[C++20][Modules] Compile Error with static template function.

[jaehyuck0103]

Hello. LLVM.

I encountered an incomprehensible compilation error related to a static template function while using C++ modules.
I provide the minimal code below to reproduce the issue.
It's my first bug report to LLVM. Please let me know if there's anything wrong.

Environment

Ubuntu clang version 21.0.0 (++20250305083430+f4878cb91612-1exp120250305083603.767)
Target: x86_64-pc-linux-gnu

Codes

// main.cpp
#include "dummy_cuda_runtime.h"
#include <cstdint>

import MyModule;

int main() {

    uint16_t *buffer;
    cudaMalloc(&buffer, 100 * sizeof(uint16_t));  // No Compile Error

    MyClass aaa;

    cudaMalloc(&buffer, 100 * sizeof(uint16_t));  // Compile Error
}

// a.cppm
module;

#include "dummy_cuda_runtime.h"
#include <cstdint>

export module MyModule;

export class MyClass {

  public:
    MyClass() {
        // If below line is not in constructor, No Compile Error
        cudaMalloc(&buffer_, 100 * sizeof(uint16_t));
    }

    uint16_t *buffer_;
};

// dummy_cuda_runtime.h
template <class T> static void cudaMalloc(T **devPtr, int size) {
    // dummy
}

# CMakeLists.txt
cmake_minimum_required(VERSION 3.30)

set(CMAKE_C_COMPILER clang)
set(CMAKE_CXX_COMPILER clang++)

project(modules-example)

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

set(CMAKE_CXX_STANDARD 20)

add_executable(demo main.cpp)
target_sources(demo PUBLIC FILE_SET all_my_modules TYPE CXX_MODULES FILES
                           a.cppm)

mkdir build
cd build
cmake .. -G Ninja
ninja

FAILED: CMakeFiles/demo.dir/main.cpp.o
/usr/lib/llvm-21/bin/clang++   -std=gnu++20 -MD -MT CMakeFiles/demo.dir/main.cpp.o -MF CMakeFiles/demo.dir/main.cpp.o.d @CMakeFiles/demo.dir/main.cpp.o.modmap -o CMakeFiles/demo.dir/main.cpp.o -c /home/jae/Projects/cpp_modules3/main.cpp
/home/jae/Projects/cpp_modules3/main.cpp:15:5: error: no matching function for call to 'cudaMalloc'
   15 |     cudaMalloc(&buffer, 100 * sizeof(uint16_t)); // Compile Error
      |     ^~~~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.

[frederick-vs-ja]

This is somehow like exposing a TU-local (translation-unit-local) entity (and should be consistently rejected when compiling a.cppm if so). See also #112294.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Jae-Hyuck Park (jaehyuck0103)

Hello. LLVM.

I encountered an incomprehensible compilation error related to a static template function while using C++ modules.
I provide the minimal code below to reproduce the issue.
It's my first bug report to LLVM. Please let me know if there's anything wrong.

Environment

Ubuntu clang version 21.0.0 (++20250305083430+f4878cb91612-1exp120250305083603.767)
Target: x86_64-pc-linux-gnu

Codes

// main.cpp
#include "dummy_cuda_runtime.h"
#include <cstdint>

import MyModule;

int main() {

    uint16_t *buffer;
    cudaMalloc(&buffer, 100 * sizeof(uint16_t));  // No Compile Error

    MyClass aaa;

    cudaMalloc(&buffer, 100 * sizeof(uint16_t));  // Compile Error
}

// a.cppm
module;

#include "dummy_cuda_runtime.h"
#include <cstdint>

export module MyModule;

export class MyClass {

  public:
    MyClass() {
        // If below line is not in constructor, No Compile Error
        cudaMalloc(&buffer_, 100 * sizeof(uint16_t));
    }

    uint16_t *buffer_;
};

// dummy_cuda_runtime.h
template <class T> static void cudaMalloc(T **devPtr, int size) {
    // dummy
}

# CMakeLists.txt
cmake_minimum_required(VERSION 3.30)

set(CMAKE_C_COMPILER clang)
set(CMAKE_CXX_COMPILER clang++)

project(modules-example)

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

set(CMAKE_CXX_STANDARD 20)

add_executable(demo main.cpp)
target_sources(demo PUBLIC FILE_SET all_my_modules TYPE CXX_MODULES FILES
                           a.cppm)

mkdir build
cd build
cmake .. -G Ninja
ninja

FAILED: CMakeFiles/demo.dir/main.cpp.o
/usr/lib/llvm-21/bin/clang++   -std=gnu++20 -MD -MT CMakeFiles/demo.dir/main.cpp.o -MF CMakeFiles/demo.dir/main.cpp.o.d @<!-- -->CMakeFiles/demo.dir/main.cpp.o.modmap -o CMakeFiles/demo.dir/main.cpp.o -c /home/jae/Projects/cpp_modules3/main.cpp
/home/jae/Projects/cpp_modules3/main.cpp:15:5: error: no matching function for call to 'cudaMalloc'
   15 |     cudaMalloc(&amp;buffer, 100 * sizeof(uint16_t)); // Compile Error
      |     ^~~~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.

[ChuanqiXu9]

This is a bug. In main you've already included the static one. Could you test if -fmodules-reduced-bmi is helpful here?

[jaehyuck0103]

@ChuanqiXu9
Good...! -fmodules-reduced-bmi removes the error.

[ChuanqiXu9]

This is somehow like exposing a TU-local (translation-unit-local) entity (and should be consistently rejected when compiling a.cppm if so). See also #112294.

It didn't expose it. Since the member function in modules are not implicitly inline. So it is fine.

[ChuanqiXu9]

@ChuanqiXu9 Good...! -fmodules-reduced-bmi removes the error.

OK. Reduced BMI is going to be the default.