[libc++][CI] Test std module in C++26 mode.

[ldionne]

libcxx/utils/ci/buildkite-pipeline.yml

214

I am kinda worried about the explosion of the number of jobs. Is there a reason why we want to keep testing the std module in C++23? For example, we don't do that for Clang modules. We only test the latest standard version with Clang modules.

[Mordante]

libcxx/utils/ci/buildkite-pipeline.yml

214

Yes and No. C++26 adds new named declarations not available in C++23. For example there are already 3 new headers accepted for C++26; hazard_pointer, rcu and text_encoding. So we need to test C++26 and C++23 separately. Based on our discussion regarding testing modules yesterday we may want to take a different approach for testing modules.
We can always build with modules enabled and run the std and std.compat module test separately in the Generic C++23/26 builds. For now I put this patch on hold; let's see what our testing strategy will be first.

[Mordante]

libcxx/utils/ci/buildkite-pipeline.yml

214

I know have a different approach where I reuse the existing job to build the module tests. So instead of adding a new job it adds a few tests to the existing job. This also solves the scalability for testing with different libc++ configurations.

[Mordante]

Reworked the patch to use a new approach.

[ldionne]

libcxx/utils/ci/buildkite-pipeline.yml

119–123

[Mordante]

Rebased, resoved merge conflicts.

[Mordante]

Rebased to test dependent patches.

libcxx/cmake/caches/Generic-cxx26.cmake

+set(LIBCXX_ENABLE_STD_MODULES ON CACHE BOOL "") # TODO MODULES Remove when enabled automatically.
 set(LIBCXX_TEST_PARAMS "std=c++26" CACHE STRING "")
 set(LIBCXXABI_TEST_PARAMS "${LIBCXX_TEST_PARAMS}" CACHE STRING "")

libcxx/docs/Modules.rst

* The following "parts disabled" configuration options are supported
 
    * ``LIBCXX_ENABLE_LOCALIZATION``
    * ``LIBCXX_ENABLE_WIDE_CHARACTERS``
 
 Some of the current limitations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
  * There is no official build system support, libc++ has experimental CMake support
- * Requires CMake 3.26
+ * Requires CMake 3.26 for C++23 support
+ * Requires CMake 3.27 for C++26 support
  * Requires Ninja 1.11
  * Requires a recent Clang 17
  * The path to the compiler may not be a symlink, ``clang-scan-deps`` does
    not handle that case properly
- * Only C++23 is tested
+ * Only C++23 and C++26 are tested
  * Libc++ is not tested with modules instead of headers
  * The module ``.cppm`` files are not installed
  * The experimental ``PSTL`` library is not supported
  * Clang supports modules using GNU extensions, but libc++ does not work using
    GNU extensions.
  * Clang:
     * Including headers after importing the ``std`` module may fail. This is
       hard to solve and there is a work-around by first including all headers

libcxx/test/libcxx/module_std.gen.py

 # newline needs to be escaped for the module partition output.
 nl = '\\\\n'
 
 # Create empty file with all parts.
 print(
     f"""\
 //--- module_std.sh.cpp
 // UNSUPPORTED{BLOCKLIT}: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED{BLOCKLIT}: c++26
 // UNSUPPORTED{BLOCKLIT}: libcpp-has-no-std-modules
 // UNSUPPORTED{BLOCKLIT}: modules-build
 
 // REQUIRES{BLOCKLIT}: has-clang-tidy
 
 // The GCC compiler flags are not always compatible with clang-tidy.
 // UNSUPPORTED{BLOCKLIT}: gcc
 

libcxx/test/lit.local.cfg

 
 
 def appendToSubstitution(substitutions, key, value):
     return [(k, v + " " + value) if k == key else (k, v) for (k, v) in substitutions]
 
 
 std = getSubstitution("%{cxx_std}", config)
 if std == "cxx26":
-    # This fails to work properly. It might be due to
-    #   CMAKE_CXX_STANDARD 26
-    # does not work in CMake 3.26, it requires the upcomming CMake 3.27.
-    # TODO MODULES test whether this is fixed with CMake 3.27.
-    std = ""
+    std = "26"
 elif std == "cxx23":
     std = "23"
 else:
     std = ""
 
 if (
     std
     and not "libcpp-has-no-std-modules" in config.available_features

libcxx/test/std/modules/std.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// TODO MODULES fix c++26
-// XFAIL: c++26
 
 // UNSUPPORTED: libcpp-has-no-std-modules
 // UNSUPPORTED: modules-build
 
 // A minimal test to validate import works.
 
 import std;
 
 int main(int, char**) {
   std::println("Hello modular world");
   return 0;
 }

libcxx/utils/ci/buildkite-pipeline.yml

- label: "GCC ${GCC_STABLE_VERSION} / C++latest"
     timeout_in_minutes: 120
 
   - label: "C++26"
     command: "libcxx/utils/ci/run-buildbot generic-cxx26"
     artifact_paths:
       - "**/test-results.xml"
       - "**/*.abilist"
     env:
-        CC: "clang-${LLVM_HEAD_VERSION}"
-        CXX: "clang++-${LLVM_HEAD_VERSION}"
+        # Note modules require and absolute path for clang-scan-deps
+        # https://github.com/llvm/llvm-project/issues/61006
+        CC: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang"
+        CXX: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang++"
+        CMAKE: "/opt/bin/cmake"
         ENABLE_CLANG_TIDY: "On"
     agents:
       queue: "libcxx-builders"
       os: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
retry:
           limit: 2
     timeout_in_minutes: 120
 
   #
   # All other supported configurations of libc++.
   #
   - wait
 
   - label: "C++23"
     command: "libcxx/utils/ci/run-buildbot generic-cxx23"
     artifact_paths:
       - "**/test-results.xml"
       - "**/*.abilist"
     env:
         CC: "clang-${LLVM_HEAD_VERSION}"
         CXX: "clang++-${LLVM_HEAD_VERSION}"
         ENABLE_CLANG_TIDY: "On"