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<title>Issue 3448: transform_view's sentinel&lt;false&gt; not comparable with iterator&lt;true&gt;</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="3448"><a href="lwg-defects.html#3448">3448</a>. <code>transform_view</code>'s <code><i>sentinel</i>&lt;false&gt;</code> not comparable with <code><i>iterator</i>&lt;true&gt;</code></h3>
<p><b>Section:</b> 25.7.9.4 <a href="https://wg21.link/range.transform.sentinel">[range.transform.sentinel]</a>, 25.7.14.4 <a href="https://wg21.link/range.join.sentinel">[range.join.sentinel]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2020-05-26 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.transform.sentinel">issues</a> in [range.transform.sentinel].</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>
A user reported that this doesn't compile:
</p>
<blockquote><pre>
#include &lt;list&gt;
#include &lt;ranges&gt;

std::list v{1, 2}; // works if std::vector
auto view1 = v | std::views::take(2);
auto view2 = view1 | std::views::transform([] (int i) { return i; });
bool b = std::ranges::cbegin(view2) == std::ranges::end(view2);
</pre></blockquote>
<p>
The comparison is supposed to use <code>operator==(<i>iterator</i>&lt;Const&gt;, <i>sentinel</i>&lt;Const&gt;)</code> 
after converting <code><i>sentinel</i>&lt;false&gt;</code> to <code><i>sentinel</i>&lt;true&gt;</code>. However, the 
<code>operator==</code> is a hidden friend so is not a candidate when comparing <code><i>iterator</i>&lt;true&gt;</code> 
with <code><i>sentinel</i>&lt;false&gt;</code>. The required conversion would only happen if we'd found the operator, 
but we can't find the operator until after the conversion happens.
<p/>
As Patrick noted, the <code>join_view</code> sentinel has a similar problem.
<p/>
The proposed wording shown below has been suggested by Casey and has been implemented and tested in GCC's libstdc++.
</p>

<p><i>[2020-07-17; Priority set to 1 in telecon]</i></p>

<p>
Should be considered together with <a href="lwg-defects.html#3406" title="elements_view::begin() and elements_view::end() have incompatible
constraints (Status: C++23)">3406</a><sup><a href="https://cplusplus.github.io/LWG/issue3406" title="Latest snapshot">(i)</a></sup> and <a href="lwg-defects.html#3449" title="take_view and take_while_view's sentinel&lt;false&gt; not comparable with their const iterator (Status: C++23)">3449</a><sup><a href="https://cplusplus.github.io/LWG/issue3449" title="Latest snapshot">(i)</a></sup>.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
  
<p>This wording is relative to <a href="https://wg21.link/n4861">N4861</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 class="note">
<p>
[<i>Drafting note:</i> The project editor is kindly asked to consider replacing editorially all of the
<p/>
"<code>using <i>Base</i> = conditional_t&lt;Const, const V, V&gt;;</code>"
<p/> 
occurrences by
<p/>
"<code>using <i>Base</i> = <i>maybe-const</i>&lt;Const, V&gt;;</code>"
<p/>
]
</p>
</blockquote>

<blockquote>
<pre>
[&hellip;]
namespace std::ranges {
[&hellip;]
  namespace views { inline constexpr <i>unspecified</i> filter = <i>unspecified</i>; }
  
  <ins>template&lt;bool Const, class T&gt;</ins>
    <ins>using <i>maybe-const</i> = conditional_t&lt;Const, const T, T&gt;; <i>// exposition-only</i></ins>
  
  <i>// 25.7.9 <a href="https://wg21.link/range.transform">[range.transform]</a>, transform view</i>
  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;
  class transform_view;
[&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.9.4 <a href="https://wg21.link/range.transform.sentinel">[range.transform.sentinel]</a>, class template <code>transform_view::<i>sentinel</i></code> 
synopsis, 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>sentinel</i> {
    [&hellip;]
    constexpr sentinel_t&lt;<i>Base</i>&gt; base() const;
    
    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
    friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);
    
    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
    friend constexpr range_difference_t&lt;<i>Base</i>&gt;
      operator-(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y)
        <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;

    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
    friend constexpr range_difference_t&lt;<i>Base</i>&gt;
      operator-(const <i>sentinel</i>&amp; y, const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x)
        <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);
</pre>
<blockquote>
<p>
-4- <i>Effects:</i> Equivalent to: <code>return x.<i>current_</i> == y.<i>end_</i>;</code>
</p>
</blockquote>
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
friend constexpr range_difference_t&lt;<i>Base</i>&gt;
  operator-(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y)
    <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> Equivalent to: <code>return x.<i>current_</i> - y.<i>end_</i>;</code>
</p>
</blockquote>
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
friend constexpr range_difference_t&lt;<i>Base</i>&gt;
  operator-(const <i>sentinel</i>&amp; y, const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x)
    <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;
</pre>
<blockquote>
<p>
-6- <i>Effects:</i> Equivalent to: <code>return y.<i>end_</i> - x.<i>current_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>


<li><p>Modify 25.7.14.4 <a href="https://wg21.link/range.join.sentinel">[range.join.sentinel]</a>, class template <code>join_view::<i>sentinel</i></code> 
synopsis, as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V&gt;
    requires view&lt;V&gt; &amp;&amp; input_range&lt;range_reference_t&lt;V&gt;&gt; &amp;&amp;
             (is_reference_v&lt;range_reference_t&lt;V&gt;&gt; ||
             view&lt;range_value_t&lt;V&gt;&gt;)
  template&lt;bool Const&gt;
  class join_view&lt;V&gt;::<i>sentinel</i> {
    [&hellip;]

    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins>
    friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);  
};
}
</pre>
</blockquote>
[&hellip;]
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins>
friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);  
</pre>
<blockquote>
<p>
-3- <i>Effects:</i> Equivalent to: <code>return x.<i>outer_</i> == y.<i>end_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2020-08-21 Tim updates PR per telecon discussion]</i></p>

<p>As noted in the PR of LWG <a href="lwg-defects.html#3406" title="elements_view::begin() and elements_view::end() have incompatible
constraints (Status: C++23)">3406</a><sup><a href="https://cplusplus.github.io/LWG/issue3406" title="Latest snapshot">(i)</a></sup>, the return type of <code>operator-</code>
should be based on the constness of the iterator rather than that of the sentinel, as
<code>sized_sentinel_for&lt;S, I&gt;</code> (24.3.4.8 <a href="https://wg21.link/iterator.concept.sizedsentinel">[iterator.concept.sizedsentinel]</a>)
requires <code>decltype(i - s)</code> to be <code>iter_difference_t&lt;I&gt;</code>.</p>

<p><i>[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]</i></p>


<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3448"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.</p>

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

<blockquote class="note">
<p>
[<i>Drafting note:</i> The project editor is kindly asked to consider replacing editorially all of the
<p/>
"<code>using <i>Base</i> = conditional_t&lt;Const, const V, V&gt;;</code>"
<p/> 
occurrences by
<p/>
"<code>using <i>Base</i> = <i>maybe-const</i>&lt;Const, V&gt;;</code>"
<p/>
]
</p>
</blockquote>

<blockquote>
<pre>
[&hellip;]
namespace std::ranges {
[&hellip;]
  namespace views { inline constexpr <i>unspecified</i> filter = <i>unspecified</i>; }
  
  <ins>template&lt;bool Const, class T&gt;</ins>
    <ins>using <i>maybe-const</i> = conditional_t&lt;Const, const T, T&gt;; <i>// exposition-only</i></ins>
  
  <i>// 25.7.9 <a href="https://wg21.link/range.transform">[range.transform]</a>, transform view</i>
  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;
  class transform_view;
[&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.9.4 <a href="https://wg21.link/range.transform.sentinel">[range.transform.sentinel]</a>, class template <code>transform_view::<i>sentinel</i></code> 
synopsis, 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>sentinel</i> {
    [&hellip;]
    constexpr sentinel_t&lt;<i>Base</i>&gt; base() const;
    
    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
    friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);
    
    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
    friend constexpr range_difference_t&lt;<del><i>Base</i></del><ins><i>maybe-const</i>&lt;OtherConst, V&gt;</ins>&gt;
      operator-(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y)
        <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;

    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
    friend constexpr range_difference_t&lt;<del><i>Base</i></del><ins><i>maybe-const</i>&lt;OtherConst, V&gt;</ins>&gt;
      operator-(const <i>sentinel</i>&amp; y, const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x)
        <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);
</pre>
<blockquote>
<p>
-4- <i>Effects:</i> Equivalent to: <code>return x.<i>current_</i> == y.<i>end_</i>;</code>
</p>
</blockquote>
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
friend constexpr range_difference_t&lt;<del><i>Base</i></del><ins><i>maybe-const</i>&lt;OtherConst, V&gt;</ins>&gt;
  operator-(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y)
    <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> Equivalent to: <code>return x.<i>current_</i> - y.<i>end_</i>;</code>
</p>
</blockquote>
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins> 
friend constexpr range_difference_t&lt;<del><i>Base</i></del><ins><i>maybe-const</i>&lt;OtherConst, V&gt;</ins>&gt;
  operator-(const <i>sentinel</i>&amp; y, const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x)
    <del>requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;</del>;
</pre>
<blockquote>
<p>
-6- <i>Effects:</i> Equivalent to: <code>return y.<i>end_</i> - x.<i>current_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>


<li><p>Modify 25.7.14.4 <a href="https://wg21.link/range.join.sentinel">[range.join.sentinel]</a>, class template <code>join_view::<i>sentinel</i></code> 
synopsis, as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V&gt;
    requires view&lt;V&gt; &amp;&amp; input_range&lt;range_reference_t&lt;V&gt;&gt; &amp;&amp;
             (is_reference_v&lt;range_reference_t&lt;V&gt;&gt; ||
             view&lt;range_value_t&lt;V&gt;&gt;)
  template&lt;bool Const&gt;
  class join_view&lt;V&gt;::<i>sentinel</i> {
    [&hellip;]

    <ins>template&lt;bool OtherConst&gt;</ins>
      <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins>
    friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);  
};
}
</pre>
</blockquote>
[&hellip;]
<pre>
<ins>template&lt;bool OtherConst&gt;</ins>
  <ins>requires sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;</ins>
friend constexpr bool operator==(const <i>iterator</i>&lt;<ins>Other</ins>Const&gt;&amp; x, const <i>sentinel</i>&amp; y);  
</pre>
<blockquote>
<p>
-3- <i>Effects:</i> Equivalent to: <code>return x.<i>outer_</i> == y.<i>end_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>

</ol>




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