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<title>Issue 4270: Diagnose misuse of std::projected::operator*</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="4270"><a href="lwg-active.html#4270">4270</a>. Diagnose misuse of <code class='backtick'>std::projected::operator*</code></h3>
<p><b>Section:</b> 24.3.6.4 <a href="https://wg21.link/projected">[projected]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2025-05-30 <b>Last modified:</b> 2025-06-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#projected">issues</a> in [projected].</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'>std::projected::operator*</code> is only intentionally used for type calculation like <code class='backtick'>std::declval</code>. 
Currently, if one attempts to call it in a potentially evaluated expression, the program is ill-formed, 
no diagnostic required because the <code class='backtick'>operator*</code> is not defined.
<p/>
It seems better to diagnose such misuse, as we are already diagnosing misuse of <code class='backtick'>std::declval</code> 
(22.2.6 <a href="https://wg21.link/declval">[declval]</a>/2). Implementation used to perform such misuse via <code class='backtick'>std::ranges::iter_move</code>, 
but libc++ and libstdc++ have already switched not to do this (see 
<a href="https://github.com/microsoft/STL/issues/5555">microsoft/STL#5555</a>).
</p>


<p id="res-4270"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>
<li><p>Modify 24.3.6.4 <a href="https://wg21.link/projected">[projected]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class I, class Proj&gt;
  struct <i>projected-impl</i> { <i>// exposition only</i>
    struct <i>type</i> { <i>// exposition only</i>
      [&hellip;]
      <i>// models weakly_incrementable</i>
      indirect_result_t&lt;Proj&amp;, I&gt; operator*() const; <del><i>// not defined</i></del>
    };
  };
  [&hellip;]
}
</pre>
<pre>
<ins>indirect_result_t&lt;Proj&amp;, I&gt; operator*() const;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Mandates</i>: This function is not odr-used (6.3 <a href="https://wg21.link/basic.def.odr">[basic.def.odr]</a>).</ins>
</p>
</blockquote>
</blockquote>
</li>
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