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<title>Issue 3969: std::ranges::fold_left_first_with_iter should be more ADL-proof</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="3969"><a href="lwg-active.html#3969">3969</a>. <code>std::ranges::fold_left_first_with_iter</code> should be more ADL-proof</h3>
<p><b>Section:</b> 26.6.18 <a href="https://wg21.link/alg.fold">[alg.fold]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-08-10 <b>Last modified:</b> 2023-11-03</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#alg.fold">active issues</a> in [alg.fold].</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.fold">issues</a> in [alg.fold].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following program is currently ill-formed, because 26.6.18 <a href="https://wg21.link/alg.fold">[alg.fold]</a>/10 requires evaluating 
<code>*init</code>, where <code>init</code> is an object of an <code>optional</code> specialization, which triggers 
ADL and finds unwanted overloads.
</p>
<blockquote><pre>
#include &lt;algorithm&gt;
#include &lt;optional&gt;

namespace myns {
  struct Foo {};
  
  void operator*(std::optional&lt;Foo&gt;&amp;);
  void operator*(const std::optional&lt;Foo&gt;&amp;);
}

int main()
{
  myns::Foo x[1]{};
  std::ranges::fold_left_first_with_iter(x, []&lt;class T&gt;(T lhs, T) { return lhs; });
}
</pre></blockquote>
<p>
I think only the member <code>operator*</code> overload is intendedly used.
</p>

<p><i>[2023-11-03; Reflector poll]</i></p>

<p>Many votes for NAD.
"Yuck, can we just use <code>.value()</code> instead?"
"The example is not good motivation, but we should ADL-proof to avoid
attempting to complete incomplete associated classes."
</p>



<p id="res-3969"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</a>.
</p>

<ol>

<li><p>Modify 26.6.18 <a href="https://wg21.link/alg.fold">[alg.fold]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;input_iterator I, sentinel_for&lt;I&gt; S,
         <i>indirectly-binary-left-foldable</i>&lt;iter_value_t&lt;I&gt;, I&gt; F&gt;
  requires constructible_from&lt;iter_value_t&lt;I&gt;, iter_reference_t&lt;I&gt;&gt;
  constexpr <i>see below</i> ranges::fold_left_first_with_iter(I first, S last, F f);
template&lt;input_range R, <i>indirectly-binary-left-foldable</i>&lt;range_value_t&lt;R&gt;, iterator_t&lt;R&gt;&gt; F&gt;
  requires constructible_from&lt;range_value_t&lt;R&gt;, range_reference_t&lt;R&gt;&gt;
  constexpr <i>see below</i> ranges::fold_left_first_with_iter(R&amp;&amp; r, F f);
</pre>
<blockquote>
<p>
-9- Let <code>U</code> be
</p>
<blockquote><pre>
decltype(ranges::fold_left(std::move(first), last, iter_value_t&lt;I&gt;(*first), f))
</pre></blockquote>
<p>
-10- <i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
if (first == last)
  return {std::move(first), optional&lt;U&gt;()};
optional&lt;U&gt; init(in_place, *first);
for (++first; first != last; ++first)
  <del>*init</del><ins>init.operator*()</ins> = invoke(f, std::move(<del>*init</del><ins>init.operator*()</ins>), *first);
return {std::move(first), std::move(init)};
</pre></blockquote>
</blockquote>
</blockquote>
</li>
</ol>





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