<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/76620>76620</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] Feature guards test too late
        </td>
    </tr>

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

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

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

<pre>
    In this example:
```
In file included from weird.cc:1:
In file included from /home/luca/wasm-compilers/build/sysroot/include/c++/v1/thread:93:
In file included from /home/luca/wasm-compilers/build/sysroot/include/c++/v1/__thread/jthread.h:18:
/home/luca/wasm-compilers/build/sysroot/include/c++/v1/__thread/thread.h:55:34: error: unknown type name 'condition_variable'
   55 |   void notify_all_at_thread_exit(condition_variable*, mutex*);
      | ^
```

The header `thread` validates the `_LIBCPP_HAS_NO_THREADS` macro _after_ including its granularized headers. We should validate the top-level header before starting to include unguarded headers. This should be done for all feature macros.
</pre>
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