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    <head>
      <base href="https://bugs.llvm.org/">
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - Headers containing just #error harm __has_include"
   href="https://bugs.llvm.org/show_bug.cgi?id=39749">39749</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Headers containing just #error harm __has_include
          </td>
        </tr>

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

        <tr>
          <th>Version</th>
          <td>unspecified
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>enhancement
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>All Bugs
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>bmerry@gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
          </td>
        </tr></table>
      <p>
        <div>
        <pre>It looks like the latest version of <experimental/optional>
(<a href="https://llvm.org/svn/llvm-project/libcxx/trunk/include/experimental/optional">https://llvm.org/svn/llvm-project/libcxx/trunk/include/experimental/optional</a>)
consists only of a #error to tell the user that it isn't supported anymore.

While a helpful error, it will presumably(*) break any code that uses
__has_include to detect whether this header is supported (as recommended by
N4480).

The same applies to <experimental/string_view>, and I'm guessing others
although I haven't checked.

(*) I haven't yet got my own build of libc++ trunk to verify experimentally, so
if there is some compiler magic to know that __has_include should be 0 for
these special cases then I'm less worried.</pre>
        </div>
      </p>


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