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New issue from Corentin: "A less hacky and more useful way to compare…
… comparison category types"
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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="4051" status="New"> | ||
| <title>A less hacky and more useful way to compare comparison category types</title> | ||
| <section><sref ref="[cmp.categories]"/></section> | ||
| <submitter>Corentin Jabot</submitter> | ||
| <date>31 Jan 2024</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| Comparison categories can only be compared to the literal <tt>0</tt>. | ||
| It does not make sense for them to be comparable to anything else, so conceptually, the design of | ||
| <paper num="P0515"/> makes sense, however in practice it's a pain point from users and implementations | ||
| alike, as the desired semantics is barely implementable. | ||
| </p> | ||
| <ul> | ||
| <li><p>One implementation strategy is to use <tt>nullptr_t</tt>. This produces warnings in implementations | ||
| because using a <tt>0</tt> pointer constant is not recommended, and filtering out these warnings would be unreliable.</p></li> | ||
| <li><p>Another implementation strategy is to require a <tt>consteval</tt> expression but</p> | ||
| <ul> | ||
| <li><p>This permits <tt>1-1</tt>, which ought to be totally fine and yet it exposes users to UB</p></li> | ||
| <li><p>It is not SFINAE friendly (and attempts at SFINAE are presumably UB).</p></li> | ||
| </ul> | ||
| </li> | ||
| </ul> | ||
| <p> | ||
| And there are use cases where SFINAE friendliness is important, notably testing frameworks. | ||
| <p/> | ||
| The status quo has engendered multiple issues being reported to at least 3 vendors | ||
| <p/> | ||
| Suggestion: | ||
| </p> | ||
| <ul> | ||
| <li><p>Allow any <tt>0</tt> constant expression</p></li> | ||
| <li><p>Require compile magic to SFINAE on non zero values</p></li> | ||
| <li><p>Remove the gratuitous UB</p></li> | ||
| </ul> | ||
| <p> | ||
| The proposed wording requires compiler magic and has been | ||
| <a href="https://godbolt.org/z/1xr9Wx38T">implemented in clang</a>. | ||
| (Other possible way to implement that would for example be a magic type attribute, or a magic type) | ||
| <p/> | ||
| Related vendor issues: | ||
| <p/> | ||
| <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96278">GCC Bugzilla issue 96278</a> | ||
| <p/> | ||
| <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100903">GCC Bugzilla issue 100903</a> | ||
| <p/> | ||
| <a href="https://github.com/llvm/llvm-project/issues/43670">LLVM issue 43670</a> | ||
| <p/> | ||
| <a href="https://github.com/llvm/llvm-project/pull/79465">LLVM pulls request 79465</a> | ||
| <p/> | ||
| <a href="https://github.com/microsoft/STL/issues/4359">Microsoft STL issue 4359</a> | ||
| <p/> | ||
| <a href="https://github.com/microsoft/STL/pull/3581">Microsoft STL pull request 3581</a> | ||
| <p/> | ||
| <a href="https://developercommunity.visualstudio.com/t/Regression:-False-positive-C7595:-std::/10509214">Visual Studio issue 10509214</a> | ||
| <p/> | ||
| <a href="https://github.com/snitch-org/snitch/issues/140">snitch issue 140</a> | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4971"/>. | ||
| </p> | ||
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| <ol> | ||
| <li><p>Modify <sref ref="[cmp.categories.pre]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <p> | ||
| -3- The relational and equality operators for the comparison category types are specified with an anonymous | ||
| parameter of unspecified type. This type shall be selected by the implementation such that these parameters | ||
| can <ins>only</ins> accept <ins>an integral constant expression evaluating to</ins><del>literal</del> <tt>0</tt> | ||
| as a corresponding argument. | ||
| <p/> | ||
| <del>[<i>Example 1</i>: <tt>nullptr_t</tt> meets this requirement. — <i>end example</i>]</del> | ||
| <p/> | ||
| <del>In this context, the behavior of a program that supplies an argument other than a literal <tt>0</tt> is undefined.</del> | ||
| </p> | ||
| </blockquote> | ||
| </li> | ||
| </ol> | ||
| </resolution> | ||
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| </issue> |