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    CWG Issue 2461</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="2461"></A><H4>2461.
  
Diagnosing non-<TT>bool</TT> type constraints
</H4>
<B>Section: </B>13.8&#160; [<A href="https://wg21.link/temp.res">temp.res</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD6
 &#160;&#160;&#160;

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

 <B>Date: </B>2020-04-20<BR>


<P>[Accepted at the November, 2020 meeting.]</P>



<P>Given the following example,</P>

<PRE>
  template &lt;typename T&gt; struct A {};
  template &lt;typename T&gt; void f() requires (sizeof(A&lt;T&gt;)) {}
</PRE>

<P>the current wording does not appear to allow diagnosis
of the program as ill-formed. In particular, 13.8 [<A href="https://wg21.link/temp.res">temp.res</A>]
bullet 8.2 says,</P>

<BLOCKQUOTE>

<P>The program is ill-formed, no diagnostic required, if:</P>

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

<LI><P>no substitution of template arguments into
a <I>type-constraint</I> or <I>requires-clause</I> would
result in a valid expression, or</P></LI>

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

</UL>

</BLOCKQUOTE>

<P>However, substitution into the <I>requires-clause</I>
in this case would result in a valid expression, but not
one that is an atomic constraint that can be checked for
satisfaction.</P>

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

<P>Change bullet 8.2 of 13.8 [<A href="https://wg21.link/temp.res">temp.res</A>] as follows:</P>

<BLOCKQUOTE>

<P>The program is ill-formed, no diagnostic required, if:</P>

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

<LI><P>
<DEL>no substitution of template arguments into
a <I>type-constraint</I> or <I>requires-clause</I> would
result in a valid expression</DEL> <INS>any
<I>constraint-expression</I> in the program, introduced
or otherwise, has (in its normal form) an atomic constraint
<I>A</I> where no satisfaction check of <I>A</I> could be
well-formed and no satisfaction check of <I>A</I> is
performed</INS>, or</P></LI>

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

</UL>

</BLOCKQUOTE>

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