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    CWG Issue 2386</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="2386"></A><H4>2386.
  
<T>tuple_size</T> requirements for structured binding
</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>Stephan T. Lavavej
 &#160;&#160;&#160;

 <B>Date: </B>2018-09-19<BR>


<P>[Accepted as a DR at the February, 2019 meeting.]</P>

<P>According to 9.7 [<A href="https://wg21.link/dcl.struct.bind#4">dcl.struct.bind</A>] paragraph 4,</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.

</BLOCKQUOTE>

<P>However, the common idiom in the library is that SFINAE tests the
presence or absence of a <TT>value</TT> member rather than the
completeness of the class; see 22.4.7 [<A href="https://wg21.link/tuple.helper">tuple.helper</A>] paragraph
4. The core language requirement should be changed to match the common
library practice.</P>

<P><B>Proposed resolution (December, 2018):</B></P>

<P>Change 9.6 [<A href="https://wg21.link/dcl.fct.def#4">dcl.fct.def</A>] paragraph 4 as follows:</P>

<BLOCKQUOTE>

Otherwise, if the <I>qualified-id</I> <TT>std::tuple_size&lt;E&gt;</TT>
names a complete <INS>class</INS> type <INS>with a
member <TT>value</TT></INS>, the expression std::tuple_size&lt;E&gt;::value
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...

</BLOCKQUOTE>

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