CMake 3.28 and C++20, error: CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND

[Boris-Rasin]

EMSDK needs to include "clang-scan-deps" tool, as CMake 3.28 with C++20 and clang compiler always runs this tool.

This CMake produces the error, test.cpp could be empty:

cmake_minimum_required(VERSION 3.5...3.28)

project(test)

set(CMAKE_CXX_STANDARD 20)

add_executable(test test.cpp)

[sbc100]

@dschuff should we start including this? This tool doesn't seem to exist on my debian-based system, is it new?

 $ which clang-scan-deps
 $ apt-cache search clang-scan-deps

[sbc100]

Ah, I found it via sudo apt-get install clang-tools-17. Does that mean that CMake 3.28 with C++20 depends not just on the clang package but also on the clang-tools package?

[dschuff]

Yes, clang-scan-deps is used with C++20 modules.
Actually we got a recent request for this, #21042 and I thought I fixed it. But looking at that change, it maybe that it only affected non-release builds. Let me check on that some more.

[sbc100]

I think maybe we don't build "clang/tools" at all yet on our builder.

[dschuff]

I went ahead and did a minimal fix for this in https://chromium-review.googlesource.com/c/emscripten-releases/+/5402952 so the next release should be good. By the way, the non-LTO builds of emscripten-releases should have this binary in them already, so if you wanted to try it out you could use emsdk to install one of them, e.g. emsdk install 3.1.56-asserts

[sxstd001]

Can you provide a simple example, including CMake configuration? I always encounter errors when compiling modules!

[sbc100]

emscripten doesn't really have support for C++20 modules yet. Its something we should probably add tests for, but for now its not something that is easy to do. Just FYI

[Boris-Rasin]

Just to note, this problem prevents using CMake 3.28 (or later) with C++20 (or later), even in a project without modules.

[sbc100]

@Boris-Rasin I'm not seeing any iseeu with simply using C++20. For example, with the above CMakeLists.txt file I can run cmake 3.2.8 just fine:

./emcmake cmake -S . -B out/test/ 
configure: cmake -S . -B out/test/  -DCMAKE_TOOLCHAIN_FILE=/usr/local/google/home/sbc/dev/wasm/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=/usr/local/google/home/sbc/dev/wasm/emsdk/node/18.20.3_64bit/bin/node
-- Configuring done (1.5s)
-- Generating done (0.0s)
-- Build files have been written to: /usr/local/google/home/sbc/dev/wasm/emscripten/out/test
$ cmake --version
cmake version 3.28.3

What problems do you see when you don't use modules?

[Boris-Rasin]

The problem I see is listed in this bug report title: "error: CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND".

This happens with provided CMake file and empty "test.cpp" file.

Which means it happens with a project that does not use modules.

The error is during build, not during CMake config.

[sbc100]

Thanks! I was able to reproduce now. Uploading a fix now.

[jpvanoosten]

Is this issue fixed on Windows?
I'm using Emscripten 3.1.64 and using CMake 3.29 and with

target_compile_features( ${TARGET} PUBLIC cxx_std_20 )

I'm still getting the CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND error.

Is this tool included in the Windows distributions?

Note: I am not using modules, but CMake doesn't seem to care.

[sbc100]

The fix for this has not yet landed.

[tuket]

Any know workaround? I can think of:

• Using CMake <= 3.27
• Using C++ <= 17

[eliemichel]

Workaround that imports the bits of PR#21987: https://github.com/eliemichel/cpp20-cmake-emscripten-template