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New issue from Jiang An: "Monadic operations should be ADL-proof"
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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="3973" status="New"> | ||
| <title>Monadic operations should be ADL-proof</title> | ||
| <section><sref ref="[expected.object.monadic]"/><sref ref="[optional.monadic]"/></section> | ||
| <submitter>Jiang An</submitter> | ||
| <date>10 Aug 2023</date> | ||
| <priority>99</priority> | ||
|
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| <discussion> | ||
| <p> | ||
| LWG <iref ref="3938"/> switched to use <tt>**this</tt> to access the value stored in <tt>std::expected</tt>. | ||
| However, as shown in LWG <iref ref="3969"/>, <tt>**this</tt> can trigger ADL and find an unwanted overload, | ||
| and thus may caused unintended behavior. | ||
| <p/> | ||
| Current implementations behave correctly (<a href="https://godbolt.org/z/3b1Tbs1jd">Godbolt link</a>): they | ||
| don't direct use <tt>**this</tt>, but use the name of the union member instead. | ||
| <p/> | ||
| Moreover, <paper num="P2407R5"/> will change the monadic operations of <tt>std::optional</tt> to use <tt>**this</tt>, | ||
| which is also problematic. | ||
| </p> | ||
| </discussion> | ||
|
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||
| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4958"/>. | ||
| </p> | ||
|
|
||
| <ol> | ||
|
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| <li><p>Modify the <sref ref="[expected.object.monadic]"/> as indicated:</p> | ||
|
|
||
| <blockquote class="note"> | ||
| <p> | ||
| [<i>Drafting note:</i> Effectively replace all occurrences of <tt>**this</tt> by <tt>operator*()</tt>.] | ||
| </p> | ||
| </blockquote> | ||
|
|
||
| <blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto and_then(F&& f) &; | ||
| template<class F> constexpr auto and_then(F&& f) const &; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -1- Let <tt>U</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype(<del>**this</del><ins>operator*()</ins>)>></tt>. | ||
| <p/> | ||
| -2- […] | ||
| <p/> | ||
| -3- […] | ||
| <p/> | ||
| -4- <i>Effects</i>: Equivalent to: | ||
| </p> | ||
| <blockquote><pre> | ||
| if (has_value()) | ||
| return invoke(std::forward<F>(f), <del>**this</del><ins>operator*()</ins>); | ||
| else | ||
| return U(unexpect, error()); | ||
| </pre></blockquote> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto and_then(F&& f) &&; | ||
| template<class F> constexpr auto and_then(F&& f) const &&; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -5- Let <tt>U</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype((std::move(<del>**this</del><ins>operator*()</ins>))>></tt>. | ||
| <p/> | ||
| -6- […] | ||
| <p/> | ||
| -7- […] | ||
| <p/> | ||
| -8- <i>Effects</i>: Equivalent to: | ||
| </p> | ||
| <blockquote><pre> | ||
| if (has_value()) | ||
| return invoke(std::forward<F>(f), std::move(<del>**this</del><ins>operator*()</ins>)); | ||
| else | ||
| return U(unexpect, std::move(error())); | ||
| </pre></blockquote> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto or_else(F&& f) &; | ||
| template<class F> constexpr auto or_else(F&& f) const &; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -9- Let <tt>G</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype(error())>></tt>. | ||
| <p/> | ||
| -10- <i>Constraints</i>: <tt>is_constructible_v<T, decltype(<del>**this</del><ins>operator*()</ins>)></tt> is <tt>true</tt>. | ||
| <p/> | ||
| -11- […] | ||
| <p/> | ||
| -12- <i>Effects</i>: Equivalent to: | ||
| </p> | ||
| <blockquote><pre> | ||
| if (has_value()) | ||
| return G(in_place, <del>**this</del><ins>operator*()</ins>); | ||
| else | ||
| return invoke(std::forward<F>(f), error()); | ||
| </pre></blockquote> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto or_else(F&& f) &&; | ||
| template<class F> constexpr auto or_else(F&& f) const &&; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -13- Let <tt>G</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>></tt>. | ||
| <p/> | ||
| -14- <i>Constraints</i>: <tt>is_constructible_v<T, decltype(std::move(<del>**this</del><ins>operator*()</ins>))></tt> is <tt>true</tt>. | ||
| <p/> | ||
| -15- […] | ||
| <p/> | ||
| -16- <i>Effects</i>: Equivalent to: | ||
| </p> | ||
| <blockquote><pre> | ||
| if (has_value()) | ||
| return G(in_place, std::move(<del>**this</del><ins>operator*()</ins>)); | ||
| else | ||
| return invoke(std::forward<F>(f), std::move(error())); | ||
| </pre></blockquote> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto transform(F&& f) &; | ||
| template<class F> constexpr auto transform(F&& f) const &; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -17- Let <tt>U</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype(<del>**this</del><ins>operator*()</ins>)>></tt>. | ||
| <p/> | ||
| -18- […] | ||
| <p/> | ||
| -19- <i>Mandates</i>: <tt>U</tt> is a valid value type for <tt>expected</tt>. If <tt>is_void_v<U></tt> is <tt>false</tt>, | ||
| the declaration | ||
| </p> | ||
| <blockquote><pre> | ||
| U u(invoke(std::forward<F>(f), <del>**this</del><ins>operator*()</ins>)); | ||
| </pre></blockquote> | ||
| <p> | ||
| is well-formed. | ||
| <p/> | ||
| -20- <i>Effects</i>: | ||
| </p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p>(20.1) — […]</p></li> | ||
| <li><p>(20.2) — Otherwise, if <tt>is_void_v<U></tt> is <tt>false</tt>, returns an <tt>expected<U, E></tt> | ||
| object whose <tt><i>has_val</i></tt> member is <tt>true</tt> and <tt><i>val</i></tt> member is direct-non-list-initialized | ||
| with <tt>invoke(std::forward<F>(f), <del>**this</del><ins>operator*()</ins>)</tt>.</p></li> | ||
| <li><p>(20.3) — Otherwise, evaluates <tt>invoke(std::forward<F>(f), <del>**this</del><ins>operator*()</ins>)</tt> | ||
| and then returns <tt>expected<U, E>()</tt>.</p></li> | ||
| </ol> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto transform(F&& f) &&; | ||
| template<class F> constexpr auto transform(F&& f) const &&; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -21- Let <tt>U</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype(std::move(<del>**this</del><ins>operator*()</ins>))>></tt>. | ||
| <p/> | ||
| -22- […] | ||
| <p/> | ||
| -23- <i>Mandates</i>: <tt>U</tt> is a valid value type for <tt>expected</tt>. If <tt>is_void_v<U></tt> is <tt>false</tt>, | ||
| the declaration | ||
| </p> | ||
| <blockquote><pre> | ||
| U u(invoke(std::forward<F>(f), std::move(<del>**this</del><ins>operator*()</ins>))); | ||
| </pre></blockquote> | ||
| <p> | ||
| is well-formed. | ||
| <p/> | ||
| -24- <i>Effects</i>: | ||
| </p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p>(24.1) — […]</p></li> | ||
| <li><p>(24.2) — Otherwise, if <tt>is_void_v<U></tt> is <tt>false</tt>, returns an <tt>expected<U, E></tt> | ||
| object whose <tt><i>has_val</i></tt> member is <tt>true</tt> and <tt><i>val</i></tt> member is direct-non-list-initialized | ||
| with <tt>invoke(std::forward<F>(f), std::move(<del>**this</del><ins>operator*()</ins>))</tt>.</p></li> | ||
| <li><p>(24.3) — Otherwise, evaluates <tt>invoke(std::forward<F>(f), std::move(<del>**this</del><ins>operator*()</ins>))</tt> | ||
| and then returns <tt>expected<U, E>()</tt>.</p></li> | ||
| </ol> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto transform_error(F&& f) &; | ||
| template<class F> constexpr auto transform_error(F&& f) const &; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -25- Let <tt>G</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype(error())>></tt>. | ||
| <p/> | ||
| -26- <i>Constraints</i>: <tt>is_constructible_v<T, decltype(<del>**this</del><ins>operator*()</ins>)></tt> is <tt>true</tt>. | ||
| <p/> | ||
| -27- <i>Mandates</i>: […] | ||
| <p/> | ||
| -28- <i>Returns</i>: If <tt>has_value()</tt> is <tt>true</tt>, <tt>expected<T, G>(in_place, <del>**this</del><ins>operator*()</ins>)</tt>; | ||
| otherwise, an <tt>expected<T, G></tt> object whose <tt><i>has_val</i></tt> member is <tt>false</tt> and | ||
| <tt><i>unex</i></tt> member is direct-non-list-initialized with <tt>invoke(std::forward<F>(f), error())</tt>. | ||
| </p> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto transform_error(F&& f) &&; | ||
| template<class F> constexpr auto transform_error(F&& f) const &&; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -29- Let <tt>G</tt> be <tt>remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>></tt>. | ||
| <p/> | ||
| -30- <i>Constraints</i>: <tt>is_constructible_v<T, decltype(std::move(<del>**this</del><ins>operator*()</ins>))></tt> is <tt>true</tt>. | ||
| <p/> | ||
| -31- <i>Mandates</i>: […] | ||
| <p/> | ||
| -32- <i>Returns</i>: If <tt>has_value()</tt> is <tt>true</tt>, <tt>expected<T, G>(in_place, std::move(<del>**this</del><ins>operator*()</ins>))</tt>; | ||
| otherwise, an <tt>expected<T, G></tt> object whose <tt><i>has_val</i></tt> member is <tt>false</tt> and | ||
| <tt><i>unex</i></tt> member is direct-non-list-initialized with <tt>invoke(std::forward<F>(f), std::move(error()))</tt>. | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
| </li> | ||
|
|
||
| <li><p>Modify the <sref ref="[optional.monadic]"/> as indicated:</p> | ||
|
|
||
|
|
||
| <blockquote class="note"> | ||
| <p> | ||
| [<i>Drafting note:</i> Effectively replace all occurrences of <tt>value()</tt> by <tt>operator*()</tt>.] | ||
| </p> | ||
| </blockquote> | ||
|
|
||
| <blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto and_then(F&& f) &; | ||
| template<class F> constexpr auto and_then(F&& f) const &; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -1- Let <tt>U</tt> be <tt>invoke_result_t<F, decltype(<del>value()</del><ins>operator*()</ins>)></tt>. | ||
| <p/> | ||
| -2- […] | ||
| <p/> | ||
| -3- <i>Effects</i>: Equivalent to: | ||
| </p> | ||
| <blockquote><pre> | ||
| if (*this) { | ||
| return invoke(std::forward<F>(f), <del>value()</del><ins>operator*()</ins>); | ||
| } else { | ||
| return remove_cvref_t<U>(); | ||
| } | ||
| </pre></blockquote> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto and_then(F&& f) &&; | ||
| template<class F> constexpr auto and_then(F&& f) const &&; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -4- Let <tt>U</tt> be <tt>invoke_result_t<F, decltype(std::move(<del>value()</del><ins>operator*()</ins>))></tt>. | ||
| <p/> | ||
| -5- […] | ||
| <p/> | ||
| -6- <i>Effects</i>: Equivalent to: | ||
| </p> | ||
| <blockquote><pre> | ||
| if (*this) { | ||
| return invoke(std::forward<F>(f), std::move(<del>value()</del><ins>operator*()</ins>)); | ||
| } else { | ||
| return remove_cvref_t<U>(); | ||
| } | ||
| </pre></blockquote> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto transform(F&& f) &; | ||
| template<class F> constexpr auto transform(F&& f) const &; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -7- Let <tt>U</tt> be <tt>remove_cv_t<invoke_result_t<F, decltype(<del>value()</del><ins>operator*()</ins>)>></tt>. | ||
| <p/> | ||
| -8- <i>Mandates</i>: <tt>U</tt> is a non-array object type other than <tt>in_place_t</tt> or <tt>nullopt_t</tt>. The declaration | ||
| </p> | ||
| <blockquote><pre> | ||
| U u(invoke(std::forward<F>(f), <del>value()</del><ins>operator*()</ins>)); | ||
| </pre></blockquote> | ||
| <p> | ||
| is well-formed for some invented variable <tt>u</tt>. | ||
| <p/> | ||
| […] | ||
| <p/> | ||
| -9- <i>Returns</i>: If <tt>*this</tt> contains a value, an <tt>optional<U></tt> object whose contained value is | ||
| direct-non-list-initialized with <tt>invoke(std::forward<F>(f), <del>value()</del><ins>operator*()</ins>)</tt>; otherwise, | ||
| <tt>optional<U>()</tt>. | ||
| </p> | ||
| </blockquote> | ||
| <pre> | ||
| template<class F> constexpr auto transform(F&& f) &&; | ||
| template<class F> constexpr auto transform(F&& f) const &&; | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -10- Let <tt>U</tt> be <tt>remove_cv_t<invoke_result_t<F, decltype(std::move(<del>value()</del><ins>operator*()</ins>))>></tt>. | ||
| <p/> | ||
| -11- <i>Mandates</i>: <tt>U</tt> is a non-array object type other than <tt>in_place_t</tt> or <tt>nullopt_t</tt>. The declaration | ||
| </p> | ||
| <blockquote><pre> | ||
| U u(invoke(std::forward<F>(f), std::move(<del>value()</del><ins>operator*()</ins>))); | ||
| </pre></blockquote> | ||
| <p> | ||
| is well-formed for some invented variable <tt>u</tt>. | ||
| <p/> | ||
| […] | ||
| <p/> | ||
| -12- <i>Returns</i>: If <tt>*this</tt> contains a value, an <tt>optional<U></tt> object whose contained value is | ||
| direct-non-list-initialized with <tt>invoke(std::forward<F>(f), std::move(<del>value()</del><ins>operator*()</ins>))</tt>; otherwise, | ||
| <tt>optional<U>()</tt>. | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
| </li> | ||
| </ol> | ||
| </resolution> | ||
|
|
||
| </issue> |