Two mutually exclusive clang flags for C++20 modules compilation are included

[aacirino]

Xmake Version

2.7.1

Operating System Version and Architecture

Fedora 36, clang 15

Describe Bug

Since some time I've had problems building C++20 projects with modules. I had to resort to adding some compile flags but got tired of it and noticed that the compiler flags generation is including two mutually exclusive flags. In xmake/rules/c++/modules/modules_support/clang.lua we have

local implicitmodulesflag = get_implicitmodulesflag(target)
local noimplicitmodulemapsflag = get_noimplicitmodulemapsflag(target)

and then

target:add("cxxflags", implicitmodulesflag, {force = true})
target:add("cxxflags", noimplicitmodulemapsflag, {force = true})

Xmake should include only the implicit modules flag, allowing us to use clang's module maps transparently. Unfortunately xmake will not build in my environment due to an error something like No rule to build the target 'lua/lapi.c' (my system is in Brazilian Portuguese so I have to guess this is how it is phrased in English). I would change the modules flag generation myself and submit a pull request if I could build xmake.
In the meantime I have to keep adding the following flags to xmake.lua:

"-fimplicit-module-maps",
"-fmodule-map-file=$(shell brew --prefix)/include/c++/v1/module.modulemap",

Expected Behavior

xmake should include only the -fimplicit-modules flag.

Project Configuration

trouble.zip

Additional Information and Error Logs

[~/git/trouble]$ xmake -bv
[ 0%]: generating.cxx.module.deps src/my_module.cpp
[ 23%]: generating.cxx.module.bmi my_module
/home/linuxbrew/.linuxbrew/opt/llvm/bin/clang -Qunused-arguments -m64 -fPIC -O3 -std=c++20 -fmodules -fbuiltin-module-map -fimplicit-modules -fno-implicit-module-maps -DNDEBUG -fmodules-cache-path=build/.gens/trouble/linux/x86_64/release/rules/modules/cache -c -x c++-module --precompile src/my_module.mpp -o build/.gens/trouble/linux/x86_64/release/rules/modules/cache/my_module.pcm
src/my_module.mpp:3:8: fatal error: module 'std' not found
import std;

1 error generated.

[waruqi]

Unfortunately xmake will not build in my environment due to an error something like No rule to build the target 'lua/lapi.c' (my system is in Brazilian Portuguese so I have to guess this is how it is phrased in English). I would change the modules flag generation myself and submit a pull request if I could build xmake.

you need download full source code (contains git submodules), see
https://xmake.io/#/guide/faq?id=how-to-debug-xmake-source-code

[waruqi]

xmake should include only the -fimplicit-modules flag.
"-fmodule-map-file=$(shell brew --prefix)/include/c++/v1/module.modulemap",

Even when I add these flags, it still doesn't seem to work.

@Arthapz Any idea about this?

/usr/bin/clang -c -x c++-module --precompile -Qunused-arguments -m64 -fPIC -O3 -std=c++20 -fmodules -fbuiltin-module-map -fimplicit-modules -fimplicit-module-maps -fmodule-map-file=/usr/lib/llvm-15/lib/clang/15.0.2/include/module.modulemap -DNDEBUG -fmodules-cache-path=build/.gens/trouble/linux/x86_64/release/rules/modules/cache -o build/.gens/trouble/linux/x86_64/release/rules/modules/cache/my_module.pcm src/my_module.mpp
/usr/bin/clang -c -Qunused-arguments -m64 -fPIC -O3 -std=c++20 -fmodules -fbuiltin-module-map -fimplicit-modules -fimplicit-module-maps -fmodule-map-file=/usr/lib/llvm-15/lib/clang/15.0.2/include/module.modulemap -DNDEBUG -fmodules-cache-path=build/.gens/trouble/linux/x86_64/release/rules/modules/cache -o build/.objs/trouble/linux/x86_64/release/src/my_module.mpp.o build/.gens/trouble/linux/x86_64/release/rules/modules/cache/my_module.pcm
checking for flags (-MMD -MF) ... ok
checking for flags (-fdiagnostics-color=always) ... ok
error: src/my_module.mpp:3:8: fatal error: module 'std' not found
import std;
~~~~~~~^~~
1 error generated.

[aacirino]

That was the smallest non-building modules project I was able to create to illustrate the issue. I modified the xmake.lua build script to include the additions that I've been using to be able to build my projects. This is the outcome of the attached build file:

[  0%]: generating.cxx.module.deps test/test.cpp
[  0%]: generating.cxx.module.deps src/my_module.cpp
[  0%]: generating.cxx.module.deps src/my_module.mpp
[ 23%]: generating.cxx.module.bmi my_module
[ 38%]: cache compiling.release src/my_module.cpp
[ 42%]: linking.release libtrouble.so
[ 85%]: cache compiling.release test/test.cpp
[ 90%]: linking.release test
[100%]: build ok!
[doctest] doctest version is "2.4.9"
[doctest] run with "--help" for options
===============================================================================
[doctest] test cases: 1 | 1 passed | 0 failed | 0 skipped
[doctest] assertions: 1 | 1 passed | 0 failed |
[doctest] Status: SUCCESS!

Please note that I had to add link flags, link dirs, include dirs and compile flags for this to work.

trouble.zip

[waruqi]

I have not linuxbrew and mold. I modified llvm path, but it does not work for me.

/usr/bin/clang -c -x c++-module --precompile -Qunused-arguments -m64 -fPIC -O3 -std=c++20 -I/usr/local/include -I/usr/lib/llvm-15/include -I/usr/lib/llvm-15/lib/clang/15.0.2/include/c++/v1 -fmodules -fcxx-modules -fmodule-maps -fimplicit-module-maps -fbuiltin-module-map -fmodule-map-file=/usr/lib/llvm-15/lib/clang/15.0.2/include/module.modulemap -fvisibility=default -fimplicit-modules -fno-implicit-module-maps -DNDEBUG -fmodules-cache-path=build/.gens/trouble/linux/x86_64/release/rules/modules/cache -o build/.gens/trouble/linux/x86_64/release/rules/modules/cache/my_module.pcm src/my_module.mpp
/usr/bin/clang -c -Qunused-arguments -m64 -fPIC -O3 -std=c++20 -I/usr/local/include -I/usr/lib/llvm-15/include -I/usr/lib/llvm-15/lib/clang/15.0.2/include/c++/v1 -fmodules -fcxx-modules -fmodule-maps -fimplicit-module-maps -fbuiltin-module-map -fmodule-map-file=/usr/lib/llvm-15/lib/clang/15.0.2/include/module.modulemap -fvisibility=default -fimplicit-modules -fno-implicit-module-maps -DNDEBUG -fmodules-cache-path=build/.gens/trouble/linux/x86_64/release/rules/modules/cache -o build/.objs/trouble/linux/x86_64/release/src/my_module.mpp.o build/.gens/trouble/linux/x86_64/release/rules/modules/cache/my_module.pcm
error: src/my_module.mpp:3:8: fatal error: module 'std' not found
import std;
~~~~~~~^~~
1 error generated.

[aacirino]

By commenting out target:add("cxxflags", noimplicitmodulemapsflag, {force = true}) in xmake/rules/c++/modules/modules_support/clang.lua this is the smallest xmake.lua build scrit that works for me:

add_rules("mode.debug", "mode.release")
set_languages("c++20")

target()
add_includedirs(
	"$(shell brew --prefix)/include/",
	"$(shell brew --prefix)/opt/llvm/include/c++/v1"
)
add_cxxflags(
	"-fvisibility=default",
	{ force = true }
)
add_ldflags("-nostdlib++", "-stdlib=libc++", "-lc++")
target_end()

target("trouble")
    set_kind("shared")
    add_files("src/*.cpp", "src/*.mpp")

target("test")
    set_kind("binary")
    add_files("test/*.cpp")
    add_deps("trouble")

[waruqi]

I cannot run this script, because I have not brew on my ubuntu.

[aacirino]

@waruqi the problem is that your compiler flags include the undesired -fno-implicit-module-maps flag. This flag excludes clang's module.modulemap files from the compilation. I managed to comment-out the generation of this flag in xmake and built it again.

[waruqi]

But now I can't use your example to test it, even if I delete -fno-implicit-module-maps, I still have the same problem.

[aacirino]

Use this build script

set_config("toolchain", "clang")
add_rules("mode.debug", "mode.release")
set_languages("c++20")

target()
add_includedirs(
	"/usr/local/include",
	"/usr/lib/llvm-15/lib/clang/15.0.2/include/c++/v1"
)
add_cxxflags(
	"-fvisibility=default",
	{ force = true }
)
add_ldflags("-nostdlib++", "-stdlib=libc++", "-lc++")
target_end()

target("trouble")
    set_kind("shared")
    add_files("src/*.cpp", "src/*.mpp")

target("test")
    set_kind("binary")
    add_files("test/*.cpp")
    add_deps("trouble")

[aacirino]

Is xmake able to get clang's installation dir?

[waruqi]

Is xmake able to get clang's installation dir?

/usr/lib/llvm-15/lib
/usr/bin/clang

[waruqi]

Use this build script

set_config("toolchain", "clang")
add_rules("mode.debug", "mode.release")
set_languages("c++20")

target()
add_includedirs(
	"/usr/local/include",
	"/usr/lib/llvm-15/lib/clang/15.0.2/include/c++/v1"
)
add_cxxflags(
	"-fvisibility=default",
	{ force = true }
)
add_ldflags("-nostdlib++", "-stdlib=libc++", "-lc++")
target_end()

target("trouble")
    set_kind("shared")
    add_files("src/*.cpp", "src/*.mpp")

target("test")
    set_kind("binary")
    add_files("test/*.cpp")
    add_deps("trouble")

/usr/bin/clang -c -x c++-module --precompile -Qunused-arguments -m64 -fPIC -O3 -std=c++20 -I/usr/local/include -I/usr/lib/llvm-15/lib/clang/15.0.2/include/c++/v1 -fvisibility=default -fmodules -fbuiltin-module-map -fimplicit-modules -fno-implicit-module-maps -DNDEBUG -fmodules-cache-path=build/.gens/trouble/linux/x86_64/release/rules/modules/cache -o build/.gens/trouble/linux/x86_64/release/rules/modules/cache/my_module.pcm src/my_module.mpp
/usr/bin/clang -c -Qunused-arguments -m64 -fPIC -O3 -std=c++20 -I/usr/local/include -I/usr/lib/llvm-15/lib/clang/15.0.2/include/c++/v1 -fvisibility=default -fmodules -fbuiltin-module-map -fimplicit-modules -fno-implicit-module-maps -DNDEBUG -fmodules-cache-path=build/.gens/trouble/linux/x86_64/release/rules/modules/cache -o build/.objs/trouble/linux/x86_64/release/src/my_module.mpp.o build/.gens/trouble/linux/x86_64/release/rules/modules/cache/my_module.pcm
checking for flags (-MMD -MF) ... ok
checking for flags (-fdiagnostics-color=always) ... ok
error: src/my_module.mpp:3:8: fatal error: module 'std' not found
import std;
~~~~~~~^~~
1 error generated.

it does not work.

[waruqi]

If it works on your machine, you can open a pr first.

[aacirino]

PR created

[waruqi]

see #3016

But I still need to solve the compatibility problem between libc++ and libstdc++.

[waruqi]

try it again. xmake update -s dev

[waruqi]

https://github.com/xmake-io/xmake/blob/7c5bd94cde348e8d656a0cb577bef83c7e20a070/tests/projects/c%2B%2B/modules/stdmodules/xmake.lua

we need use -stdlib=libc++ for clang if we use import std

add_rules("mode.debug", "mode.release")
set_languages("c++20")

add_cxxflags("clang::-stdlib=libc++")

target("mod")
    set_kind("shared")
    add_files("src/*.cpp", "src/*.mpp")

target("test")
    set_kind("binary")
    add_files("test/*.cpp")
    add_deps("mod")

[waruqi]

It should work now.