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  <title>P0454r0: Wording for a Minimal mdspan&lt;></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
 *
 * 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)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * 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
 *
 * 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)
 *
 ******************************************************************************/

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

	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;

		/* Colors */
		color: black;
		background: white 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-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		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;
		border-color: #c00;
	}

	/* 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: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* 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-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** 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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}
		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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}

		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;
		background: transparent;
	}

	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,
	#subtitle {
		/* #subtitle 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) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		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;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em 0;
	}
	[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: normal;
	}
	dfn var {
		font-style: normal;
	}

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

	del { color: red;  background-color: #FFA0A0; text-decoration: line-through; }
	ins { color: #080; background-color: #A0FFA0; text-decoration: none;         }

	del code, del pre, code del, pre del { background-color: #FFA0A0 !important; text-decoration: line-through; }
	ins code, ins pre, code ins, pre ins { background-color: #A0FFA0 !important; text-decoration: none;         }

/** 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;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	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;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

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

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

	img {
		border-style: none;
	}

	/* 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;
	}
	.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 {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* 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;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* 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;
		background: #E9FBE9;
		overflow: auto;
	}

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

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

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

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

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

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** 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: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    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;
	}

	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;
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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">P0454r0<br>Wording for a Minimal mdspan&lt;></h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Proposal, <time class="dt-updated" datetime="2016-10-10">10 October 2016</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0454r0">http://wg21.link/P0454r0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:hcedwar@sandia.gov">H. Carter Edwards</a> (<span class="p-org org">Sandia National Laboratory</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:balelbach@lbl.gov">Bryce Adelstein Lelbach</a> (<span class="p-org org">Lawrence Berkeley National Laboratory</span>)
     <dt>Audience:
     <dd>LEWG, LWG
     <dt>Project:
     <dd><span>ISO JTC1/SC22/WG21: Programming Language C++</span>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
  <div class="p-summary" data-fill-with="abstract">
   <p>Proposed wording, based on span <a data-link-type="biblio" href="#biblio-p0122r3">[P0122R3]</a> for an <code class="highlight"><span class="n">mdspan</span></code> with a core subset of the full <code class="highlight"><span class="n">mdspan</span></code> functionality.</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="#spanword"><span class="secno">1</span> <span class="content">Modified P0122R3 Wording</span></a>
    <li>
     <a href="#mdspanword"><span class="secno">2</span> <span class="content"><code class="highlight"><span class="n">mdspan</span></code> Wording</span></a>
     <ol class="toc">
      <li><a href="#dims"><span class="secno">2.1</span> <span class="content"><code class="highlight"><span class="n">dimensions</span></code> Class Template</span></a>
      <li><a href="#layreq"><span class="secno">2.2</span> <span class="content">Layout Mapping Requirements</span></a>
      <li><a href="#mdspan"><span class="secno">2.3</span> <span class="content"><code class="highlight"><span class="n">mdspan</span></code> Class Template</span></a>
      <li><a href="#laymap"><span class="secno">2.4</span> <span class="content">Layout Mapping Properties</span></a>
     </ol>
    <li><a href="#test"><span class="secno">3</span> <span class="content">Feature Testing</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="spanword"><span class="secno">1. </span><span class="content">Modified P0122R3 Wording</span><a class="self-link" href="#spanword"></a></h2>
   <p>The proposed wording changes are relative to the working draft of the standard
as of <a data-link-type="biblio" href="#biblio-n4567">[N4567]</a> with the changes proposed in <a data-link-type="biblio" href="#biblio-p0122r3">[P0122R3]</a> applied.</p>
   <p>The � character is used to denote a placeholder section number which the editor
shall determine.</p>
   <p>Apply the following changes to 23.7.� [views.general] paragraph 1:</p>
   <blockquote>
     The header <code class="highlight"><span class="o">&lt;</span><span class="n">span</span><span class="o">></span></code> defines the 
    <del>view</del>
    <ins>views</ins>
     <code class="highlight"><span class="n">span</span></code> 
    <ins>and <code class="highlight"><span class="n">mdspan</span></code></ins>
    . A <code class="highlight"><span class="n">span</span></code> is a view over a
contiguous sequence of objects, the storage of which is owned by some other
object.
    <ins> An <code class="highlight"><span class="n">mdspan</span></code> is a multidimensional view over a contiguous sequence
of objects, the storage of which is owned by some other object.</ins>
   </blockquote>
   <p>Apply the following changes to 23.7.�.1 [views.span.synop]:</p>
<pre class="highlight"><span class="k">namespace</span> <span class="n">std</span> <span class="p">{</span>

<span class="c1">// [views.constants], constants
</span><del><span class="k">constexpr</span> <span class="kt">ptrdiff_t</span> <span class="n">dynamic_extent</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span></del>
<ins><span class="k">enum</span> <span class="k">class</span> <span class="nc">dynamic_extent_tag</span> <span class="p">{</span><span class="p">}</span><span class="p">;</span>
<span class="kr">inline</span> <span class="k">constexpr</span> <span class="n">dynamic_extent_tag</span> <span class="n">dyn</span> <span class="p">{</span> <span class="o">-</span><span class="mi">1</span> <span class="p">}</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span> <span class="n">Extent</span><span class="o">></span>
  <span class="k">using</span> <span class="n">is_dynamic_dimension</span> <span class="o">=</span> <span class="k">typename</span> <span class="n">is_same</span><span class="o">&lt;</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="k">decltype</span><span class="p">(</span><span class="n">Extent</span><span class="p">)</span><span class="o">></span><span class="p">,</span>
<span class="n">dynamic_extent_tag</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">type</span><span class="p">;</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span> <span class="n">Extent</span><span class="o">></span>
  <span class="kr">inline</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_dynamic_dimension_v</span> <span class="o">=</span>
<span class="n">is_dynamic_dimension</span><span class="o">&lt;</span><span class="n">Extent</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">;</span></ins>

<span class="c1">// [views.span], class template span
</span><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span> <span class="o">=</span> <del><span class="n">dynamic_extent</span></del><ins><span class="n">dyn</span></ins><span class="o">></span>
<span class="k">class</span> <span class="nc">span</span><span class="p">;</span>

<span class="c1">// [views.span.comparison], span comparison operators
</span><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span> <span class="k">const</span>
<span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span> <span class="k">const</span>
<span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">&lt;</span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span> <span class="k">const</span>
<span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">&lt;</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span> <span class="k">const</span>
<span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">></span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span> <span class="k">const</span>
<span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">></span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span> <span class="k">const</span>
<span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="c1">// [views.span.objectrep], views of object representation
</span><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="n">span</span><span class="o">&lt;</span><span class="k">const</span> <span class="kt">char</span><span class="p">,</span> <span class="p">(</span><span class="p">(</span><del><span class="n">Extent</span> <span class="o">=</span><span class="o">=</span> <span class="n">dynamic_extent</span></del><ins><span class="n">is_dynamic_dimension_v</span><span class="o">&lt;</span><span class="n">Extent</span><span class="o">></span></ins><span class="p">)</span> <span class="o">?</span> <span class="nl">dynamic_extent</span> <span class="p">:</span>
<span class="p">(</span><span class="k">sizeof</span><span class="p">(</span><span class="n">ElementType</span><span class="p">)</span> <span class="o">*</span> <span class="n">Extent</span><span class="p">)</span><span class="p">)</span><span class="o">></span> <span class="n">as_bytes</span><span class="p">(</span><span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span> <span class="n">Extent</span><span class="o">></span> <span class="n">s</span><span class="p">)</span>
<span class="k">noexcept</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span><span class="o">></span>
  <span class="n">span</span><span class="o">&lt;</span><span class="k">const</span> <span class="kt">char</span><span class="p">,</span> <span class="p">(</span><span class="p">(</span><del><span class="n">Extent</span> <span class="o">=</span><span class="o">=</span> <span class="n">dynamic_extent</span></del><ins><span class="n">is_dynamic_dimension_v</span><span class="o">&lt;</span><span class="n">Extent</span><span class="o">></span></ins><span class="p">)</span> <span class="o">?</span> <span class="nl">dynamic_extent</span> <span class="p">:</span>
<span class="p">(</span><span class="k">sizeof</span><span class="p">(</span><span class="n">ElementType</span><span class="p">)</span> <span class="o">*</span> <span class="n">Extent</span><span class="p">)</span><span class="p">)</span><span class="o">></span> <span class="n">as_writeable_bytes</span><span class="p">(</span><span class="n">span</span><span class="o">&lt;</span><span class="n">ElementType</span><span class="p">,</span>
<span class="n">Extent</span><span class="o">></span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>

<ins><span class="c1">// [views.dimensions], class template dimensions
</span><span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Extents</span><span class="o">></span>
<span class="k">class</span> <span class="nc">dimensions</span><span class="p">;</span>

<span class="c1">// [views.dimensions.comparison], dimensions comparison operators
</span><span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Extents</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span>
                            <span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Extents</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span>
                            <span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>

<span class="c1">// [views.mdspan], class template mdspan
</span><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">Dimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Properties</span><span class="o">></span>
<span class="k">class</span> <span class="nc">mdspan</span><span class="p">;</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">is_mdspan_property</span><span class="p">;</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span>
<span class="kr">inline</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_mdspan_property_v</span> <span class="o">=</span> <span class="n">is_mdspan_property</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">;</span>

<span class="c1">// [views.layout.props], mdspan layout properties
</span><span class="k">class</span> <span class="nc">layout_right</span><span class="p">;</span>
<span class="k">class</span> <span class="nc">layout_left</span><span class="p">;</span>


<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">is_layout</span><span class="p">;</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="o">></span>
<span class="kr">inline</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_layout_v</span> <span class="o">=</span> <span class="n">is_layout</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">;</span>


<span class="p">}</span> <span class="c1">// namespace std
</span></extents...></extents...></extents...></extents...></ins></pre>
   <p>Add the following paragraph in between paragraphs 2 and 3 in 23.7.�.2 [views.span.template]:</p>
   <blockquote>
    <ins><code class="highlight"><span class="n">span</span></code> requires the type of <code class="highlight"><span class="n">Extent</span></code> to be either <code class="highlight"><span class="n">dynamic_extent_tag</span></code> or
implicitly convertible to the <code class="highlight"><span class="n">span</span></code>'s <code class="highlight"><span class="n">index_type</span></code>.</ins>
   </blockquote>
   <p>Apply the following changes to the definition of <code class="highlight"><span class="n">span</span></code> in 23.7.�.2 [views.span.template]</p>
<pre class="highlight"><span class="k">namespace</span> <span class="n">std</span> <span class="p">{</span>

<span class="c1">// A view over a contiguous, single-dimension sequence of objects
</span><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Extent</span> <span class="o">=</span> <del><span class="n">dynamic_extent</span></del><ins><span class="n">dyn</span></ins><span class="o">></span>
<span class="k">class</span> <span class="nc">span</span> <span class="p">{</span>
</pre>
<pre class="highlight">  <ins><span class="kr">inline</span></ins> <span class="k">constexpr</span> <del><span class="k">static</span> <span class="n">index_type</span></del><ins><span class="n">auto</span></ins> <span class="n">extent</span> <span class="o">=</span> <span class="n">Extent</span><span class="p">;</span>
</pre>
<pre class="highlight">  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">OtherElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">OtherExtent</span><span class="o">></span>
    <span class="k">constexpr</span> <span class="n">span</span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">OtherElementType</span><span class="p">,</span> <span class="n">OtherExtent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">OtherElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">OtherExtent</span><span class="o">></span>
    <span class="k">constexpr</span> <span class="n">span</span><span class="p">(</span><span class="n">span</span><span class="o">&lt;</span><span class="n">OtherElementType</span><span class="p">,</span> <span class="n">OtherExtent</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
</pre>
<pre class="highlight">  <span class="k">template</span> <span class="o">&lt;</span><span class="kt">ptrdiff_t</span> <span class="n">Offset</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Count</span> <span class="o">=</span> <del><span class="n">dynamic_extent</span></del><ins><span class="n">dyn</span></ins><span class="o">></span>
    <span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">Count</span><span class="o">></span> <span class="n">subspan</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">dynamic_extent</span><span class="o">></span> <span class="n">first</span><span class="p">(</span><span class="n">index_type</span> <span class="n">count</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">dynamic_extent</span><span class="o">></span> <span class="n">last</span><span class="p">(</span><span class="n">index_type</span> <span class="n">count</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
<ins>  <span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">dynamic_extent</span><span class="o">></span> <span class="n">subspan</span><span class="p">(</span><span class="n">index_type</span>
    <span class="n">offset</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span></ins>
  <span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">dynamic_extent</span><span class="o">></span> <span class="n">subspan</span><span class="p">(</span><span class="n">index_type</span>
    <span class="n">offset</span><span class="p">,</span> <span class="n">index_type</span> <span class="n">count</span><del> <span class="o">=</span> <span class="n">dynamic_extent</span></del><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
   <p>Apply the following changes to the converting copy constructor in 23.7.�.3
[views.span.cons]:</p>
   <blockquote>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">OtherElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">OtherExtent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">span</span><span class="p">(</span><span class="k">const</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">OtherElementType</span><span class="p">,</span> <span class="n">OtherExtent</span><span class="o">></span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">OtherElementType</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">OtherExtent</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">span</span><span class="p">(</span><span class="n">span</span><span class="o">&lt;</span><span class="n">OtherElementType</span><span class="p">,</span> <span class="n">OtherExtent</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
</pre>
    <p><em>Remarks:</em> These constructors shall not participate in overload resolution
unless trying to access OtherElementType through an ElementType* would meet the
rules for well-defined object access defined in 3.10/10.</p>
    <p>
     <em>Requires:</em> If 
     <del><code class="highlight"><span class="n">extent</span></code> is not equal to <code class="highlight"><span class="n">dynamic_extent</span></code></del>
     <ins><code class="highlight"><span class="n">is_dynamic_dimension_v</span><span class="o">&lt;</span><span class="n">Extent</span><span class="o">></span> <span class="o">=</span><span class="o">=</span> false</code></ins>
     ,
then <code class="highlight"><span class="n">other</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span></code> shall be equal to <code class="highlight"><span class="n">extent</span></code>.
    </p>
    <p><em>Effects:</em> Constructs a <code class="highlight"><span class="n">span</span></code> by copying the implementation data members of
another <code class="highlight"><span class="n">span</span></code>, performing suitable conversions.</p>
    <p><em>Postconditions:</em> <code class="highlight"><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">other</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="k">reinterpret_cast</span><span class="o">&lt;</span><span class="n">pointer</span><span class="o">></span><span class="p">(</span><span class="n">other</span><span class="p">.</span><span class="n">data</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
    <p><em>Complexity:</em> Constant.</p>
    <p><em>Throws:</em> Nothing.</p>
   </blockquote>
   <p>Apply the following changes to <code class="highlight"><span class="n">subspan</span><span class="p">(</span><span class="p">)</span></code> in 23.7.�.4 [views.span.sub]:</p>
   <blockquote>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="kt">ptrdiff_t</span> <span class="n">Offset</span><span class="p">,</span> <del><span class="n">ptrdiff_t</span></del><ins><span class="n">auto</span></ins> <span class="n">Count</span> <span class="o">=</span> <del><span class="n">dynamic_extent</span></del><ins><span class="n">dyn</span></ins><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">Count</span><span class="o">></span> <span class="n">subspan</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
    <p><em>Requires:</em> <code class="highlight"><span class="p">(</span><span class="n">Offset</span> <span class="o">=</span><span class="o">=</span> <span class="mi">0</span> <span class="o">|</span><span class="o">|</span> <span class="n">Offset</span> <span class="o">></span> <span class="mi">0</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">Offset</span> <span class="o">&lt;</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="p">(</span><del><span class="n">Count</span> <span class="o">=</span><span class="o">=</span> <span class="n">dynamic_extent</span></del><ins><span class="n">is_dynamic_dimension</span><span class="o">&lt;</span><span class="n">Count</span><span class="o">></span></ins> <span class="o">|</span><span class="o">|</span> <span class="n">Count</span> <span class="o">></span><span class="o">=</span> <span class="mi">0</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">Offset</span> <span class="o">+</span> <span class="n">Count</span> <span class="o">&lt;</span><span class="o">=</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
    <p>
     <em>Effects:</em> Returns a new <code class="highlight"><span class="n">span</span></code> that is a view over <code class="highlight"><span class="n">Count</span></code> elements of the
current span starting at element <code class="highlight"><span class="n">Offset</span></code>. If 
     <del><code class="highlight"><span class="n">Count</span></code> is equal to <code class="highlight"><span class="n">dynamic_extent</span></code></del>
     <ins><code class="highlight"><span class="n">is_dynamic_dimension</span><span class="o">&lt;</span><span class="n">Count</span><span class="o">></span> <span class="o">=</span><span class="o">=</span> true</code></ins>
     , then a <code class="highlight"><span class="n">span</span></code> over all elements from <code class="highlight"><span class="n">Offset</span></code> onwards is
returned.
    </p>
    <p><em>Returns:</em> <code class="highlight"><span class="n">span</span><span class="p">(</span><span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">+</span> <span class="n">Offset</span><span class="p">,</span> <del><span class="n">Count</span> <span class="o">=</span><span class="o">=</span> <span class="n">dynamic_extent</span></del><ins><span class="n">is_dynamic_dimension</span><span class="o">&lt;</span><span class="n">Count</span><span class="o">></span></ins> <span class="o">?</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">-</span> <span class="nl">Offset</span> <span class="p">:</span> <span class="n">Count</span><span class="p">)</span></code>.</p>
    <p><em>Complexity:</em> Constant.</p>
   </blockquote>
   <p>Add the following <code class="highlight"><span class="n">subspan</span><span class="p">(</span><span class="p">)</span></code> overload to 23.7.�.4 [views.span.sub]:</p>
   <blockquote>
    <ins style="display: block;">
<pre class="highlight"><span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">dynamic_extent</span><span class="o">></span> <span class="n">subspan</span><span class="p">(</span><span class="n">index_type</span>
  <span class="n">offset</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
     <p><em>Requires:</em> <code class="highlight"><span class="p">(</span><span class="n">offset</span> <span class="o">=</span><span class="o">=</span> <span class="mi">0</span> <span class="o">|</span><span class="o">|</span> <span class="n">offset</span> <span class="o">></span> <span class="mi">0</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">offset</span> <span class="o">&lt;</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
     <p><em>Effects:</em> Returns a new <code class="highlight"><span class="n">span</span></code> that is a view over all elements from <code class="highlight"><span class="n">offset</span></code> onwards.</p>
     <p><em>Returns:</em> <code class="highlight"><span class="n">span</span><span class="p">(</span><span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">+</span> <span class="n">offset</span><span class="p">,</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">-</span> <span class="n">offset</span><span class="p">)</span></code>.</p>
     <p><em>Complexity:</em> Constant.</p>
    </ins>
   </blockquote>
   <p>Apply the following changes to <code class="highlight"><span class="n">subspan</span><span class="p">(</span><span class="p">)</span></code> in 23.7.�.4 [views.span.sub]:</p>
   <blockquote>
<pre class="highlight"><span class="k">constexpr</span> <span class="n">span</span><span class="o">&lt;</span><span class="n">element_type</span><span class="p">,</span> <span class="n">dynamic_extent</span><span class="o">></span> <span class="n">subspan</span><span class="p">(</span><span class="n">index_type</span>
  <span class="n">offset</span><span class="p">,</span> <span class="n">index_type</span> <span class="n">count</span><del> <span class="o">=</span> <span class="n">dynamic_extent</span></del><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
    <p><em>Requires:</em> <code class="highlight"><span class="p">(</span><span class="n">offset</span> <span class="o">=</span><span class="o">=</span> <span class="mi">0</span> <span class="o">|</span><span class="o">|</span> <span class="n">offset</span> <span class="o">></span> <span class="mi">0</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">offset</span> <span class="o">&lt;</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="p">(</span><del><span class="n">count</span> <span class="o">=</span><span class="o">=</span> <span class="n">dynamic_extent</span> <span class="o">|</span><span class="o">|</span> </del><span class="n">count</span> <span class="o">></span><span class="o">=</span> <span class="mi">0</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">offset</span> <span class="o">+</span> <span class="n">count</span> <span class="o">&lt;</span><span class="o">=</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
    <p>
     <em>Effects:</em> Returns a new <code class="highlight"><span class="n">span</span></code> that is a view over <code class="highlight"><span class="n">count</span></code> elements of the
current span starting at element <code class="highlight"><span class="n">offset</span></code>.
     <del> If count is equal to <code class="highlight"><span class="n">dynamic_extent</span></code>, then a <code class="highlight"><span class="n">span</span></code> over all elements from <code class="highlight"><span class="n">offset</span></code> onwards is
returned.</del>
    </p>
    <p><em>Returns:</em> <code class="highlight"><span class="n">span</span><span class="p">(</span><span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">+</span> <span class="n">offset</span><span class="p">,</span> <del><span class="n">count</span> <span class="o">=</span><span class="o">=</span> <span class="n">dynamic_extent</span> <span class="o">?</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">-</span> <span class="nl">offset</span> <span class="p">:</span> </del><span class="n">count</span><span class="p">)</span></code>.</p>
    <p><em>Complexity:</em> Constant.</p>
   </blockquote>
   <h2 class="heading settled" data-level="2" id="mdspanword"><span class="secno">2. </span><span class="content"><code class="highlight"><span class="n">mdspan</span></code> Wording</span><a class="self-link" href="#mdspanword"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="dims"><span class="secno">2.1. </span><span class="content"><code class="highlight"><span class="n">dimensions</span></code> Class Template</span><a class="self-link" href="#dims"></a></h3>
   <p>Add the following section, 23.7.� [views.dimensions.template]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.� Class Template</strong> <code class="highlight"><span class="n">dimensions</span></code> <strong>[views.dimensions.template]</strong> 
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Extents</span><span class="o">></span>
<span class="k">class</span> <span class="nc">dimensions</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
  <span class="c1">// types
</span>  <span class="k">using</span> <span class="n">value_type</span> <span class="o">=</span> <span class="kt">ptrdiff_t</span><span class="p">;</span>
  <span class="k">using</span> <span class="n">index_type</span> <span class="o">=</span> <span class="kt">ptrdiff_t</span><span class="p">;</span>

  <span class="c1">// [views.dimensions.cons], constructors/assignment/destructor
</span>  <span class="k">constexpr</span> <span class="nf">dimensions</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">DynamicExtents</span><span class="o">></span>
    <span class="k">constexpr</span> <span class="n">dimensions</span><span class="p">(</span><span class="n">DynamicExtents</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">dexts</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="nf">dimensions</span><span class="p">(</span><span class="n">dimensions</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="nf">dimensions</span><span class="p">(</span><span class="n">dimensions</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="n">dimensions</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">dimensions</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="n">dimensions</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">dimensions</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>

  <span class="c1">// [views.dimensions.obs], observers
</span>  <span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank_dynamic</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="k">constexpr</span> <span class="n">size_type</span> <span class="nf">size</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="nf">is_dynamic_dimension</span><span class="p">(</span><span class="n">index_type</span> <span class="n">i</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="c1">// [views.dimensions.elem], element access
</span>  <span class="k">constexpr</span> <span class="n">value_type</span> <span class="k">operator</span><span class="p">[</span><span class="p">]</span><span class="p">(</span><span class="n">index_type</span> <span class="n">i</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="p">}</span><span class="p">;</span>
</pre>
     <p><code class="highlight"><span class="n">dimensions</span></code> is a class which contains storage for a fixed number of integer
elements, each of which represents the extent of a <em>dimension</em>, collectively
forming a multi-dimensional integer index.</p>
     <p>Each non-type template parameter specifies either a positive integral value,
indicating a <em>static dimension</em>, or a value of type <code class="highlight"><span class="n">dynamic_extent_tag</span></code>, 
indicating a <em>dynamic dimension</em> whose value will be provided at runtime.</p>
     <p>Implementations are not permitted to use additional storage, such as dynamic
memory, to allocate the contained dimensions. The contained values shall be
allocated in a region of the <code class="highlight"><span class="n">dimensions</span></code> storage suitably aligned for the type <code class="highlight"><span class="n">value_type</span></code>. [ <em>Note:</em> Implementations are not required to store the value
of static dimensions. — <em>end note</em> ]</p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.dimensions.cons]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">dimensions</span></code> <strong>Constructors, Assignment and Destructors [views.dimensions.cons]</strong> 
<pre class="highlight"><span class="k">constexpr</span> <span class="nf">dimensions</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Constructs a <code class="highlight"><span class="n">dimensions</span></code> object with all dynamic dimensions default
initialized.</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">DynamicExtents</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">dimensions</span><span class="p">(</span><span class="n">DynamicExtents</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">dexts</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Remarks:</em> This constructor shall not participate in overload resolution unless <code class="highlight"><span class="k">sizeof</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">(</span><span class="n">DynamicExtents</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">rank_dynamic</span><span class="p">(</span><span class="p">)</span></code>.</p>
     <p><em>Effects:</em> Constructs a <code class="highlight"><span class="n">dimensions</span></code> object with each dynamic dimensions
initialized from a corresponding value in <code class="highlight"><span class="n">dexts</span></code>.</p>
<pre class="highlight"><span class="k">constexpr</span> <span class="nf">dimensions</span><span class="p">(</span><span class="n">dimensions</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="nf">dimensions</span><span class="p">(</span><span class="n">dimensions</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Constructs a <code class="highlight"><span class="n">dimensions</span></code> object by copying from another <code class="highlight"><span class="n">dimensions</span></code> object.</p>
<pre class="highlight"><span class="n">dimensions</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">dimensions</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
<span class="n">dimensions</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">dimensions</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Assigns from one <code class="highlight"><span class="n">dimensions</span></code> object into another.</p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.dimensions.obs]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">dimensions</span></code> <strong>Observers [views.dimensions.obs]</strong> 
<pre class="highlight"><span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Returns the number of elements in the <code class="highlight"><span class="n">dimensions</span></code> object.</p>
     <p><em>Returns:</em> <code class="highlight"><span class="k">sizeof</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">(</span><span class="n">Extents</span><span class="p">)</span></code>.</p>
<pre class="highlight"><span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Returns the number of parameters in <code class="highlight"><span class="n">Extents</span></code> for which <code class="highlight"><span class="n">is_dynamic_dimension_v</span></code> is <code class="highlight">true</code>.</p>
<pre class="highlight"><span class="k">constexpr</span> <span class="kt">bool</span> <span class="nf">is_dynamic_dimension</span><span class="p">(</span><span class="n">index_type</span> <span class="n">i</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> If <code class="highlight"><span class="n">i</span> <span class="o">&lt;</span> <span class="n">rank</span><span class="p">(</span><span class="p">)</span></code>, returns true if the <code class="highlight"><span class="n">i</span></code>th dimension is a dynamic
dimension. Otherwise, returns false.</p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.dimensions.elem]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">dimensions</span></code> <strong>Element Access [views.dimensions.elem]</strong> 
<pre class="highlight"><span class="k">constexpr</span> <span class="n">value_type</span> <span class="k">operator</span><span class="p">[</span><span class="p">]</span><span class="p">(</span><span class="n">index_type</span> <span class="n">i</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> If <code class="highlight"><span class="n">i</span> <span class="o">&lt;</span> <span class="n">rank</span><span class="p">(</span><span class="p">)</span></code>, returns the value of the <code class="highlight"><span class="n">i</span></code>th element. Otherwise,
returns 0.</p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.dimensions.comparison]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">dimensions</span></code> <strong>Comparison Operators [views.dimensions.comparison]</strong> 
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Extents</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span>
                            <span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>
</extents...></extents...></pre>
     <p><em>Effects:</em> <code class="highlight"><span class="n">l</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span><span class="o">=</span> <span class="n">r</span><span class="p">[</span><span class="n">i</span><span class="p">]</span></code>, where <code class="highlight"><span class="n">i</span></code> is  in the range <code class="highlight"><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="n">rank</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
     <p><em>Throws:</em> Nothing.</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">auto</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Extents</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">l</span><span class="p">,</span>
                            <span class="k">const</span> <span class="n">dimensions</span><extents...><span class="o">&amp;</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span>
</extents...></extents...></pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="o">!</span><span class="p">(</span><span class="n">l</span> <span class="o">=</span><span class="o">=</span> <span class="n">r</span><span class="p">)</span><span class="p">;</span></code></p>
    </ins>
   </blockquote>
   <h3 class="heading settled" data-level="2.2" id="layreq"><span class="secno">2.2. </span><span class="content">Layout Mapping Requirements</span><a class="self-link" href="#layreq"></a></h3>
   <p>Add the following section, 23.7.� [views.layout.requirements]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.� Layout Mapping Requirements [views.layout.requirements]</strong> 
     <p>A <em>layout</em> is class that describes a mapping from a multi-dimensional index to
a <em>mapped index</em>, which is one-dimensional. <code class="highlight"><span class="n">mdspan</span></code> (23.7.�) is parameterized
in terms of layout mappings.</p>
     <p>A layout shall have an embedded template class <code class="highlight"><span class="n">mapping</span></code> which takes a single
template parameter that shall be a type which fulfills the definition of
the <code class="highlight"><span class="n">dimensions</span></code> class. <em>Layout mappings</em> are instantiations of this embedded
template class.</p>
     <p>In the following section:</p>
     <ul>
      <li data-md="">
       <p>Let <code class="highlight"><span class="n">LM</span></code> be a layout mapping and let <code class="highlight"><span class="n">lm</span></code> be an object of type <code class="highlight"><span class="n">LM</span></code>.</p>
      <li data-md="">
       <p>Let <code class="highlight"><span class="n">i</span></code> be in the range <code class="highlight"><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="n">rank</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> in order.</p>
     </ul>
     <p>The expression <code class="highlight"><span class="n">is_layout</span><span class="o">&lt;</span><span class="n">LM</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">value</span></code> shall be true.</p>
     <p>The expression <code class="highlight"><span class="n">lm</span><span class="p">.</span><span class="n">span</span><span class="p">(</span><span class="p">)</span></code> shall be well-formed and have the following
semantics:</p>
     <ul>
      <li data-md="">
       <p><em>Effects:</em> Returns <code class="highlight"><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span><span class="p">(</span><span class="n">extent</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span><span class="p">,</span> <span class="cm">/* ... */</span><span class="p">,</span> <span class="n">extent</span><span class="p">(</span><span class="n">i</span><span class="p">)</span><span class="p">)</span> <span class="o">-</span> <span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="cm">/* ... */</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span></code>.</p>
     </ul>
     <p>The expression <code class="highlight"><span class="n">LM</span><span class="o">:</span><span class="o">:</span><span class="n">is_always_strided</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code> shall be true if the difference
between two mapped indices that are consecutive is always identical.</p>
     <p>If <code class="highlight"><span class="n">LM</span><span class="o">:</span><span class="o">:</span><span class="n">is_always_strided</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code>, the expression <code class="highlight"><span class="n">lm</span><span class="p">.</span><span class="n">stride</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code> shall be
well-formed and have the following semantics:</p>
     <ul>
      <li data-md="">
       <p><em>Effects:</em> Returns the difference between two mapped indices that are
consecutive.</p>
     </ul>
     <p>The expression <code class="highlight"><span class="n">lm</span><span class="p">(</span><span class="n">is</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span></code> shall be well-formed and have the following semantics
if <code class="highlight"><span class="k">sizeof</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">(</span><span class="k">decltype</span><span class="p">(</span><span class="n">is</span><span class="p">)</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">LM</span><span class="o">:</span><span class="o">:</span><span class="n">rank</span><span class="p">(</span><span class="p">)</span></code> and all the types in <code class="highlight"><span class="k">decltype</span><span class="p">(</span><span class="n">is</span><span class="p">)</span></code> are integral:</p>
     <ul>
      <li data-md="">
       <p><em>Effects:</em> Returns the mapped index for multi-dimensional index <code class="highlight"><span class="n">is</span></code>.</p>
     </ul>
    </ins>
   </blockquote>
   <h3 class="heading settled" data-level="2.3" id="mdspan"><span class="secno">2.3. </span><span class="content"><code class="highlight"><span class="n">mdspan</span></code> Class Template</span><a class="self-link" href="#mdspan"></a></h3>
   <p>Add the following section, 23.7.� [views.mdspan.template]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.� Class template</strong> <code class="highlight"><span class="n">mdspan</span></code> <strong>[views.mdspan.template]</strong> 
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">ElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">Dimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Properties</span><span class="o">></span>
<span class="k">class</span> <span class="nc">mdspan</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
  <span class="c1">// types
</span>  <span class="k">using</span> <span class="n">element_type</span> <span class="o">=</span> <span class="n">ElementType</span><span class="p">;</span>
  <span class="k">using</span> <span class="n">index_type</span> <span class="o">=</span> <span class="kt">ptrdiff_t</span><span class="p">;</span>
  <span class="k">using</span> <span class="n">difference_type</span> <span class="o">=</span> <span class="kt">ptrdiff_t</span><span class="p">;</span>
  <span class="k">using</span> <span class="n">pointer</span> <span class="o">=</span> <span class="n">element_type</span><span class="o">*</span><span class="p">;</span>
  <span class="k">using</span> <span class="n">reference</span> <span class="o">=</span> <span class="n">element_type</span><span class="o">&amp;</span><span class="p">;</span>

  <span class="k">using</span> <span class="n">layout</span> <span class="o">=</span> <i><span class="n">implementation</span> <span class="n">defined</span></i><span class="p">;</span>
  <span class="k">using</span> <span class="n">layout_mapping</span> <span class="o">=</span> <span class="k">typename</span> <span class="n">layout</span><span class="o">:</span><span class="o">:</span><span class="k">template</span> <span class="n">mapping</span><dimensions><span class="p">;</span>

  <span class="c1">// [views.mdspan.cons], constructors/assignment/destructor 
</span>  <span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">nullptr_t</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">DynamicExtents</span><span class="o">></span>
    <span class="k">constexpr</span> <span class="n">mdspan</span><span class="p">(</span><span class="n">pointer</span> <span class="n">ptr</span><span class="p">,</span> <span class="n">DynamicExtents</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">dexts</span><span class="p">)</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">pointer</span> <span class="n">ptr</span><span class="p">,</span> <span class="n">layout_mapping</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">l</span><span class="p">)</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">pointer</span> <span class="n">ptr</span><span class="p">,</span> <span class="n">layout_mapping</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">l</span><span class="p">)</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">mdspan</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
    <span class="k">constexpr</span> <span class="n">mdspan</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
    <span class="k">constexpr</span> <span class="n">mdspan</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>

  <span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
    <span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
    <span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>

  <span class="o">~</span><span class="n">mdspan</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>

  
  <span class="c1">// [views.mdspan.domobs], domain observers
</span>  <span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank_dynamic</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="nf">is_dynamic_dimension</span><span class="p">(</span><span class="n">rank_type</span> <span class="n">r</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">extent</span><span class="p">(</span><span class="n">rank_type</span> <span class="n">r</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">size</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">length</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
  <span class="k">constexpr</span> <span class="kt">bool</span> <span class="nf">empty</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="c1">// [views.mdspan.codobs], codomain observers
</span>  <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">span</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="c1">// [views.mdspan.mapobs], mapping observers
</span>  <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">stride</span><span class="p">(</span><span class="n">rank_type</span> <span class="n">r</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="k">constexpr</span> <span class="n">layout_mapping</span> <span class="nf">mapping</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>

  <span class="c1">// [views.mdspan.elem], element access
</span>  <span class="k">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Indices</span><span class="o">></span>
    <span class="n">reference</span> <span class="k">operator</span><span class="p">(</span><span class="p">)</span><span class="p">(</span><span class="n">Indices</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">is</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>

  <span class="k">constexpr</span> <span class="n">pointer</span> <span class="nf">data</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="p">}</span><span class="p">;</span>
</dimensions></pre>
     <ol>
      <li data-md="">
       <p>An <code class="highlight"><span class="n">mdspan</span></code> is a multidimensional view over a contiguous sequence of
  objects, the storage of which is owned by some other object.</p>
      <li data-md="">
       <p><code class="highlight"><span class="n">ElementType</span></code> is required to be a complete object that is not an abstract
  class type.</p>
      <li data-md="">
       <p>In the following section:</p>
       <ul>
        <li data-md="">
         <p>Let <code class="highlight"><span class="n">i</span></code> be in the range <code class="highlight"><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">(</span><span class="n">Properties</span><span class="p">)</span><span class="p">)</span></code> in order and</p>
        <li data-md="">
         <p><code class="highlight"><span class="n">P</span><sup><span class="n">i</span></sup></code> be the <code class="highlight"><span class="n">i</span></code>th type in <code class="highlight"><span class="n">Properties</span></code>,</p>
        <li data-md="">
         <p>Let <code class="highlight"><span class="n">r</span></code> be in the range <code class="highlight"><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="n">rank</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> in order and</p>
        <li data-md="">
         <p><code class="highlight"><span class="n">Idx</span><sup><span class="n">r</span></sup></code> be the <code class="highlight"><span class="n">r</span></code>th type in a template parameter
  pack named <code class="highlight"><span class="n">Indices</span></code>,</p>
        <li data-md="">
         <p>Let <code class="highlight"><span class="n">rd</span></code> be in the range <code class="highlight"><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="n">rank_dynamic</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> in order, and</p>
        <li data-md="">
         <p><code class="highlight"><span class="n">DExt</span><sup><span class="n">rd</span></sup></code> be the <code class="highlight"><span class="n">rd</span></code>th type in a template
  parameter pack named <code class="highlight"><span class="n">DynamicExtents</span></code>.</p>
       </ul>
      <li data-md="">
       <p><code class="highlight"><span class="n">is_layout_v</span><span class="o">&lt;</span><span class="n">P</span><sup><span class="n">i</span></sup><span class="o">></span></code> shall be true for <code class="highlight"><span class="n">i</span></code>. The <code class="highlight"><span class="n">layout</span></code> type is the type in <code class="highlight"><span class="n">P</span><sup><span class="n">i</span></sup></code> for which <code class="highlight"><span class="n">is_layout_v</span><span class="o">&lt;</span><span class="n">P</span><sup><span class="n">i</span></sup><span class="o">></span> <span class="o">=</span><span class="o">=</span> true</code>. If <code class="highlight"><span class="k">sizeof</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">(</span><span class="n">Properties</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="mi">0</span></code> or <code class="highlight"><span class="n">is_layout_v</span><span class="o">&lt;</span><span class="n">P</span><sup><span class="n">i</span></sup><span class="o">></span> <span class="o">=</span><span class="o">=</span> false</code> for all <code class="highlight"><span class="n">i</span></code>, then the <code class="highlight"><span class="n">layout</span></code> type is <code class="highlight"><span class="n">layout_right</span></code>.</p>
      <li data-md="">
       <p>If <code class="highlight"><span class="n">is_mdspan_property_v</span><span class="o">&lt;</span><span class="n">P</span><sup><span class="n">i</span></sup><span class="o">></span></code> is false for any <code class="highlight"><span class="n">i</span></code>, then <code class="highlight"><span class="n">mdspan</span></code> is ill-formed.</p>
     </ol>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.mdspan.cons]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">mdspan</span></code> <strong>Constructors, Assignment and Destructors [views.mdspan.cons]</strong> 
<pre class="highlight"><span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">nullptr_t</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Remarks:</em> If <code class="highlight"><span class="o">!</span><span class="n">is_dynamic_dimension</span><span class="p">(</span><span class="n">r</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">extent</span><span class="p">(</span><span class="n">r</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="mi">0</span></code> then the program is
ill-formed.</p>
     <p><em>Effects:</em> Constructs an empty <code class="highlight"><span class="n">mdspan</span></code>.</p>
     <p><em>Postconditions:</em> <code class="highlight"><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="mi">0</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="k">nullptr</span></code></p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">DynamicExtents</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">mdspan</span><span class="p">(</span><span class="n">pointer</span> <span class="n">ptr</span><span class="p">,</span> <span class="n">DynamicExtents</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">dexts</span><span class="p">)</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="n">mdspan</span><span class="p">(</span><span class="n">ptr</span><span class="p">,</span> <span class="n">Dimensions</span><span class="p">(</span><span class="n">dexts</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span><span class="p">)</span></code>.</p>
     <p><em>Complexity:</em> Constant.</p>
<pre class="highlight"><span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">pointer</span> <span class="n">ptr</span><span class="p">,</span> <span class="n">layout_mapping</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">l</span><span class="p">)</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">pointer</span> <span class="n">ptr</span><span class="p">,</span> <span class="n">layout_mapping</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">l</span><span class="p">)</span><span class="p">;</span>
</pre>
     <p><em>Requires:</em></p>
     <ul>
      <li data-md="">
       <p><code class="highlight"><span class="n">l</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">></span><span class="o">=</span> <span class="mi">0</span></code>.</p>
      <li data-md="">
       <p>If <code class="highlight"><span class="n">ptr</span></code> is null, then <code class="highlight"><span class="n">l</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span></code> shall be 0.</p>
      <li data-md="">
       <p>If <code class="highlight"><span class="n">ptr</span></code> is not null, <code class="highlight"><span class="n">ptr</span></code> shall point to the beginning of a valid sequence
of objects of at least <code class="highlight"><span class="n">l</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span></code> length.</p>
     </ul>
     <p><em>Effects:</em> Constructs an <code class="highlight"><span class="n">mdspan</span></code> that is a view over the sequence of objects
pointed to be <code class="highlight"><span class="n">ptr</span></code> with mapping <code class="highlight"><span class="n">l</span></code>. If <code class="highlight"><span class="n">ptr</span></code> is null or <code class="highlight"><span class="n">l</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span></code> is 0, then
an empty <code class="highlight"><span class="n">mdspan</span></code> is constructed.</p>
     <p><em>Complexity:</em> Constant.</p>
     <p><em>Throws:</em> Nothing.</p>
<pre class="highlight"><span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">mdspan</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="nf">mdspan</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Constructs an <code class="highlight"><span class="n">mdspan</span></code> by copying from another <code class="highlight"><span class="n">mdspan</span></code>.</p>
     <p><em>Postconditions:</em> <code class="highlight"><span class="n">other</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">.</span><span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">.</span><span class="n">mapping</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">mapping</span><span class="p">(</span><span class="p">)</span></code></p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">mdspan</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
  <span class="k">constexpr</span> <span class="n">mdspan</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
</pre>
     <p><em>Remarks:</em> This constructor shall not participate in overload resolution unless <code class="highlight"><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">pointer</span></code> is implicitly
convertible to <code class="highlight"><span class="n">pointer</span></code>.</p>
     <p><em>Requires:</em> If <code class="highlight"><span class="o">!</span><span class="n">other</span><span class="p">.</span><span class="n">is_dynamic_dimension</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code> then <code class="highlight"><span class="n">other</span><span class="p">.</span><span class="n">extent</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code> shall be
equal to <code class="highlight"><span class="n">extent</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code>, for all <code class="highlight"><span class="n">r</span></code>.</p>
     <p><em>Effects:</em> Constructs an <code class="highlight"><span class="n">mdspan</span></code> by copying from another <code class="highlight"><span class="n">mdspan</span></code>.</p>
     <p><em>Postconditions:</em> <code class="highlight"><span class="n">other</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="k">reinterpret_cast</span><span class="o">&lt;</span><span class="n">pointer</span><span class="o">></span><span class="p">(</span><span class="n">other</span><span class="p">.</span><span class="n">data</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">.</span><span class="n">mapping</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">mapping</span><span class="p">(</span><span class="p">)</span></code></p>
     <p><em>Complexity:</em> Constant.</p>
     <p><em>Throws:</em> Nothing.</p>
<pre class="highlight"><span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
<span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Assigns from one <code class="highlight"><span class="n">mdspan</span></code> to another.</p>
     <p><em>Postconditions:</em> <code class="highlight"><span class="n">other</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">.</span><span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">.</span><span class="n">mapping</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">mapping</span><span class="p">(</span><span class="p">)</span></code></p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
  <span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">UElementType</span><span class="p">,</span> <span class="k">class</span> <span class="nc">UDimensions</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">UProperties</span><span class="o">></span>
  <span class="n">mdspan</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span><span class="p">;</span>
</pre>
     <p><em>Remarks:</em> This operator shall not participate in overload resolution unless <code class="highlight"><span class="n">mdspan</span><span class="o">&lt;</span><span class="n">UElementType</span><span class="p">,</span> <span class="n">UDimensions</span><span class="p">,</span> <span class="n">UProperties</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">pointer</span></code> is implicitly
convertible to <code class="highlight"><span class="n">pointer</span></code>.</p>
     <p><em>Requires:</em> If <code class="highlight"><span class="o">!</span><span class="n">other</span><span class="p">.</span><span class="n">is_dynamic_dimension</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code> then <code class="highlight"><span class="n">other</span><span class="p">.</span><span class="n">extent</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code> shall be
equal to <code class="highlight"><span class="n">extent</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code>, for all <code class="highlight"><span class="n">r</span></code>.</p>
     <p><em>Effects:</em> Assigns from one <code class="highlight"><span class="n">mdspan</span></code> to another.</p>
     <p><em>Postconditions:</em> <code class="highlight"><span class="n">other</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">size</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">.</span><span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">data</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">.</span><span class="n">mapping</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">mapping</span><span class="p">(</span><span class="p">)</span></code></p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.mdspan.domobs]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">mdspan</span></code> <strong>Domain Observers [views.mdspan.domobs]</strong> 
<pre class="highlight"><span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="n">layout_mapping</span><span class="o">:</span><span class="o">:</span><span class="n">rank</span><span class="p">(</span><span class="p">)</span><span class="p">;</span></code></p>
<pre class="highlight"><span class="k">static</span> <span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">rank_dynamic</span><span class="p">(</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="n">layout_mapping</span><span class="o">:</span><span class="o">:</span><span class="n">rank_dynamic</span><span class="p">(</span><span class="p">)</span><span class="p">;</span></code></p>
<pre class="highlight"><span class="k">constexpr</span> <span class="kt">bool</span> <span class="nf">is_dynamic_dimension</span><span class="p">(</span><span class="n">rank_type</span> <span class="n">r</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">mapping</span><span class="p">(</span><span class="p">)</span><span class="p">.</span><span class="n">is_dynamic_dimension</span><span class="p">(</span><span class="n">r</span><span class="p">)</span><span class="p">;</span></code></p>
<pre class="highlight"><span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">extent</span><span class="p">(</span><span class="n">rank_type</span> <span class="n">r</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">mapping</span><span class="p">(</span><span class="p">)</span><span class="p">[</span><span class="n">r</span><span class="p">]</span><span class="p">;</span></code></p>
<pre class="highlight"><span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">size</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">length</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">mapping</span><span class="p">(</span><span class="p">)</span><span class="p">.</span><span class="n">size</span><span class="p">(</span><span class="p">)</span><span class="p">;</span></code></p>
<pre class="highlight"><span class="k">constexpr</span> <span class="kt">bool</span> <span class="nf">empty</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">size</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="mi">0</span><span class="p">;</span></code></p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.mdspan.codobs]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">mdspan</span></code> <strong>Codomain Observers [views.mdspan.codobs]</strong> 
<pre class="highlight"><span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">span</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">mapping</span><span class="p">(</span><span class="p">)</span><span class="p">.</span><span class="n">span</span><span class="p">(</span><span class="p">)</span><span class="p">;</span></code></p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.mdspan.mapobs]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">mdspan</span></code> <strong>Mapping Observers [views.mdspan.mapobs]</strong> 
<pre class="highlight"><span class="k">constexpr</span> <span class="n">index_type</span> <span class="nf">stride</span><span class="p">(</span><span class="n">rank_type</span> <span class="n">r</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Remarks:</em> This function shall not participate in overload resolution if <code class="highlight"><span class="n">layout_mapping</span><span class="o">:</span><span class="o">:</span><span class="n">is_always_strided</span><span class="p">(</span><span class="n">r</span><span class="p">)</span></code> is true for all r.</p>
     <p><em>Effects:</em> Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">mapping</span><span class="p">(</span><span class="p">)</span><span class="p">.</span><span class="n">stride</span><span class="p">(</span><span class="n">r</span><span class="p">)</span><span class="p">;</span></code></p>
<pre class="highlight"><span class="k">constexpr</span> <span class="n">layout_mapping</span> <span class="nf">mapping</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Returns a copy of the <code class="highlight"><span class="n">mdspan</span></code>'s layout mapping.</p>
    </ins>
   </blockquote>
   <p>Add the following section, 23.7.� [views.mdspan.mapobs]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.�</strong> <code class="highlight"><span class="n">mdspan</span></code> <strong>Element Access [views.mdspan.elem]</strong> 
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Indices</span><span class="o">></span>
  <span class="n">reference</span> <span class="k">operator</span><span class="p">(</span><span class="p">)</span><span class="p">(</span><span class="n">Indices</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">is</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> Returns a reference to the element at position <code class="highlight"><span class="n">mapping</span><span class="p">(</span><span class="p">)</span><span class="p">(</span><span class="n">is</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span></code>.</p>
     <p><em>Complexity:</em> Constant</p>
     <p><em>Throws:</em> Nothing</p>
<pre class="highlight"><span class="k">constexpr</span> <span class="n">pointer</span> <span class="nf">data</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span><span class="p">;</span>
</pre>
     <p><em>Effects:</em> If <code class="highlight"><span class="o">!</span><span class="n">empty</span><span class="p">(</span><span class="p">)</span></code>, returns a pointer to the first element in the
sequence accessible via the <code class="highlight"><span class="n">mdspan</span></code>. Otherwise, returns <code class="highlight"><span class="k">nullptr</span></code>.</p>
     <p><em>Complexity:</em> Constant</p>
    </ins>
   </blockquote>
   <h3 class="heading settled" data-level="2.4" id="laymap"><span class="secno">2.4. </span><span class="content">Layout Mapping Properties</span><a class="self-link" href="#laymap"></a></h3>
   <p>Add the following section, 23.7.� [views.layout.props]:</p>
   <blockquote>
    <ins style="display: block;">
      <strong>23.7.� Layout Mapping Properties [views.layout.props]</strong> 
     <p>In the following section, let <code class="highlight"><span class="n">D</span></code> be an instantiation of the <code class="highlight"><span class="n">dimensions</span></code> class
template.</p>
     <p><code class="highlight"><span class="n">layout_right</span></code> shall be a layout ([views.layout.requirements]) such
that <code class="highlight"><span class="n">layout_right</span><span class="o">:</span><span class="o">:</span><span class="n">mapping</span><span class="o">&lt;</span><span class="n">D</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">.</span><span class="n">stride</span><span class="p">(</span><span class="n">D</span><span class="o">:</span><span class="o">:</span><span class="n">rank</span><span class="p">(</span><span class="p">)</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="mi">1</span></code>.</p>
     <p><code class="highlight"><span class="n">layout_left</span></code> shall be a layout such that <code class="highlight"><span class="n">layout_left</span><span class="o">:</span><span class="o">:</span><span class="n">mapping</span><span class="o">&lt;</span><span class="n">D</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">.</span><span class="n">stride</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="mi">1</span></code>.</p>
    </ins>
   </blockquote>
   <h2 class="heading settled" data-level="3" id="test"><span class="secno">3. </span><span class="content">Feature Testing</span><a class="self-link" href="#test"></a></h2>
   <p>The <code class="highlight"><span class="n">__cpp_lib_mdspan</span></code> feature test macro should be added.</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-n4567">[N4567]
   <dd>Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4567.pdf">Working Draft, Standard for Programming Language C++ Note:</a>. 9 November 2015. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4567.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4567.pdf</a>
   <dt id="biblio-p0122r3">[P0122R3]
   <dd>Neil MacIntosh. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0122r3.pdf">span: bounds-safe views for sequences of objects</a>. 10 July 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0122r3.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0122r3.pdf</a>
  </dl>