[clang] Reland "[clang][modules-driver] Add support for C++ named modules and import std" (2nd attempt) (PR #194475)

Mon Apr 27 17:11:22 PDT 2026

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-4` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/41510

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: Driver/modules-driver-import-std.cpp' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 7
rm -rf /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp && split-file /Users/buildbot/buildbot-root/llvm-project/clang/test/Driver/modules-driver-import-std.cpp /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp
# executed command: rm -rf /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp
# note: command had no output on stdout or stderr
# executed command: split-file /Users/buildbot/buildbot-root/llvm-project/clang/test/Driver/modules-driver-import-std.cpp /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp
# note: command had no output on stdout or stderr
# RUN: at line 8
mkdir -p /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu
# executed command: mkdir -p /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu
# note: command had no output on stdout or stderr
# RUN: at line 9
touch /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu/libc++.so
# executed command: touch /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu/libc++.so
# note: command had no output on stdout or stderr
# RUN: at line 10
touch /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu/libc++.a
# executed command: touch /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu/libc++.a
# note: command had no output on stdout or stderr
# RUN: at line 12
sed "s|DIR|/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp|g" /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/libc++.modules.json.in >    /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu/libc++.modules.json
# executed command: sed 's|DIR|/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp|g' /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/libc++.modules.json.in
# note: command had no output on stdout or stderr
# RUN: at line 15
mkdir -p /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/share/libc++/v1
# executed command: mkdir -p /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/share/libc++/v1
# note: command had no output on stdout or stderr
# RUN: at line 16
cat /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/std.cppm > /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/share/libc++/v1/std.cppm
# executed command: cat /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/std.cppm
# note: command had no output on stdout or stderr
# RUN: at line 17
cat /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/std.compat.cppm > /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/share/libc++/v1/std.compat.cppm
# executed command: cat /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/std.compat.cppm
# note: command had no output on stdout or stderr
# RUN: at line 50
/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/bin/clang -std=c++23 -stdlib=libc++    -fmodules-driver -Rmodules-driver -Rmodule-import    -stdlib=libc++    -resource-dir=/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu    --sysroot=/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot    -L/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/Inputs/usr/lib/x86_64-linux-gnu    --target=x86_64-linux-gnu    -c /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/main.cpp 2>&1    | sed 's:\\\\\?:/:g'    | /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/bin/FileCheck -DPREFIX=/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp /Users/buildbot/buildbot-root/llvm-project/clang/test/Driver/modules-driver-import-std.cpp
# executed command: /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/bin/clang -std=c++23 -stdlib=libc++ -fmodules-driver -Rmodules-driver -Rmodule-import -stdlib=libc++ -resource-dir=/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot/usr/lib/x86_64-linux-gnu --sysroot=/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/FakeSysroot -L/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/Inputs/usr/lib/x86_64-linux-gnu --target=x86_64-linux-gnu -c /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp/main.cpp
# note: command had no output on stdout or stderr
# error: command failed with exit status: 1
# executed command: sed 's:\\\\\?:/:g'
# note: command had no output on stdout or stderr
# executed command: /Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/bin/FileCheck -DPREFIX=/Volumes/RAMDisk/buildbot-root/aarch64-darwin/build/tools/clang/test/Driver/Output/modules-driver-import-std.cpp.tmp /Users/buildbot/buildbot-root/llvm-project/clang/test/Driver/modules-driver-import-std.cpp
# note: command had no output on stdout or stderr

--
...

```

</details>

https://github.com/llvm/llvm-project/pull/194475