<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63900>63900</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS causes bad diagnostics
        </td>
    </tr>

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

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

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

<pre>
    I'm upgrading Meta's Android codebase to C++20, and I'm building with `-D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS` globally to ease the initial transition. That somehow causes incomplete backtraces in some diagnostics. See https://godbolt.org/z/TMKWrzd59 as an example, where the error message has no reference to the actual problematic `emplace_back` call, whereas either building with `-std=c++17` (https://godbolt.org/z/1oq4q4Pb8) or removing `-D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS` (https://godbolt.org/z/6v3f7aKor) gives the expected full error.

Can we improve this? I expect it's not uncommon to use `-D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS` and other definitions during the C++20 upgrade process, and there are certain errors that are exposed only when building with C++20 (e.g. https://godbolt.org/z/cEMvdrca8), so this issue makes it much harder to figure out the cause of those errors.
</pre>
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