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<title>Issue 3664: LWG 3392 broke std::ranges::distance(a, a+3)</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="3664"><a href="lwg-defects.html#3664">3664</a>. LWG 3392 broke <code>std::ranges::distance(a, a+3)</code></h3>
<p><b>Section:</b> 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Arthur O'Dwyer <b>Opened:</b> 2022-01-23 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.iter.op.distance">issues</a> in [range.iter.op.distance].</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>
Consider the use of <code>std::ranges::distance(first, last)</code> on a simple C array.
This works fine with <code>std::distance</code>, but currently does not work with
<code>std::ranges::distance</code>.
</p>
<blockquote>
<pre>
// <a href="https://godbolt.org/z/vsxbcfEv6">godbolt link</a>
#include &lt;ranges&gt;
#include &lt;cassert&gt;

int main() {
  int a[] = {1, 2, 3};
  assert(std::ranges::distance(a, a+3) == 3);
  assert(std::ranges::distance(a, a) == 0);
  assert(std::ranges::distance(a+3, a) == -3);
}
</pre>
</blockquote>
<p>
Before LWG <a href="lwg-defects.html#3392" title="ranges::distance() cannot be used on a move-only iterator with a sized sentinel (Status: C++23)">3392</a><sup><a href="https://cplusplus.github.io/LWG/issue3392" title="Latest snapshot">(i)</a></sup>, we had a single iterator-pair overload:
</p>
<blockquote><pre>
template&lt;input_or_output_iterator I, sentinel_for&lt;I&gt; S&gt;
  constexpr iter_difference_t&lt;I&gt; distance(I first, S last);
</pre></blockquote>
<p>
which works fine for C pointers. After LWG <a href="lwg-defects.html#3392" title="ranges::distance() cannot be used on a move-only iterator with a sized sentinel (Status: C++23)">3392</a><sup><a href="https://cplusplus.github.io/LWG/issue3392" title="Latest snapshot">(i)</a></sup>, we have two
iterator-pair overloads:
</p>
<blockquote><pre>
template&lt;input_or_output_iterator I, sentinel_for&lt;I&gt; S&gt;
  requires (!sized_sentinel_for&lt;S, I&gt;)
    constexpr iter_difference_t&lt;I&gt; distance(I first, S last);

template&lt;input_or_output_iterator I, sized_sentinel_for&lt;I&gt; S&gt;
  constexpr iter_difference_t&lt;I&gt; distance(const I&amp; first, const S&amp; last);
</pre></blockquote>
<p>
and unfortunately the one we want &mdash; <code>distance(I first, S last)</code> &mdash; 
is no longer viable because [with <code>I=int*</code>, <code>S=int*</code>], we have
<code>sized_sentinel_for&lt;S, I&gt;</code> and so its constraints aren't satisfied.
So we look at the other overload [with <code>I=int[3]</code>, <code>S=int[3]</code>], but
unfortunately its constraints aren't satisfied either, because <code>int[3]</code>
is not an <code>input_or_output_iterator</code>.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>

<p>
<strong>Previous resolution [SUPERSEDED]:</strong>
</p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>. 
</p>

<blockquote class="note">
<p>
[<i>Drafting Note:</i> Thanks to Casey Carter. Notice that <code>sentinel_for&lt;S, I&gt;</code> 
already implies and subsumes <code>input_or_output_iterator&lt;I&gt;</code>, so that constraint 
wasn't doing anything; personally I'd prefer to remove it for symmetry (and to save the 
environment). Otherwise you'll have people asking why one of the <code>I</code>'s is constrained 
and the other isn't.] 
</p>
</blockquote>

<ol>
<li><p>Modify 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a>, header <code>&lt;iterator&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a>, ranges::distance</i>
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sentinel_for&lt;I&gt; S&gt;
  requires (!sized_sentinel_for&lt;S, I&gt;)
  constexpr iter_difference_t&lt;I&gt; distance(I first, S last);
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sized_sentinel_for&lt;<ins>decay_t&lt;</ins>I<ins>&gt;</ins>&gt; S&gt;
  constexpr iter_difference_t&lt;I&gt; distance(const I&amp; first, <del>const</del> S<del>&amp;</del> last);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sentinel_for&lt;I&gt; S&gt;
  requires (!sized_sentinel_for&lt;S, I&gt;)
  constexpr iter_difference_t&lt;I&gt; ranges::distance(I first, S last);
</pre>
<blockquote>
<p>
-1- <i>Preconditions:</i> <code>[first, last)</code> denotes a range.
<p/>
-2- <i>Effects:</i> Increments <code>first</code> until <code>last</code> is reached and returns the number of increments.
</p>
</blockquote>
<pre>
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sized_sentinel_for&lt;<ins>decay_t&lt;</ins>I<ins>&gt;</ins>&gt; S&gt;
  constexpr iter_difference_t&lt;I&gt; ranges::distance(const I&amp; first, <del>const</del> S<del>&amp;</del> last);
</pre>
<blockquote>
<p>
-3- <i>Effects:</i> Equivalent to: <code>return last - first;</code>
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2022-02-16; Arthur and Casey provide improved wording]</i></p>


<p><i>[Kona 2022-11-08; Move to Ready]</i></p>


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



<p id="res-3664"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>. 
</p>

<blockquote class="note">
<p>
[<i>Drafting Note:</i> Arthur thinks it's a bit "cute" of the <i>Effects:</i> element to <code>static_cast</code> 
from <code>T(&amp;)[N]</code> to <code>T* const&amp;</code> in the array case, but it does seem to do the right thing 
in all cases, and it saves us from having to use an <code>if constexpr (is_array_v...)</code> or something like that.] 
</p>
</blockquote>

<ol>
<li><p>Modify 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a>, header <code>&lt;iterator&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a>, ranges::distance</i>
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sentinel_for&lt;I&gt; S&gt;
  requires (!sized_sentinel_for&lt;S, I&gt;)
  constexpr iter_difference_t&lt;I&gt; distance(I first, S last);
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sized_sentinel_for&lt;<ins>decay_t&lt;</ins>I<ins>&gt;</ins>&gt; S&gt;
  constexpr iter_difference_t&lt;<ins>decay_t&lt;</ins>I<ins>&gt;</ins>&gt; distance(<del>const</del> I&amp;<ins>&amp;</ins> first, <del>const</del> S<del>&amp;</del> last);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sentinel_for&lt;I&gt; S&gt;
  requires (!sized_sentinel_for&lt;S, I&gt;)
  constexpr iter_difference_t&lt;I&gt; ranges::distance(I first, S last);
</pre>
<blockquote>
<p>
-1- <i>Preconditions:</i> <code>[first, last)</code> denotes a range.
<p/>
-2- <i>Effects:</i> Increments <code>first</code> until <code>last</code> is reached and returns the number of increments.
</p>
</blockquote>
<pre>
template&lt;<ins>class</ins><del>input_or_output_iterator</del> I, sized_sentinel_for&lt;<ins>decay_t&lt;</ins>I<ins>&gt;</ins>&gt; S&gt;
  constexpr iter_difference_t&lt;<ins>decay_t&lt;</ins>I<ins>&gt;</ins>&gt; ranges::distance(<del>const</del> I&amp;<ins>&amp;</ins> first, <del>const</del> S<del>&amp;</del> last);
</pre>
<blockquote>
<p>
-3- <i>Effects:</i> Equivalent to: <code>return last - <ins>static_cast&lt;const decay_t&lt;I&gt;&amp;&gt;(</ins>first<ins>)</ins>;</code>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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