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<title>Issue 4316: {can_}substitute specification is ill-formed</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="4316"><a href="lwg-active.html#4316">4316</a>. <code>{can_}substitute</code> specification is ill-formed</h3>
<p><b>Section:</b> 21.4.13 <a href="https://wg21.link/meta.reflection.substitute">[meta.reflection.substitute]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Matthias Wippich <b>Opened:</b> 2025-08-15 <b>Last modified:</b> 2025-08-17</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code class='backtick'>can_substitute</code> and <code class='backtick'>substitute</code> are currently specified in terms of splices in a template argument list:
<p/>
21.4.13 <a href="https://wg21.link/meta.reflection.substitute">[meta.reflection.substitute]</a> p3:
</p>
<blockquote><p>
<i>Returns</i>: <code class='backtick'>true</code> if <code>Z&lt;[:Args:]...&gt;</code> is a valid <i>template-id</i> (13.3 <a href="https://wg21.link/temp.names">[temp.names]</a>) 
that does not name a function whose type contains an undeduced placeholder type. Otherwise, <code class='backtick'>false</code>.
</p></blockquote>
<p>
21.4.13 <a href="https://wg21.link/meta.reflection.substitute">[meta.reflection.substitute]</a> p7:
</p>
<blockquote><p>
<i>Returns</i>: <code>^^Z&lt;[:Args:]...&gt;</code>.
</p></blockquote>
<p>
This wording was introduced in <a href="https://wg21.link/P2996R11" title=" Reflection for C++26">P2996R11</a>. However, merging in changes from 
<a href="https://wg21.link/P3687" title=" Final Adjustments to C++26 Reflection">P3687</a> "Final Adjustments to C++26 Reflection" in <a href="https://wg21.link/P2996R13" title=" Reflection for C++26">P2996R13</a> changed 
the rules for splices in this context. This makes <code class='backtick'>can_substitute</code> and <code class='backtick'>substitute</code> as specified 
currently ill-formed. We cannot use the given syntax to splice an arbitrary choice of values, 
types and templates anymore.
<p/>
While the intent seems clear, this should be rephrased to be more technically correct.
</p>


<p id="res-4316"><b>Proposed resolution:</b></p>





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