<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3182: Specification of Same could be clearer</title>
<meta property="og:title" content="Issue 3182: Specification of Same could be clearer">
<meta property="og:description" content="C++ library issue. Status: C++20">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3182.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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#C++20">C++20</a> status.</em></p>
<h3 id="3182"><a href="lwg-defects.html#3182">3182</a>. Specification of <code>Same</code> could be clearer</h3>
<p><b>Section:</b> 18.4.2 <a href="https://wg21.link/concept.same">[concept.same]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2019-01-05 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The specification of the <code>Same</code> concept in 18.4.2 <a href="https://wg21.link/concept.same">[concept.same]</a>:
</p>
<blockquote>
<pre>
template&lt;class T, class U&gt;
  concept Same = is_same_v&lt;T, U&gt;;
</pre>
<p>
-1- <code>Same&lt;T, U&gt;</code> subsumes <code>Same&lt;U, T&gt;</code> and vice versa.
</p>
</blockquote>
<p>
seems contradictory. From the concept definition alone, it is <em>not</em> the
case that <code>Same&lt;T, U&gt;</code> subsumes <code>Same&lt;U, T&gt;</code> nor vice
versa. Paragraph 1 is trying to tell us that there's some magic that provides
the stated subsumption relationship, but to a casual reader it appears to be a
mis-annotated note. We should either add a note to explain what's actually
happening here, or define the concept in such a way that it naturally
provides the specified subsumption relationship.
</p><p>
Given that there's a straightforward library implementation of the symmetric
subsumption idiom, the latter option seems preferable.
</p>

<p><i>[2019-01-20 Reflector prioritization]</i></p>

<p>Set Priority to and status to Tentatively Ready</p>


<p id="res-3182"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4791">N4791</a>.</p>

<ol>
<li><p>Change 18.4.2 <a href="https://wg21.link/concept.same">[concept.same]</a> as follows:</p>
<blockquote>
<pre>
<ins>template&lt;class T, class U&gt;</ins>
  <ins>concept <i>same-impl</i> = // exposition only</ins>
    <ins>is_same_v&lt;T, U&gt;;</ins>

template&lt;class T, class U&gt;
  concept Same = <del>is_same_v&lt;T, U&gt;</del><ins><i>same-impl</i>&lt;T, U&gt; &amp;&amp; <i>same-impl</i>&lt;U, T&gt;</ins>;
</pre>
<p>
-1- <ins>[<i>Note:</i></ins> <code>Same&lt;T, U&gt;</code> subsumes
<code>Same&lt;U, T&gt;</code> and vice versa.<ins>&mdash;<i>end note</i>]</ins>
</p>
</blockquote>
</li>
</ol>




</body>
</html>
