<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2822: Resolution for LWG 2742 introduces ambiguities</title>
<meta property="og:title" content="Issue 2822: Resolution for LWG 2742 introduces ambiguities">
<meta property="og:description" content="C++ library issue. Status: NAD">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2822.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#NAD">NAD</a> status.</em></p>
<h3 id="2822"><a href="lwg-closed.html#2822">2822</a>. Resolution for LWG 2742 introduces ambiguities</h3>
<p><b>Section:</b> 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2016-11-15 <b>Last modified:</b> 2017-01-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#string.cons">issues</a> in [string.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The new constructor added for LWG <a href="lwg-defects.html#2742" title="Inconsistent string interface taking string_view (Status: C++17)">2742</a><sup><a href="https://cplusplus.github.io/LWG/issue2742" title="Latest snapshot">(i)</a></sup> causes an ambiguity when attempting to construct from a (non-const) <code>char*</code>
</p>
<blockquote><pre>
char s[] = "whoops"; 
std::string str(s, 2, 4);
</pre></blockquote>
<p>
This is the same problem discussed in <a href="lwg-defects.html#2758" title="std::string{}.assign(&quot;ABCDE&quot;, 0, 1) is ambiguous (Status: C++17)">2758</a><sup><a href="https://cplusplus.github.io/LWG/issue2758" title="Latest snapshot">(i)</a></sup>, and the fix is the same: disable that new constructor if <code>T</code> 
is convertible to <code>const_pointer</code>, so that the old constructor is used instead.
</p>

<p>
<strong>Previous resolution [SUPERSEDED]:</strong>
</p>
<blockquote class="note">
<p>
This wording is relative to N4606.
</p>
<ol>
<li><p>Modify the new <code>basic_string(const T&amp; t, size_type pos, size_type n, const Allocator&amp; a = Allocator());</code> 
constructor in 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a> as shown:</p>

<blockquote>
<pre>
template&lt;class T&gt;
basic_string(const T&amp; t, size_type pos, size_type n, const Allocator&amp; a = Allocator());
</pre>
<blockquote>
<p>
-?- <i>Effects:</i> Creates a variable, <code>sv</code>, as if by <code>basic_string_view&lt;charT, traits&gt; sv = t;</code> and 
then behaves the same as:
</p>
<blockquote><pre>
basic_string(sv.substr(pos, n), a)
</pre></blockquote>
<p>
-?- <i>Remarks:</i> This constructor shall not participate in overload resolution unless 
<code>is_convertible_v&lt;const T&amp;, basic_string_view&lt;charT, traits&gt;&gt;</code> is <code>true</code> <ins>and 
<code>is_convertible_v&lt;const T&amp;, const charT*&gt;</code> is <code>false</code></ins>.
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2016-12-21, Jonathan comments and reommends NAD]</i></p>

<p>
Tim Song pointed out that this constructor isn't ambiguous, and I can
no longer reproduce the errors I was getting, so I think this issue
should be resolved NAD. The code snippet in the issue discussion
compiles OK without the proposed change.
</p>

<p><i>[2017-01-27 Telecon]</i></p>

<p>NAD</p>


<p id="res-2822"><b>Proposed resolution:</b></p>





</body>
</html>
