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New issue from S.B. Tam: "§[range.utility.conv.to] ranges::to may cau…
…se infinite recursion if range_value_t<C> is a non-move-constructible range"
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| <?xml version='1.0' encoding='utf-8' standalone='no'?> | ||
| <!DOCTYPE issue SYSTEM "lwg-issue.dtd"> | ||
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| <issue num="4008" status="New"> | ||
| <title>§[range.utility.conv.to] <tt>ranges::to</tt> may cause infinite recursion if <tt>range_value_t<C></tt> | ||
| is a non-move-constructible range</title> | ||
| <section><sref ref="[range.utility.conv.to]"/></section> | ||
| <submitter>S. B. Tam</submitter> | ||
| <date>08 Nov 2023</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| <sref ref="[range.utility.conv.to]"/>/2 says: | ||
| </p> | ||
| <blockquote> | ||
| <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>[…]</p></li> | ||
| </ol></li> | ||
| <li><p>(2.2) — Otherwise, if <tt>input_range<range_reference_t<R>></tt> is <tt>true</tt>: | ||
| </p> | ||
| <blockquote> | ||
| <pre> | ||
| to<C>(r | views::transform([](auto&& elem) { | ||
| return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); | ||
| }), std::forward<Args>(args)...); | ||
| </pre> | ||
| </blockquote> | ||
| </li> | ||
| <li><p>[…]</p></li> | ||
| </ol> | ||
| </blockquote> | ||
| <p> | ||
| That is, if <tt>range_reference_t<R></tt> is not convertible to <tt>range_value_t<C></tt>, and | ||
| <tt>range_reference_t<R></tt> is an input range, <tt>views::transform</tt> is applied to convert the | ||
| inner range to <tt>range_value_t<C></tt> (through <tt>to<range_value_t<C>></tt>), and then | ||
| the transformed range is converted to <tt>C</tt> (through <tt>to<C></tt>). | ||
| <p/> | ||
| Consider: | ||
| </p> | ||
| <blockquote><pre> | ||
| #include <ranges> | ||
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| struct ImmovableRange { | ||
| ImmovableRange(int*, int*); | ||
| ImmovableRange(ImmovableRange&&) = delete; | ||
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| int* begin(); | ||
| int* end(); | ||
| }; | ||
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| struct C { | ||
| ImmovableRange* begin(); | ||
| ImmovableRange* end(); | ||
| }; | ||
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| using R = int[1][2]; | ||
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| void test() { | ||
| (void)std::ranges::to<C>(R{}); | ||
| } | ||
| </pre></blockquote> | ||
| <p> | ||
| Here: | ||
| </p> | ||
| <ol> | ||
| <li><p><tt>convertible_to<range_reference_t<R>, range_value_t<C>></tt> is <tt>false</tt>.</p></li> | ||
| <li><p><tt>range_reference_t<R></tt> satisfies <tt>input_range</tt>.</p></li> | ||
| <li><p><tt>range_reference_t<R></tt> can be converted to <tt>range_value_t<C></tt> through | ||
| <tt>to<range_value_t<C>></tt>. (If it couldn't, an error would be produced immediately.)</p></li> | ||
| </ol> | ||
| <p> | ||
| So <tt>to<C></tt> is called recursively, constructing <tt>C</tt> with the transformed range (whose | ||
| <tt>range_reference_t<R></tt> is the same as <tt>range_value_t<C></tt>). For the construction | ||
| from the transformed range: | ||
| </p> | ||
| <ol> | ||
| <li><p><tt>range_reference_t<R></tt> and <tt>range_value_t<C></tt> are both <tt>ImmovableRange</tt>.</p></li> | ||
| <li><p><tt>convertible_to<range_reference_t<R>, range_value_t<C>></tt> (i.e. | ||
| <tt>convertible_to<ImmovableRange, ImmovableRange></tt>) is <tt>false</tt>.</p></li> | ||
| <li><p><tt>range_reference_t<R></tt> (i.e. <tt>ImmovableRange</tt>) satisfies <tt>input_range</tt>.</p></li> | ||
| <li><p><tt>range_reference_t<R></tt> can be converted to <tt>range_value_t<C></tt> through | ||
| <tt>to<range_value_t<C>></tt>.</p></li> | ||
| </ol> | ||
| <p> | ||
| So <tt>to<C></tt> is called recursively again, transforming the range for the second time. This time, | ||
| the transformation does not change any of the above four facts. As a result, <tt>to<C></tt> is called | ||
| yet again, leading to an infinite recursion. | ||
| <p/> | ||
| I believe this can be fixed by stop calling <tt>to<C></tt> recursively when <tt>range_reference_t<R></tt> | ||
| is the same as <tt>range_value_t<C></tt>. | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4964"/>. | ||
| </p> | ||
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| <ol> | ||
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| <li><p>Modify <sref ref="[range.utility.conv.to]"/> 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>[…]</p></li> | ||
| </ol></li> | ||
| <li><p>(2.2) — Otherwise, if <ins><tt>same_as<range_reference_t<R>, range_value_t<C>></tt> | ||
| is <tt>false</tt> and</ins> <tt>input_range<range_reference_t<R>></tt> is <tt>true</tt>: | ||
| </p> | ||
| <blockquote> | ||
| <pre> | ||
| to<C>(r | views::transform([](auto&& elem) { | ||
| return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); | ||
| }), std::forward<Args>(args)...); | ||
| </pre> | ||
| </blockquote> | ||
| </li> | ||
| <li><p>(2.3) — Otherwise, the program is ill-formed.</p></li> | ||
| </ol> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </li> | ||
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| </ol> | ||
| </resolution> | ||
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| </issue> |