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<title>Issue 3820: cartesian_product_view::iterator::prev is not quite right</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="3820"><a href="lwg-defects.html#3820">3820</a>. <code>cartesian_product_view::<i>iterator</i>::<i>prev</i></code> is not quite right</h3>
<p><b>Section:</b> 25.7.33.3 <a href="https://wg21.link/range.cartesian.iterator">[range.cartesian.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-11-08 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.cartesian.iterator">issues</a> in [range.cartesian.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>
Currently, <code>cartesian_product_view::<i>iterator</i>::<i>prev</i></code> has the following <i>Effects</i>:
</p>
<blockquote><pre>
auto&amp; it = std::get&lt;N&gt;(<i>current_</i>);
if (it == ranges::begin(std::get&lt;N&gt;(<i>parent_</i>-><i>bases_</i>))) {
  it = <i>cartesian-common-arg-end</i>(std::get&lt;N&gt;(<i>parent_</i>-&gt;<i>bases_</i>));
  if constexpr (N &gt; 0) {
    <i>prev</i>&lt;N - 1&gt;();
  }
}
--it;
</pre></blockquote>
<p>
which decrements the underlying iterator one by one using recursion. 
However, when <code>N == 0</code>, it still detects if the first iterator has reached the beginning and assigns it to 
the end, which is not only unnecessary, but also causes <code><i>cartesian-common-arg-end</i></code> to be applied to 
the first range, making it ill-formed in some cases, <a href="https://godbolt.org/z/Pd6bYnP1G">for example</a>:
</p>
<blockquote><pre>
#include &lt;ranges&gt;

int main() {
  auto r = std::views::cartesian_product(std::views::iota(0));
  r.begin() += 3; // <span style="color:red;font-weight:bolder">hard error</span>
}
</pre></blockquote>
<p>
This is because, for the first range, <code>cartesian_product_view::<i>iterator</i>::operator+=</code> only requires 
it to model <code>random_access_range</code>.
However, when <code>x</code> is negative, this function will call <code><i>prev</i></code> and indirectly calls 
<code><i>cartesian-common-arg-end</i></code>, since the latter constrains its argument to satisfy 
<code><i>cartesian-product-common-arg</i></code>, that is, <code>common_range&lt;R&gt; || (sized_range&lt;R&gt; &amp;&amp; 
random_access_range&lt;R&gt;)</code>, which is not the case for the unbounded <code>iota_view</code>, resulting in a 
hard error in <code><i>prev</i></code>'s function body.
<p/>
The proposed resolution changes the position of the <code>if constexpr</code> so that we just decrement the first 
iterator and nothing else.
</p>

<p><i>[Kona 2022-11-08; Move to Ready]</i></p>


<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3820"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify  [ranges.cartesian.iterator] as indicated:</p>

<blockquote>
<pre>
template&lt;size_t N = sizeof...(Vs)&gt;
  constexpr void <i>prev</i>();
</pre>
<blockquote>
<p>
-6- <i>Effects</i>: Equivalent to:
</p>
<pre>
auto&amp; it = std::get&lt;N&gt;(<i>current_</i>);
<ins>if constexpr (N > 0) {</ins>
  if (it == ranges::begin(std::get&lt;N&gt;(<i>parent_</i>-><i>bases_</i>))) {
    it = <i>cartesian-common-arg-end</i>(std::get&lt;N&gt;(<i>parent_</i>-&gt;<i>bases_</i>));
    <del>if constexpr (N &gt; 0) {</del>
      <i>prev</i>&lt;N - 1&gt;();
    <del>}</del>
  }
<ins>}</ins>
--it;
</pre>
</blockquote>
</blockquote>

</li>
</ol>





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