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<title>Issue 4218: Constraint recursion in basic_const_iterator's relational operators due to ADL + CWG 2369</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="4218"><a href="lwg-active.html#4218">4218</a>. Constraint recursion in <code class='backtick'>basic_const_iterator</code>'s relational operators due to ADL + CWG 2369</h3>
<p><b>Section:</b> 24.5.3.5 <a href="https://wg21.link/const.iterators.ops">[const.iterators.ops]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Patrick Palka <b>Opened:</b> 2025-03-03 <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>
Consider the example (devised by Hewill Kang)
</p>
<blockquote><pre>
using RCI = reverse_iterator&lt;basic_const_iterator&lt;vector&lt;int&gt;::iterator&gt;&gt;;
static_assert(std::totally_ordered&lt;RCI&gt;);
</pre></blockquote>
<p>
Checking <code class='backtick'>RCI</code> is <code class='backtick'>totally_ordered</code> entails checking
</p>
<blockquote><pre>
requires (RCI x) { x <i>RELOP</i> x; } for each <i>RELOP</i> in {&lt;, &gt;, &lt;=, &gt;=}
</pre></blockquote>
<p>
which we expect to be straightforwardly satisfied by <code class='backtick'>reverse_iterator</code>'s
namespace-scope operators (24.5.1.8 <a href="https://wg21.link/reverse.iter.cmp">[reverse.iter.cmp]</a>):
</p>
<blockquote><pre>
template&lt;class Iterator1, class Iterator2&gt;
  constexpr bool operator&lt;(
    const reverse_iterator&lt;Iterator1&gt;&amp; x,
    const reverse_iterator&lt;Iterator2&gt;&amp; y);
// etc
</pre></blockquote>
<p>
But due to ADL we find ourselves also considering the <code class='backtick'>basic_const_iterator</code>
relop friends (24.5.3.5 <a href="https://wg21.link/const.iterators.ops">[const.iterators.ops]</a>/24).
</p>
<blockquote><pre>
template&lt;input_iterator Iterator&gt;
class basic_const_iterator {
  template&lt;<i>not-a-const-iterator</i> I&gt;
    friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator&amp; y)
      requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;
  // etc
};
</pre></blockquote>
<p>
Before <a href="https://wg21.link/cwg2369">CWG 2369</a> these candidates would quickly get 
discarded since for the second operand RCI clearly isn't convertible to <code class='backtick'>basic_const_iterator</code>.  
But after CWG 2369 implementations must first check these operators' constraints
(with <code>Iterator = vector&lt;int&gt;::iterator</code> and <code>I = RCI</code>), which entails 
checking <code>totally_ordered&lt;RCI&gt;</code> recursively, causing the example to be ill-formed.
<p/>
The constraint recursion is diagnosed by GCC (<a href="https://godbolt.org/z/dr1dK1dnj">See godbolt demo</a>).
Other compilers accept the example because they don't implement CWG 2369, as
far as I know.
<p/>
GCC trunk works around this issue by giving these friend relational operators
a dependent second operand of the form <code>basic_const_iterator&lt;J&gt;</code> where <code class='backtick'>J</code> is
constrained to match <code class='backtick'>Iterator</code>:
</p>
<blockquote><pre>
template&lt;<i>not-a-const-iterator</i> I, same_as&lt;Iterator&gt; J>
  friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator&lt;J&gt;&amp; y)
    requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;
// etc
</pre></blockquote>
<p>
So that deduction fails earlier, before constraints get checked, for a second
operand that isn't a specialization of <code class='backtick'>basic_const_iterator</code> (or derived from one).
<p/>
LWG <a href="lwg-defects.html#3769" title="basic_const_iterator::operator== causes infinite constraint recursion (Status: C++23)">3769</a><sup><a href="https://cplusplus.github.io/LWG/issue3769" title="Latest snapshot">(i)</a></sup> is an earlier issue about constraint recursion in <code class='backtick'>basic_const_iterator</code>'s
operators, but there the recursion was independent of CWG 2369.
</p>

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

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



<p id="res-4218"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5001" title=" Working Draft, Programming Languages — C++">N5001</a>.
</p>

<ol>
<li><p>Modify 24.5.3.3 <a href="https://wg21.link/const.iterators.iterator">[const.iterators.iterator]</a>, class template <code class='backtick'>basic_const_iterator</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  [&hellip;]
  template&lt;input_iterator Iterator&gt;
  class basic_const_iterator {
    [&hellip;]
    template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
      friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
        requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
    template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
      friend constexpr bool operator&gt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
        requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
    template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
      friend constexpr bool operator&lt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
        requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
    template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
      friend constexpr bool operator&gt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
        requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 24.5.3.5 <a href="https://wg21.link/const.iterators.ops">[const.iterators.ops]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
  friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
    requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
  friend constexpr bool operator&gt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
    requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
  friend constexpr bool operator&lt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
    requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
  friend constexpr bool operator&gt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
    requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
</pre>
<blockquote>
<p>
-23- Let <code><i>op</i></code> be the operator.
<p/>
-24- <i>Effects</i>: Equivalent to: <code>return x <i>op</i> y.<i>current_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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