New issue from Giuseppe D'Angelo: "Imprecise Throws: clause in std::relocate"

Author: Dani-Hub

xml/issue4282.xml

@@ -0,0 +1,85 @@
+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4282" status="New">
+<title>Imprecise <i>Throws</i>: clause in `std::relocate`</title>
+<section>
+<sref ref="[obj.lifetime]"/>
+</section>
+<submitter>Giuseppe D'Angelo</submitter>
+<date>23 Jun 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+The current specification of `std::relocate` in <sref ref="[obj.lifetime]"/> says
+</p>
+<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
+<p>
+<i>Throws</i>: Nothing.
+</p>
+</blockquote>
+<p>
+This is imprecise. A trivially relocatable type may feature a throwing
+move constructor (or a throwing destructor); for instance, a linked-list
+implementation with an externally allocated sentinel node may have a
+throwing move constructor, but still be trivially relocatable. Such a
+type is nothrow relocatable (as per `is_nothrow_relocatable_v`, cf.
+<sref ref="[meta.unary.prop]"/>) and therefore one can call `std::relocate` 
+on objects of that type.
+<p/>
+During constant evaluation, a call to `std::relocate` is specified to
+relocate objects via move construction and destruction.
+(`std::trivially_relocate` is unavailable during constant evaluation.)
+<p/>
+Since <paper num="P3068"/> we are allowed to throw during constant evaluation, 
+and therefore it's unclear whether `std::relocate` should propagate such an
+exception to the caller or should instead make the evaluation not a
+constant expression (and therefore make the program ill-formed, since by
+construction we end up in this possibility during constant evaluation).
+<p/>
+Given the rationale brought forward for `std::relocate` in <paper num="P2786"/> 
+is to be always a nofail operation, I'm proposing a resolution that goes in
+that direction.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5008"/>.
+</p>
+
+<ol>
+<li><p>Modify <sref ref="[obj.lifetime]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+template&lt;class T&gt;
+  constexpr T* relocate(T* first, T* last, T* result);
+</pre>
+<blockquote>
+<p>
+-16 <i>Mandates</i>: [&hellip;]
+<p/>
+-17- <i>Preconditions</i>: [&hellip;]
+<p/>
+-18- <i>Effects</i>: [&hellip;]
+<p/>
+-19- <i>Returns</i>: `result + (last - first)`.
+<p/>
+-20- <i>Throws:</i>: Nothing.
+<p/>
+<ins>-?- <i>Remarks</i>: If `relocate` is called during constant evaluation, and an
+exception is thrown by a constructor or destructor of `T`, the call to `relocate` 
+is not a core constant expression (<sref ref="[expr.const]"/>).</ins>
+<p/>
+[<i>Note 3</i>: Overlapping ranges are supported. &mdash; <i>end note</i>]
+</p>
+</blockquote>
+</blockquote>
+</li>
+
+</ol>
+</resolution>
+
+</issue>