<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P2406R0: Fix `counted_iterator` interaction with input iterators</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,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

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

	:not(.head) > :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: none;
		border-bottom: 1px solid #707070;
		border-bottom: 1px solid var(--a-normal-underline);
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		border-bottom-color: #bbb;
		border-bottom-color: var(--a-visited-underline);
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		border-color: #c00;
		border-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-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* 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: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 span:not(.dfn-paneled) {
			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>
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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">P2406R0<br>Fix `counted_iterator` interaction with input iterators</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2021-07-11">2021-07-11</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://yehezkelshb.github.io/cpp_proposals/P2406-counted-iterator-and-input-iterators.html">https://yehezkelshb.github.io/cpp_proposals/P2406-counted-iterator-and-input-iterators.html</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 suggest some changes to improve the situation</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="#intro"><span class="secno">2</span> <span class="content">Intro</span></a>
    <li>
     <a href="#problem-description"><span class="secno">3</span> <span class="content">Problem description</span></a>
     <ol class="toc">
      <li><a href="#counter-argument"><span class="secno">3.1</span> <span class="content">Counter argument</span></a>
      <li><a href="#the-real-problem"><span class="secno">3.2</span> <span class="content">The real problem</span></a>
     </ol>
    <li>
     <a href="#design-of-suggested-solution"><span class="secno">4</span> <span class="content">Design of suggested solution</span></a>
     <ol class="toc">
      <li><a href="#the-main-changes"><span class="secno">4.1</span> <span class="content">The main changes</span></a>
      <li><a href="#random_access_iterator-case-kept-as-is"><span class="secno">4.2</span> <span class="content"><code class="highlight"><c- n>random_</c->­<c- n>access_</c->­<c- n>iterator</c-></code> case kept as-is</span></a>
      <li><a href="#open-question-constructing-with-0-length"><span class="secno">4.3</span> <span class="content">Open question: Constructing with 0 length</span></a>
     </ol>
    <li><a href="#proposed-wording"><span class="secno">5</span> <span class="content">Proposed Wording</span></a>
    <li><a href="#effect-on-current-state"><span class="secno">6</span> <span class="content">Effect on current state</span></a>
    <li><a href="#appendix"><span class="secno">7</span> <span class="content">Appendix</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>r0: initial revision</p>
   <h2 class="heading settled" data-level="2" id="intro"><span class="secno">2. </span><span class="content">Intro</span><a class="self-link" href="#intro"></a></h2>
    Project Euler is a project with many mathematical-related questions that are
intended to encourage the reader to write a small program to compute the result.
In this case, one of the problems there, no. 37 <a data-link-type="biblio" href="#biblio-euler">[EULER]</a>, helped reveal a
pitfall coming from the definition of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>counted_iterator</c-></code>. 
   <h2 class="heading settled" data-level="3" id="problem-description"><span class="secno">3. </span><span class="content">Problem description</span><a class="self-link" href="#problem-description"></a></h2>
    Look at this example code <a data-link-type="biblio" href="#biblio-ce-filter">[CE-FILTER]</a>: 
<pre class="highlight"><c- cp>#include</c-> &lt;ranges>
<c- cp>#include</c-> &lt;iostream>
 
<c- k>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- nl>i</c->  <c- p>:</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>10</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>10</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 9. 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>The problem mentioned in the intro is one that actually describes a filter that
return exactly 11 elements, so trying to use <code class="highlight"><c- n>take</c-><c- p>(</c-><c- mi>11</c-><c- p>)</c-></code> on it means the program
never ends even while it got 11 elements already.</p>
   <h3 class="heading settled" data-level="3.1" id="counter-argument"><span class="secno">3.1. </span><span class="content">Counter argument</span><a class="self-link" href="#counter-argument"></a></h3>
    If the user doesn’t know how many items the filter returns, it’s possible that
the filter returns less items than what <code class="highlight"><c- n>take</c-></code> tries to take, and never ends
anyway. So one might claim that the user must know the number of items returned
by the filter and then there is no point in using <code class="highlight"><c- n>take</c-></code> when they are the same. 
   <p>It still leaves the cases that there is a huge performance penalty on
incrementing the iterator one additional time, but those are maybe considered
less interesting.</p>
   <h3 class="heading settled" data-level="3.2" id="the-real-problem"><span class="secno">3.2. </span><span class="content">The real problem</span><a class="self-link" href="#the-real-problem"></a></h3>
    The real problem we see is when using input ranges, e.g. <code class="highlight"><c- n>basic_istream_view</c-></code>.
In these cases, advancing the internal iterator means hanging forever if no
additional input exists and the stream isn’t closed or eating an additional
input that can’t be retrieved anymore. For example <a data-link-type="biblio" href="#biblio-ce-istream">[CE-ISTREAM]</a>: 
<pre class="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- k>namespace</c-> <c- n>rn</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- p>;</c->
<c- k>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- nl>i</c-> <c- p>:</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 means that one can’t use ranges when parsing input, for example.</p>
   <h2 class="heading settled" data-level="4" id="design-of-suggested-solution"><span class="secno">4. </span><span class="content">Design of suggested solution</span><a class="self-link" href="#design-of-suggested-solution"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="the-main-changes"><span class="secno">4.1. </span><span class="content">The main changes</span><a class="self-link" href="#the-main-changes"></a></h3>
    We suggest changing the behavior of <code class="highlight"><c- n>counted_iterator</c-></code> operators around 0 length
so it doesn’t increment the internal iterator when reaching 0 length, and as a
result doesn’t decrement it when getting back from 0 to 1 length. This requires
changing <code class="highlight"><c- n>base</c-><c- p>()</c-></code> behavior too, including the return type to be by value. 
   <h3 class="heading settled" data-level="4.2" id="random_access_iterator-case-kept-as-is"><span class="secno">4.2. </span><span class="content"><code class="highlight"><c- n>random_</c->­<c- n>access_</c->­<c- n>iterator</c-></code> case kept as-is</span><a class="self-link" href="#random_access_iterator-case-kept-as-is"></a></h3>
   <p>To reduce the amount of changes required, we keep the current behavior for <code class="highlight"><c- n>random_</c->­<c- n>access_</c->­<c- n>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_</c->­<c- n>access_</c->­<c- n>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 this
issue.</p>
   <h3 class="heading settled" data-level="4.3" id="open-question-constructing-with-0-length"><span class="secno">4.3. </span><span class="content">Open question: Constructing with 0 length</span><a class="self-link" href="#open-question-constructing-with-0-length"></a></h3>
   <p>We don’t have a good solution for the case that user constructs <code class="highlight"><c- n>counted_iterator</c-></code> with <code class="highlight"><c- mi>0</c-></code> as argument for length. This puts it in an
inconsistent internal state, as the next operations will be <code class="highlight"><c- n>base</c-><c- p>()</c-></code> or <code class="highlight"><c- o>--</c-></code>,
and those expect the iterator to be one step back, with the changes suggested
here.</p>
   <p>Please note that <code class="highlight"><c- n>base</c-><c- p>()</c-></code> and <code class="highlight"><c- o>--</c-></code> 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>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. (This option assumes the only reason to create such
an <code class="highlight"><c- n>counted_iteraor</c-></code> is to decrement it anyway.)</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).</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>) 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). Please note that <code class="highlight"><c- n>base</c-><c- p>()</c-></code> doesn’t need any special handling here.</p>
   <h2 class="heading settled" data-level="5" id="proposed-wording"><span class="secno">5. </span><span class="content">Proposed Wording</span><a class="self-link" href="#proposed-wording"></a></h2>
   <p class="note" role="note"><span>Note:</span> Wording doesn’t include any of the options suggested for the c-tor.</p>
   <p>Under 23.5.6.3 [<strong>counted.iter.access</strong>]:</p>
   <ins> <code class="highlight"><c- k>constexpr</c-> <c- n>I</c-> <c- n>base</c-><c- p>()</c-> <c- k>const</c-> <c- o>&amp;</c-><c- p>;</c-></code><br> Effects: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>length</c-> <c- o>?</c-> <c- nl>current</c-> <c- p>:</c-> <c- n>next</c-><c- p>(</c-><c- n>current</c-><c- p>);</c-></code><br> Note: calling <code class="highlight"><c- n>base</c-><c- p>()</c-></code> twice isn’t safe when <code class="highlight"><c- n>I</c-></code> isn’t <code class="highlight"><c- n>forward_iterator</c-></code> </ins>
   <p><code class="highlight"><c- k>constexpr</c-> <c- k>const</c-> <c- n>I</c-><c- o>&amp;</c-> <c- n>base</c-><c- p>()</c-> <c- k>const</c-> <c- o>&amp;</c-><c- p>;</c-></code><br> <span class="ins"><code class="highlight"><c- k>requires</c-> <c- n>random_</c->­<c- n>access_</c->­<c- n>iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c-></code></span><br> Effects: Equivalent to: <code class="highlight"><c- k>return</c-> <c- n>current</c-><c- p>;</c-></code></p>
   <ins> <code class="highlight"><c- k>constexpr</c-> <c- n>I</c-> <c- n>base</c-><c- p>()</c-> <c- o>&amp;&amp;</c-><c- p>;</c-></code><br> Returns: <code class="highlight"><c- n>std</c->​<c- o>::</c->​<c- n>move</c-><c- p>(</c-><c- n>length</c-> <c- o>?</c-> <c- nl>current</c-> <c- p>:</c-> <c- n>next</c-><c- p>(</c-><c- n>current</c-><c- p>))</c-></code>. </ins>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>I</c-> <c- n>base</c-><c- p>()</c-> <c- o>&amp;&amp;</c-><c- p>;</c-></code><br> <span class="ins"><code class="highlight"><c- k>requires</c-> <c- n>random_</c->­<c- n>access_</c->­<c- n>iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c-></code></span><br> Returns: <code class="highlight"><c- n>std</c->​<c- o>::</c->​<c- n>move</c-><c- p>(</c-><c- n>current</c-><c- p>)</c-></code>.</p>
   <p>Under 23.5.6.5 [<strong>counted.iter.nav</strong>]:</p>
   <ins> <code class="highlight"><c- k>constexpr</c-> <c- n>counted_iterator</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>++</c-><c- p>();</c-></code><br> Preconditions: <code class="highlight"><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code>.<br> Effects: Equivalent to:<br> <code class="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-></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> </ins>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>counted_iterator</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>++</c-><c- p>();</c-></code><br> <span class="ins"><code class="highlight"><c- k>requires</c-> <c- n>random_</c->­<c- n>access_</c->­<c- n>iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c-></code></span><br> Preconditions: <code class="highlight"><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code>.<br> Effects: 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><code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>);</c-></code><br> Preconditions: <code class="highlight"><c- n>length</c-> <c- o>></c-> <c- mi>0</c-></code>.<br effectsU0003A equivalent toU0003AU0003Cbr> <code class="highlight"><c- o>--</c-><c- n>length</c-><c- p>;</c-></code><br> <span class="del"><code class="highlight"><c- k>try</c-> <c- p>{</c-> <c- k>return</c-> <c- n>current</c-><c- o>++</c-><c- p>;</c-> <c- p>}</c-></code></span><br> <span class="ins"><code class="highlight"><c- k>try</c-> <c- p>{</c-> <c- k>return</c-> <c- n>length</c-> <c- o>?</c-> <c- n>current</c-><c- o>++</c-> <c- o>:</c-> <c- n>current</c-><c- p>;</c-> <c- p>}</c-></code></span><br> <code class="highlight"><c- k>catch</c-><c- p>(...)</c-> <c- p>{</c-> <c- o>++</c-><c- n>length</c-><c- p>;</c-> <c- k>throw</c-><c- p>;</c-> <c- p>}</c-></code><br></p>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>counted_iterator</c-> <c- k>operator</c-><c- o>++</c-><c- p>(</c-><c- b>int</c-><c- p>)</c-></code><br> <code class="highlight"><c- k>requires</c-> <c- n>forward_</c->­<c- n>iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c-></code><br> Effects: Equivalent to:<br> <code class="highlight"><c- n>counted_iterator</c-> <c- n>tmp</c-> <c- o>=</c-> <c- o>*</c-><c- k>this</c-><c- p>;</c-></code><br> <code class="highlight"><c- o>++*</c-><c- k>this</c-><c- p>;</c-></code><br> <code class="highlight"><c- k>return</c-> <c- n>tmp</c-><c- p>;</c-></code><br></p>
   <ins> <code class="highlight"><c- k>constexpr</c-> <c- n>counted_iterator</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>--</c-><c- p>()</c-></code><br> <code class="highlight"><c- k>requires</c-> <c- n>bidirectional_</c->­<c- n>iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c-></code><br> Effects: Equivalent to:<br> <code class="highlight"><c- k>if</c-> <c- p>(</c-><c- n>length</c-><c- p>)</c-> <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> </ins>
   <p><code class="highlight"><c- k>constexpr</c-> <c- n>counted_iterator</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>--</c-><c- p>()</c-></code><br> <span class="ins"><code class="highlight"><c- k>requires</c-> <c- n>random_</c->­<c- n>access_</c->­<c- n>iterator</c-><c- o>&lt;</c-><c- n>I</c-><c- o>></c-><c- p>;</c-></code></span><br> Effects: 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>
   <h2 class="heading settled" data-level="6" id="effect-on-current-state"><span class="secno">6. </span><span class="content">Effect on current state</span><a class="self-link" href="#effect-on-current-state"></a></h2>
    This breaks ABI (as every inline function does). If implementers don’t accept
this change, or if WG21 feels it’s too late to apply it to C++20, we can add <code class="highlight"><c- n>lazy_counted_iterator</c-></code> (or bikeshed a better name) which has the same behavior
as <code class="highlight"><c- n>counted_iterator</c-></code> except for the mentioned changes. In additional, it
requires adding <code class="highlight"><c- n>lazy_take</c-></code> and <code class="highlight"><c- n>lazy_counted_view</c-></code> that uses it instead of <code class="highlight"><c- n>counted_iterator</c-></code>. 
   <p>In such a case, we suggest to consider requiring <code class="highlight"><c- n>forward_or_output_iterator</c-></code> for <code class="highlight"><c- n>counted_iterator</c-></code>, as it always does the wrong thing for <code class="highlight"><c- n>input_iterator</c-></code> if it doesn’t reach the <code class="highlight"><c- n>end</c-></code> of the input (while for <code class="highlight"><c- n>forward_iterator</c-></code> and <code class="highlight"><c- n>bidirectional_</c->­<c- n>iterator</c-></code> it’s mainly just a possible performance issue and
rarely an infinite loop, as mentioned, and for <code class="highlight"><c- n>output_iterator</c-></code> the
incrementing is usually no-op).</p>
   <p>At the very least, we have to warn users against using <code class="highlight"><c- n>counted_iterator</c-></code> and
its consumers with <code class="highlight"><c- n>input_iterator</c-></code>.</p>
   <h2 class="heading settled" data-level="7" id="appendix"><span class="secno">7. </span><span class="content">Appendix</span><a class="self-link" href="#appendix"></a></h2>
    It’s interesting to note that with optimizations enabled, gcc is able to "fix
the issue" <a data-link-type="biblio" href="#biblio-ce-opt">[CE-OPT]</a>. Our assumption is that it doesn’t work with more complex
cases, and anyway we can’t count on optimization to fix such an issue. 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>. 
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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/cadsr1GMj">filter+take problem example, Compiler Explorer</a>. URL: <a href="https://gcc.godbolt.org/z/cadsr1GMj">https://gcc.godbolt.org/z/cadsr1GMj</a>
   <dt id="biblio-ce-istream">[CE-ISTREAM]
   <dd><a href="https://gcc.godbolt.org/z/Eb8rdWYbP">istream problem example, Compiler Explorer</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">Optimizer magic solves filter+take issue, Compiler Explorer</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">Optimizer is right when filter really never returns, Compiler Explorer</a>. URL: <a href="https://gcc.godbolt.org/z/PvMY8WeaT">https://gcc.godbolt.org/z/PvMY8WeaT</a>
   <dt id="biblio-euler">[EULER]
   <dd><a href="https://projecteuler.net/problem=37">Truncatable primes, problem 37, Project Euler</a>. URL: <a href="https://projecteuler.net/problem=37">https://projecteuler.net/problem=37</a>
  </dl>