<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/121278>121278</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Incorrect handling of incomplete types in `std::derived_from`
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          cor3ntin
      </td>
    </tr>
</table>

<pre>
    Per https://eel.is/c++draft/meta#lib:is_base_of,  

> Derived shall be a complete type

This is generally enforced by `__is_base_of` (the builtin used to implement `is_base_of_v`)

However, in libc++, `derived_from` is defined as

https://github.com/llvm/llvm-project/blob/cb1ad985b53c87b53974e37bba60129acb294f0d/libcxx/include/__concepts/derived_from.h#L27

And because the constraints are only enforced in `__is_base_of` which is not in the immediate context of the `is_base_of_v` constraint, substituting an incomplete class in  `is_base_of_v` does not result in a substitution failure in the immediate context (which should just cause `derived_from` to not be satisfied per https://eel.is/c++draft/temp.constr.atomic#3.sentence-2) but instead makes the program ill-formed, which i believe is non-conforming.

Note that GCC diverges in all cases. MSVC and EDG exposes what I believe to be the correct behavior.

I think there are 2 solutions:

- Modify `derived_from` to use `__is_base_of` directly (which is what libstdc++ and MS STL do)
- Add a require clause to `std::is_base_of_v` to check for completeness


https://godbolt.org/z/zGrKEKEao

</pre>
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