<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P2406R2: Add `lazy_counted_iterator`</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

	/* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
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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 */
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    blockquote c-[e] { color: inherit; } /* Name.Attribute */
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    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
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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-[ni] { color: inherit; } /* Name.Entity */
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    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 44af0bf3e, updated Fri Jul 29 17:05:16 2022 -0700" name="generator">
  <link href="https://wg21.link/P2406" rel="canonical">
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<style>
.ins, ins, ins *, span.ins, span.ins * {
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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">P2406R2<br>Add `lazy_counted_iterator`</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2023-01-10">2023-01-10</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P2406">https://wg21.link/P2406</a>
     <dt class="editor">Authors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:YehezkelShB@gmail.com">Yehezkel Bernat</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:YehudaMBer@gmail.com">Yehuda Bernat</a>
     <dt>Audience:
     <dd>SG9 (RANGES)
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/YehezkelShB/cpp_proposals/blob/master/P2406-counted-iterator-and-input-iterators.bs">GitHub</a>
    </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>`counted_iterator` increments its internal iterator even when reaching its own end, which makes it unusable in some cases, especially for input iterators. This paper suggests adding `lazy_counted_iterator` alternative to be used in such cases</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="#revision-history"><span class="secno">1</span> <span class="content">Revision History</span></a>
    <li>
     <a href="#problem-description"><span class="secno">2</span> <span class="content">Problem description</span></a>
     <ol class="toc">
      <li><a href="#range-with-the-exact-number-of-items"><span class="secno">2.1</span> <span class="content">Range with the exact number of items</span></a>
      <li><a href="#input_iterator-case"><span class="secno">2.2</span> <span class="content"><code class="highlight"><c- n>input_iterator</c-></code> case</span></a>
     </ol>
    <li><a href="#current-behavior-is-what-the-standard-mandates"><span class="secno">3</span> <span class="content">Current behavior is what the standard mandates</span></a>
    <li><a href="#desired-behavior"><span class="secno">4</span> <span class="content">Desired behavior</span></a>
    <li><a href="#high-level-design-of-the-proposed-solution"><span class="secno">5</span> <span class="content">High-level design of the proposed solution</span></a>
    <li>
     <a href="#design-points-for-discussion"><span class="secno">6</span> <span class="content">Design points for discussion</span></a>
     <ol class="toc">
      <li><a href="#random_access_iterator-case"><span class="secno">6.1</span> <span class="content"><code class="highlight"><c- n>random_access_iterator</c-></code> case</span></a>
      <li><a href="#consructing-with-0-count"><span class="secno">6.2</span> <span class="content">Consructing with 0 count</span></a>
      <li><a href="#base"><span class="secno">6.3</span> <span class="content"><code class="highlight"><c- n>base</c-><c- p>()</c-></code></span></a>
      <li><a href="#return-type-of-operator-int"><span class="secno">6.4</span> <span class="content">Return type of <code class="highlight"><c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>)</c-></code></span></a>
      <li>
       <a href="#why-lazy_take-instead-of-fixing-take"><span class="secno">6.5</span> <span class="content">Why <code class="highlight"><c- n>lazy_take</c-></code> instead of fixing <code class="highlight"><c- n>take</c-></code>?</span></a>
       <ol class="toc">
        <li><a href="#improve-discoverability-and-teachability"><span class="secno">6.5.1</span> <span class="content">Improve discoverability and teachability</span></a>
       </ol>
     </ol>
    <li>
     <a href="#design-alternatives"><span class="secno">7</span> <span class="content">Design alternatives</span></a>
     <ol class="toc">
      <li><a href="#option-1---as-closer-to-counted_iterator-as-possible"><span class="secno">7.1</span> <span class="content">Option 1 - As closer to <code class="highlight"><c- n>counted_iterator</c-></code> as possible</span></a>
      <li><a href="#option-2---capped-to-forward_iterator"><span class="secno">7.2</span> <span class="content">Option 2 - Capped to <code class="highlight"><c- n>forward_iterator</c-></code></span></a>
      <li><a href="#option-3---dont-read-it-if-created-with-0-count"><span class="secno">7.3</span> <span class="content">Option 3 - Don’t read it if created with 0 count</span></a>
      <li><a href="#option-4---dont-increment-the-underlying-iterator-until-really-required"><span class="secno">7.4</span> <span class="content">Option 4 - Don’t increment the underlying iterator until really required</span></a>
      <li>
       <a href="#alternatives-for-base"><span class="secno">7.5</span> <span class="content">Alternatives for <code class="highlight"><c- n>base</c-><c- p>()</c-></code></span></a>
       <ol class="toc">
        <li><a href="#option-i---dont-provide-base"><span class="secno">7.5.1</span> <span class="content">Option I - Don’t provide <code class="highlight"><c- n>base</c-><c- p>()</c-></code></span></a>
        <li><a href="#option-ii---make-lazy_counted_iterator-non-copyable"><span class="secno">7.5.2</span> <span class="content">Option II - Make <code class="highlight"><c- n>lazy_counted_iterator</c-></code> non-copyable</span></a>
        <li><a href="#option-iii---provide-base-and-mention-it-invalidates-other-copies"><span class="secno">7.5.3</span> <span class="content">Option III - Provide <code class="highlight"><c- n>base</c-><c- p>()</c-></code> and mention it invalidates other copies</span></a>
       </ol>
     </ol>
    <li>
     <a href="#wording"><span class="secno">8</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li>
       <a href="#wording-for-lazy_counted_iterator"><span class="secno">8.1</span> <span class="content">Wording for <code class="highlight"><c- n>lazy_counted_iterator</c-></code></span></a>
       <ol class="toc">
        <li><a href="#x1-class-template-lazy_counted_iterator-lazycountediterator"><span class="secno">8.1.1</span> <span class="content">x.1 Class template <code class="highlight"><c- n>lazy_counted_iterator</c-></code> [lazy.counted.iterator]</span></a>
        <li><a href="#x2-constructors-and-conversions-lazycountediterconst"><span class="secno">8.1.2</span> <span class="content">x.2 Constructors and conversions          [lazy.counted.iter.const]</span></a>
        <li><a href="#x3-accessors-lazycountediteraccess"><span class="secno">8.1.3</span> <span class="content">x.3 Accessors              [lazy.counted.iter.access]</span></a>
        <li><a href="#x4-element-access-lazycountediterelem"><span class="secno">8.1.4</span> <span class="content">x.4 Element access         [lazy.counted.iter.elem]</span></a>
        <li><a href="#x5-navigation-lazycountediternav"><span class="secno">8.1.5</span> <span class="content">x.5 Navigation            [lazy.counted.iter.nav]</span></a>
        <li><a href="#x6-comparisons-lazycounteditercmp"><span class="secno">8.1.6</span> <span class="content">x.6 Comparisons           [lazy.counted.iter.cmp]</span></a>
        <li><a href="#x7-customizations-lazycounteditercust"><span class="secno">8.1.7</span> <span class="content">x.7 Customizations       [lazy.counted.iter.cust]</span></a>
       </ol>
      <li><a href="#wording-for-viewslazy_counted-and-lazy_take_view"><span class="secno">8.2</span> <span class="content">Wording for <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-></code> and <code class="highlight"><c- n>lazy_take_view</c-></code></span></a>
      <li>
       <a href="#wording-for-viewslazy_counted"><span class="secno">8.3</span> <span class="content">Wording for <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-></code></span></a>
       <ol class="toc">
        <li><a href="#267x-lazy-counted-view-rangelazycounted"><span class="secno">8.3.1</span> <span class="content">26.7.x Lazy counted view        [range.lazy.counted]</span></a>
       </ol>
      <li>
       <a href="#wording-for-lazy_take_view"><span class="secno">8.4</span> <span class="content">Wording for <code class="highlight"><c- n>lazy_take_view</c-></code></span></a>
       <ol class="toc">
        <li><a href="#x1-overview-rangelazytakeoverview"><span class="secno">8.4.1</span> <span class="content">x.1 Overview              [range.lazy.take.overview]</span></a>
        <li><a href="#x2-class-template-lazy_take_view-rangelazytakeview"><span class="secno">8.4.2</span> <span class="content">x.2 Class template <code class="highlight"><c- n>lazy_take_view</c-></code> [range.lazy.take.view]</span></a>
        <li><a href="#x3-class-template-lazy_take_viewsentinel-rangelazytakesentinel"><span class="secno">8.4.3</span> <span class="content">x.3 Class template <code class="highlight"><c- n>lazy_take_view</c-><c- o>::</c-><c- n>sentinel</c-></code> [range.lazy.take.sentinel]</span></a>
       </ol>
      <li>
       <a href="#opens"><span class="secno">8.5</span> <span class="content">Opens</span></a>
       <ol class="toc">
        <li><a href="#iterator_concept-depends-on-iiterator_concept"><span class="secno">8.5.1</span> <span class="content"><code class="highlight"><c- n>iterator_concept</c-></code> depends on <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_concept</c-></code></span></a>
       </ol>
     </ol>
    <li><a href="#note-about-optimization"><span class="secno">9</span> <span class="content">Note about optimization</span></a>
    <li><a href="#acknowledgements"><span class="secno">10</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="revision-history"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#revision-history"></a></h2>
   <p>r2: Integrating SG9 feedback:</p>
   <ul>
    <li data-md>
     <p>Removing references to p2578, after SG9 vote against it</p>
    <li data-md>
     <p>Fix design suggested</p>
    <li data-md>
     <p>Add design alternatives</p>
   </ul>
   <p>r1: Improving many parts, following feedback from Inbal Levi and from Reddit
users - <a data-link-type="biblio" href="#biblio-d2406r1">[D2406R1]</a></p>
   <p>r0: initial revision - <a data-link-type="biblio" href="#biblio-p2406r0">[P2406R0]</a></p>
   <h2 class="heading settled" data-level="2" id="problem-description"><span class="secno">2. </span><span class="content">Problem description</span><a class="self-link" href="#problem-description"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="range-with-the-exact-number-of-items"><span class="secno">2.1. </span><span class="content">Range with the exact number of items</span><a class="self-link" href="#range-with-the-exact-number-of-items"></a></h3>
   <p>Look at this example code <a data-link-type="biblio" href="#biblio-ce-filter">[CE-FILTER]</a>:</p>
<pre class="language-c++ highlight"><c- cp>#include</c-> &lt;ranges>
<c- cp>#include</c-> &lt;iostream>
 
<c- n>namespace</c-> <c- n>rv</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>views</c-><c- p>;</c->
 
<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- k>for</c-> <c- p>(</c-><c- k>auto</c-> <c- n>i</c->  <c- o>:</c-> <c- n>rv</c-><c- o>::</c-><c- n>iota</c-><c- p>(</c-><c- mi>0</c-><c- p>)</c->
            <c- o>|</c-> <c- n>rv</c-><c- o>::</c-><c- n>filter</c-><c- p>([](</c-><c- k>auto</c-> <c- n>i</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- mi>11</c-><c- p>;</c-> <c- p>})</c->
            <c- o>|</c-> <c- n>rv</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>11</c-><c- p>))</c->
        <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>i</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>Compiler explorer gets a timeout when trying to run this simple example, instead
of printing the numbers from 0 to 10. Running the same code locally, it runs for
very long time. Tracking the roots of the issue, the problem is that <code class="highlight"><c- n>take</c-></code> uses <code class="highlight"><c- n>counted_iterator</c-></code> when the range isn’t <code class="highlight"><c- n>random_access</c-></code> and <code class="highlight"><c- n>counted_iterator</c-></code> increments the internal iterator even if the counter has reached the requested
count. In this case, the filter never returns when trying to increment it once
again (at least not until <code class="highlight"><c- n>iota</c-></code> reaches the UB case of signed overflow).</p>
   <p>The example above is just for illustration, but we can think about cases where
it isn’t clear for the user how many items the filter is expected to return, so
limiting the output count with <code class="highlight"><c- n>take</c-></code> becomes dangerous and results in
unexpected behavior.</p>
   <p>It means <code class="highlight"><c- n>take</c-></code> isn’t usable on ranges if we don’t know in advance that there is
an extra element in the range.</p>
   <h3 class="heading settled" data-level="2.2" id="input_iterator-case"><span class="secno">2.2. </span><span class="content"><code class="highlight"><c- n>input_iterator</c-></code> case</span><a class="self-link" href="#input_iterator-case"></a></h3>
   <p>Even more common problem is when using input ranges, e.g. <code class="highlight"><c- n>basic_istream_view</c-></code>.
In most of these cases, advancing the internal iterator when reaching the count
means eating an additional input that can’t be retrieved again later, or hanging
forever if no additional input exists and the stream isn’t closed. For example <a data-link-type="biblio" href="#biblio-ce-istream">[CE-ISTREAM]</a>:</p>
<pre class="language-c++ highlight"><c- cp>#include</c-> &lt;ranges>
<c- cp>#include</c-> &lt;iostream>
<c- cp>#include</c-> &lt;sstream>
<c- cp>#include</c-> &lt;cassert>
 
<c- n>namespace</c-> <c- n>rn</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- p>;</c->
<c- n>namespace</c-> <c- n>rv</c-> <c- o>=</c-> <c- n>rn</c-><c- o>::</c-><c- n>views</c-><c- p>;</c->
 
<c- b>int</c-> <c- nf>main</c-><c- p>()</c->
<c- p>{</c->
    <c- k>auto</c-> <c- n>iss</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>istringstream</c-><c- p>(</c-><c- s>"0 1 2"</c-><c- p>);</c->
    <c- k>for</c-> <c- p>(</c-><c- k>auto</c-> <c- n>i</c-> <c- o>:</c-> <c- n>rn</c-><c- o>::</c-><c- n>istream_view</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- n>iss</c-><c- p>)</c->
                  <c- o>|</c-> <c- n>rv</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>1</c-><c- p>))</c->
        <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>i</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c->
    <c- k>auto</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
    <c- n>iss</c-> <c- o>>></c-> <c- n>i</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>i</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c-> <c- c1>// flush it in case the assert fails</c->
    <c- n>assert</c-><c- p>(</c-><c- n>i</c-> <c- o>==</c-> <c- mi>1</c-><c- p>);</c-> <c- c1>// FAILS, i == 2</c->
<c- p>}</c->
</pre>
   <p>It makes it harder to use ranges for things like parsing input, if the rest of
the stream is still to be used or we aren’t sure there is any additional element
in the stream.</p>
   <p>Seems like this was discussed in <a data-link-type="biblio" href="#biblio-range-v3-issue57">[range-v3-issue57]</a>, and there was no decision
what is the right solution.</p>
   <h2 class="heading settled" data-level="3" id="current-behavior-is-what-the-standard-mandates"><span class="secno">3. </span><span class="content">Current behavior is what the standard mandates</span><a class="self-link" href="#current-behavior-is-what-the-standard-mandates"></a></h2>
   <p>Under 23.5.6.5 [counted.iter.nav], the standard defines the behavior of <code class="highlight"><c- k>operator</c-><c- o>++</c-></code> for <code class="highlight"><c- n>counted_iterator</c-></code> as:</p>
   <p><em>Effects</em>: Equivalent to:<br> <code class="highlight"><c- o>++</c-><c- n>current</c-><c- p>;</c-></code><br> <code class="highlight"><c- o>--</c-><c- n>length</c-><c- p>;</c-></code><br> <code class="highlight"><c- k>return</c-> <c- o>*</c-><c- k>this</c-><c- p>;</c-></code><br></p>
   <p>It means that even when <code class="highlight"><c- n>length</c-></code> becomes 0, the internal iterator is
incremented, thus consuming an additional item from the range, and causing the
effects mentioned above for input iterator case or when <code class="highlight"><c- o>++</c-></code> on the internal
iterator is costly (or never returns).</p>
   <h2 class="heading settled" data-level="4" id="desired-behavior"><span class="secno">4. </span><span class="content">Desired behavior</span><a class="self-link" href="#desired-behavior"></a></h2>
   <p>As long as <code class="highlight"><c- n>counted_iterator</c-></code> is valid (not equal to <code class="highlight"><c- n>default_sentinel</c-></code>), it
must never try to access more than <code class="highlight"><c- n>n</c-></code> items (when <code class="highlight"><c- n>n</c-></code> is the given count). If
the range doesn’t have <code class="highlight"><c- n>n</c-></code> items, the behavior is kept as is, i.e. it isn’t
defined (<code class="highlight"><c- k>operator</c-><c- o>++</c-></code> might hang forever or access things that shouldn’t be
accessed etc.).</p>
   <h2 class="heading settled" data-level="5" id="high-level-design-of-the-proposed-solution"><span class="secno">5. </span><span class="content">High-level design of the proposed solution</span><a class="self-link" href="#high-level-design-of-the-proposed-solution"></a></h2>
   <p>We propose adding a new iterator type, <code class="highlight"><c- n>lazy_counted_iterator</c-></code>. This type
behaves similarly to <code class="highlight"><c- n>counted_iterator</c-></code>, with changes to its operator definition
around 0 count so it doesn’t increment the internal iterator when reaching 0
count.</p>
   <p>Additionally, this requires adding <code class="highlight"><c- n>lazy_take</c-></code> and <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-></code> that
uses the new iterator instead of <code class="highlight"><c- n>counted_iterator</c-></code>.</p>
   <p>See below (<a href="#design-alternatives">§ 7 Design alternatives</a>) for more details.</p>
   <h2 class="heading settled" data-level="6" id="design-points-for-discussion"><span class="secno">6. </span><span class="content">Design points for discussion</span><a class="self-link" href="#design-points-for-discussion"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="random_access_iterator-case"><span class="secno">6.1. </span><span class="content"><code class="highlight"><c- n>random_access_iterator</c-></code> case</span><a class="self-link" href="#random_access_iterator-case"></a></h3>
   <p>To reduce the amount of changes required, we kept the current behavior for <code class="highlight"><c- n>random_access_iterator</c-></code> case, so we don’t have to touch the additional
operators defined only for this category. The rational behind it is that for <code class="highlight"><c- n>random_access_iterator</c-></code> case we can expect the view to either have all the
items ready or able to compute all of them efficiently, so it doesn’t suffer
from an issue similar to the one <code class="highlight"><c- n>forward_iterator</c-></code> might have.</p>
   <h3 class="heading settled" data-level="6.2" id="consructing-with-0-count"><span class="secno">6.2. </span><span class="content">Consructing with 0 count</span><a class="self-link" href="#consructing-with-0-count"></a></h3>
   <p>Similarly to <code class="highlight"><c- n>counted_iterator</c-></code>, <code class="highlight"><c- n>lazy_counted_iterator</c-></code> must allow constructing
with 0 count. In most design alternatives, this puts the iterator in an
inconsistent internal state, as the underlying iterator is expected to be "one
step back".</p>
   <p>Please note that <code class="highlight"><c- n>base</c-><c- p>()</c-></code> and decrementing are the only operations involving the
state of the internal iterator and still legal for <code class="highlight"><c- n>counted_iterator</c-></code> constructed with <code class="highlight"><c- n>n</c-><c- o>==</c-><c- mi>0</c-></code>.</p>
   <p>The options we see:</p>
   <p>Option 1: Require that if <code class="highlight"><c- n>n</c-> <c- o>==</c-> <c- mi>0</c-></code>, <code class="highlight"><c- n>i</c-></code> must be decrementable, and actually
decrement it in the c-tor. Please note that this obviously works only for <code class="highlight"><c- n>bidirectional_iterator</c-></code>. Other kind of iterators can be left as UB, or just
advice against calling <code class="highlight"><c- n>base</c-><c- p>()</c-></code> on the resulted <code class="highlight"><c- n>counted_iterator</c-></code> (which
doesn’t sound correct, blocking a basic operation like <code class="highlight"><c- n>base</c-><c- p>()</c-></code>).</p>
   <p>This option assumes the only reason to create such an <code class="highlight"><c- n>counted_iterator</c-></code> is to
decrement it later, so we always expect the given iterator to be decrementable.
Obviously, we can’t really assume it. The next operation could be to test for <code class="highlight"><c- n>count</c-><c- p>()</c-></code> before doing anything else and do nothing in the <code class="highlight"><c- mi>0</c-></code> case. This
happens naturally in a pipeline if the passed range becomes empty, for example.</p>
   <p>Option 2: Require that if <code class="highlight"><c- n>n</c-><c- o>==</c-><c- mi>0</c-></code>, <code class="highlight"><c- n>i</c-></code> must be "the one before" the actual
iterator (leaving it to the user to decide how to handle, and if neither <code class="highlight"><c- o>--</c-></code> nor <code class="highlight"><c- n>base</c-><c- p>()</c-></code> are ever called on it, it doesn’t matter what the user does). This
option changes behavior of existing code so it’s isn’t relevant either.</p>
   <p>Option 3: Mark this case internally (e.g. with <code class="highlight"><c- n>length</c-><c- o>=</c-><c- mi>-1</c-></code> or a boolean flag)
and handle specially when decrementing (<code class="highlight"><c- n>length</c-></code> "jumps" to <code class="highlight"><c- mi>1</c-></code> after
decrementing the internal iterator). Implementation must be careful if -1 is
used, instead of a separated flag, as comparison operators have to consider this
case too. Using a flag, OTOH, will probably push for separated specialization of
the whole class, so for random-access iterators this member will not exist.</p>
   <h3 class="heading settled" data-level="6.3" id="base"><span class="secno">6.3. </span><span class="content"><code class="highlight"><c- n>base</c-><c- p>()</c-></code></span><a class="self-link" href="#base"></a></h3>
   <p>When reaching 0 count, if <code class="highlight"><c- n>base</c-><c- p>()</c-></code> still simply returns the underlying iterator,
it returns the "one before" iterator in most cases.</p>
   <p>If we want it to return the actual end (as users expect) and make the behavior
consistent, it means we must advance the underlying iterator first. This doesn’t
allow <code class="highlight"><c- n>base</c-><c- p>()</c-></code> to be <code class="highlight"><c- k>const</c-></code> (at least logically) and invalidates other copies
of the iterator (in case of non-forward iterator). As this option seems the most
reasonable one, we based out <a href="#alternatives-for-base">§ 7.5 Alternatives for base()</a> suggestions on it.</p>
   <p>Another option is to return by value, so this increment only a copy of the
underlying operator and keeps <code class="highlight"><c- n>base</c-><c- p>()</c-></code> as <code class="highlight"><c- k>const</c-></code> member. Besides having the
invlidation problem here (maybe worse than before, as it invalidated the current
object too, so calling <code class="highlight"><c- n>base</c-><c- p>()</c-></code> twich isn’t allowed!), this prevents using it
with move-only iterators, as mentioned in <a data-link-type="biblio" href="#biblio-lwg3391">[LWG3391]</a>.</p>
   <h3 class="heading settled" data-level="6.4" id="return-type-of-operator-int"><span class="secno">6.4. </span><span class="content">Return type of <code class="highlight"><c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>)</c-></code></span><a class="self-link" href="#return-type-of-operator-int"></a></h3>
   <p>For non-forward iterators, today counted_iterator::operator++(int) is defined
with <code class="highlight"><c- k>return</c-> <c- n>current</c-><c- o>++</c-><c- p>;</c-></code> and <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-></code>, as such an iterator might return
a different type or not return anything at all (e.g. if it’s move only
iterator). <code class="highlight"><c- n>input_or_output_iterator</c-></code> is <code class="highlight"><c- n>weakly_incrementable</c-></code>, not <code class="highlight"><c- n>incrementable</c-></code>. As in this change we don’t always increment the iterator, there
is no consistent type to return. As a result, for non-forward iterators, we
define <code class="highlight"><c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>)</c-></code> as returning <code class="highlight"><c- b>void</c-></code>.</p>
   <h3 class="heading settled" data-level="6.5" id="why-lazy_take-instead-of-fixing-take"><span class="secno">6.5. </span><span class="content">Why <code class="highlight"><c- n>lazy_take</c-></code> instead of fixing <code class="highlight"><c- n>take</c-></code>?</span><a class="self-link" href="#why-lazy_take-instead-of-fixing-take"></a></h3>
   <p>We could have change <code class="highlight"><c- n>take</c-></code> to use <code class="highlight"><c- n>lazy_counted_iterator</c-></code> when constructed with
input (non lazy) range. Besides ABI considerations, we find it wrong if <code class="highlight"><c- n>take</c-></code> used to return one type (<code class="highlight"><c- n>counted_iterator</c-></code>) and now will start returning a
different one, <code class="highlight"><c- n>lazy_counted_iterator</c-></code>, as this is source-breaking change.
Additionally, as demonstrated above, there are cases where the user wants using <code class="highlight"><c- n>lazy_counted_iterator</c-></code> on forward iterators too, but this is something that
only the user know and we can’t automatically detect and decide on behalf of
them. We can’t change all cases of <code class="highlight"><c- n>take</c-></code> to use <code class="highlight"><c- n>lazy_counted_iterator</c-></code>, due to
the differences in behavior both for lazy input iterators and forward iterators
(that are not random access), as described below.</p>
   <p>We aren’t happy with the additional burden on teachability, but we believe in
most cases users can just use <code class="highlight"><c- n>lazy_take</c-></code> and it does The Right Thing. The only
point where users must be aware of it is when they use <code class="highlight"><c- n>base</c-><c- p>()</c-></code> method, which we
expect to be quite advance usage in general. Users who care about absolute
performance, can choose using <code class="highlight"><c- n>take</c-></code> when they know it works correctly for their
case.</p>
   <h4 class="heading settled" data-level="6.5.1" id="improve-discoverability-and-teachability"><span class="secno">6.5.1. </span><span class="content">Improve discoverability and teachability</span><a class="self-link" href="#improve-discoverability-and-teachability"></a></h4>
   <p>An option mentioned in the discussions was to improve discoverability by
renaming <code class="highlight"><c- n>counted_iterator</c-></code> and <code class="highlight"><c- n>take</c-></code> to <code class="highlight"><c- n>eager_counted_iterator</c-></code> and <code class="highlight"><c- n>eager_take</c-></code> and calling the new tools <code class="highlight"><c- n>counted_iterator</c-></code> and <code class="highlight"><c- n>take</c-></code>.</p>
   <p>Another variation was to call the new tools <code class="highlight"><c- n>lazy_counted_iterator</c-></code> and <code class="highlight"><c- n>lazy_take</c-></code> as suggested here, rename the existing one to have <code class="highlight"><c- n>eager_</c-></code> prefix
and add new versions of <code class="highlight"><c- n>counted_iterator</c-></code> and <code class="highlight"><c- n>take</c-></code> that alias the <code class="highlight"><c- n>lazy_</c-></code> version for <code class="highlight"><c- n>input_iterator</c-></code> case and keep aliasing the <code class="highlight"><c- n>eager_</c-></code> version for
the rest of the cases.</p>
   <p>While leaving the user the option to choose the option that fits their specific
use-case, it still provides a safer default.</p>
   <p>We believe this option is problematic as it might be source breaking (e.g. <code class="highlight"><c- n>counted_iterator</c-> <c- n>ci</c-> <c- o>=</c-> <c- n>take</c-><c- p>(</c-><c- n>i</c-><c- p>,</c-> <c- mi>4</c-><c- p>).</c-><c- n>begin</c-><c- p>();</c-></code> for the second variation, or any
usage of interface that isn’t provided by the new tools in any variation), but
still wanted to mention this option for discussion.</p>
   <h2 class="heading settled" data-level="7" id="design-alternatives"><span class="secno">7. </span><span class="content">Design alternatives</span><a class="self-link" href="#design-alternatives"></a></h2>
   <p>Following are the design alternatives we came up with, after the discussions
happend over the previous revisions:</p>
   <h3 class="heading settled" data-level="7.1" id="option-1---as-closer-to-counted_iterator-as-possible"><span class="secno">7.1. </span><span class="content">Option 1 - As closer to <code class="highlight"><c- n>counted_iterator</c-></code> as possible</span><a class="self-link" href="#option-1---as-closer-to-counted_iterator-as-possible"></a></h3>
   <p>With this option, <code class="highlight"><c- n>lazy_counted_iterator</c-></code> is almost identical to <code class="highlight"><c- n>counted_iterator</c-></code> with the following changes:</p>
   <ol>
    <li data-md>
     <p>For <code class="highlight"><c- n>random_access_iterator</c-></code>, the behavior is the same. <br> Reason: If an iterator is random access, we expect it to know when we reach
   the end. (Question: maybe we actually want <code class="highlight"><c- n>sized_iterator</c-></code> here?)</p>
    <li data-md>
     <p>Incrementing <code class="highlight"><c- n>lazy_counted_iterator</c-></code> doesn’t increment the underlying
   iterator when reaching 0 count.</p>
    <li data-md>
     <p>Decrementing <code class="highlight"><c- n>lazy_counted_iterator</c-></code> doesn’t decrement the underlying
   iterator when current count is 0.</p>
    <li data-md>
     <p>Implementation must handle correctly <code class="highlight"><c- n>lazy_counted_iterator</c-></code> constructed with
   0 count (e.g. marking it differently so it knows to decrement the underlying
   iterator despite the previous point).</p>
   </ol>
   <h3 class="heading settled" data-level="7.2" id="option-2---capped-to-forward_iterator"><span class="secno">7.2. </span><span class="content">Option 2 - Capped to <code class="highlight"><c- n>forward_iterator</c-></code></span><a class="self-link" href="#option-2---capped-to-forward_iterator"></a></h3>
   <p>With this option, <code class="highlight"><c- n>lazy_counted_iterator</c-></code> provides <code class="highlight"><c- n>forward_iterator</c-></code> interface
(at max). This removes the need to handle decrementing correctly. (Implementations
might still choose to track the case of <code class="highlight"><c- n>lazy_counted_iterator</c-></code> constructed with 0
count for validating the precondition of iterator comparison.)</p>
   <h3 class="heading settled" data-level="7.3" id="option-3---dont-read-it-if-created-with-0-count"><span class="secno">7.3. </span><span class="content">Option 3 - Don’t read it if created with 0 count</span><a class="self-link" href="#option-3---dont-read-it-if-created-with-0-count"></a></h3>
   <p>This option is similar to option 1, but to simplify the handling of an iterator
constructed with 0 count, disallows using such an iterator with any operation
that access the underlying iterator (no <code class="highlight"><c- o>--</c-></code> or <code class="highlight"><c- n>base</c-><c- p>()</c-></code>) or comparing to
another iterator (effectively allowing only calling <code class="highlight"><c- n>count</c-><c- p>()</c-></code> and comparing to
sentinel). Please note that if we choose option III below, it means that after
calling <code class="highlight"><c- n>base</c-><c- p>()</c-></code>, same restrictions apply on that iterator, including diallowing
another call to <code class="highlight"><c- n>base</c-><c- p>()</c-></code>.</p>
   <h3 class="heading settled" data-level="7.4" id="option-4---dont-increment-the-underlying-iterator-until-really-required"><span class="secno">7.4. </span><span class="content">Option 4 - Don’t increment the underlying iterator until really required</span><a class="self-link" href="#option-4---dont-increment-the-underlying-iterator-until-really-required"></a></h3>
   <p>Increment the underlying iterator only on first dereference. This makes
dereferencing a non-<code class="highlight"><c- k>const</c-></code> operation. Additionally, copying <code class="highlight"><c- n>lazy_counted_iterator</c-></code> of non-forward iterator (that wasn’t dereferenced yet
for the current item) and then dereferencing one of them resulted with
invalidation of the other one. To improve the situation we can make <code class="highlight"><c- n>lazy_counted_iterator</c-></code> non-copyable.</p>
   <h3 class="heading settled" data-level="7.5" id="alternatives-for-base"><span class="secno">7.5. </span><span class="content">Alternatives for <code class="highlight"><c- n>base</c-><c- p>()</c-></code></span><a class="self-link" href="#alternatives-for-base"></a></h3>
   <h4 class="heading settled" data-level="7.5.1" id="option-i---dont-provide-base"><span class="secno">7.5.1. </span><span class="content">Option I - Don’t provide <code class="highlight"><c- n>base</c-><c- p>()</c-></code></span><a class="self-link" href="#option-i---dont-provide-base"></a></h4>
   <p>In this option, we just don’t provide <code class="highlight"><c- n>base</c-><c- p>()</c-></code>. Returning the underlying
iterator when count is 0 can be confusing (it depends if <code class="highlight"><c- n>lazy_counted_iterator</c-></code> was created with 0 count or reached this state by incrementing it) or invoking
the same issue we try to solve here (if we increment the iterator here).
Additionally, if we increment the underlying iterator on first access to <code class="highlight"><c- n>base</c-><c- p>()</c-></code>, it can’t be <code class="highlight"><c- k>const</c-></code> (at least not logically), it invalidates other
copies of the iterator in case of <code class="highlight"><c- n>input_iterator</c-></code> and (even if we go with
option 2 above) adds back the complexity of tracking if the iterator was
incremented already (e.g. constructed with 0 count).</p>
   <p>If we go with option 1 above, it means that for <code class="highlight"><c- n>random_access_iterator</c-></code> we keep
providing <code class="highlight"><c- n>base</c-><c- p>()</c-></code>, like <code class="highlight"><c- n>counted_iterator</c-></code> does, as there is no reason not to.</p>
   <p>Wording must be adjustment when other functionality is defined in terms of
calling <code class="highlight"><c- n>base</c-><c- p>()</c-></code> and implementation can access the underlying iterator directly
when needed.</p>
   <h4 class="heading settled" data-level="7.5.2" id="option-ii---make-lazy_counted_iterator-non-copyable"><span class="secno">7.5.2. </span><span class="content">Option II - Make <code class="highlight"><c- n>lazy_counted_iterator</c-></code> non-copyable</span><a class="self-link" href="#option-ii---make-lazy_counted_iterator-non-copyable"></a></h4>
   <p>Provide <code class="highlight"><c- n>base</c-><c- p>()</c-></code> function which increments the underlying iterator if needed.
Users are warned that calling <code class="highlight"><c- n>base</c-><c- p>()</c-></code> means non-lazy behavior. Implementation
must not invoke it if not explicitly requested (same wording adjustment is
required as in the previous option). We make <code class="highlight"><c- n>lazy_counted_iterator</c-></code> non-copyable even if the underlying iterator is copyable. If it’s non-copyable,
we don’t have to worry about invalidation (at least not directly; it still
invalidates copies of the underlying iterator).</p>
   <h4 class="heading settled" data-level="7.5.3" id="option-iii---provide-base-and-mention-it-invalidates-other-copies"><span class="secno">7.5.3. </span><span class="content">Option III - Provide <code class="highlight"><c- n>base</c-><c- p>()</c-></code> and mention it invalidates other copies</span><a class="self-link" href="#option-iii---provide-base-and-mention-it-invalidates-other-copies"></a></h4>
   <p>Provide <code class="highlight"><c- n>base</c-><c- p>()</c-></code> and specify it as invalidating other copies, leaving it to the
users to use it correctly. Implementation must increment the underlying iterator
if needed (reached 0 count and this is the first time <code class="highlight"><c- n>base</c-><c- p>()</c-></code> is called).</p>
   <h2 class="heading settled" data-level="8" id="wording"><span class="secno">8. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>Wording is for the design recommended by SG9, Option 2 + I.</p>
   <h3 class="heading settled" data-level="8.1" id="wording-for-lazy_counted_iterator"><span class="secno">8.1. </span><span class="content">Wording for <code class="highlight"><c- n>lazy_counted_iterator</c-></code></span><a class="self-link" href="#wording-for-lazy_counted_iterator"></a></h3>
   <p>Under Header <code class="highlight"><c- o>&lt;</c-><c- n>iterator</c-><c- o>></c-></code> synopsis [<strong>iterator.syn</strong>] add the new type:</p>
<pre class="language-c++ highlight"><c- c1>// [iterators.counted], counted iterators</c->
 <c- n>template</c-><c- o>&lt;</c-><c- n>input_or_output_iterator</c-> <c- n>I</c-><c- o>></c-> <c- n>class</c-> <c- n>counted_iterator</c-><c- p>;</c->             <c- c1>// freestanding</c->

 <c- n>template</c-><c- o>&lt;</c-><c- n>input_iterator</c-> <c- n>I</c-><c- o>></c->
   <c- n>requires</c-> <c- n>see</c-> <c- n>below</c->
   <c- k>struct</c-> <c- nc>iterator_traits</c-><c- o>&lt;</c-><c- n>counted_iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>>></c-><c- p>;</c->            <c- c1>// freestanding</c->
</pre>
   <ins>
<pre class="language-c++ highlight"><c- c1>// [iterators.lazy.counted], lazy counted iterators</c->
 <c- n>template</c-><c- o>&lt;</c-><c- n>input_or_output_iterator</c-> <c- n>I</c-><c- o>></c-> <c- n>class</c-> <c- n>lazy_counted_iterator</c-><c- p>;</c->             <c- c1>// freestanding</c->

 <c- n>template</c-><c- o>&lt;</c-><c- n>input_iterator</c-> <c- n>I</c-><c- o>></c->
   <c- n>requires</c-> <c- n>see</c-> <c- n>below</c->
   <c- k>struct</c-> <c- nc>iterator_traits</c-><c- o>&lt;</c-><c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>>></c-><c- p>;</c->            <c- c1>// freestanding</c->
</pre>
   </ins>
   <p>In Iterator adaptors [<strong>predef.iterators</strong>], after 25.5.7 Counted iterators [<strong>iterators.counted</strong>] add new section:</p>
   <ins> 25.5.x Lazy counted iterators     [iterators.lazy.counted] </ins>
   <p>Under this section add:</p>
   <h4 class="heading settled" data-level="8.1.1" id="x1-class-template-lazy_counted_iterator-lazycountediterator"><span class="secno">8.1.1. </span><span class="content">x.1 Class template <code class="highlight"><c- n>lazy_counted_iterator</c-></code> [lazy.counted.iterator]</span><a class="self-link" href="#x1-class-template-lazy_counted_iterator-lazycountediterator"></a></h4>
   <p>Class template <code class="highlight"><c- n>lazy_counted_iterator</c-></code> is an iterator adaptor with the same behavior
as the underlying iterator except that it keeps track of the distance to the end
of its range. It can be used together with <code class="highlight"><c- n>default_sentinel</c-></code> in calls to generic
algorithms to operate on a range of N elements starting at a given position
without needing to know the end position a priori.</p>
   <p>[Example 1:</p>
<pre class="language-c++ highlight"><c- n>list</c-><c- o>&lt;</c-><c- n>string</c-><c- o>></c-> <c- n>s</c-><c- p>;</c->
<c- c1>// populate the list s with at least 10 strings</c->
<c- n>vector</c-><c- o>&lt;</c-><c- n>string</c-><c- o>></c-> <c- n>v</c-><c- p>;</c->
<c- c1>// copies 10 strings into v:</c->
<c- n>ranges</c-><c- o>::</c-><c- n>copy</c-><c- p>(</c-><c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>s</c-><c- p>.</c-><c- n>begin</c-><c- p>(),</c-> <c- mi>10</c-><c- p>),</c-> <c- n>default_sentinel</c-><c- p>,</c-> <c- n>back_inserter</c-><c- p>(</c-><c- n>v</c-><c- p>));</c->
</pre>
   — end example] 
   <p>Given two values <code class="highlight"><c- n>i1</c-></code> and <code class="highlight"><c- n>i2</c-></code> of types <code class="highlight"><c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I1</c-><c- o>></c-></code> and <code class="highlight"><c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>></c-></code>,
let <code class="highlight"><c- n>i1base</c-></code> denote the underlying iterator of <code class="highlight"><c- n>i1</c-></code> and <code class="highlight"><c- n>i2base</c-></code> denote the underlying iterator of <code class="highlight"><c- n>i2</c-></code>. <code class="highlight"><c- n>i1</c-></code> and <code class="highlight"><c- n>i2</c-></code> refer to elements of the same sequence if and only if there exists some integer
n such that <code class="highlight"><c- n>next</c-><c- p>(</c-><c- n>i1base</c-><c- p>,</c-> <c- n>i1</c-><c- p>.</c-><c- n>count</c-><c- p>()</c-> <c- o>+</c-> <c- n>n</c-><c- p>)</c-></code> and <code class="highlight"><c- n>next</c-><c- p>(</c-><c- n>i2base</c-><c- p>,</c-> <c- n>i2</c-><c- p>.</c-><c- n>count</c-><c- p>()</c-> <c- o>+</c-> <c- n>n</c-><c- p>)</c-></code> refer to the same (possibly past-the-end) element.</p>
<pre class="language-c++ highlight"><c- n>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>input_or_output_iterator</c-> <c- n>I</c-><c- o>></c->
  <c- n>class</c-> <c- n>lazy_counted_iterator</c-> <c- p>{</c->
  <c- n>public</c-><c- o>:</c->
    <c- n>using</c-> <c- n>iterator_type</c-> <c- o>=</c-> <c- n>I</c-><c- p>;</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>iter_value_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c->                         <c- c1>// present only</c->
                        <c- c1>// if I models indirectly_readable</c->
    <c- n>using</c-> <c- n>difference_type</c-> <c- o>=</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c->
    <c- n>using</c-> <c- n>iterator_concept</c->  <c- o>=</c-> <c- n>see</c-> <c- n>below</c-><c- p>;</c->                      <c- c1>// not always present</c->
    <c- n>using</c-> <c- n>iterator_category</c-> <c- o>=</c-> <c- n>see</c-> <c- n>below</c-><c- p>;</c->                      <c- c1>// not always present</c->
    <c- n>constexpr</c-> <c- nf>lazy_counted_iterator</c-><c- p>()</c-> <c- n>requires</c-> <c- n>default_initializable</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>constexpr</c-> <c- nf>lazy_counted_iterator</c-><c- p>(</c-><c- n>I</c-> <c- n>x</c-><c- p>,</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>n</c-><c- p>);</c->
    <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>I2</c-><c- o>></c->
      <c- n>requires</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>I2</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>I</c-><c- o>></c->
        <c- n>constexpr</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>);</c->

    <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>I2</c-><c- o>></c->
      <c- n>requires</c-> <c- n>assignable_from</c-><c- o>&lt;</c-><c- n>I</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>I2</c-><c- o>&amp;></c->
        <c- n>constexpr</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>);</c->

    <c- n>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>count</c-><c- p>()</c-> <c- k>const</c-> <c- n>noexcept</c-><c- p>;</c->
    <c- n>constexpr</c-> <c- nf>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- n>operator</c-><c- o>*</c-><c- p>();</c->
    <c- n>constexpr</c-> <c- nf>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- n>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c->
      <c- n>requires</c-> <c- n>dereferenceable</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>I</c-><c- o>></c-><c- p>;</c->

    <c- n>constexpr</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>++</c-><c- p>();</c->
    <c- n>constexpr</c-> <c- b>void</c-> <c- n>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>);</c->
    <c- n>constexpr</c-> <c- n>lazy_counted_iterator</c-> <c- n>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>)</c->
      <c- n>requires</c-> <c- n>forward_iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c->

    <c- n>template</c-><c- o>&lt;</c-><c- n>common_with</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
      <c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>></c-> <c- n>operator</c-><c- o>-</c-><c- p>(</c->
        <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
    <c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>operator</c-><c- o>-</c-><c- p>(</c->
      <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel_t</c-><c- p>);</c->
    <c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>operator</c-><c- o>-</c-><c- p>(</c->
      <c- n>default_sentinel_t</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>);</c->

    <c- n>template</c-><c- o>&lt;</c-><c- n>common_with</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
      <c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c->
        <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
    <c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c->
      <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel_t</c-><c- p>);</c->

    <c- n>template</c-><c- o>&lt;</c-><c- n>common_with</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
      <c- n>friend</c-> <c- n>constexpr</c-> <c- n>strong_ordering</c-> <c- n>operator</c-><c- o>&lt;=></c-><c- p>(</c->
        <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->

    <c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_rvalue_reference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>iter_move</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>i</c-><c- p>)</c->
      <c- n>noexcept</c-><c- p>(</c-><c- n>noexcept</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>iter_move</c-><c- p>(</c-><c- n>i</c-><c- p>.</c-><c- n>current</c-><c- p>)))</c->
        <c- n>requires</c-> <c- n>input_iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c->
    <c- n>template</c-><c- o>&lt;</c-><c- n>indirectly_swappable</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
      <c- n>friend</c-> <c- n>constexpr</c-> <c- b>void</c-> <c- n>iter_swap</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>)</c->
        <c- n>noexcept</c-><c- p>(</c-><c- n>noexcept</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>iter_swap</c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><c- n>current</c-><c- p>,</c-> <c- n>y</c-><c- p>.</c-><c- n>current</c-><c- p>)));</c->

  <c- n>private</c-><c- o>:</c->
    <c- n>I</c-> <c- n>current</c-> <c- o>=</c-> <c- n>I</c-><c- p>();</c->                    <c- c1>// exposition only</c->
    <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>length</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->    <c- c1>// exposition only</c->
  <c- p>};</c->
<c- p>}</c->
</pre>
   <p>The member typedef-name <code class="highlight"><c- n>iterator_concept</c-></code> is defined if and only if the
qualified-id <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_concept</c-></code> is valid and denotes a type. In that case, <code class="highlight"><c- n>iterator_concept</c-></code> denotes</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>forward_iterator_tag</c-></code> if the type <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_concept</c-></code> models <code class="highlight"><c- n>derived_from</c-><c- o>&lt;</c-><c- n>forward_iterator_tag</c-><c- o>></c-></code>, and</p>
    <li data-md>
     <p><code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_concept</c-></code> otherwise.</p>
   </ul>
   <p>The member typedef-name <code class="highlight"><c- n>iterator_category</c-></code> is defined if and only if the
qualified-id <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_category</c-></code> is valid and denotes a type. In that case, <code class="highlight"><c- n>iterator_category</c-></code> denotes</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>forward_iterator_tag</c-></code> if the type <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_category</c-></code> models <code class="highlight"><c- n>derived_from</c-><c- o>&lt;</c-><c- n>forward_iterator_tag</c-><c- o>></c-></code>, and</p>
    <li data-md>
     <p><code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_category</c-></code> otherwise.</p>
   </ul>
   <h4 class="heading settled" data-level="8.1.2" id="x2-constructors-and-conversions-lazycountediterconst"><span class="secno">8.1.2. </span><span class="content">x.2 Constructors and conversions          [lazy.counted.iter.const]</span><a class="self-link" href="#x2-constructors-and-conversions-lazycountediterconst"></a></h4>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>I</c-> <c- n>i</c-><c- p>,</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>n</c-><c- p>);</c-></code></p>
   <p><em>Preconditions</em>: n >= 0.</p>
   <p><em>Effects</em>: Initializes <code class="highlight"><c- n>current</c-></code> with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>i</c-><c- p>)</c-></code> and <code class="highlight"><c- n>length</c-></code> with <code class="highlight"><c- n>n</c-></code>.</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>I2</c-><c- o>></c->
  <c- n>requires</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>I2</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>I</c-><c- o>></c->
    <c- n>constexpr</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>);</c->
</pre>
   <p><em>Effects</em>: Initializes <code class="highlight"><c- n>current</c-></code> with <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>current</c-></code> and <code class="highlight"><c- n>length</c-></code> with <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>length</c-></code>.</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>I2</c-><c- o>></c->
  <c- n>requires</c-> <c- n>assignable_from</c-><c- o>&lt;</c-><c- n>I</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>I2</c-><c- o>&amp;></c->
    <c- n>constexpr</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>x</c-><c- p>);</c->
</pre>
   <p><em>Effects</em>: Assigns <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>current</c-></code> to <code class="highlight"><c- n>current</c-></code> and <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>length</c-></code> to <code class="highlight"><c- n>length</c-></code>.</p>
   <p><em>Returns</em>: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
   <h4 class="heading settled" data-level="8.1.3" id="x3-accessors-lazycountediteraccess"><span class="secno">8.1.3. </span><span class="content">x.3 Accessors              [lazy.counted.iter.access]</span><a class="self-link" href="#x3-accessors-lazycountediteraccess"></a></h4>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>count</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c-></code></p>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>length</c-><c- p>;</c-></code></p>
   <h4 class="heading settled" data-level="8.1.4" id="x4-element-access-lazycountediterelem"><span class="secno">8.1.4. </span><span class="content">x.4 Element access         [lazy.counted.iter.elem]</span><a class="self-link" href="#x4-element-access-lazycountediterelem"></a></h4>
<pre class="language-c++ highlight"><c- n>constexpr</c-> <c- nf>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- n>operator</c-><c- o>*</c-><c- p>();</c->
<c- n>constexpr</c-> <c- nf>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- n>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c->
  <c- n>requires</c-> <c- n>dereferenceable</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>I</c-><c- o>></c-><c- p>;</c->
</pre>
   <p><em>Preconditions</em>: <code class="highlight"><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code> is <code class="highlight">true</code>.</p>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- o>*</c-><c- n>current</c-><c- p>;</c-></code></p>
   <h4 class="heading settled" data-level="8.1.5" id="x5-navigation-lazycountediternav"><span class="secno">8.1.5. </span><span class="content">x.5 Navigation            [lazy.counted.iter.nav]</span><a class="self-link" href="#x5-navigation-lazycountediternav"></a></h4>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>++</c-><c- p>();</c-></code></p>
   <p><em>Preconditions</em>: <code class="highlight"><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code>.</p>
   <p><em>Effects</em>: Equivalent to:</p>
<pre class="language-c++ highlight"><c- k>if</c-> <c- p>(</c-><c- n>length</c-> <c- o>></c-> <c- mi>1</c-><c- p>)</c-> <c- o>++</c-><c- n>current</c-><c- p>;</c->
<c- o>--</c-><c- n>length</c-><c- p>;</c->
<c- k>return</c-> <c- o>*</c-><c- n>this</c-><c- p>;</c->
</pre>
   <p><code class="highlight"><c- k>constexpr</c-> <c- b>void</c-> <c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>);</c-></code></p>
   <p><em>Preconditions</em>: <code class="highlight"><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code>.</p>
   <p><em>Effects</em>: Equivalent to:</p>
<pre class="language-c++ highlight"><c- o>--</c-><c- n>length</c-><c- p>;</c->
<c- n>try</c-> <c- p>{</c-> <c- k>if</c-> <c- p>(</c-><c- n>length</c-><c- p>)</c-> <c- n>current</c-><c- o>++</c-><c- p>;</c-> <c- p>}</c->
<c- n>catch</c-><c- p>(...)</c-> <c- p>{</c-> <c- o>++</c-><c- n>length</c-><c- p>;</c-> <c- n>throw</c-><c- p>;</c-> <c- p>}</c->
</pre>
<pre class="language-c++ highlight"><c- n>constexpr</c-> <c- n>lazy_counted_iterator</c-> <c- n>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>)</c->
  <c- n>requires</c-> <c- n>forward_iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c->
</pre>
   <em>Effects</em>: Equivalent to: 
<pre class="language-c++ highlight"><c- n>lazy_counted_iterator</c-> <c- n>tmp</c-> <c- o>=</c-> <c- o>*</c-><c- n>this</c-><c- p>;</c->
<c- o>++*</c-><c- n>this</c-><c- p>;</c->
<c- k>return</c-> <c- n>tmp</c-><c- p>;</c->
</pre>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>common_with</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
  <c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>></c-> <c- n>operator</c-><c- o>-</c-><c- p>(</c->
    <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <em>Preconditions</em>: <code class="highlight"><c- n>x</c-></code> and <code class="highlight"><c- n>y</c-></code> refer to elements of the same sequence ([lazy.counted.iterator]). 
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>y</c-><c- p>.</c-><c- n>length</c-> <c- o>-</c-> <c- n>x</c-><c- p>.</c-><c- n>length</c-><c- p>;</c-></code></p>
<pre class="language-c++ highlight"><c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>operator</c-><c- o>-</c-><c- p>(</c->
  <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel_t</c-><c- p>);</c->
</pre>
   <em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- o>-</c-><c- n>x</c-><c- p>.</c-><c- n>length</c-><c- p>;</c-></code> 
<pre class="language-c++ highlight"><c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>operator</c-><c- o>-</c-><c- p>(</c->
  <c- n>default_sentinel_t</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>y</c-><c- p>.</c-><c- n>length</c-><c- p>;</c-></code> 
   <h4 class="heading settled" data-level="8.1.6" id="x6-comparisons-lazycounteditercmp"><span class="secno">8.1.6. </span><span class="content">x.6 Comparisons           [lazy.counted.iter.cmp]</span><a class="self-link" href="#x6-comparisons-lazycounteditercmp"></a></h4>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>common_with</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
  <c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c->
    <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <em>Preconditions</em>: <code class="highlight"><c- n>x</c-></code> and <code class="highlight"><c- n>y</c-></code> refer to elements of the same sequence ([lazy.counted.iterator]). 
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>x</c-><c- p>.</c-><c- n>length</c-> <c- o>==</c-> <c- n>y</c-><c- p>.</c-><c- n>length</c-><c- p>;</c-></code></p>
<pre class="language-c++ highlight"><c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c->
  <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- n>default_sentinel_t</c-><c- p>);</c->
</pre>
   <em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>x</c-><c- p>.</c-><c- n>length</c-> <c- o>==</c-> <c- mi>0</c-><c- p>;</c-></code> 
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>common_with</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
  <c- n>friend</c-> <c- n>constexpr</c-> <c- n>strong_ordering</c-> <c- n>operator</c-><c- o>&lt;=></c-><c- p>(</c->
    <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <em>Preconditions</em>: <code class="highlight"><c- n>x</c-></code> and <code class="highlight"><c- n>y</c-></code> refer to elements of the same sequence ([lazy.counted.iterator]). 
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>y</c-><c- p>.</c-><c- n>length</c-> <c- o>&lt;=></c-> <c- n>x</c-><c- p>.</c-><c- n>length</c-><c- p>;</c-></code></p>
   <p>[Note 1: The argument order in the Effects: element is reversed because <code class="highlight"><c- n>length</c-></code> counts down, not up. — end note]</p>
   <h4 class="heading settled" data-level="8.1.7" id="x7-customizations-lazycounteditercust"><span class="secno">8.1.7. </span><span class="content">x.7 Customizations       [lazy.counted.iter.cust]</span><a class="self-link" href="#x7-customizations-lazycounteditercust"></a></h4>
<pre class="language-c++ highlight"><c- n>friend</c-> <c- n>constexpr</c-> <c- n>iter_rvalue_reference_t</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c->
  <c- n>iter_move</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>i</c-><c- p>)</c->
    <c- n>noexcept</c-><c- p>(</c-><c- n>noexcept</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>iter_move</c-><c- p>(</c-><c- n>i</c-><c- p>.</c-><c- n>current</c-><c- p>)))</c->
    <c- n>requires</c-> <c- n>input_iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c->
</pre>
   <em>Preconditions</em>: <code class="highlight"><c- n>i</c-><c- p>.</c-><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code> is <code class="highlight">true</code>. 
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>iter_move</c-><c- p>(</c-><c- n>i</c-><c- p>.</c-><c- n>current</c-><c- p>);</c-></code></p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>indirectly_swappable</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-> <c- n>I2</c-><c- o>></c->
  <c- n>friend</c-> <c- n>constexpr</c-> <c- b>void</c->
    <c- n>iter_swap</c-><c- p>(</c-><c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>I2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>)</c->
      <c- n>noexcept</c-><c- p>(</c-><c- n>noexcept</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>iter_swap</c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><c- n>current</c-><c- p>,</c-> <c- n>y</c-><c- p>.</c-><c- n>current</c-><c- p>)));</c->
</pre>
   <em>Preconditions</em>: Both <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code> and <code class="highlight"><c- n>y</c-><c- p>.</c-><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code> are <code class="highlight">true</code>. 
   <p><em>Effects</em>: Equivalent to <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>iter_swap</c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><c- n>current</c-><c- p>,</c-> <c- n>y</c-><c- p>.</c-><c- n>current</c-><c- p>)</c-></code>.</p>
   <h3 class="heading settled" data-level="8.2" id="wording-for-viewslazy_counted-and-lazy_take_view"><span class="secno">8.2. </span><span class="content">Wording for <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-></code> and <code class="highlight"><c- n>lazy_take_view</c-></code></span><a class="self-link" href="#wording-for-viewslazy_counted-and-lazy_take_view"></a></h3>
   <p>Under Header <code class="highlight"><c- o>&lt;</c-><c- n>ranges</c-><c- o>></c-></code> synopsis [<strong>ranges.syn</strong>] add the new types:</p>
<pre class="language-c++ highlight"><c- c1>// [range.counted], counted view</c->
<c- n>namespace</c-> <c- n>views</c-> <c- p>{</c-> <c- kr>inline</c-> <c- n>constexpr</c-> <c- n>unspecified</c-> <c- n>counted</c-> <c- o>=</c-> <c- n>unspecified</c-><c- p>;</c-> <c- p>}</c->     <c- c1>// freestanding</c->
</pre>
   <ins>
<pre class="language-c++ highlight"><c- c1>// [range.lazy.counted], lazy counted view</c->
<c- n>namespace</c-> <c- n>views</c-> <c- p>{</c-> <c- kr>inline</c-> <c- n>constexpr</c-> <c- n>unspecified</c-> <c- n>lazy_counted</c-> <c- o>=</c-> <c- n>unspecified</c-><c- p>;</c-> <c- p>}</c->     <c- c1>// freestanding</c->
</pre>
   </ins>
<pre class="language-c++ highlight"> <c- c1>// [range.take], take view</c->
<c- n>template</c-><c- o>&lt;</c-><c- n>view</c-><c- o>></c-> <c- n>class</c-> <c- n>take_view</c-><c- p>;</c->        <c- c1>// freestanding</c->

<c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c->
  <c- n>constexpr</c-> <c- b>bool</c-> <c- n>enable_borrowed_range</c-><c- o>&lt;</c-><c- n>take_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>=</c->      <c- c1>// freestanding</c->
    <c- n>enable_borrowed_range</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c->

<c- n>namespace</c-> <c- n>views</c-> <c- p>{</c-> <c- kr>inline</c-> <c- n>constexpr</c-> <c- n>unspecified</c-> <c- n>take</c-> <c- o>=</c-> <c- n>unspecified</c-><c- p>;</c-> <c- p>}</c->        <c- c1>// freestanding</c->
</pre>
   <ins>
<pre class="language-c++ highlight"> <c- c1>// [range.lazy.take], lazy take view</c->
<c- n>template</c-><c- o>&lt;</c-><c- n>view</c-><c- o>></c-> <c- n>class</c-> <c- n>lazy_take_view</c-><c- p>;</c->        <c- c1>// freestanding</c->

<c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c->
  <c- n>constexpr</c-> <c- b>bool</c-> <c- n>enable_borrowed_range</c-><c- o>&lt;</c-><c- n>lazy_take_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>=</c->      <c- c1>// freestanding</c->
    <c- n>enable_borrowed_range</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c->

<c- n>namespace</c-> <c- n>views</c-> <c- p>{</c-> <c- kr>inline</c-> <c- n>constexpr</c-> <c- n>unspecified</c-> <c- n>lazy_take</c-> <c- o>=</c-> <c- n>unspecified</c-><c- p>;</c-> <c- p>}</c->        <c- c1>// freestanding</c->
</pre>
   </ins>
   <h3 class="heading settled" data-level="8.3" id="wording-for-viewslazy_counted"><span class="secno">8.3. </span><span class="content">Wording for <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-></code></span><a class="self-link" href="#wording-for-viewslazy_counted"></a></h3>
   <p>In Range adaptors [<strong>range.adaptors</strong>], after 26.7.18 Counted view [<strong>range.counted</strong>] add new section:</p>
   <h4 class="heading settled" data-level="8.3.1" id="267x-lazy-counted-view-rangelazycounted"><span class="secno">8.3.1. </span><span class="content">26.7.x Lazy counted view        [range.lazy.counted]</span><a class="self-link" href="#267x-lazy-counted-view-rangelazycounted"></a></h4>
   <p>A counted view presents a view of the elements of the counted range
([iterator.requirements.general]) <code class="highlight"><c- n>i</c-> <c- o>+</c-> <c- p>[</c-><c- mi>0</c-><c- p>,</c-> <c- n>n</c-><c- p>)</c-></code> for an iterator <code class="highlight"><c- n>i</c-></code> and
non-negative integer <code class="highlight"><c- n>n</c-></code>.</p>
   <p>The name <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-></code> denotes a customization point object
([customization.point.object]). Let <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> be expressions, let <code class="highlight"><c- n>T</c-></code> be <code class="highlight"><c- n>decay_t</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-><c- o>></c-></code>, and let <code class="highlight"><c- n>D</c-></code> be <code class="highlight"><c- n>iter_difference_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>. If <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>F</c-><c- p>))</c-></code> does not model <code class="highlight"><c- n>convertible_to</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-></code>, <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is ill-formed.</p>
   <p>[Note 1: This case can result in substitution failure when <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> appears in the immediate context of a template instantiation. — end note]</p>
   <p>Otherwise, <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_counted</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is expression-equivalent to:</p>
   <ul>
    <li data-md>
     <p>If <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>contiguous_iterator</c-></code>, then <code class="highlight"><c- n>span</c-><c- p>(</c-><c- n>to_address</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- b>size_t</c-><c- o>></c-><c- p>(</c-><c- n>static_</c-><c- o>-</c-><c- n>cast</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-><c- p>(</c-><c- n>F</c-><c- p>)))</c-></code>.</p>
    <li data-md>
     <p>Otherwise, if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>random_access_iterator</c-></code>, then <code class="highlight"><c- n>subrange</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>E</c-> <c- o>+</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-><c- p>(</c-><c- n>F</c-><c- p>))</c-></code>, except that <code class="highlight"><c- n>E</c-></code> is evaluated only once.</p>
    <li data-md>
     <p>Otherwise, <code class="highlight"><c- n>subrange</c-><c- p>(</c-><c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>),</c-> <c- n>default_sentinel</c-><c- p>)</c-></code>.</p>
   </ul>
   <h3 class="heading settled" data-level="8.4" id="wording-for-lazy_take_view"><span class="secno">8.4. </span><span class="content">Wording for <code class="highlight"><c- n>lazy_take_view</c-></code></span><a class="self-link" href="#wording-for-lazy_take_view"></a></h3>
   <p>After 26.7.10 Take view [<strong>range.take</strong>] add new section:</p>
   <ins> 26.7.x Lazy take view               [range.lazy.take] </ins>
   <p>Under this section add:</p>
   <h4 class="heading settled" data-level="8.4.1" id="x1-overview-rangelazytakeoverview"><span class="secno">8.4.1. </span><span class="content">x.1 Overview              [range.lazy.take.overview]</span><a class="self-link" href="#x1-overview-rangelazytakeoverview"></a></h4>
   <p><code class="highlight"><c- n>lazy_take_view</c-></code> produces a view of the first N elements from another view, or all
the elements if the adapted view contains fewer than N.</p>
   <p>The name <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_take</c-></code> denotes a range adaptor object
([range.adaptor.object]). Let <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> be expressions, let <code class="highlight"><c- n>T</c-></code> be <code class="highlight"><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-><c- o>></c-></code>, and let <code class="highlight"><c- n>D</c-></code> be <code class="highlight"><c- n>range_difference_t</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-><c- o>></c-></code>. If <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>F</c-><c- p>))</c-></code> does not model <code class="highlight"><c- n>convertible_to</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-></code>, <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_take</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is ill-formed. Otherwise, the
expression <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>lazy_take</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is expression-equivalent to:</p>
   <ul>
    <li data-md>
     <p>If <code class="highlight"><c- n>T</c-></code> is a specialization of <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>empty_view</c-></code> ([range.empty.view]),
  then <code class="highlight"><c- p>((</c-><c- b>void</c-><c- p>)</c-><c- n>F</c-><c- p>,</c-> <c- n>decay</c-><c- o>-</c-><c- n>copy</c-><c- p>(</c-><c- n>E</c-><c- p>))</c-></code>, except that the evaluations of <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> are indeterminately sequenced.</p>
    <li data-md>
     <p>Otherwise, if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>random_access_range</c-></code> and <code class="highlight"><c- n>sized_range</c-></code> and is a
  specialization of <code class="highlight"><c- n>span</c-></code> ([views.span]), <code class="highlight"><c- n>basic_string_view</c-></code> ([string.view]), or <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code> ([range.subrange]), then <code class="highlight"><c- n>U</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>E</c-><c- p>)</c-> <c- o>+</c-> <c- n>std</c-><c- o>::</c-><c- n>min</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>distance</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>F</c-><c- p>))</c-></code>, except that <code class="highlight"><c- n>E</c-></code> is evaluated only
  once, where <code class="highlight"><c- n>U</c-></code> is a type determined as follows:</p>
     <ul>
      <li data-md>
       <p>if <code class="highlight"><c- n>T</c-></code> is a specialization of span, then <code class="highlight"><c- n>U</c-></code> is <code class="highlight"><c- n>span</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>::</c-><c- n>element_type</c-><c- o>></c-></code>;</p>
      <li data-md>
       <p>otherwise, if <code class="highlight"><c- n>T</c-></code> is a specialization of <code class="highlight"><c- n>basic_string_view</c-></code>, then <code class="highlight"><c- n>U</c-></code> is <code class="highlight"><c- n>T</c-></code>;</p>
      <li data-md>
       <p>otherwise, <code class="highlight"><c- n>T</c-></code> is a specialization of <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code>, and <code class="highlight"><c- n>U</c-></code> is <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-></code>;</p>
      <li data-md>
       <p>otherwise, if <code class="highlight"><c- n>T</c-></code> is a specialization of <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>iota_view</c-></code> ([range.iota.view]) that models <code class="highlight"><c- n>random_access_range</c-></code> and <code class="highlight"><c- n>sized_range</c-></code>, then <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>iota_view</c-><c- p>(</c-><c- o>*</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- o>*</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>E</c-><c- p>)</c-> <c- o>+</c-> <c- n>std</c-><c- o>::</c-> <c- n>min</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>distance</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>F</c-><c- p>)))</c-></code>, except
  that <code class="highlight"><c- n>E</c-></code> is evaluated only once.</p>
     </ul>
    <li data-md>
     <p>Otherwise, if <code class="highlight"><c- n>T</c-></code> is a specialization of <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>repeat_view</c-></code> ([range.repeat.view]):</p>
     <ul>
      <li data-md>
       <p>if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>sized_range</c-></code>, then <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>repeat</c-><c- p>(</c-><c- o>*</c-><c- n>E</c-><c- p>.</c-><c- n>value_</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>min</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>distance</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>F</c-><c- p>))</c-></code> except that <code class="highlight"><c- n>E</c-></code> is evaluated only
  once;</p>
      <li data-md>
       <p>otherwise, <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>repeat</c-><c- p>(</c-><c- o>*</c-><c- n>E</c-><c- p>.</c-><c- n>value_</c-><c- p>,</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-><c- p>(</c-><c- n>F</c-><c- p>))</c-></code>.</p>
     </ul>
    <li data-md>
     <p>Otherwise, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>lazy_take_view</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code>.</p>
   </ul>
   <p>[Example 1:</p>
<pre class="language-c++ highlight"><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>is</c-><c- p>{</c-><c- mi>0</c-><c- p>,</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>,</c-><c- mi>4</c-><c- p>,</c-><c- mi>5</c-><c- p>,</c-><c- mi>6</c-><c- p>,</c-><c- mi>7</c-><c- p>,</c-><c- mi>8</c-><c- p>,</c-><c- mi>9</c-><c- p>};</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>:</c-> <c- n>is</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>lazy_take</c-><c- p>(</c-><c- mi>5</c-><c- p>))</c->
  <c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>i</c-> <c- o>&lt;&lt;</c-> <c- sc>' '</c-><c- p>;</c-> <c- c1>// prints 0 1 2 3 4</c->
</pre>
   — end example] 
   <h4 class="heading settled" data-level="8.4.2" id="x2-class-template-lazy_take_view-rangelazytakeview"><span class="secno">8.4.2. </span><span class="content">x.2 Class template <code class="highlight"><c- n>lazy_take_view</c-></code> [range.lazy.take.view]</span><a class="self-link" href="#x2-class-template-lazy_take_view-rangelazytakeview"></a></h4>
<pre class="language-c++ highlight"><c- n>namespace</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>view</c-> <c- n>V</c-><c- o>></c->
  <c- n>class</c-> <c- n>lazy_take_view</c-> <c- o>:</c-> <c- n>public</c-> <c- n>view_interface</c-><c- o>&lt;</c-><c- n>take_view</c-><c- o>&lt;</c-><c- n>V</c-><c- o>>></c-> <c- p>{</c->
  <c- n>private</c-><c- o>:</c->
    <c- n>V</c-> <c- n>base_</c-> <c- o>=</c-> <c- n>V</c-><c- p>();</c->                                      <c- c1>// exposition only</c->
    <c- n>range_difference_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-> <c- n>count_</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->                   <c- c1>// exposition only</c->

    <c- c1>// [range.lazy.take.sentinel], class template lazy_take_view::sentinel</c->
    <c- n>template</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-> <c- n>class</c-> <c- n>sentinel</c-><c- p>;</c->                      <c- c1>// exposition only</c->

  <c- n>public</c-><c- o>:</c->
    <c- n>lazy_take_view</c-><c- p>()</c-> <c- n>requires</c-> <c- n>default_initializable</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>constexpr</c-> <c- nf>lazy_take_view</c-><c- p>(</c-><c- n>V</c-> <c- n>base</c-><c- p>,</c-> <c- n>range_difference_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-> <c- n>count</c-><c- p>);</c->

    <c- n>constexpr</c-> <c- n>V</c-> <c- n>base</c-><c- p>()</c-> <c- k>const</c-> <c- o>&amp;</c-> <c- n>requires</c-> <c- n>copy_constructible</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-> <c- p>{</c-> <c- k>return</c-> <c- n>base_</c-><c- p>;</c-> <c- p>}</c->
    <c- n>constexpr</c-> <c- n>V</c-> <c- n>base</c-><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c-> <c- p>}</c->

    <c- n>constexpr</c-> <c- k>auto</c-> <c- n>begin</c-><c- p>()</c-> <c- n>requires</c-> <c- p>(</c-><c- o>!</c-><c- n>simple</c-><c- o>-</c-><c- n>view</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
      <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_range</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>random_access_range</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
          <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c->
        <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
          <c- k>auto</c-> <c- n>sz</c-> <c- o>=</c-> <c- n>range_difference_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>(</c-><c- n>size</c-><c- p>());</c->
          <c- k>return</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>),</c-> <c- n>sz</c-><c- p>);</c->
        <c- p>}</c->
      <c- p>}</c-> <c- k>else</c-> <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_sentinel_for</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c->
        <c- k>auto</c-> <c- n>sz</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>min</c-><c- p>(</c-><c- n>count_</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>base_</c-><c- p>)</c-> <c- o>-</c-> <c- n>it</c-><c- p>);</c->
        <c- k>return</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>it</c-><c- p>),</c-> <c- n>sz</c-><c- p>);</c->
      <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>),</c-> <c- n>count_</c-><c- p>);</c->
      <c- p>}</c->
    <c- p>}</c->

    <c- n>constexpr</c-> <c- k>auto</c-> <c- n>begin</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-> <c- p>{</c->
      <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>random_access_range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
          <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c->
        <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
          <c- k>auto</c-> <c- n>sz</c-> <c- o>=</c-> <c- n>range_difference_t</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>(</c-><c- n>size</c-><c- p>());</c->
          <c- k>return</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>),</c-> <c- n>sz</c-><c- p>);</c->
        <c- p>}</c->
      <c- p>}</c-> <c- k>else</c-> <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_sentinel_for</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>auto</c-> <c- n>it</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c->
        <c- k>auto</c-> <c- n>sz</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>min</c-><c- p>(</c-><c- n>count_</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>base_</c-><c- p>)</c-> <c- o>-</c-> <c- n>it</c-><c- p>);</c->
        <c- k>return</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>it</c-><c- p>),</c-> <c- n>sz</c-><c- p>);</c->
      <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>lazy_counted_iterator</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>),</c-> <c- n>count_</c-><c- p>);</c->
      <c- p>}</c->
    <c- p>}</c->

    <c- n>constexpr</c-> <c- k>auto</c-> <c- n>end</c-><c- p>()</c-> <c- n>requires</c-> <c- p>(</c-><c- o>!</c-><c- n>simple</c-><c- o>-</c-><c- n>view</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
      <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_range</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>random_access_range</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>)</c->
          <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>)</c-> <c- o>+</c-> <c- n>range_difference_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>(</c-><c- n>size</c-><c- p>());</c->
        <c- k>else</c->
          <c- k>return</c-> <c- n>default_sentinel</c-><c- p>;</c->
      <c- p>}</c-> <c- k>else</c-> <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_sentinel_for</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>default_sentinel</c-><c- p>;</c->
      <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>sentinel</c-><c- o>&lt;</c->false<c- o>></c-><c- p>{</c-><c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>base_</c-><c- p>)};</c->
      <c- p>}</c->
    <c- p>}</c->

    <c- n>constexpr</c-> <c- k>auto</c-> <c- n>end</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-> <c- p>{</c->
      <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>random_access_range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>)</c->
          <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>base_</c-><c- p>)</c-> <c- o>+</c-> <c- n>range_difference_t</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>(</c-><c- n>size</c-><c- p>());</c->
        <c- k>else</c->
          <c- k>return</c-> <c- n>default_sentinel</c-><c- p>;</c->
      <c- p>}</c-> <c- k>else</c-> <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>sized_sentinel_for</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>>></c-><c- p>)</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>default_sentinel</c-><c- p>;</c->
      <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>sentinel</c-><c- o>&lt;</c->true<c- o>></c-><c- p>{</c-><c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>base_</c-><c- p>)};</c->
      <c- p>}</c->
    <c- p>}</c->

    <c- n>constexpr</c-> <c- k>auto</c-> <c- n>size</c-><c- p>()</c-> <c- n>requires</c-> <c- n>sized_range</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-> <c- p>{</c->
      <c- k>auto</c-> <c- n>n</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>size</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c->
      <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>min</c-><c- p>(</c-><c- n>n</c-><c- p>,</c-> <c- n>static_cast</c-><c- o>&lt;</c-><c- n>decltype</c-><c- p>(</c-><c- n>n</c-><c- p>)</c-><c- o>></c-><c- p>(</c-><c- n>count_</c-><c- p>));</c->
    <c- p>}</c->

    <c- n>constexpr</c-> <c- k>auto</c-> <c- n>size</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>sized_range</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>V</c-><c- o>></c-> <c- p>{</c->
      <c- k>auto</c-> <c- n>n</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>size</c-><c- p>(</c-><c- n>base_</c-><c- p>);</c->
      <c- k>return</c-> <c- n>ranges</c-><c- o>::</c-><c- n>min</c-><c- p>(</c-><c- n>n</c-><c- p>,</c-> <c- n>static_cast</c-><c- o>&lt;</c-><c- n>decltype</c-><c- p>(</c-><c- n>n</c-><c- p>)</c-><c- o>></c-><c- p>(</c-><c- n>count_</c-><c- p>));</c->
    <c- p>}</c->
  <c- p>};</c->

  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>R</c-><c- o>></c->
    <c- n>lazy_take_view</c-><c- p>(</c-><c- n>R</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- n>range_difference_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>)</c->
      <c- o>-></c-> <c- n>lazy_take_view</c-><c- o>&lt;</c-><c- n>views</c-><c- o>::</c-><c- n>all_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>lazy_take_view</c-><c- p>(</c-><c- n>V</c-> <c- n>base</c-><c- p>,</c-> <c- n>range_difference_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-> <c- n>count</c-><c- p>);</c-></code></p>
   <p><em>Preconditions</em>: <code class="highlight"><c- n>count</c-> <c- o>>=</c-> <c- mi>0</c-></code> is <code class="highlight">true</code>.</p>
   <p><em>Effects</em>: Initializes <code class="highlight"><c- n>base_</c-></code> with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>base</c-><c- p>)</c-></code> and <code class="highlight"><c- n>count_</c-></code> with <code class="highlight"><c- n>count</c-></code>.</p>
   <h4 class="heading settled" data-level="8.4.3" id="x3-class-template-lazy_take_viewsentinel-rangelazytakesentinel"><span class="secno">8.4.3. </span><span class="content">x.3 Class template <code class="highlight"><c- n>lazy_take_view</c-><c- o>::</c-><c- n>sentinel</c-></code> [range.lazy.take.sentinel]</span><a class="self-link" href="#x3-class-template-lazy_take_viewsentinel-rangelazytakesentinel"></a></h4>
<pre class="language-c++ highlight"><c- n>namespace</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>view</c-> <c- n>V</c-><c- o>></c->
  <c- n>template</c-><c- o>&lt;</c-><c- b>bool</c-> <c- n>Const</c-><c- o>></c->
  <c- n>class</c-> <c- n>lazy_take_view</c-><c- o>&lt;</c-><c- n>V</c-><c- o>>::</c-><c- n>sentinel</c-> <c- p>{</c->
  <c- n>private</c-><c- o>:</c->
    <c- n>using</c-> <c- n>Base</c-> <c- o>=</c-> <c- n>maybe</c-><c- o>-</c-><c- k>const</c-><c- o>&lt;</c-><c- n>Const</c-><c- p>,</c-> <c- n>V</c-><c- o>></c-><c- p>;</c->                                     <c- c1>// exposition only</c->
    <c- n>template</c-><c- o>&lt;</c-><c- b>bool</c-> <c- n>OtherConst</c-><c- o>></c->
      <c- n>using</c-> <c- n>CI</c-> <c- o>=</c-> <c- n>lazy_counted_iterator</c-><c- o>&lt;</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>maybe</c-><c- o>-</c-><c- k>const</c-><c- o>&lt;</c-><c- n>OtherConst</c-><c- p>,</c-> <c- n>V</c-><c- o>>>></c-><c- p>;</c->  <c- c1>// exposition only</c->
    <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>end_</c-> <c- o>=</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-><c- p>();</c->                             <c- c1>// exposition only</c->

  <c- n>public</c-><c- o>:</c->
    <c- n>sentinel</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>constexpr</c-> <c- n>explicit</c-> <c- n>sentinel</c-><c- p>(</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>end</c-><c- p>);</c->
    <c- n>constexpr</c-> <c- nf>sentinel</c-><c- p>(</c-><c- n>sentinel</c-><c- o>&lt;!</c-><c- n>Const</c-><c- o>></c-> <c- n>s</c-><c- p>)</c->
      <c- n>requires</c-> <c- n>Const</c-> <c- o>&amp;&amp;</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>,</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>></c-><c- p>;</c->

    <c- n>constexpr</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

    <c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>CI</c-><c- o>&lt;</c-><c- n>Const</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>,</c-> <c- k>const</c-> <c- n>sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>);</c->

    <c- n>template</c-><c- o>&lt;</c-><c- b>bool</c-> <c- n>OtherConst</c-> <c- o>=</c-> <c- o>!</c-><c- n>Const</c-><c- o>></c->
      <c- n>requires</c-> <c- n>sentinel_for</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>maybe</c-><c- o>-</c-><c- k>const</c-><c- o>&lt;</c-><c- n>OtherConst</c-><c- p>,</c-> <c- n>V</c-><c- o>>>></c->
    <c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>CI</c-><c- o>&lt;</c-><c- n>OtherConst</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>,</c-> <c- k>const</c-> <c- n>sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>);</c->
  <c- p>};</c->
<c- p>}</c->
</pre>
   <p><code class="highlight"><c- k>constexpr</c-> <c- k>explicit</c-> <c- n>sentinel</c-><c- p>(</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>end</c-><c- p>);</c-></code></p>
   <p><em>Effects</em>: Initializes <code class="highlight"><c- n>end_</c-></code> with <code class="highlight"><c- n>end</c-></code>.</p>
<pre class="language-c++ highlight"><c- n>constexpr</c-> <c- nf>sentinel</c-><c- p>(</c-><c- n>sentinel</c-><c- o>&lt;!</c-><c- n>Const</c-><c- o>></c-> <c- n>s</c-><c- p>)</c->
  <c- n>requires</c-> <c- n>Const</c-> <c- o>&amp;&amp;</c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>V</c-><c- o>></c-><c- p>,</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>>></c-><c- p>;</c->
</pre>
   <p><em>Effects</em>: Initializes <code class="highlight"><c- n>end_</c-></code> with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>s</c-><c- p>.</c-><c- n>end_</c-><c- p>)</c-></code>.</p>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>base</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c-></code></p>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>end_</c-><c- p>;</c-></code></p>
<pre class="language-c++ highlight"><c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>CI</c-><c- o>&lt;</c-><c- n>Const</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>,</c-> <c- k>const</c-> <c- n>sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>);</c->

<c- n>template</c-><c- o>&lt;</c-><c- b>bool</c-> <c- n>OtherConst</c-> <c- o>=</c-> <c- o>!</c-><c- n>Const</c-><c- o>></c->
  <c- n>requires</c-> <c- n>sentinel_for</c-><c- o>&lt;</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-><c- p>,</c-> <c- n>iterator_t</c-><c- o>&lt;</c-><c- n>maybe</c-><c- o>-</c-><c- k>const</c-><c- o>&lt;</c-><c- n>OtherConst</c-><c- p>,</c-> <c- n>V</c-><c- o>>>></c->
<c- n>friend</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>CI</c-><c- o>&lt;</c-><c- n>OtherConst</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>,</c-> <c- k>const</c-> <c- n>sentinel</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>);</c->
</pre>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>y</c-><c- p>.</c-><c- n>count</c-><c- p>()</c-> <c- o>==</c-> <c- mi>0</c-> <c- o>||</c-> <c- n>y</c-><c- p>.</c-><c- n>base</c-><c- p>()</c-> <c- o>==</c-> <c- n>x</c-><c- p>.</c-><c- n>end_</c-><c- p>;</c-></code></p>
   <h3 class="heading settled" data-level="8.5" id="opens"><span class="secno">8.5. </span><span class="content">Opens</span><a class="self-link" href="#opens"></a></h3>
   <h4 class="heading settled" data-level="8.5.1" id="iterator_concept-depends-on-iiterator_concept"><span class="secno">8.5.1. </span><span class="content"><code class="highlight"><c- n>iterator_concept</c-></code> depends on <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_concept</c-></code></span><a class="self-link" href="#iterator_concept-depends-on-iiterator_concept"></a></h4>
   <p>We followed <code class="highlight"><c- n>counted_iterator</c-></code> defintion, which defines <code class="highlight"><c- n>iterator_concept</c-></code> as
equal to <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_concept</c-></code> and only if it’s found. As other iterators always
define <code class="highlight"><c- n>iterator_concept</c-></code> depending on what <code class="highlight"><c- n>I</c-></code> models (only <code class="highlight"><c- n>iterator_category</c-></code> depends on <code class="highlight"><c- n>I</c-><c- o>::</c-><c- n>iterator_category</c-></code>), maybe <code class="highlight"><c- p>{</c-><c- n>lazy_</c-><c- p>,}</c-><c- n>counted_iterator</c-></code> should
follow this for consistency?</p>
   <h2 class="heading settled" data-level="9" id="note-about-optimization"><span class="secno">9. </span><span class="content">Note about optimization</span><a class="self-link" href="#note-about-optimization"></a></h2>
   <p>It’s interesting to note that with any level of optimization enabled (including <code class="highlight"><c- o>-</c-><c- n>Og</c-></code>!), gcc is able to "fix the issue" <a data-link-type="biblio" href="#biblio-ce-opt">[CE-OPT]</a> for the filter+take case (but
not for <code class="highlight"><c- n>input_iterator</c-></code>, of course). It’s maybe even more interesting to see
the mentioned optimization is not an optimizer bug, and when the filter will
never return another number, it doesn’t change the behavior <a data-link-type="biblio" href="#biblio-ce-opt2">[CE-OPT2]</a>.</p>
   <h2 class="heading settled" data-level="10" id="acknowledgements"><span class="secno">10. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Many thanks to the Israeli NB members for their feedback and support, in
particular Inbal Levi, Dvir Yitzchaki, Dan Raviv and Andrei Zissu. Thanks r/cpp
Reddit users for their feedback on P2406R0 <a data-link-type="biblio" href="#biblio-reddit-cpp">[reddit-cpp]</a>. Thanks SG9 members
for their feedback and guidance.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-ce-filter">[CE-FILTER]
   <dd><a href="https://gcc.godbolt.org/z/9TjbdMn3d"><cite>filter+take problem example, Compiler Explorer</cite></a>. URL: <a href="https://gcc.godbolt.org/z/9TjbdMn3d">https://gcc.godbolt.org/z/9TjbdMn3d</a>
   <dt id="biblio-ce-istream">[CE-ISTREAM]
   <dd><a href="https://gcc.godbolt.org/z/Eb8rdWYbP"><cite>istream problem example, Compiler Explorer</cite></a>. URL: <a href="https://gcc.godbolt.org/z/Eb8rdWYbP">https://gcc.godbolt.org/z/Eb8rdWYbP</a>
   <dt id="biblio-ce-opt">[CE-OPT]
   <dd><a href="https://gcc.godbolt.org/z/4dahzG8Gz"><cite>Optimizer magic solves filter+take issue, Compiler Explorer</cite></a>. URL: <a href="https://gcc.godbolt.org/z/4dahzG8Gz">https://gcc.godbolt.org/z/4dahzG8Gz</a>
   <dt id="biblio-ce-opt2">[CE-OPT2]
   <dd><a href="https://gcc.godbolt.org/z/PvMY8WeaT"><cite>Optimizer is right when filter really never returns, Compiler Explorer</cite></a>. URL: <a href="https://gcc.godbolt.org/z/PvMY8WeaT">https://gcc.godbolt.org/z/PvMY8WeaT</a>
   <dt id="biblio-d2406r1">[D2406R1]
   <dd><a href="https://isocpp.org/files/papers/D2406R1.html"><cite>D2406R1</cite></a>. URL: <a href="https://isocpp.org/files/papers/D2406R1.html">https://isocpp.org/files/papers/D2406R1.html</a>
   <dt id="biblio-lwg3391">[LWG3391]
   <dd><a href="https://cplusplus.github.io/LWG/issue3391"><cite>LWG3391 - Problems with counted_iterator/move_iterator::base() const &amp;</cite></a>. URL: <a href="https://cplusplus.github.io/LWG/issue3391">https://cplusplus.github.io/LWG/issue3391</a>
   <dt id="biblio-p2406r0">[P2406R0]
   <dd><a href="https://wg21.link/p2406r0"><cite>P2406R0</cite></a>. URL: <a href="https://wg21.link/p2406r0">https://wg21.link/p2406r0</a>
   <dt id="biblio-range-v3-issue57">[RANGE-V3-ISSUE57]
   <dd><a href="https://github.com/ericniebler/range-v3/issues/57"><cite>range-v3 - istream_range filtered with take(N) should stop reading at N</cite></a>. URL: <a href="https://github.com/ericniebler/range-v3/issues/57">https://github.com/ericniebler/range-v3/issues/57</a>
   <dt id="biblio-reddit-cpp">[REDDIT-CPP]
   <dd><a href="https://www.reddit.com/r/cpp/comments/orw4q8/wg21_july_2021_mailing/h6kqu7y"><cite>r/cpp comments on P2406R0</cite></a>. URL: <a href="https://www.reddit.com/r/cpp/comments/orw4q8/wg21_july_2021_mailing/h6kqu7y">https://www.reddit.com/r/cpp/comments/orw4q8/wg21_july_2021_mailing/h6kqu7y</a>
  </dl>