<!doctype html><html lang="en">
 <head>
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  <title>P2985R0: A type trait for detecting virtual base classes</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 82ce88815, updated Thu Sep 7 16:33:55 2023 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* Boilerplate: style-autolinks */
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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">P2985R0<br>A type trait for detecting virtual base classes</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="2023-09-28">2023-09-28</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <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>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 if a class is a virtual base of another class.

    This trait can be used in smart pointer classes, in order to constrain or optimize
    certain operations.</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="#priorart"><span class="secno">2.1</span> <span class="content">Prior art</span></a>
     </ol>
    <li>
     <a href="#design"><span class="secno">3</span> <span class="content">Design Decisions</span></a>
     <ol class="toc">
      <li><a href="#usercode"><span class="secno">3.1</span> <span class="content">Do we need this trait in the Standard Library? Can it be implemented entirely in user code?</span></a>
      <li><a href="#im"><span class="secno">3.2</span> <span class="content">What about classes that inherit virtually and non-virtually from the same base?</span></a>
      <li><a href="#selfvirtual"><span class="secno">3.3</span> <span class="content">Is a class a virtual base of itself?</span></a>
     </ol>
    <li><a href="#impact"><span class="secno">4</span> <span class="content">Impact on the Standard</span></a>
    <li><a href="#technicalspecifications"><span class="secno">5</span> <span class="content">Technical Specifications</span></a>
    <li><a href="#proposedwording"><span class="secno">6</span> <span class="content">Proposed wording</span></a>
    <li><a href="#acknowledgements"><span class="secno">7</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>
   </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>We propose the addition of the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_virtual_base_of</c-></code> type trait to the Standard Library, that complements the existing <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_base_of</c-></code> trait.
The trait aims to detect whether a given class is a virtual base class of another one, ignoring accessibility, ambiguities and cv-qualifiers, just like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_base_of</c-></code>.</p>
   <hr>
   <p>Detecting whether a class is a virtual or non-virtual base of another class has applications in the context of pointer conversions. Given a pointer <code class="highlight"><c- n>Derived</c-> <c- o>*</c-><c- n>d</c-></code> convertible to <code class="highlight"><c- n>Base</c-> <c- o>*</c-></code> (that is, <code class="highlight"><c- n>Base</c-></code> is an accessible, non-ambiguous base of <code class="highlight"><c- n>Derived</c-></code>), actually performing the conversion requires completely different implementations depending on whether <code class="highlight"><c- n>Base</c-></code> is a non-virtual or a virtual base of <code class="highlight"><c- n>Derived</c-></code>.</p>
   <p>On all common ABIs, if <code class="highlight"><c- n>Base</c-></code> is a non-virtual base of <code class="highlight"><c- n>Derived</c-></code>, then the conversion consists in a test for nullptr. Otherwise, a constant offset is applied to the pointer; the offset is entirely known at compile time. The test for nullptr is necessary to yield nullptr back in that case; on common ABIs, null pointers to objects are represented by a <code class="highlight"><c- mi>0</c-></code> value, and we have to return <code class="highlight"><c- mi>0</c-></code> (= null pointer) and not <code class="highlight"><c- mi>0</c-> <c- o>+</c-> <c- n>offset</c-></code> to honor [conv.ptr]/3 (null pointers convert to null pointers).</p>
   <p>If instead <code class="highlight"><c- n>Base</c-></code> is a virtual base of <code class="highlight"><c- n>Derived</c-></code>, then we perform the test for nullptr as usual. In case we don’t have a null pointer, we then have to determine the address of the <code class="highlight"><c- n>Base</c-></code> subobject inside the <code class="highlight"><c- n>Derived</c-></code> object. The actual mechanics depend on the ABI, but this always requires inspecting <code class="highlight"><c- n>d</c-></code>’s virtual table, which means examining <code class="highlight"><c- o>*</c-><c- n>d</c-></code> (the <em>pointee</em>). If the pointee has already been destroyed and the pointer is dangling (an "invalid pointer value", as per [basic.stc.general]/4’s definition), then this will cause problems.</p>
   <p>Technically speaking, the result of the cast is implementation-defined; [basic.stc.general]/4 says that "Any other use of an invalid pointer value has implementation-defined behavior". (In particular, we don’t think that the "Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undefined behavior." sentence applies here, as the indirection is necessary due to implementation/ABI requirements, and not language requirements.)</p>
   <p>Although no implementation seems to document their behavior, on all common ABIs, the conversion of an invalid pointer value towards a virtual base causes an illegal memory access (which may segfault, or just read "garbage", etc.); while a conversion towards a non-virtual base class does not cause any issues (see the note below).</p>
   <hr>
   <p>This behavior with pointer conversions is taken into account, for instance, in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>weak_ptr</c-></code>’s converting constructor. On all major implementations (libstdc++, libc++, MS-STL), <code class="highlight"><c- n>weak_ptr</c-></code> is implemented with this class layout:</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>weak_ptr</c->
<c- p>{</c->
    <c- n>control_block</c-> <c- o>*</c-><c- n>m_cb</c-><c- p>;</c->
    <c- n>T</c-> <c- o>*</c-><c- n>m_data</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>Let’s now consider the move-converting constructor from a <code class="highlight"><c- n>weak_ptr</c-><c- o>&lt;</c-><c- n>Y</c-><c- o>></c-></code> to <code class="highlight"><c- n>weak_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>. Simplifying, the constructor is constrained on <code class="highlight"><c- n>Y</c-><c- o>*</c-></code> being convertible to <code class="highlight"><c- n>T</c-><c- o>*</c-></code> (cf. [util.smartptr.weak.const]/6 and [util.smartptr.shared.general]/6).</p>
   <p>The "obvious" implementation:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>Y</c-><c- o>></c->
  <c- k>requires</c-> <c- n>std</c-><c- o>::</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>Y</c-> <c- o>*</c-><c- p>,</c-> <c- n>T</c-> <c- o>*></c->
    <c- n>weak_ptr</c-><c- p>(</c-><c- n>weak_ptr</c-><c- o>&lt;</c-><c- n>Y</c-><c- o>></c-> <c- o>&amp;&amp;</c-><c- n>other</c-><c- p>)</c->
    <c- o>:</c-> <c- n>m_cb</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>exchange</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>m_cb</c-><c- p>,</c-> <c- k>nullptr</c-><c- p>)),</c->
      <c- n>m_data</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>exchange</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>m_data</c-><c- p>,</c-> <c- k>nullptr</c-><c- p>))</c-> <c- c1>// &lt;-- danger</c->
      <c- p>{}</c->
</pre>
   <p>is wrong and will produce UB / crashes when converting <code class="highlight"><c- n>other</c-><c- p>.</c-><c- n>m_data</c-></code> to <code class="highlight"><c- n>m_data</c-></code>, in case 1) <code class="highlight"><c- n>T</c-></code> is a virtual base of <code class="highlight"><c- n>Y</c-></code> and 2) the managed object has already been destroyed. This is a concrete possibility -- the <em>raison d’être</em> for <code class="highlight"><c- n>weak_ptr</c-></code> is that it does <strong>not</strong> keep the managed object alive.</p>
   <p>A correct implementation therefore needs to check that the managed object is still alive:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>Y</c-><c- o>></c-> 
  <c- k>requires</c-> <c- n>std</c-><c- o>::</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>Y</c-> <c- o>*</c-><c- p>,</c-> <c- n>T</c-> <c- o>*></c->
    <c- n>weak_ptr</c-><c- p>(</c-><c- n>weak_ptr</c-><c- o>&lt;</c-><c- n>Y</c-><c- o>></c-> <c- o>&amp;&amp;</c-><c- n>other</c-><c- p>)</c->
    <c- o>:</c-> <c- n>m_cb</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>m_cb</c-><c- p>),</c->
      <c- n>m_data</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>lock</c-><c- p>().</c-><c- n>get</c-><c- p>())</c->
    <c- p>{</c->
       <c- n>other</c-><c- p>.</c-><c- n>m_cb</c-> <c- o>=</c-> <c- k>nullptr</c-><c- p>;</c-> <c- n>other</c-><c- p>.</c-><c- n>m_data</c-> <c- o>=</c-> <c- k>nullptr</c-><c- p>;</c->
    <c- p>}</c->
</pre>
   <p>The above snippet works, but now this is a pessimization for the "common" case when <code class="highlight"><c- n>T</c-></code> is a non-virtual base of <code class="highlight"><c- n>Y</c-></code>. In that case we could avoid the (relatively more expensive) <code class="highlight"><c- n>lock</c-><c- p>()</c-></code> operation and just safely offset the pointer.</p>
   <p>If an implementation wishes to optimize for this case, then they would need to detect if <code class="highlight"><c- n>T</c-></code> is a virtual base class of <code class="highlight"><c- n>Y</c-></code>, which is where they would use the type trait that we’re proposing:</p>
<pre class="highlight"><c- c1>// simple implementation for non-virtual base</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>Y</c-><c- o>></c-> 
  <c- k>requires</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>Y</c-> <c- o>*</c-><c- p>,</c-> <c- n>T</c-> <c- o>*></c-> <c- o>&amp;&amp;</c-> <c- o>!</c-><c- n>std</c-><c- o>::</c-><c- n>is_virtual_base_of_v</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Y</c-><c- o>></c-><c- p>)</c->
    <c- n>weak_ptr</c-><c- p>(</c-><c- n>weak_ptr</c-><c- o>&lt;</c-><c- n>Y</c-><c- o>></c-> <c- o>&amp;&amp;</c-><c- n>other</c-><c- p>)</c->
    <c- o>:</c-> <c- n>m_cb</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>exchange</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>m_cb</c-><c- p>,</c-> <c- k>nullptr</c-><c- p>)),</c->
      <c- n>m_data</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>exchange</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>m_data</c-><c- p>,</c-> <c- k>nullptr</c-><c- p>))</c-> 
      <c- p>{}</c->
    
<c- c1>// correct, more expensive implementation for virtual bases</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>Y</c-><c- o>></c-> 
  <c- k>requires</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>Y</c-> <c- o>*</c-><c- p>,</c-> <c- n>T</c-> <c- o>*></c-> <c- o>&amp;&amp;</c-> <c- n>std</c-><c- o>::</c-><c- n>is_virtual_base_of_v</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Y</c-><c- o>></c-><c- p>)</c->
    <c- n>weak_ptr</c-><c- p>(</c-><c- n>weak_ptr</c-><c- o>&lt;</c-><c- n>Y</c-><c- o>></c-> <c- o>&amp;&amp;</c-><c- n>other</c-><c- p>)</c->
    <c- o>:</c-> <c- n>m_cb</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>m_cb</c-><c- p>),</c->
      <c- n>m_data</c-><c- p>(</c-><c- n>other</c-><c- p>.</c-><c- n>lock</c-><c- p>().</c-><c- n>get</c-><c- p>())</c->
    <c- p>{</c->
       <c- n>other</c-><c- p>.</c-><c- n>m_cb</c-> <c- o>=</c-> <c- k>nullptr</c-><c- p>;</c-> <c- n>other</c-><c- p>.</c-><c- n>m_data</c-> <c- o>=</c-> <c- k>nullptr</c-><c- p>;</c->
    <c- p>}</c->
</pre>
   <p class="note" role="note"><span class="marker">Note:</span> On all common Standard Library implementations there is no concern for accessing <code class="highlight"><c- n>m_data</c-></code> even in case it may contain an invalid pointer value ([basic.stc.general]/4). For instance, it’s simply copied as a pointer (and not <code class="highlight"><c- n>memcpy</c-></code>’d or similar) in their implementation of <code class="highlight"><c- n>weak_ptr</c-></code>’s move constructor. This confirms our understanding that no major compiler/ABI has any "special" behavior with regards to invalid pointer values, although they don’t document their implementation-specific behavior in the area.</p>
   <hr>
   <p>A similar consideration may be applied to <code class="highlight"><c- n>observer_ptr</c-></code>’s converting constructor, which could be marked as <code class="highlight"><c- k>explicit</c-></code> if a conversion towards a virtual base class is requested (since such a conversion may be "unsafe"). We however do not have sufficient field experience with such a change to reach any conclusions.</p>
   <h3 class="heading settled" data-level="2.1" id="priorart"><span class="secno">2.1. </span><span class="content">Prior art</span><a class="self-link" href="#priorart"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-boosttypetraits" title="Boost.TypeTraits version 1.83: is_virtual_base_of">[Boost.TypeTraits]</a> has shipped a <code class="highlight"><c- n>boost</c-><c- o>::</c-><c- n>is_virtual_base_of</c-></code> type trait since at least Boost version 1.40 (2009). A similar implementation has been added to Qt as private API. The trait that we are proposing will however have slightly different semantics.</p>
   <p>A trait with the same name and semantics has also been proposed by <a data-link-type="biblio" href="#biblio-n3987" title="Yet another set of C++ type traits.">[N3987]</a>, although in a different context (reflection), and together with many more traits. We are unsure of the status of that paper, which looks more like a thought experiment than an actual proposal.</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="usercode"><span class="secno">3.1. </span><span class="content">Do we need this trait in the Standard Library? Can it be implemented entirely in user code?</span><a class="self-link" href="#usercode"></a></h3>
   <p>This trait has been indeed implemented in user code, but the detection achieved this way is <em>incomplete</em> (see below). The implementation is definitely "experts-only", and therefore would benefit from being provided by the Standard Library.</p>
   <p>The core of the detection in Boost and Qt combines these two checks:</p>
   <ol>
    <li data-md>
     <p>whether <code class="highlight"><c- p>(</c-><c- n>Base</c-> <c- o>*</c-><c- p>)</c-><c- n>std</c-><c- o>::</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>Derived</c-> <c- o>*></c-><c- p>()</c-></code> is well formed. If it is, then <code class="highlight"><c- n>Base</c-></code> is an unambiguous base of <code class="highlight"><c- n>Derived</c-></code>. This will be true if <code class="highlight"><c- n>Base</c-></code> is a virtual base class of <code class="highlight"><c- n>Derived</c-></code> (and <code class="highlight"><c- n>Derived</c-></code> doesn’t also inherit from <code class="highlight"><c- n>Base</c-></code> non-virtually). The cast notation is used in order to ignore accessibility ([expr.cast]/4).</p>
    <li data-md>
     <p>whether <code class="highlight"><c- p>(</c-><c- n>Derived</c-> <c- o>*</c-><c- p>)</c-><c- n>std</c-><c- o>::</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>Base</c-> <c- o>*></c-><c- p>()</c-></code> is <strong>ill</strong> formed. This is the case if <code class="highlight"><c- n>Base</c-></code> is an ambiguous or virtual base of <code class="highlight"><c- n>Derived</c-></code> (again ignoring accessibility).</p>
   </ol>
   <p>With the first check we establish that <code class="highlight"><c- n>Base</c-></code> is not an ambiguous base, and combined with the second check, we therefore establish that it must be a virtual base.</p>
   <h3 class="heading settled" data-level="3.2" id="im"><span class="secno">3.2. </span><span class="content">What about classes that inherit virtually and non-virtually from the same base?</span><a class="self-link" href="#im"></a></h3>
   <p>The detection in usercode has a shortcoming: if <code class="highlight"><c- n>Derived</c-></code> inherits virtually <em>and</em> non-virtually from <code class="highlight"><c- n>Base</c-></code>, then 1. fails, but <code class="highlight"><c- n>Base</c-></code> still is "a" virtual base class. Such a situation is perfectly legitimate and allowed by the core language.</p>
   <p>Consider for instance the example in Note 6 in [class.mi]:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>B</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>};</c->
<c- k>class</c-> <c- nc>X</c-> <c- o>:</c-> <c- k>virtual</c-> <c- k>public</c-> <c- n>B</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>};</c->
<c- k>class</c-> <c- nc>Y</c-> <c- o>:</c-> <c- k>virtual</c-> <c- k>public</c-> <c- n>B</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>};</c->
<c- k>class</c-> <c- nc>Z</c-> <c- o>:</c-> <c- k>public</c-> <c- n>B</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>};</c->
<c- k>class</c-> <c- nc>AA</c-> <c- o>:</c-> <c- k>public</c-> <c- n>X</c-><c- p>,</c-> <c- k>public</c-> <c- n>Y</c-><c- p>,</c-> <c- k>public</c-> <c- n>Z</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>};</c->
</pre>
   <p>In this example <code class="highlight"><c- n>AA</c-></code> inherits from <code class="highlight"><c- n>B</c-></code> twice: once virtually (through <code class="highlight"><c- n>X</c-></code> and <code class="highlight"><c- n>Y</c-></code>) and once non-virtually (through <code class="highlight"><c- n>Z</c-></code>). Therefore, what should <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_virtual_base_of_v</c-><c- o>&lt;</c-><c- n>B</c-><c- p>,</c-> <c- n>A</c-><c- o>></c-></code> yield? There are two possible lines of reasoning:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>B</c-></code> is <strong>a</strong> virtual base class (considering all possible inheritance paths); therefore the trait should yield <code class="highlight">true</code>;</p>
    <li data-md>
     <p><code class="highlight"><c- n>B</c-></code> is not a virtual base class in at least one inheritance path; therefore the trait should yield <code class="highlight">false</code>.</p>
   </ol>
   <p>(In other words: with 1. we would be asking if <code class="highlight"><c- n>B</c-></code> is a virtual base class, and with 2. we would be asking if <code class="highlight"><c- n>B</c-></code> is a virtual base class and also not a non-virtual base class.)</p>
   <p>We believe that the following considerations apply here:</p>
   <ul>
    <li data-md>
     <p>Each choice has some merits. 1. is what the core language says; however 2. is what Boost.TypeTraits and similar code actually detect (and have done so for a very long time). We are not aware of any implementation in user code that is able to do detection no. 1, nor viable implementation strategies.</p>
    <li data-md>
     <p>Whether the trait does 1. or 2. does not ultimately affect the main use case of the trait (pointer conversions). In the example, <code class="highlight"><c- n>AA</c-><c- o>*</c-></code> is simply not convertible to <code class="highlight"><c- n>B</c-><c- o>*</c-></code> because the conversion is ambiguous, so for those use cases it won’t matter at all what the trait detects.</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_base_of</c-></code> already somehow deviates from the base class definition in core language, as the core language doesn’t allow a class to be a base of itself ([class.derived.general]/2, "A <em>class-or-decltype</em> shall denote a (possibly cv-qualified) class type that is not an incompletely defined class").</p>
   </ul>
   <p>In this proposal we feel more comfortable at embracing the core language definition of virtual base class, and therefore we would propose choice no. 1 (that is, for the trait to detect that <code class="highlight"><c- n>B</c-></code> <em>is</em> a virtual base class of <code class="highlight"><c- n>AA</c-></code>), despite the prior art in Boost and Qt.</p>
   <p>We also don’t believe it is going to be necessary to offer a trait that does detection no. 2, for two reasons. First, having fine-grained detections sounds more like task for a reflection system rather than for type traits in the standard library. (For instance, there aren’t multiple "versions" of <code class="highlight"><c- n>is_base_of</c-></code> that check for accessibility, multiple inheritance from the same class, and so on. The trait does one thing and one thing only.) Second, as discussed above, we do not have any use case for it, and would not risk introducing two traits that will just cause confusion to end users.</p>
   <h3 class="heading settled" data-level="3.3" id="selfvirtual"><span class="secno">3.3. </span><span class="content">Is a class a virtual base of itself?</span><a class="self-link" href="#selfvirtual"></a></h3>
   <p>We are aware that, although no class is a base class of itself, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_base_of_v</c-><c- o>&lt;</c-><c- n>X</c-><c- p>,</c-> <c- n>X</c-><c- o>></c-></code> is actually <code class="highlight">true</code>. While one can consider <code class="highlight"><c- n>is_base_of</c-></code> to model a general inheritance/is-A relationship between types, the check for virtual inheritance is actually much more specific, and we strongly feel that the trait should yield what the core language describes here: no class is a virtual base class of itself.</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 extension; it adds a type trait to <code class="highlight"><c- o>&lt;</c-><c- n>type_traits</c-><c- o>></c-></code>. There are no changes required in the core language.</p>
   <p>Vendors are expected to implement the trait through internal compiler hooks, as they already do for other similar type traits (e.g. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_base_of</c-></code>), for performance, correctness and maintenance reasons.</p>
   <h2 class="heading settled" data-level="5" id="technicalspecifications"><span class="secno">5. </span><span class="content">Technical Specifications</span><a class="self-link" href="#technicalspecifications"></a></h2>
   <p>All the proposed changes are relative to <a data-link-type="biblio" href="#biblio-n4958" title="Working Draft, Programming Languages — C++">[N4958]</a>.</p>
   <h2 class="heading settled" data-level="6" id="proposedwording"><span class="secno">6. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposedwording"></a></h2>
   <p>Add to the list in [version.syn]:</p>
   <blockquote>
<pre class="highlight"><ins><c- cp>#define __cpp_lib_is_virtual_base_of YYYYMML </c-><c- c1>// also in &lt;type_traits></c->
</ins></pre>
   </blockquote>
   <p>Modify [meta.type.synop] as shown.</p>
   <p>Add to first [meta.rel] block:</p>
   <blockquote>
<pre class="highlight">  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Base</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Derived</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_base_of</c-><c- p>;</c->
<ins>  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Base</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Derived</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_virtual_base_of</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <p>And to the second block:</p>
   <blockquote>
<pre class="highlight">  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Base</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Derived</c-><c- o>></c->
    <c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_base_of_v</c->
      <c- o>=</c-> <c- n>is_base_of</c-><c- o>&lt;</c-><c- n>Base</c-><c- p>,</c-> <c- n>Derived</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>Base</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Derived</c-><c- o>></c->
    <c- k>constexpr</c-> <c- b>bool</c-> <c- n>is_virtual_base_of_v</c->
      <c- o>=</c-> <c- n>is_virtual_base_of</c-><c- o>&lt;</c-><c- n>Base</c-><c- p>,</c-> <c- n>Derived</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c-></ins>
</pre>
   </blockquote>
   <p>Insert a new row in Table 49 in [meta.rel], after the one for <code class="highlight"><c- n>is_base_of</c-></code>:</p>
   <blockquote>
    <table>
     <thead>
     <tbody>
      <tr>
       <td>
        <ins><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Base</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Derived</c-><c- o>></c-> <c- k>struct</c-> <c- nc>is_virtual_base_of</c-><c- p>;</c-></code></ins>
       <td>
        <ins><code class="highlight"><c- n>Base</c-></code> is a virtual base class of <code class="highlight"><c- n>Derived</c-></code> ([class.mi]) without regard to cv-qualifiers.</ins>
       <td>
        <ins>If <code class="highlight"><c- n>Base</c-></code> and <code class="highlight"><c- n>Derived</c-></code> are non-union class types, <code class="highlight"><c- n>Derived</c-></code> shall be a complete type. [<em>Note 2</em>: Virtual base classes that are private, protected or ambiguous are, nonetheless, virtual base classes. — <em>end note</em>] [<em>Note 3</em>: A class is never a virtual base class of itself. — <em>end note</em>]</ins>
    </table>
   </blockquote>
   <h2 class="heading settled" data-level="7" id="acknowledgements"><span class="secno">7. </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-boosttypetraits">[Boost.TypeTraits]
   <dd><a href="https://www.boost.org/doc/libs/1_83_0/libs/type_traits/doc/html/boost_typetraits/reference/is_virtual_base_of.html"><cite>Boost.TypeTraits version 1.83: is_virtual_base_of</cite></a>. URL: <a href="https://www.boost.org/doc/libs/1_83_0/libs/type_traits/doc/html/boost_typetraits/reference/is_virtual_base_of.html">https://www.boost.org/doc/libs/1_83_0/libs/type_traits/doc/html/boost_typetraits/reference/is_virtual_base_of.html</a>
   <dt id="biblio-n3987">[N3987]
   <dd>C. Silva, D. Auresco. <a href="https://wg21.link/n3987"><cite>Yet another set of C++ type traits.</cite></a>. 7 May 2014. URL: <a href="https://wg21.link/n3987">https://wg21.link/n3987</a>
   <dt id="biblio-n4958">[N4958]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4958"><cite>Working Draft, Programming Languages — C++</cite></a>. 14 August 2023. URL: <a href="https://wg21.link/n4958">https://wg21.link/n4958</a>
  </dl>