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    CWG Issue 2982</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="2982"></A><H4>2982.
  
Deduction in <I>type-constraint</I>s
</H4>
<B>Section: </B>13.10.3.7&#160; [<A href="https://wg21.link/temp.deduct.decl">temp.deduct.decl</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD7
 &#160;&#160;&#160;

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

 <B>Date: </B>2023-03-30<BR>


<P>[Accepted as a DR at the June, 2025 meeting.]</P>



<P>Consider:</P>

<PRE>
  template&lt;typename T, typename U&gt; concept C = true;
  template&lt;typename T&gt; C&lt;T&gt; auto f() { return 0; }
  template C&lt;int&gt; auto f();
</PRE>

<P>Is that valid?  Or is the following needed:</P>

<PRE>
  template C&lt;int&gt; auto f&lt;int&gt;();
</PRE>
<P>The specification neither discusses deduction from a
<I>type-constraint</I> nor treating it as a non-deduced context.</P>

<P><B>Proposed resolution (no objections during CWG reflector review starting 2025-03-18) (approved by CWG 2025-06-20):</B></P>

<P>Change in 13.10.3.6 [<A href="https://wg21.link/temp.deduct.type#5">temp.deduct.type</A>] paragraph 5 as follows:</P>

<BLOCKQUOTE>

The non-deduced contexts are:
<UL>
<LI>The <I>nested-name-specifier</I> of a type that was specified
using a <I>qualified-id</I>.</LI>
<LI>A <I>pack-index-specifier</I> or a <I>pack-index-expression</I>.</LI>
<LI class="ins">A <I>type-constraint</I>.</LI>
<LI>The <I>expression</I> of a <I>decltype-specifier</I>.</LI>
<LI>...</LI>
</UL>

</BLOCKQUOTE>

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