New issue from Hewill: "Range adaptor closure object is underspecifie…

…d for its return type"

xml/issue3981.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="3981" status="New">
+<title>Range adaptor closure object is underspecified for its return type</title>
+<section><sref ref="[range.adaptor.object]"/></section>
+<submitter>Hewill Kang</submitter>
+<date>22 Aug 2023</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+In order to provide pipe support for user-defined range adaptors, <paper num="P2387R3"/> 
+removed the specification that the adaptor closure object returns a view, which conforms to the wording of <tt>ranges::to</tt>.
+<p/>
+However, the current wording seems to be too low-spec so that the range adaptor closure object can return any type 
+or even <tt>void</tt>. This makes it possible to break the previous specification when returning types that don't make sense, 
+<a href="https://godbolt.org/z/Y4nvjY3xj">for example</a>:
+</p>
+<blockquote>
+<pre>
+#include &lt;ranges&gt;
+
+struct Closure : std::ranges::range_adaptor_closure&lt;Closure&gt; {
+  struct NonCopyable {
+    NonCopyable(const NonCopyable&amp;) = delete;
+  };
+
+  const NonCopyable&amp; operator()(std::ranges::range auto&amp;&amp;);
+};
+
+auto r = std::views::iota(0) | Closure{}; // <span style="color:red;font-weight:bolder">hard error in stdlibc++ and MSVC-STL</span>
+</pre></blockquote>
+<p>
+Above, since the return type of the pipeline operator is declared as <tt>auto</tt>, this causes the deleted 
+copy constructor to be invoked in the function body and produces a hard error.
+<p/>
+The proposed resolution adds a specification for the range adaptor closure object to return a <i>cv</i>-unqualified class type.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N4958"/>.
+</p>
+
+<ol>
+
+<li><p>Modify the <sref ref="[range.adaptor.object]"/> as indicated:</p>
+
+<blockquote>
+<p>
+-1- A <i>range adaptor closure object</i> is a unary function object that accepts a range argument. For a range adaptor
+closure object <tt>C</tt> and an expression <tt>R</tt> such that <tt>decltype((R))</tt> models <tt>range</tt>, the 
+following expressions are equivalent:
+</p>
+<p>[&hellip;]</p>
+<p>
+-2- Given an object <tt>t</tt> of type <tt>T</tt>, where
+</p>
+<ol style="list-style-type: none">
+<li><p>(2.1) &mdash; <tt>t</tt> is a unary function object that accepts a range argument <ins>and returns a 
+<i>cv</i>-unqualified class object</ins>,</p></li>
+<p>[&hellip;]</p>
+</ol>
+<p>
+then the implementation ensures that <tt>t</tt> is a range adaptor closure object.
+</p>
+</blockquote>
+
+</li>
+
+</ol>
+</resolution>
+
+</issue>