New issue from Jiang An: "Contradictory specification of std::tuple_size"

Author: Dani-Hub

xml/issue4040.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4040" status="New">
+<title>Contradictory specification of <tt>std::tuple_size</tt></title>
+<section><sref ref="[ostream.formatted.print]"/></section>
+<submitter>Jiang An</submitter>
+<date>20 Jan 2024</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+<sref ref="[tuple.helper]"/>/1 currently says:
+</p>
+<blockquote>
+<p>
+All specializations of <tt>tuple_size</tt> meet the <i>Cpp17UnaryTypeTrait</i> requirements 
+(<sref ref="[meta.rqmts]"/>) with a base characteristic of <tt>integral_constant&lt;size_t, N&gt;</tt> 
+for some <tt>N</tt>.
+</p>
+</blockquote>
+<p>
+which is probably wrong for <tt>std::tuple_size&lt;int&gt;</tt> and 
+<tt>std::tuple_size&lt;std::vector&lt;int&gt;&gt;</tt> whose definitions are not provided, and 
+contradicts with <sref ref="[tuple.helper]"/>/4
+</p>
+<blockquote>
+<p>
+[&hellip;] Otherwise, it has no member <tt>value</tt>.
+</p>
+</blockquote>
+<p>
+Also, there's currently implementation divergence on whether <tt>std::tuple_size&lt;<i>cv</i> T&gt;</tt> 
+is a complete type when <tt>std::tuple_size&lt;T&gt;::value</tt> is absent (related to LWG <iref ref="2770"/>). 
+Some implementations (e.g. 
+<a href="https://github.com/llvm/llvm-project/blob/b1d4265a5f5345065e5c2aedf50787da05ddcc08/libcxx/include/__tuple/tuple_size.h#L29-L47">libc++</a> 
+and <a href="https://github.com/gcc-mirror/gcc/blob/615e25c82de97acc17ab438f88d6788cf7ffe1d6/libstdc%2B%2B-v3/include/bits/utility.h#L55-L71">libstdc++</a>) 
+treat <sref ref="[tuple.helper]"/>/4 and <sref ref="[depr.tuple]"/>/2 as "<tt>std::tuple_size&lt;<i>cv</i> T&gt;</tt> 
+is generated from partial specializations only if <tt>std::tuple_size&lt;T&gt;</tt> meets the aforementioned conditions", 
+while others (e.g. <a href="https://github.com/microsoft/STL/blob/442029c6fa37f1b6f9203357de09672d5704077c/stl/inc/utility#L598-L612">MSVC STL</a>) 
+always consider <tt>std::tuple_size&lt;<i>cv</i> T&gt;</tt> needs to be defined. The latter 
+reading seems to be justified by <sref ref="[contents]"/>/1 and <sref ref="[tuple.syn]"/>.
+</p>
+</discussion>
+
+<resolution>
+</resolution>
+
+</issue>