<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/107191>107191</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] Invalid character literal accepted in C++ during preprocessing after P2621R2.
        </td>
    </tr>

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

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

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

<pre>
    After [P2621R2](https://github.com/cplusplus/draft/commit/47dbc585f6297f03518e81bfe59d4b414ac64fb2), which is a defect report, the following program is ill-formed in C++ (UB beforehand):

```
#define X '\N'
int main() {}
```

This is ill-formed already in translation phase 2 when lexing into preprocessing tokens, because `\N` not followed by `{` can't be a _named-universal-character_ an `\N` can also not begin any _escape-sequence_. Therefore `'\N'` can't be a _character-literal_ and `'` will form a single-character preprocessing token by itself. [[lex.pptoken]/2](https://eel.is/c++draft/lex#pptoken-2.sentence-4) makes this ill-formed (UB before P2621R2).

The C++ status page claims that P2621R2 is already supported, but Clang compiles this without diagnostic (https://godbolt.org/z/WxvcfPj8a).
</pre>
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