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    CWG Issue 2140</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="2140"></A><H4>2140.
  
Lvalue-to-rvalue conversion of <TT>std::nullptr_t</TT>
</H4>
<B>Section: </B>7.3.2&#160; [<A href="https://wg21.link/conv.lval">conv.lval</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD4
 &#160;&#160;&#160;

 <B>Submitter: </B>Richard Smith
 &#160;&#160;&#160;

 <B>Date: </B>2015-06-12<BR>


<P>[Adopted at the February, 2016 meeting.]</P>



<P>The current rules in 7.3.2 [<A href="https://wg21.link/conv.lval#2">conv.lval</A>] paragraph 2 do
not require fetching the value in memory of an object of type
<TT>std::nullpt_t</TT> in order to produce its prvalue.
This choice has implications that may not have been
considered for questions like whether use of
a <TT>std::nullptr_t</TT> that is an inactive member of a
union results in undefined behavior or whether a volatile
<TT>std::nullptr_t</TT> variable in a discarded-value
expression produces a side effect.</P>

<P><B>Proposed resolution (January, 2016):</B></P>

<P>Change 7.3.2 [<A href="https://wg21.link/conv.lval#2.3">conv.lval</A>] bullet 2.3 as follows:</P>

<BLOCKQUOTE>

<P>...In all other cases, the result of the conversion is
determined according to the following rules:</P>

<UL>
<LI><P>If <TT>T</TT> is (possibly cv-qualified)
<TT>std::nullptr_t</TT>, the result is a null pointer constant
(7.3.12 [<A href="https://wg21.link/conv.ptr">conv.ptr</A>]). <INS>[<I>Note:</I> Since no value
is fetched from memory, there is no side effect for a volatile
access (6.10.1 [<A href="https://wg21.link/intro.execution">intro.execution</A>]), and an inactive member of a
union (11.5 [<A href="https://wg21.link/class.union">class.union</A>]) may be accessed.
&#8212;<I>end note</I>]</INS>
</P></LI>

<LI><P>...</P></LI>

</UL>

</BLOCKQUOTE>

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