<!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>P2636R1: References to ranges should always be viewable</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

	/* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

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

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version b0de3432c, updated Thu Oct 13 09:16:59 2022 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
td {
  vertical-align: middle;
}
</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
}
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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">P2636R1<br>References to ranges should always be viewable</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2022-10-14">2022-10-14</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <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, SG9
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </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 that the `views::all` adaptor and the `viewable_range` concept accept lvalue references to all ranges (including move-only views).</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 - 2022-10-14</span></a>
      <li><a href="#changelog-r0"><span class="secno">1.2</span> <span class="content">Revision 0 - 2022-09-14</span></a>
     </ol>
    <li>
     <a href="#introduction"><span class="secno">2</span> <span class="content">Introduction</span></a>
     <ol class="toc">
      <li><a href="#problem"><span class="secno">2.1</span> <span class="content">The problem (TL;DR)</span></a>
      <li><a href="#solution"><span class="secno">2.2</span> <span class="content">The solution (TL;DR)</span></a>
     </ol>
    <li>
     <a href="#examples"><span class="secno">3</span> <span class="content">Detailed examples</span></a>
     <ol class="toc">
      <li><a href="#from_lvalues"><span class="secno">3.1</span> <span class="content">Views from lvalue reference</span></a>
      <li><a href="#composition2"><span class="secno">3.2</span> <span class="content">View composition (starting from lvalue reference)</span></a>
      <li><a href="#from_rvalues"><span class="secno">3.3</span> <span class="content">Views from rvalues</span></a>
      <li><a href="#composition"><span class="secno">3.4</span> <span class="content">View composition (starting from rvalue)</span></a>
      <li><a href="#generic"><span class="secno">3.5</span> <span class="content">Generic code</span></a>
     </ol>
    <li>
     <a href="#proposal"><span class="secno">4</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#wording"><span class="secno">4.1</span> <span class="content">Wording</span></a>
      <li><a href="#impact"><span class="secno">4.2</span> <span class="content">Impact</span></a>
     </ol>
    <li>
     <a href="#workarounds"><span class="secno">5</span> <span class="content">"Workarounds"</span></a>
     <ol class="toc">
      <li><a href="#views_ref"><span class="secno">5.1</span> <span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>views</c-><c- o>::</c-><c- n>ref</c-></code></span></a>
     </ol>
    <li>
     <a href="#lifetime"><span class="secno">6</span> <span class="content">Lifetime</span></a>
     <ol class="toc">
      <li><a href="#lifetime1"><span class="secno">6.1</span> <span class="content">Returning views on local variables</span></a>
      <li><a href="#lifetime1①"><span class="secno">6.2</span> <span class="content">Returning views on in-parameters</span></a>
     </ol>
    <li><a href="#meaning"><span class="secno">7</span> <span class="content">Implication for the "meaning" of views</span></a>
    <li><a href="#summary"><span class="secno">8</span> <span class="content">Summary</span></a>
    <li><a href="#ack"><span class="secno">9</span> <span class="content">Acknowledgments</span></a>
    <li><a href="#references"><span class="secno">10</span> <span class="content">References</span></a>
   </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 - 2022-10-14</span><a class="self-link" href="#changelog-r1"></a></h3>
   <ul>
    <li data-md>
     <p>Some language corrections and smaller additions to text.</p>
    <li data-md>
     <p>Added <a href="#solution">"The solution (TL;DR)"</a>.</p>
    <li data-md>
     <p>Added <a href="#meaning">"Implication for the "meaning" of views"</a>.</p>
    <li data-md>
     <p>Added section on <a href="#views_ref"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>views</c-><c- o>::</c-><c- n>ref</c-></code></a>.</p>
    <li data-md>
     <p>Feature test macro added to proposed wording.</p>
   </ul>
   <h3 class="heading settled" data-level="1.2" id="changelog-r0"><span class="secno">1.2. </span><span class="content">Revision 0 - 2022-09-14</span><a class="self-link" href="#changelog-r0"></a></h3>
   <ul>
    <li data-md>
     <p>Initial version.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="introduction"><span class="secno">2. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
   <p>When dealing with views, two concepts are important: <code class="highlight"><c- n>view</c-></code> ("ranges that are views themselves") and <code class="highlight"><c- n>viewable_range</c-></code> ("ranges that can be turned into views").
Initially, the requirements for the first were very strict, encompassing only non-owning, semi-regular types. The second concept encompassed "views and any lvalue-references to ranges".</p>
   <p>Over time, both concepts were relaxed considerably. Views can now be move-only (<a href="https://wg21.link/p1456">P1456</a>) and owning (<a href="https://wg21.link/p2415">P2415</a>), rvalues of non-views are now viewable––in fact most ranges people encounter are now viewable.
However, in one particular case, the concepts are now stricter than before: lvalue-references to certain ranges, move-only views, are not viewable. This means that while almost all non-view ranges can be easily composed with view adaptors, certain <em>views</em> now do not qualify for this.</p>
   <p>This issue was briefly discussed on the LWG reflector in 2020 where another member highlighted the wide-ranging impact of this change:</p>
   <blockquote>
    <p>I’m concerned about having components that work with ranges but not
views, given that views are supposed to be just ranges with additional
properties.</p>
   </blockquote>
   <p>A possible "fix" was mentioned in that discussion:</p>
   <blockquote>
    <p>I can envision an alternate design in which views:all(E) does admit move-only lvalue views [...]
I think that design change needs LEWG approval [...]</p>
   </blockquote>
   <p>But apparently there was no support for this change or no will to pursue it further. This paper picks up that discussion and argues why such a change would indeed make a lot of sense.</p>
   <h3 class="heading settled" data-level="2.1" id="problem"><span class="secno">2.1. </span><span class="content">The problem (TL;DR)</span><a class="self-link" href="#problem"></a></h3>
<pre class="highlight"><c- b>void</c-> <c- nf>foobar</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>forward_range</c-> <c- k>auto</c-> <c- o>&amp;&amp;</c-> <c- n>r</c-><c- p>)</c->
<c- p>{</c->
    <c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>r</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <p>In the original C++20 standard and in multiple shipped compiler versions, the above code was well-formed for all <code class="highlight"><c- n>forward_range</c-></code>s.
Since P1456, the function template is now underconstrained and will become ill-formed for certain inputs.</p>
   <h3 class="heading settled" data-level="2.2" id="solution"><span class="secno">2.2. </span><span class="content">The solution (TL;DR)</span><a class="self-link" href="#solution"></a></h3>
   <p>Change the definition of <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>all</c-></code> (and <code class="highlight"><c- n>viewable_range</c-></code>) so that:</p>
   <ul>
    <li data-md>
     <p>rvalues are moved</p>
    <li data-md>
     <p>lvalues are copied if that is possible and cheap</p>
    <li data-md>
     <p>
      other lvalues are wrapped in ref_view 
      <del>except lvalues to views</del>
     </p>
   </ul>
   <p>This means that all lvalue ranges become viewable. The only non-viewable ranges are non-movable rvalues.</p>
   <h2 class="heading settled" data-level="3" id="examples"><span class="secno">3. </span><span class="content">Detailed examples</span><a class="self-link" href="#examples"></a></h2>
   <p>To illustrate the different behaviours, I have chosen to provide examples that start out with (1) a copyable view, (2) a copyable non-view range, (3) a move-only non-view range, and (4) a move-only view.</p>
   <p>As example types for non-copyable ranges, I use <code class="highlight"><c- n>Generator</c-></code> and <code class="highlight"><c- n>GeneratorView</c-></code>. <code class="highlight"><c- n>Generator</c-></code> could refer to a user-provided type that generates objects upon iteration (e.g. records of data from a stream); it is a move-only single-pass input-range.</p>
   <p>The type <code class="highlight"><c- n>GeneratorView</c-></code> is the same as <code class="highlight"><c- n>Generator</c-></code> except that it also inherits from the empty base class <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>view_base</c-></code>.
This has no influence on the semantics of the type other than it begin treated as a view by current standard libraries.
While we might suggest to users that such ranges should always be implemented as views, we cannot assume that this will always be the case (there are ranges that are neither containers nor views!).</p>
   <h3 class="heading settled" data-level="3.1" id="from_lvalues"><span class="secno">3.1. </span><span class="content">Views from lvalue reference</span><a class="self-link" href="#from_lvalues"></a></h3>
   <table>
    <tbody>
     <tr>
      <td> (1) 
      <td>
<pre class="highlight"><c- n>string_view</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</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>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> <code class="highlight"><c- n>s</c-></code> is <code class="highlight"><c- n>decay_copy</c-></code>d 
     <tr>
      <td> (2) 
      <td>
<pre class="highlight"><c- n>string</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</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>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> <code class="highlight"><c- n>s</c-></code> is bound by <code class="highlight"><c- n>ref_view</c-></code> 
     <tr>
      <td> (3) 
      <td>
<pre class="highlight"><c- n>Generator</c-> <c- n>g</c-><c- p>{};</c->
<c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>g</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> <code class="highlight"><c- n>g</c-></code> is bound by <code class="highlight"><c- n>ref_view</c-></code> 
     <tr>
      <td>
       <p style="color:Red;">(4)</p>
      <td>
<pre class="highlight"><c- n>GeneratorView</c-> <c- n>g</c-><c- p>{};</c->
<c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>g</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td>
       <p style="color:Red;">currently ill-formed</p>
   </table>
   <p><b>Before <a href="https://wg21.link/p1456">P1456</a> and <a href="https://wg21.link/p2415">P2415</a>, all of these snippets would have been well-formed.</b> Currently, (4) is not, because lvalue references to move-only views are not viewable.</p>
   <p><em>This also means that code is rejected by current compilers that was accepted by previous versions of the same compiler</em> (GCC, MSVC). While this is true for other C++20 features, the particular changes to the view concepts were discussed mainly as relaxing constraints and not as introducing new ones.</p>
   <h3 class="heading settled" data-level="3.2" id="composition2"><span class="secno">3.2. </span><span class="content">View composition (starting from lvalue reference)</span><a class="self-link" href="#composition2"></a></h3>
   <table>
    <tbody>
     <tr>
      <td> (1) 
      <td>
<pre class="highlight"><c- n>string_view</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>};</c->
<c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>s</c->  <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td> <code class="highlight"><c- n>s</c-></code> is <code class="highlight"><c- n>decay_copy</c-></code>d<br> <code class="highlight"><c- n>v1</c-></code> is <code class="highlight"><c- n>decay_copy</c-></code>d 
     <tr>
      <td> (2) 
      <td>
<pre class="highlight"><c- n>string</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>};</c->
<c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>s</c->  <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td> <code class="highlight"><c- n>s</c-></code> is bound by <code class="highlight"><c- n>ref_view</c-></code><br> <code class="highlight"><c- n>v1</c-></code> is <code class="highlight"><c- n>decay_copy</c-></code>d 
     <tr>
      <td> (3) 
      <td>
<pre class="highlight"><c- n>Generator</c-> <c- n>g</c-><c- p>{};</c->
<c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>g</c->  <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td> <code class="highlight"><c- n>g</c-></code> is bound by <code class="highlight"><c- n>ref_view</c-></code> <br> <code class="highlight"><c- n>v1</c-></code> is <code class="highlight"><c- n>decay_copy</c-></code>d 
     <tr>
      <td>
       <p style="color:Red;">(4)</p>
      <td>
<pre class="highlight"><c- n>GeneratorView</c-> <c- n>g</c-><c- p>{};</c->
<c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>g</c->  <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td>
       <p style="color:Red;">currently ill-formed</p>
   </table>
   <p>(4) is still ill-formed; the rest are well-formed.</p>
   <h3 class="heading settled" data-level="3.3" id="from_rvalues"><span class="secno">3.3. </span><span class="content">Views from rvalues</span><a class="self-link" href="#from_rvalues"></a></h3>
   <table>
    <tbody>
     <tr>
      <td> (1) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>string_view</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> string_view is <code class="highlight"><c- n>decay_copy</c-></code>d 
     <tr>
      <td> (2) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>string</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> string is bound by <code class="highlight"><c- n>owning_view</c-></code> 
     <tr>
      <td> (3) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>Generator</c-><c- p>{}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> generator is bound by <code class="highlight"><c- n>owning_view</c-></code> 
     <tr>
      <td> (4) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>GeneratorView</c-><c- p>{}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> generator is <code class="highlight"><c- n>decay_copy</c-></code>d (actually a move) 
   </table>
   <p>All of these snippets are currently well-formed code. In the original ranges proposal, only (1) would have been well-formed, because the others are rvalues of non-views; (4) would not have been regarded as a view, because it is move-only.</p>
   <h3 class="heading settled" data-level="3.4" id="composition"><span class="secno">3.4. </span><span class="content">View composition (starting from rvalue)</span><a class="self-link" href="#composition"></a></h3>
   <table>
    <tbody>
     <tr>
      <td> (1) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>string_view</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td> string_view is <code class="highlight"><c- n>decay_copy</c-></code>d<br> <code class="highlight"><c- n>v1</c-></code> is <code class="highlight"><c- n>decay_copy</c-></code>d 
     <tr>
      <td>
       <p style="color:Red;">(2)</p>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>string</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td>
        string is bound by <code class="highlight"><c- n>owning_view</c-></code> 
       <p style="color:Red;">line 2 is currently ill-formed</p>
     <tr>
      <td>
       <p style="color:Red;">(3)</p>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>Generator</c-><c- p>{}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td>
        generator is bound by <code class="highlight"><c- n>owning_view</c-></code> 
       <p style="color:Red;">line 2 is currently ill-formed</p>
     <tr>
      <td>
       <p style="color:Red;">(4)</p>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>GeneratorView</c-><c- p>{}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
</pre>
      <td>
        generator is <code class="highlight"><c- n>decay_copy</c-></code>d (actually a move) 
       <p style="color:Red;">line 2 is currently ill-formed</p>
   </table>
   <p>(2)-(4) are not well-formed, because <code class="highlight"><c- n>v1</c-></code> in the respective snippet is move-only.  Note, how the well-formed snippets in this example are different from <a href="#composition2">those created from lvalues</a>. If the <code class="highlight"><c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>)</c-></code> had been appended to the first line and no <code class="highlight"><c- n>v2</c-></code> created, it would have worked. This means that––contrary to what many previous ranges guides suggest––it makes an important difference now whether you compose the view incrementally or in one line.
And it appears that "creating in one line" needs to be recommended.</p>
   <h3 class="heading settled" data-level="3.5" id="generic"><span class="secno">3.5. </span><span class="content">Generic code</span><a class="self-link" href="#generic"></a></h3>
   <p>Returning to the problem of generic code, assume the following:</p>
   <p>(definitions for <code class="highlight"><c- n>to_upper</c-></code> and <code class="highlight"><c- n>to_lower</c-></code> omitted)</p>
<pre class="highlight"><c- b>void</c-> <c- nf>print_up_low</c-><c- p>(</c-><c- n>ranges</c-><c- o>::</c-><c- n>forward_range</c-> <c- k>auto</c-> <c- o>&amp;&amp;</c-> <c- n>rng</c-><c- p>)</c->
<c- p>{</c->
    <c- k>for</c-> <c- p>(</c-><c- b>char</c-> <c- n>c</c-> <c- o>:</c-> <c- n>rng</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- n>to_upper</c-><c- p>))</c->
        <c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>c</c-><c- p>;</c->
    <c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c->
    <c- k>for</c-> <c- p>(</c-><c- b>char</c-> <c- n>c</c-> <c- o>:</c-> <c- n>rng</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- n>to_lower</c-><c- p>))</c->
        <c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>c</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <table>
    <tbody>
     <tr>
      <td> (1a) 
      <td>
<pre class="highlight"><c- n>print_up_low</c-><c- p>(</c-><c- n>string_view</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>});</c->
<c- n>print_up_low</c-><c- p>(</c-><c- n>string_view</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>));</c->
</pre>
      <td> both calls are well-formed 
     <tr>
      <td>
       <p style="color:Red;">(2a)</p>
      <td>
<pre class="highlight"><c- n>print_up_low</c-><c- p>(</c-><c- n>string</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>});</c->
<c- n>print_up_low</c-><c- p>(</c-><c- n>string</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>}</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>));</c->
</pre>
      <td>
        first call is well-formed<br> 
       <p style="color:Red;">second call is ill-formed</p>
     <tr>
      <td> (1b) 
      <td>
<pre class="highlight"><c- n>string_view</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>};</c->
<c- n>print_up_low</c-><c- p>(</c-><c- n>s</c-><c- p>);</c->
<c- n>print_up_low</c-><c- p>(</c-><c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>));</c->
</pre>
      <td> both calls are well-formed 
     <tr>
      <td> (2b) 
      <td>
<pre class="highlight"><c- n>string</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>};</c->
<c- n>print_up_low</c-><c- p>(</c-><c- n>s</c-><c- p>);</c->
<c- n>print_up_low</c-><c- p>(</c-><c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>));</c->
</pre>
      <td> both calls are well-formed 
   </table>
   <p>These examples show that it is easy to run into problems with "move-only views", even when just using simple adaptors on types likes strings and string_views.
Is the behaviour shown above intended? Do we expect developers to understand why 1 of 8 calls to <code class="highlight"><c- n>print_up_low</c-><c- p>()</c-></code> is ill-formed and why?</p>
   <p>Curiously, it is now the example of "creating a view in one line" that is ill-formed, while previous examples suggested that exactly this usage pattern increases the likelihood of the code being well-formed.</p>
   <h2 class="heading settled" data-level="4" id="proposal"><span class="secno">4. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>This papers suggests <strong>making all previously shown ill-formed snippets well-formed</strong> by changing the <code class="highlight"><c- n>viewable_range</c-></code> concept and the definition of the <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>all</c-></code> adaptor.</p>
   <h3 class="heading settled" data-level="4.1" id="wording"><span class="secno">4.1. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h3>
   <p><a href="https://eel.is/c++draft/range.req#range.refinements-6">26.4.5 Other range refinements [range.refinements]</a></p>
   <p>Change the definition of <code class="highlight"><c- n>viewable_range</c-></code> to:</p>
   <blockquote>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>concept</c-> <c- nc>viewable_</c->­<c- n>range</c-> <c- o>=</c->
    <c- n>range</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>&amp;&amp;</c->
    <c- p>((</c-><c- n>view</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c-> <c- n>constructible_</c->­<c- n>from</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>T</c-><c- o>></c-><c- p>)</c-> <c- o>||</c->
</pre>
    <del>
<pre class="highlight"><c- p>(</c-><c- o>!</c-><c- n>view</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c-> <c- p>(</c-><c- n>is_lvalue_reference_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>||</c-> <c- p>(</c-><c- n>movable</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c-> <c- o>!</c-><c- n>is</c-><c- o>-</c-><c- n>initializer</c-><c- o>-</c-><c- n>list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>))));</c->
</pre>
    </del>
    <ins>
<pre class="highlight"><c- n>is_lvalue_reference_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>||</c-> <c- p>(</c-><c- n>movable</c-><c- o>&lt;</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-> <c- o>&amp;&amp;</c-> <c- o>!</c-><c- n>is</c-><c- o>-</c-><c- n>initializer</c-><c- o>-</c-><c- n>list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>));</c->
</pre>
    </ins>
   </blockquote>
   <p><a href="https://eel.is/c++draft/range.all#general">[26.7.6.1 General [range.all.general]]</a></p>
   <p>Change the definition of <code class="highlight"><c- nl>views</c-><c- p>:</c-><c- n>all</c-></code> to</p>
   <blockquote>
    <ul>
     <li>
      (2.1)    decay-copy(E) 
      <ins>if that expression is well-formed and</ins>
       the decayed type of E models view.
     <li>(2.2)    Otherwise, ref_­view{E} if that expression is well-formed.
     <li>(2.3)    Otherwise, owning_­view{E}.
    </ul>
   </blockquote>
   <p><a href="https://wg21.link/version.syn">[17.3.2 version.syn]</a></p>
   <p>Bump the <code class="highlight"><c- n>__cpp_lib_ranges</c-></code> version.</p>
   <h3 class="heading settled" data-level="4.2" id="impact"><span class="secno">4.2. </span><span class="content">Impact</span><a class="self-link" href="#impact"></a></h3>
   <p>These changes are <em>breaking</em> insofar as all concept changes are always potentially breaking. However, this proposal very clearly relaxes the constraints on <code class="highlight"><c- n>viewable_range</c-></code> so the scope is broadened, and in practice more code will work than before and not less.
In fact, this will <em>unbreak</em> code that was well-formed in earlier published versions of the IS (and is well-formed in released versions of GCC and MSVC) but that has been broken in the meantime.</p>
   <p>Should LEWG accept this proposal for C++23, it could ask LWG to denote it as a C++20 defect fix.
There is precedent for this, because the <code class="highlight"><c- n>view</c-></code> and <code class="highlight"><c- n>viewable_range</c-></code> concepts have already been changed multiple times with wide-ranging modifications being back-ported to C++20 and even older compiler versions (GCC10, GCC11); although technically these changes were feature changes more than fixes.
The current changes are much smaller in scope and re-establish initial C++20 behaviour.</p>
   <h2 class="heading settled" data-level="5" id="workarounds"><span class="secno">5. </span><span class="content">"Workarounds"</span><a class="self-link" href="#workarounds"></a></h2>
   <p>For the previously shown ill-formed snippets, it is possible to work around the problem by any or either of the following methods:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>move</c-></code>-ing the initial range into the pipe (resulting in an <code class="highlight"><c- n>decay_copy</c-></code> or <code class="highlight"><c- n>owning_view</c-></code>); or</p>
    <li data-md>
     <p>manually wrapping the initial range in <code class="highlight"><c- n>ref_view</c-></code></p>
   </ol>
   <p>Since most users never directly interact with <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>all</c-></code>, <code class="highlight"><c- n>ref_view</c-></code> and <code class="highlight"><c- n>owning_view</c-></code>, I assume that it will be very hard to explain where (and which of) these workarounds are necessary. Furthermore, this creates a burden on generic code that will either need to <code class="highlight"><c- k>if</c-> <c- k>constexpr</c-></code> on ranges passed to it by reference or blindly wrap them in <code class="highlight"><c- n>ref_view</c-></code> to make them viewable.</p>
   <p>Note also that "just adding <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>ref_view</c-></code>" in a generic context may have undesired side-effects.
Even for ranges that are already viewable, it is not a no-op; e.g. it prevents copyable views from being captured by value which changes their behaviour with regard to <code class="highlight"><c- k>const</c-></code> (they are now shallow const but would have been deep const before).
It also changes type-specific behaviour in view adaptor objects like the "same-type optimisations" in <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>take</c-></code> and <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>drop</c-></code>.</p>
   <h3 class="heading settled" data-level="5.1" id="views_ref"><span class="secno">5.1. </span><span class="content"><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>views</c-><c- o>::</c-><c- n>ref</c-></code></span><a class="self-link" href="#views_ref"></a></h3>
   <p>It was suggested that having <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>ref</c-></code> as a shortcut for <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>ref_view</c-></code> would alleviate the problems presented here.
While I do not object to having such a view factory, I do not see how it solves the fundamental problem of a bad default.</p>
   <p>It is of no consequence to almost all algorithms whether they are operating on a move-only view or a copyable range, so we shouldn’t require a workaround.</p>
   <h2 class="heading settled" data-level="6" id="lifetime"><span class="secno">6. </span><span class="content">Lifetime</span><a class="self-link" href="#lifetime"></a></h2>
   <p>Previous mailing list comments suggested that the changes proposed here would create issues with view lifetime.
The following two examples explore some of those implications.</p>
   <h3 class="heading settled" data-level="6.1" id="lifetime1"><span class="secno">6.1. </span><span class="content">Returning views on local variables</span><a class="self-link" href="#lifetime1"></a></h3>
   <table>
    <tbody>
     <tr>
      <th> Nr. 
      <th> Code 
      <th> Status quo 
      <th> This proposal 
     <tr>
      <td> (1) 
      <td>
<pre class="highlight"><c- n>string_view</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>};</c->
<c- k>return</c-> <c- n>s</c->  <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td> not dangling 
      <td> not dangling 
     <tr>
      <td> (2) 
      <td>
<pre class="highlight"><c- n>string</c-> <c- n>s</c-><c- p>{</c-><c- s>"foobar"</c-><c- p>};</c->
<c- k>return</c-> <c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td>
       <p style="color:Orange;">dangling</p>
      <td>
       <p style="color:Orange;">dangling</p>
     <tr>
      <td> (3) 
      <td>
<pre class="highlight"><c- n>Generator</c-> <c- n>g</c-><c- p>{};</c->
<c- k>return</c-> <c- n>g</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td>
       <p style="color:Orange;">dangling</p>
      <td>
       <p style="color:Orange;">dangling</p>
     <tr>
      <td>
       <p style="color:Red;">(4)</p>
      <td>
<pre class="highlight"><c- n>GeneratorView</c-> <c- n>g</c-><c- p>{};</c->
<c- k>return</c-> <c- n>g</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
</pre>
      <td>
       <p style="color:Red;">ill-formed</p>
      <td>
       <p style="color:Orange;">dangling</p>
   </table>
   <p>Returning a view created from a local variable can create a dangling reference. This has always been the case and is inherent to <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>ref_view</c-></code> (and independent of the underlying range). While it could be argued that preventing it for <em>some ranges</em> is beneficial, it is unclear why this should only be the case for move-only views and not other ranges; especially since example (2) is likely to appear more often than example (4) and the range in (3) is semantically identical to the range in (4).</p>
   <p><em>On a sidenote:</em> Now that we have move-only owning views, one could make an argument for making all uses of <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>ref_view</c-></code> explicit (removing the option from <code class="highlight"><c- n>views</c-><c- o>::</c-><c- n>all</c-></code> and recommending that users <code class="highlight"><c- n>move</c-></code> into a view composition by default or explicitly opt into having a reference if desired). However, such a change would be quite disruptive, so pursuing this argument at this point seems futile.</p>
   <h3 class="heading settled" data-level="6.2" id="lifetime1①"><span class="secno">6.2. </span><span class="content">Returning views on in-parameters</span><a class="self-link" href="#lifetime1①"></a></h3>
   <table>
    <tbody>
     <tr>
      <th> Nr. 
      <th> Code 
      <th> Status quo 
      <th> This proposal 
     <tr>
      <td> (1) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>foo</c-><c- p>(</c-><c- n>string_view</c-> <c- o>&amp;</c-> <c- n>s</c-><c- p>)</c->
<c- p>{</c->
  <c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
  <c- k>return</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
<c- p>}</c->
</pre>
      <td> not dangling 
      <td> not dangling 
     <tr>
      <td> (2) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>foo</c-><c- p>(</c-><c- n>string</c-> <c- o>&amp;</c-> <c- n>s</c-><c- p>)</c->
<c- p>{</c->
  <c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>s</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
  <c- k>return</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
<c- p>}</c->
</pre>
      <td> not dangling 
      <td> not dangling 
     <tr>
      <td> (3) 
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>foo</c-><c- p>(</c-><c- n>Generator</c-> <c- o>&amp;</c-> <c- n>g</c-><c- p>)</c->
<c- p>{</c->
  <c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>g</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
  <c- k>return</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
<c- p>}</c->
</pre>
      <td> not dangling 
      <td> not dangling 
     <tr>
      <td>
       <p style="color:Red;">(4)</p>
      <td>
<pre class="highlight"><c- k>auto</c-> <c- n>foo</c-><c- p>(</c-><c- n>GeneratorView</c-> <c- o>&amp;</c-> <c- n>g</c-><c- p>)</c->
<c- p>{</c->
  <c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>g</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>take</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->
  <c- k>return</c-> <c- n>v1</c-> <c- o>|</c-> <c- n>views</c-><c- o>::</c-><c- n>transform</c-><c- p>(</c-><c- d>/**/</c-><c- p>);</c->
<c- p>}</c->
</pre>
      <td>
       <p style="color:Red;">ill-formed</p>
      <td>
       <p style="color:Green;">well-formed &amp; not dangling</p>
   </table>
   <p>Example (4) would become well-formed code through the changes proposed here. Contrary to what one may assume, the example will not return a dangling reference: while <code class="highlight"><c- n>s</c-></code> is move-only, <code class="highlight"><c- n>v1</c-></code> will be a <code class="highlight"><c- n>ref_view</c-></code> to <code class="highlight"><c- n>s</c-></code> and thus copyable; in the return statement, it is copied into the returned view, so the composition does not dangle.</p>
   <h2 class="heading settled" data-level="7" id="meaning"><span class="secno">7. </span><span class="content">Implication for the "meaning" of views</span><a class="self-link" href="#meaning"></a></h2>
   <p>Objections to this proposal include that it would distort the meaning of views and that <em>»[t]he entire point of a view is that this is something that should be taken by value and owned«</em>.
There is some irony here, because this criticism comes from the same people who removed all other defining features from views (default-constructibility, cheap copyability, not owning)!
But I do not disagree with those changes and want to argue instead that (a) views are not <em>generally</em> taken by value; and (b) the view concept is not useful in most generic code anyway.</p>
   <p>In fact, this argument is already made in <a href="https://wg21.link/p2415">P2415</a>:</p>
   <blockquote>
    <p>The initial thought […] might be that […] we write algorithms that take views by value:
[…] and we definitely do encourage people to take <em>specific</em> views by value - such as <code class="highlight"><c- n>span</c-></code> and <code class="highlight"><c- n>string_view</c-></code> […]
Either way, we very much want range-based algorithms to be able to operate on, well, ranges, so these are always written instead to take ranges by forwarding reference:</p>
   </blockquote>
   <blockquote>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- n>input_range</c-> <c- n>R</c-><c- o>></c->
<c- b>void</c-> <c- n>some_algo</c-><c- p>(</c-><c- n>R</c-><c- o>&amp;&amp;</c-> <c- n>r</c-><c- p>);</c->
</pre>
   </blockquote>
   <p>→ we do not actually take views by value in generic code, and we do not use the view concept to constrain algorithms.</p>
   <p>As far as the "meaning of a view" is concerned, the authors of <a href="https://wg21.link/p1456">P1456</a> conclude a paragraph by writing:</p>
   <blockquote>
    <p>the benefit that views provide: cheap construction of range adaptor pipelines.</p>
   </blockquote>
   <p>I would argue that this is probably the only remaining useful definition of "view", and it is exactly what the changes proposed here try to make more consistent.</p>
   <p>By strongly relaxing the constraints of <code class="highlight"><c- n>view</c-></code> and <code class="highlight"><c- n>viewable_range</c-></code>, <a href="https://wg21.link/p1456">P1456</a> and <a href="https://wg21.link/p2415">P2415</a> made it possible to use range adaptors without having to know how these concepts are defined or what they "mean".
That’s great, because most people do not need to know (and we have trouble explaining it ourselves 😉)!</p>
   <p>There is, however, an exception to that rule where the "details" (what is a view? [when] is it copyable? why should I care?) leak out, and this paper fixes that.</p>
   <h2 class="heading settled" data-level="8" id="summary"><span class="secno">8. </span><span class="content">Summary</span><a class="self-link" href="#summary"></a></h2>
   <p>Treating move-only views as fundamentally different from copyable views when evaluating view-ability, creates a source of user error and frustration.
I agree with the changes made to views, but we now have problems explaining what a view even is (<a href="https://wg21.link/p2415">P2415</a>); having two sets of views with different usage patterns definitely won’t help.
This does not only affect exotic, user-defined types, but can become obvious even in toy examples created with strings.</p>
   <p>The difference becomes even more pronounced with standalone move-only ranges that can be implemented as views but where we cannot assume that they always will be. For those types, it is unclear what benefit <strong>opting into</strong> the view concept provides––if that only makes the type <strong>opt out</strong> of easy view composition.</p>
   <p>Further complications arise in generic code, which doesn’t care about the view concept or ownership at all and could previously assume that all lvalue ranges are viewable. It now needs workarounds which are easy to forget and come with their own side-effects.</p>
   <p>While it is true that all direct and indirect uses of <code class="highlight"><c- n>ranges</c-><c- o>::</c-><c- n>ref_view</c-></code> entail the possibility of dangling references, it is unclear why this is considered more dangerous for move-only views than other ranges.
I hope to have alleviated some concerns in this regard and shown that the proposed increase in consistency and usability provides a clear benefit.</p>
   <h2 class="heading settled" data-level="9" id="ack"><span class="secno">9. </span><span class="content">Acknowledgments</span><a class="self-link" href="#ack"></a></h2>
   <p>I want to thank Barry Revzin and Tim Song for their work on revising views and for their comments on the current topic. Thanks also to others for their feedback on the draft of this paper.</p>
   <p>Thanks to my employer, deCODE Genetics, for sponsoring my WG21 time.</p>
   <h2 class="heading settled" data-level="10" id="references"><span class="secno">10. </span><span class="content">References</span><a class="self-link" href="#references"></a></h2>
   <ul>
    <li data-md>
     <p>Short 2020 discussion in LWG: <a href="https://lists.isocpp.org/lib/2020/09/17353.php">https://lists.isocpp.org/lib/2020/09/17353.php</a></p>
    <li data-md>
     <p>Short 2022 discussion in LWG: <a href="https://lists.isocpp.org/lib/2022/08/23438.php">https://lists.isocpp.org/lib/2022/08/23438.php</a></p>
    <li data-md>
     <p>Brief mention in minutes: <a href="https://wiki.edg.com/bin/view/Wg21telecons2021/LWG-issues-20210115">https://wiki.edg.com/bin/view/Wg21telecons2021/LWG-issues-20210115</a></p>
    <li data-md>
     <p>GCC issue with other examples: <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106669">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106669</a></p>
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