<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3587: std::three_way_comparable_with&lt;T, U, void&gt; can be satisfied but can't be modeled</title>
<meta property="og:title" content="Issue 3587: std::three_way_comparable_with&lt;T, U, void&gt; can be satisfied but can't be modeled">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3587.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#New">New</a> status.</em></p>
<h3 id="3587"><a href="lwg-active.html#3587">3587</a>. <code>std::three_way_comparable_with&lt;T, U, void&gt;</code> can be satisfied but can't be modeled</h3>
<p><b>Section:</b> 17.12.4 <a href="https://wg21.link/cmp.concept">[cmp.concept]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2021-09-06 <b>Last modified:</b> 2021-10-23</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#cmp.concept">issues</a> in [cmp.concept].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Due to the current specification of <code>std::common_comparison_category</code>,
<code><i>compare-as</i>&lt;void, void&gt;</code> is satisfied. And thus given two types <code>T</code>
and <code>U</code> with regular <code>==</code>, <code>!=</code>, <code>&lt;</code>, <code>&gt;</code>, <code>&lt;=</code>,
and <code>&gt;=</code> but irregular <code>&lt;=&gt;</code> that has return type <code>void</code>,
<code>std::three_way_comparable_with&lt;T, U, void&gt;</code> is satisfied.
<p/>
However, in this situation <code>std::three_way_comparable_with&lt;T, U, void&gt;</code>
can't be modeled, because <code>Cat(t &lt;=&gt; u) == Cat(C(t) &lt;=&gt; C(u))</code>
is ill-formed if <code>Cat</code> is <code>void</code> (see <a href="https://gcc.godbolt.org/z/5c4839qz1">this godbolt link</a>).
<p/>
I don't think this issue has any impact on meaningful codes, but it seems to be a hole in the
current specification.
<p/>
It seems that <code><i>compares-as</i>&lt;void, void&gt;</code> should be changed to <code>false</code>.
<code><i>compares-as</i>&lt;NonComparisonCategoryType, void&gt;</code> might need refinement.
<p/>
(Note: The same problem does not exists for <code>std::three_way_comparable</code>, because
<code>std::three_way_comparable&lt;T, void&gt;</code> can be modeled if the <code>&lt;=&gt;</code> on <code>T</code>
has "regular" definition or its return type is not a comparison category type, but the return value
has "regular" semantics)
</p>

<p><i>[2021-09-20; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3587"><b>Proposed resolution:</b></p>





</body>
</html>
