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New issue from Jiang An: "std::is_(nothrow_)convertible should be rew…
…orded to avoid dependence on the return statement"
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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="4028" status="New"> | ||
| <title><tt>std::is_(nothrow_)convertible</tt> should be reworded to avoid dependence on the <tt>return</tt> statement</title> | ||
| <section><sref ref="[meta.rel]"/></section> | ||
| <submitter>Jiang An</submitter> | ||
| <date>18 Dec 2023</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| The current specification for <tt>std::is_convertible</tt> is sensitive to the requirements for the <tt>return</tt> | ||
| statements. As a result, the requirements were accidentally changed by <paper num="P0135R1"/> and then changed back by | ||
| CWG issue <a href="https://wg21.link/cwg2426">2426</a>. The current revision of <paper num="P2748"/> also plans to | ||
| change the wording for <tt>std::is_convertible</tt> to avoid actual behavioral changing. | ||
| <p/> | ||
| IMO it's better to specify <tt>std::is_convertible</tt> in a such way that is independent to <tt>return</tt> statements. | ||
| The proposed resolution matches what mainstream implementations do, and should resolve LWG <iref ref="3400"/> together. | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4971"/>. | ||
| </p> | ||
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| <ol> | ||
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| <li><p>Modify <sref ref="[meta.rel]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <table border="1"> | ||
| <caption>Table 49 — Type relationship predicates [tab:meta.rel]</caption> | ||
| <tr style="text-align:center"> | ||
| <th>Template</th> | ||
| <th>Condition</th> | ||
| <th>Comments</th> | ||
| </tr> | ||
| <tr> | ||
| <td>[…]</td> | ||
| <td>[…]</td> | ||
| <td>[…]</td> | ||
| </tr> | ||
| <tr> | ||
| <td> | ||
| <pre> | ||
| template<class From, class To> | ||
| struct is_convertible; | ||
| </pre> | ||
| </td> | ||
| <td><i>see below</i></td> | ||
| <td> | ||
| <tt>From</tt> and <tt>To</tt> shall be<br/> | ||
| complete types, <tt><i>cv</i> void</tt>, or arrays of<br/> | ||
| unknown bound. | ||
| </td> | ||
| </tr> | ||
| <tr> | ||
| <td> | ||
| <pre> | ||
| template<class From, class To> | ||
| struct is_nothrow_convertible; | ||
| </pre> | ||
| </td> | ||
| <td> | ||
| <tt>is_convertible_v<From,<br/> | ||
| To></tt> is <tt>true</tt> and <del>the</del><br/> | ||
| <del>conversion, as defined by</del><br/> | ||
| <del><tt>is_convertible</tt>, is known</del><br/> | ||
| <del>not to throw any exceptions</del><br/> | ||
| <ins>either both <tt>From</tt> and <tt>To</tt><br/> | ||
| are <tt><i>cv</i> void</tt>, or the function call<br/> | ||
| expression used for specifying <tt>is_convertible</tt><br/> | ||
| is non-throwing</ins> (<sref ref="[expr.unary.noexcept]"/>) | ||
| </td> | ||
| <td> | ||
| <tt>From</tt> and <tt>To</tt> shall be<br/> | ||
| complete types, <tt><i>cv</i> void</tt>, or arrays of<br/> | ||
| unknown bound. | ||
| </td> | ||
| </tr> | ||
| <tr> | ||
| <td>[…]</td> | ||
| <td>[…]</td> | ||
| <td>[…]</td> | ||
| </tr> | ||
| </table> | ||
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| <p> | ||
| -5- The predicate condition for a template specialization <tt>is_convertible<From, To></tt> | ||
| shall be satisfied if and only if <del>the return expression in the following code would be well-formed, | ||
| including any implicit conversions to the return type of the function:</del> | ||
| </p> | ||
| <blockquote><pre> | ||
| <del>To test() { | ||
| return declval<From>(); | ||
| }</del> | ||
| </pre></blockquote> | ||
| <p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p><ins>(?.1) — either both <tt>From</tt> and <tt>To</tt> are <tt><i>cv</i> void</tt>, or</ins></p></li> | ||
| <li><p><ins>(?.2) — <tt>To</tt> is neither array nor function type and the function call expression | ||
| <tt><i>conv-dest</i>(declval<From>())</tt> would be well-formed when treated as an unevaluated operand, | ||
| where <tt><i>conv-dest</i></tt> is a hypothetical function declared as</ins></p> | ||
| <blockquote><pre> | ||
| <ins>void <i>conv-dest</i>(To) noexcept;</ins> | ||
| </pre></blockquote> | ||
| <p> | ||
| <ins>.</ins> | ||
| </p> | ||
| </li> | ||
| </ol> | ||
| <p/> | ||
| [<i>Note 2</i>: This requirement gives well-defined results for reference types, array types, function types, | ||
| and <i>cv</i> <tt>void</tt>. — <i>end note</i>] | ||
| <p/> | ||
| Access checking is performed in a context unrelated to <tt>To</tt> and <tt>From</tt>. Only the validity of the | ||
| immediate context of the expression of the <del>return statement (<sref ref="[stmt.return]"/>)</del> | ||
| <ins>function call expression (<sref ref="[expr.call]"/>)</ins> (including initialization of the | ||
| <del>returned</del><ins>parameter</ins> object or reference) is considered. | ||
| </p> | ||
| </blockquote> | ||
| </li> | ||
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| </ol> | ||
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| </resolution> | ||
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| </issue> |