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<title>Issue 4226: to_input_view::iterator cannot be compared to its const sentinel</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#New">New</a> status.</em></p>
<h3 id="4226"><a href="lwg-active.html#4226">4226</a>. <code>to_input_view::<i>iterator</i></code> cannot be compared to its <code class='backtick'>const</code> sentinel</h3>
<p><b>Section:</b> 25.7.35 <a href="https://wg21.link/range.to.input">[range.to.input]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-03-15 <b>Last modified:</b> 2025-06-13</p>
<p><b>Priority: </b>2
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code class='backtick'>to_input_view</code> was recently added to the working draft by <a href="https://wg21.link/P3137R3" title=" views::to_input">P3137R3</a>.
<p/>
Consider: (<a href="https://godbolt.org/z/7s1hPscrG">demo</a>):
</p>
<blockquote><pre>
#include &lt;ranges&gt;

int main() {
  auto r = std::views::single(0)
         | std::views::chunk(1)
         | std::views::to_input;

  r.begin() == std::as_const(r).end(); // <span style="color:red;font-weight:bolder">#1, error</span>
  r.begin() ==               r.cend(); // <span style="color:red;font-weight:bolder">#2, error</span>
}
</pre></blockquote>
<p>
In <code>#1</code>, <code>r.begin()</code> returns <code>to_input_view&lt;chunk_view&gt;::<i>iterator</i>&lt;false&gt;</code>,
while the latter returns <code>chunk_view::<i>iterator</i>&lt;true&gt;</code>.
Since the former can only be compared with <code>chunk_view::<i>iterator</i>&lt;false&gt;</code> that cannot
be converted from <code>chunk_view::<i>iterator</i>&lt;true&gt;</code>, the two are incomparable.
<p/>
This can be fixed by adding the following overload to <code>to_input_view::<i>iterator</i></code>:
</p>
<blockquote><pre>
template&lt;bool OtherConst = !Const&gt;
  requires sentinel_for&lt;sentinel_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;
friend constexpr bool operator==(const <i>iterator</i>&amp; x, const sentinel_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&amp; y)
{ return x.<i>current_</i> == y; }
</pre></blockquote>
<p>
Unfortunately, it still doesn't resolve <code>#2</code>, because <code>r.cend()</code> returns
<code>basic_const_iterator&lt;chunk_view::iterator&lt;true&gt;&gt;</code>, which cannot be compared to any
non-copyable iterators as its <code>operator==(const S&amp; s)</code> requires <code>S</code> to be a 
sentinel type, which rules out <code>to_input_view::<i>iterator</i></code>, so the constraint is not satisfied.
<p/>
I believe we may need to introduce a custom sentinel class for <code>to_input_view</code>.
</p>

<p><i>[2025-06-13; Reflector poll]</i></p>

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



<p id="res-4226"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>

<li><p>Modify 25.7.35.2 <a href="https://wg21.link/range.to.input.view">[range.to.input.view]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V&gt;
    requires view&lt;V&gt;
  class to_input_view : public view_interface&lt;to_input_view&lt;V&gt;&gt; {
    V <i>base_</i> = V();                          // <i>exposition only</i>

    template&lt;bool Const&gt;
    class <i>iterator</i>;                         // <i>exposition only</i>
    <ins>template&lt;bool Const&gt;
    class <i>sentinel</i>;                         // <i>exposition only</i></ins>
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr auto end() requires (!<i>simple-view</i>&lt;V&gt;);
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to:</ins>
</p>
<pre>
<ins>return <i>sentinel</i>&lt;false&gt;(ranges::end(<i>base_</i>));</ins>
</pre>
</blockquote>
<pre>
constexpr auto end() const requires range&lt;const V&gt;;
</pre>
<blockquote>
<p>
-4- <i>Effects</i>: Equivalent to:
</p>
<pre>
return <ins><i>sentinel</i>&lt;true&gt;(</ins>ranges::end(<i>base_</i>)<ins>)</ins>;
</pre>
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.35.3 <a href="https://wg21.link/range.to.input.iterator">[range.to.input.iterator]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V&gt;
    requires view&lt;V&gt;
  template&lt;bool Const&gt;
  class to_input_view&lt;V&gt;::<i>iterator</i> {
    [&hellip;]
    <del>friend constexpr bool operator==(const <i>iterator</i>&amp; x, const sentinel_t&lt;<i>Base</i>&gt;&amp; y);

    friend constexpr difference_type operator-(const sentinel_t&lt;<i>Base</i>&gt;&amp; y, const <i>iterator</i>&amp; x)
      requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;;
    friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const sentinel_t&lt;<i>Base</i>&gt;&amp; y)
      requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;;</del>
    [&hellip;]
  };
}
</pre>
</blockquote>
[&hellip;]
<pre><del>
friend constexpr bool operator==(const <i>iterator</i>&amp; x, const sentinel_t&lt;<i>Base</i>&gt;&amp; y);</del>
</pre>
<blockquote>
<p>
<del>-7- <i>Returns</i>: <code>x.<i>current_</i> == y</code>.</del>
</p>
</blockquote>
<pre><del>
friend constexpr difference_type operator-(const sentinel_t&lt;<i>Base</i>&gt;&amp; y, const <i>iterator</i>&amp; x)
  requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;;</del>
</pre>
<blockquote>
<p>
<del>-8- <i>Returns</i>: <code>y - x.<i>current_</i></code>.</del>
</p>
</blockquote>
<pre><del>
friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const sentinel_t&lt;<i>Base</i>&gt;&amp; y)
  requires sized_sentinel_for&lt;sentinel_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;;</del>
</pre>
<blockquote>
<p>
<del>-9- <i>Returns</i>: <code>x.<i>current_</i> - y</code>.</del>
</p>
</blockquote>
</blockquote>
</li>


<li><p>Add [range.to.input.sentinel] after [range.to.input.iterator] as indicated:</p>

<blockquote>
<blockquote>
<pre><ins>
namespace std::ranges {
  template&lt;input_range V&gt;
    requires view&lt;V&gt;
  template&lt;bool Const&gt;
  class to_input_view&lt;V&gt;::<i>sentinel</i> {
    using <i>Base</i> = <i>maybe-const</i>&lt;Const, V&gt;;                         // <i>exposition only</i>
    sentinel_t&lt;<i>Base</i>&gt; <i>end_</i> = sentinel_t&lt;<i>Base</i>&gt;();                 // <i>exposition only</i>
    constexpr explicit <i>sentinel</i>(sentinel_t&lt;<i>Base</i>&gt; end);          // <i>exposition only</i>

  public:
    <i>sentinel</i>() = default;
    constexpr <i>sentinel</i>(<i>sentinel</i>&lt;!Const&gt; other)
      requires Const &amp;&amp; convertible_to&lt;sentinel_t&lt;V&gt;, sentinel_t&lt;<i>Base</i>&gt;&gt;;

    constexpr sentinel_t&lt;Base&gt; base() const;

    template&lt;bool OtherConst&gt;
      requires sentinel_for&lt;sentinel_t&lt;Base&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;
    friend constexpr bool operator==(const <i>iterator</i>&lt;OtherConst&gt;&amp; x, const <i>sentinel</i>&amp; y);

    template&lt;bool OtherConst&gt;
      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;
    friend constexpr range_difference_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;
      operator-(const <i>iterator</i>&lt;OtherConst&gt;&amp; x, const <i>sentinel</i>&amp; y);

    template&lt;bool OtherConst&gt;
      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;
    friend constexpr range_difference_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;
      operator-(const <i>sentinel</i>&amp; x, const <i>iterator</i>&lt;OtherConst&gt;&amp; y);
  };
}</ins>
</pre>
</blockquote>
<pre><ins>
constexpr explicit <i>sentinel</i>(sentinel_t&lt;<i>Base</i>&gt; end);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Initializes <code><i>end_</i></code> with <code>std::move(end)</code>.</ins>
</p>
</blockquote>
<pre><ins>
constexpr <i>sentinel</i>(<i>sentinel</i>&lt;!Const&gt; other)
  requires Const &amp;&amp; convertible_to&lt;sentinel_t&lt;V&gt;, sentinel_t&lt;<i>Base</i>&gt;&gt;;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Initializes <code><i>end_</i></code> with <code>std::move(other.<i>end_</i>)</code>.</ins>
</p>
</blockquote>
<pre><ins>
constexpr sentinel_t&lt;Base&gt; base() const;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return <i>end_</i>;</code></ins>
</p>
</blockquote>
<pre><ins>
template&lt;bool OtherConst&gt;
  requires sentinel_for&lt;sentinel_t&lt;Base&gt;, iterator_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;&gt;
friend constexpr bool operator==(const <i>iterator</i>&lt;OtherConst&gt;&amp; x, const <i>sentinel</i>&amp; y);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return x.<i>current_</i> == y.<i>end_</i>;</code></ins>
</p>
</blockquote>
<pre><ins>
template&lt;bool OtherConst&gt;
  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;
friend constexpr range_difference_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;
  operator-(const <i>iterator</i>&lt;OtherConst&gt;&amp; x, const <i>sentinel</i>&amp; y);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return x.<i>current_</i> - y.<i>end_</i>;</code></ins>
</p>
</blockquote>
<pre><ins>
template&lt;bool OtherConst&gt;
  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;
friend constexpr range_difference_t&lt;<i>maybe-const</i>&lt;OtherConst, V&gt;&gt;
  operator-(const <i>sentinel</i>&amp; x, const <i>iterator</i>&lt;OtherConst&gt;&amp; y);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Effects</i>: Equivalent to: <code>return x.<i>end_</i> - y.<i>current_</i>;</code></ins>
</p>
</blockquote>
</blockquote>
</li>
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