<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P2613R0: Add the missing empty to mdspan</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;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom: 3px solid #054572;
			border-bottom: 3px solid var(--toclink-underline);
			margin-bottom: -3px;
		}
		#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 fb1e763a4, updated Tue Mar 1 13:13:50 2022 -0800" name="generator">
  <link href="https://wg21.link/P2613R0" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="1e29943dde399639cdb470c02e0a743fea5b1299" name="document-revision">
<style>
table, th, td { border:1px solid grey; }
</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">P2613R0<br>Add the missing <code class="highlight"><c- n>empty</c-></code> to <code class="highlight"><c- n>mdspan</c-></code></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="2022-06-22">2022-06-22</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P2613R0">https://wg21.link/P2613R0</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:winmikedows@hotmail.com">Yihe Li</a>
     <dt>Audience:
     <dd>LEWG, LWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Target:
     <dd>C++23
     <dt>Source:
     <dd><a href="https://github.com/Mick235711/wg21-papers/blob/main/P2613R0.bs">github.com/Mick235711/wg21-papers/blob/main/P2613R0.bs</a>
     <dt>Issue Tracking:
     <dd><a href="https://github.com/Mick235711/wg21-papers/issues">GitHub Mick235711/wg21-papers</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </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="#revisions"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r0"><span class="secno">1.1</span> <span class="content">R0</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li><a href="#design"><span class="secno">3</span> <span class="content">Design</span></a>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li>
       <a href="#mdspan.mdspan"><span class="secno">4.1</span> <span class="content">24.7.� Class template <code class="highlight"><c- n>mdspan</c-></code> [mdspan.mdspan]</span></a>
       <ol class="toc">
        <li><a href="#mdspan.mdspan.overview"><span class="secno">4.1.1</span> <span class="content">24.7.�.1 Overview [mdspan.mdspan.overview]</span></a>
        <li><a href="#mdspan.mdspan.members"><span class="secno">4.1.2</span> <span class="content">24.7.�.3 Members [mdspan.mdspan.members]</span></a>
       </ol>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <p>This paper propose to fix a defect in <a data-link-type="biblio" href="#biblio-p0009r17">[P0009R17]</a>. During its LWG review, I found that even though the proposed <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>mdspan</c-></code> type have a <code class="highlight"><c- n>size</c-><c- p>()</c-></code> member function,
it does not have an <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> member function, which makes it distinct from nearly all other STL containers. So this paper propose to add the missing member to increase consistency and performance of common operations.</p>
   <h2 class="heading settled" data-level="1" id="revisions"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#revisions"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="r0"><span class="secno">1.1. </span><span class="content">R0</span><a class="self-link" href="#r0"></a></h3>
    Initial revision. 
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Consistency among library types is important and makes user interaction intuitive. Also, adding <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> does have performance optimization chance in common implementations.
If we survey the current STL containers, we can summarize as a table for their <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> and <code class="highlight"><c- n>size</c-><c- p>()</c-></code> behavior:</p>
   <table>
    <thead>
     <tr>
      <td>Having <code class="highlight"><c- n>size</c-><c- p>()</c-></code> without <code class="highlight"><c- n>empty</c-><c- p>()</c-></code>
      <td>Having both <code class="highlight"><c- n>size</c-><c- p>()</c-></code> and <code class="highlight"><c- n>empty</c-><c- p>()</c-></code>
      <td>Having <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> without <code class="highlight"><c- n>size</c-><c- p>()</c-></code>
    <tbody>
     <tr>
      <td><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bitset</c-></code><br><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-></code>
      <td><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-></code><br><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>list</c-></code><br><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>deque</c-></code><br><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code><br>...
      <td><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward_list</c-></code>
   </table>
   <p>We can see a clear trend in the STL to provide <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> with <code class="highlight"><c- n>size</c-><c- p>()</c-></code> concurrently whenever possible. The only containers in STL that has <code class="highlight"><c- n>size</c-><c- p>()</c-></code> but not <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> are some of the fixed-sized containers like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bitset</c-></code>, for which <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> does not makes sense. Conversely, even though <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward_list</c-></code> cannot provide <code class="highlight"><c- n>size</c-><c- p>()</c-></code> due to not storing this information, it still provides <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> for convenience. More importantly, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code> provide an <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> too, and since <code class="highlight"><c- n>mdspan</c-></code> is modeled as a higher-dimensional <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code>, it should have an <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> alongside its <code class="highlight"><c- n>size</c-><c- p>()</c-></code> too.</p>
   <p>Apart from the consistency motivation, the introduction of <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> can have real performance gains to users, since under the as-if wording it is just specified as "the same behaviour as <code class="highlight"><c- n>size</c-><c- p>()</c-> <c- o>==</c-> <c- mi>0</c-></code>", but
a good implementation can avoid calling <code class="highlight"><c- n>size</c-><c- p>()</c-></code> entirely. For example, since <code class="highlight"><c- n>size</c-><c- p>()</c-></code> is specified (through <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>extents</c-><c- o>::</c-><i><c- n>fwd</c-><c- o>-</c-><c- n>prod</c-><c- o>-</c-><c- n>of</c-><c- o>-</c-><c- n>extents</c-></i></code>) to perform a multiplication on all the extents, <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> can simply be implemented as checking whether all the extents are greater than zero, which can be substantially faster than multiplication. This possibility of better performance is also why we were told to rely on <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> instead of doing <code class="highlight"><c- n>size</c-><c- p>()</c-> <c- o>==</c-> <c- mi>0</c-></code>. Not including <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> will both create surprise for users, and also force them to resort to a worse approach to check emptiness.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design</span><a class="self-link" href="#design"></a></h2>
   <p>The design of the member function follows that of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code>. Which means that it is marked <code class="highlight"><c- k>const</c-></code>, <code class="highlight"><c- k>noexcept</c-></code>, and <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code>.</p>
   <p>In theory, since this is a pure addition, it can be done after C++23 ships. However, I would argue (and the <code class="highlight"><c- n>mdspan</c-></code> authors also agree) that not having both <code class="highlight"><c- n>empty</c-><c- p>()</c-></code> and <code class="highlight"><c- n>size</c-><c- p>()</c-></code> in <a data-link-type="biblio" href="#biblio-p0009r17">[P0009R17]</a> is simply a design mistake, and delay the addition will cause significant user confusion.</p>
   <h2 class="heading settled" data-level="4" id="wording"><span class="secno">4. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>The wording below is based on <a data-link-type="biblio" href="#biblio-p0009r17">[P0009R17]</a>.</p>
   <h3 class="heading settled" data-level="4.1" id="mdspan.mdspan"><span class="secno">4.1. </span><span class="content">24.7.� Class template <code class="highlight"><c- n>mdspan</c-></code> [mdspan.mdspan]</span><a class="self-link" href="#mdspan.mdspan"></a></h3>
   <h4 class="heading settled" data-level="4.1.1" id="mdspan.mdspan.overview"><span class="secno">4.1.1. </span><span class="content">24.7.�.1 Overview [mdspan.mdspan.overview]</span><a class="self-link" href="#mdspan.mdspan.overview"></a></h4>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ElementType</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Extents</c-><c- p>,</c-> <c- k>class</c-> <c- nc>LayoutPolicy</c-><c- p>,</c-> <c- k>class</c-> <c- nc>AccessorPolicy</c-><c- o>></c->
<c- k>class</c-> <c- nc>mdspan</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
  <c- k>using</c-> <c- n>extents_type</c-> <c- o>=</c-> <c- n>Extents</c-><c- p>;</c->
  <c- k>using</c-> <c- n>layout_type</c-> <c- o>=</c-> <c- n>LayoutPolicy</c-><c- p>;</c->
  <c- k>using</c-> <c- n>accessor_type</c-> <c- o>=</c-> <c- n>AccessorPolicy</c-><c- p>;</c->
  <c- k>using</c-> <c- n>mapping_type</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>layout_type</c-><c- o>::</c-><c- k>template</c-> <c- n>mapping</c-><c- o>&lt;</c-><c- n>extents_type</c-><c- o>></c-><c- p>;</c->
  <c- k>using</c-> <c- n>element_type</c-> <c- o>=</c-> <c- n>ElementType</c-><c- p>;</c->
  <c- k>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>remove_cv_t</c-><c- o>&lt;</c-><c- n>element_type</c-><c- o>></c-><c- p>;</c->
  <c- k>using</c-> <c- n>size_type</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>extents_type</c-><c- o>::</c-><c- n>size_type</c-> <c- p>;</c->
  <c- k>using</c-> <c- n>rank_type</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>extents_type</c-><c- o>::</c-><c- n>rank_type</c-> <c- p>;</c->
  <c- k>using</c-> <c- n>pointer</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>accessor_type</c-><c- o>::</c-><c- n>pointer</c-><c- p>;</c->
  <c- k>using</c-> <c- n>reference</c-> <c- o>=</c-> <c- k>typename</c-> <c- nc>accessor_type</c-><c- o>::</c-><c- n>reference</c-><c- p>;</c->

  <c- k>static</c-> <c- k>constexpr</c-> <c- n>rank_type</c-> <c- nf>rank</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>extents_type</c-><c- o>::</c-><c- n>rank</c-><c- p>();</c-> <c- p>}</c->
  <c- k>static</c-> <c- k>constexpr</c-> <c- n>rank_type</c-> <c- nf>rank_dynamic</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>extents_type</c-><c- o>::</c-><c- n>rank_dynamic</c-><c- p>();</c-> <c- p>}</c->
  <c- k>static</c-> <c- k>constexpr</c-> <c- b>size_t</c-> <c- nf>static_extent</c-><c- p>(</c-><c- n>rank_type</c-> <c- n>r</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>extents_type</c-><c- o>::</c-><c- n>static_extent</c-><c- p>(</c-><c- n>r</c-><c- p>);</c-> <c- p>}</c->
  <c- k>constexpr</c-> <c- n>size_type</c-> <c- nf>extent</c-><c- p>(</c-><c- n>rank_type</c-> <c- n>r</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- n>extents</c-><c- p>().</c-><c- n>extent</c-><c- p>(</c-><c- n>r</c-><c- p>);</c-> <c- p>}</c->

  <c- c1>// [mdspan.mdspan.ctor], mdspan Constructors</c->
  <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>();</c->
  <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- k>const</c-> <c- n>mdspan</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
  <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>mdspan</c-><c- o>&amp;&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>SizeTypes</c-><c- o>></c->
    <c- k>explicit</c-> <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>pointer</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>SizeTypes</c-><c- p>...</c-> <c- n>exts</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>SizeType</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>explicit</c-><c- p>(</c-><c- n>N</c-> <c- o>!=</c-> <c- n>rank_dynamic</c-><c- p>())</c->
    <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>pointer</c-> <c- n>p</c-><c- p>,</c-> <c- n>span</c-><c- o>&lt;</c-><c- n>SizeType</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-> <c- n>exts</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>SizeType</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>explicit</c-><c- p>(</c-><c- n>N</c-> <c- o>!=</c-> <c- n>rank_dynamic</c-><c- p>())</c->
    <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>pointer</c-> <c- n>p</c-><c- p>,</c-> <c- k>const</c-> <c- n>array</c-><c- o>&lt;</c-><c- n>SizeType</c-><c- p>,</c-> <c- n>N</c-><c- o>>&amp;</c-> <c- n>exts</c-><c- p>);</c->
  <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>pointer</c-> <c- n>p</c-><c- p>,</c-> <c- k>const</c-> <c- n>extents_type</c-><c- o>&amp;</c-> <c- n>ext</c-><c- p>);</c->
  <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>pointer</c-> <c- n>p</c-><c- p>,</c-> <c- k>const</c-> <c- n>mapping_type</c-><c- o>&amp;</c-> <c- n>m</c-><c- p>);</c->
  <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>pointer</c-> <c- n>p</c-><c- p>,</c-> <c- k>const</c-> <c- n>mapping_type</c-><c- o>&amp;</c-> <c- n>m</c-><c- p>,</c-> <c- k>const</c-> <c- n>accessor_type</c-><c- o>&amp;</c-> <c- n>a</c-><c- p>);</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>OtherElementType</c-><c- p>,</c-> <c- k>class</c-> <c- nc>OtherExtents</c-><c- p>,</c-> 
           <c- k>class</c-> <c- nc>OtherLayoutPolicy</c-><c- p>,</c-> <c- k>class</c-> <c- nc>OtherAccessorPolicy</c-><c- o>></c->
    <c- k>explicit</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>)</c->
    <c- k>constexpr</c-> <c- n>mdspan</c-><c- p>(</c->
      <c- k>const</c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>OtherElementType</c-><c- p>,</c-> <c- n>OtherExtents</c-><c- p>,</c-> 
                   <c- n>OtherLayoutPolicy</c-><c- p>,</c-> <c- n>OtherAccessorPolicy</c-><c- o>>&amp;</c-> <c- n>other</c-><c- p>);</c->

  <c- k>constexpr</c-> <c- n>mdspan</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>mdspan</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
  <c- k>constexpr</c-> <c- n>mdspan</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>mdspan</c-><c- o>&amp;&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

  <c- c1>// [mdspan.mdspan.members], mdspan members</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>SizeTypes</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>reference</c-> <c- k>operator</c-><c- p>[](</c-><c- n>SizeTypes</c-><c- p>...</c-> <c- n>indices</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>SizeType</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>reference</c-> <c- k>operator</c-><c- p>[](</c-><c- n>span</c-><c- o>&lt;</c-><c- n>SizeType</c-><c- p>,</c-> <c- n>rank</c-><c- p>()</c-><c- o>></c-> <c- n>indices</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>SizeType</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>reference</c-> <c- k>operator</c-><c- p>[](</c-><c- k>const</c-> <c- n>array</c-><c- o>&lt;</c-><c- n>SizeType</c-><c- p>,</c-> <c- n>rank</c-><c- p>()</c-><c- o>>&amp;</c-> <c- n>indices</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->

  <c- k>constexpr</c-> <c- b>size_t</c-> <c- nf>size</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
  <ins><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- n>empty</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c-></ins>

  <c- k>friend</c-> <c- k>constexpr</c-> <c- b>void</c-> <c- nf>swap</c-><c- p>(</c-><c- n>mdspan</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- n>mdspan</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

  <c- k>constexpr</c-> <c- k>const</c-> <c- n>extents_type</c-><c- o>&amp;</c-> <c- nf>extents</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <i><c- n>map_</c-></i><c- p>.</c-><c- n>extents</c-><c- p>();</c-> <c- p>}</c->
  <c- k>constexpr</c-> <c- k>const</c-> <c- n>pointer</c-><c- o>&amp;</c-> <c- nf>data</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <i><c- n>ptr_</c-></i><c- p>;</c-> <c- p>}</c->
  <c- k>constexpr</c-> <c- k>const</c-> <c- n>mapping_type</c-><c- o>&amp;</c-> <c- nf>mapping</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <i><c- n>map_</c-></i><c- p>;</c-> <c- p>}</c->
  <c- k>constexpr</c-> <c- k>const</c-> <c- n>accessor_type</c-><c- o>&amp;</c-> <c- nf>accessor</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <i><c- n>acc_</c-></i><c- p>;</c-> <c- p>}</c->

  <c- k>static</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- nf>is_always_unique</c-><c- p>()</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>mapping_type</c-><c- o>::</c-><c- n>is_always_unique</c-><c- p>();</c->
  <c- p>}</c->
  <c- k>static</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- nf>is_always_contiguous</c-><c- p>()</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>mapping_type</c-><c- o>::</c-><c- n>is_always_contiguous</c-><c- p>();</c->
  <c- p>}</c->
  <c- k>static</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- nf>is_always_strided</c-><c- p>()</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>mapping_type</c-><c- o>::</c-><c- n>is_always_strided</c-><c- p>();</c->
  <c- p>}</c->

  <c- k>constexpr</c-> <c- b>bool</c-> <c- nf>is_unique</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c->
    <c- k>return</c-> <i><c- n>map_</c-></i><c- p>.</c-><c- n>is_unique</c-><c- p>();</c->
  <c- p>}</c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- nf>is_contiguous</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c->
    <c- k>return</c-> <i><c- n>map_</c-></i><c- p>.</c-><c- n>is_contiguous</c-><c- p>();</c->
  <c- p>}</c->
  <c- k>constexpr</c-> <c- b>bool</c-> <c- nf>is_strided</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c->
    <c- k>return</c-> <i><c- n>map_</c-></i><c- p>.</c-><c- n>is_strided</c-><c- p>();</c->
  <c- p>}</c->
  <c- k>constexpr</c-> <c- n>size_type</c-> <c- nf>stride</c-><c- p>(</c-><c- n>rank_type</c-> <c- n>r</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
    <c- k>return</c-> <i><c- n>map_</c-></i><c- p>.</c-><c- n>stride</c-><c- p>(</c-><c- n>r</c-><c- p>);</c->
  <c- p>}</c->

<c- k>private</c-><c- o>:</c->
  <c- n>accessor_type</c-> <i><c- n>acc_</c-></i><c- p>;</c-> <c- c1>// exposition only</c->
  <c- n>mapping_type</c-> <i><c- n>map_</c-></i><c- p>;</c-> <c- c1>// exposition only</c->
  <c- n>pointer</c-> <i><c- n>ptr_</c-></i><c- p>;</c-> <c- c1>// exposition only</c->
<c- p>};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>CArray</c-><c- o>></c->
<c- k>requires</c-><c- p>(</c-><c- n>is_array_v</c-><c- o>&lt;</c-><c- n>CArray</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>rank_v</c-><c- o>&lt;</c-><c- n>CArray</c-><c- o>>==</c-><c- mi>1</c-><c- p>)</c->
<c- n>mdspan</c-><c- p>(</c-><c- n>CArray</c-><c- o>&amp;</c-><c- p>)</c->
  <c- o>-></c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>remove_all_extents_t</c-><c- o>&lt;</c-><c- n>CArray</c-><c- o>></c-><c- p>,</c-> <c- n>extents</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>extent_v</c-><c- o>&lt;</c-><c- n>CArray</c-><c- p>,</c-> <c- mi>0</c-><c- o>>>></c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>Pointer</c-><c- o>></c->
<c- k>requires</c-><c- p>(</c-><c- o>!</c-><c- n>is_array_v</c-><c- o>&lt;</c-><c- n>Pointer</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>is_pointer_v</c-><c- o>&lt;</c-><c- n>Pointer</c-><c- o>></c-><c- p>)</c->
<c- n>mdspan</c-><c- p>(</c-><c- n>Pointer</c-><c- o>&amp;</c-><c- p>)</c->
  <c- o>-></c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>remove_pointer_t</c-><c- o>&lt;</c-><c- n>Pointer</c-><c- o>></c-><c- p>,</c-> <c- n>extents</c-><c- o>&lt;</c-><c- b>size_t</c-><c- o>>>></c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>ElementType</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Integrals</c-><c- o>></c->
<c- k>requires</c-><c- p>((</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>Integrals</c-><c- p>,</c-> <c- b>size_t</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- p>...)</c-> <c- o>&amp;&amp;</c-> <c- k>sizeof</c-><c- p>...(</c-><c- n>Integrals</c-><c- p>)</c-> <c- o>></c-> <c- mi>0</c-><c- p>)</c->
<c- k>explicit</c-> <c- n>mdspan</c-><c- p>(</c-><c- n>ElementType</c-><c- o>*</c-><c- p>,</c-> <c- n>Integrals</c-><c- p>...)</c->
  <c- o>-></c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>ElementType</c-><c- p>,</c-> <c- n>dextents</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>...(</c-><c- n>Integrals</c-><c- p>)</c-><c- o>>></c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>ElementType</c-><c- p>,</c-> <c- k>class</c-> <c- nc>SizeType</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- n>mdspan</c-><c- p>(</c-><c- n>ElementType</c-><c- o>*</c-><c- p>,</c-> <c- n>span</c-><c- o>&lt;</c-><c- n>SizeType</c-><c- p>,</c-> <c- n>N</c-><c- o>></c-><c- p>)</c->
  <c- o>-></c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>ElementType</c-><c- p>,</c-> <c- n>dextents</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>>></c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>ElementType</c-><c- p>,</c-> <c- k>class</c-> <c- nc>SizeType</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
<c- n>mdspan</c-><c- p>(</c-><c- n>ElementType</c-><c- o>*</c-><c- p>,</c-> <c- k>const</c-> <c- n>array</c-><c- o>&lt;</c-><c- n>SizeType</c-><c- p>,</c-> <c- n>N</c-><c- o>>&amp;</c-><c- p>)</c->
  <c- o>-></c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>ElementType</c-><c- p>,</c-> <c- n>dextents</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>N</c-><c- o>>></c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>ElementType</c-><c- p>,</c-> <c- k>class</c-> <c- nc>MappingType</c-><c- o>></c->
<c- n>mdspan</c-><c- p>(</c-><c- n>ElementType</c-><c- o>*</c-><c- p>,</c-> <c- k>const</c-> <c- n>MappingType</c-><c- o>&amp;</c-><c- p>)</c->
  <c- o>-></c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>ElementType</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>MappingType</c-><c- o>::</c-><c- n>extents_type</c-><c- p>,</c->
            <c- k>typename</c-> <c- nc>MappingType</c-><c- o>::</c-><c- n>layout_type</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>ElementType</c-><c- p>,</c-> <c- k>class</c-> <c- nc>MappingType</c-><c- p>,</c-> <c- k>class</c-> <c- nc>AccessorType</c-><c- o>></c->
<c- n>mdspan</c-><c- p>(</c-><c- n>ElementType</c-><c- o>*</c-><c- p>,</c-> <c- k>const</c-> <c- n>MappingType</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>AccessorType</c-><c- o>&amp;</c-><c- p>)</c->
  <c- o>-></c-> <c- n>mdspan</c-><c- o>&lt;</c-><c- n>ElementType</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>MappingType</c-><c- o>::</c-><c- n>extents_type</c-><c- p>,</c-> 
            <c- k>typename</c-> <c- nc>MappingType</c-><c- o>::</c-><c- n>layout_type</c-><c- p>,</c-> <c- n>AccessorType</c-><c- o>></c-><c- p>;</c->

<c- p>}</c->
</pre>
   <h4 class="heading settled" data-level="4.1.2" id="mdspan.mdspan.members"><span class="secno">4.1.2. </span><span class="content">24.7.�.3 Members [mdspan.mdspan.members]</span><a class="self-link" href="#mdspan.mdspan.members"></a></h4>
    [...] 
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>size_type</c-> <c- nf>size</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
   <p><em>Precondition</em>: The size of the multidimensional index space <code class="highlight"><c- n>extents</c-><c- p>()</c-></code> is a representable value of type <code class="highlight"><c- n>size_type</c-></code> ([basic.fundamental]).</p>
   <p><em>Returns</em>: <code class="highlight"><c- n>extents</c-><c- p>().</c-><i><c- n>fwd</c-><c- o>-</c-><c- n>prod</c-><c- o>-</c-><c- n>of</c-><c- o>-</c-><c- n>extents</c-></i><c- p>(</c-><c- n>rank</c-><c- p>())</c-></code>.</p>
<pre class="highlight"><ins><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- n>empty</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->
</ins></pre>
   <ins><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>size</c-><c- p>()</c-> <c- o>==</c-> <c- mi>0</c-><c- p>;</c-></code></ins>
<pre class="language-cpp highlight"><c- k>friend</c-> <c- k>constexpr</c-> <c- b>void</c-> <c- nf>swap</c-><c- p>(</c-><c- n>mdspan</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- n>mdspan</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <p><em>Effects</em>: Equivalent to:</p>
<pre class="highlight"><c- n>swap</c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><i><c- n>ptr_</c-></i><c- p>,</c-> <c- n>y</c-><c- p>.</c-><i><c- n>ptr_</c-></i><c- p>);</c->
<c- n>swap</c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><i><c- n>map_</c-></i><c- p>,</c-> <c- n>y</c-><c- p>.</c-><i><c- n>map_</c-></i><c- p>);</c->
<c- n>swap</c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><i><c- n>acc_</c-></i><c- p>,</c-> <c- n>y</c-><c- p>.</c-><i><c- n>acc_</c-></i><c- p>);</c->
</pre>
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  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-p0009r17">[P0009R17]
   <dd>Christian Trott; et al. <a href="https://wg21.link/p0009r17"><cite>MDSPAN</cite></a>. URL: <a href="https://wg21.link/p0009r17">https://wg21.link/p0009r17</a>
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