<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/129894>129894</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Regression] [Modules] Forward declaration cannot be attached to other modules
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          yujincheng08
      </td>
    </tr>
</table>

<pre>
    ### Min repro
Godbolt Link https://godbolt.org/z/hKTo45oro


### Additional notes
The same codes compile on clang 18: https://godbolt.org/z/185WM1arf

Maybe @ChuanqiXu9?
</pre>
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