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New issue from Alex Guteniev: "copy_if, remove_copy, remove_copy_if, unique_copy have too strong preconditions"
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‎xml/issue4262.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="4262" status="New">
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<title>`copy_if`, `remove_copy`, `remove_copy_if`, `unique_copy` have too strong preconditions</title>
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<section><sref ref="[alg.copy]"/><sref ref="[alg.remove]"/><sref ref="[alg.unique]"/></section>
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<submitter>Alex Guteniev</submitter>
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<date>12 May 2025</date>
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<priority>99</priority>
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<discussion>
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<p>
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<sref ref="[alg.copy]"/>/16 , <sref ref="[alg.remove]"/>/11, <sref ref="[alg.unique]"/>/8.1 all say:
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</p>
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<blockquote>
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<p>
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<i>Preconditions</i>: The ranges `[first, last)` and `[result, result + (last - first))` do not overlap.
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</p>
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</blockquote>
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<p>
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These algorithms may produce fewer elements than `(last - first)`. If this is known in advance,
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a smaller output range may be used, so that the precondition will not be satisfied.
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<p/>
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Example from LLVM test suite
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(<a href="https://github.com/llvm/llvm-project/blob/79210feb2993ff9a79ef11f8a7016a527d4fcf22/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy_if.pass.cpp#L160-L162">
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/libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy_if.pass.cpp</a>):
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</p>
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<blockquote><pre>
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std::array&lt;S, 4&gt; in = {{{4, 2}, {1, 3}, {3, 4}, {3, 5}}};
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std::array&lt;S, 2&gt; out;
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auto ret = std::ranges::copy_if(in.begin(), in.end(), out.begin(), [](int i) { return i == 3; }, &amp;S::val);
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</pre></blockquote>
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<p>
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I think there should be a weaker precondition, like <sref ref="[alg.copy]"/>/2:
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</p>
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<blockquote>
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<p>
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<i>Preconditions</i>: `result` is not in the range `[first, last)`.
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</p>
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</blockquote>
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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="N5008"/>.
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</p>
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<ol>
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<li><p>Modify <sref ref="[alg.copy]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class InputIterator, class OutputIterator, class Predicate&gt;
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constexpr OutputIterator copy_if(InputIterator first, InputIterator last,
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OutputIterator result, Predicate pred);
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[&hellip;]
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template&lt;input_range R, weakly_incrementable O, class Proj = identity,
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indirect_unary_predicate&lt;projected&lt;iterator_t&lt;R&gt;, Proj&gt;&gt; Pred&gt;
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requires indirectly_copyable&lt;iterator_t&lt;R&gt;, O&gt;
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constexpr ranges::copy_if_result&lt;borrowed_iterator_t&lt;R&gt;, O&gt;
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ranges::copy_if(R&amp;&amp; r, O result, Pred pred, Proj proj = {});
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</pre>
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<blockquote>
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<p>
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-15- Let <tt><i>E</i></tt> be: [&hellip;]
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<p/>
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-16 <i>Preconditions</i>: <ins>`result` is not in the range `[first, last)`</ins><del>The ranges `[first, last)` and `[result, result + (last - first))` do not overlap</del>.
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</p>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify <sref ref="[alg.remove]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class InputIterator, class OutputIterator,
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class T = iterator_traits&lt;InputIterator&gt;::value_type>
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constexpr OutputIterator
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remove_copy(InputIterator first, InputIterator last,
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OutputIterator result, const T&amp; value);
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[&hellip;]
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template&lt;input_range R, weakly_incrementable O, class Proj = identity,
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indirect_unary_predicate&lt;projected&lt;iterator_t&lt;R&gt;, Proj&gt;&gt; Pred&gt;
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requires indirectly_copyable&lt;iterator_t&lt;R&gt;, O&gt;
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constexpr ranges::remove_copy_if_result&lt;borrowed_iterator_t&lt;R&gt;, O&gt;
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ranges::remove_copy_if(R&amp;&amp; r, O result, Pred pred, Proj proj = {});
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</pre>
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<blockquote>
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<p>
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-8- Let <tt><i>E</i></tt> be [&hellip;]
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<p/>
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[&hellip;]
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<p/>
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-11 <i>Preconditions</i>: <ins>`result` is not in the range `[first, last)`</ins><del>The ranges `[first, last)` and `[result, result + (last - first))` do not overlap</del>.
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</p>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify <sref ref="[alg.unique]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class InputIterator, class OutputIterator&gt;
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constexpr OutputIterator
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unique_copy(InputIterator first, InputIterator last,
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OutputIterator result);
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[&hellip;]
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template&lt;input_range R, weakly_incrementable O, class Proj = identity,
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indirect_equivalence_relation&lt;projected&lt;iterator_t&lt;R&gt;, Proj&gt;&gt; C = ranges::equal_to&gt;
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requires indirectly_copyable&lt;iterator_t&lt;R&gt;, O&gt; &amp;&amp;
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(forward_iterator&lt;iterator_t&lt;R&gt;&gt; ||
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(input_iterator&lt;O&gt; &amp;&amp; same_as&lt;range_value_t&lt;R&gt;, iter_value_t&lt;O&gt;&gt;) ||
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indirectly_copyable_storable&lt;iterator_t&lt;R&gt;, O&gt;)
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constexpr ranges::unique_copy_result&lt;borrowed_iterator_t&lt;R&gt;, O&gt;
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ranges::unique_copy(R&amp;&amp; r, O result, C comp = {}, Proj proj = {});
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</pre>
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<blockquote>
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<p>
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-6- Let `pred` be [&hellip;]
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<p/>
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-7- <i>Mandates</i>: [&hellip;]
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<p/>
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-8 <i>Preconditions</i>:
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</p>
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<ol style="list-style-type: none">
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<li><p>(8.1) &mdash; <ins><ins>`result` is not in the range `[first, last)`</ins></ins><del>The ranges `[first, last)` and `[result, result + (last - first))` do not overlap</del>.</p></li>
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<li><p>(8.2) &mdash; [&hellip;]</p></li>
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</ol>
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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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