<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/64682>64682</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Add diagnostic for undocumented builtins
        </td>
    </tr>

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

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

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

<pre>
    Non-user facing builtins could be too fragile to be used by regular users, leading to crashes.
Builtins for coroutines are one example of that (see #60863 and linked issues).
@AaronBallman proposed a generic diagnostic to catch this: https://github.com/llvm/llvm-project/issues/60863#issuecomment-1677257472
Possible phrasing: `__builtin_xxx is not documented and can crash the compiler if used incorrectly.`
False-positives are going to be suppressed by generic mechanism of suppressing diagnostics for system headers (`-isystem`).
I wonder, though, how this is going to impact compile times.

Since this is intentionally a low-effort mitigation for low-priority crashes, should it be added to the pool of good first issues?
</pre>
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