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<title>Issue 3034: P0767R1 breaks previously-standard-layout types</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="3034"><a href="lwg-defects.html#3034">3034</a>. P0767R1 breaks previously-standard-layout types</h3>
<p><b>Section:</b> 21.3.9.7 <a href="https://wg21.link/meta.trans.other">[meta.trans.other]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2017-11-12 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.trans.other">issues</a> in [meta.trans.other].</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>
<a href="https://wg21.link/p0767r1">P0767R1 "Expunge POD"</a> changed the
requirement for several library types from "POD" to "trivial." Since these types
no longer provide/require the standard-layout portion of "POD," the change breaks:
<ul>
<li>user classes with a member of such a type that were standard-layout in C++17</li>
<li>implementations of <code>basic_string</code> and <code>basic_string_view</code> that
expect character types to be both trivial and standard layout.</li>
</ul>
It appears this breakage was not intentional and not discussed in LWG.
</p>
<p>
The fix is straight-forward: apply an additional standard-layout requirement to
the affected types:
<ul>
<li><code>max_align_t</code></li>
<li>the <code>type</code> member of specializations of <code>aligned_storage</code></li>
<li>the <code>type</code> member of specializations of <code>aligned_union</code></li>
<li>the char-like objects used by <code>basic_string</code> and <code>basic_string_view</code>.</li>
</ul>
(Albeit the potential for breakage with <code>max_align_t</code> is admittedly small.)
</p>

<p><i>[
2017-11-14 Moved to Tentatively Ready after 8 positive votes for P0 on c++std-lib.
]</i></p>

<p><i>[2018-3-17 Adopted in Jacksonville]</i></p>



<p id="res-3034"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4700">N4700</a> +
<a href="https://wg21.link/p0767r1">P0767R1</a>.
</p>
<ol>
<li><p>Change in 17.2.4 <a href="https://wg21.link/support.types.layout">[support.types.layout]</a> paragraph 5:</p>
<p>
The type <code>max_align_t</code> is a trivial <ins>standard-layout</ins> type whose
alignment requirement is at least as great as that of every scalar type, and whose
alignment requirement is supported in every context (6.8.3 <a href="https://wg21.link/basic.align">[basic.align]</a>).
</p>
</li>
<li><p>Change the table in 21.3.9.7 <a href="https://wg21.link/meta.trans.other">[meta.trans.other]</a> as indicated:</p>
<p>
<code>aligned_storage</code><br/>
The member typedef <code>type</code> shall be a trivial <ins>standard-layout</ins>
type suitable for use as uninitialized storage for any object whose size is at
most <code>Len</code> and whose alignment is a divisor of <code>Align</code>.
</p>
<p>
<code>aligned_union</code><br/>
The member typedef <code>type</code> shall be a trivial <ins>standard-layout</ins>
type suitable for use as uninitialized storage for any object whose type is listed
in <code>Types</code>; its size shall be at least <code>Len</code>.
</p>
</li>
<li><p>Change 27.1 <a href="https://wg21.link/strings.general">[strings.general]</a> paragraph 1 as indicated:</p>
<p>
This Clause describes components for manipulating sequences of any non-array
trivial <ins>standard-layout</ins> (6.9 <a href="https://wg21.link/basic.types">[basic.types]</a>) type. Such types
are called char-like types, and objects of char-like types are called char-like
objects or simply characters.
</p>
</li>
</ol>





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