C++ 20 modules simply don't work with MSVC 2022 and CMake: cannot 'import' neither standard nor custom headers. Nothing can be imported in module files, only included. To reproduce: <details><summary>math.ixx:</summary> ``` export module math; import <iostream>; export int add(int a, int b){ return a + b; std::cout << a; } ``` </details> <details><summary>main.cpp:</summary> ``` import math; int main(){ int a=1; int b=2; int absum = add(a, b); return absum * 0; } ``` </details> <details><summary>CMakeLists.txt:</summary> ``` cmake_minimum_required(VERSION 3.26) project(CXXModules LANGUAGES CXX) set(CMAKE_CXX_STANDARD 20) set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1) set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a") add_executable(math_mod math.cpp) target_sources(math_mod PUBLIC FILE_SET cxx_modules TYPE CXX_MODULES FILES math.ixx ) ``` </details> <details><summary>Output with Ninja:</summary> ``` >------ Rebuild All started: Project: module, Configuration: x64-debug ------ [1/1] Cleaning all built files... Cleaning... 0 files. [1/6] Scanning M:\Temp\Cpp23-examples-main\cpp20\module\math.cpp for CXX dependencies [2/6] Scanning M:\Temp\Cpp23-examples-main\cpp20\module\math.ixx for CXX dependencies math.ixx [3/6] Generating CXX dyndep file CMakeFiles\math_mod.dir\CXX.dd [4/6] Building CXX object CMakeFiles\math_mod.dir\math.ixx.obj FAILED: CMakeFiles/math_mod.dir/math.ixx.obj CMakeFiles/math_mod.dir/math.ifc C:\PROGRA~1\MIB055~1\2022\COMMUN~1\VC\Tools\MSVC\1435~1.322\bin\Hostx64\x64\cl.exe /nologo /TP /DWIN32 /D_WINDOWS /EHsc /Ob0 /Od /RTC1 -std:c++20 -MDd -Zi /showIncludes @CMakeFiles\math_mod.dir\math.ixx.obj.modmap /FoCMakeFiles\math_mod.dir\math.ixx.obj /FdCMakeFiles\math_mod.dir\ /FS -c M:\Temp\Cpp23-examples-main\cpp20\module\math.ixx math.ixx M:\Temp\Cpp23-examples-main\cpp20\module\math.ixx(3): error C7612: could not find header unit for 'C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\iostream' M:\Temp\Cpp23-examples-main\cpp20\module\math.ixx(7): error C2039: 'cout': is not a member of 'std' predefined C++ types (compiler internal)(343): note: see declaration of 'std' M:\Temp\Cpp23-examples-main\cpp20\module\math.ixx(7): error C2065: 'cout': undeclared identifier ninja: build stopped: subcommand failed. Rebuild All failed. ``` </details> Including iostream instead of importing solves the problem, but this defeats the purpose of using C++20 modules. <details><summary>MSBuild builds the above code just fine:</summary> ``` Rebuild started... 1>------ Rebuild All started: Project: mod, Configuration: Debug x64 ------ 1>Scanning sources for module dependencies... 1>math.ixx 1>iostream 1>Compiling... 1>iostream 1>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\atomic(325,35): warning C5260: the constant variable 'std::memory_order const (* `std::memory_order __cdecl std::_Combine_cas_memory_orders(std::memory_order,std::memory_order)'::`2'::_Combined_memory_orders)[6]' has internal linkage in an included header file context, but external linkage in imported header unit context; consider declaring it 'inline' as well if it will be shared across translation units, or 'static' to express intent to use it local to this translation unit 1>math.ixx 1>math.cpp 1>mod.vcxproj -> M:\Temp\Cpp23-examples-main\cpp20\module\mod\x64\Debug\mod.exe 1>Done building project "mod.vcxproj". ========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ========== ========== Rebuild started at 1:31 PM and took 02.080 seconds ========== ``` If this helps, attached is the 'Detailed' MSBuild output. [MSBuild_log_Detailed.txt](/uploads/7d0b46d289b96f72d62b7953e5454561/MSBuild_log_Detailed.txt) </details> <details><summary>Adding /scanDependencies- compiler flag results in another problem:</summary> the .obj files are just not created. So build still fails: ``` >------ Rebuild All started: Project: module, Configuration: x64-debug ------ [1/1] Cleaning all built files... Cleaning... 5 files. [1/6] Scanning M:\Temp\Cpp23-examples-main\cpp20\module\math.cpp for CXX dependencies [2/6] Scanning M:\Temp\Cpp23-examples-main\cpp20\module\math.ixx for CXX dependencies math.ixx [3/6] Generating CXX dyndep file CMakeFiles\math_mod.dir\CXX.dd [4/6] Building CXX object CMakeFiles\math_mod.dir\math.ixx.obj math.ixx { "version": 1, "revision": 0, "rules": [ { "primary-output": "CMakeFiles\\math_mod.dir\\math.ixx.obj", "outputs": [ "M:\\Temp\\Cpp23-examples-main\\cpp20\\module\\out\\build\\x64-debug\\CMakeFiles\\math_mod.dir\\vc140.pdb", "-", "CMakeFiles\\math_mod.dir\\math.ifc" ], "provides": [ { "logical-name": "math", "source-path": "m:\\temp\\cpp23-examples-main\\cpp20\\module\\math.ixx", "is-interface": true } ], "requires": [ { "logical-name": "iostream", "source-path": "C:\\Program Files\\Microsoft Visual Studio\\2022\\Community\\VC\\Tools\\MSVC\\14.35.32215\\include\\iostream", "lookup-method": "include-angle", "unique-on-source-path": true } ] } ] } [5/6] Building CXX object CMakeFiles\math_mod.dir\math.cpp.obj { "version": 1, "revision": 0, "rules": [ { "primary-output": "CMakeFiles\\math_mod.dir\\math.cpp.obj", "outputs": [ "M:\\Temp\\Cpp23-examples-main\\cpp20\\module\\out\\build\\x64-debug\\CMakeFiles\\math_mod.dir\\vc140.pdb", "-" ], "requires": [ { "logical-name": "math" } ] } ] } [6/6] Linking CXX executable math_mod.exe FAILED: math_mod.exe cmd.exe /C "cd . && C:\Python310\Lib\site-packages\cmake\data\bin\cmake.exe -E vs_link_exe --intdir=CMakeFiles\math_mod.dir --rc=C:\PROGRA~2\WI3CF2~1\10\bin\100220~1.0\x64\rc.exe --mt=C:\PROGRA~2\WI3CF2~1\10\bin\100220~1.0\x64\mt.exe --manifests -- C:\PROGRA~1\MIB055~1\2022\COMMUN~1\VC\Tools\MSVC\1435~1.322\bin\Hostx64\x64\link.exe /nologo CMakeFiles\math_mod.dir\math.cpp.obj CMakeFiles\math_mod.dir\math.ixx.obj /out:math_mod.exe /implib:math_mod.lib /pdb:math_mod.pdb /version:0.0 /machine:x64 /debug /INCREMENTAL /subsystem:console kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ." LINK Pass 1: command "C:\PROGRA~1\MIB055~1\2022\COMMUN~1\VC\Tools\MSVC\1435~1.322\bin\Hostx64\x64\link.exe /nologo CMakeFiles\math_mod.dir\math.cpp.obj CMakeFiles\math_mod.dir\math.ixx.obj /out:math_mod.exe /implib:math_mod.lib /pdb:math_mod.pdb /version:0.0 /machine:x64 /debug /INCREMENTAL /subsystem:console kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:CMakeFiles\math_mod.dir/intermediate.manifest CMakeFiles\math_mod.dir/manifest.res" failed (exit code 1104) with the following output: LINK : math_mod.exe not found or not built by the last incremental link; performing full link M:\Temp\Cpp23-examples-main\cpp20\module\out\build\x64-debug\LINK : fatal error LNK1104: cannot open file 'CMakeFiles\math_mod.dir\math.cpp.obj' ninja: build stopped: subcommand failed. Rebuild All failed. ``` </details>