[clangd] clangd doesn't work with some file names with --experimental-modules-support option

[FSK-idk]

Here I have an example repo: https://github.com/FSK-idk/clangd-test

When I build the project and look at the file somefile.cppm in particular, I don't see any clangd info:

 

However, if I do one of the following, the info will come back:

• comment import std; in the file types.cppm
• rename the file somefile.cppm, for example, to asd.cppm (or to some other name, but there is a chance that clangd will not work with the new name either)
• add a new file asd.cppm with the content export module asd; (same here, maybe some other name)
• remove --experimental-modules-support option (but then module info won't be available, but at least highlight will work)

Here is what it looks like if I rename somefile.cppm to asd.cppm:

 

Versions

❯ clangd --version
clangd version 20.1.7
Features: linux
Platform: x86_64-pc-linux-gnu
❯ clang --version 
clang version 20.1.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

I also tested it with 21.0.0git version I built today and got the same result.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: FSK (FSK-idk)

Here I have an example repo: https://github.com/FSK-idk/clangd-test

When I build the project and look at the file somefile.cppm in particular, I don't see any clangd info:

<img width="1332" height="306" alt="Image" src="https://github.com/user-attachments/assets/4c239d9c-95cf-4970-930b-62a59affeb72" />
&nbsp;

However, if I do one of the following, the info will come back:

• comment import std; in the file types.cppm
• rename the file somefile.cppm, for example, to asd.cppm (or to some other name, but there is a chance that clangd will not work with the new name either)
• add a new file asd.cppm with the content export module asd; (same here, maybe some other name)
• remove --experimental-modules-support option (but then module info won't be available, but at least highlight will work)

Here is what it looks like if I rename somefile.cppm to asd.cppm:

<img width="1331" height="310" alt="Image" src="https://github.com/user-attachments/assets/544df3f1-65bd-4836-94d9-e56a712c1291" />
&nbsp;

Versions

❯ clangd --version
clangd version 20.1.7
Features: linux
Platform: x86_64-pc-linux-gnu
❯ clang --version 
clang version 20.1.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

I also tested it with 21.0.0git version I built today and got the same result.

[llvmbot]

@llvm/issue-subscribers-clangd

Author: FSK (FSK-idk)

Here I have an example repo: https://github.com/FSK-idk/clangd-test

When I build the project and look at the file somefile.cppm in particular, I don't see any clangd info:

<img width="1332" height="306" alt="Image" src="https://github.com/user-attachments/assets/4c239d9c-95cf-4970-930b-62a59affeb72" />
&nbsp;

However, if I do one of the following, the info will come back:

• comment import std; in the file types.cppm
• rename the file somefile.cppm, for example, to asd.cppm (or to some other name, but there is a chance that clangd will not work with the new name either)
• add a new file asd.cppm with the content export module asd; (same here, maybe some other name)
• remove --experimental-modules-support option (but then module info won't be available, but at least highlight will work)

Here is what it looks like if I rename somefile.cppm to asd.cppm:

<img width="1331" height="310" alt="Image" src="https://github.com/user-attachments/assets/544df3f1-65bd-4836-94d9-e56a712c1291" />
&nbsp;

Versions

❯ clangd --version
clangd version 20.1.7
Features: linux
Platform: x86_64-pc-linux-gnu
❯ clang --version 
clang version 20.1.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

I also tested it with 21.0.0git version I built today and got the same result.

[ChuanqiXu9]

how is your compilation database

[FSK-idk]

This is the file when it doesn't work:
compile_commands.json

And this is the file after renaming:
compile_commands2.json

[ChuanqiXu9]

Maybe you can try to take a look yourself at ModulesBuilder.cpp and --log=verbose. As I don't have the time recently. I feel the implementation is straightforward and maybe you're able to fix it.

[FSK-idk]

Ok, I'll try

[FSK-idk]

So I tried using clangd trunk with debug build type (previously it was release).
It doesn't even runs buildModuleFile (ModuleBuilder.cpp).

I looked at the logs and saw the following:

Scanning modules dependencies for /usr/share/libc++/v1/std.cppm failed: In file included from /usr/share/libc++/v1/std.cppm:202:
/usr/share/libc++/v1/std/array.inc:16:22: warning: '<=>' is a single token in C++20; add a space to avoid a change in behavior [-Wc++20-compat]
In file included from /usr/share/libc++/v1/std.cppm:213:
/usr/share/libc++/v1/std/chrono.inc:44:32: warning: '<=>' is a single token in C++20; add a space to avoid a change in behavior [-Wc++20-compat]
/usr/share/libc++/v1/std/chrono.inc:300:51: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
/usr/share/libc++/v1/std/chrono.inc:303:51: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
In file included from /usr/share/libc++/v1/std.cppm:223:
/usr/share/libc++/v1/std/coroutine.inc:20:22: warning: '<=>' is a single token in C++20; add a space to avoid a change in behavior [-Wc++20-compat]
...

This is because FinalCommandLine in DependencyScanningWorker::computeDependencies (DependencyScanningWorker.cpp) doesn't have any standard version flags. It defaults to c++11, so it causes errors.

Also, FinalCommandLine has other flags that I specified in CMakePresets.json and ignores -std=c++26. However, for other files (except std.cppm) it passes all flags specified in CMakePresets.json.

To solve this problem, I needed to create a .clangd file with the contents:

CompileFlags:
  Add: [-std=c++26]

Only after that -std=c++26 appears in FinalCommandLine. It's a bit confusing that compile_commands.json already has this flag, but why would I need to explicitly pass it in the config file?

It seems to be working now, but the logs show the following error:

Failed to find compilation database for /usr/share/libc++/v1/std.cppm
Failed to build module std; due to No compile command for /usr/share/libc++/v1/std.cppm

I found out that the problem is that it can't find the directory where compile_commands.json is located. In particular, SearchDirs in DirectoryBasedGlobalCompilationDatabase::lookupCDB (GlobalCompilationDataBase.cpp) only contains:

/usr/share/libc++
/usr/share
/usr
/

To solve this problem it's needed to add the --compile-commands-dir=build option to clangd. The std.cppm is outside of this project, so I think that's why it can't find the compile commands.

After this clangd no longer produces errors.

I put the corrections I made into the fixed branch.

I got all the information from clangd log. I used log(), and if it is not available (e.g. in clang/ directory), I changed the necessary functions and passed std::string& dbg to them to get information about what is going on there.

It's hard for me to do more because compilation takes a long time on my not-so-great notebook (btw, every time I compiled clangd for the first time with debug build type, I got a kernel panic).

[ChuanqiXu9]

So I tried using clangd trunk with debug build type (previously it was release). It doesn't even runs buildModuleFile (ModuleBuilder.cpp).

I looked at the logs and saw the following:

Scanning modules dependencies for /usr/share/libc++/v1/std.cppm failed: In file included from /usr/share/libc++/v1/std.cppm:202:
/usr/share/libc++/v1/std/array.inc:16:22: warning: '<=>' is a single token in C++20; add a space to avoid a change in behavior [-Wc++20-compat]
In file included from /usr/share/libc++/v1/std.cppm:213:
/usr/share/libc++/v1/std/chrono.inc:44:32: warning: '<=>' is a single token in C++20; add a space to avoid a change in behavior [-Wc++20-compat]
/usr/share/libc++/v1/std/chrono.inc:300:51: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
/usr/share/libc++/v1/std/chrono.inc:303:51: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
In file included from /usr/share/libc++/v1/std.cppm:223:
/usr/share/libc++/v1/std/coroutine.inc:20:22: warning: '<=>' is a single token in C++20; add a space to avoid a change in behavior [-Wc++20-compat]
...

This is because FinalCommandLine in DependencyScanningWorker::computeDependencies (DependencyScanningWorker.cpp) doesn't have any standard version flags. It defaults to c++11, so it causes errors.

Also, FinalCommandLine has other flags that I specified in CMakePresets.json and ignores -std=c++26. However, for other files (except std.cppm) it passes all flags specified in CMakePresets.json.

To solve this problem, I needed to create a .clangd file with the contents:

CompileFlags:
  Add: [-std=c++26]

Only after that -std=c++26 appears in FinalCommandLine. It's a bit confusing that compile_commands.json already has this flag, but why would I need to explicitly pass it in the config file?

It seems to be working now, but the logs show the following error:

Failed to find compilation database for /usr/share/libc++/v1/std.cppm
Failed to build module std; due to No compile command for /usr/share/libc++/v1/std.cppm

I found out that the problem is that it can't find the directory where compile_commands.json is located. In particular, SearchDirs in DirectoryBasedGlobalCompilationDatabase::lookupCDB (GlobalCompilationDataBase.cpp) only contains:

/usr/share/libc++
/usr/share
/usr
/

To solve this problem it's needed to add the --compile-commands-dir=build option to clangd. The std.cppm is outside of this project, so I think that's why it can't find the compile commands.

After this clangd no longer produces errors.

I put the corrections I made into the fixed branch.

I got all the information from clangd log. I used log(), and if it is not available (e.g. in clang/ directory), I changed the necessary functions and passed std::string& dbg to them to get information about what is going on there.

It's hard for me to do more because compilation takes a long time on my not-so-great notebook (btw, every time I compiled clangd for the first time with debug build type, I got a kernel panic).

Excellent!

For logging, it will be helpful to use --log=verbose. For building, it is faster to build release version without debug info.