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<title>Issue 3868: Constrained algorithms should not require output_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#LEWG">LEWG</a> status.</em></p>
<h3 id="3868"><a href="lwg-active.html#3868">3868</a>. Constrained algorithms should not require <code>output_iterator</code></h3>
<p><b>Section:</b> 26.7.5 <a href="https://wg21.link/alg.replace">[alg.replace]</a>, 26.7.6 <a href="https://wg21.link/alg.fill">[alg.fill]</a> <b>Status:</b> <a href="lwg-active.html#LEWG">LEWG</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-01-29 <b>Last modified:</b> 2024-06-28</p>
<p><b>Priority: </b>4
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.replace">issues</a> in [alg.replace].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#LEWG">LEWG</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In C++20, <code>output_iterator</code> is required to support <code>*o++ = t</code> mainly for backward compatibility,
that is to say, in addition to avoiding needless breakage, this semantic is actually not very useful, as it can 
be equivalently replaced by <code>*o = t; ++o;</code>.
<p/>
This is reflected in the current implementation for constrained algorithms in libstdc++ and MSVC-STL. 
Even if the algorithm explicitly requires <code>output_iterator</code>, there is no code of the form <code>*o++ = t</code> in practice,
and the latter of a more generic form is used instead.
<p/>
It seems to me that constrained algorithms should never require <code>output_iterator</code>, since there really isn't
any desirable reason to use <code>*o++ = t</code> in the new iterator system.
It would be more appropriate to relax <code>output_iterator</code> to <code>weakly_incrementable</code> 
(or <code>input_or_output_iterator</code>) and <code>indirectly_writable</code>,
given that many constrained algorithms already do that.
</p>

<p><i>[2023-02-06; Reflector poll]</i></p>

<p>
Set priority to 4 after reflector poll. Send to LEWG.
Several votes for NAD.
</p>

<p><i>[St. Louis 2024-06-28; LWG and SG9 joint session]</i></p>

<p>
Poll: SG9 and LWG believe this is not a defect, but we do agree that there are inconsistencies that need a paper to address more thoroughly
<pre>
|SF| F| N| A|SA|
| 6| 2| 0| 0| 0|
</pre>
</p>



<p id="res-3868"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>
<li><p>Modify 26.4 <a href="https://wg21.link/algorithm.syn">[algorithm.syn]</a>, header <code>&lt;algorithm&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
#include &lt;initializer_list&gt;     // <i>see 17.11.2 <a href="https://wg21.link/initializer.list.syn">[initializer.list.syn]</a></i>

namespace std {
  [&hellip;]
  namespace ranges {
    template&lt;class I, class O&gt;
      using replace_copy_result = in_out_result&lt;I, O&gt;;

    template&lt;input_iterator I, sentinel_for&lt;I&gt; S, class T1, class T2,
             <ins>weakly_incrementable</ins><del>output_iterator&lt;const T2&amp;&gt;</del> O, class Proj = identity&gt;
      requires <ins>indirectly_writable&lt;O, const T2&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;I, O&gt; &amp;&amp;
               indirect_binary_predicate&lt;ranges::equal_to, projected&lt;I, Proj&gt;, const T1*&gt;
      constexpr replace_copy_result&lt;I, O&gt;
        replace_copy(I first, S last, O result, const T1&amp; old_value, const T2&amp; new_value,
                     Proj proj = {});
    template&lt;input_range R, class T1, class T2, <ins>weakly_incrementable</ins><del>output_iterator&lt;const T2&amp;&gt;</del> O,
             class Proj = identity&gt;
      requires <ins>indirectly_writable&lt;O, const T2&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;iterator_t&lt;R&gt;, O&gt; &amp;&amp;
               indirect_binary_predicate&lt;ranges::equal_to,
                                         projected&lt;iterator_t&lt;R&gt;, Proj&gt;, const T1*&gt;
      constexpr replace_copy_result&lt;borrowed_iterator_t&lt;R&gt;, O&gt;
        replace_copy(R&amp;&amp; r, O result, const T1&amp; old_value, const T2&amp; new_value,
                     Proj proj = {});

    template&lt;class I, class O&gt;
      using replace_copy_if_result = in_out_result&lt;I, O&gt;;

    template&lt;input_iterator I, sentinel_for&lt;I&gt; S, class T, <ins>weakly_incrementable</ins><del>output_iterator&lt;const T&amp;&gt;</del> O,
             class Proj = identity, indirect_unary_predicate&lt;projected&lt;I, Proj&gt;&gt; Pred&gt;
      requires <ins>indirectly_writable&lt;O, const T&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;I, O&gt;
      constexpr replace_copy_if_result&lt;I, O&gt;
        replace_copy_if(I first, S last, O result, Pred pred, const T&amp; new_value,
                        Proj proj = {});
    template&lt;input_range R, class T, <ins>weakly_incrementable</ins><del>output_iterator&lt;const T&amp;&gt;</del> O, class Proj = identity,
             indirect_unary_predicate&lt;projected&lt;iterator_t&lt;R&gt;, Proj&gt;&gt; Pred&gt;
      requires <ins>indirectly_writable&lt;O, const T&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;iterator_t&lt;R&gt;, O&gt;
      constexpr replace_copy_if_result&lt;borrowed_iterator_t&lt;R&gt;, O&gt;
        replace_copy_if(R&amp;&amp; r, O result, Pred pred, const T&amp; new_value,
                        Proj proj = {});
  }
  [&hellip;]
  namespace ranges {
    template&lt;class T, <ins>input_or_output_iterator</ins><del>output_iterator&lt;const T&amp;&gt;</del> O, sentinel_for&lt;O&gt; S&gt;
      <ins>requires indirectly_writable&lt;O, const T&amp;&gt;</ins>
      constexpr O fill(O first, S last, const T&amp; value);
    template&lt;class T, output_range&lt;const T&amp;&gt; R&gt;
      constexpr borrowed_iterator_t&lt;R&gt; fill(R&amp;&amp; r, const T&amp; value);
    template&lt;class T, <ins>input_or_output_iterator</ins><del>output_iterator&lt;const T&amp;&gt;</del> O&gt;
      <ins>requires indirectly_writable&lt;O, const T&amp;&gt;</ins>
      constexpr O fill_n(O first, iter_difference_t&lt;O&gt; n, const T&amp; value);
  }
  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 26.7.5 <a href="https://wg21.link/alg.replace">[alg.replace]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;input_iterator I, sentinel_for&lt;I&gt; S, class T1, class T2, <ins>weakly_incrementable</ins><del>output_iterator&lt;const T2&amp;&gt;</del> O, 
         class Proj = identity&gt;
  requires <ins>indirectly_writable&lt;O, const T2&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;I, O&gt; &amp;&amp;
           indirect_binary_predicate&lt;ranges::equal_to, projected&lt;I, Proj&gt;, const T1*&gt;
constexpr ranges::replace_copy_result&lt;I, O&gt;
  ranges::replace_copy(I first, S last, O result, const T1&amp; old_value, const T2&amp; new_value,
                       Proj proj = {});
template&lt;input_range R, class T1, class T2, <ins>weakly_incrementable</ins><del>output_iterator&lt;const T2&amp;&gt;</del> O,
         class Proj = identity&gt;
  requires <ins>indirectly_writable&lt;O, const T2&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;iterator_t&lt;R&gt;, O&gt; &amp;&amp;
           indirect_binary_predicate&lt;ranges::equal_to, projected&lt;iterator_t&lt;R&gt;, Proj&gt;, const T1*&gt;
constexpr ranges::replace_copy_result&lt;borrowed_iterator_t&lt;R&gt;, O&gt;
  ranges::replace_copy(R&amp;&amp; r, O result, const T1&amp; old_value, const T2&amp; new_value,
                       Proj proj = {});

template&lt;input_iterator I, sentinel_for&lt;I&gt; S, class T, <ins>weakly_incrementable</ins><del>output_iterator&lt;const T&amp;&gt;</del> O,
         class Proj = identity, indirect_unary_predicate&lt;projected&lt;I, Proj&gt;&gt; Pred&gt;
  requires <ins>indirectly_writable&lt;O, const T&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;I, O&gt;
constexpr ranges::replace_copy_if_result&lt;I, O&gt;
  ranges::replace_copy_if(I first, S last, O result, Pred pred, const T&amp; new_value,
                          Proj proj = {});
template&lt;input_range R, class T, <ins>weakly_incrementable</ins><del>output_iterator&lt;const T&amp;&gt;</del> O, class Proj = identity,
         indirect_unary_predicate&lt;projected&lt;iterator_t&lt;R&gt;, Proj&gt;&gt; Pred&gt;
  requires <ins>indirectly_writable&lt;O, const T&amp;&gt; &amp;&amp;</ins> indirectly_copyable&lt;iterator_t&lt;R&gt;, O&gt;
constexpr ranges::replace_copy_if_result&lt;borrowed_iterator_t&lt;R&gt;, O&gt;
  ranges::replace_copy_if(R&amp;&amp; r, O result, Pred pred, const T&amp; new_value,
                          Proj proj = {});

</pre>
<blockquote>
<p>
-6- Let <i>E</i> be
<p/>
[&hellip;]
</p>
</blockquote>

</blockquote>
</li>


<li><p>Modify 26.7.6 <a href="https://wg21.link/alg.fill">[alg.fill]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;class T, <ins>input_or_output_iterator</ins><del>output_iterator&lt;const T&amp;&gt;</del> O, sentinel_for&lt;O&gt; S&gt;
  <ins>requires indirectly_writable&lt;O, const T&amp;&gt;</ins>
  constexpr O ranges::fill(O first, S last, const T&amp; value);
template&lt;class T, output_range&lt;const T&amp;&gt; R&gt;
  constexpr borrowed_iterator_t&lt;R&gt; ranges::fill(R&amp;&amp; r, const T&amp; value);
template&lt;class T, <ins>input_or_output_iterator</ins><del>output_iterator&lt;const T&amp;&gt;</del> O&gt;
  <ins>requires indirectly_writable&lt;O, const T&amp;&gt;</ins>
  constexpr O ranges::fill_n(O first, iter_difference_t&lt;O&gt; n, const T&amp; value);
</pre>
<blockquote>
<p>
-1- Let <i>N</i> be max(0, <code>n</code>) for the <code>fill_n</code> algorithms, and <code>last - first</code> for the <code>fill</code> algorithms.
<p/>
[&hellip;]
</p>
</blockquote>

</blockquote>
</li>

</ol>




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