<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/85283>85283</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc][math][c23] Implement C23 math functions nextup, nextdown.
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            good first issue
      </td>
    </tr>

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

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

<pre>
    These functions are defined in C23 standard: section 7.12.11.5 and 7.12.11.6 in https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf

Could simply be implemented using `fputil::nextafter` template.

Need to support the following data types: float, double, long double, float128.
</pre>
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