"warning: std and std.compat modules not found!" on Ubuntu 24.04 using llvm18

[nata11]

Hello, I have both Mac OS (using brew install llvm@18) and Ubuntu 24.04 (llvm's apt repository) with llvm18. I am testing std modules example code from https://github.com/xmake-io/xmake/tree/dev/tests/projects/c%2B%2B/modules/stdmodules_cpp_only
I am using Xmake version 3.0.2 on both machines.
Clang versions are 18.1.8 on both machines.

On MacOs I used the toolchain command to switch toolchains for the project : xmake f --toolchain=llvm --sdk=/usr/local/opt/llvm@18

The build works and I did not get any error or warning.

But on Linux I used the toolchain switch command : xmake f --toolchain=llvm --sdk=/usr/lib/llvm-18/
Here the build also happens and the program runs but xmake spits out a warning:
warning: std and std.compat modules not found! maybe try to add --sdk=<PATH/TO/LLVM> or install libc++

What is the reason for this warning and how can I get rid of the warning?
Please see the verbose build logs for both Mac and Ubuntu below.

---

Mac OS
$ xmake --verbose
checking for Xcode directory ... /Applications/Xcode.app
checking for SDK version of Xcode for macosx (x86_64) ... 15.5
checking for clang ... /usr/local/opt/llvm@18/bin/clang
checking for the c++ compiler (cxx) ... clang
checking for /usr/local/opt/llvm@18/bin/clang ... ok
checking for flags (-fPIC) ... ok
checking for flags (clang_modules) ... ok
checking for flags (clang_modules_ts) ... no
checking for /usr/local/opt/llvm@18/bin/clang ... ok
checking for flags (clang_print_library_module_manifest_path) ... no
checking for flags (-fvisibility-inlines-hidden) ... ok
checking for flags (-O3) ... ok
checking for flags (-std=c++26) ... ok
checking for flags (-DNDEBUG) ... ok
[ 2%]: <stdmodules_cpp_only> generating.module.deps /usr/local/opt/llvm@18/share/libc++/v1/std.compat.cppm
[ 2%]: <stdmodules_cpp_only> generating.module.deps /usr/local/opt/llvm@18/share/libc++/v1/std.cppm
[ 2%]: <stdmodules_cpp_only> generating.module.deps src/main.cpp
checking for /usr/local/opt/llvm@18/bin/clang-scan-deps ... ok
/usr/local/opt/llvm@18/bin/clang-scan-deps --format=p1689 -- /usr/local/opt/llvm@18/bin/clang -x c++ -Qunused-arguments --target=x86_64-apple-macos15.5 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -c /usr/local/opt/llvm@18/share/libc++/v1/std.compat.cppm -o build/.objs/stdmodules_cpp_only/macosx/x86_64/release/usr/local/opt/llvm@18/share/libc++/v1/std.compat.cppm.o
/usr/local/opt/llvm@18/bin/clang-scan-deps --format=p1689 -- /usr/local/opt/llvm@18/bin/clang -x c++ -Qunused-arguments --target=x86_64-apple-macos15.5 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -c /usr/local/opt/llvm@18/share/libc++/v1/std.cppm -o build/.objs/stdmodules_cpp_only/macosx/x86_64/release/usr/local/opt/llvm@18/share/libc++/v1/std.cppm.o
/usr/local/opt/llvm@18/bin/clang-scan-deps --format=p1689 -- /usr/local/opt/llvm@18/bin/clang -x c++ -Qunused-arguments --target=x86_64-apple-macos15.5 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -c src/main.cpp -o build/.objs/stdmodules_cpp_only/macosx/x86_64/release/src/main.cpp.o
checking for flags (clang_module_file) ... ok
checking for flags (clang_module_output) ... ok
[ 17%]: <stdmodules_cpp_only> compiling.module.bmi.release std
/usr/local/opt/llvm@18/bin/clang -c -Qunused-arguments --target=x86_64-apple-macos15.5 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -x c++-module --precompile -Wno-include-angled-in-module-purview -Wno-reserved-module-identifier -Wno-deprecated-declarations -fmodule-output=build/.gens/stdmodules_cpp_only/macosx/x86_64/release/rules/bmi/cache/interfaces/3b1eb589/std.pcm -o build/.gens/stdmodules_cpp_only/macosx/x86_64/release/rules/bmi/cache/interfaces/3b1eb589/std.pcm /usr/local/opt/llvm@18/share/libc++/v1/std.cppm
checking for flags (-fdiagnostics-color=always) ... ok
[ 25%]: compiling.release /usr/local/opt/llvm@18/share/libc++/v1/std.cppm
/usr/local/opt/llvm@18/bin/clang -c -Qunused-arguments --target=x86_64-apple-macos15.5 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -x c++ -Wno-include-angled-in-module-purview -Wno-reserved-module-identifier -Wno-deprecated-declarations -o build/.objs/stdmodules_cpp_only/macosx/x86_64/release/usr/local/opt/llvm@18/share/libc++/v1/std.cppm.o /usr/local/opt/llvm@18/share/libc++/v1/std.cppm
[ 43%]: compiling.release src/main.cpp
/usr/local/opt/llvm@18/bin/clang -c -Qunused-arguments --target=x86_64-apple-macos15.5 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -fmodule-file=std=build/.gens/stdmodules_cpp_only/macosx/x86_64/release/rules/bmi/cache/interfaces/3b1eb589/std.pcm -o build/.objs/stdmodules_cpp_only/macosx/x86_64/release/src/main.cpp.o src/main.cpp
checking for flags (-MMD -MF) ... ok
checking for clang++ ... /usr/local/opt/llvm@18/bin/clang++
checking for the linker (ld) ... clang++
checking for /usr/local/opt/llvm@18/bin/clang++ ... ok
checking for flags (-fPIC) ... ok
[ 60%]: linking.release stdmodules_cpp_only
/usr/local/opt/llvm@18/bin/clang++ -o build/macosx/x86_64/release/stdmodules_cpp_only build/.objs/stdmodules_cpp_only/macosx/x86_64/release/src/main.cpp.o build/.objs/stdmodules_cpp_only/macosx/x86_64/release/usr/local/opt/llvm@18/share/libc++/v1/std.cppm.o --target=x86_64-apple-macos15.5 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.5.sdk -Wl,-x -Wl,-dead_strip
[100%]: build ok, spent 9.288s

Ubuntu 24.04
xmake --verbose
checking for flags (-fvisibility-inlines-hidden) ... ok
checking for flags (-O3) ... ok
checking for flags (-std=c++26) ... ok
checking for flags (-DNDEBUG) ... ok
[ 1%]: <stdmodules_cpp_only> generating.module.deps src/main.cpp
checking for /usr/lib/llvm-18/bin/clang-scan-deps ... ok
/usr/lib/llvm-18/bin/clang-scan-deps --format=p1689 -- /usr/lib/llvm-18/bin/clang -x c++ -Qunused-arguments -m64 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -c src/main.cpp -o build/.objs/stdmodules_cpp_only/linux/x86_64/release/src/main.cpp.o
[ 31%]: compiling.release src/main.cpp
/usr/lib/llvm-18/bin/clang -c -Qunused-arguments -m64 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++26 -DNDEBUG -o build/.objs/stdmodules_cpp_only/linux/x86_64/release/src/main.cpp.o src/main.cpp
checking for flags (-MMD -MF) ... ok
checking for flags (-fdiagnostics-color=always) ... ok
checking for clang++ ... /usr/lib/llvm-18/bin/clang++
checking for the linker (ld) ... clang++
checking for /usr/lib/llvm-18/bin/clang++ ... ok
checking for flags (-fPIC) ... ok
[ 52%]: linking.release stdmodules_cpp_only
/usr/lib/llvm-18/bin/clang++ -o build/linux/x86_64/release/stdmodules_cpp_only build/.objs/stdmodules_cpp_only/linux/x86_64/release/src/main.cpp.o -m64 -s
[100%]: build ok, spent 1.682s
warning: std and std.compat modules not found! maybe try to add --sdk=<PATH/TO/LLVM> or install libc++