<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/99296>99296</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang shouldn't accept extraneous template headers as an extension
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang:frontend
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          zyn0217
      </td>
    </tr>
</table>

<pre>
    (This is uncovered while working on https://github.com/llvm/llvm-project/pull/75697)

https://github.com/llvm/llvm-project/commit/65911498eff34ac4fc2f9c250efd49aa51c35f85 results in an extension where Clang accepts templates with redundant template headers. However, this is clearly ill-formed and no other compilers would accept it. I think we should probably remove the extension in `Sema::MatchTemplateParametersToScopeSpecifier()` and reject such templates in the end.

https://compiler-explorer.com/z/TcPGjWMGM
</pre>
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