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<title>Issue 3733: ranges::to misuses cpp17-input-iterator</title>
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<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++23">C++23</a> status.</em></p>
<h3 id="3733"><a href="lwg-defects.html#3733">3733</a>. <code>ranges::to</code> misuses <code><i>cpp17-input-iterator</i></code></h3>
<p><b>Section:</b> 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2022-07-10 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.utility.conv.to">active issues</a> in [range.utility.conv.to].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.utility.conv.to">issues</a> in [range.utility.conv.to].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>ranges::to</code> uses <code><i>cpp17-input-iterator</i>&lt;iterator_t&lt;R&gt;&gt;</code> to check 
whether an iterator is a <i>Cpp17InputIterator</i>, which misbehaves if there is a 
<code>std::iterator_traits</code> specialization for that iterator (e.g. if the iterator is a 
<code>std::common_iterator</code>).
</p>
<blockquote><pre>
struct MyContainer {
    template&lt;class Iter&gt;
    MyContainer(Iter, Iter);

    char* begin();
    char* end();
};

auto nul_terminated = std::views::take_while([](char ch) { return ch != '\0'; });
auto c = nul_terminated("") | std::views::common | std::ranges::to&lt;MyContainer&gt;();  // <span style="color:red;font-weight:bolder">error</span>
</pre></blockquote>
<p>
I believe that <code>ranges::to</code> should instead use 
<code>derived_from&lt;typename iterator_traits&lt;iterator_t&lt;R&gt;&gt;::iterator_category, input_iterator_tag&gt;</code>, 
which correctly detects the iterator category of a <code>std::common_iterator</code>.
</p>

<p><i>[2022-08-23; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>

<li><p>Modify 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class C, input_range R, class... Args&gt; requires (!view&lt;C&gt;)
  constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Returns</i>: An object of type <code>C</code> constructed from the elements of <code>r</code> in the following manner:
</p>
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; If <code>convertible_to&lt;range_reference_t&lt;R&gt;, range_value_t&lt;C&gt;&gt;</code> is <code>true</code>:</p>
<ol style="list-style-type: none">
<li><p>(1.1.1) &mdash; If <code>constructible_from&lt;C, R, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
<li><p>(1.1.2) &mdash; Otherwise, if <code>constructible_from&lt;C, from_range_t, R, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(from_range, std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
<li><p>(1.1.3) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p>(1.1.3.1) &mdash; <code>common_range&lt;R&gt;</code> is <code>true</code>,</p></li>
<li><p>(1.1.3.2) &mdash; <code><del><i>cpp17-input-iterator</i></del><ins>derived_from&lt;typename 
iterator_traits</ins>&lt;iterator_t&lt;R&gt;&gt;<ins>::iterator_category, input_iterator_tag&gt;</ins></code> 
is <code>true</code>, and</p></li>
<li><p>(1.1.3.3) &mdash; <code>constructible_from&lt;C, iterator_t&lt;R&gt;, sentinel_t&lt;R&gt;, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(ranges::begin(r), ranges::end(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
</ol>
</li>
<li><p>(1.1.4) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p>(1.1.4.1) &mdash; <code>constructible_from&lt;C, Args...&gt;</code> is <code>true</code>, and</p></li>
<li><p>(1.1.4.2) &mdash; <code><i>container-insertable</i>&lt;C, range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:</p>
<blockquote><pre>
[&hellip;]
</pre></blockquote></li>
</ol>
</li>
</ol>
</li>
<li><p>(1.2) &mdash; Otherwise, if <code>input_range&lt;range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:</p>
<blockquote><pre>
to&lt;C&gt;(r | views::transform([](auto&amp;&amp; elem) {
  return to&lt;range_value_t&lt;C&gt;&gt;(std::forward&lt;decltype(elem)&gt;(elem));
}), std::forward&lt;Args&gt;(args)...);
</pre></blockquote></li>
<li><p>(1.3) &mdash; Otherwise, the program is ill-formed.</p></li>
</ol>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2022-08-27; Hewill Kang reopens and suggests a different resolution]</i></p>

<p>
This issue points out that the standard misuses <code><i>cpp17-input-iterator</i></code> to check 
whether the iterator meets the requirements of <i>Cpp17InputIterator</i>, and proposes to use 
<code>iterator_traits&lt;I&gt;::iterator_category</code> to check the iterator's category directly, 
which may lead to the following potential problems:
<p/>
First, for the range types that model both <code>common_range</code> and <code>input_range</code>, 
the expression <code>iterator_traits&lt;I&gt;::iterator_category</code> may not be valid, consider
</p>
<blockquote><pre>
#include &lt;ranges&gt;

struct I {
  using difference_type = int;
  using value_type = int;
  int operator*() const;
  I&amp; operator++();
  void operator++(int);
  bool operator==(const I&amp;) const;
  bool operator==(std::default_sentinel_t) const;
};

int main() {
  auto r = std::ranges::subrange(I{}, I{});
  auto v = r | std::ranges::to&lt;std::vector&lt;int&gt;&gt;(0);
}
</pre></blockquote>
<p>
Although <code>I</code> can serve as its own sentinel, it does not model 
<code><i>cpp17-input-iterator</i></code> since postfix <code>operator++</code> returns <code>void</code>, 
which causes <code>iterator_traits&lt;R&gt;</code> to be an empty class, making the 
expression <code>derived_from&lt;iterator_traits&lt;I&gt;::iterator_category, input_iterator_tag&gt;</code> ill-formed.
<p/>
Second, for <code>common_iterator</code>, <code>iterator_traits&lt;I&gt;::iterator_category</code> 
does not guarantee a strictly correct iterator category in the current standard.
<p/>
For example, when the above <code>I::operator*</code> returns a non-<code>copyable</code> object that 
can be converted to <code>int</code>, this makes <code>common_iterator&lt;I, default_sentinel_t&gt;</code> 
unable to synthesize a C++17-conforming postfix <code>operator++</code>, however, 
<code>iterator_traits&lt;common_iterator&lt;I, S&gt;&gt;::iterator_category</code> will still 
give <code>input_iterator_tag</code> even though it's not even a C++17 iterator.
<p/>
Another example is that for <code>input_iterator</code>s with difference type of integer-class type, 
the difference type of the <code>common_iterator</code> wrapped on it is still of the integer-class type, 
but the <code>iterator_category</code> obtained by the <code>iterator_traits</code> is <code>input_iterator_tag</code>. 
<p/>
The proposed resolution only addresses the first issue since I believe that the problem with 
<code>common_iterator</code> requires a paper.
</p>

<p><i>[2023-01-11; LWG telecon]</i></p>

<p>
Set status to Tentatively Ready (poll results F6/A0/N1)
</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3733"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class C, input_range R, class... Args&gt; requires (!view&lt;C&gt;)
  constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Returns</i>: An object of type <code>C</code> constructed from the elements of <code>r</code> in the following manner:
</p>
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; If <code>convertible_to&lt;range_reference_t&lt;R&gt;, range_value_t&lt;C&gt;&gt;</code> is <code>true</code>:</p>
<ol style="list-style-type: none">
<li><p>(1.1.1) &mdash; If <code>constructible_from&lt;C, R, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
<li><p>(1.1.2) &mdash; Otherwise, if <code>constructible_from&lt;C, from_range_t, R, Args...&gt;</code> is <code>true</code>:</p>
<blockquote><code>C(from_range, std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
<li><p>(1.1.3) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p>(1.1.3.1) &mdash; <code>common_range&lt;R&gt;</code> is <code>true</code>,</p></li>
<li><p>(1.1.3.2) &mdash; <del><code><i>cpp17-input-iterator</i></code></del><ins>if the <i>qualified-id</i>  
<code>iterator_traits</code></ins><code>&lt;iterator_t&lt;R&gt;&gt;</code><ins><code>::iterator_category</code> </ins>is<ins> 
<del><code>true</code></del> valid and denotes a type that models <code>derived_from&lt;input_iterator_tag&gt;</code></ins>, and</p></li>
<li><p>(1.1.3.3) &mdash; <code>constructible_from&lt;C, iterator_t&lt;R&gt;, sentinel_t&lt;R&gt;, Args...&gt;</code>:</p>
<blockquote><code>C(ranges::begin(r), ranges::end(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
</ol>
</li>
<li><p>(1.1.4) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p>(1.1.4.1) &mdash; <code>constructible_from&lt;C, Args...&gt;</code> is <code>true</code>, and</p></li>
<li><p>(1.1.4.2) &mdash; <code><i>container-insertable</i>&lt;C, range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:</p>
<blockquote><pre>
[&hellip;]
</pre></blockquote></li>
</ol>
</li>
</ol>
</li>
<li><p>(1.2) &mdash; Otherwise, if <code>input_range&lt;range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:</p>
<blockquote><pre>
to&lt;C&gt;(r | views::transform([](auto&amp;&amp; elem) {
  return to&lt;range_value_t&lt;C&gt;&gt;(std::forward&lt;decltype(elem)&gt;(elem));
}), std::forward&lt;Args&gt;(args)...);
</pre></blockquote></li>
<li><p>(1.3) &mdash; Otherwise, the program is ill-formed.</p></li>
</ol>
</blockquote>
</blockquote>
</li>
</ol>





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