<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/87811>87811</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Document implementation-defined behavior with UCN escape sequences in C
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            documentation,
            c99
      </td>
    </tr>

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

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

<pre>
    A universal character name (UCN) is a kind of escape sequence. C99 6.4.4.4p10 has this to say: ... The value of an integer character constant containing more than one character (e.g., 'ab'), or containing a character or escape sequence that does not map to a single-byte execution character, is implementation-defined. ...

Clang [defines the implementation-defined behavior](https://github.com/llvm/llvm-project/blob/c7eede5d4c1c8e0c20337f3f3c9003f59fe5ccd5/clang/lib/Lex/LiteralSupport.cpp#L1734) in C to be the same as the [required behavior](https://eel.is/c++draft/lex.ccon#3.1.sentence-2) in C++.

We should document this outside of the source code.
</pre>
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