import std module with gcc-15

As mention in this gcc patches, the std module is built in gcc development branch. I build from source and try to use it in cmake project but it show std module not found.

This is my minimal example:

cmake_minimum_required(VERSION 3.30 FATAL_ERROR)

set(CMAKE_CXX_STANDARD 23)
if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.30)
    set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "0e5b6991-d74f-4b3d-a41c-cf096e0b2508")
endif()

set(CMAKE_CXX_STANDARD_REQUIRED OFF)
set(CMAKE_CXX_MODULE_STD 1)

project(module-test VERSION 0.1.0 LANGUAGES CXX)

add_executable(module-test main.cpp)
target_link_options(module-test PRIVATE -L/home/leenhawk/tools/gcc/lib -Wl,-rpath,/home/leenhawk/tools/gcc/lib)
set(CMAKE_VERBOSE_MAKEFILE ON)

import std;

int main(int, char**){
    std::cout << "Hello, from module-test!\n";
}

I have found libstdc++.module.json in the ~/tools/gcc/lib, how to use it?

The teminal output:

cmake] CMake Error in CMakeLists.txt:
[cmake]   The "CXX_MODULE_STD" property on the target "module-test" requires that the
[cmake]   "__CMAKE::CXX23" target exist, but it was not provided by the toolchain.
[cmake]   Reason:
[cmake] 
[cmake]     Toolchain does not support discovering `import std` support
[cmake] 
[cmake] 
[cmake] -- Generating done (0.0s)

I use CMake 3.31.1. And there is command line:

/home/leenhawk/tools/gcc/bin/g++ main.cpp -L/home/leenhawk/tools/gcc/lib -Wl,-rpath,/home/leenhawk/tools/gcc/lib -std=c++2b  -fmodules-ts

In module imported at main.cpp:1:1:
std: error: failed to read compiled module: No such file or directory
std: note: compiled module file is ‘gcm.cache/std.gcm’
std: note: imports must be built before being imported
std: fatal error: returning to the gate for a mechanical issue
compilation terminated.

Solution

CMake 3.31.1 comes with 2 files Clang-CXX-CXXImportStd.cmake and MSVC-CXX-CXXImportStd.cmake but no GNU-CXX-CXXImportStd.cmake. It will probably be added at some point in the future, or you could try writing one based on the 2 examples.

In the meantime, you have to compile the module by hand first. Something like (ignore the error message from the linker)

g++ -std=gnu++26 -fmodules -fsearch-include-path bits/std.cc

After which you can compile your file normally

g++ -std=gnu++26 -fmodules a.cc

The JSON file basically contains instructions so CMake (or other) can guess this first command.