definition with same mangled name when using c++20 modules, templates and libstdc++

[TroyKomodo]

main.cpp

import pollcoro;
import helper;

int main() {
    pollcoro::block_on(helper::async(0));
}

This fails to compile with the following:

/nix/store/90lsdrdffwgf3kjd4sdxjchchg44cr02-clang-wrapper-20.1.8/bin/clang++   -O3 -O3 -DNDEBUG -std=c++20 -fvisibility=hidden -fvisibility-inlines-hidden -MD -MT CMakeFiles/thing.dir/src/main.cpp.o -MF CMakeFiles/thing.dir/src/main.cpp.o.d @CMakeFiles/thing.dir/src/main.cpp.o.modmap -o CMakeFiles/thing.dir/src/main.cpp.o -c /home/troy/github/paravision/tmp/src/main.cpp
In module 'pollcoro' imported from /home/troy/github/paravision/tmp/src/main.cpp:8:
In module 'pollcoro:mutex' imported from /home/troy/github/paravision/tmp/build/_deps/pollcoro-src/src/pollcoro.cppm:46:
/nix/store/y28c83zz73yr4vwz1fsl4nsrn6yz5fj0-gcc-14.3.0/include/c++/14.3.0/x86_64-unknown-linux-gnu/bits/gthr-default.h:313:1: error: definition with same mangled name '_ZL18__gthread_active_pv' as another definition
  313 | __gthread_active_p (void)
      | ^
/nix/store/y28c83zz73yr4vwz1fsl4nsrn6yz5fj0-gcc-14.3.0/include/c++/14.3.0/x86_64-unknown-linux-gnu/bits/gthr-default.h:313:1: note: previous definition is here
  313 | __gthread_active_p (void)
      | ^
1 error generated.
ninja: build stopped: subcommand failed.
error: Recipe `build` failed on line 2 with exit code 1

helper.cppm

module;

#include <utility>

export module helper;

import pollcoro;

export namespace helper {
template<typename T>
auto async(T value) {
    auto [awaitable, setter] = pollcoro::single_event<T>();

    setter.set(std::move(value));

    return std::move(awaitable);
}

} // helper

pollcoro is https://github.com/troyKomodo/pollcoro

The following 2 fix the error:

• adding #include <thread> to main.cpp
• removing the template on helper in helper.cppm

Reproducible repo: https://github.com/TroyKomodo/cxx-compiler-bug

Possibly related:
#137533

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Troy Benson (TroyKomodo)

Details

`main.cpp` ```cxx import pollcoro; import helper;

int main() {
pollcoro::block_on(helper::async(0));
}

This fails to compile with the following:

```sh
/nix/store/90lsdrdffwgf3kjd4sdxjchchg44cr02-clang-wrapper-20.1.8/bin/clang++   -O3 -O3 -DNDEBUG -std=c++20 -fvisibility=hidden -fvisibility-inlines-hidden -MD -MT CMakeFiles/thing.dir/src/main.cpp.o -MF CMakeFiles/thing.dir/src/main.cpp.o.d @<!-- -->CMakeFiles/thing.dir/src/main.cpp.o.modmap -o CMakeFiles/thing.dir/src/main.cpp.o -c /home/troy/github/paravision/tmp/src/main.cpp
In module 'pollcoro' imported from /home/troy/github/paravision/tmp/src/main.cpp:8:
In module 'pollcoro:mutex' imported from /home/troy/github/paravision/tmp/build/_deps/pollcoro-src/src/pollcoro.cppm:46:
/nix/store/y28c83zz73yr4vwz1fsl4nsrn6yz5fj0-gcc-14.3.0/include/c++/14.3.0/x86_64-unknown-linux-gnu/bits/gthr-default.h:313:1: error: definition with same mangled name '_ZL18__gthread_active_pv' as another definition
  313 | __gthread_active_p (void)
      | ^
/nix/store/y28c83zz73yr4vwz1fsl4nsrn6yz5fj0-gcc-14.3.0/include/c++/14.3.0/x86_64-unknown-linux-gnu/bits/gthr-default.h:313:1: note: previous definition is here
  313 | __gthread_active_p (void)
      | ^
1 error generated.
ninja: build stopped: subcommand failed.
error: Recipe `build` failed on line 2 with exit code 1

helper.cppm

module;

#include <utility>

export module helper;

import pollcoro;

export namespace helper {
template<typename T>
auto async(T value) {
    auto [awaitable, setter] = pollcoro::single_event<T>();

    setter.set(std::move(value));

    return std::move(awaitable);
}

} // helper

pollcoro is https://github.com/troyKomodo/pollcoro

The following 2 fix the error:

• adding #include <thread> to main.cpp
• removing the template on helper in helper.cppm

Reproducible repo: https://github.com/TroyKomodo/cxx-compiler-bug

Possibly related:
#137533

[ChuanqiXu9]

Could you try the latest trunk?

[pinskia]

Libstdc++ was not fixed until gcc 15 for this issue:
https://gcc.gnu.org/cgit/gcc/commit/?id=6a4d1c374eed177eceb12a50f3b25bd20f8b347a

[TroyKomodo]

Could you try the latest trunk?

Of clang? is there a way to get precompiled binaries or should i just compile it from src?

Libstdc++ was not fixed until gcc 15 for this issue: gcc.gnu.org/cgit/gcc/commit?id=6a4d1c374eed177eceb12a50f3b25bd20f8b347a

I see, perhaps I am missing something here. My understanding was that if someone imports a module which includes other headers the import shouldn't be required to add any includes to their import location. Why does the ODR violation disappear when I include it directly in my code or not use templates?

Using gcc15 libstdc++ does solve the issue.

[ChuanqiXu9]

Could you try the latest trunk?

Of clang? is there a way to get precompiled binaries or should i just compile it from src?

If you're on Ubuntu or Debian, you can look at https://apt.llvm.org/ Otherwise you need to compile it from src.

I see, perhaps I am missing something here. My understanding was that if someone imports a module which includes other headers the import shouldn't be required to add any includes to their import location. Why does the ODR violation disappear when I include it directly in my code or not use templates?

Using gcc15 libstdc++ does solve the issue.

I feel this may be a clang bug instead of TU-local things. It will be much more helpful to provide a minimal reproducer without any STL things

[ChuanqiXu9]

BTW, we prefer smaller reproducer. The smaller the reproducer is, it is easier to be fixed. Generally we want a reproducer without any dependencies. The best is without any dependencies with the std lib too. e.g., #130080

[TroyKomodo]

I will try get a minimal, it's pretty tricky. I only run into it very rarely.

[ChuanqiXu9]

I will try get a minimal, it's pretty tricky. I only run into it very rarely.

If you're able to compile from source, I suggest you to compile trunk with #172430 to verify if this is the same issue with others.