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  <title>P2769R3: get_element customization point object</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

	/* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

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

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
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    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
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    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 6270e4735, updated Tue Aug 6 12:12:30 2024 -0700" name="generator">
  <link href="https://wg21.link/P2769R3" rel="canonical">
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  <meta content="8e2ffc70e15d0c465f3cc104a7ac19a76f5f0855" name="revision">
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<style>
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2769R3<br>get_element customization point object</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-10-16">2024-10-16</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P2769R3">https://wg21.link/P2769R3</a>
     <dt class="editor">Authors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ruslan.arutyunyan@intel.com">Ruslan Arutyunyan</a> (<span class="p-org org">Intel</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:alexey.kukanov@intel.com">Alexey Kukanov</a> (<span class="p-org org">Intel</span>)
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper introduces a CPO to read elements of tuple like objects, and uses it to define a generic <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept.</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="#motivation"><span class="secno">1</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#true_motivation"><span class="secno">1.1</span> <span class="content">Allowing user-defined tuples, or "true" motivation :)</span></a>
      <li>
       <a href="#motivating_use_case"><span class="secno">1.2</span> <span class="content">The original motivating use case</span></a>
       <ol class="toc">
        <li><a href="#projection_alternative"><span class="secno">1.2.1</span> <span class="content">Projections-based alternative</span></a>
        <li><a href="#desired_approach"><span class="secno">1.2.2</span> <span class="content">The desired approach</span></a>
        <li><a href="#not_views_elements"><span class="secno">1.2.3</span> <span class="content">Why not <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>views</c-><c- o>::</c-><c- n>elements</c-></code></span></a>
       </ol>
      <li><a href="#radix_sort"><span class="secno">1.3</span> <span class="content">Radix sort use case</span></a>
      <li><a href="#other_use_cases"><span class="secno">1.4</span> <span class="content">Other use cases</span></a>
     </ol>
    <li>
     <a href="#proposed_api"><span class="secno">2</span> <span class="content">Proposed API</span></a>
     <ol class="toc">
      <li><a href="#possible_implementation"><span class="secno">2.1</span> <span class="content">Possible implementation</span></a>
      <li>
       <a href="#tuple_like_concept"><span class="secno">2.2</span> <span class="content"><code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept</span></a>
       <ol class="toc">
        <li><a href="#tuple_like_with_get_element"><span class="secno">2.2.1</span> <span class="content"><code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept generalization with <code class="highlight"><c- n>get_element</c-></code></span></a>
       </ol>
     </ol>
    <li>
     <a href="#design_decisions"><span class="secno">3</span> <span class="content">Design considerations</span></a>
     <ol class="toc">
      <li><a href="#ranges_get"><span class="secno">3.1</span> <span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get</c-></code></span></a>
      <li><a href="#breaking_code"><span class="secno">3.2</span> <span class="content">Breaking code example</span></a>
      <li><a href="#get_element_std_variant"><span class="secno">3.3</span> <span class="content"><code class="highlight"><c- n>get_element</c-></code> for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>variant</c-></code> and other non-tuple-like types</span></a>
     </ol>
    <li>
     <a href="#other_papers_connections"><span class="secno">4</span> <span class="content">Connections with other papers</span></a>
     <ol class="toc">
      <li><a href="#p2547_connection"><span class="secno">4.1</span> <span class="content">Connection with <span title="Language support for customisable functions">[P2547R1]</span></span></a>
      <li><a href="#p2141_connection"><span class="secno">4.2</span> <span class="content">Connection with <span title="Aggregates are named tuples">[P2141R1]</span></span></a>
     </ol>
    <li><a href="#further_work"><span class="secno">5</span> <span class="content">Further discussion</span></a>
    <li>
     <a href="#wording"><span class="secno">6</span> <span class="content">Formal wording</span></a>
     <ol class="toc">
      <li><a href="#wording_tuple_like"><span class="secno">6.1</span> <span class="content">Modify Concept <i>tuple-like</i> <span>[<strong>tuple.like</strong>] </span></span></a>
      <li><a href="#wording_tuple_syn"><span class="secno">6.2</span> <span class="content">Modify Header <code class="highlight"><c- o>&lt;</c-><c- n>tuple</c-><c- o>></c-></code> synopsis <span>[<strong>tuple.syn</strong>] </span></span></a>
      <li><a href="#wording_tuple_sections"><span class="secno">6.3</span> <span class="content">Add the following sections into <span>[<strong>tuple</strong>]</span></span></a>
      <li><a href="#wording_tuple_get_elem"><span class="secno">6.4</span> <span class="content">Add the following wording into [<strong>tuple.cust.get_elem</strong>]</span></a>
      <li><a href="#wording_feature_macro"><span class="secno">6.5</span> <span class="content">Add feature test macro to the end of <span>[<strong>version.syn</strong>]</span></span></a>
      <li><a href="#modify_constructor"><span class="secno">6.6</span> <span class="content">Modify <code class="highlight"><c- n>tuple</c-></code> construct <span>[<strong>tuple.cnstr</strong>]</span></span></a>
      <li><a href="#modify_assignment"><span class="secno">6.7</span> <span class="content">Modify <code class="highlight"><c- n>tuple</c-></code> assignment <span>[<strong>tuple.assign</strong>]</span></span></a>
      <li><a href="#modify_tuple_cat"><span class="secno">6.8</span> <span class="content">Modify <code class="highlight"><c- n>tuple_cat</c-></code> in tuple creation <span>[<strong>tuple.creation</strong>]</span></span></a>
      <li><a href="#modify_tuple_apply"><span class="secno">6.9</span> <span class="content">Modify <code class="highlight"><c- n>apply</c-></code> in <span>[<strong>tuple.apply</strong>]</span></span></a>
      <li><a href="#modify_tuple_rel"><span class="secno">6.10</span> <span class="content">Modify relation operators in <span>[<strong>tuple.rel</strong>]</span></span></a>
      <li><a href="#modify_range_elements_iterator"><span class="secno">6.11</span> <span class="content">Modify <span>[<strong>range.elements.iterator</strong>]</span></span></a>
      <li><a href="#allocator_uses_construction"><span class="secno">6.12</span> <span class="content">Modify <code class="highlight"><c- n>uses_allocator_construction_args</c-></code> in <span>[<strong>allocator.uses.construction</strong>]</span></span></a>
      <li><a href="#pairs_pair"><span class="secno">6.13</span> <span class="content">Modify <code class="highlight"><c- n>pair</c-></code> in <span>[<strong>pairs.pair</strong>]</span></span></a>
      <li><a href="#mdspan_sub_helpers"><span class="secno">6.14</span> <span class="content">Modify "Exposition-only helpers" in <span>[<strong>mdspan.sub.helpers</strong>]</span></span></a>
     </ol>
    <li>
     <a href="#revision_history"><span class="secno">7</span> <span class="content">Revision history</span></a>
     <ol class="toc">
      <li><a href="#r2_r3"><span class="secno">7.1</span> <span class="content">R2 => R3</span></a>
      <li><a href="#r1_r2"><span class="secno">7.2</span> <span class="content">R1 => R2</span></a>
      <li><a href="#r0_r1"><span class="secno">7.3</span> <span class="content">R0 => R1</span></a>
     </ol>
    <li>
     <a href="#polls"><span class="secno">8</span> <span class="content">Polls</span></a>
     <ol class="toc">
      <li><a href="#sg9_issaquah_2023"><span class="secno">8.1</span> <span class="content">SG9 polls, Issaquah 2023</span></a>
      <li><a href="#lewg_telecon_2024_01_23"><span class="secno">8.2</span> <span class="content">Library Evolution Telecon 2024-01-23</span></a>
      <li><a href="#polls_lewg_st_louis_2024"><span class="secno">8.3</span> <span class="content">Library Evolution, St. Louis 2024</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">9</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="motivation"><span class="secno">1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="true_motivation"><span class="secno">1.1. </span><span class="content">Allowing user-defined tuples, or "true" motivation :)</span><a class="self-link" href="#true_motivation"></a></h3>
   <p>The section addresses the LEWG feedback. While we always had in mind that with the <code class="highlight"><c- n>get_element</c-></code> facility we can make a generic concept for <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> types, we assumed it to be developed as a separate
proposal. However, based on the LEWG poll (<a href="#lewg_telecon_2024_01_23">§ 8.2 Library Evolution Telecon 2024-01-23</a>), the <code class="highlight"><c- n>get_element</c-></code> paper should enable
user-defined tuples to make its motivation strong enough.</p>
   <p>So, let’s go to the problem statement. Today the C++ standard defines <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> types as only 5 types
from the <code class="highlight"><c- n>std</c-></code> namespace:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pair</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>complex</c-></code>(since C++26)</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code></p>
   </ul>
   <p>That sounds like a huge limitation for a generic library because, in principle, user-defined types could be treated like
tuples. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_size</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_element</c-></code> are already customizable by users. The problematic part is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> which is not a customization point.</p>
   <p>Furthermore, there is already partial support for user-defined tuples in the language. For example the <em>structured binding</em> language feature has special rules for finding a <code class="highlight"><c- n>get</c-></code> function for an arbitrary type (in a nutshell, either as a
non-static-member-function or by argument-dependent lookup).</p>
   <p>Unfortunately, rules are different in different places in the standard today. For example, <em>has-tuple-element</em> exposition-only concept for <code class="highlight"><c- n>elements_view</c-></code> allows only the 5 <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> types listed above and does not
consider user-defined types.</p>
   <p><a data-link-type="biblio" href="#biblio-p2165r4" title="Compatibility between tuple, pair and tuple-like objects">[P2165R4]</a> added constraints for existing APIs (like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>apply</c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple_cat</c-></code>, etc.) to take <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> and also provides better compatibility between <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> objects
by adding extra APIs, which is great. The unfortunate part of the story, however, is that the mentioned APIs are still
limited by the definition of the <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept (the 5 standard types).</p>
   <p>In this paper, we proposed a new customization point object, <code class="highlight"><c- n>get_element</c-></code>, which can be used to extend the tuple protocol
to user-defined types. Since this facility uses argument-dependent lookup for <code class="highlight"><c- n>get</c-></code>, using it to redefine <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> should not be a breaking change for the vast majority of the code.
See the example in <a href="#breaking_code">§ 3.2 Breaking code example</a> section for more details.</p>
   <p>For the following (simplified) code snippet:</p>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- nn>user</c-> <c- p>{</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>struct</c-> <c- nc>my_tuple_like</c->
<c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- n>my_tuple_like</c-><c- p>(</c-><c- n>T</c-> <c- n>tt</c-><c- p>,</c-> <c- n>U</c-> <c- n>uu</c-><c- p>)</c-> <c- o>:</c-> <c- n>t</c-><c- p>(</c-><c- n>tt</c-><c- p>),</c-> <c- n>u</c-><c- p>(</c-><c- n>uu</c-><c- p>)</c-> <c- p>{}</c->
<c- k>private</c-><c- o>:</c->
    <c- n>T</c-> <c- n>t</c-><c- p>;</c->
    <c- n>U</c-> <c- n>u</c-><c- p>;</c->

    <c- k>template</c-> <c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>I</c-><c- o>></c->
    <c- k>friend</c-> <c- k>auto</c-> <c- n>get</c-><c- p>(</c-><c- n>my_tuple_like</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>></c-> <c- n>t_like</c-><c- p>)</c->
    <c- p>{</c->
        <c- k>static_assert</c-> <c- p>(</c-><c- n>I</c-> <c- o>==</c-> <c- mi>0</c-> <c- o>||</c-> <c- n>I</c-> <c- o>==</c-> <c- mi>1</c-><c- p>);</c->
        <c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c-><c- n>I</c-> <c- o>==</c-> <c- mi>0</c-><c- p>)</c->
            <c- k>return</c-> <c- n>t_like</c-><c- p>.</c-><c- n>t</c-><c- p>;</c->
        <c- k>else</c-> <c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c-><c- n>I</c-> <c- o>==</c-> <c- mi>1</c-><c- p>)</c->
            <c- k>return</c-> <c- n>t_like</c-><c- p>.</c-><c- n>u</c-><c- p>;</c->
    <c- p>}</c->
<c- p>};</c->

<c- p>}</c-> <c- c1>// namespace user</c->

<c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>user</c-><c- o>::</c-><c- n>my_tuple_like</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>>></c-> <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>integral_constant</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>,</c-> <c- mi>2</c-><c- o>></c-> <c- p>{};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- mi>0</c-><c- p>,</c-> <c- n>user</c-><c- o>::</c-><c- n>my_tuple_like</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>>></c-> <c- p>{</c->
    <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->
<c- p>};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>struct</c-> <c- nc>tuple_element</c-><c- o>&lt;</c-><c- mi>1</c-><c- p>,</c-> <c- n>user</c-><c- o>::</c-><c- n>my_tuple_like</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>>></c-> <c- p>{</c->
    <c- k>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>U</c-><c- p>;</c->
<c- p>};</c->

<c- p>}</c-> <c- c1>// namespace std</c->
</pre>
   <p>please see the Before-After table</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- p>[</c-><c- n>x</c-><c- p>,</c-> <c- n>y</c-><c- p>]</c-> <c- o>=</c-> <c- n>user</c-><c- o>::</c-><c- n>my_tuple_like</c-><c- p>{</c-><c- mi>3</c-><c- p>,</c-><c- mi>3</c-><c- p>};</c->

<c- c1>// This code does not compile</c->
<c- c1>// std::apply([](auto x, auto y) {</c->
<c- c1>//     return x + y;</c->
<c- c1>// }, user::my_tuple_like{3,3});</c->
</pre>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- p>[</c-><c- n>x</c-><c- p>,</c-> <c- n>y</c-><c- p>]</c-> <c- o>=</c-> <c- n>user</c-><c- o>::</c-><c- n>my_tuple_like</c-><c- p>{</c-><c- mi>3</c-><c- p>,</c-><c- mi>3</c-><c- p>};</c->

<c- c1>// Works fine, assuming that std::apply uses std::get_element</c->
<c- n>std</c-><c- o>::</c-><c- n>apply</c-><c- p>([](</c-><c- k>auto</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- n>y</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c->
<c- p>},</c-> <c- n>user</c-><c- o>::</c-><c- n>my_tuple_like</c-><c- p>{</c-><c- mi>3</c-><c- p>,</c-><c- mi>3</c-><c- p>});</c->
</pre>
   </table>
   <p>Of course, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>apply</c-></code> is just an example. <code class="highlight"><c- n>my_tuple_like</c-></code> would work with any API that supports <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> types.</p>
   <h3 class="heading settled" data-level="1.2" id="motivating_use_case"><span class="secno">1.2. </span><span class="content">The original motivating use case</span><a class="self-link" href="#motivating_use_case"></a></h3>
   <p>Having <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pair</c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tuple</c-></code> and other <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> types as the value types for algorithms
creates a plenty of opportunities. With special views, such as <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>elements_view</c-></code>, we can
specify which tuple elements to access when iterating over collections of such objects. However,
we cannot easily use a predicate to make a decision based on only some of tuple elements, for example keys or values.</p>
   <p>Let’s consider the following example:</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- n>y</c-><c- p>)</c->
<c- p>{</c->
    <c- c1>// key-based sorting</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>)</c-> <c- o>&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>y</c-><c- p>);</c->
<c- p>});</c->
</pre>
   <p>As we can see, users should spell some extra syntax out to achieve the necessary goal,
comparing to what is described in <a href="#desired_approach">§ 1.2.2 The desired approach</a>. The example above can be considered
simplified; in real practice users might also need to think of e.g. adding references
to lambda parameters to avoid copying.</p>
   <p>The code above can be rewritten with <em>structured binding</em>:</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- n>y</c-><c- p>)</c->
<c- p>{</c->
    <c- c1>// key-based sorting</c->
    <c- k>auto</c-> <c- p>[</c-><c- n>key1</c-><c- p>,</c-> <c- n>value1</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-><c- p>;</c->
    <c- k>auto</c-> <c- p>[</c-><c- n>key2</c-><c- p>,</c-> <c- n>value2</c-><c- p>]</c-> <c- o>=</c-> <c- n>y</c-><c- p>;</c->
    <c- k>return</c-> <c- n>key1</c-> <c- o>&lt;</c-> <c- n>key2</c-><c- p>;</c->
<c- p>});</c->
</pre>
   <p>Though one could say that it makes code simpler or at least more readable, on the other hand,
its syntax forces the programmer to give names to otherwise unneeded variables, which is often
considered a bad practice.</p>
   <p>With <a data-link-type="biblio" href="#biblio-p2169r3" title="A Nice Placeholder With No Name">[P2169R3]</a> the situation with unused variables for structured binding becomes better but
still might require the user to write a quite amount of underscores depending on the use case:</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>,</c-><c- mi>1</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>,</c-><c- mi>4</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>,</c-><c- mi>7</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- n>y</c-><c- p>)</c->
<c- p>{</c->
    <c- c1>// key-based sorting</c->
    <c- k>auto</c-> <c- p>[</c-><c- n>key1</c-><c- p>,</c-> <c- n>_</c-><c- p>,</c-> <c- n>_</c-><c- p>,</c-> <c- n>_</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-><c- p>;</c->
    <c- k>auto</c-> <c- p>[</c-><c- n>key2</c-><c- p>,</c-> <c- n>_</c-><c- p>,</c-> <c- n>_</c-><c- p>,</c-> <c- n>_</c-><c- p>]</c-> <c- o>=</c-> <c- n>y</c-><c- p>;</c->
    <c- k>return</c-> <c- n>key1</c-> <c- o>&lt;</c-> <c- n>key2</c-><c- p>;</c->
<c- p>});</c->
</pre>
   <h4 class="heading settled" data-level="1.2.1" id="projection_alternative"><span class="secno">1.2.1. </span><span class="content">Projections-based alternative</span><a class="self-link" href="#projection_alternative"></a></h4>
   <p>Projections provide another option to achieve the same behavior:</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>{},</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>)</c->
<c- p>{</c->
    <c- c1>// key-based sorting</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>);</c->
<c- p>});</c->
</pre>
   <p>A variant that properly handles references would use a generic lambda:</p>
<pre class="highlight"><c- p>[](</c-><c- k>auto</c-><c- o>&amp;&amp;</c-> <c- n>x</c-><c- p>)</c-> <c- o>-></c-> <c- k>auto</c-><c- o>&amp;&amp;</c->
<c- p>{</c->
    <c- c1>// key-based sorting</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><c- n>x</c-><c- p>)</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>));</c->
<c- p>}</c->
</pre>
   <p>While this code achieves the desired result, it requires more syntactic boilerplate
(lambda, forwarding etc.) than the useful code.</p>
   <h4 class="heading settled" data-level="1.2.2" id="desired_approach"><span class="secno">1.2.2. </span><span class="content">The desired approach</span><a class="self-link" href="#desired_approach"></a></h4>
   <p>The nicest way to get what we want would be:</p>
<pre class="language-cpp highlight"><c- c1>// The code that does not work because std::get is not fully instantiated</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>{},</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>);</c->
</pre>
   <p>But it doesn’t work because <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> is a function template, and one cannot pass function
templates as arguments without instantiating them.</p>
   <h4 class="heading settled" data-level="1.2.3" id="not_views_elements"><span class="secno">1.2.3. </span><span class="content">Why not <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>views</c-><c- o>::</c-><c- n>elements</c-></code></span><a class="self-link" href="#not_views_elements"></a></h4>
   <p>The necessary result cannot be achieved with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>views</c-><c- o>::</c-><c- n>elements</c-></code>, which
would apply the filter for all operations on the input data, including element swap
(for sort algorithm), while we need it to be only be applied for the comparator.</p>
   <table>
    <tbody>
     <tr>
      <th>std::ranges::views::elements
      <th>Desired behavior
     <tr>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- c1>// views::keys is an alias to views::elements</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-> <c- o>|</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>views</c-><c- o>::</c-><c- n>keys</c-><c- p>,</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- n>y</c-><c- p>)</c->
<c- p>{</c->
    <c- k>return</c-> <c- n>x</c-> <c- o>&lt;</c-> <c- n>y</c-><c- p>;</c->
<c- p>});</c->

<c- k>for</c-> <c- p>(</c-><c- k>auto</c-><c- o>&amp;</c-> <c- n>x</c-> <c- o>:</c-> <c- n>v</c-><c- p>)</c->
<c- p>{</c->
    <c- k>auto</c-> <c- p>[</c-><c- n>key</c-><c- p>,</c-> <c- n>val</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"Key = "</c-> <c- o>&lt;&lt;</c-> <c- n>key</c-> <c- o>&lt;&lt;</c-> <c- s>", Value = "</c-> <c- o>&lt;&lt;</c-> <c- n>val</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
<c- p>}</c->

<c- c1>// Output (only keys are sorted):</c->
<c- c1>// Key = 1, Value = 1</c->
<c- c1>// Key = 2, Value = 4</c->
<c- c1>// Key = 3, Value = 7</c->
</pre>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->

<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- n>y</c-><c- p>)</c->
<c- p>{</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>)</c-> <c- o>&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>y</c-><c- p>);</c-> <c- c1>// key-based sorting</c->
<c- p>});</c->

<c- k>for</c-> <c- p>(</c-><c- k>auto</c-><c- o>&amp;</c-> <c- n>x</c-> <c- o>:</c-> <c- n>v</c-><c- p>)</c->
<c- p>{</c->
    <c- k>auto</c-> <c- p>[</c-><c- n>key</c-><c- p>,</c-> <c- n>val</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"Key = "</c-> <c- o>&lt;&lt;</c-> <c- n>key</c-> <c- o>&lt;&lt;</c-> <c- s>", Value = "</c-> <c- o>&lt;&lt;</c-> <c- n>val</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
<c- p>}</c->

<c- c1>// Output (values are sorted based on keys):</c->
<c- c1>// Key = 1, Value = 7</c->
<c- c1>// Key = 2, Value = 4</c->
<c- c1>// Key = 3, Value = 1</c->
</pre>
   </table>
   <h3 class="heading settled" data-level="1.3" id="radix_sort"><span class="secno">1.3. </span><span class="content">Radix sort use case</span><a class="self-link" href="#radix_sort"></a></h3>
   <p>Counting-based sorts, and Radix Sort in particular, provide another motivating use case.
Today it is not possible to have a C++ standard conformant implementation that uses
Radix Sort algorithm underneath because the complexity of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sort</c-></code> is defined as
the number of comparator calls, while counting-based sorts do not use a comparator at all.</p>
   <p>However, the industry needs Radix Sort for performance reasons. Implementations of C++ standard
parallel algorithms, such as oneAPI Data Parallel C++ Library (oneDPL) and CUDA Thrust, use Radix Sort
conditionally under the hood of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>sort</c-></code>, checking data types of the input and the comparator.
In this case, a special comparator is of no help to sort values by keys, and projections seem the only viable option.</p>
   <p>That makes the proposed API applicable wider than just with the C++ standard library use cases.</p>
   <h3 class="heading settled" data-level="1.4" id="other_use_cases"><span class="secno">1.4. </span><span class="content">Other use cases</span><a class="self-link" href="#other_use_cases"></a></h3>
   <p>With the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>zip_view</c-></code> appearance in the standard the easy use of projection for <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> objects might become even more important because its dereferenceable type is exactly <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code>.</p>
   <p>Another example where it would be convenient to pass <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> to a function we discovered in <a href="https://brevzin.github.io/c++/2023/05/23/map-api/">a blog article</a> by Barry Revzin.
Let’s assume there is a function that returns <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> over a <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> type:</p>
<pre class="language-cpp highlight"><c- k>auto</c-> <c- n>find</c-><c- p>(</c-><c- n>Key</c-> <c- k>const</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-> <c- o>-></c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>pair</c-><c- o>&lt;</c-><c- n>Key</c-> <c- k>const</c-><c- p>,</c-> <c- n>Value</c-><c- o>></c-> <c- k>const</c-><c- o>&amp;></c-><c- p>;</c->
</pre>
   <p>Now imagine the caller wants to use only one element out of the returned <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> object, or some
default value in case the received <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> is empty. Applying <code class="highlight"><c- n>value_or</c-></code> directly to the result would require
providing default values for all tuple elements, so it makes sense to convert it first to a single-element optional
with <code class="highlight"><c- n>transform</c-></code>, and only then get the value with <code class="highlight"><c- n>value_or</c-></code>:</p>
<pre class="language-cpp highlight"><c- n>find</c-><c- p>(</c-><c- n>key</c-><c- p>).</c-><c- n>transform</c-><c- p>(</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>).</c-><c- n>value_or</c-><c- p>(</c-><c- mi>-1</c-><c- p>);</c->
</pre>
   <p>except, quoting the author, "of course that you can’t just pass <code class="highlight"><c- n>get</c-><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-></code> like that, so it’s not as easy at it should be."</p>
   <h2 class="heading settled" data-level="2" id="proposed_api"><span class="secno">2. </span><span class="content">Proposed API</span><a class="self-link" href="#proposed_api"></a></h2>
   <p>We propose the following API:</p>
<pre class="language-cpp highlight"><c- kr>inline</c-> <c- k>namespace</c-> <c- d>/* unspecified */</c->
<c- p>{</c->
    <c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- o>></c->
    <c- kr>inline</c-> <c- k>constexpr</c-> <c- d>/* unspecified */</c-> <c- n>get_element</c-> <c- o>=</c-> <c- d>/* unspecified */</c-><c- p>;</c->
<c- p>}</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- k>auto</c-> <c- n>get_key</c-> <c- o>=</c-> <c- n>get_element</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>;</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- k>auto</c-> <c- n>get_value</c-> <c- o>=</c-> <c- n>get_element</c-><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>;</c->
</pre>
   <p>With that API the motivating use case code with the desired behavior would be:</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>{},</c-> <c- n>std</c-><c- o>::</c-><c- n>get_element</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>);</c->
</pre>
   <p>or even</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>{},</c-> <c- n>std</c-><c- o>::</c-><c- n>get_key</c-><c- p>);</c->
</pre>
   <p>Let’s look at comparison tables (a.k.a. Tony Tables):</p>
   <p>Comparison of proposed API with comparator-based version</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>,</c-> <c- k>auto</c-> <c- n>y</c-><c- p>)</c->
<c- p>{</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>)</c-> <c- o>&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>y</c-><c- p>);</c-> <c- c1>// key-based sorting</c->
<c- p>});</c->
</pre>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>{},</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get_key</c-><c- p>);</c->
</pre>
   </table>
   <p>Comparison of proposed API with projections-based version</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>{},</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>)</c->
<c- p>{</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>);</c-> <c- c1>// key-based sorting</c->
<c- p>});</c->
</pre>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>v</c-><c- p>{{</c-><c- mi>3</c-><c- p>,</c-><c- mi>1</c-><c- p>},{</c-><c- mi>2</c-><c- p>,</c-><c- mi>4</c-><c- p>},{</c-><c- mi>1</c-><c- p>,</c-><c- mi>7</c-><c- p>}};</c->
<c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>sort</c-><c- p>(</c-><c- n>v</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>less</c-><c- p>{},</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get_key</c-><c- p>);</c->
</pre>
   </table>
   <h3 class="heading settled" data-level="2.1" id="possible_implementation"><span class="secno">2.1. </span><span class="content">Possible implementation</span><a class="self-link" href="#possible_implementation"></a></h3>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- nn>std</c->
<c- p>{</c->
<c- k>namespace</c-> <c- nn>__detail</c->
<c- p>{</c->
<c- k>template</c-> <c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>_Ip</c-><c- o>></c->
<c- k>struct</c-> <c- nc>__get_element_fn</c->
<c- p>{</c->
    <c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>_TupleLike</c-><c- o>></c-> <c- k>requires</c-> <c- k>requires</c-> <c- p>{</c->
        <c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>tuple_size</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>>>::</c-><c- n>type</c-><c- p>;</c->
    <c- p>}</c->
    <c- k>auto</c-> <c- k>operator</c-><c- p>()(</c-><c- n>_TupleLike</c-><c- o>&amp;&amp;</c-> <c- n>__tuple_like</c-><c- p>)</c-> <c- k>const</c-> <c- o>-></c->
         <c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c->
    <c- p>{</c->
        <c- k>constexpr</c-> <c- b>bool</c-> <c- n>__get_member_well_formed</c-> <c- o>=</c-> <c- k>requires</c-> <c- p>{</c->
            <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>></c-><c- p>(</c-><c- n>__tuple_like</c-><c- p>).</c-><c- k>template</c-> <c- n>get</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-><c- p>();</c->
        <c- p>};</c->
        <c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c-><c- n>__get_member_well_formed</c-><c- p>)</c->
        <c- p>{</c->
            <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>></c-><c- p>(</c-><c- n>__tuple_like</c-><c- p>).</c-><c- k>template</c-> <c- n>get</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-><c- p>();</c->
        <c- p>}</c->
        <c- k>return</c-> <c- n>get</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>></c-><c- p>(</c-><c- n>__tuple_like</c-><c- p>));</c->
    <c- p>}</c->
<c- p>};</c->
<c- p>}</c-> <c- c1>// namespace __detail</c->

<c- kr>inline</c-> <c- k>namespace</c-> <c- nn>__get_element_namespace</c->
<c- p>{</c->
<c- k>template</c-> <c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>_Ip</c-><c- o>></c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- n>__detail</c-><c- o>::</c-><c- n>__get_element_fn</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-> <c- n>get_element</c-><c- p>;</c->
<c- p>}</c-> <c- c1>// inline namespace __get_element_namespace</c->

<c- kr>inline</c-> <c- k>constexpr</c-> <c- k>auto</c-> <c- n>get_key</c-> <c- o>=</c-> <c- n>get_element</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>;</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- k>auto</c-> <c- n>get_value</c-> <c- o>=</c-> <c- n>get_element</c-><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>;</c->
<c- p>}</c-> <c- c1>// namespace std</c->
</pre>
   <h3 class="heading settled" data-level="2.2" id="tuple_like_concept"><span class="secno">2.2. </span><span class="content"><code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept</span><a class="self-link" href="#tuple_like_concept"></a></h3>
   <p>With the proposed <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get_element</c-></code> CPO, the <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept can be generalized to cover
wider range of types rather than just the listed standard types.</p>
   <h4 class="heading settled" data-level="2.2.1" id="tuple_like_with_get_element"><span class="secno">2.2.1. </span><span class="content"><code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept generalization with <code class="highlight"><c- n>get_element</c-></code></span><a class="self-link" href="#tuple_like_with_get_element"></a></h4>
   <p>With <code class="highlight"><c- n>get_element</c-></code> we can define an exposition only helper concept <em>can-get-tuple-element</em> in the following way:</p>
<pre class="highlight"><c- c1>// necessary to check if std::tuple_size_v is well-formed before using it</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>concept</c-> <i><c- nc>has</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>size</c-></i> <c- o>=</c-> <c- c1>// exposition only</c->
    <c- k>requires</c-> <c- p>{</c->
        <c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>type</c-><c- p>;</c->
    <c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>I</c-><c- o>></c->
<c- k>concept</c-> <i><c- nc>can</c-><c- o>-</c-><c- n>get</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>element</c-></i> <c- o>=</c-> <c- c1>// exposition only</c->
    <i><c- n>has</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>size</c-></i><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c->
    <c- k>requires</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c->
        <c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>;</c->
        <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>get_element</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>))</c-> <c- p>}</c-> <c- o>-></c->
            <c- n>std</c-><c- o>::</c-><c- n>convertible_to</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>>&amp;></c-><c- p>;</c->
    <c- p>};</c->
</pre>
   <p>Then the <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept can use <em>can-get-tuple-element</em> and do something like:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>concept</c-> <i><c- nc>tuple</c-><c- o>-</c-><c- n>like</c-></i> <c- o>=</c-> <c- c1>// exposition only</c->
    <i><c- n>has</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>size</c-></i><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c->
    <c- p>[]</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-><c- p>...</c-> <c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>index_sequence</c-><c- o>&lt;</c-><c- n>I</c-><c- p>...</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>return</c-> <c- p>(...</c-> <c- o>&amp;&amp;</c-> <i><c- n>can</c-><c- o>-</c-><c- n>get</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>element</c-></i><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>I</c-><c- o>></c-><c- p>);</c->
    <c- p>}</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>make_index_sequence</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>{});</c->

</pre>
   <h2 class="heading settled" data-level="3" id="design_decisions"><span class="secno">3. </span><span class="content">Design considerations</span><a class="self-link" href="#design_decisions"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="ranges_get"><span class="secno">3.1. </span><span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get</c-></code></span><a class="self-link" href="#ranges_get"></a></h3>
   <p>Alternative design was considered to name the proposed API <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get</c-></code>.</p>
   <p>Given that we discussed this option several times in C++ committee and it didn’t even have a weak support to pursue,
authors think that keeping the detailed information is no more necessary. Please see <a data-link-type="biblio" href="#biblio-p2769r2" title="get_element customization point object">[P2769R2]</a> for more information.</p>
   <h3 class="heading settled" data-level="3.2" id="breaking_code"><span class="secno">3.2. </span><span class="content">Breaking code example</span><a class="self-link" href="#breaking_code"></a></h3>
   <p>With the proposed API some user code might be broken because <code class="highlight"><c- n>get_element</c-></code> makes an ADL call underneath.
Consider the following example:</p>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- nn>my</c-> <c- p>{</c->
    <c- k>struct</c-> <c- nc>type</c-><c- p>{};</c->

    <c- k>template</c-> <c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>I</c-><c- p>,</c-> <c- k>typename</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
    <c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- n>get</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- n>Args</c-><c- p>...</c-><c- o>>&amp;</c-> <c- n>t</c-><c- p>)</c->
    <c- p>{</c->
        <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>);</c->
    <c- p>}</c->

<c- p>}</c->

<c- b>int</c-> <c- n>main</c-><c- p>()</c->
<c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- n>my</c-><c- o>::</c-><c- n>type</c-><c- o>></c-> <c- n>t</c-><c- p>;</c->

    <c- c1>// In the standard std::apply uses std::get</c->
    <c- c1>// If std::apply would use std::get_element it's compile-time error because ADL call within get_element</c->
    <c- c1>// will find both std::get and my::get, thus it ends up with ambiguity</c->
    <c- n>std</c-><c- o>::</c-><c- n>apply</c-><c- p>([](</c-><c- k>auto</c-><c- p>)</c-> <c- p>{</c-> <c- d>/* do something */</c-> <c- p>},</c-> <c- n>t</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <p>However, such potential break is very unlikely. In particular, there is no user-defined type involved in the <code class="highlight"><c- n>get</c-></code> overload in <code class="highlight"><c- n>my</c-></code> namespace. If such code in <code class="highlight"><c- n>my</c-></code> namespace was deployed, it’s likely to be already broken.
Indeed, while <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> is not a customization point, it is often used unqualified (and therefore found by ADL),
such that the code with in <code class="highlight"><c- n>my</c-></code> namespace already leads to ambiguity.</p>
   <p>This potential breaking change could be mitigated by checking in the <code class="highlight"><c- n>get_element</c-></code> implementation if the function argument type
is one of the four types <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> now works with (ignoring <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>complex</c-></code> supported only since C++26).
The implementation (with pseudo-code) is provided below for the sake of completeness.
However, the authors do not think any preventive measures are required because of the reasons mentioned above,
and therefore do <strong>not</strong> recommend this solution.</p>
<pre class="language-cpp highlight"><c- k>namespace</c-> <c- nn>std</c->
<c- p>{</c->
<c- k>namespace</c-> <c- nn>__detail</c->
<c- p>{</c->
<c- k>template</c-> <c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- b>size_t</c-> <c- n>_Ip</c-><c- o>></c->
<c- k>struct</c-> <c- nc>__get_element_fn</c->
<c- p>{</c->
    <c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>_TupleLike</c-><c- o>></c->
    <c- k>auto</c-> <c- k>operator</c-><c- p>()(</c-><c- n>_TupleLike</c-><c- o>&amp;&amp;</c-> <c- n>__tuple_like</c-><c- p>)</c-> <c- k>const</c-> <c- o>-></c->
         <c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c->
    <c- p>{</c->
        <c- k>constexpr</c-> <c- b>bool</c-> <c- n>__get_member_well_formed</c-> <c- o>=</c-> <c- k>requires</c-> <c- p>{</c->
            <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>></c-><c- p>(</c-><c- n>__tuple_like</c-><c- p>).</c-><c- k>template</c-> <c- n>get</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-><c- p>();</c->
        <c- p>};</c->
        <c- c1>// using __type = std::remove_cvref_t&lt;_TupleLike>'</c->
        <c- c1>// if constexpr (__type is std::tuple or std::pair or std::array or</c->
        <c- c1>//                 std::ranges::subrange)</c->
        <c- c1>// {</c->
        <c- c1>//     std::get&lt;_Ip>(std::forward&lt;_TupleLike>(__tuple_like));</c->
        <c- c1>// }</c->
        <c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c-><c- n>__get_member_well_formed</c-><c- p>)</c->
        <c- p>{</c->
            <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>></c-><c- p>(</c-><c- n>__tuple_like</c-><c- p>).</c-><c- k>template</c-> <c- n>get</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-><c- p>();</c->
        <c- p>}</c->
        <c- k>return</c-> <c- n>get</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>></c-><c- p>(</c-><c- n>__tuple_like</c-><c- p>));</c->
    <c- p>}</c->
<c- p>};</c->
<c- p>}</c-> <c- c1>// namespace __detail</c->
<c- p>}</c-> <c- c1>// namespace std</c->
</pre>
   <h3 class="heading settled" data-level="3.3" id="get_element_std_variant"><span class="secno">3.3. </span><span class="content"><code class="highlight"><c- n>get_element</c-></code> for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>variant</c-></code> and other non-tuple-like types</span><a class="self-link" href="#get_element_std_variant"></a></h3>
   <p>During the discussion in LEWG (St. Louis, 2024) we got a feedback that it’s better to constraint <code class="highlight"><c- n>get_element</c-></code> with <code class="highlight"><c- n>tuple_size</c-></code> and possibly <code class="highlight"><c- n>tuple_element</c-></code>. The idea was to not use it for something that is not <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code>, e.g., <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>variant</c-></code>.</p>
   <p>The current proposal purposefully does not consider <code class="highlight"><c- n>get_element</c-></code> to be applicable in all the same contexts
where <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> is used, for the following reasons:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> has overloads that allow to access <code class="highlight"><c- n>tuple</c-></code> elements by type, while <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get_element</c-></code> doesn’t.</p>
    <li data-md>
     <p>It is hard to imagine generic code that works with either <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>variant</c-></code> because it is always safe to access any element of a <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> object whereas for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>variant</c-></code> only one alternative is "active" and safe to access. Restricting <code class="highlight"><c- n>get_element</c-></code> can therefore prevent
potential UB in generic code.</p>
   </ul>
   <p>Moreover, <code class="highlight"><c- n>get_element</c-></code> could use <code class="highlight"><c- n>tuple_size</c-></code> to require that the specified element index is within the size
of the given <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> type, not relying on <code class="highlight"><c- n>get</c-></code> to have such check.</p>
   <p>Thus, the recommendation is to constrain <code class="highlight"><c- n>get_element</c-></code> with <code class="highlight"><c- n>tuple_size</c-></code>. It should be sufficient without <code class="highlight"><c- n>tuple_element</c-></code>; structured binding does the same.</p>
   <h2 class="heading settled" data-level="4" id="other_papers_connections"><span class="secno">4. </span><span class="content">Connections with other papers</span><a class="self-link" href="#other_papers_connections"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="p2547_connection"><span class="secno">4.1. </span><span class="content">Connection with <a data-link-type="biblio" href="#biblio-p2547r1" title="Language support for customisable functions">[P2547R1]</a></span><a class="self-link" href="#p2547_connection"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p2547r1" title="Language support for customisable functions">[P2547R1]</a> uses <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> as the example and a good candidate to be a customizable function.
Authors plan to ship the customizable functions proposal first and deal with customizing
standard library functions later. That means we should not expect that examples in this paper
automatically would be transformed to customizable functions when it will land.</p>
   <p>Moreover, at this time the authors of <a data-link-type="biblio" href="#biblio-p2547r1" title="Language support for customisable functions">[P2547R1]</a> don’t see how to introduce customizable functions
with the same names (e.g. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code>) without the ABI break, so they will likely need to choose
different names.</p>
   <h3 class="heading settled" data-level="4.2" id="p2141_connection"><span class="secno">4.2. </span><span class="content">Connection with <a data-link-type="biblio" href="#biblio-p2141r1" title="Aggregates are named tuples">[P2141R1]</a></span><a class="self-link" href="#p2141_connection"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p2141r1" title="Aggregates are named tuples">[P2141R1]</a>'s main goal is allow aggregates being interpreted as <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code>. At the same time, it
touches the <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept making it as generic as for the types structured binding can work with.
It also adds yet another <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> overload that works with any <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> object except those that
are already in the <code class="highlight"><c- n>std</c-><c- o>::</c-></code> namespace.</p>
   <p>With <a data-link-type="biblio" href="#biblio-p2141r1" title="Aggregates are named tuples">[P2141R1]</a> being adopted <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-></code> does the right thing and works with <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> object, so we
may use just <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- n>_Ip</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>_TupleLike</c-><c- o>></c-><c- p>(</c-><c- n>__tuple_like</c-><c- p>))</c-></code> within the implementation of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get_element</c-></code> instead of the unqualified <code class="highlight"><c- n>get</c-></code> call.</p>
   <p>Independently of <a data-link-type="biblio" href="#biblio-p2141r1" title="Aggregates are named tuples">[P2141R1]</a> <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get_element</c-></code> brings its own value by covering the described motivation use-cases.
Furthermore, in the standard there are already precedences of having two similar things with slightly different
semantics, for example, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>less</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>less</c-></code>, where the latter is not even a CPO.</p>
   <p><a data-link-type="biblio" href="#biblio-p2141r1" title="Aggregates are named tuples">[P2141R1]</a> also gives another way to generalize the <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> concept (via structured binding).</p>
   <h2 class="heading settled" data-level="5" id="further_work"><span class="secno">5. </span><span class="content">Further discussion</span><a class="self-link" href="#further_work"></a></h2>
   <ul>
    <li data-md>
     <p>Broader implementation experience (should be done before meeting in Poland, 2024)</p>
    <li data-md>
     <p>Make <code class="highlight"><i><c- n>tuple</c-><c- o>-</c-><c- n>like</c-></i></code> a public concept</p>
    <li data-md>
     <p>Consider requiring the index for <code class="highlight"><c- n>get_element</c-></code> to be within the size of the tuple</p>
   </ul>
   <h2 class="heading settled" data-level="6" id="wording"><span class="secno">6. </span><span class="content">Formal wording</span><a class="self-link" href="#wording"></a></h2>
   <p>Below, substitute the � character with a number the editor finds appropriate for the table, paragraph, section or sub-section.</p>
   <h3 class="heading settled" data-level="6.1" id="wording_tuple_like"><span class="secno">6.1. </span><span class="content">Modify Concept <i>tuple-like</i> <a href="https://eel.is/c++draft/tuple.like">[<strong>tuple.like</strong>] </a></span><a class="self-link" href="#wording_tuple_like"></a></h3>
   <blockquote>
    <ins>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>concept</c-> <i><c- nc>has</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>size</c-></i> <c- o>=</c->  <i><c- c1>// exposition only</c-></i>
  <c- k>requires</c-> <c- p>{</c->
      <c- k>typename</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>type</c-><c- p>;</c->
  <c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>I</c-><c- o>></c->
<c- k>concept</c-> <i><c- nc>can</c-><c- o>-</c-><c- n>get</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>element</c-></i> <c- o>=</c-> <i><c- c1>// exposition only</c-></i>
  <i><c- n>has</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>size</c-></i><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c->
  <c- k>requires</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c->
      <c- k>typename</c-> <c- nc>tuple_element_t</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>;</c->
      <c- p>{</c-> <c- n>get_element</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>))</c-> <c- p>}</c-> <c- o>-></c->
        <c- n>convertible_to</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>>&amp;></c-><c- p>;</c->
  <c- p>};</c->
</pre>
    </ins>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c->
<c- k>concept</c-> <i><c- nc>tuple</c-><c- o>-</c-><c- n>like</c-></i> <c- o>=</c-> <del><i><c- n>see</c-><c- o>-</c-><c- n>below</c-></i></del> <i><c- c1>// exposition only</c-></i>
    <ins><i><c- n>has</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>size</c-></i><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c-></ins>
    <ins><c- p>[]</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>...</c-> <c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>index_sequence</c-><c- o>&lt;</c-><c- n>I</c-><c- p>...</c-><c- o>></c-><c- p>)</c-> <c- p>{</c-></ins>
        <ins><c- k>return</c-> <c- p>(...</c-> <c- o>&amp;&amp;</c-> <i><c- n>can</c-><c- o>-</c-><c- n>get</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>element</c-></i><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>I</c-><c- o>></c-><c- p>);</c-></ins>
    <ins><c- p>}</c-> <c- p>(</c-><c- n>make_index_sequence</c-><c- o>&lt;</c-><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>>></c-><c- p>{});</c-></ins>
</pre>
    <del> A type <code class="highlight"><c- n>T</c-></code> models and satisfies the exposition-only concept <em>tuple-like</em> if <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> is a specialization of <code class="highlight"><c- n>array</c-></code>, <code class="highlight"><c- n>complex</c-></code>, <code class="highlight"><c- n>pair</c-></code>, <code class="highlight"><c- n>tuple</c-></code>,
or <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-></code>. </del>
   </blockquote>
   <h3 class="heading settled" data-level="6.2" id="wording_tuple_syn"><span class="secno">6.2. </span><span class="content">Modify Header <code class="highlight"><c- o>&lt;</c-><c- n>tuple</c-><c- o>></c-></code> synopsis <a href="https://eel.is/c++draft/tuple.syn">[<strong>tuple.syn</strong>] </a></span><a class="self-link" href="#wording_tuple_syn"></a></h3>
   <blockquote>
     [...] 
<pre class="highlight"><c- c1>// [tuple.helper], tuple helper classes</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>constexpr</c-> <c- b>size_t</c-> <c- n>tuple_size_v</c-> <c- o>=</c-> <c- n>tuple_size</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c->
</pre>
    <ins>
<pre class="highlight"><c- kr>inline</c-> <c- k>namespace</c-> <c- d>/* unspecified */</c-> <c- p>{</c->
    <c- k>template</c-> <c- o>&lt;</c-><c- b>size_t</c-> <c- n>I</c-><c- o>></c->
    <c- kr>inline</c-> <c- k>constexpr</c-> <c- d>/* unspecified */</c-> <c- n>get_element</c-> <c- o>=</c-> <c- d>/* unspecified */</c-><c- p>;</c->
<c- p>}</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- k>auto</c-> <c- n>get_key</c-> <c- o>=</c-> <c- n>get_element</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>;</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- k>auto</c-> <c- n>get_value</c-> <c- o>=</c-> <c- n>get_element</c-><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>;</c->
</pre>
    </ins>
   </blockquote>
   <h3 class="heading settled" data-level="6.3" id="wording_tuple_sections"><span class="secno">6.3. </span><span class="content">Add the following sections into <a href="http://eel.is/c++draft/tuple">[<strong>tuple</strong>]</a></span><a class="self-link" href="#wording_tuple_sections"></a></h3>
   <blockquote>
     [...]<br> � Element access [tuple.elem]<br> 
    <ins> <span>� Customization Point Objects [tuple.cust]
    � <code class="highlight"><c- n>get_element</c-></code> [tuple.cust.get_elem]</span> </ins>
   </blockquote>
   <h3 class="heading settled" data-level="6.4" id="wording_tuple_get_elem"><span class="secno">6.4. </span><span class="content">Add the following wording into [<strong>tuple.cust.get_elem</strong>]</span><a class="self-link" href="#wording_tuple_get_elem"></a></h3>
   <ins>
    <blockquote>
      The name <code class="highlight"><c- n>get_element</c-></code> denotes a customization point object
(<a href="https://eel.is/c++draft/customization.point.object">[<b>customization.point.object</b>])</a>.
Given an integral constant <code class="highlight"><c- n>I</c-></code> of type <code class="highlight"><c- b>size_t</c-></code> and a subexpression <code class="highlight"><c- n>E</c-></code> having type <code class="highlight"><c- n>T</c-></code>: 
     <ul>
      <li data-md>
       <p>If <code class="highlight"><c- k>typename</c-> <c- nc>tuple_size</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>>::</c-><c- n>type</c-></code> does not denote a valid type, <code class="highlight"><c- n>get_element</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is ill-formed.</p>
      <li data-md>
       <p>Otherwise, if <code class="highlight"><c- n>E</c-><c- p>.</c-><c- n>get</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>()</c-></code> is a valid expression, <code class="highlight"><c- n>get_element</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is expression equivalent to <code class="highlight"><c- n>E</c-><c- p>.</c-><c- n>get</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>()</c-></code></p>
      <li data-md>
       <p>Otherwise,  if <code class="highlight"><c- n>E</c-></code> has class type and <code class="highlight"><c- n>get</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is a valid expression where the meaning of <code class="highlight"><c- n>get</c-></code> is established as-if by performing argument-dependent lookup only
 (<a href="https://eel.is/c++draft/basic.lookup.argdep">[<b>basic.lookup.argdep]</b>)</a>, <code class="highlight"><c- n>get_element</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is expression equivalent to <code class="highlight"><c- n>get</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code></p>
      <li data-md>
       <p>Otherwise, <code class="highlight"><c- n>get_element</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>E</c-><c- p>)</c-></code> is ill-formed.</p>
     </ul>
    </blockquote>
   </ins>
   <h3 class="heading settled" data-level="6.5" id="wording_feature_macro"><span class="secno">6.5. </span><span class="content">Add feature test macro to the end of <a href="http://eel.is/c++draft/version.syn">[<strong>version.syn</strong>]</a></span><a class="self-link" href="#wording_feature_macro"></a></h3>
   <ins>
    <blockquote>
      [...] 
<pre class="highlight"><c- cp>#define __cpp_lib_get_element_customization_point  20����L</c->
 <i><c- c1>// also in </c-></i><c- c1>&lt;tuple></c-><i><c- c1>, </c-></i><c- c1>&lt;utility></c-><i><c- c1>, </c-></i><c- c1>&lt;array></c-><i><c- c1>, </c-></i><c- c1>&lt;ranges></c->
</pre>
     <p>[...]</p>
    </blockquote>
   </ins>
   <h3 class="heading settled" data-level="6.6" id="modify_constructor"><span class="secno">6.6. </span><span class="content">Modify <code class="highlight"><c- n>tuple</c-></code> construct <a href="http://eel.is/c++draft/tuple.cnstr">[<strong>tuple.cnstr</strong>]</a></span><a class="self-link" href="#modify_constructor"></a></h3>
   <blockquote>
     <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>UTuple</c-><c- o>></c-></code><br>   <code class="highlight"><c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>)</c-> <c- n>tuple</c-><c- p>(</c-><c- n>UTuple</c-><c- o>&amp;&amp;</c-> <c- n>u</c-><c- p>);</c-></code> 
    <p>Let <code class="highlight"><c- n>I</c-></code> be the pack <code class="highlight"><c- mi>0</c-><c- p>,</c-> <c- mi>1</c-><c- p>,</c-> …<c- p>,</c-> <c- p>(</c-><c- k>sizeof</c-><c- p>...(</c-><c- n>Types</c-><c- p>)</c-> <c- o>-</c-> <c- mi>1</c-><c- p>)</c-></code>.</p>
    <p><em>Constraints</em>:</p>
    <ul>
     <li data-md>
      <p><code class="highlight"><c- n>different</c-><c- o>-</c-><c- n>from</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- p>,</c-> <c- n>tuple</c-><c- o>></c-></code> ([range.utility.helpers]) is <code class="highlight">true</code>,</p>
     <li data-md>
      <p><code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-></code> is not a specialization of <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-></code>,</p>
     <li data-md>
      <p><code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>Types</c-><c- p>)</c-></code> equals <code class="highlight"><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>>></c-></code>,</p>
     <li data-md>
      <p><code class="highlight"><c- p>(</c-><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)))</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- p>...)</c-></code> is <code class="highlight">true</code>, and</p>
     <li data-md>
      <p>either <code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>Types</c-><c- p>)</c-></code> is not 1, or (when <code class="highlight"><c- n>Types</c-><c- p>...</c-></code> expands to <code class="highlight"><c- n>T</c-></code>) <code class="highlight"><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-></code> and <code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>UTuple</c-><c- o>></c-></code> are both <code class="highlight">false</code>.</p>
    </ul>
    <p>Effects: For all <em>i</em>, initializes the <em>i</em>th element of <code class="highlight"><c- o>*</c-><c- k>this</c-></code> with <code class="highlight"><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)).</c-></code></p>
    <p>Remarks: The expression inside explicit is equivalent to: <code class="highlight"><c- o>!</c-><c- p>(</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>))),</c-> <c- n>Types</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- p>...)</c-></code> The constructor is defined as deleted if <code class="highlight"><c- p>(</c-><c- n>reference_constructs_from_temporary_v</c-><c- o>&lt;</c-><c- n>Types</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span> <code class="highlight"><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)))</c-><c- o>></c-> <c- o>||</c-> <c- p>...)</c-></code> is <code class="highlight">true</code>.</p>
   </blockquote>
   <h3 class="heading settled" data-level="6.7" id="modify_assignment"><span class="secno">6.7. </span><span class="content">Modify <code class="highlight"><c- n>tuple</c-></code> assignment <a href="http://eel.is/c++draft/tuple.assign">[<strong>tuple.assign</strong>]</a></span><a class="self-link" href="#modify_assignment"></a></h3>
   <blockquote>
     <code class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>UTuple</c-><c- o>></c-></code><br>   <code class="highlight"><c- k>constexpr</c-> <c- n>tuple</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>UTuple</c-><c- o>&amp;&amp;</c-> <c- n>u</c-><c- p>);</c-></code> 
    <p><em>Constraints</em>:</p>
    <ul>
     <li data-md>
      <p><code class="highlight"><c- n>different</c-><c- o>-</c-><c- n>from</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- p>,</c-> <c- n>tuple</c-><c- o>></c-></code> ([range.utility.helpers]) is <code class="highlight">true</code>,</p>
     <li data-md>
      <p><code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-></code> is not a specialization of <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-></code>,</p>
     <li data-md>
      <p><code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>Types</c-><c- p>)</c-></code> equals <code class="highlight"><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>>></c-></code>, and,</p>
     <li data-md>
      <p><code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- n>Ti</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code> for all <em>i</em>.</p>
    </ul>
    <p><em>Effects</em>: For all <em>i</em>, assigns <code class="highlight"><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>))</c-></code> to <code class="highlight"><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-></code>.</p>
    <p><em>Returns</em>: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
    <p><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>UTuple</c-><c- o>></c-></code><br>   <code class="highlight"><c- k>constexpr</c-> <c- k>const</c-> <c- n>tuple</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>UTuple</c-><c- o>&amp;&amp;</c-> <c- n>u</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></code></p>
    <p><em>Constraints</em>:</p>
    <ul>
     <li data-md>
      <p><code class="highlight"><c- n>different</c-><c- o>-</c-><c- n>from</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- p>,</c-> <c- n>tuple</c-><c- o>></c-></code> ([range.utility.helpers]) is <code class="highlight">true</code>,</p>
     <li data-md>
      <p><code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-></code> is not a specialization of <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-></code>,</p>
     <li data-md>
      <p><code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>Types</c-><c- p>)</c-></code> equals <code class="highlight"><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>>></c-></code>, and,</p>
     <li data-md>
      <p><code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Ti</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code> for all <em>i</em>.</p>
    </ul>
    <p><em>Effects</em>: For all <em>i</em>, assigns <code class="highlight"><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>))</c-></code> to <code class="highlight"><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-></code>.</p>
    <p><em>Returns</em>: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
   </blockquote>
   <h3 class="heading settled" data-level="6.8" id="modify_tuple_cat"><span class="secno">6.8. </span><span class="content">Modify <code class="highlight"><c- n>tuple_cat</c-></code> in tuple creation <a href="https://eel.is/c++draft/tuple.creation">[<strong>tuple.creation</strong>]</a></span><a class="self-link" href="#modify_tuple_cat"></a></h3>
   <blockquote>
     <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>-</c-><c- n>like</c-><c- p>...</c-> <c- n>Tuples</c-><c- o>></c-></code><br>   <code class="highlight"><c- k>constexpr</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>CTypes</c-><c- p>...</c-><c- o>></c-> <c- n>tuple_cat</c-><c- p>(</c-><c- n>Tuples</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>tpls</c-><c- p>);</c-></code> 
    <p>Let <em>n</em> be <code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>Tuples</c-><c- p>)</c-></code>. For every integer <code class="highlight"><c- mi>0</c-> <c- o>&lt;=</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>n</c-></code>:</p>
    <ul>
     <li data-md>
      <p>Let <code class="highlight"><c- n>Ti</c-></code> be the ith type in <code class="highlight"><c- n>Tuples</c-></code>.</p>
     <li data-md>
      <p>Let <code class="highlight"><c- n>Ui</c-></code> be <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Ti</c-><c- o>></c-></code>.</p>
     <li data-md>
      <p>Let <code class="highlight"><c- n>tpi</c-></code> be the <em>i</em>th element in the function parameter pack <code class="highlight"><c- n>tpls</c-></code>.</p>
     <li data-md>
      <p>Let <code class="highlight"><c- n>Si</c-></code> be <code class="highlight"><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>Ui</c-><c- o>></c-></code>.</p>
     <li data-md>
      <p>Let <code class="highlight"><c- n>Eki</c-></code> be <code class="highlight"><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>k</c-><c- p>,</c-> <c- n>Ui</c-><c- o>></c-></code>.</p>
     <li data-md>
      <p>Let <code class="highlight"><c- n>eki</c-></code> be <code class="highlight"><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>k</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Ti</c-><c- o>></c-><c- p>(</c-><c- n>tpi</c-><c- p>))</c-></code>.</p>
     <li data-md>
      <p>Let <code class="highlight"><c- n>Elemsi</c-></code> be a pack of the types <code class="highlight"><c- n>E0i</c-><c- p>,...,</c-><c- n>ESi</c->−<c- mi>1</c-><c- n>i</c-></code>.</p>
     <li data-md>
      <p>Let <code class="highlight"><c- n>elemsi</c-></code> be a pack of the expressions <code class="highlight"><c- n>e0i</c-></code>,...,<code class="highlight"><c- n>eSi</c->−<c- mi>1</c-><c- n>i</c-></code>.</p>
    </ul>
    <p>The types in <code class="highlight"><c- n>CTypes</c-></code> are equal to the ordered sequence of the expanded packs of types <code class="highlight"><c- n>Elems0</c-><c- p>...,</c-> <c- n>Elems1</c-><c- p>...,</c-> <c- p>...,</c-> <c- n>Elemsn</c->−<c- mf>1.</c-><c- p>..</c-></code>.
Let <code class="highlight"><c- n>celems</c-></code> be the ordered sequence of the expanded packs of expressions <code class="highlight"><c- n>elems0</c-><c- p>...,</c-> <c- p>...,</c-> <c- n>elemsn</c->−<c- mf>1.</c-><c- p>..</c-></code>.</p>
    <p><em>Mandates</em>: <code class="highlight"><c- p>(</c-><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>CTypes</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>celems</c-><c- p>)</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- p>...)</c-></code> is <code class="highlight">true</code>.</p>
    <p><em>Returns</em>: <code class="highlight"><c- n>tuple</c-><c- o>&lt;</c-><c- n>CTypes</c-><c- p>...</c-><c- o>></c-><c- p>(</c-><c- n>celems</c-><c- p>...)</c-></code></p>
   </blockquote>
   <h3 class="heading settled" data-level="6.9" id="modify_tuple_apply"><span class="secno">6.9. </span><span class="content">Modify <code class="highlight"><c- n>apply</c-></code> in <a href="https://eel.is/c++draft/tuple.apply">[<strong>tuple.apply</strong>]</a></span><a class="self-link" href="#modify_tuple_apply"></a></h3>
   <blockquote>
     <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>F</c-><c- p>,</c-> <c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>Tuple</c-><c- o>></c-></code><br>   <code class="highlight"><c- k>constexpr</c-> <c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- n>apply</c-><c- p>(</c-><c- n>F</c-><c- o>&amp;&amp;</c-> <c- n>f</c-><c- p>,</c-> <c- n>Tuple</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>);</c-></code> 
    <p><em>Effects</em>: Given the exposition-only function template: <span></span></p>
<pre class="highlight"><code>
<c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->
  <c- k>template</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- k><c- k>class</c-></c-> <c- nc><c- nc>F</c-></c-><c- p><c- p>,</c-></c-> <c- n><c- n>tuple</c-></c-><c- o><c- o>-</c-></c-><c- n><c- n>like</c-></c-> <c- n><c- n>Tuple</c-></c-><c- p><c- p>,</c-></c-> <c- b><c- b>size_t</c-></c-><c- p><c- p>...</c-></c-> <c- n><c- n>I</c-></c-><c- o><c- o>></c-></c-></code>
  <c- k>constexpr</c-> <c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- n>apply</c-><c- o>-</c-><c- n>impl</c-><c- p>(</c-><c- n>F</c-><c- o>&amp;&amp;</c-> <c- n>f</c-><c- p>,</c-> <c- n>Tuple</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>,</c-> <c- n>index_sequence</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- n><c- n>I</c-></c-><c- p><c- p>...</c-></c-><c- o><c- o>></c-></c-></code><c- p>)</c-> <c- p>{</c->
                                                                        <i><c- c1>// exposition only</c-></i>
    <c- k>return</c-> <c- n>INVOKE</c-><c- p>(</c-><code class="highlight"><c- n><c- n>std</c-></c-><c- o><c- o>::</c-></c-><c- n><c- n>forward</c-></c-><c- o><c- o>&lt;</c-></c-><c- n><c- n>F</c-></c-><c- o><c- o>></c-></c-><c- p><c- p>(</c-></c-><c- n><c- n>f</c-></c-><c- p><c- p>)</c-></c-></code><c- p>,</c-> <c- n>get</c-><ins><c- n>_element</c-></ins><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- n><c- n>I</c-></c-><c- o><c- o>></c-></c-><c- p><c- p>(</c-></c-><c- n><c- n>std</c-></c-><c- o><c- o>::</c-></c-><c- n><c- n>forward</c-></c-><c- o><c- o>&lt;</c-></c-><c- n><c- n>Tuple</c-></c-><c- o><c- o>></c-></c-><c- p><c- p>(</c-></c-><c- n><c- n>t</c-></c-><c- p><c- p>))...);</c-></c-></code>     <i><c- c1>// see [func.require]</c-></i>
  <c- p>}</c->
<c- p>}</c->
</code></pre>
    <p> Equivalent to: <span></span></p>
<pre class="highlight"><code> <c- k>return</c-> <c- n>apply</c-><c- o>-</c-><c- n>impl</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><f><c- p>(</c-><c- n>f</c-><c- p>),</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><tuple><c- p>(</c-><c- n>t</c-><c- p>),</c->
                   <c- n>make_index_sequence</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- n><c- n>tuple_size_v</c-></c-><c- o><c- o>&lt;</c-></c-><c- n><c- n>remove_reference_t</c-></c-><c- o><c- o>&lt;</c-></c-><c- n><c- n>Tuple</c-></c-><c- o><c- o>>>></c-></c-><c- p><c- p>{});</c-></c-></code></tuple></f></code></pre>
    <p></p>
    <p><em>Remarks</em>: Let <code class="highlight"><c- n>I</c-></code> be the pack <code class="highlight"><c- mi>0</c-><c- p>,</c-> <c- mi>1</c-><c- p>,</c-> <c- p>...,</c-> <c- p>(</c-><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>Tuple</c-><c- o>>></c-> <c- o>-</c-> <c- mi>1</c-><c- p>)</c-></code>. The exception specification is equivalent to:</p>
<pre class="highlight"><c- k>noexcept</c-><c- p>(</c-><c- n>invoke</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>F</c-><c- o>></c-><c- p>(</c-><c- n>f</c-><c- p>),</c-> <c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Tuple</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>))...))</c-></pre>
   </blockquote>
   <blockquote>
     <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>Tuple</c-><c- o>></c-></code> 
<pre class="highlight"><code>  <c- k>constexpr</c-> <c- n>T</c-> <c- nf>make_from_tuple</c-><c- p>(</c-><c- n>Tuple</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>);</c-></code></pre>
    <p><em>Mandates</em>: If <code class="highlight"><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>Tuple</c-><c- o>>></c-></code> is 1, then<br>   <code class="highlight"><c- n>reference_constructs_from_temporary_vT</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>Tuple</c-><c- o>></c-><c- p>()))</c-><c- o>></c-></code> is <code class="highlight">false</code>.</p>
    <p>Effects: Given the exposition-only function template:</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>Tuple</c-><c- p>,</c-> <c- b>size_t</c-><c- p>...</c-> <c- n>I</c-><c- o>></c->
    <c- k>requires</c-> <c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>Tuple</c-><c- o>></c-><c- p>()))...</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>T</c-> <c- n>make</c-><c- o>-</c-><c- n>from</c-><c- o>-</c-><c- n>tuple</c-><c- o>-</c-><c- n>impl</c-><c- p>(</c-><c- n>Tuple</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>,</c-> <c- n>index_sequence</c-><c- o>&lt;</c-><c- n>I</c-><c- p>...</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->   <c- c1>// exposition only</c->
    <c- k>return</c-> <c- n>T</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Tuple</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>))...);</c->
  <c- p>}</c->
<c- p>}</c->
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="6.10" id="modify_tuple_rel"><span class="secno">6.10. </span><span class="content">Modify relation operators in <a href="https://eel.is/c++draft/tuple.rel">[<strong>tuple.rel</strong>]</a></span><a class="self-link" href="#modify_tuple_rel"></a></h3>
   <blockquote>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>TTypes</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>UTypes</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>TTypes</c-><c- p>...</c-><c- o>>&amp;</c-> <c- n>t</c-><c- p>,</c-> <c- k>const</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>UTypes</c-><c- p>...</c-><c- o>>&amp;</c-> <c- n>u</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>TTypes</c-><c- p>,</c-> <c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>UTuple</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>TTypes</c-><c- p>...</c-><c- o>>&amp;</c-> <c- n>t</c-><c- p>,</c-> <c- k>const</c-> <c- n>UTuple</c-><c- o>&amp;</c-> <c- n>u</c-><c- p>);</c->
</pre>
    <p>For the first overload let <code class="highlight"><c- n>UTuple</c-></code> be <code class="highlight"><c- n>tuple</c-><c- o>&lt;</c-><c- n>UTypes</c-><c- p>...</c-><c- o>></c-></code>.</p>
    <p><em>Constraints</em>: For all <code class="highlight"><c- n>i</c-></code>, where <code class="highlight"><c- mi>0</c->  <c- o>&lt;=</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- k>sizeof</c-><c- p>...(</c-><c- n>TTypes</c-><c- p>)</c-></code>, <code class="highlight"><c- n>get</c-><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>)</c-> <c- o>==</c-> <c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)</c-></code> is a valid expression and <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>get</c-><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>)</c-> <c- o>==</c-> <c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>))</c-></code> models boolean-testable. <code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>TTypes</c-><c- p>)</c-></code> equals <code class="highlight"><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-></code>.</p>
    <p><em>Returns</em>: <code class="highlight">true</code> if <code class="highlight"><c- n>get</c-><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>)</c-> <c- o>==</c-> <c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>i</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)</c-></code> for all <code class="highlight"><c- n>i</c-></code>, otherwise <code class="highlight">false</code>.</p>
    <p>[Note 1: If <code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>TTypes</c-><c- p>)</c-></code> equals zero, returns <code class="highlight">true</code>. — end note]</p>
    <p><em>Remarks</em>:</p>
    <ul>
     <li data-md>
      <p>The elementary comparisons are performed in order from the zeroth index upwards.
No comparisons or element accesses are performed after the first equality comparison that evaluates to <code class="highlight">false</code>.</p>
     <li data-md>
      <p>The second overload is to be found via argument-dependent lookup ([basic.lookup.argdep]) only.</p>
    </ul>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>TTypes</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>UTypes</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>common_comparison_category_t</c-><c- o>&lt;</c-><c- n>synth</c-><c- o>-</c-><c- n>three</c-><c- o>-</c-><c- n>way</c-><c- o>-</c-><c- n>result</c-><c- o>&lt;</c-><c- n>TTypes</c-><c- p>,</c-> <c- n>UTypes</c-><c- o>></c-><c- p>...</c-><c- o>></c->
    <c- k>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>TTypes</c-><c- p>...</c-><c- o>>&amp;</c-> <c- n>t</c-><c- p>,</c-> <c- k>const</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>UTypes</c-><c- p>...</c-><c- o>>&amp;</c-> <c- n>u</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>TTypes</c-><c- p>,</c-> <c- n>tuple</c-><c- o>-</c-><c- n>like</c-> <c- n>UTuple</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>common_comparison_category_t</c-><c- o>&lt;</c-><c- n>synth</c-><c- o>-</c-><c- n>three</c-><c- o>-</c-><c- n>way</c-><c- o>-</c-><c- n>result</c-><c- o>&lt;</c-><c- n>TTypes</c-><c- p>,</c-> <c- n>Elems</c-><c- o>></c-><c- p>...</c-><c- o>></c->
    <c- k>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>tuple</c-><c- o>&lt;</c-><c- n>TTypes</c-><c- p>...</c-><c- o>>&amp;</c-> <c- n>t</c-><c- p>,</c-> <c- k>const</c-> <c- n>UTuple</c-><c- o>&amp;</c-> <c- n>u</c-><c- p>);</c->
</pre>
    <p>For the second overload, <code class="highlight"><c- n>Elems</c-></code> denotes the pack of types <code class="highlight"><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- mi>0</c-><c- p>,</c-> <c- n>UTuple</c-><c- o>></c-><c- p>,</c-> <c- n>tuple_element_t</c-><c- o>&lt;</c-><c- mi>1</c-><c- p>,</c-> <c- n>UTuple</c-><c- o>></c-><c- p>,</c-> …<c- p>,</c-> <c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>tuple_size_v</c-><c- o>&lt;</c-><c- n>UTuple</c-><c- o>></c-> <c- o>-</c-> <c- mi>1</c-><c- p>,</c-> <c- n>UTuple</c-><c- o>></c-></code>.</p>
    <p><em>Effects</em>: Performs a lexicographical comparison between <code class="highlight"><c- n>t</c-></code> and <code class="highlight"><c- n>u</c-></code>. If <code class="highlight"><c- k>sizeof</c-><c- p>...(</c-><c- n>TTypes</c-><c- p>)</c-></code> equals zero, returns <code class="highlight"><c- n>strong_ordering</c->​<c- o>::</c->​<c- n>equal</c-></code>.</p>
    <p>Otherwise, equivalent to: <code class="highlight"><c- k>if</c-> <c- p>(</c-><c- k>auto</c-> <c- n>c</c-> <c- o>=</c-> <c- n>synth</c-><c- o>-</c-><c- n>three</c-><c- o>-</c-><c- n>way</c-><c- p>(</c-><c- n>get</c-><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>t</c-><c- p>),</c-> <c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>));</c-> <c- n>c</c-> <c- o>!=</c-> <c- mi>0</c-><c- p>)</c-> <c- k>return</c-> <c- n>c</c-><c- p>;</c-></code> <code class="highlight"><c- k>return</c-> <c- n>ttail</c-> <c- o>&lt;=></c-> <c- n>utail</c-><c- p>;</c-></code></p>
    <p>where <code class="highlight"><c- n>rtail</c-></code> for some <code class="highlight"><c- n>r</c-></code> is a tuple containing all but the first element of <code class="highlight"><c- n>r</c-></code>.</p>
    <p><em>Remarks</em>: The second overload is to be found via argument-dependent lookup ([basic.lookup.argdep]) only.</p>
   </blockquote>
   <h3 class="heading settled" data-level="6.11" id="modify_range_elements_iterator"><span class="secno">6.11. </span><span class="content">Modify <a href="https://eel.is/c++draft/range.elements.iterator">[<strong>range.elements.iterator</strong>]</a></span><a class="self-link" href="#modify_range_elements_iterator"></a></h3>
   <blockquote>
     The member <em>typedef-name</em> <code class="highlight"><c- n>iterator_category</c-></code> is defined if and only if <em>Base</em> models <code class="highlight"><c- n>forward_range</c-></code>.
In that case, <code class="highlight"><c- n>iterator_category</c-></code> is defined as follows: Let <code class="highlight"><c- n>C</c-></code> denote the type <code class="highlight"><c- n>iterator_traits</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>></c->​<c- o>::</c->​<c- n>iterator_category</c-><c- p>.</c-></code> 
    <ul>
     <li data-md>
      <p>If <code class="highlight"><c- n>std</c->​<c- o>::</c->​<c- n>get</c-></code><span><ins><code class="highlight"><c- n>_element</c-></code></ins></span><code class="highlight"><c- o>&lt;</c-><c- n>N</c-><c- o>></c-><c- p>(</c-><c- o>*</c-><c- n>current_</c-><c- p>)</c-></code> is an rvalue, <code class="highlight"><c- n>iterator_category</c-></code> denotes <code class="highlight"><c- n>input_iterator_tag</c-></code>.</p>
     <li data-md>
      <p>Otherwise, if C models <code class="highlight"><c- n>derived_from</c-><c- o>&lt;</c-><c- n>random_access_iterator_tag</c-><c- o>></c-></code>, <code class="highlight"><c- n>iterator_category</c-></code> denotes <code class="highlight"><c- n>random_access_iterator_tag</c-><c- p>.</c-></code></p>
     <li data-md>
      <p>Otherwise, <code class="highlight"><c- n>iterator_category</c-></code> denotes <code class="highlight"><c- n>C</c-></code>.</p>
    </ul>
    <p><code class="highlight"><c- k>static</c-> <c- k>constexpr</c-> <c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> </code><i><code class="highlight"><c- n>get</c-><c- o>-</c-><c- n>element</c-></code></i><code class="highlight"><c- p>(</c-><c- k>const</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>&amp;</c-> <c- n>i</c-><c- p>);</c-></code></p>
    <p><em>Effects</em>: Equivalent to:</p>
<pre class="highlight"><c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c-><c- n>is_reference_v</c-><c- o>&lt;</c-><c- n>range_reference_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>></c-><c- p>)</c-> <c- p>{</c->
  <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>get</c-><ins><code class="highlight"><c- n><c- n>_element</c-></c-></code></ins><c- o>&lt;</c-><c- n>N</c-><c- o>></c-><c- p>(</c-><c- o>*</c-><c- n>i</c-><c- p>);</c->
<c- p>}</c-> <c- k>else</c-> <c- p>{</c->
  <c- k>using</c-> <c- n>E</c-> <c- o>=</c-> <c- n>remove_cv_t</c-><c- o>&lt;</c-><c- n>tuple_element_t</c-><c- o>&lt;</c-><c- n>N</c-><c- p>,</c-> <c- n>range_reference_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>>></c-><c- p>;</c->
  <c- k>return</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>get</c-><ins><code class="highlight"><c- n><c- n>_element</c-></c-></code></ins><c- o>&lt;</c-><c- n>N</c-><c- o>></c-><c- p>(</c-><c- o>*</c-><c- n>i</c-><c- p>));</c->
<c- p>}</c->
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="6.12" id="allocator_uses_construction"><span class="secno">6.12. </span><span class="content">Modify <code class="highlight"><c- n>uses_allocator_construction_args</c-></code> in <a href="https://eel.is/c++draft/allocator.uses.construction">[<strong>allocator.uses.construction</strong>]</a></span><a class="self-link" href="#allocator_uses_construction"></a></h3>
   <blockquote>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Alloc</c-><c- p>,</c-> <i><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i> <c- n>P</c-><c- o>></c->
  <c- k>constexpr</c-> <c- k>auto</c-> <c- n>uses_allocator_construction_args</c-><c- p>(</c-><c- k>const</c-> <c- n>Alloc</c-><c- o>&amp;</c-> <c- n>alloc</c-><c- p>,</c-> <c- n>P</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <p><em>Constraints</em>: <code class="highlight"><c- n>remove_cv_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> is a specialization of <code class="highlight"><c- n>pair</c-></code> and <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-></code> is not a specialization of <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-><c- p>.</c-></code></p>
    <p><em>Effects</em>: Equivalent to:</p>
<pre class="highlight"><c- k>return</c-> <c- n>uses_allocator_construction_args</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>alloc</c-><c- p>,</c-> <c- n>piecewise_construct</c-><c- p>,</c->
                                      <c- n>forward_as_tuple</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>))),</c->
                                      <c- n>forward_as_tuple</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>))));</c->
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="6.13" id="pairs_pair"><span class="secno">6.13. </span><span class="content">Modify <code class="highlight"><c- n>pair</c-></code> in <a href="https://eel.is/c++draft/pairs.pair">[<strong>pairs.pair</strong>]</a></span><a class="self-link" href="#pairs_pair"></a></h3>
   <blockquote>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U1</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U2</c-><c- o>></c-> <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>)</c-> <c- n>pair</c-><c- p>(</c-><c- n>pair</c-><c- o>&lt;</c-><c- n>U1</c-><c- p>,</c-> <c- n>U2</c-><c- o>>&amp;</c-> <c- n>p</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U1</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U2</c-><c- o>></c-> <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>)</c-> <c- n>pair</c-><c- p>(</c-><c- k>const</c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>U1</c-><c- p>,</c-> <c- n>U2</c-><c- o>>&amp;</c-> <c- n>p</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U1</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U2</c-><c- o>></c-> <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>)</c-> <c- n>pair</c-><c- p>(</c-><c- n>pair</c-><c- o>&lt;</c-><c- n>U1</c-><c- p>,</c-> <c- n>U2</c-><c- o>>&amp;&amp;</c-> <c- n>p</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U1</c-><c- p>,</c-> <c- k>class</c-> <c- nc>U2</c-><c- o>></c-> <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>)</c-> <c- n>pair</c-><c- p>(</c-><c- k>const</c-> <c- n>pair</c-><c- o>&lt;</c-><c- n>U1</c-><c- p>,</c-> <c- n>U2</c-><c- o>>&amp;&amp;</c-> <c- n>p</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><i><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i> <c- n>P</c-><c- o>></c-> <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>see</c-> <c- n>below</c-><c- p>)</c-> <c- n>pair</c-><c- p>(</c-><c- n>P</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>);</c->
</pre>
    <p>
     Let <code class="highlight"><i><c- n>FWD</c-></i><c- p>(</c-><c- n>u</c-><c- p>)</c-></code> be <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><c- n>u</c-><c- p>)</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)</c-></code> 
     <ins>and let <code class="highlight"><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></code> be <code class="highlight"><c- n>get</c-></code> for the first four overloads and <code class="highlight"><c- n>get_element</c-></code> for the last overload.</ins>
    </p>
    <p><em>Constraints:</em></p>
    <ul>
     <li data-md>
      <p>For the last overload <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-></code> is not a specialization of <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-></code></p>
     <li data-md>
      <p><code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>T1</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code>, and</p>
     <li data-md>
      <p><code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>T2</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code>.</p>
    </ul>
    <p><em>Effects</em>: Initializes first with <code class="highlight"><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>))</c-></code> and second with <code class="highlight"><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>))</c-></code>.</p>
    <p><em>Remarks</em>: The expression inside explicit is equivalent to: <code class="highlight"><c- o>!</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>))),</c-> <c- n>T1</c-><c- o>></c-> <c- o>||</c-></code> <code class="highlight"><c- o>!</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>))),</c-> <c- n>T2</c-><c- o>></c-></code></p>
    <p>The constructor is defined as deleted if</p>
    <p><code class="highlight"><c- n>reference_constructs_from_temporary_v</c-><c- o>&lt;</c-><c- n>first_type</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-> <c- o>||</c-></code> <code class="highlight"><c- n>reference_constructs_from_temporary_v</c-><c- o>&lt;</c-><c- n>second_type</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><ins><i><c- n>get</c-><c- o>-</c-><c- n>elem</c-></i></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><i><c- n>FWD</c-></i><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-></code></p>
    <p>is <code class="highlight">true</code>.</p>
   </blockquote>
   <blockquote>
    <p><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><i><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i> <c- n>P</c-><c- o>></c-> <c- k>constexpr</c-> <c- n>pair</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>P</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>);</c-></code></p>
    <p><em>Constraints</em>:</p>
    <ul>
     <li data-md>
      <p><code class="highlight"><i><c- n>different</c-><c- o>-</c-><c- n>from</c-></i><c- o>&lt;</c-><c- n>P</c-><c- p>,</c-> <c- n>pair</c-><c- o>></c-></code> ([range.utility.helpers]) is <code class="highlight">true</code>,</p>
     <li data-md>
      <p><code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-></code> is not a specialization of <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-></code>,</p>
     <li data-md>
      <p><code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- n>T1</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code>, and</p>
     <li data-md>
      <p><code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- n>T2</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code>.</p>
    </ul>
    <p>Effects: Assigns <code class="highlight"><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>))</c-></code> to <code class="highlight"><c- n>first</c-></code> and <code class="highlight"><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>))</c-></code> to <code class="highlight"><c- n>second</c-></code>.
Returns: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
    <p><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><i><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i> <c- n>P</c-><c- o>></c-> <c- k>constexpr</c-> <c- k>const</c-> <c- n>pair</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>P</c-><c- o>&amp;&amp;</c-> <c- n>p</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></code></p>
    <p><em>Constraints</em>:</p>
    <ul>
     <li data-md>
      <p><code class="highlight"><i><c- n>different</c-><c- o>-</c-><c- n>from</c-></i><c- o>&lt;</c-><c- n>P</c-><c- p>,</c-> <c- n>pair</c-><c- o>></c-></code> ([range.utility.helpers]) is <code class="highlight">true</code>,</p>
     <li data-md>
      <p><code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-></code> is not a specialization of <code class="highlight"><c- n>ranges</c->​<c- o>::</c->​<c- n>subrange</c-></code>,</p>
     <li data-md>
      <p><code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>T1</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code>, and</p>
     <li data-md>
      <p><code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>T2</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>)))</c-><c- o>></c-></code> is <code class="highlight">true</code>.</p>
    </ul>
    <p><em>Effects</em>: Assigns <code class="highlight"><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>))</c-></code> to <code class="highlight"><c- n>first</c-></code> and <code class="highlight"><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><c- n>std</c->​<c- o>::</c->​<c- n>forward</c-><c- o>&lt;</c-><c- n>P</c-><c- o>></c-><c- p>(</c-><c- n>p</c-><c- p>))</c-></code> to <code class="highlight"><c- n>second</c-></code>. <em>Returns</em>: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
   </blockquote>
   <h3 class="heading settled" data-level="6.14" id="mdspan_sub_helpers"><span class="secno">6.14. </span><span class="content">Modify "Exposition-only helpers" in <a href="https://eel.is/c++draft/mdspan.sub.helpers">[<strong>mdspan.sub.helpers</strong>]</a></span><a class="self-link" href="#mdspan_sub_helpers"></a></h3>
   <blockquote>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>T</c-> <i><c- n>de</c-><c- o>-</c-><c- n>ice</c-></i><c- p>(</c-><c- n>T</c-> <c- n>val</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>val</c-><c- p>;</c-> <c- p>}</c->
<c- k>template</c-><c- o>&lt;</c-><c- n>integral</c-><c- o>-</c-><c- n>constant</c-><c- o>-</c-><c- n>like</c-> <c- n>T</c-><c- o>></c->
  <c- k>constexpr</c-> <c- k>auto</c-> <i><c- n>de</c-><c- o>-</c-><c- n>ice</c-></i><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>T</c-><c- o>::</c-><c- n>value</c-><c- p>;</c-> <c- p>}</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>IndexType</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>k</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>SliceSpecifiers</c-><c- o>></c->
  <c- k>constexpr</c-> <c- n>IndexType</c-> <c- n>first_</c-><c- p>(</c-><c- n>SliceSpecifiers</c-><c- p>...</c-> <c- n>slices</c-><c- p>);</c->
</pre>
    <p><em>Mandates</em>: <code class="highlight"><c- n>IndexType</c-></code> is a signed or unsigned integer type.</p>
    <p>Let <i>ϕk</i> denote the following value:</p>
    <ul>
     <li data-md>
      <p><i>sk</i> if <i>Sk</i> models <code class="highlight"><c- n>convertible_to</c-><c- o>&lt;</c-><c- n>IndexType</c-><c- o>></c-><c- p>;</c-></code></p>
     <li data-md>
      <p>otherwise, <code class="highlight"><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>0</c-><c- o>></c-><c- p>(</c-><i><c- n>sk</c-></i><c- p>)</c-></code> if <i>Sk</i> models <code class="highlight"><i><c- n>index</c-><c- o>-</c-><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i><c- o>&lt;</c-><c- n>IndexType</c-><c- o>></c-></code>;</p>
     <li data-md>
      <p>otherwise, <code class="highlight"><i><c- n>de</c-><c- o>-</c-><c- n>ice</c-></i><c- p>(</c-><i><c- n>sk</c-></i><c- p>.</c-><c- n>offset</c-><c- p>)</c-></code> if <i>Sk</i> is a specialization of <code class="highlight"><c- n>strided_slice</c-></code>;</p>
     <li data-md>
      <p>otherwise, <code class="highlight"><c- mi>0</c-></code>.</p>
    </ul>
    <p>Preconditions: <i>ϕk</i> is representable as a value of type <code class="highlight"><c- n>IndexType</c-></code>.</p>
    <p>Returns: <code class="highlight"><c- n>extents</c-><c- o>&lt;</c-><c- n>IndexType</c-><c- o>></c->​<c- o>::</c-><i>​<c- n>index</c-><c- o>-</c-><c- n>cast</c-><c- p>(</c-><c- n>ϕk</c-><c- p>)</c-></i></code>.</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>k</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Extents</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>SliceSpecifiers</c-><c- o>></c->
  <c- k>constexpr</c-> <c- k>auto</c-> <c- n>last_</c-><c- p>(</c-><c- k>const</c-> <c- n>Extents</c-><c- o>&amp;</c-> <c- n>src</c-><c- p>,</c-> <c- n>SliceSpecifiers</c-><c- p>...</c-> <c- n>slices</c-><c- p>);</c->
</pre>
    <p><em>Mandates</em>: <code class="highlight"><c- n>Extents</c-></code> is a specialization of extents.
Let <code class="highlight"><c- n>index_type</c-></code> be typename <code class="highlight"><c- n>Extents</c->​<c- o>::</c->​<c- n>index_type</c-><c- p>.</c-></code> Let <i>λk</i> denote the following value:</p>
    <ul>
     <li data-md>
      <p><code class="highlight"><i><c- n>de</c-><c- o>-</c-><c- n>ice</c-><c- p>(</c-><c- n>sk</c-><c- p>)</c-></i> <c- o>+</c-> <c- mi>1</c-></code> if <i>Sk</i> models <code class="highlight"><c- n>convertible_to</c-><c- o>&lt;</c-><c- n>index_type</c-><c- o>></c-><c- p>;</c-></code> otherwise</p>
     <li data-md>
      <p><code class="highlight"><c- n>get</c-><ins><c- n>_element</c-></ins><c- o>&lt;</c-><c- mi>1</c-><c- o>></c-><c- p>(</c-><i><c- n>sk</c-></i><c- p>)</c-></code> if <i>Sk</i> models <code class="highlight"><i><c- n>index</c-><c- o>-</c-><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i><c- o>&lt;</c-><c- n>index_type</c-><c- o>></c-></code>; otherwise</p>
     <li data-md>
      <p><code class="highlight"><i><c- n>de</c-><c- o>-</c-><c- n>ice</c-></i><c- p>(</c-><i><c- n>sk</c-></i><c- p>.</c-><c- n>offset</c-><c- p>)</c-> <c- o>+</c-> <i><c- n>de</c-><c- o>-</c-><c- n>ice</c-></i><c- p>(</c-><i><c- n>sk</c-></i><c- p>.</c-><c- n>extent</c-><c- p>)</c-></code> if <i>Sk</i> is a specialization
of <code class="highlight"><c- n>strided_slice</c-></code>; otherwise</p>
     <li data-md>
      <p><code class="highlight"><c- n>src</c-><c- p>.</c-><c- n>extent</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code>.</p>
    </ul>
    <p>Preconditions: <i>λk</i> is representable as a value of type <code class="highlight"><c- n>index_type</c-></code>.</p>
    <p>Returns: <code class="highlight"><c- n>Extents</c->​<c- o>::</c-><i>​<c- n>index</c-><c- o>-</c-><c- n>cast</c-><c- p>(</c-><c- n>λk</c-><c- p>)</c-></i></code>.</p>
   </blockquote>
   <h2 class="heading settled" data-level="7" id="revision_history"><span class="secno">7. </span><span class="content">Revision history</span><a class="self-link" href="#revision_history"></a></h2>
   <h3 class="heading settled" data-level="7.1" id="r2_r3"><span class="secno">7.1. </span><span class="content">R2 => R3</span><a class="self-link" href="#r2_r3"></a></h3>
   <ul>
    <li data-md>
     <p>Remove the details about <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get</c-></code> design alternative</p>
    <li data-md>
     <p>Align the behavior of <code class="highlight"><c- n>get_element</c-></code> with <em>structured binding</em></p>
    <li data-md>
     <p>Add support for value categories for <em>tuple-like</em> concept</p>
    <li data-md>
     <p>Modify <code class="highlight"><i><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i></code> and <code class="highlight"><i><c- n>index</c-><c- o>-</c-><c- n>pair</c-><c- o>-</c-><c- n>like</c-></i></code> to use <code class="highlight"><c- n>get_element</c-></code></p>
    <li data-md>
     <p>Constrain <code class="highlight"><c- n>get_element</c-></code> with <code class="highlight"><c- n>tuple_size</c-></code></p>
   </ul>
   <h3 class="heading settled" data-level="7.2" id="r1_r2"><span class="secno">7.2. </span><span class="content">R1 => R2</span><a class="self-link" href="#r1_r2"></a></h3>
   <ul>
    <li data-md>
     <p>Add extra motivation to allow user-defined tuples in the standard</p>
    <li data-md>
     <p>Propose changes to <em>tuple-like</em> concept with wording</p>
   </ul>
   <h3 class="heading settled" data-level="7.3" id="r0_r1"><span class="secno">7.3. </span><span class="content">R0 => R1</span><a class="self-link" href="#r0_r1"></a></h3>
   <ul>
    <li data-md>
     <p>Address the "structured binding unused variables" questions with <a data-link-type="biblio" href="#biblio-p2169r3" title="A Nice Placeholder With No Name">[P2169R3]</a></p>
    <li data-md>
     <p>Add ruminations about possible relaxation of the <em>tuple-like</em> concept</p>
    <li data-md>
     <p>Add connections to <a data-link-type="biblio" href="#biblio-p2547r1" title="Language support for customisable functions">[P2547R1]</a> and <a data-link-type="biblio" href="#biblio-p2141r1" title="Aggregates are named tuples">[P2141R1]</a></p>
    <li data-md>
     <p>Apply an approach to minimize ABI and API breaking for the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get</c-></code> name</p>
    <li data-md>
     <p>Add wording and a feature test macro for the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get_element</c-></code> API</p>
   </ul>
   <h2 class="heading settled" data-level="8" id="polls"><span class="secno">8. </span><span class="content">Polls</span><a class="self-link" href="#polls"></a></h2>
   <h3 class="heading settled" data-level="8.1" id="sg9_issaquah_2023"><span class="secno">8.1. </span><span class="content">SG9 polls, Issaquah 2023</span><a class="self-link" href="#sg9_issaquah_2023"></a></h3>
   <p><strong>POLL</strong>: The solution proposed in the paper "P2769: <code class="highlight"><c- n>get_element</c-></code> customization point object" should be renamed to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>get</c-></code>.</p>
   <table>
    <tbody>
     <tr>
      <th>SF
      <th>F
      <th>N
      <th>A
      <th>SA
     <tr>
      <td> 1
      <td>2
      <td>1
      <td>2
      <td> 1
   </table>
   <p><strong>POLL</strong>: The solution proposed in the paper "P2769: <code class="highlight"><c- n>get_element</c-></code> customization point object" should be moved out of the <code class="highlight"><c- n>ranges</c-></code> namespace (<code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>get_element</c-></code>).</p>
   <table>
    <tbody>
     <tr>
      <th>SF
      <th>F
      <th>N
      <th>A
      <th>SA
     <tr>
      <td> 2
      <td>4
      <td>0
      <td>1
      <td> 0
   </table>
   <h3 class="heading settled" data-level="8.2" id="lewg_telecon_2024_01_23"><span class="secno">8.2. </span><span class="content">Library Evolution Telecon 2024-01-23</span><a class="self-link" href="#lewg_telecon_2024_01_23"></a></h3>
   <p><strong>POLL</strong>: <a data-link-type="biblio" href="#biblio-p2769r1" title="get_element customization point object">[P2769R1]</a> (<code class="highlight"><c- n>get_element</c-></code> customization point object) needs to allow for user tuple-likes before it can ship</p>
   <table>
    <tbody>
     <tr>
      <th>SF
      <th>F
      <th>N
      <th>A
      <th>SA
     <tr>
      <td> 3
      <td>3
      <td>4
      <td>2
      <td> 0
   </table>
   <p><strong>POLL</strong>: LEWG should spend more time on <a data-link-type="biblio" href="#biblio-p2769r1" title="get_element customization point object">[P2769R1]</a> (<code class="highlight"><c- n>get_element</c-></code> customization point object)</p>
   <table>
    <tbody>
     <tr>
      <th>SF
      <th>F
      <th>N
      <th>A
      <th>SA
     <tr>
      <td> 5
      <td>4
      <td>0
      <td>2
      <td> 0
   </table>
   <h3 class="heading settled" data-level="8.3" id="polls_lewg_st_louis_2024"><span class="secno">8.3. </span><span class="content">Library Evolution, St. Louis 2024</span><a class="self-link" href="#polls_lewg_st_louis_2024"></a></h3>
   <p><strong>POLL</strong>: Remove the <code class="highlight"><c- n>get_key</c-></code> and <code class="highlight"><c- n>get_value</c-></code> variables.</p>
   <p><strong>Outcome</strong>: Unanimous dissent.</p>
   <p><strong>POLL</strong>: Rename <code class="highlight"><c- n>get_key</c-></code> and <code class="highlight"><c- n>get_value</c-></code> to <code class="highlight"><c- n>get_first</c-></code> and <code class="highlight"><c- n>get_second</c-></code>, respectively.</p>
   <p><strong>Outcome</strong>: Unanimous dissent.</p>
   <p><strong>POLL</strong>: <code class="highlight"><c- n>get_element</c-></code> should align with the structured binding protocol by attempting to find member functions named <code class="highlight"><c- n>get</c-></code>.</p>
   <table>
    <tbody>
     <tr>
      <th>SF 
      <th>F 
      <th>N 
      <th>A 
      <th>SA 
     <tr>
      <td> 5 
      <td>5 
      <td>1 
      <td>2 
      <td> 0 
   </table>
   <h2 class="heading settled" data-level="9" id="acknowledgements"><span class="secno">9. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <ul>
    <li data-md>
     <p>Thanks to Casey Carter for providing <code class="highlight"><c- n>adl_hook</c-></code> trick. See <a data-link-type="biblio" href="#biblio-p2769r2" title="get_element customization point object">[P2769R2]</a> for more details.</p>
    <li data-md>
     <p>Thanks to Corentin Jabot for continuous contribution that improved <code class="highlight"><c- n>get_element</c-></code> proposal.</p>
    <li data-md>
     <p>Thanks to Benjamin Brock for paper review and design discussions.</p>
    <li data-md>
     <p>Thanks to Mark Hoemmen for confirming wording changes for <code class="highlight"><c- n>submdspan</c-></code>.</p>
   </ul>
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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-p2141r1">[P2141R1]
   <dd>Antony Polukhin. <a href="https://wg21.link/p2141r1"><cite>Aggregates are named tuples</cite></a>. 3 May 2023. URL: <a href="https://wg21.link/p2141r1">https://wg21.link/p2141r1</a>
   <dt id="biblio-p2165r4">[P2165R4]
   <dd>Corentin Jabot. <a href="https://wg21.link/p2165r4"><cite>Compatibility between tuple, pair and tuple-like objects</cite></a>. 15 July 2022. URL: <a href="https://wg21.link/p2165r4">https://wg21.link/p2165r4</a>
   <dt id="biblio-p2169r3">[P2169R3]
   <dd>Corentin Jabot, Michael Park. <a href="https://wg21.link/p2169r3"><cite>A Nice Placeholder With No Name</cite></a>. 15 December 2022. URL: <a href="https://wg21.link/p2169r3">https://wg21.link/p2169r3</a>
   <dt id="biblio-p2547r1">[P2547R1]
   <dd>Lewis Baker, Corentin Jabot, Gašper Ažman. <a href="https://wg21.link/p2547r1"><cite>Language support for customisable functions</cite></a>. 16 July 2022. URL: <a href="https://wg21.link/p2547r1">https://wg21.link/p2547r1</a>
   <dt id="biblio-p2769r1">[P2769R1]
   <dd>Ruslan Arutyunyan, Alexey Kukanov. <a href="https://wg21.link/p2769r1"><cite>get_element customization point object</cite></a>. 17 May 2023. URL: <a href="https://wg21.link/p2769r1">https://wg21.link/p2769r1</a>
   <dt id="biblio-p2769r2">[P2769R2]
   <dd>Ruslan Arutyunyan, Alexey Kukanov. <a href="https://wg21.link/p2769r2"><cite>get_element customization point object</cite></a>. 26 June 2024. URL: <a href="https://wg21.link/p2769r2">https://wg21.link/p2769r2</a>
  </dl>