clang++ segfault when using import std in module

[jpc0]

While building a module that has import std I get a segfault in clang++ 20 building with a very new cmake.

I am using the flags -stdlib=libc++ -Xclang -fno-validate-pch and the linker flags -stdlib=libc++ -lc++abi. I have also tested with onl the -stdlib=libc++ flag and -stdlib=libc++ -lc++abi linker flags and get the same error.

The CMake output before the stackdump is:

/usr/bin/clang++-20   -stdlib=libc++ -g -std=gnu++23 -MD -MT CMakeFiles/test.dir/test.cpp.o -MF CMakeFiles/test.dir/test.cpp.o.d @CMakeFiles/test.dir/test.cpp.o.modmap -o CMakeFiles/test.dir/test.cpp.o -c /home/jpc0/devel/git/debug/clang20/test.cpp
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.

I am running on Ubuntu 24.04.1 LTS under WSL if that makes any difference. I do not currenly have a different configuration of OS to test under.

I have a git repo here that has a basic reproduction as well as the relevant preprocessed source and associated run script.

In the repo run:

cmake -S . --preset debug
cmake --build build

and the crash will happen. All variables passed to cmake is in the CMakePresets debug preset.

Ubuntu clang version 20.0.0 (++20250120081916+d35d7f4b13c0-1~exp1~20250120202047.676)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-20/bin

cmake version 3.31.4-g98de8ec

I got clang from the ubuntu repo added video the llvm install script: deb http://apt.llvm.org/noble/ llvm-toolchain-noble main

[jpc0]

If I replace import std with #include <print> in the relevate section of the module header there is no crash and everything works as expected.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Jean-Pierre Coetzee (jpc0)

While building a module that has `import std` I get a segfault in clang++ 20 building with a very new cmake.

I am using the flags -stdlib=libc++ -Xclang -fno-validate-pch and the linker flags -stdlib=libc++ -lc++abi. I have also tested with onl the -stdlib=libc++ flag and -stdlib=libc++ -lc++abi linker flags and get the same error.

The CMake output before the stackdump is:

/usr/bin/clang++-20   -stdlib=libc++ -g -std=gnu++23 -MD -MT CMakeFiles/test.dir/test.cpp.o -MF CMakeFiles/test.dir/test.cpp.o.d @<!-- -->CMakeFiles/test.dir/test.cpp.o.modmap -o CMakeFiles/test.dir/test.cpp.o -c /home/jpc0/devel/git/debug/clang20/test.cpp
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.

I am running on Ubuntu 24.04.1 LTS under WSL if that makes any difference. I do not currenly have a different configuration of OS to test under.

I have a git repo here that has a basic reproduction as well as the relevant preprocessed source and associated run script.

In the repo run:

cmake -S . --preset debug
cmake --build build

and the crash will happen. All variables passed to cmake is in the CMakePresets debug preset.

Ubuntu clang version 20.0.0 (++20250120081916+d35d7f4b13c0-1~exp1~20250120202047.676)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-20/bin

cmake version 3.31.4-g98de8ec

I got clang from the ubuntu repo added video the llvm install script: deb http://apt.llvm.org/noble/ llvm-toolchain-noble main

[ChuanqiXu9]

What would happen if you remove -Xclang -fno-validate-pch?

[jpc0]

I did mention it in the original post but same result.

[koplas]

This does not crash on an older version for me:

clang version 19.1.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

@jpc0 Can you check if this is a regression? Maybe it was introduced with 5a0181f.

git tag --contains 5a0181f568e56e37df80d0f74eca4775776fa8cd returns:

llvmorg-20-init
llvmorg-21-init

Update: It doesn't crash for me on this version

clang version 21.0.0git (https://github.com/llvm/llvm-project.git 53b48301eb3bb4aba63678e9b5eb7cbea7745e34)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin