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    CWG Issue 1741</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="1741"></A><H4>1741.
  
odr-use of class object in lvalue-to-rvalue conversion
</H4>
<B>Section: </B>6.3&#160; [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++14
 &#160;&#160;&#160;

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

 <B>Date: </B>2013-08-21<BR><BR>


<A href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3903.html#CA28">N3690 comment
  CA&#160;28<BR></A>

<P>[Moved to DR at the February, 2014 meeting.]</P>



<P>According to 7.3.2 [<A href="https://wg21.link/conv.lval#2">conv.lval</A>] paragraph 2,</P>

<BLOCKQUOTE>

if the glvalue has a class type, the [lvalue-to-rvalue] conversion
copy-initializes a temporary of type <TT>T</TT> from the glvalue and the
result of the conversion is a prvalue for the temporary.

</BLOCKQUOTE>

<P>The implications of such a conversion for odr-use do not appear to have
been factored into 6.3 [<A href="https://wg21.link/basic.def.odr#3">basic.def.odr</A>] paragraph 3, which exempts
constant objects that are immediately lvalue-to-rvalue converted.  For
example, given</P>

<PRE>
  struct S { int n; };
  struct T {
    static constexpr S s = {};
  };
  void f(...);
  void g() { f(T::s); }
</PRE>

<P>Does this odr-use <TT>T::s</TT>, requiring it to have a definition,
because of binding it to the reference parameter of <TT>S</TT>'s
copy constructor?  How about</P>

<PRE>
  struct S { int n; };
  void f(...);
  void g() {
    constexpr S s = {};
    [] { f(s); };
  }
</PRE>

<P>Does <TT>s</TT> need to be captured?  There is implementation variance
on both these examples.</P>

<P><B>Proposed resolution (September, 2013):</B></P>

<P>Change 6.3 [<A href="https://wg21.link/basic.def.odr#3">basic.def.odr</A>] paragraph 3 as follows:</P>

<BLOCKQUOTE>

A variable <TT>x</TT> whose name appears as a potentially-evaluated
expression <TT>ex</TT> is <I>odr-used</I> unless <DEL><TT>x</TT> satisfies
the requirements for appearing in a constant expression
(7.7 [<A href="https://wg21.link/expr.const">expr.const</A>])</DEL> <INS>applying the lvalue-to-rvalue
conversion (7.3.2 [<A href="https://wg21.link/conv.lval">conv.lval</A>]) to <TT>x</TT> yields a constant
expression (7.7 [<A href="https://wg21.link/expr.const">expr.const</A>]) that does not invoke any non-trivial
functions</INS> and, if <TT>x</TT> is an object, <TT>ex</TT> is an element
of the set of potential results of an expression <TT>e</TT>, where either
the lvalue-to-rvalue conversion (7.3.2 [<A href="https://wg21.link/conv.lval">conv.lval</A>]) is applied
to <TT>e</TT>, or <TT>e</TT> is a discarded-value expression (
Clause 7 [<A href="https://wg21.link/expr">expr</A>]). <TT>this</TT> is odr-used...

</BLOCKQUOTE>

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