<html>
    <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 - valueless_by_exception() should unconditionally return false if all the constructors are noexcept"
   href="https://bugs.llvm.org/show_bug.cgi?id=39072">39072</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>valueless_by_exception() should unconditionally return false if all the constructors are noexcept
          </td>
        </tr>

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

        <tr>
          <th>Version</th>
          <td>7.0
          </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>antoshkka@gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
          </td>
        </tr></table>
      <p>
        <div>
        <pre>Right now the variant::valueless_by_exception() is defined like that:

  constexpr bool valueless_by_exception() const noexcept {
    return index() == variant_npos;
  }


Above code is quite complex for the optimizer. It may be profitable to detect
variant types that never have valueless_by_exception() state and re-implement
the function:

  constexpr bool valueless_by_exception() const noexcept {
    return !__always_valid::value /*compile-time constant*/ && index() ==
variant_npos;
  }


Such change would improve assembly for multiple functions including
comparisons, swap, get and constructors.</pre>
        </div>
      </p>


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