<!doctype html><html lang="en">
 <head>
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  <title>P3780R0: Detecting bitwise trivially relocatable types</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

/******************************************************************************/
/*                                    Body                                    */
/******************************************************************************/

	html {
	}

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;			  /* limit line length to 50em for readability   */
		margin: 0 auto;				/* center text within page                    */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag    */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em; }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small; }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;	/* Inherit the font family. */
		line-height: 1.2;		/* Keep wrapped headings compact */
		hyphens: manual;		/* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#profile-and-date {
		/* #profile-and-date is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	border-left: 0.5em solid #DEF;
	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 border-radius: .5em;
	 margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	 margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	 margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	margin-left: -2em;
	}
	dl.switch {
	padding-left: 2em;
	}
	dl.switch > dt {
	text-indent: -1.5em;
	margin-top: 1em;
	}
	dl.switch > dt + dt {
	margin-top: 0;
	}
	dl.switch > dt::before {
	content: '\21AA';
	padding: 0 0.5em 0 0;
	display: inline-block;
	width: 1em;
	text-align: right;
	line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: inherit;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

	/* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

	/* For autogen numbers, add
	  .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,	.sidefigure img,	figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
		height: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em;
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em; }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote,
	.amendment, .correction, .addition {
		margin: 1em auto;
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	  or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #e0cb52;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	  or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52e052;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary {
		color: hsl(120, 70%, 30%);
		color: var(--noteheading-text);
	}
	/* Add .note::before { content: "Note "; } for autogen label,
	  or use class="marker" to mark up the label in source. */

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
		border-bottom: 1px solid var(--defrow-border);
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def td.footnote {
		padding-top: 0.6em;
	}
	table.def td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		<table class="data"> highlights structural relationships in a table
		when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		Use class="complex data" for particularly complicated tables --
		(This will draw more lines: busier, but clearer.)

		Use class="long" on table cells with paragraph-like contents
		(This will adjust text alignment accordingly.)
		Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-color: var(--datacell-border);
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		border-top: 1px solid var(--datacell-border);
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 618007319, updated Thu Jun 26 17:23:55 2025 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="dark light" name="color-scheme">
<style>
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P3780R0<br>Detecting bitwise trivially relocatable types</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal,
    <time class="dt-updated" datetime="2025-06-30">2025-06-30</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Issue Tracking:
     <dd><a href="#issues-index">Inline In Spec</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:giuseppe.dangelo@kdab.com">Giuseppe D'Angelo</a>
     <dt>Audience:
     <dd>EWG, LEWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>We propose a new type trait that detects whether a type can be trivially relocated, and such trivial relocation does not alter any part of the object representation. This trait has direct use cases when creating types that use byte buffers to manage objects, such as certain implementations of <code class="highlight"><c- n>optional</c-></code>, <code class="highlight"><c- n>inplace_vector</c-></code>, <code class="highlight"><c- n>any</c-></code>, <code class="highlight"><c- n>function</c-></code> and similar.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#changelog"><span class="secno">1</span> <span class="content">Changelog</span></a>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivation and scope</span></a>
     <ol class="toc">
      <li>
       <a href="#usecase"><span class="secno">2.1</span> <span class="content">The need of detecting bitwise trivial relocations</span></a>
       <ol class="toc">
        <li><a href="#QVariant"><span class="secno">2.1.1</span> <span class="content">Example: <code class="highlight"><c- n>QVariant</c-></code></span></a>
        <li><a href="#optional"><span class="secno">2.1.2</span> <span class="content">Example: <code class="highlight"><c- n>optional</c-></code></span></a>
        <li><a href="#inplacevector"><span class="secno">2.1.3</span> <span class="content">Example: <code class="highlight"><c- n>inplace_vector</c-></code></span></a>
        <li><a href="#qlist"><span class="secno">2.1.4</span> <span class="content">Example: <code class="highlight"><c- n>QList</c-></code></span></a>
        <li><a href="#lwgissue"><span class="secno">2.1.5</span> <span class="content">Resolving LWG4283</span></a>
       </ol>
      <li>
       <a href="#aside"><span class="secno">2.2</span> <span class="content">Aside: is trivial relocation... trivial?</span></a>
       <ol class="toc">
        <li><a href="#polls"><span class="secno">2.2.1</span> <span class="content">Polls during the <span title="std::is_trivially_relocatable">[P1144R13]</span> review in Sofia</span></a>
       </ol>
     </ol>
    <li>
     <a href="#design"><span class="secno">3</span> <span class="content">Design decisions</span></a>
     <ol class="toc">
      <li><a href="#whyanewtrait"><span class="secno">3.1</span> <span class="content">Do we need a new trait? Can users implement this feature?</span></a>
      <li><a href="#whynotbool"><span class="secno">3.2</span> <span class="content">Why a new type trait, rather than a non-template boolean variable / a macro / etc.?</span></a>
     </ol>
    <li><a href="#impact"><span class="secno">4</span> <span class="content">Impact on the Standard</span></a>
    <li>
     <a href="#wording"><span class="secno">5</span> <span class="content">Proposed Wording</span></a>
     <ol class="toc">
      <li><a href="#feature-testing"><span class="secno">5.1</span> <span class="content">Feature-testing macro</span></a>
      <li><a href="#delta"><span class="secno">5.2</span> <span class="content">Wording</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">6</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Changelog</span><a class="self-link" href="#changelog"></a></h2>
   <ul>
    <li data-md>
     <p>R0</p>
     <ul>
      <li data-md>
       <p>First submission</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation and scope</span><a class="self-link" href="#motivation"></a></h2>
   <p><b><a data-link-type="biblio" href="#biblio-p2786r13" title="Trivial Relocatability For C++26">[P2786R13]</a> ("Trivial Relocatability For C++26") has been approved
for inclusion in C++26.</b> The paper added several new language and
library facilities to create, detect and manipulate trivially
relocatable types.</p>
   <p>A specific feature of <a data-link-type="biblio" href="#biblio-p2786r13" title="Trivial Relocatability For C++26">[P2786R13]</a>’s design is the <strong>ability of
trivially relocate polymorphic types.</strong></p>
   <p>In theory, a polymorphic type is just a type with a "hidden pointer";
from a certain point of view, it’s as if objects of polymorphic type
contain a non-static data member of pointer type, managed by
the implementation, that points to a given’s object virtual table.
(They may contain more than one such pointer in case of multiple
inheritance, but this does not change the discussion in any way.)</p>
   <p>Since pointer types are trivially relocatable, it seems natural to
think that polymorphic types should be trivially relocatable as well.</p>
   <p>However pointers to virtual tables add a complication. Simplifying, on
some architectures/ABIs (such as <code class="highlight"><c- n>arm64e</c-></code>) these pointers are
<em>authenticated</em>, meaning that some bits of the pointer value are used
to store a cryptographic signature. This signature gets verified when
one accesses the virtual table pointer (for instance by calling a
virtual function).</p>
   <p>One of the inputs of the signature is target address of the pointer
(that is, the address of the virtual table). Another input, which is
crucial to this story, is the <em>address of the virtual table pointer
itself</em>.</p>
   <p><strong>This means that, on those architectures, relocating a polymorphic
object by using a simple byte copy would produce an object whose
virtual table pointer would fail to authenticate.</strong> This is because the
new virtual table pointer is now located at a different address than
the old one (as it has been moved in memory through relocation), and
since the address is part of the signature, and the signature has been
copied bitwise, the signature is now invalid for the new address.</p>
   <p><a data-link-type="biblio" href="#biblio-p2786r13" title="Trivial Relocatability For C++26">[P2786R13]</a> solves this problem (and allows "universal" trivial
relocation of polymorphic types) because trivial relocation happens
through a new library entry point: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>trivially_relocate</c-></code>. This
entry point is strongly typed (unlike <code class="highlight"><c- n>memcpy</c-></code> or <code class="highlight"><c- n>memmove</c-></code>). On the
architectures that feature this form of pointer signature the
implementation of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>trivially_relocate</c-></code> is going to take care of
re-signing the virtual table pointers as part of the relocation
process.</p>
   <p>For more information about these mechanisms, please refer to the
excellent write-up in P3751R0; to chapter §15.1 of <a data-link-type="biblio" href="#biblio-p2786r13" title="Trivial Relocatability For C++26">[P2786R13]</a>; and to
Apple’s documentation of pointer authentication available
<a href="https://developer.apple.com/documentation/security/preparing-your-app-to-work-with-pointer-authentication">here</a>.</p>
   <p class="issue" id="issue-c8fe2963"><a class="self-link" href="#issue-c8fe2963"></a> add a link to the whitepaper on pointer auth once it’s available.</p>
   <p><strong>Summarizing: in C++26 polymorphic types can be trivially relocatable,
and this quality is not platform or implementation-specific.</strong></p>
   <h3 class="heading settled" data-level="2.1" id="usecase"><span class="secno">2.1. </span><span class="content">The need of detecting bitwise trivial relocations</span><a class="self-link" href="#usecase"></a></h3>
   <p>In some contexts it is necessary to detect whether trivial relocation
is akin to a mere byte copy of an object’s representation, or if
instead <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>trivially_relocate</c-></code> needs to do some extra work (such as
the just discussed re-signing of pointers to virtual tables).</p>
   <p>All these context have one in common the fact that one has a wrapper
object that manages an array of bytes. Those bytes are then used as
storage for dynamically created objects. Do note that such a design is
extremely common: it can be used to implement (non-constexpr friendly)
<code class="highlight"><c- n>optional</c-></code>, <code class="highlight"><c- n>inplace_vector</c-></code>, <code class="highlight"><c- n>any</c-></code> (with a small object optimization),
and many other vocabulary types. The wrapper class may be a class
template that knows the type of the object(s) it will hold (<code class="highlight"><c- n>optional</c-></code>,
<code class="highlight"><c- n>inplace_vector</c-></code>) or be even type-erased (<code class="highlight"><c- n>any</c-></code>); this does not affect
our discussion.</p>
   <p><strong>The layout of these wrapper classes looks like this:</strong></p>
<pre class="highlight"><c- k>class</c-> <c- nc>Wrapper</c->
<c- p>{</c->
    <c- k>alignas</c-><c- p>(</c-><c- n>A</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>byte</c-> <c- n>storage</c-><c- p>[</c-><c- n>N</c-><c- p>];</c->
    <c- d>/* ... other data ... */</c->

<c- k>public</c-><c- o>:</c->
    <c- n>Wrapper</c-><c- p>();</c->
    <c- o>~</c-><c- n>Wrapper</c-><c- p>();</c->
    <c- n>Wrapper</c-><c- p>(</c-><c- n>Wrapper</c-> <c- o>&amp;&amp;</c-><c- p>);</c->
    <c- d>/* other special member functions, rest of the API, etc. */</c->
<c- p>};</c->
</pre>
   <p>Problems arise if we want to make <code class="highlight"><c- n>Wrapper</c-></code> a trivially relocatable
class. First and foremost, we need to add the new
<code class="highlight"><c- n>trivially_relocatable_if_eligible</c-></code> keyword to the class definition:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>Wrapper</c-> <c- n>trivially_relocatable_if_eligible</c->
<c- p>{</c->
    <c- k>alignas</c-><c- p>(</c-><c- n>A</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>byte</c-> <c- n>storage</c-><c- p>[</c-><c- n>N</c-><c- p>];</c->
    <c- d>/* ... other data ... */</c->

<c- k>public</c-><c- o>:</c->
    <c- n>Wrapper</c-><c- p>();</c->
    <c- o>~</c-><c- n>Wrapper</c-><c- p>();</c->
    <c- n>Wrapper</c-><c- p>(</c-><c- n>Wrapper</c-> <c- o>&amp;&amp;</c-><c- p>);</c->
    <c- d>/* other special member functions, rest of the API, etc. */</c->
<c- p>};</c->
</pre>
   <p>The keyword is necessary due to the presence of user-defined destructor
and move operations. The non-static data members of <code class="highlight"><c- n>Wrapper</c-></code> are all
of trivially relocatable type (storage of bytes, and some other
metadata which is usually integers and pointers), and therefore
<code class="highlight"><c- n>Wrapper</c-></code> is eligible for trivial relocation (see <a href="https://wg21.link/class.prop#3">[class.prop]/3</a>); with the
addition of the keyword, it is trivially relocatable.</p>
   <p>We also need some other requirement:</p>
   <ul>
    <li data-md>
     <p>If <code class="highlight"><c- n>Wrapper</c-></code> is <em>not</em> a class template (e.g. <code class="highlight"><c- n>any</c-></code>), we may decide
to forbid construction from <code class="highlight"><c- n>T</c-></code> objects that are not of trivially
relocatable types, or decide to always heap-allocate them instead
of storing them directly in the internal storage (see
<a href="#QVariant">§ 2.1.1 Example: QVariant</a>).
Otherwise, one risks trivially relocating a <code class="highlight"><c- n>Wrapper</c-></code> object when
it contains a non-trivially relocatable type, and that verges on UB
(see also <a href="#lwgissue">§ 2.1.5 Resolving LWG4283</a>).</p>
    <li data-md>
     <p>If <code class="highlight"><c- n>Wrapper</c-></code> is a class template (e.g. <code class="highlight"><c- n>optional</c-></code>), then we can
make it <em>conditionally</em> trivially relocatable depending on <code class="highlight"><c- n>T</c-></code>. For
instance, we can make it inherit from a conditionally trivially
relocatable helper class (see also §13.2 of <a data-link-type="biblio" href="#biblio-p2786r13" title="Trivial Relocatability For C++26">[P2786R13]</a>):</p>
<pre class="highlight"><c- c1>// trivially relocatable</c->
<c- k>template</c-> <c- o>&lt;</c-><c- b>bool</c-> <c- n>b</c-><c- o>></c->
<c- k>struct</c-> <c- nc>trivially_relocatable_if</c-> <c- p>{};</c->

<c- c1>// not trivially relocatable</c->
<c- k>template</c-> <c- o>&lt;></c->
<c- k>struct</c-> <c- nc>trivially_relocatable_if</c-><c- o>&lt;</c->false<c- o>></c-> <c- p>{</c->
  <c- o>~</c-><c- n>trivially_relocatable_if</c-><c- p>()</c-> <c- p>{}</c-> <c- c1>// deliberately not =default!</c->
<c- p>};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>Wrapper</c-> <c- n>trivially_relocatable_if_eligible</c->
  <c- o>:</c-> <c- n>trivially_relocatable_if</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c->
<c- p>{</c->
  <c- d>/* ... */</c->
<c- p>};</c->
</pre>
   </ul>
   <p>Ultimately, either choice (template or non-template) does not affect
our discussion.</p>
   <p>The point is that the design of all these wrapper datatypes has a
problem with C++26’s trivial relocation model; specifically, <b>the fact
that the objects held into the internal storage are of trivially
relocatable type is a <i>necessary but not sufficient condition</i> to
make <code class="highlight"><c- n>Wrapper</c-></code> itself trivially relocatable.</b></p>
   <p>A polymorphic type can be trivially relocatable, and stored into
<code class="highlight"><c- n>Wrapper</c-></code>’s internal buffer. However, attempting to trivially relocate
a <code class="highlight"><c- n>Wrapper</c-></code> object that contains such an object of polymoprhic type on
an architecture that employs pointer authentication will ultimately
result in a runtime crash (UB), because the virtual table pointer in
the wrapped object would <em>not</em> be re-signed.</p>
   <p>We therefore need a more stringent requirement for a type to be stored
into <code class="highlight"><c- n>Wrapper</c-></code>’s internal buffer.</p>
   <hr>
   <p>One strategy would be to exclude polymorphic types from being stored
into the small buffer (that is, to effectively treat polymorphic types
as if they were non-trivially relocatable types).</p>
   <p><strong>This however is limiting and possibly confusing,</strong> for at least two
reasons. First, on platforms where polymorphic types which do not
employ any pointer authentication, there is no need of having this
additional requirement on the types storable in the small buffer.
Second, the check for "is polymorphic" does not immediately convey the
nature of the problem.</p>
   <p><b>For these reasons we are proposing a novel trait, that we are going
to call <code class="highlight"><c- n>is_bitwise_trivially_relocatable</c-></code>.</b> This trait is going to
be satisfied by types that are trivially relocatable, and also for
which trivial relocation is "bitwise"; that is, for which trivial
relocation can be achieved without any changes to an object’s
representation.</p>
   <p>For types that are both polymorphic and trivially relocatable, the
value of this trait is going to depend on the implementation. This is
already the case with <code class="highlight"><c- k>union</c-></code>s containing polymorphic types (see <a href="https://wg21.link/class.prop#3">[class.prop/3]</a>), so the idea
is not novel in the core language.</p>
   <h4 class="heading settled" data-level="2.1.1" id="QVariant"><span class="secno">2.1.1. </span><span class="content">Example: <code class="highlight"><c- n>QVariant</c-></code></span><a class="self-link" href="#QVariant"></a></h4>
   <p>The <code class="highlight"><c- n>QVariant</c-></code> class in Qt
(<a href="https://doc.qt.io/qt-6/qvariant.html">documentation</a>) is a
type-erased single object wrapper, very similar to the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>any</c-></code> class
from the Standard Library.</p>
   <p>There are a few aspects of <code class="highlight"><c- n>QVariant</c-></code>’s design that are important to
discuss:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>QVariant</c-></code> is <em>unconditionally trivially relocatable</em>. In Qt this is
realized via the Qt’s specific trivial relocation macros, but of
course in C++26 this would be achieved by using the new language
keywords.</p>
    <li data-md>
     <p><code class="highlight"><c- n>QVariant</c-></code> features a small object optimization, just like the
<code class="highlight"><c- n>Wrapper</c-></code>example discussed before. When an object is stored into a
<code class="highlight"><c- n>QVariant</c-></code>, if the object is small enough, not overaligned, etc.
<em>and</em> it’s of trivially relocatable type, then it gets stored
directly into <code class="highlight"><c- n>QVariant</c-></code>’s small buffer. Otherwise, the object is
stored in heap-allocated storage, and the <code class="highlight"><c- n>QVariant</c-></code> object stores
a pointer to it.</p>
   </ol>
   <p>In other words, QVariant class layout looks like this (please note,
this is a huge simplification; the actual code is available
<a href="https://codebrowser.dev/qt6/qtbase/src/corelib/kernel/qvariant.h.html#64">here</a>):</p>
<pre class="highlight"><c- k>class</c-> <c- nc>QVariant</c-> <c- n>trivially_relocatable_if_eligible</c->
<c- p>{</c->
    <c- c1>// Storage: pointer to heap-allocated object, or small buffer</c->
    <c- k>union</c-> <c- nc>Storage</c-> <c- p>{</c->
      <c- b>void</c-> <c- o>*</c-><c- n>m_ptr</c-> <c- o>=</c-> <c- k>nullptr</c-><c- p>;</c->
      <c- k>alignas</c-><c- p>(</c-><c- n>QVariantSmallBufferAlignment</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>byte</c-> <c- n>m_buffer</c-><c- p>[</c-><c- n>QVariantSmallBufferSize</c-><c- p>];</c->
    <c- p>}</c-> <c- n>m_storage</c-><c- p>;</c->

    <c- c1>// Pointer to a data structure with pointers to functions to do</c->
    <c- c1>// type-aware manipulation of the storage (copy, move, destroy,</c->
    <c- c1>// assign, etc.), answer type queries, and so on:</c->
    <c- k>const</c-> <c- n>detail</c-><c- o>::</c-><c- n>type_interface</c-> <c- o>*</c-><c- n>m_type_interface</c-> <c- o>=</c-> <c- k>nullptr</c-><c- p>;</c->

<c- k>public</c-><c- o>:</c->
    <c- n>QVariant</c-><c- p>();</c->
    <c- o>~</c-><c- n>QVariant</c-><c- p>();</c->
    <c- n>QVariant</c-><c- p>(</c-><c- k>const</c-> <c- n>QVariant</c-> <c- o>&amp;</c-><c- p>);</c->
    <c- n>QVariant</c-><c- p>(</c-><c- n>QVariant</c-> <c- o>&amp;&amp;</c-><c- p>);</c->

    <c- c1>// ... other special member functions, constructors from a generic</c->
    <c- c1>// T, rest of the API, etc.; not important for the discussion.</c->
<c- p>};</c->

<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>QVariant</c-><c- o>></c-><c- p>);</c-> <c- c1>// OK</c->
</pre>
   <p>Again, the requirement that the wrapped type needs to be trivially
relocatable in order to be stored into the small storage is
insufficient: <code class="highlight"><c- n>T</c-></code> needs to be <em>bitwise</em> trivially relocatable.</p>
   <h4 class="heading settled" data-level="2.1.2" id="optional"><span class="secno">2.1.2. </span><span class="content">Example: <code class="highlight"><c- n>optional</c-></code></span><a class="self-link" href="#optional"></a></h4>
   <p>Consider this (non-constexpr friendly) implementation of <code class="highlight"><c- n>optional</c-></code>,
from this <a data-link-type="biblio" href="#biblio-nonconstexproptionalblog" title="Non-constexpr Optional and trivial relocation">blog post</a> by Arthur O’Dwyer,
with some code omitted for brevity:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- p>[[</c-><c- n>trivially_relocatable</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>)]]</c-> <c- n>Optional</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
  <c- k>explicit</c-> <c- n>Optional</c-><c- p>()</c-> <c- p>{}</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
  <c- b>void</c-> <c- n>emplace</c-><c- p>(</c-><c- n>Args</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>args</c-><c- p>)</c-> <c- p>{</c->
    <c- k>if</c-> <c- p>(</c-><c- n>engaged_</c-><c- p>)</c-> <c- p>{</c->
      <c- n>std</c-><c- o>::</c-><c- n>destroy_at</c-><c- p>(</c-><c- n>t</c-><c- p>());</c->
      <c- n>engaged_</c-> <c- o>=</c-> false<c- p>;</c->
    <c- p>}</c->
    <c- n>std</c-><c- o>::</c-><c- n>construct_at</c-><c- p>(</c-><c- n>t</c-><c- p>(),</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Args</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>)...);</c->
    <c- n>engaged_</c-> <c- o>=</c-> true<c- p>;</c->
  <c- p>}</c->

  <c- n>T</c-><c- o>&amp;</c-> <c- n>value</c-><c- p>()</c-> <c- p>{</c-> <c- n>assert</c-><c- p>(</c-><c- n>engaged_</c-><c- p>);</c-> <c- k>return</c-> <c- o>*</c-><c- k>reinterpret_cast</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-><c- p>(</c-><c- n>data_</c-><c- p>);</c-> <c- p>}</c->
  <c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>value</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- n>assert</c-><c- p>(</c-><c- n>engaged_</c-><c- p>);</c-> <c- k>return</c-> <c- o>*</c-><c- k>reinterpret_cast</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>T</c-><c- o>*></c-><c- p>(</c-><c- n>data_</c-><c- p>);</c-> <c- p>}</c->

  <c- c1>// ... rest of the API ...</c->

<c- k>private</c-><c- o>:</c->
  <c- n>T</c-> <c- o>*</c-><c- n>t</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- k>reinterpret_cast</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-><c- p>(</c-><c- n>data_</c-><c- p>);</c-> <c- p>}</c->

  <c- k>alignas</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- b>char</c-> <c- n>data_</c-><c- p>[</c-><c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)];</c->
  <c- b>bool</c-> <c- n>engaged_</c-> <c- o>=</c-> false<c- p>;</c->
<c- p>};</c->
</pre>
   <p>Apart from using <a data-link-type="biblio" href="#biblio-p1144r13" title="std::is_trivially_relocatable">[P1144R13]</a> trivial relocation syntax, this
<code class="highlight"><c- n>Optional</c-></code> type is yet another instance of <code class="highlight"><c- n>Wrapper</c-></code>. An <code class="highlight"><c- n>Optional</c-></code>
wrapping a trivially relocatable polymorphic type may misbehave if
it gets trivially relocated.</p>
   <h4 class="heading settled" data-level="2.1.3" id="inplacevector"><span class="secno">2.1.3. </span><span class="content">Example: <code class="highlight"><c- n>inplace_vector</c-></code></span><a class="self-link" href="#inplacevector"></a></h4>
   <p>A non-constexpr friendly <code class="highlight"><c- n>inplace_vector</c-></code> is just a generalization of
the just shown <code class="highlight"><c- n>Optional</c-></code>, holding storage for up to N elements
rather than just one. A quick modification of the previous example
yields:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- k>class</c-> <c- nc>inplace_vector</c->
    <c- o>:</c-> <c- n>trivially_relocatable_if</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c->
<c- p>{</c->
<c- k>public</c-><c- o>:</c->
  <c- k>explicit</c-> <c- n>inplace_vector</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

  <c- n>T</c-> <c- o>&amp;</c-><c- nf>push_back</c-><c- p>(</c-><c- k>const</c-> <c- n>T</c-> <c- o>&amp;</c-><c- n>value</c-><c- p>)</c->
  <c- p>{</c->
      <c- k>if</c-> <c- p>(</c-><c- n>m_size</c-> <c- o>==</c-> <c- n>capacity</c-><c- p>())</c-> <c- k>throw</c-> <c- n>std</c-><c- o>::</c-><c- n>bad_alloc</c-><c- p>();</c->
      <c- n>std</c-><c- o>::</c-><c- n>construct_at</c-><c- p>(</c-><c- n>t</c-><c- p>()[</c-><c- n>m_size</c-><c- p>],</c-> <c- n>value</c-><c- p>);</c->
      <c- k>return</c-> <c- n>t</c-><c- p>()[</c-><c- n>m_size</c-><c- o>++</c-><c- p>];</c->
  <c- p>}</c->

  <c- c1>// ... rest of the API ...</c->

<c- k>private</c-><c- o>:</c->
  <c- n>T</c-> <c- o>*</c-><c- n>t</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- k>reinterpret_cast</c-><c- o>&lt;</c-><c- n>T</c-> <c- o>*></c-><c- p>(</c-><c- n>m_data</c-><c- p>);</c-> <c- p>}</c->

  <c- k>alignas</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>byte</c-> <c- n>m_data</c-><c- p>[</c-><c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- o>*</c-> <c- n>N</c-><c- p>];</c->
  <c- b>size_t</c-> <c- n>m_size</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>The very same issue is present here.</p>
   <h4 class="heading settled" data-level="2.1.4" id="qlist"><span class="secno">2.1.4. </span><span class="content">Example: <code class="highlight"><c- n>QList</c-></code></span><a class="self-link" href="#qlist"></a></h4>
   <p>In Qt 4 and 5 <code class="highlight"><c- n>QList</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> managed a dynamic array of "blocks", with each
block being a few bytes. (In Qt 6 the implementation is completely
different.)</p>
   <p>For a type <code class="highlight"><c- n>T</c-></code> which was small enough, not overaligned, and trivially
relocatable, objects of that type were stored <em>directly</em> into each
block (possibly with extra padding). Objects of types that didn’t meet
these criteria were individually allocated on the heap, and the blocks
stored pointers to them.</p>
   <p>With a few stylistic changes, the code looked like this:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>QList</c->
<c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- c1>// Tag type, used for tag dispatching depending on the actual layout</c->
    <c- k>struct</c-> <c- nc>MemoryLayout</c->
        <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>conditional</c-><c- o>&lt;</c->
            <c- o>!</c-><c- n>QTypeInfo</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>isTriviallyRelocatable</c-> <c- o>||</c-> <c- n>QTypeInfo</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>isLarge</c-><c- p>,</c->
            <c- n>QListData</c-><c- o>::</c-><c- n>IndirectLayout</c-><c- p>,</c->
            <c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>conditional</c-><c- o>&lt;</c->
                <c- k>sizeof</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- o>==</c-> <c- k>sizeof</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-><c- p>),</c->
                <c- n>QListData</c-><c- o>::</c-><c- n>ArrayCompatibleLayout</c-><c- p>,</c->
                <c- n>QListData</c-><c- o>::</c-><c- n>InlineWithPaddingLayout</c->
             <c- o>>::</c-><c- n>type</c-><c- o>>::</c-><c- n>type</c-> <c- p>{};</c->
    <c- d>/* ... */</c->
<c- p>};</c->

</pre>
   <p>(<a href="https://codebrowser.dev/qt5/qtbase/src/corelib/tools/qlist.h.html#QList::MemoryLayout">Source</a>.)</p>
   <p>The main idea behind this implementation strategy has to do with type
erasure; specifically, <b>the management of the dynamic array backing a
<code class="highlight"><c- n>QList</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> object was type erased</b>. Allocation and reallocation of the
array happened through calls to <code class="highlight"><c- n>realloc</c-></code>; inserting and erasing in the
middle would "open a window" and "close a window" by calling <code class="highlight"><c- n>memmove</c-></code>
on the tail of the array, that is, shifting the tail’s elements around.</p>
   <p>In other words, <code class="highlight"><c- n>QList</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> was a random access container, but traded
speed of access and overall memory usage (compared to something like a
<code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>) for a much smaller generated code size and better
compilation times.</p>
   <p>The implementation of <code class="highlight"><c- n>QList</c-></code> had the same issue of <code class="highlight"><c- n>Wrapper</c-></code>: <b>a
<code class="highlight"><c- n>QList</c-><c- o>&lt;</c-><c- n>Polymorphic</c-><c- o>></c-></code> must not store objects of type <code class="highlight"><c- n>Polymorphic</c-></code>
directly inside each block unless they’re <em>bitwise</em> trivially
relocatable.</b></p>
   <h4 class="heading settled" data-level="2.1.5" id="lwgissue"><span class="secno">2.1.5. </span><span class="content">Resolving LWG4283</span><a class="self-link" href="#lwgissue"></a></h4>
   <p>The introduction of the trait affects the resolution of <a data-link-type="biblio" href="#biblio-lwg4283" title="std::trivially_relocate needs stronger preconditions on &quot;nested&quot; objects with dynamic lifetime">[LWG4283]</a>.</p>
   <p>This issue argues that the specification of
<code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>trivially_relocate</c-></code> in <a href="http://wg21.link/obj.lifetime">[obj.lifetime]</a> is incomplete:
there is no precondition that covers the fact that
trivially-relocatable objects like <code class="highlight"><c- n>Wrapper</c-></code> must be providing storage
to objects of <strong>bitwise</strong> trivially relocatable types, otherwise the
behavior is undefined.</p>
   <p>With the trait that we are proposing, we would change the proposed
resolution of the issue to something much more simple and direct,
like this:</p>
   <blockquote>
    
<em>Preconditions:</em>

    <ul>
     <li>[...]
     <li>
      <ins>(10.4) For each object <code class="highlight"><c- n>o</c-></code> whose storage is being provided for
([intro.object]) by objects in the <code class="highlight"><c- p>[</c-><c- n>first</c-><c- p>,</c-> <c- n>last</c-><c- p>)</c-></code> range, let <code class="highlight"><c- n>O</c-></code> be
the type of <code class="highlight"><c- n>o</c-></code>; <code class="highlight"><c- n>is_bitwise_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>O</c-><c- o>></c-></code> is
<code class="highlight">true</code>.</ins>
    </ul>
   </blockquote>
   <p>There is no need for the second change that explicitly mentions
polymorphic types, nor to mention <code class="highlight"><c- k>union</c-></code>s at all (as proposed on the
Library reflector.)</p>
   <h3 class="heading settled" data-level="2.2" id="aside"><span class="secno">2.2. </span><span class="content">Aside: is trivial relocation... trivial?</span><a class="self-link" href="#aside"></a></h3>
   <p>Historically trivial relocation has always been implemented in
third-party libraries in terms of byte operations.</p>
   <p>Lacking support from the core language, libraries introduced ways to
mark types as "trivially relocatable" (macros, template variables to
specialize, etc.), and exploited "UB that works in practice" by using
<code class="highlight"><c- n>memcpy</c-></code>/<code class="highlight"><c- n>memmove</c-></code> to move in memory trivially relocatable objects,
<code class="highlight"><c- n>realloc</c-></code> to reallocate contiguous buffers of trivially relocatable
objects, and so on. <a data-link-type="biblio" href="#biblio-p1144r11" title="std::is_trivially_relocatable">[P1144R11]</a> and <a data-link-type="biblio" href="#biblio-p3559r0" title="Trivial relocation: One trait or two?">[P3559R0]</a> contain extensive
surveys of such libraries.</p>
   <p>Setting aside the fact that all such library solutions are formally UB,
the evolution of C++ proposals for trivial relocation "diverged" from
the status quo in at least two aspects:</p>
   <ol>
    <li data-md>
     <p>the semantic split of "trivial relocation" and "replacement". Most
libraries (see <a data-link-type="biblio" href="#biblio-p3559r0" title="Trivial relocation: One trait or two?">[P3559R0]</a>) combine both semantics under one trait.
As highlighted by <a data-link-type="biblio" href="#biblio-p3233r0" title="Issues with P2786 (Trivial Relocatability For C++26)">[P3233R0]</a>, this semantic combination is
necessary to use trivial relocation to optimize not just vector
reallocation, but also vector erase, insertion, swaps in algorithms,
and so on (see also <a data-link-type="biblio" href="#biblio-p3278r0" title="Analysis of interaction between relocation, assignment, and swap">[P3278R0]</a>).
It is fair to argue that the two type properties are, indeed,
independent properties, and therefore should be modeled separately.
This design got incorporated in <a data-link-type="biblio" href="#biblio-p2786r7" title="Trivial Relocatability For C++26">[P2786R7]</a> revision 7.</p>
    <li data-md>
     <p>The fact that trivial relocation may not be equivalent to a byte
copy. This is a much recent development, driven by the introduction
of pointer authentication on new platforms, while preserving the
ability of polymorphic types to be trivially relocatable.</p>
   </ol>
   <p>One may argue that C++26’s trivial relocation is not "truly" trivial,
because in general it is not equivalent to a byte copy. On the other
hand, one may counter-argue that it’s trivial in the sense that no
user-defined code is executed. We don’t think this particular debate is
productive.</p>
   <p><strong>What however is really important is the upgrade story for all these
libraries</strong>: how can they switch from their own custom traits and
algorithms towards standardized solutions?</p>
   <p>Here we see a potential conflict: many libraries today define "trivial
relocatable" as a combination of <em>bitwise trivially relocatable</em> and
<em>replaceable</em>. C++26’s trivially relocatable type traits do not give
exactly these semantics, and this may impede adoption of the new
facilities; or, worse, it may introduce regressions in code that was
<em>expecting</em> bitwise trivial relocatability, but gets ported to just
check for trivial relocatability.</p>
   <p>We can also foresee that some libraries (e.g. Qt) will continue to use
"UB that works in practice" because of deficiencies of C++26’s APIs.
For instance, it is reasonable to expect that Qt will keep using
<code class="highlight"><c- n>realloc</c-></code> to e.g. grow a buffer containing bitwise trivially
relocatable objects, as that provides a huge benefit over allocating a
new buffer, trivially relocating in there, and deallocating the old
buffer. However, C++26 is lacking a trivially relocation-aware version
of <code class="highlight"><c- n>realloc</c-></code>, nor it granted <code class="highlight"><c- n>realloc</c-></code> (or similar APIs) the ability to
automatically bitwise trivially relocate objects in the memory being
reallocated.</p>
   <p>In conclusion, adding a type trait that closely models what these
libraries have been assuming in the past (20+ years, in the case of Qt)
will likely provide a better upgrade path for them.</p>
   <h4 class="heading settled" data-level="2.2.1" id="polls"><span class="secno">2.2.1. </span><span class="content">Polls during the <a data-link-type="biblio" href="#biblio-p1144r13" title="std::is_trivially_relocatable">[P1144R13]</a> review in Sofia</span><a class="self-link" href="#polls"></a></h4>
   <p>During the LEWG review in Sofia of <a data-link-type="biblio" href="#biblio-p1144r13" title="std::is_trivially_relocatable">[P1144R13]</a> a good part of the
discussion focused on the fact that C++26’s trivial relocatability does
not necessarily imply bitwise trivial relocability.</p>
   <p>Some raised the idea of a separate trait; this poll was taken:</p>
   <blockquote>
    
<strong>POLL:</strong> We encourage more exploration on a trait and/or a function
equivalent to "bitwise_trivially_relocate" that behaves in a similar
way to what’s described in "3.13 [obj.lifetime]" section on P1144R13
(enable library utils to realloc/memcpy).


<pre class="highlight"><c- n>SF</c-> <c- o>|</c-> <c- n>F</c-> <c- o>|</c-> <c- n>N</c-> <c- o>|</c-> <c- n>A</c-> <c- o>|</c-> <c- n>SA</c->
<c- mi>6</c->  <c- o>|</c-> <c- mi>7</c-> <c- o>|</c-> <c- mi>4</c-> <c- o>|</c-> <c- mi>6</c-> <c- o>|</c-> <c- mi>2</c->
</pre>
    <p>Outcome: Feeling of the room poll. Some want more exploration (very weakly)</p>
   </blockquote>
   <p>While this paper proposes such a trait, please do note that we are
<em>not</em> proposing to allow <code class="highlight"><c- n>memcpy</c-></code>, <code class="highlight"><c- n>realloc</c-></code> etc. to perform trivial
relocation.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design decisions</span><a class="self-link" href="#design"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="whyanewtrait"><span class="secno">3.1. </span><span class="content">Do we need a new trait? Can users implement this feature?</span><a class="self-link" href="#whyanewtrait"></a></h3>
   <p>In principle, users can directly detect whether they’re on a platform
where trivial relocation may not result in a byte copy, for instance by
checking platform-specific macros. At the time of this writing, to the
best of our knowledge, <code class="highlight"><c- n>arm64e</c-></code> is the only ABI that features such a
pointer authentication scheme; users could therefore check if
<code class="highlight"><c- n>__arm64e__</c-></code> is defined.</p>
   <p>An in-language portable solution is also possible. As discussed in
<a data-link-type="biblio" href="#biblio-p2786r13" title="Trivial Relocatability For C++26">[P2786R13]</a> in chapter §15.1, <code class="highlight"><c- k>union</c-></code>s containing polymorphic types
may or may not be trivially relocatable.</p>
   <p>The problem with unions and polymorphic types is exactly the same as
the one illustrated in <a href="#usecase">§ 2.1 The need of detecting bitwise trivial relocations</a>. Given an union like this one:</p>
<pre class="highlight"><c- k>union</c-> <c- nc>U</c->
<c- p>{</c->
    <c- b>int</c-> <c- n>m_int</c-><c- p>;</c->
    <c- n>Polymorphic</c-> <c- n>m_poly</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>the implementation cannot know which member is active. Therefore, when
trivially relocating an object of type <code class="highlight"><c- n>U</c-></code>, should the implementation
just copy the byte representation (if the <code class="highlight"><c- n>m_int</c-></code> member is active) or
copy the byte representation <strong>and also</strong> re-sign the virtual table
pointer (if the <code class="highlight"><c- n>m_poly</c-></code> member is active)?</p>
   <p>Note that "guessing wrong" breaks the program, so that is not a
possibility. On the other hand, if the target platform does not have
pointer authentication, there there is no issue with trivially
relocating the union, as we can just do a byte copy.</p>
   <p>As already mentioned, this issue got enshrined in <a href="https://wg21.link/class.prop#3">[class.prop/3]</a>: it is
implementation-defined whether the <code class="highlight"><c- n>U</c-></code> <code class="highlight"><c- k>union</c-></code> is trivially relocatable
or not.</p>
   <p>We can use this knowledge to build a detector:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>union</c-> <c- nc>Detector</c->
<c- p>{</c->
    <c- b>int</c-> <c- n>dummy</c-><c- p>;</c->
    <c- n>T</c-> <c- n>t</c-><c- p>;</c->
<c- p>};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_bitwise_trivially_relocatable_v</c->
    <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>Detector</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>;</c->
</pre>
   <p>In conclusion, building the proposed facilities is indeed theoretically
possible in user code.</p>
   <p>In practice, <strong>we reject the idea that users should reinvent this
"gadget" in any library that provides type-erasure facilities.</strong> That
is to say, <strong>we firmly believe that such detection has to be built in
the Standard Library.</strong> Relying on the fact that a polymorphic type is
bitwise trivially relocatable if and only if a union containing a
member of that type is trivially relocatable requires some extremely
specialized knowledge.</p>
   <h3 class="heading settled" data-level="3.2" id="whynotbool"><span class="secno">3.2. </span><span class="content">Why a new type trait, rather than a non-template boolean variable / a macro / etc.?</span><a class="self-link" href="#whynotbool"></a></h3>
   <p>An alternative design could be to simply have a predefined macro or a
<em>non-template</em> <code class="highlight"><c- k>constexpr</c-></code> <code class="highlight"><c- b>bool</c-></code> variable that reflects the
platform-specific property of "is relocation bitwise for polymorphic
types".</p>
   <p>Although they would achieve the same goal, we believe that a new type
trait is much more expressive than combining checks.</p>
   <p>For instance, in order to check whether a type <code class="highlight"><c- n>T</c-></code> can be stored into
<code class="highlight"><c- n>QVariant</c-></code>’s small buffer implementation (see <a href="#QVariant">§ 2.1.1 Example: QVariant</a>), compare
the proposed new trait:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- n>CanBeStoredInQVariantSmallBuffer</c-> <c- o>=</c->
  <c- n>std</c-><c- o>::</c-><c- n>is_bitwise_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;&amp;</c->
  <c- d>/* check alignment, size, ... */</c-><c- p>;</c->
</pre>
   <p>with the alternative:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- n>CanBeStoredInQVariantSmallBuffer</c-> <c- o>=</c->
  <c- n>std</c-><c- o>::</c-><c- n>is_trivially_relocatable_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;&amp;</c->
  <c- p>(</c-><c- o>!</c-><c- n>std</c-><c- o>::</c-><c- n>is_polymorphic_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>||</c-> <c- n>std</c-><c- o>::</c-><c- n>trivial_relocation_is_bitwise</c-><c- p>)</c-> <c- o>&amp;&amp;</c->
  <c- d>/* check alignment, size, ... */</c-><c- p>;</c->
</pre>
   <p>The proposed trait makes it much more clear what is the requirement on
<code class="highlight"><c- n>T</c-></code> that we are looking for, without the need to deploying a material
implication that we are in one of the problematic cases (polymorphic
types). The second form could use the implication operator proposed by
<a data-link-type="biblio" href="#biblio-p2971r3" title="Implication for C++">[P2971R3]</a> for some more clarity, but we still strongly prefer the
first form.</p>
   <h2 class="heading settled" data-level="4" id="impact"><span class="secno">4. </span><span class="content">Impact on the Standard</span><a class="self-link" href="#impact"></a></h2>
   <p>This proposal is a pure library addition; it adds a new trait (class
template and associated variable template) to the <code class="highlight"><c- o>&lt;</c-><c- n>type_traits</c-><c- o>></c-></code>
header.</p>
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">5. </span><span class="content">Proposed Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>All the proposed changes are relative to <a data-link-type="biblio" href="#biblio-n5008" title="Working Draft, Programming Languages — C++">[N5008]</a>.</p>
   <h3 class="heading settled" data-level="5.1" id="feature-testing"><span class="secno">5.1. </span><span class="content">Feature-testing macro</span><a class="self-link" href="#feature-testing"></a></h3>
   <p>In <a href="https://wg21.link/version.syn">[version.syn]</a>, bump the
value of the <code class="highlight"><c- n>__cpp_lib_trivially_relocatable</c-></code> macro to match the date
of adoption of the present proposal:</p>
   <blockquote>
<pre class="highlight"><c- cp>#define __cpp_lib_trivially_relocatable             </c-><del><c- cp>202502L</c-></del><ins><c- cp>??????L</c-></ins>
  <c- c1>// freestanding, also in &lt;memory>, &lt;type_traits></c->
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="5.2" id="delta"><span class="secno">5.2. </span><span class="content">Wording</span><a class="self-link" href="#delta"></a></h3>
   <p>In <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a>, amend the <code class="highlight"><c- o>&lt;</c-><c- n>type_traits</c-><c- o>></c-></code> synopsis as shown:</p>
   <blockquote>
<pre class="highlight"><c- c1>// [meta.unary.prop], type properties</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_const</c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_volatile</c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_trivially_copyable</c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_trivially_relocatable</c-><c- p>;</c->
<ins><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_bitwise_trivially_relocatable</c-><c- p>;</c-></ins>
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_replaceable</c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_standard_layout</c-><c- p>;</c->
</pre>
   </blockquote>
   <p>Add the relative variable template definition:</p>
   <blockquote>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_trivially_relocatable_v</c-> <c- o>=</c-> <c- n>is_trivially_relocatable</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c->
<ins><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_bitwise_trivially_relocatable_v</c-> <c- o>=</c->
    <c- n>is_bitwise_trivially_relocatable</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c-></ins>
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_standard_layout_v</c-> <c- o>=</c-> <c- n>is_standard_layout</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c->
</pre>
   </blockquote>
   <p>Add a new row to Table 54 ("Type property predicates") in <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a>, just
after the row for <code class="highlight"><c- n>is_trivially_relocatable</c-></code>:</p>
   <blockquote>
    <table>
     <thead>
     <tbody>
      <tr>
       <th>Template
       <th>Condition
       <th>Preconditions
      <tr>
       <td>
        <ins><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_bitwise_trivially_relocatable</c-><c- p>;</c-></code></ins>
       <td>
        <ins><code class="highlight"><c- n>is_trivially_relocatable</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> is <code class="highlight">true</code>, and trivially relocating
objects of type <code class="highlight"><c- n>T</c-></code> by means of calling <code class="highlight"><c- n>trivially_relocate</c-></code> ([obj.lifetime])
does not change the object representation of the objects being relocated.</ins>
        
<br><br>

        <ins>[<em>Note �</em>: Depending on the implementation, objects of types that are polymorphic and trivially relocatable may change object representation after being relocated by a call to <code class="highlight"><c- n>trivially_relocate</c-></code>. — <em>end note</em>]</ins>
       <td>
        <ins><code class="highlight"><c- n>remove_all_extents_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> shall be a complete type or <em>cv</em> <code class="highlight"><c- b>void</c-></code>.</ins>
    </table>
   </blockquote>
   <h2 class="heading settled" data-level="6" id="acknowledgements"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to KDAB for supporting this work.</p>
   <p>All remaining errors are ours and ours only.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-lwg4283">[LWG4283]
   <dd>Giuseppe D'Angelo. <a href="https://cplusplus.github.io/LWG/issue4283"><cite>std::trivially_relocate needs stronger preconditions on "nested" objects with dynamic lifetime</cite></a>. URL: <a href="https://cplusplus.github.io/LWG/issue4283">https://cplusplus.github.io/LWG/issue4283</a>
   <dt id="biblio-n5008">[N5008]
   <dd>Thomas Köppe. <a href="https://wg21.link/n5008"><cite>Working Draft, Programming Languages — C++</cite></a>. 15 March 2025. URL: <a href="https://wg21.link/n5008">https://wg21.link/n5008</a>
   <dt id="biblio-nonconstexproptionalblog">[NonConstexprOptionalBlog]
   <dd>Arthur O'Dwyer. <a href="https://quuxplusone.github.io/blog/2025/05/09/siren-song-3-optional/"><cite>Non-constexpr Optional and trivial relocation</cite></a>. URL: <a href="https://quuxplusone.github.io/blog/2025/05/09/siren-song-3-optional/">https://quuxplusone.github.io/blog/2025/05/09/siren-song-3-optional/</a>
   <dt id="biblio-p1144r11">[P1144R11]
   <dd>Arthur O'Dwyer. <a href="https://wg21.link/p1144r11"><cite>std::is_trivially_relocatable</cite></a>. 15 May 2024. URL: <a href="https://wg21.link/p1144r11">https://wg21.link/p1144r11</a>
   <dt id="biblio-p1144r13">[P1144R13]
   <dd>Arthur O'Dwyer, Artur Bać, Daniel Liam Anderson, Enrico Mauro, Jody Hagins, Michael Steffens, Stéphane Janel, Vinnie Falco, Walter E. Brown, Will Wray. <a href="https://wg21.link/p1144r13"><cite>std::is_trivially_relocatable</cite></a>. 15 May 2025. URL: <a href="https://wg21.link/p1144r13">https://wg21.link/p1144r13</a>
   <dt id="biblio-p2786r13">[P2786R13]
   <dd>Pablo Halpern, Joshua Berne, Corentin Jabot, Pablo Halpern, Lori Hughes. <a href="https://wg21.link/p2786r13"><cite>Trivial Relocatability For C++26</cite></a>. 14 February 2025. URL: <a href="https://wg21.link/p2786r13">https://wg21.link/p2786r13</a>
   <dt id="biblio-p2786r7">[P2786R7]
   <dd>Mungo Gill, Alisdair Meredith, Joshua Berne. <a href="https://wg21.link/p2786r7"><cite>Trivial Relocatability For C++26</cite></a>. 17 September 2024. URL: <a href="https://wg21.link/p2786r7">https://wg21.link/p2786r7</a>
   <dt id="biblio-p2971r3">[P2971R3]
   <dd>Walter E Brown. <a href="https://wg21.link/p2971r3"><cite>Implication for C++</cite></a>. 13 January 2025. URL: <a href="https://wg21.link/p2971r3">https://wg21.link/p2971r3</a>
   <dt id="biblio-p3233r0">[P3233R0]
   <dd>Giuseppe D'Angelo. <a href="https://wg21.link/p3233r0"><cite>Issues with P2786 (Trivial Relocatability For C++26)</cite></a>. 16 April 2024. URL: <a href="https://wg21.link/p3233r0">https://wg21.link/p3233r0</a>
   <dt id="biblio-p3278r0">[P3278R0]
   <dd>Nina Ranns. <a href="https://wg21.link/p3278r0"><cite>Analysis of interaction between relocation, assignment, and swap</cite></a>. 22 May 2024. URL: <a href="https://wg21.link/p3278r0">https://wg21.link/p3278r0</a>
   <dt id="biblio-p3559r0">[P3559R0]
   <dd>Arthur O'Dwyer. <a href="https://wg21.link/p3559r0"><cite>Trivial relocation: One trait or two?</cite></a>. 8 January 2025. URL: <a href="https://wg21.link/p3559r0">https://wg21.link/p3559r0</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> add a link to the whitepaper on pointer auth once it’s available. <a class="issue-return" href="#issue-c8fe2963" title="Jump to section">↵</a></div>
  </div>