在macOS上用AppleClang16+CMake配置C++20模块遇错求助

问题描述 #

测试C++20模块时遭遇CMake报错，参考官方示例后问题仍未解决，错误信息如下：

CMake Error in CMakeLists.txt:
The target named "foo" has C++ sources that may use modules, but the compiler does not provide a way to discover the import graph dependencies.
See the cmake-cxxmodules(7) manual for details. Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable scanning.

环境信息 #

• CMake版本：3.30.5
• Apple Clang版本：16.0.0 (clang-1600.0.26.4)
• Ninja版本：1.12.1

使用的CMakeLists.txt #

cmake_minimum_required(VERSION 3.28)
project(std_module_example CXX)

# Turning off extensions avoids an issue with the clang 16 compiler
# clang 17 and greater can avoid this setting
set(CMAKE_CXX_EXTENSIONS OFF)
# Set the version of C++ for the project
set(CMAKE_CXX_STANDARD 20)
# Create a library
add_library(foo)
# Add the module file to the library
target_sources(foo
  PUBLIC
    FILE_SET CXX_MODULES FILES
      foo.cxx
)
# Create an executable
add_executable(hello main.cxx)
# Link to the library foo
target_link_libraries(hello foo)

执行命令及输出 #

% CXX=clang++ CC=clang cmake -G Ninja -S . -B build
-- The CXX compiler identification is AppleClang 16.0.0.16000026
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.2s)
CMake Error in CMakeLists.txt:
  The target named "foo" has C++ sources that may use modules, but the
  compiler does not provide a way to discover the import graph dependencies.
  See the cmake-cxxmodules(7) manual for details.  Use the
  CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable scanning.


-- Generating done (0.0s)
CMake Generate step failed.  Build files cannot be regenerated correctly.

已尝试设置CMAKE_CXX_SCAN_FOR_MODULES为ON或OFF，均无效果。

解决方法 #

1. 理解Apple Clang 16的模块支持限制 #

Apple Clang 16对C++20模块的支持未适配CMake默认的模块依赖扫描机制，CMake的FILE_SET CXX_MODULES语法依赖编译器提供的模块扫描接口，但该版本的Apple Clang未完整实现相关功能，导致扫描失败。

2. 改用手动编译模块的方式 #

放弃CMake自动扫描模块的语法，手动处理模块编译与依赖：

修改后的CMakeLists.txt示例：

cmake_minimum_required(VERSION 3.28)
project(std_module_example CXX)

set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_CXX_STANDARD 20)

# 编译foo模块生成pcm接口文件
add_library(foo MODULE)
target_sources(foo PRIVATE foo.cxx)
target_compile_options(foo PRIVATE -std=c++20 -fmodules)
# 指定模块接口文件输出目录
set_target_properties(foo PROPERTIES
  CXX_MODULE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/modules
)

# 编译可执行文件并引用模块接口
add_executable(hello main.cxx)
target_compile_options(hello PRIVATE -std=c++20 -fmodules -fmodule-file=${CMAKE_BINARY_DIR}/modules/foo.pcm)
target_link_libraries(hello foo)

3. 升级编译器到Apple Clang 17+ #

Apple Clang 17及以上版本完整支持CMake的C++模块扫描机制，升级Xcode到15+版本即可获得对应编译器，此时原示例的CMakeLists.txt可直接正常工作。

4. 验证模块文件格式正确性 #

确保模块文件foo.cxx符合标准C++模块语法：

export module foo;

export void say_hello() {
  // 实现代码
}

主文件main.cxx正确导入模块：

import foo;

int main() {
  say_hello();
  return 0;
}

内容的提问来源于stack exchange，提问作者Heikki Taavettila