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    CWG Issue 2339</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="2339"></A><H4>2339.
  
Underspecified template arguments in structured bindings
</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>Daveed Vandevoorde
 &#160;&#160;&#160;

 <B>Date: </B>2017-03-29<BR>


<P>[Accepted as a DR at the June, 2018 (Rapperswil) meeting.]</P>

<P>According to 9.7 [<A href="https://wg21.link/dcl.struct.bind#3">dcl.struct.bind</A>] paragraph 3,</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>.

</BLOCKQUOTE>

<P>It is not clear what <TT>i</TT> is in this description, and in
particular, its type is not specified.</P>

<P><B>Proposed resolution, March, 2018:</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. <INS>Let <TT>i</TT> be an index prvalue of type
<TT>std::size_t</TT> corresponding to <TT>v</TT><I><SUB>i</SUB></I>.</INS>
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...

</BLOCKQUOTE>

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