### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 25, 2025, 4:36pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/2 "2025-10-25T16:36:11Z")
### I can’t understand why libstdc++ is searched?
I have tried it with every available cmake version 4.x and llvm version 20, 21, and 22!
```bash
root ➜ /home/builder/workdir (feature/import-std-in-cmake-3-30) $ /root/llvm/bin/clang++ -print-file-name=libc++.modules.json
/root/llvm/bin/../lib/x86_64-unknown-linux-gnu/libc++.modules.json
root ➜ /home/builder/workdir (feature/import-std-in-cmake-3-30) $ cat /root/llvm/bin/../lib/x86_64-unknown-linux-gnu/libc++.modules.json
{
"version": 1,
"revision": 1,
"modules": [
{
"logical-name": "std",
"source-path": "../../share/libc++/v1/std.cppm",
"is-std-library": true,
"local-arguments": {
"system-include-directories": [
"../../share/libc++/v1"
]
}
},
{
"logical-name": "std.compat",
"source-path": "../../share/libc++/v1/std.compat.cppm",
"is-std-library": true,
"local-arguments": {
"system-include-directories": [
"../../share/libc++/v1"
]
}
}
]
}
root ➜ /home/builder/workdir (feature/import-std-in-cmake-3-30) $
```
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [October 25, 2025, 6:46pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/3 "2025-10-25T18:46:25Z")
… and I can compile the `libc++.modules.json` contents:
```bash
root ➜ /home/builder/workdir (feature/use_cxx_module_std) $ ninja -C build/dev clean
ninja: Entering directory `build/dev'
[1/1] Cleaning all built files...
Cleaning... 20 files.
root ➜ /home/builder/workdir (feature/use_cxx_module_std) $ ninja -C build/dev stdc++ -v
ninja: Entering directory `build/dev'
[1/6] "/usr/bin/clang-scan-deps-20" -format=p1689 -- /usr/lib/llvm-20/bin/clang++ -D_LIBCPP_HAS_NO_LOCALIZATION -O3 -DNDEBUG -std=gnu++26 -fexperimental-library -stdlib=libc++ -Wno-reserved-module-identifier -x c++ /usr/lib/llvm-20/share/libc++/v1/std.compat.cppm -c -o module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o -resource-dir "/usr/lib/llvm-20/lib/clang/20" -MT module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o.ddi -MD -MF module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o.ddi.d > module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o.ddi.tmp && mv module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o.ddi.tmp module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o.ddi
[2/6] "/usr/bin/clang-scan-deps-20" -format=p1689 -- /usr/lib/llvm-20/bin/clang++ -D_LIBCPP_HAS_NO_LOCALIZATION -O3 -DNDEBUG -std=gnu++26 -fexperimental-library -stdlib=libc++ -Wno-reserved-module-identifier -x c++ /usr/lib/llvm-20/share/libc++/v1/std.cppm -c -o module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o -resource-dir "/usr/lib/llvm-20/lib/clang/20" -MT module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o.ddi -MD -MF module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o.ddi.d > module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o.ddi.tmp && mv module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o.ddi.tmp module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o.ddi
[3/6] /root/cmake/cmake-4.0.4-linux-x86_64/bin/cmake -E cmake_ninja_dyndep --tdi=module/tests/CMakeFiles/stdc++.dir/CXXDependInfo.json --lang=CXX --modmapfmt=clang --dd=module/tests/CMakeFiles/stdc++.dir/CXX.dd @module/tests/CMakeFiles/stdc++.dir/CXX.dd.rsp
[4/6] /usr/bin/ccache /usr/lib/llvm-20/bin/clang++ -D_LIBCPP_HAS_NO_LOCALIZATION -O3 -DNDEBUG -std=gnu++26 -fexperimental-library -stdlib=libc++ -Wno-reserved-module-identifier -MD -MT module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o -MF module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o.d @module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o.modmap -o module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o -c /usr/lib/llvm-20/share/libc++/v1/std.cppm
[5/6] /usr/bin/ccache /usr/lib/llvm-20/bin/clang++ -D_LIBCPP_HAS_NO_LOCALIZATION -O3 -DNDEBUG -std=gnu++26 -fexperimental-library -stdlib=libc++ -Wno-reserved-module-identifier -MD -MT module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o -MF module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o.d @module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o.modmap -o module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o -c /usr/lib/llvm-20/share/libc++/v1/std.compat.cppm
[6/6] : && /root/cmake/cmake-4.0.4-linux-x86_64/bin/cmake -E rm -f module/tests/libstdc++.a && /usr/lib/llvm-20/bin/llvm-ar qc module/tests/libstdc++.a module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.cppm.o module/tests/CMakeFiles/stdc++.dir/usr/lib/llvm-20/share/libc++/v1/std.compat.cppm.o && /usr/lib/llvm-20/bin/llvm-ranlib module/tests/libstdc++.a && :
root ➜ /home/builder/workdir (feature/use_cxx_module_std) $ uname -a
Linux fe5675bfd498 6.10.14-linuxkit #1 SMP PREEMPT_DYNAMIC Tue Oct 14 07:34:37 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
root ➜ /home/builder/workdir (feature/use_cxx_module_std) $ cat /etc/issue
Ubuntu 24.04.2 LTS \n \l
root ➜ /home/builder/workdir (feature/use_cxx_module_std) $
```
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 16, 2026, 12:24pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/4 "2026-07-16T12:24:12Z")
### Update: it is still a problem under `ubuntu:24.04` and `clang++-23` installed
```bash
root@240450b65960:/home/builder/workdir# clang++-23 -print-file-name=libc++.modules.json
/lib/x86_64-linux-gnu/libc++.modules.json
root@240450b65960:/home/builder/workdir# which clang++
/usr/bin/clang++
root@240450b65960:/home/builder/workdir# clang++ --version
Ubuntu clang version 23.0.0 (++20260707081847+70646dd3eda3-1~exp1~20260707082012.1709)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-23/bin
root@240450b65960:/home/builder/workdir# cat /lib/x86_64-linux-gnu/libc++.modules.json
{
"version": 1,
"revision": 1,
"modules": [
{
"logical-name": "std",
"source-path": "../share/libc++/v1/std.cppm",
"is-std-library": true,
"local-arguments": {
"system-include-directories": [
"../share/libc++/v1"
]
}
},
{
"logical-name": "std.compat",
"source-path": "../share/libc++/v1/std.compat.cppm",
"is-std-library": true,
"local-arguments": {
"system-include-directories": [
"../share/libc++/v1"
]
}
}
]
}
root@240450b65960:/home/builder/workdir#
root@240450b65960:/home/builder/workdir# find /usr -name '*.cppm'
/usr/lib/llvm-23/share/libc++/v1/std.compat.cppm
/usr/lib/llvm-23/share/libc++/v1/std.cppm
##################################
find /usr/lib/llvm-23 -name 'libc*.json'
/usr/lib/llvm-23/lib/libc++.modules.json
-- FMT_USE_MODULES==ON
-- CMAKE_CXX_MODULE_STD=ON
-- CMAKE_CXX_COMPILER_IMPORT_STD=23;26
-- Module tests to be built: module-test
-- Configuring done (13.4s)
CMake Error:
Cannot find source file:
/lib/share/libc++/v1/std.compat.cppm
CMake Error in CMakeLists.txt:
No SOURCES given to target: @cmake_cxx_std
CMake Generate step failed. Build files cannot be regenerated correctly.
gmake: *** [GNUmakefile:90: .init] Error 1
```
There exists a workaround while building a docker image:
### Fix broken libc++ module metadata path from [apt.llvm.org](http://apt.llvm.org) Ubuntu package
```dockerfile
RUN sed -i \
's#\.\./share/libc++#/usr/lib/llvm-'${LLVM_VERSION}'/share/libc++#' \
/lib/x86_64-linux-gnu/libc++.modules.json
```
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 16, 2026, 3:35pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/5 "2026-07-16T15:35:10Z")
here is my used `Dockerfile`:
```dockerfile
# SPDX-License-Identifier: GPL-3.0-or-later
# Copyright © 2025 GSI Helmholtzzentrum fuer Schwerionenforschung GmbH
# Matthias Kretz
FROM ghcr.io/mattkretz/cplusplus-ci/gcc14
ENV DEBIAN_FRONTEND=noninteractive
ENV LLVM_VERSION=23
# NOTE: pipx ensurepath -> appends /root/.local/bin to the PATH environment variable.
ENV PATH="/root/.local/bin:${PATH}"
RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|apt-key add - && \
add-apt-repository "deb http://apt.llvm.org/noble/ llvm-toolchain-noble main" && \
apt-get update && \
apt-get install -y --no-install-recommends \
bash bash-completion vim tree less symlinks direnv \
build-essential curl git wget libssl-dev \
doxygen graphviz python3-pip python-is-python3 pipx \
zip ccache \
clang-${LLVM_VERSION} \
clang-tidy-${LLVM_VERSION} \
clang-format-${LLVM_VERSION} \
libc++-${LLVM_VERSION}-dev \
libc++abi-${LLVM_VERSION}-dev && \
apt-get install -y --no-install-recommends libclang-rt-${LLVM_VERSION}-dev && \
update-alternatives --install /usr/bin/clang clang /usr/bin/clang-${LLVM_VERSION} ${LLVM_VERSION}0 \
--slave /usr/bin/clang++ clang++ /usr/bin/clang++-${LLVM_VERSION} && \
update-alternatives --install /usr/bin/clang-tidy clang-tidy /usr/bin/clang-tidy-${LLVM_VERSION} ${LLVM_VERSION}0 && \
update-alternatives --install /usr/bin/run-clang-tidy run-clang-tidy /usr/bin/run-clang-tidy-${LLVM_VERSION} ${LLVM_VERSION}0 && \
update-alternatives --install /usr/bin/clang-format clang-format /usr/bin/clang-format-${LLVM_VERSION} ${LLVM_VERSION}0 && \
apt-get clean
# QUICKFIX: ajust the broken relative paths inside libc++.modules.json
RUN sed -i \
's#\.\./share/libc++#/usr/lib/llvm-'${LLVM_VERSION}'/share/libc++#' \
/lib/x86_64-linux-gnu/libc++.modules.json
RUN pipx install cmake && \
pipx install --include-deps cmake-format && \
pipx install codespell && \
pipx install conan && \
pipx install gcovr && \
pipx install gersemi && \
pipx install ninja && \
pipx install pre-commit && \
pipx install uv
WORKDIR /home/builder/workdir
``
```
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 16, 2026, 3:55pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/6 "2026-07-16T15:55:16Z")
And this is my build environment needed:
```makefile
ifeq (${hostSystemName},Linux)
# clang++ -print-file-name=libc++.modules.json
# /lib/x86_64-linux-gnu/libc++.modules.json
# /usr/lib/llvm-23/lib/libc++.modules.json -> ../../x86_64-linux-gnu/libc++.modules.json
# /usr/lib/llvm-23/share/libc++/v1/std.compat.cppm
# /usr/lib/llvm-23/share/libc++/v1/std.cppm
# possible other QUICKFIX: /lib/share -> /usr/lib/llvm-23/share
STDLIB:=libc++
export LLVM_DIR:=/usr/lib/llvm-23
export PATH:=${LLVM_DIR}/bin:${PATH}
export CXX:=clang++-23
export CXXFLAGS:=-stdlib=$(STDLIB)
export LDFLAGS:=-L$(LLVM_DIR)/lib/c++ # XXX -lc++abi
export CMAKE_CXX_STDLIB_MODULES_JSON:=$(shell clang++-23 -print-file-name=libc++.modules.json)
# export CMAKE_CXX_STDLIB_MODULES_JSON:=${LLVM_DIR}/lib/c++/$(STDLIB).modules.json
endif
```
---
### Author:  [@vito.gamberini](https://discourse.cmake.org/u/vito.gamberini)
#### Post date: [July 16, 2026, 4:55pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/7 "2026-07-16T16:55:07Z")
`CMAKE_CXX_STDLIB_MODULES_JSON` combined with a custom std.modules.json is the intended mechanism to fix a compiler or stdlib package which contains a bogus module manifest.
CMake can’t force packagers to accurately record the location of the std module.
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 16, 2026, 4:58pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/8 "2026-07-16T16:58:47Z")
Thanks, for `clang++` it works, but for `g++-16` I see this:
```bash
sed -i \
's#\.\./include/c++#/opt/gcc-16/include/c++#' \
/opt/gcc-16/lib64/libstdc++.modules.json
CXX=g++-16 cmake --preset ci-Linux -D CMAKE_CXX_STDLIB_MODULES_JSON=/opt/gcc-16/lib64/libstdc++.modules.json --log-level=VERBOSE --fresh
Preset CMake variables:
CMAKE_BUILD_TYPE="Release"
CMAKE_CXX_EXTENSIONS:BOOL="FALSE"
CMAKE_CXX_FLAGS_DEBUG="-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -fcf-protection=full -fstack-clash-protection -Wall -Wextra -Wpedantic -Wno-conversion -Wno-sign-conversion -Wcast-qual -Wformat=2 -Wundef -Wno-float-equal -Wshadow -Wcast-align -Wunused -Wnull-dereference -Wdouble-promotion -Wno-implicit-fallthrough -Wextra-semi -Woverloaded-virtual -Wnon-virtual-dtor -Wno-old-style-cast"
CMAKE_CXX_SCAN_FOR_MODULES:BOOL="TRUE"
CMAKE_CXX_STANDARD="23"
CMAKE_CXX_STANDARD_REQUIRED:BOOL="TRUE"
CMAKE_EXE_LINKER_FLAGS_DEBUG="-Wl,--allow-shlib-undefined,--as-needed,-z,noexecstack,-z,relro,-z,now"
CMAKE_EXPORT_COMPILE_COMMANDS:BOOL="TRUE"
CMAKE_INSTALL_PREFIX:PATH="/home/builder/workdir/stagedir"
CMAKE_MESSAGE_LOG_LEVEL="VERBOSE"
CMAKE_PREFIX_PATH:STRING="/home/builder/workdir/stagedir"
CMAKE_SHARED_LINKER_FLAGS_DEBUG="-Wl,--allow-shlib-undefined,--as-needed,-z,noexecstack,-z,relro,-z,now"
CMAKE_VERIFY_INTERFACE_HEADER_SETS:BOOL="TRUE"
FMT_DEVELOPER_MODE:BOOL="TRUE"
-- The CXX compiler identification is GNU 17.0.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/g++-16 - skipped
-- Detecting CXX compile features
CMake Warning (experimental) at /root/.local/share/pipx/venvs/cmake/lib/python3.12/site-packages/cmake/data/share/cmake-4.4/Modules/Compiler/CMakeCommonCompilerMacros.cmake:226 (cmake_language):
CMake's support for `import std;` in C++23 and newer is experimental. It
is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
/root/.local/share/pipx/venvs/cmake/lib/python3.12/site-packages/cmake/data/share/cmake-4.4/Modules/CMakeDetermineCompilerSupport.cmake:157 (cmake_cxx_find_modules_json)
/root/.local/share/pipx/venvs/cmake/lib/python3.12/site-packages/cmake/data/share/cmake-4.4/Modules/CMakeTestCXXCompiler.cmake:81 (CMAKE_DETERMINE_COMPILER_SUPPORT)
CMakeLists.txt:13 (project)
This warning is for project developers. Use -Wno-author or -Wno-experimental
to suppress it.
-- Detecting CXX compile features - done
-- CMAKE_CXX_COMPILER_IMPORT_STD=23;26
-- CMAKE_CXX_SCAN_FOR_MODULES=TRUE
-- CMAKE_CXX_MODULE_STD=ON
-- CPPdefinitions=FMT_MODULE;FMT_IMPORT_STD
-- CPM: Adding package fmt@12.2.0 (12.2.0)
-- Populating fmt
-- Configuring done (0.1s)
-- Generating done (0.0s)
-- Build files have been written to: /home/builder/workdir/build/_deps/fmt-subbuild
[0/7] Performing update step for 'fmt-populate'
-- Already at requested tag: 12.2.0
[1/7] No patch step for 'fmt-populate'
[3/7] No configure step for 'fmt-populate'
[4/7] No build step for 'fmt-populate'
[5/7] No install step for 'fmt-populate'
[6/7] No test step for 'fmt-populate'
[7/7] Completed 'fmt-populate'
-- FMT_USE_MODULES=ON
-- CMAKE_CXX_MODULE_STD=ON
-- CMAKE_CXX_COMPILER_IMPORT_STD=23;26
-- Module tests to be built: module-test
-- Configuring done (0.6s)
-- Generating done (0.1s)
-- Build files have been written to: /home/builder/workdir/build
root@cd8acd0382d0:/home/builder/workdir# ninja -C build/
ninja: Entering directory `build/'
[0/2] Re-checking globbed directories...
[1/19] Scanning /opt/gcc-16/include/c++/17.0.0/bits/std.cc for CXX dependencies
FAILED: [code=1] CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.cc.o.ddi
/usr/bin/g++-16 -O3 -DNDEBUG -std=c++23 -stdlib=libstdc++ -E -x c++ /opt/gcc-16/include/c++/17.0.0/bits/std.cc -MT CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.cc.o.ddi -MD -MF CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.cc.o.ddi.d -fmodules-ts -fdeps-file=CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.cc.o.ddi -fdeps-target=CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.cc.o -fdeps-format=p1689r5 -o CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.cc.o.ddi.i
g++-16: error: unrecognized command-line option '-stdlib=libstdc++'
[2/19] Scanning /opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc for CXX dependencies
FAILED: [code=1] CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc.o.ddi
/usr/bin/g++-16 -O3 -DNDEBUG -std=c++23 -stdlib=libstdc++ -E -x c++ /opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc -MT CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc.o.ddi -MD -MF CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc.o.ddi.d -fmodules-ts -fdeps-file=CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc.o.ddi -fdeps-target=CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc.o -fdeps-format=p1689r5 -o CMakeFiles/@cmake_cxx_std.dir/opt/gcc-16/include/c++/17.0.0/bits/std.compat.cc.o.ddi.i
g++-16: error: unrecognized command-line option '-stdlib=libstdc++'
[3/19] Scanning /home/builder/workdir/module/fmt.cppm for CXX dependencies
FAILED: [code=1] CMakeFiles/fmt.dir/module/fmt.cppm.o.ddi
/usr/bin/g++-16 -DFMT_IMPORT_STD -DFMT_MODULE -I/home/builder/workdir/build/_deps/fmt-src/include -O3 -DNDEBUG -std=c++23 -stdlib=libstdc++ -E -x c++ /home/builder/workdir/module/fmt.cppm -MT CMakeFiles/fmt.dir/module/fmt.cppm.o.ddi -MD -MF CMakeFiles/fmt.dir/module/fmt.cppm.o.ddi.d -fmodules-ts -fdeps-file=CMakeFiles/fmt.dir/module/fmt.cppm.o.ddi -fdeps-target=CMakeFiles/fmt.dir/module/fmt.cppm.o -fdeps-format=p1689r5 -o CMakeFiles/fmt.dir/module/fmt.cppm.o.ddi.i
g++-16: error: unrecognized command-line option '-stdlib=libstdc++'
[4/19] Scanning /home/builder/workdir/tests/header-only-test.cc for CXX dependencies
FAILED: [code=1] tests/CMakeFiles/header-only-test.dir/header-only-test.cc.o.ddi
/usr/bin/g++-16 -DFMT_IMPORT_STD -DFMT_MODULE -I/home/builder/workdir/build/_deps/fmt-src/include -O3 -DNDEBUG -std=c++23 -stdlib=libstdc++ -E -x c++ /home/builder/workdir/tests/header-only-test.cc -MT tests/CMakeFiles/header-only-test.dir/header-only-test.cc.o.ddi -MD -MF tests/CMakeFiles/header-only-test.dir/header-only-test.cc.o.ddi.d -fmodules-ts -fdeps-file=tests/CMakeFiles/header-only-test.dir/header-only-test.cc.o.ddi -fdeps-target=tests/CMakeFiles/header-only-test.dir/header-only-test.cc.o -fdeps-format=p1689r5 -o tests/CMakeFiles/header-only-test.dir/header-only-test.cc.o.ddi.i
g++-16: error: unrecognized command-line option '-stdlib=libstdc++'
[5/19] Scanning /home/builder/workdir/module/tests/module-test.cc for CXX dependencies
FAILED: [code=1] module/tests/CMakeFiles/module_module-test.dir/module-test.cc.o.ddi
/usr/bin/g++-16 -DFMT_IMPORT_STD -DFMT_MODULE -I/home/builder/workdir/build/_deps/fmt-src/include -O3 -DNDEBUG -std=c++23 -stdlib=libstdc++ -E -x c++ /home/builder/workdir/module/tests/module-test.cc -MT module/tests/CMakeFiles/module_module-test.dir/module-test.cc.o.ddi -MD -MF module/tests/CMakeFiles/module_module-test.dir/module-test.cc.o.ddi.d -fmodules-ts -fdeps-file=module/tests/CMakeFiles/module_module-test.dir/module-test.cc.o.ddi -fdeps-target=module/tests/CMakeFiles/module_module-test.dir/module-test.cc.o -fdeps-format=p1689r5 -o module/tests/CMakeFiles/module_module-test.dir/module-test.cc.o.ddi.i
g++-16: error: unrecognized command-line option '-stdlib=libstdc++'
ninja: build stopped: subcommand failed.
root@cd8acd0382d0:/home/builder/workdir#
root@cd8acd0382d0:/home/builder/workdir# cmake --version && ninja --version && g++-16 --version
cmake version 4.4.0
CMake suite maintained and supported by Kitware (kitware.com/cmake).
1.13.0.git.kitware.jobserver-pipe-1
g++-16 (GCC) 17.0.0 20260716 (experimental)
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
root@cd8acd0382d0:/home/builder/workdir#
```
---
### Author:  [@vito.gamberini](https://discourse.cmake.org/u/vito.gamberini)
#### Post date: [July 16, 2026, 5:02pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/9 "2026-07-16T17:02:44Z")
Extremely odd that the GCC scanner is throwing an error on `-stdlib=libstdc++`. You can open an issue with an MRE but this smells like a compiler bug if the scanner really can’t handle the stdlib argument under some conditions.
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 16, 2026, 5:06pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/10 "2026-07-16T17:06:09Z")
not sure if the GCC was right build in docker?
what is the filename of the GCC scanner?
```docker
# SPDX-License-Identifier: GPL-3.0-or-later
# Copyright © 2025 GSI Helmholtzzentrum fuer Schwerionenforschung GmbH
# Matthias Kretz
FROM ghcr.io/mattkretz/cplusplus-ci/gcc15
ENV DEBIAN_FRONTEND=noninteractive
RUN apt-get install -y --no-install-recommends \
autoconf2.69 dwz lzma xz-utils libzstd-dev zlib1g-dev systemtap-sdt-dev \
binutils:native gettext nvptx-tools amdgcn-tools-18 \
texinfo sharutils procps netbase libisl-dev libmpc-dev libmpfr-dev libgmp-dev lib32z1-dev libx32z1-dev && \
apt-get clean && \
mkdir -p /root/gcc-16/obj && \
cd /root/gcc-16 && \
curl -L https://github.com/gcc-mirror/gcc/archive/master.tar.gz \
| tar -xz --strip-components=1 && \
mkdir /root/binutils && cd /root/binutils && \
curl -L https://sourceware.org/pub/binutils/releases/binutils-2.45.tar.gz \
| tar -xz --strip-components=1 && \
cd /root/gcc-16 && \
ln -s ../binutils/bfd ../binutils/gas ../binutils/binutils ../binutils/ld ../binutils/libctf \
../binutils/libsframe ../binutils/opcodes . && \
cd /root/gcc-16/obj && \
../configure --enable-languages=c,c++,lto --prefix=/opt/gcc-16 --enable-checking=release --disable-bootstrap && \
make BOOT_CFLAGS=-march=x86-64-v3 -j3 && \
make -j3 install-strip && \
update-alternatives --install /usr/bin/gcc-16 gcc-16 /opt/gcc-16/bin/gcc 160 --slave /usr/bin/g++-16 g++-16 /opt/gcc-16/bin/g++ && \
update-alternatives --install /usr/bin/gcc gcc /opt/gcc-16/bin/gcc 160 --slave /usr/bin/g++ g++ /opt/gcc-16/bin/g++ && \
cd /root && rm -rf gcc-16 && \
apt-get purge --autoremove -y \
libzstd-dev zlib1g-dev systemtap-sdt-dev \
libisl-dev libmpc-dev libmpfr-dev libgmp-dev lib32z1-dev libx32z1-dev
# NOTE: pipx ensurepath -> appends /root/.local/bin to the PATH environment variable.
ENV PATH="/root/.local/bin:${PATH}"
RUN pipx install cmake && \
pipx install --include-deps cmake-format && \
pipx install codespell && \
pipx install conan && \
pipx install gcovr && \
pipx install gersemi && \
pipx install ninja && \
pipx install pre-commit && \
pipx install uv
RUN sed -i \
's#\.\./include/c++#/opt/gcc-16/include/c++#' \
/opt/gcc-16/lib64/libstdc++.modules.json
WORKDIR /home/builder/workdir
```
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 16, 2026, 5:27pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/11 "2026-07-16T17:27:45Z")
It was my fault; now it works with GCC too:
```diff
root@cd8acd0382d0:/home/builder/workdir# git diff
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 32958a2..094e6d9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -64,8 +64,10 @@ if(CMAKE_GENERATOR STREQUAL "Ninja" AND FMT_USE_MODULES)
CMAKE_CXX_COMPILER_ID STREQUAL "GNU"
AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 15.0
)
- add_compile_options(-stdlib=libstdc++)
- add_link_options(-stdlib=libstdc++)
+ if(APPLE)
+ add_compile_options(-stdlib=libstdc++)
+ add_link_options(-stdlib=libstdc++)
+ endif()
endif()
# Tell CMake that we explicitly want `import std`.
```
---
### Author:  [@vito.gamberini](https://discourse.cmake.org/u/vito.gamberini)
#### Post date: [July 16, 2026, 5:29pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/12 "2026-07-16T17:29:17Z")
For clarity, GCC doesn’t have a separate scanner binary. GCC and MSVC use the compiler as a scanner.
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 16, 2026, 5:32pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/13 "2026-07-16T17:32:11Z")
> [@vito.gamberini](#):
>
> You can open an issue with an MRE but this smells like a compiler bug if the scanner really can’t handle the stdlib argument under some conditions.
Do you mean a `CMake` issue?
---
### Author:  [@vito.gamberini](https://discourse.cmake.org/u/vito.gamberini)
#### Post date: [July 16, 2026, 8:04pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/14 "2026-07-16T20:04:22Z")
If you think it’s something CMake is doing wrong, yes, I mean a CMake issue. If you can isolate it to “gcc isn’t accepting this combination of command line flags, which I think it should”; then it’s a GCC bug and goes to their tracker.
---
### Author:  [@ClausKlein](https://discourse.cmake.org/u/ClausKlein)
#### Post date: [July 17, 2026, 4:37pm UTC](https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-linux-with-clang/15262/15 "2026-07-17T16:37:13Z")
Done: [cmake/cmake#27962](https://gitlab.kitware.com/cmake/cmake/-/work_items/27962)