C++ 23 module support & import std

[MatthieuMv]

When building with C++ 23 standard modules, the '-fbuiltin-module-map' clang argument seems not to work properly.
Any help to resolve this issue would be greatly appreciated, thank you !

C++ Code

// Main.cxx
import std;
int main(void)
{
    std::cout << "Hello world" << std::endl;
    return 0;
}

Version of emscripten/emsdk:

$ em++ -v
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.50 (047b82506d6b471873300a5e4d1e690420b582d0)
clang version 18.0.0 (https://github.com/llvm/llvm-project 14028ec0a62210d68a4dd7a046ac79c8c3b7727e)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/ptah/Tools/emsdk/upstream/bin

Failing command line in full:

$ em++ -std=c++23 -stdlib=libc++ -fmodules -fbuiltin-module-map Main.cxx
Main.cxx:1:9: fatal error: module 'std' not found
    1 | import  std;  //everything in std namespace is in here
      | ~~~~~~~~^~~
1 error generated.
em++: error: '/home/ptah/Tools/emsdk/upstream/bin/clang++ -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -std=c++23 -stdlib=libc++ -fmodules -fbuiltin-module-map Main.cxx -c -o /tmp/emscripten_temp__bnx4x9u/Main_0.o' failed (returned 1)

** Working command line in full using clang:**

$ clang++ -std=c++23 -stdlib=libc++ -fmodules -fbuiltin-module-map Main.cxx

Full link command and output with -v appended:

$ em++ -v -std=c++23 -stdlib=libc++ -fmodules -fbuiltin-module-map Main.cxx 
 "/home/ptah/Tools/emsdk/upstream/bin/clang++" -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -v -std=c++23 -stdlib=libc++ -fmodules -fbuiltin-module-map Main.cxx -c -o /tmp/emscripten_temp_5ll0y5z5/Main_0.o
clang version 18.0.0 (https://github.com/llvm/llvm-project 14028ec0a62210d68a4dd7a046ac79c8c3b7727e)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/ptah/Tools/emsdk/upstream/bin
 (in-process)
 "/home/ptah/Tools/emsdk/upstream/bin/clang-18" -cc1 -triple wasm32-unknown-emscripten -emit-obj -mrelax-all -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name Main.cxx -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/ptah/Projects/LexoMusic/Native/Test -v -fcoverage-compilation-dir=/home/ptah/Projects/LexoMusic/Native/Test -resource-dir /home/ptah/Tools/emsdk/upstream/lib/clang/18 -D EMSCRIPTEN -isysroot /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /home/ptah/Tools/emsdk/upstream/lib/clang/18/include -internal-isystem /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include -std=c++23 -fdeprecated-macro -ferror-limit 19 -fvisibility=default -fgnuc-version=4.2.1 -fmodules -fimplicit-module-maps -fmodules-cache-path=/home/ptah/.cache/clang/ModuleCache -fmodule-map-file=/home/ptah/Tools/emsdk/upstream/lib/clang/18/include/module.modulemap -fmodules-validate-system-headers -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_5ll0y5z5/Main_0.o -x c++ Main.cxx
clang -cc1 version 18.0.0 based upon LLVM 18.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
 /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
 /home/ptah/Tools/emsdk/upstream/lib/clang/18/include
 /home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
Main.cxx:1:9: fatal error: module 'std' not found
    1 | import  std;  //everything in std namespace is in here
      | ~~~~~~~~^~~
1 error generated.
em++: error: '/home/ptah/Tools/emsdk/upstream/bin/clang++ -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/ptah/Tools/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -v -std=c++23 -stdlib=libc++ -fmodules -fbuiltin-module-map Main.cxx -c -o /tmp/emscripten_temp_5ll0y5z5/Main_0.o' failed (returned 1)

[kripken]

The std header is pretty recent in LLVM (less than a year) and I am guessing has not been in the release of libc++ used in Emscripten yet.

[sbc100]

LLVM 18 branched a couple of days ago so perhaps once it gets released and we update libc++ this will start working

[LeenHawk]

Just checking if there's any progress here 🙂 We're excited to see how this evolves!

[sbc100]

There has been a lot of progress on supported C++ modules in general. See #23542 and #21987.

However using libc++ itself via the std module seems like something that is still very experimental even upstream.
See https://libcxx.llvm.org/Modules.html. It seems like this support still requires some special cmake setup.

In other words, if you want to use C++ modules yourself you should be good to go. If you want to use the std module it seems like there is still work to be done, which is why I'm leaving this bug open for now.

[sxstd001]

There has been a lot of progress on supported C++ modules in general. See #23542 和 #21987。

However using libc++ itself via the std module seems like something that is still very experimental even upstream. See https://libcxx.llvm.org/Modules.html. It seems like this support still requires some special cmake setup.

In other words, if you want to use C++ modules yourself you should be good to go. If you want to use the std module it seems like there is still work to be done, which is why I'm leaving this bug open for now.

Can you write an example of using C++20 modules? I using emcc v4.0.3 still clang++ error: unknown argument: '-fmodules-ts'

[sbc100]

See the tests added in #23542. I believe -fmodules-ts is deprecated and you can use -fmodules.

[sbc100]

See the tests added in #23542. I believe -fmodules-ts is deprecated and you can use -fmodules.

BTW, this is just how upstream clang behaves and not related to emcc per-say.

[sxstd001]

This is too important.
clang19:
set(CMAKE_CXX_EXTENSIONS 0)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fmodule-file=std=/root/std.pcm"
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fmodules -fbuiltin-module-map")

emsripten not supported;

[sbc100]

@sxstd001 using the std module is not yet supported by emscripten. You can use you down C++20 though.

[sxstd001]

When will it be possible to support it?

[sbc100]

From the investigation I did it does not look simple. Somebody with a good understanding the current state of libc++ might be able to get it working.