<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/98199>98199</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            missing diagnostic for reopening an ambiguous namespace
        </td>
    </tr>

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

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

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

<pre>
    ```c++
namespace internal {
  void Bar() {}
}  // namespace internal

inline namespace stay_away {
namespace internal {}  // namespace internal
}  // namespace stay_away

namespace internal {
   void Bar() {}
}
```

The final reopening of namespace `internal` should be invalid: lookup for the name `internal` is ambiguous. But clang accepts this and instead creates a second `::internal` namespace (a third `internal` namespace overall), and treats the two `Bar`s as being unrelated. (Ultimately this fails in IR generation due to a mangled name collision.)

Presumably we're incorrectly suppressing diagnostics in the lookup for the name `internal`.
</pre>
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