<!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>P2809R3: Trivial infinite loops are not Undefined Behavior</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

	/* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    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 d765c696b, updated Fri Mar 8 15:58:52 2024 -0800" name="generator">
  <link href="http://wg21.link/P2809r3" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="8144e3186b8dc787c703c1a7fdd04ba5ae1ffb93" name="revision">
  <meta content="dark light" name="color-scheme">
<style>
table, th, td { border: 2px solid grey; }
ins { color: #080; }
</style>
<style>/* Boilerplate: style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}

@media (prefers-color-scheme: dark) {
    :root {
        --selflink-text: black;
        --selflink-bg: silver;
        --selflink-hover-text: white;
    }
}
</style>
<style>/* Boilerplate: style-colors */
/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
:root {
    color-scheme: light dark;

    --text: black;
    --bg: white;

    --unofficial-watermark: url(https://www.w3.org/StyleSheets/TR/2016/logos/UD-watermark);

    --logo-bg: #1a5e9a;
    --logo-active-bg: #c00;
    --logo-text: white;

    --tocnav-normal-text: #707070;
    --tocnav-normal-bg: var(--bg);
    --tocnav-hover-text: var(--tocnav-normal-text);
    --tocnav-hover-bg: #f8f8f8;
    --tocnav-active-text: #c00;
    --tocnav-active-bg: var(--tocnav-normal-bg);

    --tocsidebar-text: var(--text);
    --tocsidebar-bg: #f7f8f9;
    --tocsidebar-shadow: rgba(0,0,0,.1);
    --tocsidebar-heading-text: hsla(203,20%,40%,.7);

    --toclink-text: var(--text);
    --toclink-underline: #3980b5;
    --toclink-visited-text: var(--toclink-text);
    --toclink-visited-underline: #054572;

    --heading-text: #005a9c;

    --hr-text: var(--text);

    --algo-border: #def;

    --del-text: red;
    --del-bg: transparent;
    --ins-text: #080;
    --ins-bg: transparent;

    --a-normal-text: #034575;
    --a-normal-underline: #bbb;
    --a-visited-text: var(--a-normal-text);
    --a-visited-underline: #707070;
    --a-hover-bg: rgba(75%, 75%, 75%, .25);
    --a-active-text: #c00;
    --a-active-underline: #c00;

    --blockquote-border: silver;
    --blockquote-bg: transparent;
    --blockquote-text: currentcolor;

    --issue-border: #e05252;
    --issue-bg: #fbe9e9;
    --issue-text: var(--text);
    --issueheading-text: #831616;

    --example-border: #e0cb52;
    --example-bg: #fcfaee;
    --example-text: var(--text);
    --exampleheading-text: #574b0f;

    --note-border: #52e052;
    --note-bg: #e9fbe9;
    --note-text: var(--text);
    --noteheading-text: hsl(120, 70%, 30%);
    --notesummary-underline: silver;

    --assertion-border: #aaa;
    --assertion-bg: #eee;
    --assertion-text: black;

    --advisement-border: orange;
    --advisement-bg: #fec;
    --advisement-text: var(--text);
    --advisementheading-text: #b35f00;

    --warning-border: red;
    --warning-bg: hsla(40,100%,50%,0.95);
    --warning-text: var(--text);

    --amendment-border: #330099;
    --amendment-bg: #F5F0FF;
    --amendment-text: var(--text);
    --amendmentheading-text: #220066;

    --def-border: #8ccbf2;
    --def-bg: #def;
    --def-text: var(--text);
    --defrow-border: #bbd7e9;

    --datacell-border: silver;

    --indexinfo-text: #707070;

    --indextable-hover-text: black;
    --indextable-hover-bg: #f7f8f9;

    --outdatedspec-bg: rgba(0, 0, 0, .5);
    --outdatedspec-text: black;
    --outdated-bg: maroon;
    --outdated-text: white;
    --outdated-shadow: red;

    --editedrec-bg: darkorange;
}

@media (prefers-color-scheme: dark) {
    :root {
        --text: #ddd;
        --bg: black;

        --unofficial-watermark: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='400' height='400'%3E%3Cg fill='%23100808' transform='translate(200 200) rotate(-45) translate(-200 -200)' stroke='%23100808' stroke-width='3'%3E%3Ctext x='50%25' y='220' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EUNOFFICIAL%3C/text%3E%3Ctext x='50%25' y='305' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EDRAFT%3C/text%3E%3C/g%3E%3C/svg%3E");

        --logo-bg: #1a5e9a;
        --logo-active-bg: #c00;
        --logo-text: white;

        --tocnav-normal-text: #999;
        --tocnav-normal-bg: var(--bg);
        --tocnav-hover-text: var(--tocnav-normal-text);
        --tocnav-hover-bg: #080808;
        --tocnav-active-text: #f44;
        --tocnav-active-bg: var(--tocnav-normal-bg);

        --tocsidebar-text: var(--text);
        --tocsidebar-bg: #080808;
        --tocsidebar-shadow: rgba(255,255,255,.1);
        --tocsidebar-heading-text: hsla(203,20%,40%,.7);

        --toclink-text: var(--text);
        --toclink-underline: #6af;
        --toclink-visited-text: var(--toclink-text);
        --toclink-visited-underline: #054572;

        --heading-text: #8af;

        --hr-text: var(--text);

        --algo-border: #456;

        --del-text: #f44;
        --del-bg: transparent;
        --ins-text: #4a4;
        --ins-bg: transparent;

        --a-normal-text: #6af;
        --a-normal-underline: #555;
        --a-visited-text: var(--a-normal-text);
        --a-visited-underline: var(--a-normal-underline);
        --a-hover-bg: rgba(25%, 25%, 25%, .2);
        --a-active-text: #f44;
        --a-active-underline: var(--a-active-text);

        --borderedblock-bg: rgba(255, 255, 255, .05);

        --blockquote-border: silver;
        --blockquote-bg: var(--borderedblock-bg);
        --blockquote-text: currentcolor;

        --issue-border: #e05252;
        --issue-bg: var(--borderedblock-bg);
        --issue-text: var(--text);
        --issueheading-text: hsl(0deg, 70%, 70%);

        --example-border: hsl(50deg, 90%, 60%);
        --example-bg: var(--borderedblock-bg);
        --example-text: var(--text);
        --exampleheading-text: hsl(50deg, 70%, 70%);

        --note-border: hsl(120deg, 100%, 35%);
        --note-bg: var(--borderedblock-bg);
        --note-text: var(--text);
        --noteheading-text: hsl(120, 70%, 70%);
        --notesummary-underline: silver;

        --assertion-border: #444;
        --assertion-bg: var(--borderedblock-bg);
        --assertion-text: var(--text);

        --advisement-border: orange;
        --advisement-bg: #222218;
        --advisement-text: var(--text);
        --advisementheading-text: #f84;

        --warning-border: red;
        --warning-bg: hsla(40,100%,20%,0.95);
        --warning-text: var(--text);

        --amendment-border: #330099;
        --amendment-bg: #080010;
        --amendment-text: var(--text);
        --amendmentheading-text: #cc00ff;

        --def-border: #8ccbf2;
        --def-bg: #080818;
        --def-text: var(--text);
        --defrow-border: #136;

        --datacell-border: silver;

        --indexinfo-text: #aaa;

        --indextable-hover-text: var(--text);
        --indextable-hover-bg: #181818;

        --outdatedspec-bg: rgba(255, 255, 255, .5);
        --outdatedspec-text: black;
        --outdated-bg: maroon;
        --outdated-text: white;
        --outdated-shadow: red;

        --editedrec-bg: darkorange;
    }
    /* In case a transparent-bg image doesn't expect to be on a dark bg,
       which is quite common in practice... */
    img { background: white; }
}
</style>
<style>/* Boilerplate: style-counters */
body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}
</style>
<style>/* Boilerplate: style-issues */
a[href].issue-return {
    float: right;
    float: inline-end;
    color: var(--issueheading-text);
    font-weight: bold;
    text-decoration: none;
}
</style>
<style>/* Boilerplate: style-md-lists */
/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}
</style>
<style>/* Boilerplate: style-selflinks */
:root {
    --selflink-text: white;
    --selflink-bg: gray;
    --selflink-hover-text: black;
}
.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
.example > a.self-link,
.note > a.self-link,
.issue > a.self-link {
    /* These blocks are overflow:auto, so positioning outside
       doesn't work. */
    left: auto;
    right: 0;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: var(--selflink-bg);
    color: var(--selflink-text);
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: var(--selflink-hover-text);
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }
</style>
<style>/* Boilerplate: style-syntax-highlighting */
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }

.highlight:not(.idl) { background: rgba(0, 0, 0, .03); }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */

@media (prefers-color-scheme: dark) {
    .highlight:not(.idl) { background: rgba(255, 255, 255, .05); }

    c-[a] { color: #d33682 } /* Keyword.Declaration */
    c-[b] { color: #d33682 } /* Keyword.Type */
    c-[c] { color: #2aa198 } /* Comment */
    c-[d] { color: #2aa198 } /* Comment.Multiline */
    c-[e] { color: #268bd2 } /* Name.Attribute */
    c-[f] { color: #b58900 } /* Name.Tag */
    c-[g] { color: #cb4b16 } /* Name.Variable */
    c-[k] { color: #d33682 } /* Keyword */
    c-[l] { color: #657b83 } /* Literal */
    c-[m] { color: #657b83 } /* Literal.Number */
    c-[n] { color: #268bd2 } /* Name */
    c-[o] { color: #657b83 } /* Operator */
    c-[p] { color: #657b83 } /* Punctuation */
    c-[s] { color: #6c71c4 } /* Literal.String */
    c-[t] { color: #6c71c4 } /* Literal.String.Single */
    c-[u] { color: #6c71c4 } /* Literal.String.Double */
    c-[ch] { color: #2aa198 } /* Comment.Hashbang */
    c-[cp] { color: #2aa198 } /* Comment.Preproc */
    c-[cpf] { color: #2aa198 } /* Comment.PreprocFile */
    c-[c1] { color: #2aa198 } /* Comment.Single */
    c-[cs] { color: #2aa198 } /* Comment.Special */
    c-[kc] { color: #d33682 } /* Keyword.Constant */
    c-[kn] { color: #d33682 } /* Keyword.Namespace */
    c-[kp] { color: #d33682 } /* Keyword.Pseudo */
    c-[kr] { color: #d33682 } /* Keyword.Reserved */
    c-[ld] { color: #657b83 } /* Literal.Date */
    c-[nc] { color: #268bd2 } /* Name.Class */
    c-[no] { color: #268bd2 } /* Name.Constant */
    c-[nd] { color: #268bd2 } /* Name.Decorator */
    c-[ni] { color: #268bd2 } /* Name.Entity */
    c-[ne] { color: #268bd2 } /* Name.Exception */
    c-[nf] { color: #268bd2 } /* Name.Function */
    c-[nl] { color: #268bd2 } /* Name.Label */
    c-[nn] { color: #268bd2 } /* Name.Namespace */
    c-[py] { color: #268bd2 } /* Name.Property */
    c-[ow] { color: #657b83 } /* Operator.Word */
    c-[mb] { color: #657b83 } /* Literal.Number.Bin */
    c-[mf] { color: #657b83 } /* Literal.Number.Float */
    c-[mh] { color: #657b83 } /* Literal.Number.Hex */
    c-[mi] { color: #657b83 } /* Literal.Number.Integer */
    c-[mo] { color: #657b83 } /* Literal.Number.Oct */
    c-[sa] { color: #6c71c4 } /* Literal.String.Affix */
    c-[sb] { color: #6c71c4 } /* Literal.String.Backtick */
    c-[sc] { color: #6c71c4 } /* Literal.String.Char */
    c-[dl] { color: #6c71c4 } /* Literal.String.Delimiter */
    c-[sd] { color: #6c71c4 } /* Literal.String.Doc */
    c-[se] { color: #6c71c4 } /* Literal.String.Escape */
    c-[sh] { color: #6c71c4 } /* Literal.String.Heredoc */
    c-[si] { color: #6c71c4 } /* Literal.String.Interpol */
    c-[sx] { color: #6c71c4 } /* Literal.String.Other */
    c-[sr] { color: #6c71c4 } /* Literal.String.Regex */
    c-[ss] { color: #6c71c4 } /* Literal.String.Symbol */
    c-[fm] { color: #268bd2 } /* Name.Function.Magic */
    c-[vc] { color: #cb4b16 } /* Name.Variable.Class */
    c-[vg] { color: #cb4b16 } /* Name.Variable.Global */
    c-[vi] { color: #cb4b16 } /* Name.Variable.Instance */
    c-[vm] { color: #cb4b16 } /* Name.Variable.Magic */
    c-[il] { color: #657b83 } /* Literal.Number.Integer.Long */
}
</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2809R3<br>Trivial infinite loops are not Undefined Behavior</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-03-21">2024-03-21</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P2809r3">http://wg21.link/P2809r3</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:cxx@jfbastien.com">JF Bastien</a> (<span class="p-org org">Woven by Toyota</span>)
     <dt>Audience:
     <dd>SG22, CWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
     <dt>Source:
     <dd><a href="https://github.com/jfbastien/papers/blob/master/source/P2809r3.bs">github.com/jfbastien/papers/blob/master/source/P2809r3.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </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="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li>
     <a href="#rev"><span class="secno">2</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r0"><span class="secno">2.1</span> <span class="content">Revision 0</span></a>
      <li><a href="#r1"><span class="secno">2.2</span> <span class="content">Revision 1</span></a>
      <li><a href="#r2"><span class="secno">2.3</span> <span class="content">Revision 2</span></a>
      <li><a href="#r3"><span class="secno">2.4</span> <span class="content">Revision 3</span></a>
     </ol>
    <li>
     <a href="#pedantry"><span class="secno">3</span> <span class="content">Standards</span></a>
     <ol class="toc">
      <li><a href="#cpp"><span class="secno">3.1</span> <span class="content">C++ Standard</span></a>
      <li><a href="#c"><span class="secno">3.2</span> <span class="content">C Standard</span></a>
      <li><a href="#freebird"><span class="secno">3.3</span> <span class="content">Freestanding in C++</span></a>
     </ol>
    <li><a href="#yawn"><span class="secno">4</span> <span class="content">History</span></a>
    <li><a href="#why"><span class="secno">5</span> <span class="content">Rationale</span></a>
    <li><a href="#lang-word"><span class="secno">6</span> <span class="content">Language Wording</span></a>
    <li><a href="#effect"><span class="secno">7</span> <span class="content">Effect</span></a>
    <li>
     <a href="#alt"><span class="secno">8</span> <span class="content">Alternative</span></a>
     <ol class="toc">
      <li><a href="#lang-alt"><span class="secno">8.1</span> <span class="content">Alternative Language Wording</span></a>
      <li><a href="#lib-alt"><span class="secno">8.2</span> <span class="content">Alternative Library Wording</span></a>
     </ol>
    <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="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>C and C++ diverge in their definition of forward progress guarantees, and have done so since C++11 and C11 added a memory model and acknowledged that threads exist. Both committees have been working together to reduce needless differences. <a data-link-type="biblio" href="#biblio-p2735r0" title="C xor C++ Programming">[P2735r0]</a> and <a data-link-type="biblio" href="#biblio-n2644" title="Programming languages — a common C/C++ core specification">[N2644]</a> perform such harmonization.</p>
   <p>C does not make iteration statements whose controlling expression is a constant expression Undefined Behavior, whereas C++ does. C++ implementations therefore assume that even "trivial" infinite loops must terminate.</p>
   <p>This is unfortunate because using an infinite loop is a common idiom in low-level programming, particularly when a bare-metal system or kernel must halt progress. It would be easy for programmers to satisfy the C++ requirements on infinite loops, and it would be easy for compilers to diagnose "invalid" infinite loops. Neither programmers nor implementations have done so. Instead, programmers decry C++'s obtuseness, and implementations upset developers by optimizing away their infinite loops (marking them as unreachable, leading to surrounding code being optimized away).</p>
   <p>For example, this code prints <code class="highlight"><c- n>Hello</c-><c- p>,</c-> <c- n>World</c-><c- o>!</c-></code> in recent versions of clang (<a href="https://godbolt.org/z/d1WP4KP99">see example</a>):</p>
<pre class="highlight"><c- cp>#include</c-> &lt;iostream>

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
  <c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- p>;</c-> 
<c- p>}</c->

<c- b>void</c-> <c- nf>unreachable</c-><c- p>()</c-> <c- p>{</c->
  <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"Hello world!"</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>It is easy for the C++ Standards Committee to instead align with C, and allow "trivial" infinite loops as C does. However, care must be taken when doing so. C++ has good reasons to specify that infinite loops are undefined behavior: it is desirable to express forward progress guarantees in a programming language because it unlocks a design space in concurrency and parallelism software and hardware design. An 80 minute deep-dive in this topic is available in <a data-link-type="biblio" href="#biblio-forwardprogress" title="Forward Progress in C++">[ForwardProgress]</a>. A definition of "trivial" which continues supporting this design space is important to reach.</p>
   <p>Other programming languages, compilers, and hardware explicitly or de-facto rely on the C++ specification of its abstract machine, including the behavior infinite loops. Some by borrowing from C++'s specification, others by simply using compilers that implement C++ as their optimizer (see <a data-link-type="biblio" href="#biblio-llvmprogressdiscussion" title="[LangRef] Document forward-progress requirement (Abandoned)">[LLVMProgressDiscussion]</a>, and <a href="https://github.com/rust-lang/rust/issues/28728">a Rust bug of this shape</a>), and yet others by being designed around C++ programs. It is therefore likely that a broader specification for an abstract machine, outside of C++, is needed to truly resolve problems as those tackled in this paper.</p>
   <h2 class="heading settled" data-level="2" id="rev"><span class="secno">2. </span><span class="content">Revision History</span><a class="self-link" href="#rev"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="r0"><span class="secno">2.1. </span><span class="content">Revision 0</span><a class="self-link" href="#r0"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p2809r0" title="Trivial infinite loops are not Undefined Behavior">[P2809r0]</a> proposed matching the C semantics. It was discussed at the 2023 Varna meeting and received feedback from SG1 against matching C semantics, as explained below.</p>
   <p><strong>Poll:</strong> SG1 in Varna: Support what C supports (paper P2809R0 as is)</p>
   <table>
    <thead>
     <tr>
      <td>SF
      <td>F
      <td>N
      <td>A
      <td>SA
    <tbody>
     <tr>
      <td> 0 
      <td> 5 
      <td> 5 
      <td> 6 
      <td> 2 
   </table>
   <h3 class="heading settled" data-level="2.2" id="r1"><span class="secno">2.2. </span><span class="content">Revision 1</span><a class="self-link" href="#r1"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p2809r1" title="Trivial infinite loops are not Undefined Behavior">[P2809r1]</a> incorporated feedback from the C++ committee.</p>
   <p>It was seen in Varna.</p>
   <p><strong>Poll:</strong> EWG approves of the direction of D2809R1R3 (Trivial infinite loops are not undefined behavior), and would like to see it again with a more mature wording, reflecting the feedback given.</p>
   <table>
    <thead>
     <tr>
      <td>SF
      <td>F
      <td>N
      <td>A
      <td>SA
    <tbody>
     <tr>
      <td> 16 
      <td> 17 
      <td> 4 
      <td> 0 
      <td> 0 
   </table>
   <p><strong>Result:</strong> Consensus</p>
   <h3 class="heading settled" data-level="2.3" id="r2"><span class="secno">2.3. </span><span class="content">Revision 2</span><a class="self-link" href="#r2"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p2809r2" title="Trivial infinite loops are not Undefined Behavior">[P2809r2]</a> went through LEWG name bikeshedding, and chose a name.</p>
   <p>EWG reviewed this on Kona on Monday.</p>
   <p><strong>Poll:</strong> forward P2809r2 “Trivial infinite loops are not undefined behavior” option #1 (after adding “function calls” to the list) to CWG for inclusion in C++26.</p>
   <table>
    <thead>
     <tr>
      <td>SF
      <td>F
      <td>N
      <td>A
      <td>SA
    <tbody>
     <tr>
      <td> 3 
      <td> 11 
      <td> 3 
      <td> 3 
      <td> 1 
   </table>
   <p><strong>Poll:</strong> forward P2809r2 “Trivial infinite loops are not undefined behavior” option #2 to CWG for inclusion in C++26.</p>
   <table>
    <thead>
     <tr>
      <td>SF
      <td>F
      <td>N
      <td>A
      <td>SA
    <tbody>
     <tr>
      <td> 6 
      <td> 11 
      <td> 3 
      <td> 1 
      <td> 0 
   </table>
   <p><strong>Result:</strong> Consensus.</p>
   <h3 class="heading settled" data-level="2.4" id="r3"><span class="secno">2.4. </span><span class="content">Revision 3</span><a class="self-link" href="#r3"></a></h3>
   <p>P2809r3 considers the freestanding behavior, and applies wording changes on the language side to follow CWG guidance. It was seen and approved by LEWG.</p>
   <p>However, CWG suggested changes which then removed the need for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield_forever</c-><c- p>()</c-></code>, making LEWG moot.</p>
   <h2 class="heading settled" data-level="3" id="pedantry"><span class="secno">3. </span><span class="content">Standards</span><a class="self-link" href="#pedantry"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="cpp"><span class="secno">3.1. </span><span class="content">C++ Standard</span><a class="self-link" href="#cpp"></a></h3>
   <p>Since C++11, the Standard’s forward progress guarantees have been defined in <strong>[intro.progress]</strong> as follows:</p>
   <blockquote>
     The implementation may assume that any thread will eventually do one of the following: 
    <ul>
     <li data-md>
      <p>terminate,</p>
     <li data-md>
      <p>make a call to a library I/O function,</p>
     <li data-md>
      <p>perform an access through a <code class="highlight"><c- k>volatile</c-></code> glvalue, or</p>
     <li data-md>
      <p>perform a synchronization operation or an atomic operation.</p>
    </ul>
    <p>[<em>Note:</em> This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven. —<em>end note</em>]</p>
   </blockquote>
   <p>This forward progress guarantee applies to the following iteration statements:</p>
   <blockquote> <code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>condition</em> <code class="highlight"><c- p>)</c-></code> <em>statement</em><br> <code class="highlight"><c- k>do</c-></code> <em>statement</em> <code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>;</c-></code><br> <code class="highlight"><c- k>for</c-></code> <code class="highlight"><c- p>(</c-></code> <em>init-statement</em> <em>condition</em><sub><em>opt</em></sub> <code class="highlight"><c- p>;</c-></code> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>)</c-></code> <em>statement</em><br> <code class="highlight"><c- k>for</c-></code> <code class="highlight"><c- p>(</c-></code> <em>init-statement</em><sub><em>opt</em></sub> <em>for-range-declaration</em> <code class="highlight"><c- o>:</c-></code> <em>for-range-initializer</em> <code class="highlight"><c- p>)</c-></code> <em>statement</em><br> </blockquote>
   <h3 class="heading settled" data-level="3.2" id="c"><span class="secno">3.2. </span><span class="content">C Standard</span><a class="self-link" href="#c"></a></h3>
   <p>Since C11, the Standard’s forward progress guarantees have been defined in <strong>Iteration statements</strong> (section 6.8.5 of C17) as follows:</p>
   <blockquote>
     An iteration statement may be assumed by the implementation to terminate if its controlling expression is not a constant expression<sup>†</sup>, and none of the following operations are performed in its body, controlling expression or (in the case of a <code class="highlight"><c- k>for</c-></code> statement) its <em>expression-3</em><sup>‡</sup>: 
    <ul>
     <li data-md>
      <p>input/output operations</p>
     <li data-md>
      <p>accessing a <code class="highlight"><c- k>volatile</c-></code> object</p>
     <li data-md>
      <p>synchronization or atomic operations.</p>
    </ul>
    <p><sup>†</sup> An omitted controlling expression is replaced by a nonzero constant, which is a constant expression.</p>
    <p><sup>‡</sup> This is intended to allow compiler transformations such as removal of empty loops even when termination cannot be proven.</p>
   </blockquote>
   <p>This forward progress guarantee applies to the following iteration statements:</p>
   <blockquote> <code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em> <code class="highlight"><c- p>)</c-></code> <em>statement</em><br> <code class="highlight"><c- k>do</c-></code> <em>statement</em> <code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>;</c-></code><br> <code class="highlight"><c- k>for</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>;</c-></code> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>;</c-></code> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>)</c-></code> <em>statement</em><br> <code class="highlight"><c- k>for</c-></code> <code class="highlight"><c- p>(</c-></code> <em>declaration</em> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>;</c-></code> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>)</c-></code> <em>statement</em><br> </blockquote>
   <p>By omission, iteration statements are undefined if the controlling expression does not meet the criteria above.</p>
   <h3 class="heading settled" data-level="3.3" id="freebird"><span class="secno">3.3. </span><span class="content">Freestanding in C++</span><a class="self-link" href="#freebird"></a></h3>
   <p>Freestanding implementations aren’t given the same guarantees. Indeed, <strong>[intro.multithread.general]</strong> states:</p>
   <blockquote> Under a hosted implementation, a C++ program can have more than one thread running concurrently. </blockquote>
   <p>Which means that a freestanding implementation can have no threads running concurrently.</p>
   <p>Combined with <strong>[intro.progress]</strong>:</p>
   <blockquote> For a thread of execution providing <em>concurrent forward progress guarantees</em>, the implementation ensures that the thread will eventually make progress for as long as it has not terminated. </blockquote>
   <p>We conclude that freestanding implementations can decide to loosen forward progress guarantees.</p>
   <h2 class="heading settled" data-level="4" id="yawn"><span class="secno">4. </span><span class="content">History</span><a class="self-link" href="#yawn"></a></h2>
   <p>C99 6.8.5p4 states:</p>
   <blockquote> An iteration statement causes a statement called the loop body to be executed repeatedly until the controlling expression compares equal to <code class="highlight"><c- mi>0</c-></code>. </blockquote>
   <p>C99 therefore disallows compilers from assuming that loops will eventually terminate.</p>
   <p>C++11 added a full memory model to C++, allowing C++ to speak of threads and inter-thread communications in a useful manner. This forward progress guarantee does away with the C99 guarantee above. A key to concurrency and parallelism is understanding when forward progress is guaranteed, because without forward progress some concurrent and parallel algorithms do not work. For example, a pair of producer / consumer threads will never consume data if the producer does not have forward progress.</p>
   <p>This memory model work was imported into the C11 standard through <a data-link-type="biblio" href="#biblio-n1349" title="Parallel memory sequencing model proposal">[N1349]</a>. There were discussions on the topic of forward progress:</p>
   <ul>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-n1509" title="Optimizing away infinite loops">[N1509]</a> proposed eliminating the paragraph on forward progress from C1X.</p>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-n1528" title="Why undefined behavior for infinite loops?">[N1528]</a> responded to N1509, explaining the C1X wording.</p>
   </ul>
   <p>These two papers were discussed, and WG14 took the following decision:</p>
   <blockquote>
     <strong>ACTION</strong> Douglas to work with Larry to come up with the proposed words are: 
    <p>Change 6.8.5p6 as follows:</p>
    <blockquote> An iteration statement whose controlling expression is not a constant expression (or omitted), that performs no input/output operations, does not access <code class="highlight"><c- k>volatile</c-></code> objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a <code class="highlight"><c- k>for</c-></code> statement) its <em>expressionX3</em>, may be assumed by the implementation to terminate. </blockquote>
    <p><strong>Decision:</strong> Adopt N1509, as modified above, to the C1X WP. result: 14 favor, 0 neutral, 1 against</p>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="why"><span class="secno">5. </span><span class="content">Rationale</span><a class="self-link" href="#why"></a></h2>
   <p>The forward progress guarantees were added to C++11 and C11 along with a memory model. This addition of forward progress guarantees reflected existing practice in compilers, and did not simply serve concurrency and parallelism goals. Indeed, compilers had been assuming that loops can terminate to enable optimizations that transform loops and reorder store instructions and other side-effects. Proving that loops terminate is generally equivalent to solving the halting problem. The compilers were therefore disregarding the C99 wording which said that loops terminate when their controlling expression becomes <code class="highlight"><c- mi>0</c-></code>, and were therefore non-conformant.</p>
   <p>The standards added the forward progress guarantees to change an optimization problem from "solve the halting problem" to "there will be observable side effects in the forms of termination, I/O, <code class="highlight"><c- k>volatile</c-></code>, and/or atomic synchronization, any other operation can be reordered". The former is generally impossible to solve, whereas the latter is eminently tractable.</p>
   <p>Forward progress guarantees therefore helped program concurrency and parallelism, and standardized existing practice which unlocked performance in compiler optimizations.</p>
   <p>However, developers do sometimes rely on infinite loops, for valid reasons. When they do so, the loops have controlling expressions which are constant expressions. Such controlling expressions are known at compile-time by definition, and are therefore easy for a compiler to detect, and mark a loop as not open to optimizations which assume termination.</p>
   <p>That said, an issue arises with the current wording in C, because this loop:</p>
<pre class="highlight"><c- k>while</c-> <c- p>(</c-><c- n>cond</c-><c- p>)</c-> <c- p>{</c->
  <c- c1>// ...</c->
<c- p>}</c->
</pre>
   <p>is not equivalent to that loop:</p>
<pre class="highlight"><c- k>while</c-> <c- p>(</c->true<c- p>)</c-> <c- p>{</c->
  <c- k>if</c-> <c- p>(</c-><c- o>!</c-><c- n>cond</c-><c- p>)</c->
    <c- k>break</c-><c- p>;</c->
  <c- c1>// ...</c->
<c- p>}</c->
</pre>
   <p>Because its controlling expression is not a constant expression. This difference is surprising because the loops look syntactically equivalent. The same applies for any loop which has a constant expression controlling expression that converts to <code class="highlight">true</code>, and has a <code class="highlight"><c- k>return</c-></code>, <code class="highlight"><c- n>longjmp</c-></code>, or (potentially nested) <code class="highlight"><c- k>throw</c-></code>.</p>
   <p>We could conside a change such as adding an implicit <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>()</c-></code> to all loops whose controlling expression is a constant expression, but doing so would needlessly pessimize code. This pessimization would likely be measured in multiples for GPU code. Indeed, without the <code class="highlight"><c- n>yield</c-></code> some GPU hardware would <code class="highlight"><c- n>yield_forever</c-></code> forward progress on <em>all</em> warps in a thread block if one of the warps were to have an infinite loop. This behavior is acceptable while infinite loops are undefined behavior, but pessimizes loops which aren’t infinite (and therefore don’t exhibit forward progress issues).</p>
   <p>The approach with <code class="highlight"><c- n>yield</c-></code> also causes problems pertaining to code motion: a compiler is no longer allowed to hoist code from below a loops whose controlling expression is a constant expression to above it, nor merge loops whose whose controlling expression is a constant expression. This is also a pessimization which affects more than "trivial" infinite loops.</p>
   <p>A change is therefore required which does not match the C semantics, and C would then need to be updated to match. It should be pointed out that C technically does not have forward progress guarantees, but hints at maybe having some guarantees, thefore changes to C ought to consider also adding forward progress guarantees.</p>
   <p>The proposed wording captures very narrowly a few forms of infinite loops and makes them equivalent to a repeated call to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code>. This behavior is implementation-defined in freestanding implementations, because some freestanding implementations do not offer forward progress guarantees and require explicit cooperation, adding a call to <code class="highlight"><c- n>yield</c-></code> would therefore add cooperation where none was intended. By doing so, the proposal does not pessimize certain types of loops as C does, and continues to allow an implementation to assume that most loops will terminate.</p>
   <p>The proposed wording also adds to the list of allowed assumptions that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>()</c-></code> will be called. It was a missing form of valid non-forward-progress in threads that was discovered while discussing this paper in SG1.</p>
   <p>This paper does not affect other forms of backwards control flow such as <code class="highlight"><c- k>goto</c-></code>, <code class="highlight"><c- n>setjmp</c-></code>/<code class="highlight"><c- n>longjmp</c-></code>, infinite tail recursion, nor looping with signal handlers. While they are also forms of loops, infinite versions of them are not idiomatic in C and C++. Functional programmers might be upset at this fact.</p>
   <p>CWG pointed out that some loops might be infinite at compile-time but not runtime, and felt very good about themselves. Such code is ill-formed were it executed at compile-time, but the proposed transformation initially inadvertently made them well-formed. One example of this would be in implementations where floating-point behavior differs between compile-time and runtime environments. Another comes in with cases such as:</p>
<pre class="highlight"><code><c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- mi>42</c-><c- p>;</c->
<c- k>while</c-> <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_constant_evaluated</c-><c- p>()</c-> <c- o>||</c-> <c- o>--</c-><c- n>x</c-><c- p>)</c-> <c- p>;</c->
</code></pre>
   <p>The objective of this paper was to make <em>trivial</em> infinite loops not become undefined behavior. Code as above has probably never been written by anyone, ever, before this paper’s publication. It is certainly not useful, besides its use as a way to show off one’s appreciation of minutia. That said, the change proposed in this paper has a sensible outcome for this horrendous code: the loop will now have an implicit call to <code class="highlight"><c- n>yield</c-></code> at runtime.</p>
   <h2 class="heading settled" data-level="6" id="lang-word"><span class="secno">6. </span><span class="content">Language Wording</span><a class="self-link" href="#lang-word"></a></h2>
   <p>Modify <strong>[stmt.iter.general]</strong>, adding the following paragraph at its end:</p>
   <blockquote>
     8.6.1 General <br> [...] <br> 
    <ins>A <em>trivially empty iteration statement</em> is an iteration statement matching one of the following forms:</ins>
     <br> 
    <ul>
     <li data-md>
      <ins><code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>;</c-></code></ins>
     <li data-md>
      <ins><code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>{</c-> <c- p>}</c-></code></ins>
     <li data-md>
      <ins><code class="highlight"><c- k>do</c-></code> <code class="highlight"><c- p>;</c-></code> <code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>;</c-></code></ins>
     <li data-md>
      <ins><code class="highlight"><c- k>do</c-></code> <code class="highlight"><c- p>{</c-> <c- p>}</c-></code> <code class="highlight"><c- k>while</c-></code> <code class="highlight"><c- p>(</c-></code> <em>expression</em> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>;</c-></code></ins>
     <li data-md>
      <ins><code class="highlight"><c- k>for</c-></code> <code class="highlight"><c- p>(</c-></code> <em>init-statement</em> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>;</c-></code> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>;</c-></code></ins>
     <li data-md>
      <ins><code class="highlight"><c- k>for</c-></code> <code class="highlight"><c- p>(</c-></code> <em>init-statement</em> <em>expression</em><sub><em>opt</em></sub> <code class="highlight"><c- p>;</c-></code> <code class="highlight"><c- p>)</c-></code> <code class="highlight"><c- p>{</c-> <c- p>}</c-></code></ins>
    </ul>
    <ins>The <em>controlling expression</em> of a trivially empty iteration statement is the <em>expression</em> of a <code class="highlight"><c- k>while</c-></code>, <code class="highlight"><c- k>do</c-></code>, or <code class="highlight"><c- k>for</c-></code> statement (or <code class="highlight">true</code>, if the <code class="highlight"><c- k>for</c-></code> statement has no <em>expression</em>).</ins>
     <br> 
    <ins>A <em>trivial infinite loop</em> is a trivially empty iteration statement for which the converted controlling expression is a constant expression, when interpreted as a <em>constant-expression</em> (<strong>[expr.const]</strong>), and evaluates to <code class="highlight">true</code>.</ins>
     <br> 
    <ins>The <em>statement</em> of a trivial infinite loop is replaced with a call to the function <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-></code> (<strong>[thread.thread.this]</strong>); it is implementation-defined whether this replacement occurs on freestanding implementations.</ins>
     <br> 
    <ins>[<em>Note:</em> In a freestanding environment, concurrent forward progress is not guaranteed; such systems therefore require explicit cooperation. A call to <code class="highlight"><c- n>yield</c-></code> can add implicit cooperation where none was otherwise intended. —<em>end note</em>]</ins>
   </blockquote>
   <p>Modify <strong>[intro.progress]</strong> as follows:</p>
   <blockquote>
     The implementation may assume that any thread will eventually do one of the following: 
    <ul>
     <li data-md>
      <p>terminate,</p>
     <li data-md>
      <ins>invoke the function <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-></code> (<strong>[thread.thread.this]</strong>),</ins>
     <li data-md>
      <p>make a call to a library I/O function,</p>
     <li data-md>
      <p>
       perform an access through a <code class="highlight"><c- k>volatile</c-></code> glvalue,
       <del> or</del>
      </p>
     <li data-md>
      <p>
       perform a synchronization operation or an atomic operation
       <del>.</del>
       <ins>, or</ins>
      </p>
     <li data-md>
      <ins>continue execution of a trivial infinite loop (<strong>[stmt.iter.general</strong>]).</ins>
    </ul>
    <p>
     [<em>Note:</em> This is intended to allow compiler transformations such as removal
     <ins>, merging, and reordering</ins>
      of empty loops, even when termination cannot be proven. 
     <ins>An affordance is made for trivial infinite loops, which cannot be removed nor reordered.</ins>
      —<em>end note</em>]
    </p>
   </blockquote>
   <h2 class="heading settled" data-level="7" id="effect"><span class="secno">7. </span><span class="content">Effect</span><a class="self-link" href="#effect"></a></h2>
   <p>Here are examples of the effects of this proposed change:</p>
   <table>
    <thead>
     <tr>
      <th>Code
      <th>Current equivalence in C++
      <th>Current equivalence in C
      <th>Equivalence with Proposed Language Wording
      <th>Equivalence with Alternate Language Wording
      <th>Note
    <tbody>
     <tr>
      <td>
<pre class="highlight"><code><c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- d>/* nothing */</c-><c- p>;</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>thrd_yield</c-><c- p>();</c-></code> 
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> Idiomatic trivial infinite loop 
     <tr>
      <td>
<pre class="highlight"><code><c- k>for</c-> <c- p>(;;)</c->
    <c- d>/* nothing */</c-><c- p>;</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>thrd_yield</c-><c- p>();</c-></code> 
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> Idiomatic trivial infinite loop 
     <tr>
      <td>
<pre class="highlight"><code><c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- s>"a meaningless statement"</c-><c- p>;</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>thrd_yield</c-><c- p>();</c-></code> 
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> Trivial infinite loop, including a meaningless statement in the loop body 
     <tr>
      <td>
<pre class="highlight"><code><c- b>bool</c-> <c- n>done</c-><c- p>{</c->false<c- p>};</c->
<c- k>while</c-> <c- p>(</c-><c- o>!</c-><c- n>done</c-><c- p>)</c->
    <c- d>/* nothing */</c-><c- p>;</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> The iteration statement’s controlling expression isn’t a constant expression which converts to <code class="highlight">true</code> 
     <tr>
      <td>
<pre class="highlight"><code><c- k>constexpr</c-> <c- b>bool</c-> <c- n>done</c-><c- p>{</c->false<c- p>};</c->
<c- k>while</c-> <c- p>(</c-><c- o>!</c-><c- n>done</c-><c- p>)</c->
    <c- d>/* nothing */</c-><c- p>;</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>thrd_yield</c-><c- p>();</c-></code> 
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> The iteration statement’s controlling expression is a constant expression which converts to <code class="highlight">true</code> 
     <tr>
      <td>
<pre class="highlight"><code><c- b>bool</c-> <c- n>done</c-><c- p>{</c->false<c- p>};</c->
<c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- k>if</c-> <c- p>(</c-><c- n>done</c-><c- p>)</c->
        <c- k>break</c-><c- p>;</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>thrd_yield</c-><c- p>();</c-></code> 
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> Contains <code class="highlight"><c- k>break</c-><c- p>;</c-></code>, the iteration statement’s controlling expression is a constant expression which converts to <code class="highlight">true</code> 
     <tr>
      <td>
<pre class="highlight"><code><c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c->false<c- p>)</c->
        <c- k>break</c-><c- p>;</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>thrd_yield</c-><c- p>();</c-></code> 
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- k>while</c-> <c- p>(</c-><c- mi>1</c-><c- p>)</c-> <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></code> 
      <td> The <code class="highlight"><c- k>if</c-> <c- k>constexpr</c-></code> is unevaluated, the <code class="highlight"><c- k>break</c-><c- p>;</c-></code> is therefore not relevant 
     <tr>
      <td>
<pre class="highlight"><code><c- b>void</c-> <c- nf>external</c-><c- p>();</c->
<c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>external</c-><c- p>();</c->
</code></pre>
      <td>
        unchanged (undefined behavior if <code class="highlight"><c- n>external</c-></code> doesn’t contain a side-effect) 
<pre class="highlight"><code><c- b>void</c-> <c- nf>external</c-><c- p>();</c->
<c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>external</c-><c- p>();</c->
</code></pre>
      <td>
        unchanged (undefined behavior if <code class="highlight"><c- n>external</c-></code> doesn’t contain a side-effect) 
<pre class="highlight"><code><c- b>void</c-> <c- nf>external</c-><c- p>();</c->
<c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>external</c-><c- p>();</c->
</code></pre>
      <td>
        unchanged (undefined behavior if <code class="highlight"><c- n>external</c-></code> doesn’t contain a side-effect) 
<pre class="highlight"><code><c- b>void</c-> <c- nf>external</c-><c- p>();</c->
<c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>external</c-><c- p>();</c->
</code></pre>
      <td>
        unchanged (undefined behavior if <code class="highlight"><c- n>external</c-></code> doesn’t contain a side-effect) 
<pre class="highlight"><code><c- b>void</c-> <c- nf>external</c-><c- p>();</c->
<c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>external</c-><c- p>();</c->
</code></pre>
      <td> <code class="highlight"><c- n>external</c-></code> might contain a side-effect, the loop cannot be assumed to be infinite 
     <tr>
      <td>
<pre class="highlight"><code><c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c->
</code></pre>
      <td>
        undefined behavior 
<pre class="highlight"><code><c- n>std</c-><c- o>::</c-><c- n>unreachable</c-><c- p>();</c->
</code></pre>
      <td>
        unchanged 
<pre class="highlight"><code><c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>thrd_yield</c-><c- p>();</c->
</code></pre>
      <td>
<pre class="highlight"><code><c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c->
</code></pre>
      <td>
<pre class="highlight"><code><c- k>while</c-> <c- p>(</c->true<c- p>)</c->
    <c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c->
</code></pre>
      <td> a loop without side-effects which nonetheless yields provides forward progress 
   </table>
   <h2 class="heading settled" data-level="8" id="alt"><span class="secno">8. </span><span class="content">Alternative</span><a class="self-link" href="#alt"></a></h2>
   <h3 class="heading settled" data-level="8.1" id="lang-alt"><span class="secno">8.1. </span><span class="content">Alternative Language Wording</span><a class="self-link" href="#lang-alt"></a></h3>
   <p>Per EWG discussion in <a href="#r2">§ 2.3 Revision 2</a>, this is not the preferred option, <a href="#lang-word">§ 6 Language Wording</a> is preferred.</p>
   <p>Modify <strong>[intro.progress]</strong> as follows:</p>
   <blockquote>
     The implementation may assume that any thread will eventually do one of the following: 
    <ul>
     <li data-md>
      <p>terminate,</p>
     <li data-md>
      <ins>call <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>()</c-></code>,</ins>
     <li data-md>
      <p>make a call to a library I/O function,</p>
     <li data-md>
      <p>perform an access through a <code class="highlight"><c- k>volatile</c-></code> glvalue, or</p>
     <li data-md>
      <p>perform a synchronization operation or an atomic operation.</p>
    </ul>
    <p>
     [<em>Note:</em> This is intended to allow compiler transformations such as removal
     <ins>, merging, and reordering</ins>
      of empty loops, even when termination cannot be proven. —<em>end note</em>]
    </p>
    <ins> Notwithstanding the above, the implementation shall implement an iteration statement (<em>[stmt.iter]</em>) as equivalent to a call to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield_forever</c-><c- p>()</c-></code> if: </ins>
    <ins>
     <ul>
      <li data-md>
       <p>the iteration statement, nor any of the subexpresssions (<em>[intro.execution]</em>), do not enclose an access through a <code class="highlight"><c- k>volatile</c-></code> glvalue, nor a synchronization operation or an atomic operation, and</p>
      <li data-md>
       <p>the iteration statement’s controlling expression is a converted constant expression (<em>[expr.const]</em>) that converts to <code class="highlight">true</code>, where the controlling expression is the <em>condition</em> of a <code class="highlight"><c- k>while</c-></code> or <code class="highlight"><c- k>for</c-></code> statement, or the <code class="highlight"><c- n>expression</c-></code> of a <code class="highlight"><c- k>do</c-></code> statement, and</p>
      <li data-md>
       <p>the iteration statement encloses none of:</p>
       <ul>
        <li data-md>
         <p>a top-level <code class="highlight"><c- k>break</c-></code> statement (<em>[stmt.break]</em>),</p>
        <li data-md>
         <p>a <code class="highlight"><c- k>goto</c-></code> statement (<em>[stmt.goto]</em>) whose label (<em>[stmt.label]</em>) is located outside of the iteration statement’s substatement,</p>
        <li data-md>
         <p>a <code class="highlight"><c- k>return</c-></code> statement (<em>[stmt.return]</em>),</p>
        <li data-md>
         <p>a <code class="highlight"><c- k>co_return</c-></code> statement (<em>[stmt.return.coroutine]</em>),</p>
        <li data-md>
         <p>a <code class="highlight"><c- k>co_await</c-></code> expression (<em>[expr.await]</em>),</p>
        <li data-md>
         <p>a <code class="highlight"><c- k>co_yield</c-></code> expression (<em>[expr.yield]</em>),</p>
        <li data-md>
         <p>a potentially-throwing expression (<em>[except.spec]</em>), and</p>
        <li data-md>
         <p>a function call (<em>[expr.call]</em>).</p>
       </ul>
     </ul>
    </ins>
    <ins>It is implementation-defined whether a program in a freestanding environment is required to implement the above behavior of iteration statements.</ins>
     <br> 
    <ins>[<em>Note:</em> This is intended to allow programmers to use idiomatic trivial infinite loops without these loops being undefined behavior per the above. —<em>end note</em>]</ins>
     <br> 
    <ins>[<em>Note:</em> In a freestanding environment, concurrent forward progress is not guaranteed. An implementation is encouraged to match the behavior of potentially infinite iteration statements with its implementation-defined concurrent behavior. A freestanding environment which implements <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>yield</c-></code> is encouraged to also provide <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>yield_forever</c-></code>, and is encouraged to offer the behavior described above. —<em>end note</em>]</ins>
   </blockquote>
   <h3 class="heading settled" data-level="8.2" id="lib-alt"><span class="secno">8.2. </span><span class="content">Alternative Library Wording</span><a class="self-link" href="#lib-alt"></a></h3>
   <p>Per CWG discussion in <a href="#r3">§ 2.4 Revision 3</a>, this library change is not proposed. It was originally proposed along with <a href="#lang-alt">§ 8.1 Alternative Language Wording</a>.</p>
   <p>Modify <strong>[thread.threads]</strong> as follows:</p>
   <blockquote>
     [thread.threads] describes components that can be used to create and manage threads. 
<pre class="highlight"><code><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->

  <c- c1>// ...</c->

  <c- c1>// [thread.thread.this], namespace this_thread</c->
  <c- k>namespace</c-> <c- nn>this_thread</c-> <c- p>{</c->
    <c- kr>thread</c-><c- o>::</c-><c- n>id</c-> <c- nf>get_id</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->

    <c- b>void</c-> <c- nf>yield</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
    <ins>
    <c- p>[[</c-><c- n>noreturn</c-><c- p>]]</c-> <c- b>void</c-> <c- n>yield_forever</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
    </ins>
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Clock</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Duration</c-><c- o>></c->
      <c- b>void</c-> <c- n>sleep_until</c-><c- p>(</c-><c- k>const</c-> <c- n>chrono</c-><c- o>::</c-><c- n>time_point</c-><c- o>&lt;</c-><c- n>Clock</c-><c- p>,</c-> <c- n>Duration</c-><c- o>>&amp;</c-> <c- n>abs_time</c-><c- p>);</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Rep</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Period</c-><c- o>></c->
      <c- b>void</c-> <c- n>sleep_for</c-><c- p>(</c-><c- k>const</c-> <c- n>chrono</c-><c- o>::</c-><c- n>duration</c-><c- o>&lt;</c-><c- n>Rep</c-><c- p>,</c-> <c- n>Period</c-><c- o>>&amp;</c-> <c- n>rel_time</c-><c- p>);</c->
  <c- p>}</c->

<c- p>}</c->
</code></pre>
   </blockquote>
   <p>Add a paragraph to section <strong>[thread.thread.this]</strong> as follows:</p>
   <blockquote>
<pre class="highlight"><code><ins>
  <c- p>[[</c-><c- n>noreturn</c-><c- p>]]</c-> <c- b>void</c-> <c- n>yield_forever</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
</ins>
</code></pre>
    <ins><em>Effects</em>: Equivalent to:</ins>
<pre class="highlight"><code><ins><c- k>while</c-> <c- p>(</c->true<c- p>)</c-></ins>
  <ins><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-><c- p>();</c-></ins>
</code></pre>
    <ins><em>Recommended practice:</em> A freestanding environment which implements <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield</c-></code> should also provide <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>yield_forever</c-></code>.</ins>
   </blockquote>
  </main>
<script>
(function() {
  "use strict";
  var collapseSidebarText = '<span aria-hidden="true">←</span> '
                          + '<span>Collapse Sidebar</span>';
  var expandSidebarText   = '<span aria-hidden="true">→</span> '
                          + '<span>Pop Out Sidebar</span>';
  var tocJumpText         = '<span aria-hidden="true">↑</span> '
                          + '<span>Jump to Table of Contents</span>';

  var sidebarMedia = window.matchMedia('screen and (min-width: 78em)');
  var autoToggle   = function(e){ toggleSidebar(e.matches) };
  if(sidebarMedia.addListener) {
    sidebarMedia.addListener(autoToggle);
  }

  function toggleSidebar(on) {
    if (on == undefined) {
      on = !document.body.classList.contains('toc-sidebar');
    }

    /* Don't scroll to compensate for the ToC if we're above it already. */
    var headY = 0;
    var head = document.querySelector('.head');
    if (head) {
      // terrible approx of "top of ToC"
      headY += head.offsetTop + head.offsetHeight;
    }
    var skipScroll = window.scrollY < headY;

    var toggle = document.getElementById('toc-toggle');
    var tocNav = document.getElementById('toc');
    if (on) {
      var tocHeight = tocNav.offsetHeight;
      document.body.classList.add('toc-sidebar');
      document.body.classList.remove('toc-inline');
      toggle.innerHTML = collapseSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, 0 - tocHeight);
      }
      tocNav.focus();
      sidebarMedia.addListener(autoToggle); // auto-collapse when out of room
    }
    else {
      document.body.classList.add('toc-inline');
      document.body.classList.remove('toc-sidebar');
      toggle.innerHTML = expandSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, tocNav.offsetHeight);
      }
      if (toggle.matches(':hover')) {
        /* Unfocus button when not using keyboard navigation,
           because I don't know where else to send the focus. */
        toggle.blur();
      }
    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn't exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn't standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don't have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we're at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can't find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>
  <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-forwardprogress">[ForwardProgress]
   <dd>Olivier Giroux. <a href="https://www.youtube.com/watch?v=CuWM-OrPitw"><cite>Forward Progress in C++</cite></a>. 2022-09-12. URL: <a href="https://www.youtube.com/watch?v=CuWM-OrPitw">https://www.youtube.com/watch?v=CuWM-OrPitw</a>
   <dt id="biblio-llvmprogressdiscussion">[LLVMProgressDiscussion]
   <dd>Nikita Popov. <a href="https://reviews.llvm.org/D65718"><cite>[LangRef] Document forward-progress requirement (Abandoned)</cite></a>. 2019-08-04. URL: <a href="https://reviews.llvm.org/D65718">https://reviews.llvm.org/D65718</a>
   <dt id="biblio-n1349">[N1349]
   <dd>Clark Nelson; Hans-J. Boehm; Lawrence Crowl. <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1349.htm"><cite>Parallel memory sequencing model proposal</cite></a>. 2009-02-23. URL: <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1349.htm">https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1349.htm</a>
   <dt id="biblio-n1509">[N1509]
   <dd>Douglas Walls. <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1509.pdf"><cite>Optimizing away infinite loops</cite></a>. 2010-09-02. URL: <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1509.pdf">https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1509.pdf</a>
   <dt id="biblio-n1528">[N1528]
   <dd>Hans Boehm. <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1528.htm"><cite>Why undefined behavior for infinite loops?</cite></a>. 2010-10-27. URL: <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1528.htm">https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1528.htm</a>
   <dt id="biblio-n2644">[N2644]
   <dd>Jens Gustedt. <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2644.pdf"><cite>Programming languages — a common C/C++ core specification</cite></a>. 2021-01-20. URL: <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2644.pdf">https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2644.pdf</a>
   <dt id="biblio-p2735r0">[P2735r0]
   <dd>Aaron Ballman. <a href="https://wg21.link/p2735r0"><cite>C xor C++ Programming</cite></a>. 5 December 2022. URL: <a href="https://wg21.link/p2735r0">https://wg21.link/p2735r0</a>
   <dt id="biblio-p2809r0">[P2809r0]
   <dd>JF Bastien. <a href="https://wg21.link/p2809r0"><cite>Trivial infinite loops are not Undefined Behavior</cite></a>. 15 March 2023. URL: <a href="https://wg21.link/p2809r0">https://wg21.link/p2809r0</a>
   <dt id="biblio-p2809r1">[P2809r1]
   <dd>JF Bastien. <a href="https://wg21.link/p2809r1"><cite>Trivial infinite loops are not Undefined Behavior</cite></a>. 18 June 2023. URL: <a href="https://wg21.link/p2809r1">https://wg21.link/p2809r1</a>
   <dt id="biblio-p2809r2">[P2809r2]
   <dd>JF Bastien. <a href="https://wg21.link/p2809r2"><cite>Trivial infinite loops are not Undefined Behavior</cite></a>. 14 October 2023. URL: <a href="https://wg21.link/p2809r2">https://wg21.link/p2809r2</a>
  </dl>