No SOURCES given to target: CMAKECXX26 on ubuntu 25.04

ClausKlein · June 16, 2025, 11:26am

import std does also not work with `gcc-15` on `ubuntu 25.04`!

builder@5b11ff1f2350:~/workdir$ cmake -S . -B build -G Ninja --fresh
-- The CXX compiler identification is GNU 15.0.1
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/g++-15 - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (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):
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:36 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY - Success
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY - Success
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR - Success
-- Configuring done (0.9s)
CMake Error at build/CMakeFiles/4.0.3/CMakeCXXCompiler.cmake:119 (target_sources):
  Cannot find source file:

    /usr/lib/gcc/x86_64-linux-gnu/include/c++/15/bits/std.cc
Call Stack (most recent call first):
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/CMakeTestCXXCompiler.cmake:91 (include)
  CMakeLists.txt:36 (project)


CMake Error in CMakeLists.txt:
  No SOURCES given to target: __CMAKE__CXX26@synth_228b59946540


CMake Generate step failed.  Build files cannot be regenerated correctly.
builder@5b11ff1f2350:~/workdir$ find /usr -iname 'std.cc'
/usr/include/c++/15/bits/std.cc
builder@5b11ff1f2350:~/workdir$ uname -a
Linux 5b11ff1f2350 6.10.14-linuxkit #1 SMP PREEMPT_DYNAMIC Thu Mar 20 16:36:58 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
builder@5b11ff1f2350:~/workdir$ cat /etc/issue
Ubuntu 25.04 \n \l

builder@5b11ff1f2350:~/workdir$ cat /usr/lib/gcc/x86_64-linux-gnu/15/libstdc++.modules.json
{
  "version": 1,
  "revision": 1,
  "modules": [
    {
      "logical-name": "std",
      "source-path": "../include/c++/15/bits/std.cc",
      "is-std-library": true
    },
    {
      "logical-name": "std.compat",
      "source-path": "../include/c++/15/bits/std.compat.cc",
      "is-std-library": true
    }
  ]
}
builder@5b11ff1f2350:~/workdir$

ClausKlein · June 16, 2025, 11:59am

import std does also not work with `clang-20` on `ubuntu 25.04`!

builder@98757d772b6f:~/workdir$ CXX=clang++ cmake -S . -B build -G Ninja --fresh
-- The CXX compiler identification is Clang 20.1.2
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++ - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (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):
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:36 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY
CMake Error at /home/builder/workdir/build/CMakeFiles/4.0.3/CMakeCXXCompiler.cmake:111 (target_sources):
  Cannot find source file:

    /usr/lib/gcc/x86_64-linux-gnu/include/c++/15/bits/std.cc
Call Stack (most recent call first):
  /home/builder/workdir/build/CMakeFiles/CMakeScratch/TryCompile-u2A8qJ/CMakeLists.txt:2 (project)


CMake Error at /home/builder/workdir/build/CMakeFiles/4.0.3/CMakeCXXCompiler.cmake:103 (add_library):
  No SOURCES given to target: __cmake_cxx23
Call Stack (most recent call first):
  /home/builder/workdir/build/CMakeFiles/CMakeScratch/TryCompile-u2A8qJ/CMakeLists.txt:2 (project)


CMake Error at /home/builder/workdir/build/CMakeFiles/4.0.3/CMakeCXXCompiler.cmake:127 (add_library):
  No SOURCES given to target: __cmake_cxx26
Call Stack (most recent call first):
  /home/builder/workdir/build/CMakeFiles/CMakeScratch/TryCompile-u2A8qJ/CMakeLists.txt:2 (project)


CMake Error at /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/Internal/CheckSourceCompiles.cmake:104 (try_compile):
  Failed to generate test project build system.
Call Stack (most recent call first):
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/Internal/CheckCompilerFlag.cmake:18 (cmake_check_source_compiles)
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/CheckCompilerFlag.cmake:56 (cmake_check_compiler_flag)
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/GenerateExportHeader.cmake:244 (check_compiler_flag)
  /usr/local/lib/python3.13/dist-packages/cmake/data/share/cmake-4.0/Modules/GenerateExportHeader.cmake:425 (_test_compiler_hidden_visibility)
  CMakeLists.txt:50 (generate_export_header)


-- Configuring incomplete, errors occurred!
builder@98757d772b6f:~/workdir$ find /usr -name std.cppm
/usr/lib/llvm-20/share/libc++/v1/std.cppm
builder@98757d772b6f:~/workdir$ find /usr -name '*modules.json'
/usr/lib/gcc/x86_64-linux-gnu/15/libstdc++.modules.json
/usr/lib/llvm-20/lib/libc++.modules.json
builder@98757d772b6f:~/workdir$ cat /usr/lib/llvm-20/lib/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"
        ]
      }
    }
  ]
}
builder@98757d772b6f:~/workdir$

ben.boeckel · June 16, 2025, 2:53pm

Looks like a packaging problem to me; the mentioned source files should be included in the -dev package that provides the stdlib modules’ .json file.

Or, perhaps, the paths are not correct? In any case, this is an Ubuntu issue based on the information provided.

ClausKlein · June 16, 2025, 3:22pm

For the gcc-15 case, I agree.

But in the clang-20 case, it seems a CMake problem, the /usr/lib/llvm-20/lib/libc++.modules.json is correct!

ben.boeckel · June 16, 2025, 5:28pm

Can you please trace the paths made in Clang-CXX-CXXImportStd.cmake to see where CMake goes astray?

ClausKlein · June 16, 2025, 6:38pm

cmake.log (40.7 KB)

My CMakeLists.txt:

cmake_minimum_required(VERSION 4.0)

if(APPLE)
    set(CMAKE_CXX_COMPILER /usr/local/bin/g++-15)

    # set(LLVM_ROOT /usr/local/Cellar/llvm/20.1.6)
    # set(CMAKE_CXX_COMPILER ${LLVM_ROOT}/bin/clang++)
    # add_link_options(-L${LLVM_ROOT}/lib/c++)
endif()

if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
    add_compile_options(-stdlib=libc++)
    add_link_options(-stdlib=libc++)
endif()

set(CMAKE_CXX_STANDARD 26)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS ON)

set(CMAKE_SKIP_TEST_ALL_DEPENDENCY FALSE)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

if(CMAKE_VERSION VERSION_LESS 4.0.3)
    # cmake v4.0.2:
    set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD a9e1cf81-9932-4810-974b-6eccaf14e457)
else()
    # cmake release branch
    set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD d0edc3af-4c50-42ea-a356-e2862fe7a444)
endif()
set(CMAKE_CXX_MODULE_STD ON)

# Ensure non-empty default build type for single-config
get_property(isMultiConfig GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
if(NOT isMultiConfig)
    set(CMAKE_BUILD_TYPE Debug CACHE STRING "Build type")
endif()
set(CMAKE_DEBUG_POSTFIX _debug)

project(example-modules VERSION 0.1.0 LANGUAGES CXX)

set(CMAKE_CXX_VISIBILITY_PRESET hidden)
set(CMAKE_VISIBILITY_INLINES_HIDDEN TRUE)

add_library(Algo SHARED)
target_sources(
    Algo
    PRIVATE algo-impl.cpp
    PUBLIC FILE_SET CXX_MODULES FILES algo-interface.cppm
)
target_compile_features(Algo PUBLIC cxx_std_23)

include(GenerateExportHeader)
generate_export_header(Algo)
target_sources(
    Algo
    PUBLIC
        FILE_SET HEADERS
        BASE_DIRS ${CMAKE_CURRENT_BINARY_DIR}
        FILES ${CMAKE_CURRENT_BINARY_DIR}/algo_export.h
)
add_executable(algo-example algo-example.cpp)
target_link_libraries(algo-example PRIVATE Algo)
enable_testing()
add_test(NAME algo-example COMMAND algo-example)

include(GNUInstallDirs)
install(
    TARGETS Algo
    EXPORT Algo
    FILE_SET HEADERS
    FILE_SET CXX_MODULES
        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/my_package/src
    CXX_MODULES_BMI
        DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/my_package/bmi-$<CONFIG>
    # Other things to install
)
install(
    EXPORT Algo
    DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/my_package
    FILE algo-targets.cmake
    NAMESPACE Algo
    CXX_MODULES_DIRECTORY
    algo
)

ben.boeckel · June 16, 2025, 7:37pm

I don’t think these get used at all during compiler detection; I see no libc++ usage in the logs you’ve provided, so they’re both using the broken libstdc++.modules.json file.

ClausKlein · June 16, 2025, 7:39pm

How may I control to use libc++ with clang++?

This is magic on linux:

builder@3be174728abe:~/workdir$ /usr/bin/clang++-20  -print-file-name=libstdc++.modules.json
/usr/lib/gcc/x86_64-linux-gnu/15/libstdc++.modules.json
builder@3be174728abe:~/workdir$ /usr/bin/clang++-20  -print-file-name=libc++.modules.json
/usr/lib/llvm-20/bin/../lib/libc++.modules.json
builder@3be174728abe:~/workdir$ 

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
builder@3be174728abe:~/workdir$ g++-15 -print-file-name=libstdc++.modules.json
/usr/lib/gcc/x86_64-linux-gnu/15/libstdc++.modules.json
builder@3be174728abe:~/workdir$ g++-15 -print-file-name=libc++.modules.json
libc++.modules.json
builder@3be174728abe:~/workdir$

ClausKlein · June 16, 2025, 7:53pm

an other miracle on OSX:

bash-5.2$ g++-15 -print-file-name=libc++.modules.json
libc++.modules.json
bash-5.2$ g++-15 -print-file-name=libstdc++.modules.json
/usr/local/Cellar/gcc/15.1.0/bin/../lib/gcc/current/gcc/x86_64-apple-darwin23/15/../../../libstdc++.modules.json

bash-5.2$ clang++ -print-file-name=libstdc++.modules.json
libstdc++.modules.json
bash-5.2$ clang++ -print-file-name=libc++.modules.json
libc++.modules.json
bash-5.2$ clang++ -print-file-name=c++/libc++.modules.json
/usr/local/Cellar/llvm/20.1.6/lib/clang/20/../../c++/libc++.modules.json
bash-5.2$

ben.boeckel · June 17, 2025, 5:01am

I used CXXFLAGS=-stdlib=libc++ in the environment.

ClausKlein · June 17, 2025, 5:10am

But the CMAKE_CXX_STANDARD_LIBRARY is set in CMakeDetermineComilerId.cmake:

  set(CMAKE_${lang}_STANDARD_LIBRARY "")
  if ("x${lang}" STREQUAL "xCXX" AND
      EXISTS "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${lang}-DetectStdlib.h" AND
      ("x${CMAKE_${lang}_COMPILER_ID}" STREQUAL "xClang" AND
       "x${CMAKE_${lang}_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU") OR
      ("x${CMAKE_${lang}_COMPILER_ID}" STREQUAL "xGNU"))
    # See #20851 for a proper abstraction for this.
    execute_process(
      COMMAND "${CMAKE_${lang}_COMPILER}"
        ${CMAKE_${lang}_COMPILER_ID_ARG1}
        ${CMAKE_CXX_COMPILER_ID_FLAGS_LIST}
        -E
        -x c++-header
        "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${lang}-DetectStdlib.h"
        -o - # Write to stdout.
      OUTPUT_VARIABLE _lang_stdlib_out
      ERROR_VARIABLE _lang_stdlib_err
      RESULT_VARIABLE _lang_stdlib_res
      ERROR_STRIP_TRAILING_WHITESPACE)
    if (_lang_stdlib_res EQUAL 0)
      string(REGEX REPLACE ".*CMAKE-STDLIB-DETECT: (.+)\n.*" "\\1" "CMAKE_${lang}_STANDARD_LIBRARY" "${_lang_stdlib_out}")
    endif ()
  endif ()

default set to:
Modules/CMakeDetermineCompilerId.cmake(434): set(CMAKE_CXX_STANDARD_LIBRARY libstdc++ PARENT_SCOPE )

But this helps:

CXX=clang++-20 CXXFLAGS=-stdlib=libc++ cmake -G Ninja -S . -B build --trace-expand --trace-source=CMakeDetermineCompilerId.cmake --fresh

ClausKlein · June 17, 2025, 6:25am

So I have to set for `clang-20`

if($ENV{CXX} MATCHES "clang" OR CMAKE_CXX_COMPILER MATCHES "clang")
    set(ENV{CXXFLAGS} -stdlib=libc++)
endif()

before project(...) in my CMakeLists.txt!

And for `gcc-15` this workaround helps:

sudo mkdir -p /usr/lib/gcc/x86_64-linux-gnu/include/c++/15
cd /usr/lib/gcc/x86_64-linux-gnu/include/c++/15
sudo ln -s /usr/include/c++/15/bits .

No SOURCES given to target: __CMAKE__CXX26 on ubuntu 25.04

import std does also not work with gcc-15 on ubuntu 25.04!

import std does also not work with clang-20 on ubuntu 25.04!

So I have to set for clang-20

And for gcc-15 this workaround helps:

No SOURCES given to target: CMAKECXX26 on ubuntu 25.04

import std does also not work with `gcc-15` on `ubuntu 25.04`!

import std does also not work with `clang-20` on `ubuntu 25.04`!

So I have to set for `clang-20`

And for `gcc-15` this workaround helps: