[llvm-bugs] [Bug 126500] [clang, CUDA, modules] Issues Compiling a CUDA Source File with C++ Modules

[LLVM Bugs via llvm-bugs]

Issue	126500
Summary	[clang, CUDA, modules] Issues Compiling a CUDA Source File with C++ Modules
Labels	clang
Assignees
Reporter	LukasBreitwieser

    Hi All,

I ran into issues compiling a simple CUDA source file  that includes a header (see `mymodule.h`) if modules are enabled.  Further investigation showed that the issue persists even after removing all CUDA related code (see `main.cu`). Compiling it with `c++20` errors out due to an ambiguous call to `operator new`, while the compiler crashes with `c++17`. Here the compilation logs and generated  `*.pcm` files: [logs-and-module-cache.tar.gz](https://github.com/user-attachments/files/18732220/logs-and-module-cache.tar.gz)

File `main.cu`
```cpp
#include "mymodule.h"
#include <iostream>

int main() {
  std::cout << GetMessage() << std::endl;
  // Problem shows up even with all CUDA related code removed.
  return 0;
}
```

File `mymodule.h`
```cpp
#ifndef MYMODULE_H
#define MYMODULE_H
#include <string>
// uncommenting the following line somehow "fixes" the issue.
//#include <iostream> 

inline std::string GetMessage() { 
 return "Hello world!";
}

#endif // MYMODULE_H
```

File `module.modulemap`
```
module mymodule {
  header "mymodule.h"
  export *
}
```

Compiler invocation:
```bash
clang++ \
  -v \
  -std=c++20 \
  -x cuda \
  --cuda-gpu-arch=sm_86 \
  -lcuda \
  -lcudart \
 -fmodules \
  -fmodule-map-file=module.modulemap \
 -fmodules-cache-path=/tmp/clang-modules \
  -o main \
 main.cu
```

LLVM commit
```
commit 2b55ef187cb6029eed43d7f4c0a3640c32691b31 (HEAD -> main, origin/main, origin/HEAD)
Author: Florian Hahn <f...@fhahn.com>
Date:   Wed Jan 29 10:50:01 2025 +0000

    [VPlan] Add helper to run VPlan passes, verify after run (NFC). (#123640)
...
```

LLVM configuration
```bash
cmake -G Ninja \
    -DCMAKE_BUILD_TYPE=Release \
 -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
    -DLLVM_ENABLE_PROJECTS="clang" \
    -DLLVM_TARGETS_TO_BUILD="host;NVPTX" \
 <path-to-llvm-project-dir>/llvm
```

System Info
- OS: `Ubuntu 20.04.1 LTS`
- Compiler used to compile llvm: `g++ (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0`
- CUDA: `12.6`
- GPU: single NVIDIA Geforce RTX 3060 with compute capability 8.6

I am happy to provide more information if needed.
Thank you in advance for taking a look and any hints on how to best debug and fix it.

Best,
Lukas

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