<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3983: ranges::to adaptors are underconstrained</title>
<meta property="og:title" content="Issue 3983: ranges::to adaptors are underconstrained">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3983.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="3983"><a href="lwg-active.html#3983">3983</a>. <code>ranges::to</code> adaptors are underconstrained</h3>
<p><b>Section:</b> 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-08-23 <b>Last modified:</b> 2023-11-03</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The <code>ranges::to</code> adaptor returns a range adaptor closure object that stores a copy of the decayed parameter pack.
<p/>
However, the function signature does not constrain the type of arguments, which leads to hard errors within the function 
body when they cannot be copied (<a href="https://godbolt.org/z/9f8q3zvb5">demo</a>):
</p>
<blockquote><pre>
#include &lt;ranges&gt;
#include &lt;vector&gt;

struct NonMovable {
  NonMovable() = default;
  NonMovable(NonMovable&amp;&amp;) = delete;
};

int main() {
  auto r = std::ranges::to&lt;std::vector&gt;(NonMovable{}); // <span style="color:red;font-weight:bolder">hard error in MSVC-STL and libc++</span>
}
</pre></blockquote>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/N4958" title=" Working Draft, Programming Languages — C++">N4958</a>.
</p>

<ol>

<li><p>Modify 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a>, header <code>&lt;ranges&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
#include &lt;compare&gt;              // <i>see <a href="https://wg21.link/compare.syn">[compare.syn]</a></i>
#include &lt;initializer_list&gt;     // <i>see <a href="https://wg21.link/initializer.list.syn">[initializer.list.syn]</a></i>
#include &lt;iterator&gt;             // <i>see <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a></i>

namespace std::ranges {
  [&hellip;]
  // <i>25.5.7 <a href="https://wg21.link/range.utility.conv">[range.utility.conv]</a>, range conversions</i>
  template&lt;class C, input_range R, class... Args&gt; requires (!view&lt;C&gt;&gt;)
    constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);                                          // <i>freestanding</i>
  template&lt;template&lt;class...&gt; class C, input_range R, class... Args&gt;
    constexpr auto to(R&amp;&amp; r, Args&amp;&amp;... args);                                       // <i>freestanding</i>
  template&lt;class C, class... Args&gt;
    requires (!view&lt;C&gt;) <ins>&amp;&amp; (constructible_from&lt;decay_t&lt;Args&gt;, Args&gt; &amp;&amp; ...)</ins>
    constexpr auto to(Args&amp;&amp;... args);                                              // <i>freestanding</i>
  template&lt;template&lt;class...&gt; class C, class... Args&gt;
    <ins>requires (constructible_from&lt;decay_t&lt;Args&gt;, Args&gt; &amp;&amp; ...)</ins>
    constexpr auto to(Args&amp;&amp;... args);                                              // <i>freestanding</i>
  [&hellip;]
}
</pre>
</blockquote>

</li>

<li><p>Modify 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class C, class... Args&gt;
  requires (!view&lt;C&gt;) <ins>&amp;&amp; (constructible_from&lt;decay_t&lt;Args&gt;, Args&gt; &amp;&amp; ...)</ins>
  constexpr auto to(Args&amp;&amp;... args);
template&lt;template&lt;class...&gt; class C, class... Args&gt;
  <ins>requires (constructible_from&lt;decay_t&lt;Args&gt;, Args&gt; &amp;&amp; ...)</ins>
  constexpr auto to(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Mandates</i>: For the first overload, <code>C</code> is a cv-unqualified class type.
<p/>
-2- <i>Returns</i>: A range adaptor closure object (25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>) <code>f</code> that is a 
perfect forwarding call wrapper (22.10.4 <a href="https://wg21.link/func.require">[func.require]</a>) with the following properties:
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>

</li>

</ol>
</blockquote>

<p><i>[2023-11-03; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
Votes split between NAD and using <em>Mandates</em> instead of constraints.
</p>


<p><i>[2023-09-28; Hewill provides alternative wording]</i></p>

<p>
The new wording form is consistent with the current wording, that is, it has a similar structure 
with 25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a> p1 and 25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a> p8.
</p>



<p id="res-3983"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4958" title=" Working Draft, Programming Languages — C++">N4958</a>.
</p>

<ol>

<li><p>Modify 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class C, class... Args&gt; requires (!view&lt;C&gt;)
  constexpr auto to(Args&amp;&amp;... args);
template&lt;template&lt;class...&gt; class C, class... Args&gt;
  constexpr auto to(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Mandates</i>: For the first overload, <code>C</code> is a cv-unqualified class type.
<p/>
-2- <i>Returns</i>: A range adaptor closure object (25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>) <code>f</code> that is a 
perfect forwarding call wrapper (22.10.4 <a href="https://wg21.link/func.require">[func.require]</a>) with the following properties:
</p>
<ol style="list-style-type:none">
<li><p>(2.1) &mdash; It has no target object.</p></li>
<li><p>(2.2) &mdash; Its bound argument entities <code>bound_args</code> consist of objects of types <code>decay_t&lt;Args&gt;...</code> 
direct-non-list-initialized with <code>std::forward&lt;Args&gt;(args)...</code>, respectively. </p></li>
<li><p>(2.3) &mdash; Its call pattern is <code>to&lt;C&gt;(r, bound_args...)</code>, where <code>r</code> is the argument used in a 
function call expression of <code>f</code>.</p></li>
</ol>
<p><ins>
-?- <i>Remarks</i>: The expression <code>to(args...)</code> is well-formed if and only if the initialization of the bound argument entities 
of the result, as specified above, are all well-formed.
</ins></p>
</blockquote>
</blockquote>

</li>

</ol>





</body>
</html>
