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<title>Issue 3483: transform_view::iterator's difference is overconstrained</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="3483"><a href="lwg-defects.html#3483">3483</a>. <code>transform_view::<i>iterator</i></code>'s difference is overconstrained</h3>
<p><b>Section:</b> 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a>, 25.7.23.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-09-04 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.transform.iterator">issues</a> in [range.transform.iterator].</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>
The difference operation for <code>transform_view::<i>iterator</i></code> is specified in 
25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> as:
</p>
<blockquote>
<pre>
friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires random_access_range&lt;<i>Base</i>&gt;;
</pre>
<blockquote>
<p>
-22- <i>Effects:</i> Equivalent to: <code>return x.<i>current_</i> - y.<i>current_</i>;</code>
</p>
</blockquote>
</blockquote>
<p>
The member <code><i>current_</i></code> is an iterator of type <code>iterator_t&lt;<i>Base</i>&gt;</code>, where 
<code><i>Base</i></code> is <code>V</code> for <code>transform_view&lt;V, F&gt;::<i>iterator</i>&lt;false&gt;</code> 
and <code>const V</code> for <code>transform_view&lt;V, F&gt;::<i>iterator</i>&lt;true&gt;</code>. The difference 
of iterators that appears in the above <i>Effects:</i> element is notably well-defined if their type models 
<code>sized_sentinel_for&lt;iterator_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</code> which 
<code>random_access_range&lt;<i>Base</i>&gt;</code> refines. This overstrong requirement seems to be simply the 
result of an oversight; it has been present since <a href="https://wg21.link/p0789r0">P0789R0</a>, without 
&mdash; to my recollection &mdash; ever having been discussed. We should relax this requirement to provide 
difference capability for <code>transform_view</code>'s iterators whenever the underlying iterators do.
</p>

<p><i>[2020-09-08; Reflector discussion]</i></p>

<p>
During reflector discussions it was observed that <code>elements_view::<i>iterator</i></code> has the same issue
and the proposed wording has been extended to cover this template as well.
</p>

<p><i>[2020-09-13; Reflector prioritization]</i></p>

<p>
Set priority to 0 and status to Tentatively Ready after seven votes in favour during reflector discussions.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3483"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, copy_constructible F&gt;
    requires view&lt;V&gt; &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;   
             regular_invocable&lt;F&amp;, range_reference_t&lt;V&gt;&gt; &amp;&amp;
             <i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;V&gt;&gt;&gt;
  template&lt;bool Const&gt;
  class transform_view&lt;V, F&gt;::<i>iterator</i> {
  public:
    [&hellip;]
    friend constexpr <i>iterator</i> operator-(<i>iterator</i> i, difference_type n)
      requires random_access_range&lt;<i>Base</i>&gt;;
    friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
      requires <del>random_access_range&lt;<i>Base</i>&gt;</del><ins>sized_sentinel_for&lt;iterator_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</ins>;
    [&hellip;]
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires <del>random_access_range&lt;<i>Base</i>&gt;</del><ins>sized_sentinel_for&lt;iterator_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</ins>;
</pre>
<blockquote>
<p>
-22- <i>Effects:</i> <code>return x.<i>current_</i> - y.<i>current_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.23.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; <i>has-tuple-element</i>&lt;range_value_t&lt;V&gt;, N&gt; &amp;&amp;
             <i>has-tuple-element</i>&lt;remove_reference_t&lt;range_reference_t&lt;V&gt;&gt;, N&gt;
  template&lt;bool Const&gt;
  class elements_view&lt;V, N&gt;::<i>iterator</i> {  <i>// exposition only</i>
    [&hellip;]
    friend constexpr <i>iterator</i> operator-(<i>iterator</i> x, difference_type y)
      requires random_access_range&lt;<i>Base</i>&gt;;
    friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
      requires <del>random_access_range&lt;<i>Base</i>&gt;</del><ins>sized_sentinel_for&lt;iterator_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</ins>;
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr difference_type operator-(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires <del>random_access_range&lt;<i>Base</i>&gt;</del><ins>sized_sentinel_for&lt;iterator_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</ins>;
</pre>
<blockquote>
<p>
-21- <i>Effects:</i> <code>return x.<i>current_</i> - y.<i>current_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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