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<title>Issue 2859: Definition of reachable in [ptr.launder] misses pointer arithmetic from pointer-interconvertible object</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++20">C++20</a> status.</em></p>
<h3 id="2859"><a href="lwg-defects.html#2859">2859</a>. Definition of <em>reachable</em> in [ptr.launder] misses pointer arithmetic from pointer-interconvertible object</h3>
<p><b>Section:</b> 17.6.5 <a href="https://wg21.link/ptr.launder">[ptr.launder]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Hubert Tong <b>Opened:</b> 2017-01-31 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#ptr.launder">issues</a> in [ptr.launder].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Given:
</p>
<blockquote><pre>
struct A { int x, y; };
A a[100];
</pre></blockquote>
<p>
The bytes which compose <code>a[3]</code> can be reached from <code>&amp;a[2].x</code>:
<code>reinterpret_cast&lt;A *&gt;(&amp;a[2].x) + 1</code> points to <code>a[3]</code>,
however, the definition of "reachable" in [ptr.launder] does not encompass this case.
</p>

<p><i>[2017-03-04, Kona]</i></p>

<p>Set priority to 2. Assign this (and <a href="lwg-closed.html#2860" title="launder and base class subobjects (Status: NAD)">2860</a><sup><a href="https://cplusplus.github.io/LWG/issue2860" title="Latest snapshot">(i)</a></sup>) to Core.</p>

<p><i>[2017-08-14, CWG telecon note]</i></p>

<p>
CWG is fine with the proposed resolution.  
</p>

<p><i>[2020-02 Status to Immediate on Thursday night in Prague.]</i></p>




<p id="res-2859"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4618">N4618</a>.</p>

<ol>
<li><p>Modify 17.6.5 <a href="https://wg21.link/ptr.launder">[ptr.launder]</a> as indicated:</p>
<blockquote>
<pre>
template &lt;class T&gt; constexpr T* launder(T* p) noexcept;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-3- <i>Remarks:</i> An invocation of this function may be used in a core constant expression whenever the value
of its argument may be used in a core constant expression. A byte of storage <ins><code><i>b</i></code></ins> is 
reachable through a pointer value that points to an object <code><i>Y</i></code> if <ins>there is an object 
<code><i>Z</i></code>, pointer-interconvertible with <code><i>Y</i></code>, such that <code><i>b</i></code></ins> <del>it</del> 
is within the storage occupied by <code><i><ins>Z</ins><del>Y</del></i></code>, <del>an object that is 
pointer-interconvertible with <code><i>Y</i></code>,</del> or the immediately-enclosing array object if 
<code><i><ins>Z</ins><del>Y</del></i></code> is an array element. The program is ill-formed if <code>T</code> is a function type or 
(possibly <i>cv</i>-qualified) <code>void</code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>





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