Undefined symbols for architecture x86_64: "initializer for module std" with g++-15

Reproduce the failure with the CMake and Ninja command shown in the issue, then inspect CMakeLists.txt and examples/scope-module.cpp, especially the scope-module target. Trace how the C++ standard module is configured and linked; done means the target links successfully on the reported macOS and GCC 15.2 setup.

see too https://discourse.cmake.org/t/cxx-modules-import-std-does-still-not-work-on-osx-with-g-15/15265
and https://discourse.cmake.org/t/header-only-libraries-and-c-20-modules/10680/13

bash-5.3$ make distclean
rm -rf build
bash-5.3$ make
cmake -S . -B build -G Ninja --log-level=DEBUG -D CMAKE_BUILD_TYPE=Release \
	 -D CMAKE_EXPERIMENTAL_CXX_IMPORT_STD="d0edc3af-4c50-42ea-a356-e2862fe7a444" \
	 -D CMAKE_CXX_STDLIB_MODULES_JSON=/usr/local/Cellar/gcc/15.2.0/lib/gcc/current/libstdc++.modules.json \
	 -D CMAKE_CXX_STANDARD=23 -D CMAKE_CXX_EXTENSIONS=ON -D CMAKE_CXX_STANDARD_REQUIRED=ON \
	 -D CMAKE_CXX_MODULE_STD=ON \
	 -D CMAKE_INSTALL_MESSAGE=LAZY # XXX -D CMAKE_SKIP_INSTALL_RULES=ON # --fresh
-- The CXX compiler identification is GNU 15.2.0
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/bin/g++-15 - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /usr/local/share/cmake/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):
  /usr/local/share/cmake/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /usr/local/share/cmake/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:5 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
Compiler is: GNU version:  15.2.0
cmake is: 4.2.0 modules scan :  1
-- Executing download step for catch2
'/usr/local/bin/cmake' '-E' 'rm' '-rf' '/Users/clausklein/Workspace/cpp/beman-project/scope/build/_deps/catch2-src'
'/usr/local/bin/git' 'clone' '--no-checkout' '--config' 'advice.detachedHead=false' 'https://github.com/catchorg/Catch2.git' 'catch2-src'
Cloning into 'catch2-src'...
'/usr/local/bin/git' 'checkout' 'v3.11.0' '--'
HEAD is now at b3fb4b9f v3.11.0
'/usr/local/bin/git' 'submodule' 'update' '--recursive' '--init'
'/usr/local/bin/cmake' '-E' 'copy' '/Users/clausklein/Workspace/cpp/beman-project/scope/build/CMakeFiles/fc-stamp/catch2/catch2-gitinfo.txt' '/Users/clausklein/Workspace/cpp/beman-project/scope/build/CMakeFiles/fc-stamp/catch2/catch2-gitclone-lastrun.txt'
-- Executing update step for catch2
-- Already at requested tag: v3.11.0
-- Executing patch step for catch2
-- Performing Test HAVE_FLAG__ffile_prefix_map__Users_clausklein_Workspace_cpp_beman_project_scope_build__deps_catch2_src__
-- Performing Test HAVE_FLAG__ffile_prefix_map__Users_clausklein_Workspace_cpp_beman_project_scope_build__deps_catch2_src__ - Success
Tests to be built: scope_success;scope_exit;scope_fail;unique_resource;module
Examples to be built: scope_example;unique_resource;unique_resource-file;scope-module
-- Configuring done (14.0s)
-- Generating done (0.1s)
-- Build files have been written to: /Users/clausklein/Workspace/cpp/beman-project/scope/build
ln -sf build/compile_commands.json .
ninja -C build
ninja: Entering directory `build'
[251/260] Building CXX object examples/CMakeFiles/scope-module.dir/scope-module.cpp.o
/Users/clausklein/Workspace/cpp/beman-project/scope/examples/scope-module.cpp: In function 'int main()':
/Users/clausklein/Workspace/cpp/beman-project/scope/examples/scope-module.cpp:39:37: warning: name-independent declarations only available with '-std=c++2c' or '-std=gnu++2c' [-Wc++26-extensions]
   39 |         beman::scope::scope_success _([&success_ran] { success_ran = true; });
      |                                     ^
/Users/clausklein/Workspace/cpp/beman-project/scope/examples/scope-module.cpp:40:37: warning: name-independent declarations only available with '-std=c++2c' or '-std=gnu++2c' [-Wc++26-extensions]
   40 |         beman::scope::scope_fail    _([&fail_ran]    { fail_ran = true;    });
      |                                     ^
[252/260] Linking CXX executable examples/scope-module
FAILED: [code=1] examples/scope-module 
: && /usr/local/bin/g++-15 -stdlib=libstdc++ -O3 -DNDEBUG -Wl,-search_paths_first -Wl,-headerpad_max_install_names examples/CMakeFiles/scope-module.dir/scope-module.cpp.o -o examples/scope-module  libbeman.scope.a && :
Undefined symbols for architecture x86_64:
  "initializer for module std", referenced from:
      __GLOBAL__sub_I_scope_module.cpp in scope-module.cpp.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
[253/260] Building CXX object _deps/catch2-build/src/CMakeFiles/Catch2.dir/catch2/matchers/catch_matchers_string.cpp.o
ninja: build stopped: subcommand failed.
make: *** [all] Error 1
bash-5.3$