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New issue from Jiang An: "Should we reject std::bind_front<42>() and …
…its friends?"
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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="3979" status="New"> | ||
| <title>Should we reject <tt>std::bind_front<42>()</tt> and its friends?</title> | ||
| <section><sref ref="[func.not.fn]"/><sref ref="[func.bind.partial]"/></section> | ||
| <submitter>Jiang An</submitter> | ||
| <date>22 Aug 2023</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| <tt>std::bind_front<42>()</tt> seems to be currently well-formed, but the result isn't invocable with any arguments. | ||
| Given we are already detecting types and values of the NTTP for new overloads of <tt>std::bind_front</tt>, | ||
| <tt>std::bind_back</tt>, and <tt>std::not_fn</tt>, it might be better to diagnose such cases when NTTP is of a scalar type. | ||
| <p/> | ||
| Note that we might need to separately handle NTTP of scalar and class types for these functions when | ||
| <a href="https://wg21.link/CWG2459">CWG2459</a> is not resolved, because it might be necessary to avoid copying a | ||
| template parameter object. | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4958"/>. | ||
| </p> | ||
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| <ol> | ||
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| <li><p>Modify the <sref ref="[func.not.fn]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <pre> | ||
| template<auto f> constexpr <i>unspecified</i> not_fn() noexcept; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -6- […] | ||
| <p/> | ||
| -7- <i>Mandates</i>: If <del><tt>is_pointer_v<F> || is_member_pointer_v<F></tt> is <tt>true</tt>, then | ||
| <tt>f != nullptr</tt> is <tt>true</tt></del><ins><tt>is_scalar_v<F></tt> is <tt>true</tt>, then either | ||
| <tt>is_pointer_v<F> && is_function_v<remove_pointer_t<F>></tt> is <tt>true</tt> or | ||
| <tt>is_member_pointer_v<F></tt> is <tt>true</tt>, and <tt>f != nullptr</tt> is <tt>true</tt> in either case</ins>. | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </li> | ||
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| <li><p>Modify the <sref ref="[func.bind.partial]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <pre> | ||
| template<auto f, class... Args> | ||
| constexpr <i>unspecified</i> bind_front(Args&&... args); | ||
| template<auto f, class... Args> | ||
| constexpr <i>unspecified</i> bind_back(Args&&... args); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -6- […] | ||
| <p/> | ||
| -7- <i>Mandates</i>: | ||
| </p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p>(7.1) — […]</p></li> | ||
| <li><p>(7.2) — […]</p></li> | ||
| <li><p>(7.3) — if <del><tt>is_pointer_v<F> || is_member_pointer_v<F></tt> is <tt>true</tt>, then | ||
| <tt>f != nullptr</tt> is <tt>true</tt></del><ins><tt>is_scalar_v<F></tt> is <tt>true</tt>, then either | ||
| <tt>is_pointer_v<F> && is_function_v<remove_pointer_t<F>></tt> is <tt>true</tt> or | ||
| <tt>is_member_pointer_v<F></tt> is <tt>true</tt>, and <tt>f != nullptr</tt> is <tt>true</tt> in either case</ins>.</p></li> | ||
| </ol> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </li> | ||
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| </ol> | ||
| </resolution> | ||
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| </issue> |