<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2289: constexpr guarantees of defaulted functions still insufficient</title>
<meta property="og:title" content="Issue 2289: constexpr guarantees of defaulted functions still insufficient">
<meta property="og:description" content="C++ library issue. Status: Open">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2289.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#Open">Open</a> status.</em></p>
<h3 id="2289"><a href="lwg-active.html#2289">2289</a>. <code>constexpr</code> guarantees of defaulted functions still insufficient</h3>
<p><b>Section:</b> 22.3.2 <a href="https://wg21.link/pairs.pair">[pairs.pair]</a>, 22.4.4.2 <a href="https://wg21.link/tuple.cnstr">[tuple.cnstr]</a>, 30.5 <a href="https://wg21.link/time.duration">[time.duration]</a> <b>Status:</b> <a href="lwg-active.html#Open">Open</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2013-09-09 <b>Last modified:</b> 2020-06-13</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#pairs.pair">active issues</a> in [pairs.pair].</p>
<p><b>View all other</b> <a href="lwg-index.html#pairs.pair">issues</a> in [pairs.pair].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Open">Open</a> status.</p>
<p><b>Discussion:</b></p>
<p>
During the acceptance of <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3471.html">N3471</a> and
some similar <code>constexpr</code> papers, specific wording was added to <code>pair</code>, <code>tuple</code>, and other templates
that were intended to impose implementation constraints that ensure that the observable <code>constexpr</code> "character"
of a defaulted function template is solely determined by the required expressions of the user-provided types when instantiated,
for example:
</p>

<blockquote><p>
The defaulted move and copy constructor, respectively, of pair shall be a <code>constexpr</code> function if and only if
all required element-wise initializations for copy and move, respectively, would satisfy the requirements for
a <code>constexpr</code> function.
</p></blockquote>

<p>
This wording doesn't require enough, especially since the core language via CWG 1358 does now support <code>constexpr</code>
function template instantiations, even if such function cannot appear in a constant expression (as specified in 7.7 <a href="https://wg21.link/expr.const">[expr.const]</a>) 
or as a constant initializer of that object (as specified in  [basic.start.init]). The wording should be 
improved and should require valid uses in constant expressions and as constant initializers instead.
</p>

<p><i>[Lenexa 2015-05-05]</i></p>

<p>STL : notice order of move/copy and copy/move with "respectively".</p>
<p>General word-smithing; ask for updated wording</p>
<p>Are we happy with this with changes we are suggesting?</p>
<p>unanimous</p>

<p><i>[2016-12-14, Daniel comments]</i></p>

<p>
LWG <a href="lwg-active.html#2833" title="Library needs to specify what it means when it declares a function constexpr (Status: Open)">2833</a><sup><a href="https://cplusplus.github.io/LWG/issue2833" title="Latest snapshot">(i)</a></sup> overlaps considerably and both should be resolved together.
</p>

<strong>Previous resolution from Daniel [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to N3691.</p>

<ol>
<li><p>Change 22.3.2 <a href="https://wg21.link/pairs.pair">[pairs.pair]</a> p2 as indicated:</p>

<blockquote>
<p>
-2- <del>The defaulted move and copy constructor, respectively, of pair shall be a <code>constexpr</code> function if and only if
all required element-wise initializations for copy and move, respectively, would satisfy the requirements for
a <code>constexpr</code> function</del><ins>An invocation of the move or copy constructor of <code>pair</code> shall be a constant expression 
(7.7 <a href="https://wg21.link/expr.const">[expr.const]</a>) if all required element-wise initializations would be constant expressions. An invocation of the 
move or copy constructor of <code>pair</code> shall be a constant initializer for that <code>pair</code> object ( [basic.start.init])
if all required element-wise initializations would be constant initializers for the respective subobjects</ins>.
</p>
</blockquote>
</li>

<li><p>Change 22.4.4.2 <a href="https://wg21.link/tuple.cnstr">[tuple.cnstr]</a> p2 as indicated:</p>

<blockquote>
<p>
-2- <del>The defaulted move and copy constructor, respectively, of <code>tuple</code> shall be a <code>constexpr</code> function if 
and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for
a <code>constexpr</code> function. The defaulted move and copy constructor of <code>tuple&lt;&gt;</code> shall be <code>constexpr</code> 
functions</del><ins>An invocation of the move or copy constructor of <code>tuple</code> shall be a constant expression (7.7 <a href="https://wg21.link/expr.const">[expr.const]</a>)
if all required element-wise initializations would be constant expressions. An invocation of the move or copy constructor of 
<code>tuple</code> shall be a constant initializer for that <code>tuple</code> object ( [basic.start.init]) if all 
required element-wise initializations would be constant initializers for the respective subobjects. An invocation of the 
move or copy constructor of <code>tuple&lt;&gt;</code> shall be a constant expression, or a constant initializer for that 
<code>tuple&lt;&gt;</code> object, respectively, if the function argument would be constant expression</ins>.
</p>
</blockquote>
</li>

<li><p>Change 30.5 <a href="https://wg21.link/time.duration">[time.duration]</a> p7 as indicated:</p>

<blockquote>
<p>
-7- <i>Remarks:</i> <del>The defaulted copy constructor of duration shall be a <code>constexpr</code> function if and only if
the required initialization of the member <code>rep_</code> for copy and move, respectively, would satisfy the
requirements for a <code>constexpr</code> function.</del><ins>An invocation of the copy constructor of <code>duration</code> shall 
be a constant expression (7.7 <a href="https://wg21.link/expr.const">[expr.const]</a>) if the required initialization of the member <code>rep_</code> would be a constant expression.
An invocation of the copy constructor of <code>duration</code> shall be a constant initializer for that <code>duration</code> object 
( [basic.start.init]) if the required initialization of the member <code>rep_</code> would be constant initializers 
for this subobject</ins>.
</p>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2020-06-08 Nina Dinka Ranns comments]</i></p>

<p>
The revised wording provided by LWG <a href="lwg-active.html#2833" title="Library needs to specify what it means when it declares a function constexpr (Status: Open)">2833</a><sup><a href="https://cplusplus.github.io/LWG/issue2833" title="Latest snapshot">(i)</a></sup> should resolve this issue as well.
</p>


<p id="res-2289"><b>Proposed resolution:</b></p>






</body>
</html>
