'
     steps:
       - uses: actions/checkout@v4
       - name: ${{ matrix.config }}
@@ -179,8 +179,8 @@ jobs:
       - name: ${{ matrix.config }}
         run: libcxx/utils/ci/run-buildbot ${{ matrix.config }}
         env:
-          CC: clang-19
-          CXX: clang++-19
+          CC: clang-20
+          CXX: clang++-20
       - uses: actions/upload-artifact@26f96dfa697d77e81fd5907df203aa23a56210a8 # v4.3.0
         if: always()
         with:
diff --git a/libcxx/docs/index.rst b/libcxx/docs/index.rst
index a9610cbb4db3a4..8d076928fe9c7f 100644
--- a/libcxx/docs/index.rst
+++ b/libcxx/docs/index.rst
@@ -130,7 +130,7 @@ 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
+Clang        18, 19, 20-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>`_
diff --git a/libcxx/include/__configuration/compiler.h b/libcxx/include/__configuration/compiler.h
index 80ece22bb50bd6..cf459a0619b23c 100644
--- a/libcxx/include/__configuration/compiler.h
+++ b/libcxx/include/__configuration/compiler.h
@@ -33,8 +33,8 @@
 // Warn if a compiler version is used that is not supported anymore
 // LLVM RELEASE Update the minimum compiler versions
 #  if defined(_LIBCPP_CLANG_VER)
-#    if _LIBCPP_CLANG_VER < 1700
-#      warning "Libc++ only supports Clang 17 and later"
+#    if _LIBCPP_CLANG_VER < 1800
+#      warning "Libc++ only supports Clang 18 and later"
 #    endif
 #  elif defined(_LIBCPP_APPLE_CLANG_VER)
 #    if _LIBCPP_APPLE_CLANG_VER < 1500
diff --git a/libcxx/test/libcxx/clang_tidy.gen.py b/libcxx/test/libcxx/clang_tidy.gen.py
index 5e84fbbb9913f3..f4f905f579784f 100644
--- a/libcxx/test/libcxx/clang_tidy.gen.py
+++ b/libcxx/test/libcxx/clang_tidy.gen.py
@@ -18,7 +18,8 @@
 from libcxx.header_information import lit_header_restrictions, public_headers
 
 for header in public_headers:
-  print(f"""\
+    print(
+        f"""\
 //--- {header}.sh.cpp
 
 // REQUIRES: has-clang-tidy
@@ -36,4 +37,5 @@
 // RUN: %{{clang-tidy}} %s --warnings-as-errors=* -header-filter=.* --config-file=%{{libcxx-dir}}/.clang-tidy -- -Wweak-vtables %{{compile_flags}} -fno-modules
 
 #include <{header}>
-""")
+"""
+    )

[Zingam]

@robincaloudis The failing tests related to span should be fixed in a separate PR. I'm not sure why they are failing in C++20/23 mode with Clang20 suddenly. Here is a blog post detailing the ordeal: https://quuxplusone.github.io/blog/2021/10/03/p2447-span-from-initializer-list/

[ldionne]

I'd like to land #110303 before we tackle any kind of upgrade. That PR will make it a lot easier (and safer) to deploy a new image onto the bots.