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    CWG Issue 953</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="953"></A><H4>953.
  
Rvalue references and function viability
</H4>
<B>Section: </B>12.2.4.2.5&#160; [<A href="https://wg21.link/over.ics.ref">over.ics.ref</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD2
 &#160;&#160;&#160;

 <B>Submitter: </B>Mike Miller
 &#160;&#160;&#160;

 <B>Date: </B>18 August, 2009<BR>


<P>[Voted into WP at March, 2010 meeting.]</P>

<P>According to 12.2.4.2.5 [<A href="https://wg21.link/over.ics.ref">over.ics.ref</A>] paragraphs 3-4,</P>

<BLOCKQUOTE>

<P>A standard conversion sequence cannot be formed if it requires
binding an lvalue reference to non-const to an rvalue (except when
binding an implicit object parameter; see the special rules for that
case in 12.2.2 [<A href="https://wg21.link/over.match.funcs">over.match.funcs</A>]).  [<I>Note:</I> this means, for
example, that a candidate function cannot be a viable function if it
has a non-<TT>const</TT> lvalue reference parameter (other than the
implicit object parameter) and the corresponding argument is a
temporary or would require one to be created to initialize the lvalue
reference (see 9.5.4 [<A href="https://wg21.link/dcl.init.ref">dcl.init.ref</A>]). &#8212;<I>end note</I>]</P>

<P>Other restrictions on binding a reference to a particular argument
that are not based on the types of the reference and the argument do
not affect the formation of a standard conversion sequence,
however.</P>

</BLOCKQUOTE>

<P>Because this section does not mention attempting to bind an
rvalue reference to an lvalue, such a &#8220;conversion sequence&#8221;
might be selected as best and result in an ill-formed program.  It
should, instead, be treated like trying to bind an lvalue reference
to non-const to an rvalue, making the function non-viable.</P>

<P><B>Proposed resolution (November, 2009):</B></P>

<P>Change 12.2.4.2.5 [<A href="https://wg21.link/over.ics.ref#3">over.ics.ref</A>] paragraph 3 as follows:</P>

<BLOCKQUOTE>

<DEL>A</DEL> <INS>Except for an implicit object parameter, for
which see 12.2.2 [<A href="https://wg21.link/over.match.funcs">over.match.funcs</A>], a</INS> standard
conversion sequence cannot be formed if it requires binding an
lvalue reference to non-<TT>const</TT> to an rvalue <DEL>(except
when binding an implicit object parameter; see the special rules
for that case in 12.2.2 [<A href="https://wg21.link/over.match.funcs">over.match.funcs</A>])</DEL> <INS>or
binding an rvalue reference to an lvalue</INS>.  [<I>Note:</I>
this means, for example, that a candidate function cannot be a
viable function if it has a non-<TT>const</TT> lvalue reference
parameter (other than the implicit object parameter) and the
corresponding argument is a temporary or would require one to be
created to initialize the lvalue reference (see
9.5.4 [<A href="https://wg21.link/dcl.init.ref">dcl.init.ref</A>]). &#8212;<I>end note</I>]

</BLOCKQUOTE>

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