<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/140448>140448</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++][test] `flat_set` std test uses `EmplaceUnsafeContainer` which is non-standard
        </td>
    </tr>

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

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

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

<pre>
    `EmplaceUnsafeContainer` is used in tests for `flat_set` and `flat_map` to test the ability to have basic exception guarantee when underlying container throws on `insert` / `emplace` of a single element.

https://github.com/llvm/llvm-project/blob/ffb9bbfd0745dc22e1fd6edd7b62f72b91f4f6de/libcxx/test/std/containers/container.adaptors/flat_helpers.h#L122

Per [[container.reqmts]/66](https://eel.is/c++draft/container.reqmts#66), a sequence container should provide strong exception guarantee in this case.

So supporting underlying  containers that don't provide that is nonstandard extension, and should be tested in libcxx test suite instead.
</pre>
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