Test if import std; is possible is wrong implemented #34
Description
Only cmake -D CMAKE_CXX_MODULE_STD=ON the std module may be build if possible!
bash-5.3$ make build
cmake -S . -B build -G Ninja -D CMAKE_SKIP_INSTALL_RULES=ON \
-D CMAKE_EXPERIMENTAL_CXX_IMPORT_STD="d0edc3af-4c50-42ea-a356-e2862fe7a444" \
-D CMAKE_CXX_STANDARD=23 -D CMAKE_CXX_EXTENSIONS=ON -D CMAKE_CXX_STANDARD_REQUIRED=ON \
-D BUILD_TESTING=ON -D BOOST_INCLUDE_LIBRARIES='any;type_index;test' \
-D CMAKE_CXX_STDLIB_MODULES_JSON=/usr/local/Cellar/llvm/21.1.6/lib/c++/libc++.modules.json \
-D BOOST_USE_MODULES=ON #XXX -D CMAKE_CXX_MODULE_STD=OFF \
# XXX --fresh
-- The CXX compiler identification is Clang 21.1.6
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/Cellar/llvm/21.1.6/bin/clang++ - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /Users/clausklein/.local/share/cmake-4.2/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (cmake_language):
CMake's support for `import std;` in C++23 and newer is experimental. It
is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
/Users/clausklein/.local/share/cmake-4.2/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
/Users/clausklein/.local/share/cmake-4.2/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
CMakeLists.txt:20 (project)
This warning is for project developers. Use -Wno-dev to suppress it.
-- Detecting CXX compile features - done
-- CMAKE_CXX_COMPILER_IMPORT_STD=23;26
-- CMAKE_CXX_STANDARD=23
-- CMAKE_CXX_MODULE_STD=
-- CMAKE_CXX_SCAN_FOR_MODULES=ON
-- Boost: using system layout: include, bin, lib, lib/cmake, share
-- Boost: using CMake 4.2.0-gd4539f6
-- Boost: Release build, static libraries, MPI OFF, Python OFF, testing ON
-- Boost: libraries included: any;type_index;test
-- Using `import std;`
-- Using `import std;`
-- Configuring done (1.7s)
-- Generating done (0.3s)
-- Build files have been written to: /Users/clausklein/Workspace/cpp/boost-git/build
bash-5.3$ make
ln -sf build/compile_commands.json .
ninja -C build
ninja: Entering directory `build'
[0/2] Re-checking globbed directories...
[107/123] Building CXX object libs/type_index/CMakeFiles/boost_type_index.dir/modules/boost_type_index.cppm.o
FAILED: [code=1] libs/type_index/CMakeFiles/boost_type_index.dir/modules/boost_type_index.cppm.o libs/type_index/CMakeFiles/boost_type_index.dir/boost.type_index.pcm
ccache /usr/local/Cellar/llvm/21.1.6/bin/clang++ -DBOOST_TYPE_INDEX_USE_STD_MODULE -DBOOST_USE_MODULES -I/Users/clausklein/Workspace/cpp/boost-git/libs/type_index/include -I/Users/clausklein/Workspace/cpp/boost-git/libs/config/include -I/Users/clausklein/Workspace/cpp/boost-git/libs/container_hash/include -I/Users/clausklein/Workspace/cpp/boost-git/libs/describe/include -I/Users/clausklein/Workspace/cpp/boost-git/libs/mp11/include -I/Users/clausklein/Workspace/cpp/boost-git/libs/throw_exception/include -I/Users/clausklein/Workspace/cpp/boost-git/libs/assert/include -O3 -DNDEBUG -std=gnu++23 -fvisibility=hidden -fvisibility-inlines-hidden -MD -MT libs/type_index/CMakeFiles/boost_type_index.dir/modules/boost_type_index.cppm.o -MF libs/type_index/CMakeFiles/boost_type_index.dir/modules/boost_type_index.cppm.o.d @libs/type_index/CMakeFiles/boost_type_index.dir/modules/boost_type_index.cppm.o.modmap -o libs/type_index/CMakeFiles/boost_type_index.dir/modules/boost_type_index.cppm.o -c /Users/clausklein/Workspace/cpp/boost-git/libs/type_index/modules/boost_type_index.cppm
/Users/clausklein/Workspace/cpp/boost-git/libs/type_index/modules/boost_type_index.cppm:42:8: fatal error: module 'std' not found
42 | import std;
| ~~~~~~~^~~
1 error generated.
ninja: build stopped: subcommand failed.
make: *** [all] Error 1
bash-5.3$