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    CWG Issue 2806</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="2806"></A><H4>2806.
  
Make a <I>type-requirement</I> a type-only context
</H4>
<B>Section: </B>13.8.1&#160; [<A href="https://wg21.link/temp.res.general">temp.res.general</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Barry Revzin
 &#160;&#160;&#160;

 <B>Date: </B>2023-10-10<BR>


<P>[Accepted as a DR at the November, 2023 meeting.]</P>

<P>Consider:</P>

<PRE>
  template &lt;typename T&gt;
  concept C = requires {
    typename T::type&lt;void&gt;;   // template <SPAN CLASS="cmnt">required?</SPAN>
  };
</PRE>

<P>There is implementation divergence: gcc accepts, clang and MSVC reject.</P>

<P>A <I>type-requirement</I> ought to be a type-only context.</P>

<P><B>Proposed resolution (approved by CWG 2023-10-20):</B></P>

<OL>
<LI>
<P>Change in 7.5.8.3 [<A href="https://wg21.link/expr.prim.req.type#1">expr.prim.req.type</A>] paragraph 1 as follows:</P>

<BLOCKQUOTE>

A <I>type-requirement</I> asserts the validity of a type.
<INS>The component names of a <I>type-requirement</I> are those of
its <I>nested-name-specifier</I> (if any) and <I>type-name</I>.</INS>

[<I>Note 1:</I> The enclosing <I>requires-expression</I> will evaluate
to <TT>false</TT> if substitution of template arguments
fails. &#8212;<I>end note</I>]

</BLOCKQUOTE>
</LI>

<LI>
<P>Change in 13.8.1 [<A href="https://wg21.link/temp.res.general#4">temp.res.general</A>] paragraph 4 as follows:</P>

<BLOCKQUOTE>

A qualified or unqualified name is said to be in a <I>type-only context</I>
if it is the terminal name of
<UL>
<LI>a <I>typename-specifier</I>, <INS><I>type-requirement</I>,</INS>
<I>nested-name-specifier</I>, <I>elaborated-type-specifier</I>,
<I>class-or-decltype</I>, or</LI>
<LI>...</LI>
</UL>

</BLOCKQUOTE>
</LI>

</OL>


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