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    CWG Issue 2446</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="2446"></A><H4>2446.
  
Questionable type-dependency of <I>concept-id</I>s
</H4>
<B>Section: </B>13.8.3.3&#160; [<A href="https://wg21.link/temp.dep.expr">temp.dep.expr</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++20
 &#160;&#160;&#160;

 <B>Submitter: </B>Hubert Tong
 &#160;&#160;&#160;

 <B>Date: </B>2019-02-03<BR>


<P>[Accepted at the February, 2020 (Prague) meeting.]</P>



<P>The rules for type-dependency do not appear to take the
<TT>bool</TT> type associated with <I>concept-id</I>s into
account. For example:</P>

<PRE>
  template &lt;typename T&gt; concept C = true;
  template &lt;typename T&gt; struct A;
  template &lt;&gt; struct A&lt;bool&gt; { using type = bool; };

  template &lt;typename T&gt;
  void f(A&lt;decltype(C&lt;T&gt;)&gt;::type); //<SPAN CLASS="cmnt"> error: needs </SPAN>typename<SPAN CLASS="cmnt"> to avoid vexing parse</SPAN>
</PRE>

<P><B>Proposed resolution (February, 2020):</B></P>

<OL>
<LI><P>Change 13.8.3.3 [<A href="https://wg21.link/temp.dep.expr#3">temp.dep.expr</A>] paragraph 3
as follows:</P></LI>

<BLOCKQUOTE>

<P>An <I>id-expression</I> is type-dependent if <INS>it is
not a <I>concept-id</I> and</INS> it contains</P>

<UL><LI><P>...</P></LI></UL>

</BLOCKQUOTE>

<LI><P>Change 13.8.3.4 [<A href="https://wg21.link/temp.dep.constexpr#3">temp.dep.constexpr</A>] paragraph 3 as follows:</P></LI>

<BLOCKQUOTE>

<P>An <I>id-expression</I> is value-dependent if:</P>

<UL>
<LI><P><INS>it is a <I>concept-id</I> and any of its
arguments are dependent,</INS></P></LI>

<LI><P>it is type-dependent,</P></LI>

<LI><P>...</P></LI>

</UL>

</BLOCKQUOTE>

</OL>

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