<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/54269>54269</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [C++20] [Modules] Multiple definition in partitions
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            c++20,
            clang:modules
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          ChuanqiXu9
      </td>
    </tr>
</table>

<pre>
    The following example would cause a multiple definition bug:
```
// mod.cppm
export module mod;
export import :parta;
export int useimpl() {
    return a();
}

// parta.cppm
export module mod:parta;

export int a() {
    return 3;
}

// main.cpp
#include <iostream>
import mod;

int main() {
    std::cout << useimpl() << "\n";
}
```

And run the following command:
```
clang++ -std=c++20 --precompile -fprebuilt-module-path=. -c parta.cppm -o mod-parta.pcm
clang++ -std=c++20 --precompile -fprebuilt-module-path=. -c mod.cppm -o mod.pcm -fmodule-file=mod-parta.pcm
clang++ -std=c++20 -fprebuilt-module-path=. main.cpp mod.pcm mod-parta.pcm
```

It would show a multiple definition error:
```
/usr/bin/ld: /tmp/mod-b8409e.o: in function `a()':
mod.pcm:(.text+0x0): multiple definition of `a()'; /tmp/main-624958.o:main.cpp:(.text+0x0): first defined here
/usr/bin/ld: /tmp/mod-parta-31e801.o: in function `a()':
mod-parta.pcm:(.text+0x0): multiple definition of `a()'; /tmp/main-624958.o:main.cpp:(.text+0x0): first defined here
```

@urnathan @iains 
</pre>
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