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 <head>
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  <title>P1664R1: reconstructible_range - a concept for putting ranges back together</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

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

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

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

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

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

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

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

  /* Do something nice. */

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

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

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

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

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

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

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

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

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

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

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

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

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

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

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

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		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 {
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/******************************************************************************/
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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">P1664R1<br>reconstructible_range - a concept for putting ranges back together</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-11-07">2019-11-07</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:phdofthehouse@gmail.com">JeanHeyd Meneide</a>
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:h2@fsfe.org">Hannes Hauswedell</a>
     <dt>Audience:
     <dd>LEWG, LWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Target:
     <dd>C++20
     <dt>Latest:
     <dd><a href="https://thephd.github.io/vendor/future_cxx/papers/d1664.html">https://thephd.github.io/vendor/future_cxx/papers/d1664.html</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>This paper proposes new concepts to the Standard Library for ranges called Reconstructible Ranges for the purpose of ensuring a range/view broken down into its two iterators can be "glued" back together using a constructor taking its iterator and sentinel type.</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="#changelog"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#changelog-r1"><span class="secno">1.1</span> <span class="content">Revision 1 - November 6th, 2019</span></a>
      <li><a href="#changelog-r0"><span class="secno">1.2</span> <span class="content">Revision 0 - August, 5th, 2019</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#design"><span class="secno">3</span> <span class="content">Design</span></a>
     <ol class="toc">
      <li><a href="#design-all"><span class="secno">3.1</span> <span class="content">Should this apply to all Ranges?</span></a>
      <li><a href="#design-applicable"><span class="secno">3.2</span> <span class="content">Applicability</span></a>
      <li><a href="#design-dual"><span class="secno">3.3</span> <span class="content">Two Concepts</span></a>
     </ol>
    <li><a href="#impact"><span class="secno">4</span> <span class="content">Impact</span></a>
    <li>
     <a href="#wording"><span class="secno">5</span> <span class="content">Proposed Changes</span></a>
     <ol class="toc">
      <li><a href="#wording-feature"><span class="secno">5.1</span> <span class="content">Feature Test Macro</span></a>
      <li><a href="#wording-intent"><span class="secno">5.2</span> <span class="content">Intent</span></a>
      <li><a href="#wording-library"><span class="secno">5.3</span> <span class="content">Proposed Library Wording</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">6</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="changelog"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="changelog-r1"><span class="secno">1.1. </span><span class="content">Revision 1 - November 6th, 2019</span><a class="self-link" href="#changelog-r1"></a></h3>
   <ul>
    <li data-md>
     <p>Change to <code class="highlight"><c- n>snake_case</c-></code> concepts.</p>
    <li data-md>
     <p>Update wording to target <a data-link-type="biblio" href="#biblio-n4835">[n4835]</a>.</p>
    <li data-md>
     <p>Moved by LWG! 🎉</p>
   </ul>
   <h3 class="heading settled" data-level="1.2" id="changelog-r0"><span class="secno">1.2. </span><span class="content">Revision 0 - August, 5th, 2019</span><a class="self-link" href="#changelog-r0"></a></h3>
   <ul>
    <li data-md>
     <p>Initial release.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <table>
    <tbody>
     <tr>
      <th>Currently
      <th>With Proposal
     <tr>
      <td>
<pre class="language-c++ highlight"><c- n>vector</c-> <c- n>vec</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- n>span</c-> <c- n>s</c-><c- p>{</c-><c- n>vec</c-><c- p>.</c-><c- n>data</c-><c- p>(),</c-> <c- mi>5</c-><c- p>};</c->
<c- n>span</c-> <c- n>v</c-> <c- o>=</c-> <c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>)</c->
           <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>);</c->
</pre>
       ❌ - Compilation error 
<pre class="language-c++ highlight"><c- n>vector</c-> <c- n>vec</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- n>span</c-> <c- n>s</c-><c- p>{</c-><c- n>vec</c-><c- p>.</c-><c- n>data</c-><c- p>(),</c-> <c- mi>5</c-><c- p>};</c->
<c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>)</c->
           <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>);</c->
</pre>
       ⚠️ - Compiles, but <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>v</c-><c- p>)</c-></code> is <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>take_view</c-><c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>drop_view</c-><c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>take_view</c-><c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>drop_view</c-><c- o>&lt;</c-><c- n>span</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>dynamic_extent</c-><c- o>>>>>></c-></code> 
      <td>
<pre class="language-c++ highlight"><c- n>vector</c-> <c- n>vec</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- n>span</c-> <c- n>s</c-><c- p>{</c-><c- n>vec</c-><c- p>.</c-><c- n>data</c-><c- p>(),</c-> <c- mi>5</c-><c- p>};</c->
<c- n>span</c-> <c- n>v</c-> <c- o>=</c-> <c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>)</c->
           <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>);</c->
</pre>
       ✔️ - Compiles and works with no extra template instantiations 
<pre class="language-c++ highlight"><c- n>vector</c-> <c- n>vec</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- n>span</c-> <c- n>s</c-><c- p>{</c-><c- n>vec</c-><c- p>.</c-><c- n>data</c-><c- p>(),</c-> <c- mi>5</c-><c- p>};</c->
<c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>)</c->
           <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>10</c-><c- p>);</c->
</pre>
       ✔️ - Compiles and works with no extra templates. <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>v</c-><c- p>)</c-></code> is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code> 
     <tr>
      <td>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>u8string</c-> <c- n>name</c-> <c- o>=</c-> <c- s>"𐌀𐌖𐌋𐌄𐌑𐌉·𐌌𐌄𐌕𐌄𐌋𐌉𐌑 𐑡𐑹𐑡 ·𐑚𐑻𐑯𐑸𐑛 ·𐑖𐑷"</c-><c- p>;</c->
<c- n>char16_t</c-> <c- n>conversion_buffer</c-><c- p>[</c-><c- mi>432</c-><c- p>];</c->

<c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-> <c- n>name_view</c-><c- p>(</c-><c- n>name</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>char16_t</c-><c- o>></c-> <c- n>output</c-><c- p>(</c-><c- n>conversion_buffer</c-><c- p>,</c-> <c- mi>432</c-><c- p>);</c->

<c- k>auto</c-> <c- n>encoding_result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>text</c-><c- o>::</c-><c- n>transcode</c-><c- p>(</c-><c- n>input</c-><c- p>,</c-> <c- n>output</c-><c- p>);</c->
<c- c1>// compiler error here !!</c->
<c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-> <c- n>unprocessed_code_units</c-> 
	<c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>input</c-><c- p>;</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>char16_t</c-><c- o>></c-> <c- n>unconsumed_output</c-> 
	<c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>output</c-><c- p>;</c->
</pre>
       ❌ - Compilation error 
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>u8string</c-> <c- n>name</c-> <c- o>=</c-> <c- s>"𐌀𐌖𐌋𐌄𐌑𐌉·𐌌𐌄𐌕𐌄𐌋𐌉𐌑 𐑡𐑹𐑡 ·𐑚𐑻𐑯𐑸𐑛 ·𐑖𐑷"</c-><c- p>;</c->
<c- n>char16_t</c-> <c- n>conversion_buffer</c-><c- p>[</c-><c- mi>432</c-><c- p>];</c->

<c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-> <c- n>name_view</c-><c- p>(</c-><c- n>name</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>char16_t</c-><c- o>></c-> <c- n>output</c-><c- p>(</c-><c- n>conversion_buffer</c-><c- p>,</c-> <c- mi>432</c-><c- p>);</c->

<c- k>auto</c-> <c- n>encoding_result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>text</c-><c- o>::</c-><c- n>transcode</c-><c- p>(</c-><c- n>input</c-><c- p>,</c-> <c- n>output</c-><c- p>);</c->
<c- k>auto</c-> <c- n>unprocessed_code_units</c-> <c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>input</c-><c- p>;</c->
<c- k>auto</c-> <c- n>unconsumed_output</c-> <c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>output</c-><c- p>;</c->
</pre>
       ⚠️ - Compiles, but <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>unprocessed_code_units</c-><c- p>)</c-></code> is <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-><c- o>::</c-><c- n>iterator</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-><c- o>::</c-><c- n>iterator</c-><c- o>></c-></code> and <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>unconsumed_output</c-><c- p>)</c-></code> is <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- b>char16_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>dynamic_extent</c-><c- o>>::</c-><c- n>iterator</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- b>char16_t</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>dynamic_extent</c-><c- o>>::</c-><c- n>iterator</c-><c- o>></c-></code> 
      <td>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>u8string</c-> <c- n>name</c-> <c- o>=</c-> <c- s>"𐌀𐌖𐌋𐌄𐌑𐌉·𐌌𐌄𐌕𐌄𐌋𐌉𐌑 𐑡𐑹𐑡 ·𐑚𐑻𐑯𐑸𐑛 ·𐑖𐑷"</c-><c- p>;</c->
<c- n>char16_t</c-> <c- n>conversion_buffer</c-><c- p>[</c-><c- mi>432</c-><c- p>];</c->

<c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-> <c- n>name_view</c-><c- p>(</c-><c- n>name</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>char16_t</c-><c- o>></c-> <c- n>output</c-><c- p>(</c-><c- n>conversion_buffer</c-><c- p>,</c-> <c- mi>432</c-><c- p>);</c->

<c- k>auto</c-> <c- n>encoding_result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>text</c-><c- o>::</c-><c- n>transcode</c-><c- p>(</c-><c- n>input</c-><c- p>,</c-> <c- n>output</c-><c- p>);</c->
<c- c1>// compiler error here !!</c->
<c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-> <c- n>unprocessed_code_units</c-> 
	<c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>input</c-><c- p>;</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>char16_t</c-><c- o>></c-> <c- n>unconsumed_output</c-> 
	<c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>output</c-><c- p>;</c->
</pre>
       ✔️ - Compiles and works, types match input. 
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>u8string</c-> <c- n>name</c-> <c- o>=</c-> <c- s>"𐌀𐌖𐌋𐌄𐌑𐌉·𐌌𐌄𐌕𐌄𐌋𐌉𐌑 𐑡𐑹𐑡 ·𐑚𐑻𐑯𐑸𐑛 ·𐑖𐑷"</c-><c- p>;</c->
<c- n>char16_t</c-> <c- n>conversion_buffer</c-><c- p>[</c-><c- mi>432</c-><c- p>];</c->

<c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-> <c- n>name_view</c-><c- p>(</c-><c- n>name</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>char16_t</c-><c- o>></c-> <c- n>output</c-><c- p>(</c-><c- n>conversion_buffer</c-><c- p>,</c-> <c- mi>432</c-><c- p>);</c->

<c- k>auto</c-> <c- n>encoding_result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>text</c-><c- o>::</c-><c- n>transcode</c-><c- p>(</c-><c- n>input</c-><c- p>,</c-> <c- n>output</c-><c- p>);</c->
<c- k>auto</c-> <c- n>unprocessed_code_units</c-> <c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>input</c-><c- p>;</c->
<c- k>auto</c-> <c- n>unconsumed_output</c-> <c- o>=</c-> <c- n>encoding_result</c-><c- p>.</c-><c- n>output</c-><c- p>;</c->
</pre>
       ✔️ - Compiles and works, where <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>unprocessed_code_units</c-><c- p>)</c-></code> is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>u8string_view</c-></code> and <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>unconsumed_output</c-><c- p>)</c-></code> is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- b>char16_t</c-><c- o>></c-></code>. 
   </table>
   <p>Currently in C++, there is no Generic ("with a capital G") way to take a range apart with its iterators and put it back together. That is, the following code is not guaranteed to work:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>Range</c-><c- o>></c->
<c- k>auto</c-> <c- n>operate_on_and_return_updated_range</c-> <c- p>(</c-><c- n>Range</c-><c- o>&amp;&amp;</c-> <c- n>range</c-><c- p>)</c-> <c- p>{</c->
	<c- k>using</c-> <c- n>uRange</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Range</c-><c- o>></c-><c- p>;</c->
	<c- k>if</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>empty</c-><c- p>(</c-><c- n>range</c-><c- p>))</c-> <c- p>{</c->
		<c- c1>// ... the below errors</c->
		<c- k>return</c-> <c- n>uRange</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Range</c-><c- o>></c-><c- p>(</c-><c- n>range</c-><c- p>));</c->
	<c- p>}</c->
	<c- d>/* perform some work with the</c->
<c- d>	iterators or similar */</c->
	<c- k>auto</c-> <c- n>first</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>range</c-><c- p>);</c->
	<c- k>auto</c-> <c- n>last</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>range</c-><c- p>);</c->
	<c- k>if</c-> <c- p>(</c-><c- o>*</c-><c- n>first</c-> <c- o>==</c-> <c- n>u</c->'\<c- mh>0xEF</c->'<c- p>)</c-> <c- p>{</c->
		<c- c1>// ...</c->
		<c- n>std</c-><c- o>::</c-><c- n>advance</c-><c- p>(</c-><c- n>first</c-><c- p>,</c-> <c- mi>3</c-><c- p>);</c->
		<c- c1>// ...</c->
	<c- p>}</c->
	<c- c1>// ... algorithm finished,</c->
	<c- c1>// return the "updated" range!</c->

	<c- c1>// ... but the below errors</c->
	<c- k>return</c-> <c- n>uRange</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>first</c-><c- p>),</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>last</c-><c- p>));</c->
<c- p>}</c->

<c- b>int</c-> <c- n>main</c-> <c- p>()</c-> <c- p>{</c->
	<c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>meow_view</c-> <c- o>=</c-> <c- s>"나는 유리를 먹을 수 있어요. 그래도 아프지 않아요"</c-><c- p>;</c->
	<c- c1>// this line will error</c->
	<c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>sub_view</c-> <c- o>=</c-> <c- n>operate_on_and_return_updated_range</c-><c- p>(</c-><c- n>meow_view</c-><c- p>);</c->
	<c- k>return</c-> <c- mi>0</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>The current fix is to employ <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>S</c-><c- p>,</c-> <c- n>K</c-><c- o>></c-></code> to return a generic subrange:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>Range</c-><c- o>></c->
<c- k>auto</c-> <c- n>operate_on_and_return_updated_range</c-> <c- p>(</c-><c- n>Range</c-><c- o>&amp;&amp;</c-> <c- n>range</c-><c- p>)</c-> <c- p>{</c->
	<c- k>using</c-> <c- n>uRange</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Range</c-><c- o>></c-><c- p>;</c->
	<c- k>using</c-> <c- n>I</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>Iterator</c-><c- o>&lt;</c-><c- n>uRange</c-><c- o>></c-><c- p>;</c->
	<c- k>using</c-> <c- n>S</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>Sentinel</c-><c- o>&lt;</c-><c- n>uRange</c-><c- o>></c-><c- p>;</c->
	<c- k>using</c-> <c- n>Result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>S</c-><c- o>></c-><c- p>;</c->
	<c- k>if</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>empty</c-><c- p>(</c-><c- n>range</c-><c- p>))</c-> <c- p>{</c->
		<c- k>return</c-> <c- n>uRange</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Range</c-><c- o>></c-><c- p>(</c-><c- n>range</c-><c- p>));</c->
	<c- p>}</c->
	<c- c1>// perform some work with the</c->
	<c- c1>// iterators or similar</c->
	<c- k>auto</c-> <c- n>first</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>begin</c-><c- p>(</c-><c- n>range</c-><c- p>);</c->
	<c- k>auto</c-> <c- n>last</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>range</c-><c- p>);</c->
	<c- k>if</c-> <c- p>(</c-><c- o>*</c-><c- n>first</c-> <c- o>==</c-> <c- n>u</c->'\<c- mh>0xEF</c->'<c- p>)</c-> <c- p>{</c->
		<c- c1>// ...</c->
		<c- n>std</c-><c- o>::</c-><c- n>advance</c-><c- p>(</c-><c- n>first</c-><c- p>,</c-> <c- mi>3</c-><c- p>);</c->
		<c- c1>// ...</c->
	<c- p>}</c->
	<c- c1>// ... algorithm finished,</c->
	<c- c1>// return the "updated" range!</c->

	<c- c1>// now it works!</c->
	<c- k>return</c-> <c- n>Result</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>first</c-><c- p>),</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>last</c-><c- p>));</c->
<c- p>}</c->

<c- b>int</c-> <c- n>main</c-> <c- p>()</c-> <c- p>{</c->
	<c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>meow_view</c-> <c- o>=</c-> <c- s>"나는 유리를 먹을 수 있어요. 그래도 아프지 않아요"</c-><c- p>;</c->
	<c- k>auto</c-> <c- n>sub_view</c-> <c- o>=</c-> <c- n>operate_on_and_return_updated_range</c-><c- p>(</c-><c- n>meow_view</c-><c- p>);</c->
	<c- c1>// decltype(sub_view) ==</c->
	<c- c1>//   std::ranges::subrange&lt;std::string_view::iterator,std::string_view::iterator></c->
	<c- c1>//   which is nowhere close to ideal.</c->
	<c- k>return</c-> <c- mi>0</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>This makes it work with any two pair of iterators, but quickly becomes undesirable from an interface point of view. If a user passes in a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Extent</c-><c- o>></c-></code> or a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>Char</c-><c- p>,</c-> <c- n>Traits</c-><c- o>></c-></code> that interface and information is entirely lost to the user of the above function. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>Iterator</c-><c- p>,</c-> <c- n>Sentinel</c-><c- p>,</c-> <c- n>Kind</c-><c- o>></c-></code> does not -- and cannot/should not -- mimic the interface of the view it was created from other than what information comes from its iterators: it is the barebones idea of a pair-of-iterators/iterator-sentinel style of range. This is useful in the generic sense that if a library developer must work with iterators, they can always rely on creation of a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code> of the iterator and sentinel.</p>
   <p>Unfortunately, this decreases usability for end users. Users who have, for example a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string_view</c-></code>, would prefer to have the same type after calling an algorithm. There is little reason why the original type needs to be discarded if it supports being put back together from its iterators. This break-it-apart-and-then-generic-subrange approach also discards any range-specific storage optimizations and layout considerations, leaving us with the most bland kind of range similar to the "pair of iterators" model. Compilation time goes up as well: users must spawn a fresh <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>S</c-><c- p>,</c-> <c- n>K</c-><c- o>></c-></code> for every different set of iterator/sentinel/kind triplet, or handle deeply nested templates in templates as the input types. This makes it impossible to compile interfaces as dynamic libraries without having to explicitly materialize or manually cajole a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code> into something more palatable for the regular world.</p>
   <p>There is also a problem where there are a wide variety of ranges that could conceivably meet this criterion, but do not. The author of the very first draft of this paper was not the only one to see utility in such operations. <a data-link-type="biblio" href="#biblio-p1739r0">[p1739r0]</a> does much the same that this paper does, without the introduction of a concept to formalize the behavior it presents. In particular, it selects views which can realistically have their return types changed to match the input range and operations being performed (or a similarly powerful alternative) by asking whether they are constructible from a subrange of the iterators with the expressions acted upon. This paper does not depend on any other papers, but note that the changes from <a data-link-type="biblio" href="#biblio-p1739r0">[p1739r0]</a>, <a data-link-type="biblio" href="#biblio-p1391r2">[p1391r2]</a> and <a data-link-type="biblio" href="#biblio-p1394r2">[p1394r2]</a> all follow down to the logical conclusion laid out here:</p>
   <ul>
    <li data-md>
     <p>Ranges should be reconstructible from their iterators (or subrange of their iterators) where applicable;</p>
    <li data-md>
     <p>and, reconstructible ranges serve a useful purpose in generic algorithms, including not losing information and returning it in a much more cromulent and desirable form.</p>
   </ul>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design</span><a class="self-link" href="#design"></a></h2>
   <p>The design is simple and is given in 2 exposition-only concepts added to the standard:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- o>></c->
<c- n>concept</c-> <c- n>pair</c-><c- o>-</c-><c- n>reconstructible</c-><c- o>-</c-><c- n>range</c-> <c- o>=</c->
    <c- n>ranges</c-><c- o>::</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c->
    <c- n>forwarding</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-> <c- o>&amp;&amp;</c->
    <c- n>constructible_from</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- o>></c->
<c- n>concept</c-> <c- n>reconstructible</c-><c- o>-</c-><c- n>range</c-> <c- o>=</c->
    <c- n>ranges</c-><c- o>::</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c->
    <c- n>forwarding</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-> <c- o>&amp;&amp;</c->
    <c- n>constructible_from</c-><c- o>&lt;</c-><c- n>R</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>>></c-><c- p>;</c->
</pre>
   <p>It is the formalization that a range can be constructed from its begin iterator and end iterator/sentinel. It also provides an exposition-only concept for allowing a range to be constructed from a <code class="highlight"><c- n>subrange</c-></code> of its iterator/sentinel pair. This allows a developer to propagate the input type’s properties after modifying its iterators for some underlying work, algorithm or other effect. This concept is also the basis of the idea behind <a data-link-type="biblio" href="#biblio-p1739r0">[p1739r0]</a>.</p>
   <p>Both concepts require that the type with any references removed model the exposition-only concept <code class="highlight"><c- n>forwarding</c-><c- o>-</c-><c- n>range</c-></code>. This ensures that the validity of the iterators is in fact independent of the lifetime of the range they originate from and that a "reconstructed" range does not depend on the original. We remove reference before performing this check, because all reference types that model <code class="highlight"><c- n>Range</c-></code> also model <code class="highlight"><c- n>forwarding</c-><c- o>-</c-><c- n>range</c-></code> and the intent of the proposed changes is narrower: (re)construction is assumed to be in constant time (this typically implies that <code class="highlight"><c- n>R</c-></code> also models <code class="highlight"><c- n>View</c-></code>, but it is sufficient to check <code class="highlight"><c- n>forwarding</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-></code>). Note that this explicitly excludes types like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- k>const</c-> <c- o>&amp;</c-></code> from being reconstructible.</p>
   <h3 class="heading settled" data-level="3.1" id="design-all"><span class="secno">3.1. </span><span class="content">Should this apply to all Ranges?</span><a class="self-link" href="#design-all"></a></h3>
   <p>Not all ranges can meet this requirement. Some ranges contain state which cannot be trivially propagated into the iterators, or state that cannot be reconstructed from the iterator/sentinel pair itself. However, most of the common ranges representing unbounded views, empty views, iterations viewing some section of non-owned storage, or similar can all be constructed from their iterator/iterator or iterator/sentinel pair.</p>
   <p>For example <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>single_view</c-></code> contains a <a href="http://eel.is/c++draft/range.semi.wrap">exposition *semiregular-box* template type (ranges.semi.wrap)</a> which holds a value to iterate over. It would not be possible to reconstruct the exact same range (e.g., iterators pointing to the exact same object) with the semi-regular wrapper.</p>
   <h3 class="heading settled" data-level="3.2" id="design-applicable"><span class="secno">3.2. </span><span class="content">Applicability</span><a class="self-link" href="#design-applicable"></a></h3>
   <p>There are many ranges to which this is applicable, but only a handful in the standard library need or satisfy this. If <a data-link-type="biblio" href="#biblio-p1391r2">[p1391r2]</a> and <a data-link-type="biblio" href="#biblio-p1394r2">[p1394r2]</a> are accepted, then the two most important view types -- <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Extent</c-><c- o>></c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>Char</c-><c- p>,</c-> <c- n>Traits</c-><c- o>></c-></code> -- will model both concepts. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>Iterator</c-><c- p>,</c-> <c- n>Sentinel</c-><c- p>,</c-> <c- n>Kind</c-><c- o>></c-></code> already fits this as well. By formalizing concepts in the standard, we can dependably and reliably assert that these properties continue to hold for these ranges. The ranges to which this would be helpfully applicable to in the current standard and proposals space are:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code> (already reconstructible)</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code> (currently under consideration, <a data-link-type="biblio" href="#biblio-p1394r2">[p1394r2]</a>);</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>basic_string_view</c-></code> (currently under consideration, <a data-link-type="biblio" href="#biblio-p1391r2">[p1391r2]</a>);</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>empty_view</c-></code> (proposed here);</p>
    <li data-md>
     <p>and, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>iota_view</c-></code> (proposed here).</p>
   </ul>
   <p>The following range adaptor closure objects will make use of the concepts in determining the type of the returned range:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>drop</c-></code>;</p>
    <li data-md>
     <p><code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take</c-></code>.</p>
   </ul>
   <p>There are also upcoming ranges from <a data-link-type="biblio" href="#biblio-range-v3">[range-v3]</a> and elsewhere that could model this concept:</p>
   <ul>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-p1255r4">[p1255r4]</a>'s <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>ref_maybe_view</c-></code>;</p>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-p0009r9">[p0009r9]</a>'s <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>mdspan</c-></code>;</p>
    <li data-md>
     <p>and, soon to be proposed by this author for the purposes of output range algorithms, <a data-link-type="biblio" href="#biblio-range-v3">[range-v3]</a>'s <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>unbounded_view</c-></code>.</p>
   </ul>
   <p>And there are further range adaptor closure objects that could make use of this concept:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>slice</c-></code>, <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take_exactly</c-></code>, <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>drop_exactly</c-></code> and <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take_last</c-></code> from <a data-link-type="biblio" href="#biblio-range-v3">[range-v3]</a></p>
   </ul>
   <p>Note that these changes will greatly aid other algorithm writers who want to preserve the same input ranges. In the future, it may be beneficial to provide more than just an exposition-only concept to check, but rather a function in the standard in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-></code> of the form <code class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>Range</c-><c- o>></c-> <c- n>reconstruct</c-><c- p>(</c-><c- n>Iterator</c-><c- p>,</c-> <c- n>Sentinel</c-><c- p>);</c-></code>, whose goal is to check if it is possible to reconstruct the <code class="highlight"><c- n>Range</c-></code> or otherwise return a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code>.</p>
   <p>This paper does not propose this at this time because concepts -- and the things that rely on them -- must remain stable from now and into infinity. It is better as an exposition-only named set of helpers whose semantics we are a little more free to improve or be adapted, rather than hard-and-fast functions and named concepts which are impossible to fix or improve in the future due to the code which may rely on it.</p>
   <h3 class="heading settled" data-level="3.3" id="design-dual"><span class="secno">3.3. </span><span class="content">Two Concepts</span><a class="self-link" href="#design-dual"></a></h3>
   <p>By giving these ranges <code class="highlight"><c- n>Iterator</c-><c- p>,</c-> <c- n>Sentinel</c-></code>, and/or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>Iterator</c-><c- p>,</c-> <c- n>Sentinel</c-><c- o>></c-></code> constructors, we can enable a greater degree of interface fidelity without having to resort to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code> for all generic algorithms. There should be a preference for <code class="highlight"><c- n>Type</c-><c- p>(</c-><c- n>Iterator</c-><c- p>,</c-> <c- n>Sentinel</c-><c- p>)</c-></code> constructors, because one-argument constructors have extremely overloaded meanings in many containers and some views and may result in having to fight with other constructor calls in a complicated overload set. It also produces less compiler boilerplate to achieve the same result of reconstructing the range when one does not have to go through <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>I</c-><c- p>,</c-> <c- n>S</c-><c- p>,</c-> <c- n>K</c-><c- o>></c-></code>. However, it is important to attempt to move away from the iterator, sentinel model being deployed all the time: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-></code> offers a single type that can accurately represent the intent and can be fairly easy to constrain overload sets on (most of the time).</p>
   <p>This paper includes two concepts that cover both reconstructible methods.</p>
   <h2 class="heading settled" data-level="4" id="impact"><span class="secno">4. </span><span class="content">Impact</span><a class="self-link" href="#impact"></a></h2>
   <p>Originally, the impact of this feature was perceived to be small and likely not necessary to work into C++20. Indeed: this paper originally targeted C++23 with the intent of slowly working through existing ranges and range implementations and putting the concept and the manifestation of concepts in range libraries, particularly range-v3, over time.</p>
   <p>This changed in the face of <a data-link-type="biblio" href="#biblio-p1739r0">[p1739r0]</a>. Hauswedell’s paper here makes it clear there are usability and API wins that are solved by this concept for APIs that are already in the working draft <strong>today</strong>, and that not having the concept has resulted in interface inconsistency and ad-hoc, one-off fixes to fit limited problem domains without any respite to routines which have a desire to preserve the input types into their algorithms. Since this paper’s concept is likely to change interfaces API return values in a beneficial but ultimately breaking manner, this paper’s consideration was brought up to be presented as a late C++20 paper for the purpose of fixing the interface as soon as possible.</p>
   <p>Note that this is a separate concept. It is not to be added to the base <code class="highlight"><c- n>Range</c-></code> concept, or added to any other concept. It is to be applied separately to the types which can reasonably support it for the benefit of algorithms and code which can enhance the quality of their implementation.</p>
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">5. </span><span class="content">Proposed Changes</span><a class="self-link" href="#wording"></a></h2>
   <p>The following wording is relative to the latest C++ Draft paper.</p>
   <h3 class="heading settled" data-level="5.1" id="wording-feature"><span class="secno">5.1. </span><span class="content">Feature Test Macro</span><a class="self-link" href="#wording-feature"></a></h3>
   <p>This paper results in an exposition-only concept to help guide the further development of standard ranges and simplify their usages in generic contexts. There is one proposed feature test macro, <code class="highlight"><c- n>__cpp_lib_range_constructors</c-></code>, which is to be input into the standard and then explicitly updated every time a constructor from a pre-existing type is changed to reflect the new wording. We hope that by putting this in the standard early, most incoming ranges will be checked for compatibility with <code class="highlight"><c- n>pair</c-><c- o>-</c-><c- n>reconstructible</c-><c- o>-</c-><c- n>range</c-></code> and <code class="highlight"><c- n>reconstructible</c-><c- o>-</c-><c- n>range</c-></code>. This paper also notes that both <a data-link-type="biblio" href="#biblio-p1394r2">[p1394r2]</a> and <a data-link-type="biblio" href="#biblio-p1391r2">[p1391r2]</a> do not add wording for feature test macros, making this paper’s feature test macro the ideal addition to encapsulate all reconstructible range changes.</p>
   <h3 class="heading settled" data-level="5.2" id="wording-intent"><span class="secno">5.2. </span><span class="content">Intent</span><a class="self-link" href="#wording-intent"></a></h3>
   <p>The intent of this wording is to provide greater generic coding guarantees and optimizations by allowing for a class of ranges and views that model the new exposition-only definitions of a reconstructible range:</p>
   <ul>
    <li data-md>
     <p>add a new feature test macro for reconstructible ranges to cover constructor changes;</p>
    <li data-md>
     <p>add two new exposition-only requirements to <a href="http://eel.is/c++draft/range.req">[range.req</a>];</p>
    <li data-md>
     <p>add expression checks to <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take</c-></code> to reconstruct the range, similar to <a data-link-type="biblio" href="#biblio-p1739r0">[p1739r0]</a>;</p>
    <li data-md>
     <p>add expression checks to <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>drop</c-></code> to reconstruct the range, similar to <a data-link-type="biblio" href="#biblio-p1739r0">[p1739r0]</a>;</p>
    <li data-md>
     <p>add expression checks to <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>counted</c-></code> to create a span, similar to <a data-link-type="biblio" href="#biblio-p1739r0">[p1739r0]</a>;</p>
    <li data-md>
     <p>add constructors for reconstructing the range to <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>empty_view</c-></code>;</p>
    <li data-md>
     <p>add constructors for reconstructing the range to <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>iota_view</c-></code>;</p>
    <li data-md>
     <p>and, make <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>iota_view</c-></code> model <em>forwarding-range</em> by adding <code class="highlight"><c- k>friend</c-></code> overloads of <code class="highlight"><c- n>begin</c-><c- p>()</c-></code> and <code class="highlight"><c- n>end</c-><c- p>()</c-></code>.</p>
   </ul>
   <p>If <em>forwarding-range</em> is changed to the <code class="highlight"><c- n>safe_range</c-></code> concept name, then this entire proposal will rename all its uses of <em>forwarding-range</em>.</p>
   <p>For ease of reading, the necessary portions of other proposal’s wording is duplicated here, with the changes necessary for the application of reconstructible range concepts. Such sections are clearly marked.</p>
   <h3 class="heading settled" data-level="5.3" id="wording-library"><span class="secno">5.3. </span><span class="content">Proposed Library Wording</span><a class="self-link" href="#wording-library"></a></h3>
   <p>Add a feature test macro <code class="highlight"><c- n>__cpp_lib_reconstructible_range</c-></code>.</p>
   <p>Insert into §24.4.2 Ranges <a href="http://eel.is/c++draft/range.req#range.range">[range.range</a>]'s after paragraph 7, one additional paragraph:</p>
   <blockquote>
    <ins>
     <sup>8</sup> The exposition-only <i>pair-reconstructible-range</i> and <i>reconstructible-range</i> concepts describe the requirements on ranges that are efficiently constructible from values of their iterator and sentinel types. 
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- o>></c->
<c- n>concept</c-> <c- n>pair</c-><c- o>-</c-><c- n>reconstructible</c-><c- o>-</c-><c- n>range</c-> <c- o>=</c->    <c- c1>// exposition only</c->
    <c- n>ranges</c-><c- o>::</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c->
    <c- n>forwarding</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-> <c- o>&amp;&amp;</c->
    <c- n>constructible_from</c-><c- o>&lt;</c->
        <c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> 
        <c- n>ranges</c-><c- o>::</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> 
        <c- n>ranges</c-><c- o>::</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c->
    <c- o>></c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- o>></c->
<c- n>concept</c-> <c- n>reconstructible</c-><c- o>-</c-><c- n>range</c-> <c- o>=</c->        <c- c1>// exposition only</c->
    <c- n>ranges</c-><c- o>::</c-><c- n>range</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-> <c- o>&amp;&amp;</c->
    <c- n>forwarding</c-><c- o>-</c-><c- n>range</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c-> <c- o>&amp;&amp;</c->
    <c- n>constructible_from</c-><c- o>&lt;</c->
        <c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> 
        <c- n>ranges</c-><c- o>::</c-><c- n>subrange</c-><c- o>&lt;</c-><c- n>ranges</c-><c- o>::</c-><c- n>iterator_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>sentinel_t</c-><c- o>&lt;</c-><c- n>R</c-><c- o>>></c->
    <c- o>></c-><c- p>;</c->
</pre>
    </ins>
   </blockquote>
   <p>Add to §24.6.1.2 Class template <code class="highlight"><c- n>empty_view</c-></code> <a href="http://eel.is/c++draft/range.empty.view">[range.empty.view</a>], a constructor in the synopsis and a new paragraph for constructor definitions:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
    <c- n>requires</c-> <c- n>is_object_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c->
  <c- k>class</c-> <c- nc>empty_view</c-> <c- o>:</c-> <c- k>public</c-> <c- n>view_interface</c-><c- o>&lt;</c-><c- n>empty_view</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- p>{</c->
  <c- k>public</c-><c- o>:</c->
</pre>
    <ins>
<pre class="highlight">    <c- n>empty_view</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- nf>empty_view</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>first</c-><c- p>,</c-> <c- n>T</c-><c- o>*</c-> <c- n>last</c-><c- p>);</c->
</pre>
    </ins>
<pre class="highlight">    <c- k>static</c-> <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- nf>begin</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> <c- k>nullptr</c-><c- p>;</c-> <c- p>}</c->
    <c- k>static</c-> <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- nf>end</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> <c- k>nullptr</c-><c- p>;</c-> <c- p>}</c->
    <c- k>static</c-> <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- nf>data</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> <c- k>nullptr</c-><c- p>;</c-> <c- p>}</c->
    <c- k>static</c-> <c- k>constexpr</c-> <c- b>ptrdiff_t</c-> <c- nf>size</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c->
    <c- k>static</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- nf>empty</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> true<c- p>;</c-> <c- p>}</c->

    <c- k>friend</c-> <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- nf>begin</c-><c- p>(</c-><c- n>empty_view</c-><c- p>)</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> <c- k>nullptr</c-><c- p>;</c-> <c- p>}</c->
    <c- k>friend</c-> <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- nf>end</c-><c- p>(</c-><c- n>empty_view</c-><c- p>)</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- k>return</c-> <c- k>nullptr</c-><c- p>;</c-> <c- p>}</c->
  <c- p>};</c->
<c- p>}</c->
</pre>
    <ins>
     <p><strong>24.6.1.3</strong> <strong>Constructors</strong> <span style="float:right">[<strong>range.empty.cons</strong>]</span></p>
<pre class="highlight"><c- k>constexpr</c-> <c- nf>empty_view</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-> <c- n>first</c-><c- p>,</c-> <c- n>T</c-><c- o>*</c-> <c- n>last</c-><c- p>);</c->
</pre>
     <p><sup>1</sup> <i>Expects:</i> <code class="highlight"><c- n>first</c-> <c- o>==</c-> <c- n>last</c-></code> is <code class="highlight">true</code>.</p>
     <p><sup>2</sup> <i>Effects:</i> None.</p>
     <p><sup>3</sup> <i>Throws:</i> Nothing.</p>
    </ins>
   </blockquote>
   <p>Add to §24.6.3 Class template <code class="highlight"><c- n>iota_view</c-></code> <a href="http://eel.is/c++draft/range.iota.view">[range.iota.view</a>]'s paragraph 2, a constructor to the synopsis:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-><c- o>::</c-><c- n>ranges</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>I</c-><c- o>></c->
    <c- n>concept</c-> <c- n>Decrementable</c-> <c- o>=</c->     <c- c1>// exposition only</c->
      <c- n>see</c-> <c- n>below</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>I</c-><c- o>></c->
    <c- n>concept</c-> <c- n>Advanceable</c-> <c- o>=</c->       <c- c1>// exposition only</c->
      <c- n>see</c-> <c- n>below</c-><c- p>;</c->

  <c- k>template</c-><c- o>&lt;</c-><c- n>WeaklyIncrementable</c-> <c- n>W</c-><c- p>,</c-> <c- n>Semiregular</c-> <c- n>Bound</c-> <c- o>=</c-> <c- n>unreachable_sentinel_t</c-><c- o>></c->
    <c- n>requires</c-> <c- n>weakly</c-><c- o>-</c-><c- n>equality</c-><c- o>-</c-><c- n>comparable</c-><c- o>-</c-><c- n>with</c-><c- o>&lt;</c-><c- n>W</c-><c- p>,</c-> <c- n>Bound</c-><c- o>></c->
  <c- k>class</c-> <c- nc>iota_view</c-> <c- o>:</c-> <c- k>public</c-> <c- n>view_interface</c-><c- o>&lt;</c-><c- n>iota_view</c-><c- o>&lt;</c-><c- n>W</c-><c- p>,</c-> <c- n>Bound</c-><c- o>>></c-> <c- p>{</c->
  <c- k>private</c-><c- o>:</c->
    <c- c1>// [range.iota.iterator], class iota_­view​::​iterator</c->
    <c- k>struct</c-> <c- n>iterator</c-><c- p>;</c->            <c- c1>// exposition only</c->
    <c- c1>// [range.iota.sentinel], class iota_­view​::​sentinel</c->
    <c- k>struct</c-> <c- n>sentinel</c-><c- p>;</c->            <c- c1>// exposition only</c->
    <c- n>W</c-> <c- n>value_</c-> <c- o>=</c-> <c- n>W</c-><c- p>();</c->             <c- c1>// exposition only</c->
    <c- n>Bound</c-> <c- n>bound_</c-> <c- o>=</c-> <c- n>Bound</c-><c- p>();</c->     <c- c1>// exposition only</c->
  <c- k>public</c-><c- o>:</c->
    <c- n>iota_view</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- k>explicit</c-> <c- nf>iota_view</c-><c- p>(</c-><c- n>W</c-> <c- n>value</c-><c- p>);</c->
    <c- k>constexpr</c-> <c- nf>iota_view</c-><c- p>(</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>W</c-><c- o>></c-> <c- n>value</c-><c- p>,</c->
                        <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Bound</c-><c- o>></c-> <c- n>bound</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight">    <c- k>constexpr</c-> <c- nf>iota_view</c-><c- p>(</c-><c- n>iterator</c-> <c- n>first</c-><c- p>,</c-> <c- n>sentinel</c-> <c- n>last</c-><c- p>)</c-> <c- o>:</c-> <c- n>iota_view</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>,</c-> <c- n>last</c-><c- p>.</c-><c- n>bound_</c-><c- p>)</c-> <c- p>{}</c->
</pre>
    </ins>
<pre class="highlight">    <c- k>constexpr</c-> <c- n>iterator</c-> <c- nf>begin</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- n>sentinel</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- n>iterator</c-> <c- nf>end</c-><c- p>()</c-> <c- k>const</c-> <c- n>requires</c-> <c- n>Same</c-><c- o>&lt;</c-><c- n>W</c-><c- p>,</c-> <c- n>Bound</c-><c- o>></c-><c- p>;</c->
</pre>
    <ins>
<pre class="highlight">    <c- k>constexpr</c-> <c- k>friend</c-> <c- n>iterator</c-> <c- nf>begin</c-><c- p>(</c-><c- n>iota_view</c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>v</c-><c- p>.</c-><c- n>begin</c-><c- p>();</c-> <c- p>}</c->
    <c- k>constexpr</c-> <c- k>friend</c-> <c- k>auto</c-> <c- nf>end</c-><c- p>(</c-><c- n>iota_view</c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>v</c-><c- p>.</c-><c- n>end</c-><c- p>();</c-> <c- p>}</c->
</pre>
    </ins>
<pre class="highlight">    <c- k>constexpr</c-> <c- k>auto</c-> <c- nf>size</c-><c- p>()</c-> <c- k>const</c->
      <c- n>requires</c-> <c- p>(</c-><c- n>Same</c-><c- o>&lt;</c-><c- n>W</c-><c- p>,</c-> <c- n>Bound</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>Advanceable</c-><c- o>&lt;</c-><c- n>W</c-><c- o>></c-><c- p>)</c-> <c- o>||</c->
               <c- p>(</c-><c- n>Integral</c-><c- o>&lt;</c-><c- n>W</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>Integral</c-><c- o>&lt;</c-><c- n>Bound</c-><c- o>></c-><c- p>)</c-> <c- o>||</c->
               <c- n>SizedSentinel</c-><c- o>&lt;</c-><c- n>Bound</c-><c- p>,</c-> <c- n>W</c-><c- o>></c->
    <c- p>{</c-> <c- k>return</c-> <c- n>bound_</c-> <c- o>-</c-> <c- n>value_</c-><c- p>;</c-> <c- p>}</c->
  <c- p>};</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>W</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Bound</c-><c- o>></c->
    <c- n>requires</c-> <c- p>(</c-><c- o>!</c-><c- n>Integral</c-><c- o>&lt;</c-><c- n>W</c-><c- o>></c-> <c- o>||</c-> <c- o>!</c-><c- n>Integral</c-><c- o>&lt;</c-><c- n>Bound</c-><c- o>></c-> <c- o>||</c-> <c- n>is_signed_v</c-><c- o>&lt;</c-><c- n>W</c-><c- o>></c-> <c- o>==</c-> <c- n>is_signed_v</c-><c- o>&lt;</c-><c- n>Bound</c-><c- o>></c-><c- p>)</c->
  <c- n>iota_view</c-><c- p>(</c-><c- n>W</c-><c- p>,</c-> <c- n>Bound</c-><c- p>)</c-> <c- o>-></c-> <c- n>iota_view</c-><c- o>&lt;</c-><c- n>W</c-><c- p>,</c-> <c- n>Bound</c-><c- o>></c-><c- p>;</c->
<c- p>}</c->
</pre>
   </blockquote>
   <p>Modify §24.7.6.4 <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take</c-></code> <a href="http://eel.is/c++draft/range.take.adaptor">[range.take.adaptor</a>] as follows:</p>
   <blockquote>
    <p>
     <sup>1</sup> The name <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take</c-></code> denotes a range adaptor object ([range.adaptor.object]). 
     <del>For some subexpressions <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code>, the expression <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is expression-equivalent to <code class="highlight"><c- n>take_</c->­<c- n>view</c-><c- p>{</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>}</c-></code>.</del>
    </p>
    <p>
     <ins><sup>2</sup> Let <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> be expressions, and let T 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>. The expression <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is expression-equivalent to:</ins>
    </p>
    <dl>
     <dd>
      <ins>— <code class="highlight"><c- n>T</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>min</c-><c- o>&lt;</c-><c- n>range_</c->­<c- n>difference_</c->­<c- n>t</c-><c- o>&lt;</c-><c- k>decltype</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>size</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>F</c-><c- p>)}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>random_access_range</c-></code>, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>sized_range</c-></code> and <i>pair-reconstructible-range</i>, except that <code class="highlight"><c- n>E</c-></code> is only evaluated once.</ins>
     <dd>
      <ins>— Otherwise, <code class="highlight"><c- n>T</c-><c- p>{</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</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>min</c-><c- o>&lt;</c-><c- n>range_</c->­<c- n>difference_</c->­<c- n>t</c-><c- o>&lt;</c-><c- k>decltype</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>size</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>F</c-><c- p>)}}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>random_access_range</c-></code>, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>sized_range</c-></code> and <i>reconstructible-range</i>, except that <code class="highlight"><c- n>E</c-></code> is only evaluated once.</ins>
     <dd>
      <ins>— Otherwise, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>take_</c->­<c- n>view</c-><c- p>{</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>}</c-></code>.</ins>
    </dl>
   </blockquote>
   <p>Modify §24.7.8 <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>drop</c-></code> <a href="http://eel.is/c++draft/range.drop.adaptor">[range.drop.adaptor</a>] as follows:</p>
   <blockquote>
    <p>
     <sup>1</sup> The name <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>drop</c-></code> denotes a range adaptor object ([range.adaptor.object]). 
     <del>For some subexpressions E and F, the expression views::drop(E, F) is expression-equivalent to drop_­view{E, F}.</del>
     <ins> Let <code class="highlight"><c- n>E</c-></code> and <code class="highlight"><c- n>F</c-></code> be expressions, and 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>. Then, the expression <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>drop</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is expression-equivalent to:</ins>
    </p>
    <dl>
     <dd>
      <ins>— <code class="highlight"><c- n>T</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>min</c-><c- o>&lt;</c-><c- n>range_</c->­<c- n>difference_</c->­<c- n>t</c-><c- o>&lt;</c-><c- k>decltype</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>size</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>F</c-><c- p>),</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>E</c-><c- p>)}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>random_access_range</c-></code>, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>sized_range</c-></code> and <i>pair-reconstructible-range</i>, except that <code class="highlight"><c- n>E</c-></code> is only evaluated once.</ins>
     <dd>
      <ins>— Otherwise, <code class="highlight"><c- n>T</c-><c- p>{</c-><c- n>ranges</c-><c- o>::</c-><c- n>subrange</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>min</c-><c- o>&lt;</c-><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-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>size</c-><c- p>(</c-><c- n>E</c-><c- p>),</c-> <c- n>F</c-><c- p>),</c-> <c- n>ranges</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>E</c-><c- p>)}}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>random_access_range</c-></code>, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>sized_range</c-></code> and <i>reconstructible-range</i>, except that <code class="highlight"><c- n>E</c-></code> is only evaluated once.</ins>
     <dd>
      <ins>— Otherwise, <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>drop_</c->­<c- n>view</c-><c- p>{</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>}</c-></code>.</ins>
    </dl>
   </blockquote>
   <p>Modify §24.7.12 <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>counted</c-></code> as follows:</p>
   <blockquote>
    <p><sup>1</sup> The name <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>counted</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, and let <code class="highlight"><c- n>T</c-></code> be <code class="highlight"><c- n>decay_</c->­<c- n>t</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>((</c-><c- n>E</c-><c- p>))</c-><c- o>></c-></code>. Then the expression <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>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>
    <dl>
     <dd>
      <ins>— If <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>input_or_output_iterator</c-></code> and <code class="highlight"><c- k>decltype</c-><c- p>((</c-><c- n>F</c-><c- p>))</c-></code> models <code class="highlight"><c- n>convertible_to</c-><c- o>&lt;</c-><c- n>iter_</c->­<c- n>difference_</c->­<c- n>t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-></code>,</ins>
     <dl>
      <dd>
       <ins>— <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- n>static_</c->­<c- n>cast</c-><c- o>&lt;</c-><c- n>iter_</c->­<c- n>difference_</c->­<c- n>t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>(</c-><c- n>F</c-><c- p>)}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>contiguous_iterator</c-></code>.</ins>
      <dd>
       — 
       <ins>Otherwise, </ins>
       <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- n>static_</c->­<c- n>cast</c-><c- o>&lt;</c-><c- n>iter_</c->­<c- n>difference_</c->­<c- n>t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>(</c-><c- n>F</c-><c- p>)}</c-></code> if <code class="highlight"><c- n>T</c-></code> models <code class="highlight"><c- n>random_access_iterator</c-></code>
       <ins>, except that <code class="highlight"><c- n>E</c-></code> is only evaluated once</ins>
       .
      <dd>— Otherwise, <code class="highlight"><c- n>subrange</c-><c- p>{</c-><c- n>counted_</c->­<c- n>iterator</c-><c- p>{</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>},</c-> <c- n>default_</c->­<c- n>sentinel</c-><c- p>}</c-></code>.
     </dl>
     <dd>— Otherwise, <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>counted</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>F</c-><c- p>)</c-></code> is ill-formed. [ <i>Note</i>: This case can result in substitution failure when <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>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. —<i> end note</i>]
    </dl>
   </blockquote>
   <h2 class="heading settled" data-level="6" id="acknowledgements"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to Corentin Jabot, Christopher DiBella, and Hannes Hauswedell for pointing me to p1035 and p1739 to review both papers and combine some of their ideas in here. Thanks to Eric Niebler for prompting me to think of the generic, scalable solution to this problem rather than working on one-off fixes for individuals views.</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-n4835">[N4835]
   <dd>Richard Smith. <a href="https://wg21.link/n4835">Working Draft, Standard for Programming Language C++</a>. 8 October 2019. URL: <a href="https://wg21.link/n4835">https://wg21.link/n4835</a>
   <dt id="biblio-p0009r9">[P0009R9]
   <dd>H. Carter Edwards, Bryce Adelstein Lelbach, Daniel Sunderland, David Hollman, Christian Trott, Mauro Bianco, Ben Sander, Athanasios Iliopoulos, John Michopoulos, Mark Hoemmen. <a href="https://wg21.link/p0009r9">mdspan: A Non-Owning Multidimensional Array Reference</a>. 20 January 2019. URL: <a href="https://wg21.link/p0009r9">https://wg21.link/p0009r9</a>
   <dt id="biblio-p1255r4">[P1255R4]
   <dd>Steve Downey. <a href="https://wg21.link/p1255r4">A view of 0 or 1 elements: view::maybe</a>. 17 June 2019. URL: <a href="https://wg21.link/p1255r4">https://wg21.link/p1255r4</a>
   <dt id="biblio-p1391r2">[P1391R2]
   <dd>Corentin Jabot. <a href="https://wg21.link/p1391r2">Range constructor for std::string_view</a>. 17 June 2019. URL: <a href="https://wg21.link/p1391r2">https://wg21.link/p1391r2</a>
   <dt id="biblio-p1394r2">[P1394R2]
   <dd>Corentin Jabot, Casey Carter. <a href="https://wg21.link/p1394r2">Range constructor for std::span</a>. 17 June 2019. URL: <a href="https://wg21.link/p1394r2">https://wg21.link/p1394r2</a>
   <dt id="biblio-p1739r0">[P1739R0]
   <dd>Hannes Hauswedell. <a href="https://wg21.link/p1739r0">Type erasure for forwarding ranges in combination with "subrange-y" view adaptors</a>. 15 June 2019. URL: <a href="https://wg21.link/p1739r0">https://wg21.link/p1739r0</a>
   <dt id="biblio-range-v3">[RANGE-V3]
   <dd>Eric Niebler; Casey Carter. <a href="https://github.com/ericniebler/range-v3">range-v3</a>. June 11th, 2019. URL: <a href="https://github.com/ericniebler/range-v3">https://github.com/ericniebler/range-v3</a>
  </dl>