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New issue from Patrick Palka: "Constraint recursion in basic_const_iterator's relational operators due to ADL + CWG 2369"
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‎xml/issue4218.xml‎

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<?xml version='1.0' encoding='utf-8' standalone='no'?>
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<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
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<issue num="4218" status="New">
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<title>Constraint recursion in `basic_const_iterator`'s relational operators due to ADL + CWG 2369</title>
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<section>
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<sref ref="[const.iterators.ops]"/>
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</section>
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<submitter>Patrick Palka</submitter>
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<date>03 Mar 2025</date>
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<priority>99</priority>
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<discussion>
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<p>
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Consider the example (devised by Hewill Kang)
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</p>
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<blockquote><pre>
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using RCI = reverse_iterator&lt;basic_const_iterator&lt;vector&lt;int&gt;::iterator&gt;&gt;;
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static_assert(std::totally_ordered&lt;RCI&gt;);
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</pre></blockquote>
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<p>
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Checking `RCI` is `totally_ordered` entails checking
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</p>
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<blockquote><pre>
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requires (RCI x) { x <i>RELOP</i> x; } for each <i>RELOP</i> in {&lt;, &gt;, &lt;=, &gt;=}
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</pre></blockquote>
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<p>
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which we expect to be straightforwardly satisfied by `reverse_iterator`'s
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namespace-scope operators (<sref ref="[reverse.iter.cmp]"/>):
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</p>
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<blockquote><pre>
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template&lt;class Iterator1, class Iterator2&gt;
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constexpr bool operator&lt;(
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const reverse_iterator&lt;Iterator1&gt;&amp; x,
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const reverse_iterator&lt;Iterator2&gt;&amp; y);
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// etc
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</pre></blockquote>
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<p>
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But due to ADL we find ourselves also considering the `basic_const_iterator`
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relop friends (<sref ref="[const.iterators.ops]"/>/24).
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</p>
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<blockquote><pre>
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template&lt;input_iterator Iterator&gt;
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class basic_const_iterator {
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template&lt;<i>not-a-const-iterator</i> I&gt;
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friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;
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// etc
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};
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</pre></blockquote>
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<p>
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Before <a href="https://wg21.link/cwg2369">CWG 2369</a> these candidates would quickly get
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discarded since for the second operand RCI clearly isn't convertible to `basic_const_iterator`.
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But after CWG 2369 implementations must first check these operators' constraints
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(with <tt>Iterator = vector&lt;int&gt;::iterator</tt> and <tt>I = RCI</tt>), which entails
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checking <tt>totally_ordered&lt;RCI&gt;</tt> recursively, causing the example to be ill-formed.
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<p/>
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The constraint recursion is diagnosed by GCC (<a href="https://godbolt.org/z/dr1dK1dnj">See godbolt demo</a>).
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Other compilers accept the example because they don't implement CWG 2369, as
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far as I know.
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<p/>
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GCC trunk works around this issue by giving these friend relational operators
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a dependent second operand of the form <tt>basic_const_iterator&lt;J&gt;</tt> where `J` is
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constrained to match `Iterator`:
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</p>
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<blockquote><pre>
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template&lt;<i>not-a-const-iterator</i> I, same_as&lt;Iterator&gt; J>
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friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator&lt;J&gt;&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;
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// etc
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</pre></blockquote>
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<p>
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So that deduction fails earlier, before constraints get checked, for a second
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operand that isn't a specialization of `basic_const_iterator` (or derived from one).
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<P/>
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LWG <iref ref="3769"/> is an earlier issue about constraint recursion in `basic_const_iterator`'s
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operators, but there the recursion was independent of CWG 2369.
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</p>
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</discussion>
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<resolution>
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<p>
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This wording is relative to <paper num="N5001"/>.
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</p>
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<ol>
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<li><p>Modify <sref ref="[const.iterators.iterator]"/>, class template `basic_const_iterator` synopsis, as indicated:</p>
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<blockquote>
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<pre>
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namespace std {
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[&hellip;]
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template&lt;input_iterator Iterator&gt;
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class basic_const_iterator {
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[&hellip;]
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&gt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&lt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&gt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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[&hellip;]
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};
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}
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</pre>
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</blockquote>
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</li>
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<li><p>Modify <sref ref="[const.iterators.ops]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&lt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&gt;(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&lt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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template&lt;<i>not-a-const-iterator</i> I<ins>, same_as&lt;Iterator&gt; J</ins>&gt;
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friend constexpr bool operator&gt;=(const I&amp; x, const basic_const_iterator<ins>&lt;J&gt;</ins>&amp; y)
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requires random_access_iterator&lt;Iterator&gt; &amp;&amp; totally_ordered_with&lt;Iterator, I&gt;;
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</pre>
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<blockquote>
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<p>
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-23- Let <tt><i>op</i></tt> be the operator.
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<p/>
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-24- <i>Effects</i>: Equivalent to: <tt>return x <i>op</i> y.<i>current_</i>;</tt>
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</p>
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</blockquote>
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</blockquote>
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</li>
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</ol>
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</resolution>
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</issue>

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