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    CWG Issue 2313</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="2313"></A><H4>2313.
  
Redeclaration of structured binding reference variables
</H4>
<B>Section: </B>9.7&#160; [<A href="https://wg21.link/dcl.struct.bind">dcl.struct.bind</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD5
 &#160;&#160;&#160;

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

 <B>Date: </B>2016-08-12<BR>


<P>[Accepted as a DR at the November, 2017 meeting.]</P>



<P>According to the current rules for structured binding declarations,
the user-defined case declares the bindings as variables of reference
type. This presumably makes an example like the following valid:</P>

<PRE>
   auto [a] = std::tuple&lt;int&gt;(0);
   extern int &amp;&amp;a; //<SPAN CLASS="cmnt"> ok, redeclaration, could even be in a different TU </SPAN>
</PRE>

<P>This seems unreasonable, especially in light of the fact that it only
works for the user-defined case and not the built-in case (where the
bindings are not modeled as references).</P>

<P><B>Proposed resolution (August, 2017):</B></P>

<P>Change 9.7 [<A href="https://wg21.link/dcl.struct.bind#3">dcl.struct.bind</A>] paragraph 3 as follows:</P>

<BLOCKQUOTE>

Otherwise, if the <I>qualified-id</I> <TT>std::tuple_size&lt;E&gt;</TT>
names a complete type, the
expression <TT>std::tuple_size&lt;E&gt;::value</TT> shall be a well-formed
integral constant expression and the number of elements in
the <I>identifier-list</I> shall be equal to the value of that
expression. The <I>unqualified-id</I> <TT>get</TT> is looked up in the
scope of <TT>E</TT> by class member access lookup
(_N4868_.6.5.6 [<A href="https://wg21.link/basic.lookup.classref">basic.lookup.classref</A>]), and if that finds at least one declaration,
the initializer is <TT>e.get&lt;i&gt;()</TT>.  Otherwise, the initializer
is <TT>get&lt;i&gt;(e)</TT> where <TT>get</TT> is looked up in the associated
namespaces (6.5.4 [<A href="https://wg21.link/basic.lookup.argdep">basic.lookup.argdep</A>]). In either case,
<TT>get&lt;i&gt;</TT> is interpreted as a <I>template-id</I>. [<I>Note:</I>
Ordinary unqualified lookup (6.5.3 [<A href="https://wg21.link/basic.lookup.unqual">basic.lookup.unqual</A>]) is not performed.
&#8212;<I>end note</I>] In either case, <TT>e</TT> is an lvalue if the type
of the entity <TT>e</TT> is an lvalue reference and an xvalue
otherwise. Given the type <TT>T</TT><SUB><I>i</I></SUB> designated
by <TT>std::tuple_element&lt;i, E&gt;::type</TT> ,
<DEL>each <TT>v</TT><SUB><I>i</I></SUB> is a variable</DEL>
<INS>variables are introduced with unique
names <TT>r</TT><SUB><I>i</I></SUB></INS> of type &#8220;reference
to <TT>T</TT><SUB><I>i</I></SUB> &#8221; initialized with the initializer
<INS>(9.5.4 [<A href="https://wg21.link/dcl.init.ref">dcl.init.ref</A>])</INS>, where the reference is an lvalue
reference if the initializer is an lvalue and an rvalue reference
otherwise<INS>. Each <TT>v</TT><SUB><I>i</I></SUB> is the name of an lvalue
of type <TT>T</TT><SUB><I>i</I></SUB> that refers to the object bound
to <TT>r</TT><SUB><I>i</I></SUB></INS>; the referenced type
is <TT>T</TT><SUB><I>i</I></SUB>.

</BLOCKQUOTE>

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