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    <body><span class="vcard"><a class="email" href="mailto:dblaikie@gmail.com" title="David Blaikie <dblaikie@gmail.com>"> <span class="fn">David Blaikie</span></a>
</span> changed
          <a class="bz_bug_link 
          bz_status_RESOLVED  bz_closed"
   title="RESOLVED INVALID - Rejects valid: reading of an inactive union member that shares common initial sequence with the active one"
   href="https://bugs.llvm.org/show_bug.cgi?id=43921">bug 43921</a>
          <br>
             <table border="1" cellspacing="0" cellpadding="8">
          <tr>
            <th>What</th>
            <th>Removed</th>
            <th>Added</th>
          </tr>

         <tr>
           <td style="text-align:right;">Status</td>
           <td>NEW
           </td>
           <td>RESOLVED
           </td>
         </tr>

         <tr>
           <td style="text-align:right;">Resolution</td>
           <td>---
           </td>
           <td>INVALID
           </td>
         </tr>

         <tr>
           <td style="text-align:right;">CC</td>
           <td>
                
           </td>
           <td>dblaikie@gmail.com
           </td>
         </tr></table>
      <p>
        <div>
            <b><a class="bz_bug_link 
          bz_status_RESOLVED  bz_closed"
   title="RESOLVED INVALID - Rejects valid: reading of an inactive union member that shares common initial sequence with the active one"
   href="https://bugs.llvm.org/show_bug.cgi?id=43921#c1">Comment # 1</a>
              on <a class="bz_bug_link 
          bz_status_RESOLVED  bz_closed"
   title="RESOLVED INVALID - Rejects valid: reading of an inactive union member that shares common initial sequence with the active one"
   href="https://bugs.llvm.org/show_bug.cgi?id=43921">bug 43921</a>
              from <span class="vcard"><a class="email" href="mailto:dblaikie@gmail.com" title="David Blaikie <dblaikie@gmail.com>"> <span class="fn">David Blaikie</span></a>
</span></b>
        <pre>8.20 [expr.const] p2

"An expression e is a core constant expression unless the evaluation of e,
following the rules of the abstract machine (4.6), would evaluate one of the
following expressions:
...
* an lvalue-to-rvalue conversion (7.1) that is applied to a glvalue that refers
to a non-active member of a union or a subobject thereof;"

It's not undefined behavior, but it isn't allowed in a constant expression.
(the wording of the error message hints at this by saying "is not allowed in a
constant expression"</pre>
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