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| <title>Issue 4018: ranges::to's copy branch is underconstrained</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><a name="4018" href="lwg-active.html#4018">4018</a>. <tt>ranges::to</tt>'s copy branch is underconstrained</h3> | ||
| <p><b>Section:</b> 26.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a> | ||
| <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-11-25 <b>Last modified:</b> 2023-11-25</p> | ||
| <p><b>Priority: </b>Not Prioritized | ||
| </p> | ||
| <p><b>View other</b> <a href="lwg-index-open.html#range.utility.conv.to">active issues</a> in [range.utility.conv.to].</p> | ||
| <p><b>View all other</b> <a href="lwg-index.html#range.utility.conv.to">issues</a> in [range.utility.conv.to].</p> | ||
| <p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p> | ||
| <p><b>Discussion:</b></p> | ||
| <p> | ||
| Unlike other branches that return a prvalue <tt>C</tt>, this branch's <tt>C</tt> is returned by elidable move, | ||
| indicating that <tt>C</tt> needs to be move constructible (<a href="https://godbolt.org/z/dfqnx6Txq">demo</a>): | ||
| </p> | ||
| <blockquote><pre> | ||
| #include <ranges> | ||
| #include <vector> | ||
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| struct nonmovable { | ||
| nonmovable() = default; | ||
| nonmovable(const nonmovable&) = delete; | ||
| nonmovable& operator=(const nonmovable&) = delete; | ||
| }; | ||
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| template<class T> | ||
| struct nonmovable_vector : std::vector<T>, nonmovable { }; | ||
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| int main() { | ||
| int arr[] = {42}; | ||
| auto v = std::ranges::to<nonmovable_vector<int>>(arr); // <span style="color:red;font-weight:bolder">hard error</span> | ||
| } | ||
| </pre></blockquote> | ||
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| <p id="res-4018"><b>Proposed resolution:</b></p> | ||
| <p> | ||
| This wording is relative to <a href="https://wg21.link/N4964">N4964</a>. | ||
| </p> | ||
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| <ol> | ||
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| <li><p>Modify 26.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> as indicated:</p> | ||
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| <blockquote> | ||
| <pre> | ||
| template<class C, input_range R, class... Args> requires (!view<C>) | ||
| constexpr C to(R&& r, Args&&... args); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -1- <i>Mandates</i>: <tt>C</tt> is a cv-unqualified class type. | ||
| </p> | ||
| <p> | ||
| -2- <i>Returns</i>: An object of type <tt>C</tt> constructed from the elements of <tt>r</tt> in the following manner: | ||
| </p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p>(2.1) — If <tt>C</tt> does not satisfy <tt>input_range</tt> or | ||
| <tt>convertible_to<range_reference_t<R>, range_value_t<C>></tt> is <tt>true</tt>:</p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p>(2.1.1) — […]</p></li> | ||
| <li><p>(2.1.2) — […]</p></li> | ||
| <li><p>(2.1.3) — […]</p></li> | ||
| <li><p>(2.1.4) — Otherwise, if</p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p><ins>(2.1.4.?) — <tt>move_constructible<C></tt> is <tt>true</tt>,</ins></p></li> | ||
| <li><p>(2.1.4.1) — <tt>constructible_from<C, Args...></tt> is <tt>true</tt>, and</p></li> | ||
| <li><p>(2.1.4.2) — <tt><i>container-insertable</i><C, range_reference_t<R>></tt> is <tt>true</tt>:</p> | ||
| <blockquote><pre> | ||
| C c(std::forward<Args>(args)...); | ||
| if constexpr (sized_range<R> && <i>reservable-container</i><C>) | ||
| c.reserve(static_cast<range_size_t<C>>(ranges::size(r))); | ||
| ranges::copy(r, <i>container-inserter</i><range_reference_t<R>>(c)); | ||
| </pre></blockquote></li> | ||
| </ol> | ||
| </li> | ||
| <li><p>(2.1.5) — Otherwise, the program is ill-formed.</p></li> | ||
| </ol></li> | ||
| <li><p>(2.2) — […]</p></li> | ||
| <li><p>(2.3) — Otherwise, the program is ill-formed.</p></li> | ||
| </ol> | ||
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