<!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>P3570R2: optional variants in sender/receiver</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 ac5ea272d, updated Fri Dec 6 15:45:15 2024 -0800" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="46c2b9a937d95dcb4929a93a72833fd19bafd766" name="revision">
  <meta content="dark light" name="color-scheme">
<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">P3570R2<br>optional variants in sender/receiver</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="2025-06-18">2025-06-18</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://fabio.fracassi.de/">Fabio Fracassi</a> (<a class="p-org org" href="http://code.berlin">Code University of Applied Sciences</a>) <a class="u-email email" href="mailto:fabio@fracassi.de">fabio@fracassi.de</a>
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Some async interfaces for concurrent queues would be well served by using <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> when used in a coroutine context. However when using sender/receiver interfaces directly
the variant style visitor based on overloading has several benefits.
This paper explores if and how we can get the best interface for each use case.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#motivation"><span class="secno">1</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#opts"><span class="secno">2</span> <span class="content">Exploration</span></a>
     <ol class="toc">
      <li><a href="#opt1"><span class="secno">2.1</span> <span class="content">Option 1: custom awaiter</span></a>
      <li>
       <a href="#opt2"><span class="secno">2.2</span> <span class="content">Option 2: generalized value transform</span></a>
       <ol class="toc">
        <li><a href="#opt2a1"><span class="secno">2.2.1</span> <span class="content">Option 2.a1: Always transform</span></a>
        <li><a href="#opt2a2"><span class="secno">2.2.2</span> <span class="content">Option 2.a2 Opt-Out</span></a>
        <li><a href="#opt2b"><span class="secno">2.2.3</span> <span class="content">Option 2.b Opt-In</span></a>
       </ol>
     </ol>
    <li><a href="#prop"><span class="secno">3</span> <span class="content">Proposal</span></a>
    <li>
     <a href="#spec"><span class="secno">4</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#spec-version"><span class="secno">4.1</span> <span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>version</c-><c- o>></c-></code> synopsis [version.syn]</span></a>
      <li><a href="#header-execution-synopsis-executionsyn"><span class="secno">4.2</span> <span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>execution</c-><c- o>></c-></code> synopsis [execution.syn]</span></a>
      <li>
       <a href="#spec-exec"><span class="secno">4.3</span> <span class="content">Execution control library [exec]</span></a>
       <ol class="toc">
        <li><a href="#executionas_awaitable-execasawaitable"><span class="secno">4.3.1</span> <span class="content">execution​::​as_awaitable [exec.as.awaitable]</span></a>
       </ol>
     </ol>
    <li><a href="#impl"><span class="secno">5</span> <span class="content">Implementation Experience</span></a>
    <li><a href="#acknowledgements"><span class="secno">6</span> <span class="content">Acknowledgements</span></a>
    <li><a href="#apxA"><span class="secno"></span> <span class="content">Appendix A (Workarounds)</span></a>
    <li><a href="#apxB"><span class="secno"></span> <span class="content">Appendix B (Prototype)</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <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="motivation"><span class="secno">1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>This paper is a reaction to the Library Evolution(LEWG) review of the Concurrent Queues paper <a data-link-type="biblio" href="#biblio-p0260r13" title="C++ Concurrent Queues">[P0260R13]</a>,
specifiaclly the review of revision 13. The return type of <code class="highlight"><c- n>async_pop</c-><c- p>()</c-></code> has been contentious 
for a while, and is has been blocking consensus of a otherwise highly anticipated facility.</p>
   <p>The contention stems from the fact that the <code class="highlight"><c- n>async_pop</c-><c- p>()</c-></code> (and other senders) can be used in two
different contexts. It can be used in chain of sender adaptors or can be awaited on in a coroutine.
During the review it became clear that depending on the context LEWG would prefer a slightly 
different usage pattern.
Consider the following examples (adapted from <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p0260r13.html#Examples">C++ Concurrent Queues § Examples</a>) for the prefered usage:</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>coro_grep</c-><c- p>(</c-><c- n>stdx</c-><c- o>::</c-><c- n>coqueue</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>fs</c-><c- o>::</c-><c- n>path</c-><c- o>>&amp;</c-> <c- n>file_queue</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>needle</c-><c- p>)</c-> 
  <c- o>-></c-> <c- n>stdp</c-><c- o>::</c-><c- n>task</c-><c- o>&lt;</c-><c- b>void</c-><c- o>></c-> 
<c- p>{</c->
  <c- k>while</c-> <c- p>(</c-><c- k>auto</c-> <c- n>fname</c-> <c- o>=</c-> <c- n>co_await</c-> <c- n>file_queue</c-><c- p>.</c-><c- n>async_pop</c-><c- p>())</c-> <c- p>{</c->
    <c- n>grep_file</c-><c- p>(</c-><c- o>*</c-><c- n>fname</c-><c- p>,</c-> <c- n>needle</c-><c- p>);</c->              <c- c1>// queue has data</c->
  <c- p>}</c->
                              <c- c1>// queue closed</c->
  <c- n>co_return</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>particularly note line 4 and 5 in the coroutine example above and line 6 to 10 in the S/R example below</p>
<pre class="language-c++ highlight"><c- n>stdexec</c-><c- o>::</c-><c- n>sender</c-> <c- k>auto</c-> <c- n>sr_grep</c-><c- p>(</c-><c- k>auto</c-> <c- n>scheduler</c->
              <c- p>,</c-> <c- n>stdx</c-><c- o>::</c-><c- n>coqueue</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>fs</c-><c- o>::</c-><c- n>path</c-><c- o>>&amp;</c-> <c- n>file_queue</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>needle</c-><c- p>)</c-> 
<c- p>{</c->
  <c- k>return</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>repeat_effect_until</c-><c- p>(</c->
    <c- n>stdexec</c-><c- o>::</c-><c- n>starts_on</c-><c- p>(</c-><c- n>scheduler</c->
      <c- p>,</c-> <c- n>files</c-><c- p>.</c-><c- n>async_pop</c-><c- p>()</c->                                  
      <c- o>|</c-> <c- n>stdp</c-><c- o>::</c-><c- n>overload</c-><c- p>(</c->
        <c- p>[]()</c-> <c- o>-></c-> <c- b>bool</c-> <c- p>{</c-> <c- k>return</c-> true<c- p>;</c-> <c- p>},</c->                  <c- c1>// queue closed</c->
        <c- p>[</c-><c- n>needle</c-><c- p>](</c-><c- n>std</c-><c- o>::</c-><c- n>fs</c-><c- o>::</c-><c- n>path</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>fname</c-><c- p>)</c-> <c- o>-></c-> <c- b>bool</c-> <c- p>{</c->  <c- c1>// queue has data</c->
          <c- k>return</c-> <c- n>grep_file</c-><c- p>(</c-><c- n>fname</c-><c- p>,</c-> <c- n>needle</c-><c- p>);</c->
        <c- p>});</c->
    <c- p>));</c->
<c- p>}</c->
</pre>
   <p>We can make either of these interfaces work, depending on the completion signatures that the sender
that is returned from <code class="highlight"><c- n>async_pop</c-><c- p>()</c-></code> supports. To support the ideal coroutine interface the <code class="highlight"><c- n>async</c-><c- o>-</c-><c- n>pop</c-><c- o>-</c-><c- n>sender</c-></code> should have the completion signature <code class="highlight"><c- n>set_value_t</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>)</c-></code>, to support 
the ideal S/R interface the completion signatures would need to be <code class="highlight"><c- n>set_value_t</c-><c- p>()</c-></code> and <code class="highlight"><c- n>set_value_t</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code>.
Whichever we support slightly penalizes the interface for the other, see <a href="#apxA">Appendix A (Workarounds)</a>.</p>
   <p>With this paper we want to explore if and how we could support the ideal interface for both contexts.</p>
   <h2 class="heading settled" data-level="2" id="opts"><span class="secno">2. </span><span class="content">Exploration</span><a class="self-link" href="#opts"></a></h2>
   <p>To understand a path toward a solution we need to take a short look into how senders/receivers
interact with coroutines.</p>
   <p>The sender is not directly used in the coroutine, but is transformed into an <code class="highlight"><c- n>awaitable</c-></code>.
The transformation is initiated by the coroutine task type (as proposed in <a data-link-type="biblio" href="#biblio-p3552r0" title="Add a Coroutine Lazy Type">[P3552R0]</a>) 
which uses S/Rs <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>as_awaitable</c-></code> to do the actual transformation mechanics.</p>
   <h3 class="heading settled" data-level="2.1" id="opt1"><span class="secno">2.1. </span><span class="content">Option 1: custom awaiter</span><a class="self-link" href="#opt1"></a></h3>
   <p>The simplest option is to exploit that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>as_awaitable</c-></code> provides a customization point, allowing
a sender to provide its own custom <code class="highlight"><c- n>awaiter</c-></code> type.</p>
   <p>This is strightforward, and allows us to get the ideal interface for both contexts. It involves very
little machinery, and in fact even bypasses most of the S/R machinery, most likely allowing for 
good compile times and runtime efficiency.</p>
   <p>However the custom awaiter will only be used if the <code class="highlight"><c- n>async</c-><c- o>-</c-><c- n>pop</c-><c- o>-</c-><c- n>sender</c-></code> is used directly.
The result of <code class="highlight"><c- n>async_pop</c-></code> can not be adapted by any sender adaptors. This will severely limit
implementation freedom, and would probably cause some complicated code duplication, e.g. in the 
coroutine task type.</p>
   <p>it would also make the abstraction brittle for the user, since the preferred interface would only
work on unadapted senders.</p>
   <h3 class="heading settled" data-level="2.2" id="opt2"><span class="secno">2.2. </span><span class="content">Option 2: generalized value transform</span><a class="self-link" href="#opt2"></a></h3>
   <p>The second option is to hook into the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>as_awaitable</c-></code> machinery in a more generic fashion.</p>
   <p>Currently the C++ Working draft spells out several potential transformations in <a data-link-type="biblio" href="#biblio-execasawaitable" title="[exec.as.awaitable]">[exec.as.awaitable]</a>,
that are tried one after the other. The one that allowes senders with a single value completion 
signature to be used seamlessly is described in §7.3 which consist of two exposition only facilities, 
an <code class="highlight"><c- n>awaitable</c-><c- o>-</c-><c- n>sender</c-></code> concept and a <code class="highlight"><c- n>sender</c-><c- o>-</c-><c- n>awaitable</c-></code> type.</p>
   <p>We could add another protential transformation right after that single sender step to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>as_awaitable</c-></code>.
This step would take an exposition only type trait <code class="highlight"><c- n>completion</c-><c- o>-</c-><c- n>adaptor</c-></code> that provides a 
sender adaptor (for the motivating usecase from <a data-link-type="biblio" href="#biblio-p0260r13" title="C++ Concurrent Queues">[P0260R13]</a> that adaptor would be <code class="highlight"><c- n>into_optional</c-></code> as described in <a data-link-type="biblio" href="#biblio-p3552r0" title="Add a Coroutine Lazy Type">[P3552R0]</a> Sec 4.4). <code class="highlight"><c- n>as_awaitable</c-></code> would now check if the original sender 
that has been adapted with the transformation provided by the <code class="highlight"><c- n>completion</c-><c- o>-</c-><c- n>adaptor</c-></code> trait 
conforms to the <code class="highlight"><c- n>awaitable</c-><c- o>-</c-><c- n>sender</c-></code> concept in which case it would provide the <code class="highlight"><c- n>sender</c-><c- o>-</c-><c- n>awaitable</c-></code> for the adapted sender.</p>
   <p>The <code class="highlight"><c- n>into_optional</c-></code> transform takes a sender with a <code class="highlight"><c- n>set_value_t</c-><c- p>()</c-></code> and <code class="highlight"><c- n>set_value_t</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code> and changes that into a <code class="highlight"><c- n>set_value_t</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>)</c-></code> completion signature.</p>
   <p>There are three options for how we could use the <code class="highlight"><c- n>completion</c-><c- o>-</c-><c- n>adaptor</c-></code> trait:</p>
   <h4 class="heading settled" data-level="2.2.1" id="opt2a1"><span class="secno">2.2.1. </span><span class="content">Option 2.a1: Always transform</span><a class="self-link" href="#opt2a1"></a></h4>
   <p>The type trait is exposition only and defaults to <code class="highlight"><c- n>into_optional</c-></code>. No specialisations are provided. <code class="highlight"><c- n>as_awaitable</c-></code> will alway try the <code class="highlight"><c- n>into_optional</c-></code> transformation.</p>
   <p>This means all senders that have compatible value completion signatures will automatically be
usable in coroutines with the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> return type. 
Users wont be able to customize the behavior. We would have to fully specify <code class="highlight"><c- n>into_optional</c-></code></p>
   <h4 class="heading settled" data-level="2.2.2" id="opt2a2"><span class="secno">2.2.2. </span><span class="content">Option 2.a2 Opt-Out</span><a class="self-link" href="#opt2a2"></a></h4>
   <p>Same as <a href="#opt2a1">§ 2.2.1 Option 2.a1: Always transform</a>, but with a opt-out mechanism.</p>
   <p>The type trait is exposition only and defaults to <code class="highlight"><c- n>into_optional</c-></code>. A specialisations is provided,
that will query the sender type with a new environment property <code class="highlight"><c- n>get_await_completion_adaptor</c-></code>. 
The (original) sender can provide such a property like this:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>env</c-> <c- p>{</c-> <c- k>auto</c-> <c- n>query</c-><c- p>(</c-><c- n>get_await_completion_adaptor_t</c-><c- p>)</c-> <c- k>const</c-> <c- o>-></c-> <c- n>std</c-><c- o>::</c-><c- n>false_type</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>get_env</c-><c- p>()</c-> <c- k>const</c-> <c- n>noexcept</c-> <c- o>-></c-> <c- n>env</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c->
</pre>
   <p>In this case the trait specialisation will return a special type that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>awaitable</c-></code> uses to skip
the adaption step.</p>
   <p>This means all senders that have compatible value completion signatures will automatically be
usable in coroutines with the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> return type, but users could opt out in case 
the optional semantic do not make sense for a certain sender. We would have to fully specify <code class="highlight"><c- n>into_optional</c-></code></p>
   <h4 class="heading settled" data-level="2.2.3" id="opt2b"><span class="secno">2.2.3. </span><span class="content">Option 2.b Opt-In</span><a class="self-link" href="#opt2b"></a></h4>
   <p>Same as <a href="#opt2a1">§ 2.2.1 Option 2.a1: Always transform</a>, but only if the sender explicitly opts-in.</p>
   <p>The type trait is exposition only and defaults to an exposition only type that will make <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>as_awaitable</c-></code> skip the adaption step (i.e. same the behavior as the current status quo).
A specialisations is provided, that will query the sender type with a new environment property <code class="highlight"><c- n>get_await_completion_adaptor</c-></code>, that will return the adaptor type from the query.</p>
   <p>The (original) sender can provide such a property like this:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>env</c-> <c- p>{</c-> <c- k>auto</c-> <c- n>query</c-><c- p>(</c-><c- n>get_await_completion_adaptor_t</c-><c- p>)</c-> <c- k>const</c-> <c- o>-></c-> <c- n>into_optional_t</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>get_env</c-><c- p>()</c-> <c- k>const</c-> <c- n>noexcept</c-> <c- o>-></c-> <c- n>env</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c->
</pre>
   <p>Note that in this approach the senders environment property provides a the adaper type directly.
This gives users full control over which adaption they want, so the mechanism would not be 
restricted to a specific transformation.</p>
   <p>This means that senders that have compatible value completion signatures will not automatically be
usable in coroutines with the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> return type, but sender providers could opt in when  
the optional semantic does make sense for a certain sender, as for example the motivating <code class="highlight"><c- n>async</c-><c- o>-</c-><c- n>pop</c-><c- o>-</c-><c- n>sender</c-></code>.</p>
   <h2 class="heading settled" data-level="3" id="prop"><span class="secno">3. </span><span class="content">Proposal</span><a class="self-link" href="#prop"></a></h2>
   <p>A custom awaiter as described in <a href="#opt1">§ 2.1 Option 1: custom awaiter</a> solves our imediate problem, but does so only for the
specific case, and seems overly brittle.</p>
   <p><a href="#opt2a1">§ 2.2.1 Option 2.a1: Always transform</a> solves our problem in a generic fashion, but might cause new problems, because it
automatically changes signatures of senders, that might be ill suited for this transformation. 
The same is true for <a href="#opt2a2">§ 2.2.2 Option 2.a2 Opt-Out</a>, but it would give such ill suited senders a way out.</p>
   <p>We propse to go with <a href="#opt2b">§ 2.2.3 Option 2.b Opt-In</a>. There is no clear evidence how often the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> semantics are inappropriate for senders that otherwise fullfill the completion signatures, so we 
propose to go with the conservative option that will allow senders to opt-into the semantics when
appropriate, but never automatically.</p>
   <p>This Option also has the benefit of being fully generic, with very localized changes,
and no need define the actual adaption type as part of either this paper or the lazy task type. 
Concurrent queues could either use a generic <code class="highlight"><c- n>into_optional</c-></code> type or a custom adaptor for that 
purpose. This also leaves the design space open for the concurrent queues to provide different 
adaptors such as e.g. disengage_on_close or disengage_on_empty.</p>
   <h2 class="heading settled" data-level="4" id="spec"><span class="secno">4. </span><span class="content">Wording</span><a class="self-link" href="#spec"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="spec-version"><span class="secno">4.1. </span><span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>version</c-><c- o>></c-></code> synopsis [version.syn]</span><a class="self-link" href="#spec-version"></a></h3>
   <p>Bump the value of the existing feature test macro <code class="highlight"><c- n>__cpp_lib_senders</c-></code>.</p>
   <p><code class="highlight"><c- cp>#define __cpp_lib_senders &lt;bump value> </c-><c- c1>// also in &lt;execution></c-></code></p>
   <h3 class="heading settled" data-level="4.2" id="header-execution-synopsis-executionsyn"><span class="secno">4.2. </span><span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>execution</c-><c- o>></c-></code> synopsis [execution.syn]</span><a class="self-link" href="#header-execution-synopsis-executionsyn"></a></h3>
   <p>Add the declarations of <code class="highlight"><c- n>get_await_completion_adaptor_t</c-></code> and <code class="highlight"><c- n>get_await_completion_adaptor</c-></code> to
the <code class="highlight"><c- o>&lt;</c-><c- n>execution</c-><c- o>></c-></code> synopsis.</p>
   <blockquote>
<pre class="highlight"><code><c- k>struct</c-> <c- nc>get_domain_t</c-> <c- p>{</c-> <c- n>unspecified</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>get_scheduler_t</c-> <c- p>{</c-> <c- n>unspecified</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>get_delegation_scheduler_t</c-> <c- p>{</c-> <c- n>unspecified</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>get_forward_progress_guarantee_t</c-> <c- p>{</c-> <c- n>unspecified</c-> <c- p>};</c->
<c- k>template</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- k><c- k>class</c-></c-> <c- nc><c- nc>CPO</c-></c-><c- o><c- o>></c-></c-></code>
  <c- k>struct</c-> <c- nc>get_completion_scheduler_t</c-> <c- p>{</c-> <c- n>unspecified</c-> <c- p>};</c->
<ins><c- k>struct</c-> <c- nc>get_await_completion_adaptor_t</c-> <c- p>{</c-> <c- n>unspecified</c-> <c- p>};</c-></ins>

<c- kr>inline</c-> <c- k>constexpr</c-> <c- n>get_domain_t</c-> <c- n>get_domain</c-><c- p>{};</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- n>get_scheduler_t</c-> <c- n>get_scheduler</c-><c- p>{};</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- n>get_delegation_scheduler_t</c-> <c- n>get_delegation_scheduler</c-><c- p>{};</c->
<c- k>enum</c-> <c- k>class</c-> <c- nc>forward_progress_guarantee</c-><c- p>;</c->
<c- kr>inline</c-> <c- k>constexpr</c-> <c- n>get_forward_progress_guarantee_t</c-> <c- n>get_forward_progress_guarantee</c-><c- p>{};</c->
<c- k>template</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- k><c- k>class</c-></c-> <c- nc><c- nc>CPO</c-></c-><c- o><c- o>></c-></c-></code>
  <c- k>constexpr</c-> <c- n>get_completion_scheduler_t</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- n><c- n>CPO</c-></c-><c- o><c- o>></c-></c-></code> <c- n>get_completion_scheduler</c-><c- p>{};</c->
<ins><c- kr>inline</c-> <c- k>constexpr</c-> <c- n>get_await_completion_adaptor_t</c-> <c- n>get_await_completion_adaptor</c-><c- p>{};</c-></ins>

</code></pre>
   </blockquote>
   <h3 class="heading settled" data-level="4.3" id="spec-exec"><span class="secno">4.3. </span><span class="content">Execution control library [exec]</span><a class="self-link" href="#spec-exec"></a></h3>
   <p>Insert new subsection in [exec.queries] after <a href="3.5.9">exec.get.compl.sched</a>:</p>
   <blockquote>
    <ins><strong>3.5.10 execution​::get_await_completion_adaptor [exec.get.await.adapt]</strong></ins>
    <ol>
     <li data-md>
      <ins><code class="highlight"><c- n>get_await_completion_adaptor</c-></code> asks a queryable object for its associated awaitable completion adaptor.</ins>
     <li data-md>
      <ins>The name <code class="highlight"><c- n>get_await_completion_adaptor</c-></code> denotes a query object.
For a subexpression <code class="highlight"><c- n>env</c-></code>, <code class="highlight"><c- n>get_await_completion_adaptor</c-><c- p>(</c-><c- n>env</c-><c- p>)</c-></code> is 
expression-equivalent to <code class="highlight"><i><c- n>MANDATE</c-><c- o>-</c-><c- n>NOTHROW</c-></i><c- p>(</c-><i><c- n>AS</c-><c- o>-</c-><c- n>CONST</c-></i><c- p>(</c-><c- n>env</c-><c- p>).</c-><c- n>query</c-><c- p>(</c-><c- n>get_await_completion_adaptor</c-><c- p>))</c-></code>.</ins>
     <li data-md>
      <ins><code class="highlight"><c- n>forwarding_query</c-><c- p>(</c-><c- n>execution</c->​<c- o>::</c-><c- n>get_await_completion_adaptor</c-><c- p>)</c-></code> is a core constant expression and has value <code class="highlight">true</code>.</ins>
    </ol>
   </blockquote>
   <h4 class="heading settled" data-level="4.3.1" id="executionas_awaitable-execasawaitable"><span class="secno">4.3.1. </span><span class="content">execution​::​as_awaitable [exec.as.awaitable]</span><a class="self-link" href="#executionas_awaitable-execasawaitable"></a></h4>
   <p>in [exec.as.awaitable] (33.13.1) in §1 immediately after <i><code class="highlight"><c- n>awaitable</c-><c- o>-</c-><c- n>sender</c-></code></i> add the following exposition only concept:</p>
   <blockquote>
<pre class="highlight"><code><ins><c- k>template</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- k><c- k>class</c-></c-> <c- nc><c- nc>Sndr</c-></c-><c- o><c- o>></c-></c-></code>
<c- k>concept</c-> <i><c- nc>has</c-><c- o>-</c-><c- n>queryable</c-><c- o>-</c-><c- n>await</c-><c- o>-</c-><c- n>completion</c-><c- o>-</c-><c- n>adaptor</c-></i>  <c- c1>// exposition only</c->
      <c- o>=</c-> <c- n>sender</c-><code class="highlight"><c- o><c- o>&lt;</c-></c-><c- n><c- n>Sndr</c-></c-><c- o><c- o>></c-></c-></code>
        <c- o>&amp;&amp;</c-> <c- k>requires</c-><c- p>(</c-><c- n>Sndr</c-><c- o>&amp;&amp;</c-> <c- n>sender</c-><c- p>)</c-> <c- p>{</c->
        <c- n>get_await_completion_adaptor</c-><c- p>(</c-><c- n>get_env</c-><c- p>(</c-><c- n>sender</c-><c- p>));</c->
      <c- p>};</c->
</ins></code></pre>
   </blockquote>
   <p>Change [exec.as.awaitable] (33.13.1).§7, inserting a new item in between (7.2) and (7.3) as follows:</p>
   <blockquote>
     (7.2) Otherwise, (void(p), expr) if <code class="highlight"><i><c- n>is</c-><c- o>-</c-><c- n>awaitable</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>Expr</c-><c- p>,</c-> <c- n>U</c-><c- o>></c-></code> is <code class="highlight">true</code>, where <code class="highlight"><c- n>U</c-></code> is an unspecified 
        class type that is not <code class="highlight"><c- n>Promise</c-></code> and that lacks a member named <code class="highlight"><c- n>await_transform</c-></code>.
        Preconditions: <code class="highlight"><i><c- n>is</c-><c- o>-</c-><c- n>awaitable</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>Expr</c-><c- p>,</c-> <c- n>Promise</c-><c- o>></c-></code> is <code class="highlight">true</code> and the expression <code class="highlight"><c- k>co_await</c-> <c- n>expr</c-></code> in a coroutine with promise type <code class="highlight"><c- n>U</c-></code> is expression-equivalent to the same expression in a 
        coroutine with promise type <code class="highlight"><c- n>Promise</c-></code>. 
    <p>
     (7.3) 
     <ins>Otherwise, <code class="highlight"><i><c- n>sender</c-><c- o>-</c-><c- n>awaitable</c-></i><c- p>{</c-><i><c- n>adapted</c-><c- o>-</c-><c- n>expr</c-></i><c- p>,</c-> <c- n>p</c-><c- p>}</c-></code> if <code class="highlight"><i><c- n>has</c-><c- o>-</c-><c- n>queryable</c-><c- o>-</c-><c- n>await</c-><c- o>-</c-><c- n>completion</c-><c- o>-</c-><c- n>adaptor</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-></code> and <code class="highlight"><i><c- n>awaitable</c-><c- o>-</c-><c- n>sender</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- k>decltype</c-><c- p>((</c-></code><code class="highlight"><i><c- n>adapted</c-><c- o>-</c-><c- n>expr</c-></i></code><code class="highlight"><c- p>)),</c-> <c- n>Promise</c-><c- o>></c-></code> are both satisfied,
               where <i><code class="highlight"><c- n>adapted</c-><c- o>-</c-><c- n>expr</c-></code></i> is <code class="highlight"><c- n>get_await_completion_adaptor</c-><c- p>(</c-><c- n>get_env</c-><c- p>(</c-><c- n>expr</c-><c- p>))(</c-><c- n>expr</c-><c- p>)</c-></code>, 
               except that <code class="highlight"><c- n>expr</c-></code> is evaluated only once.</ins>
    </p>
    <ins>(7.4)</ins>
     Otherwise, <code class="highlight"><i><c- n>sender</c-><c- o>-</c-><c- n>awaitable</c-></i><c- p>{</c-><c- n>expr</c-><c- p>,</c-> <c- n>p</c-><c- p>}</c-></code> if <code class="highlight"><i><c- n>awaitable</c-><c- o>-</c-><c- n>sender</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>Expr</c-><c- p>,</c-> <c- n>Promise</c-><c- o>></c-></code> is <code class="highlight">true</code>. 
    <p>(7.5) Otherwise, (void(p), expr).</p>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="impl"><span class="secno">5. </span><span class="content">Implementation Experience</span><a class="self-link" href="#impl"></a></h2>
   <p><a href="#opt1">§ 2.1 Option 1: custom awaiter</a>, <a href="#opt2a1">§ 2.2.1 Option 2.a1: Always transform</a> and <a href="#opt2b">§ 2.2.3 Option 2.b Opt-In</a> have been prototyped on top of <a data-link-type="biblio" href="#biblio-exec26" title="beman.execution: Building Block For Asynchronous Programs">[exec26]</a>.
The implementation is strightforward, see <a href="#apxB">Appendix B (Prototype)</a>.</p>
   <h2 class="heading settled" data-level="6" id="acknowledgements"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Dietmar Kühl, for his implementation of senders/receivers, and for talking through some of the initial ideas.
Ian Petersen, for helping me with the opt-in mechanism.</p>
   <h2 class="heading settled" id="apxA"><span class="content">Appendix A (Workarounds)</span><a class="self-link" href="#apxA"></a></h2>
   <p>Workaround for the coroutine case if <code class="highlight"><c- n>async</c-><c- o>-</c-><c- n>pop</c-><c- o>-</c-><c- n>sender</c-></code> supports <code class="highlight"><c- n>set_value_t</c-><c- p>()</c-></code> and <code class="highlight"><c- n>set_value_t</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-></code></p>
<pre class="language-c++ highlight"><c- k>while</c-> <c- p>(</c-><c- k>auto</c-> <c- n>fname</c-> <c- o>=</c-> <c- n>co_await</c-> <c- n>file_queue</c-><c- p>.</c-><c- n>async_pop</c-><c- p>()</c-> <c- o>|</c-> <c- n>stdp</c-><c- o>::</c-><c- n>into_optional</c-><c- p>())</c-> <c- p>{</c->
  <c- n>grep_file</c-><c- p>(</c-><c- o>*</c-><c- n>fname</c-><c- p>,</c-> <c- n>needle</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <p>The drawback here is that everyone needs to remember the additional <code class="highlight"><c- o>|</c-> <c- n>stdp</c-><c- o>::</c-><c- n>into_optional</c-><c- p>()</c-></code> for
what is most likely a very common application programming usecase.</p>
   <p>The code for sender/receiver if <code class="highlight"><c- n>async</c-><c- o>-</c-><c- n>pop</c-><c- o>-</c-><c- n>sender</c-></code> supports <code class="highlight"><c- n>set_value_t</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>)</c-></code></p>
<pre class="language-c++ highlight"><c- n>files</c-><c- p>.</c-><c- n>async_pop</c-><c- p>()</c-> <c- o>|</c-> <c- n>stdp</c-><c- o>::</c-><c- n>overload</c-><c- p>(</c->
  <c- p>[</c-><c- n>needle</c-><c- p>](</c-><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>fs</c-><c- o>::</c-><c- n>path</c-> <c- k>const</c-><c- o>>&amp;</c-> <c- n>fname</c-><c- p>)</c-> <c- o>-></c-> <c- b>bool</c-> <c- p>{</c->
    <c- k>if</c-> <c- p>(</c-><c- o>!</c-><c- n>fname</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> false<c- p>;</c-> <c- p>}</c->

    <c- k>return</c-> <c- n>grep_file</c-><c- p>(</c-><c- o>*</c-><c- n>fname</c-><c- p>,</c-> <c- n>needle</c-><c- p>);</c->
  <c- p>});</c->
</pre>
   <p>This has two drawbacks, it introduces the potential for UB, since users could forget to check the
optionals engaged state. The wrapping and unwrapping of the optional could also possibly introduce
some overhead.</p>
   <p>Concievably we could also introduce a special <code class="highlight"><c- n>overload</c-></code> function for this case, which would get rid
of the first drawback.</p>
<pre class="language-c++ highlight"><c- n>files</c-><c- p>.</c-><c- n>async_pop</c-><c- p>()</c->                                  
<c- o>|</c-> <c- n>stdp</c-><c- o>::</c-><c- n>overload_optional</c-><c- p>(</c->
  <c- p>[]()</c-> <c- o>-></c-> <c- b>bool</c-> <c- p>{</c-> <c- k>return</c-> true<c- p>;</c-> <c- p>},</c->
  <c- p>[</c-><c- n>needle</c-><c- p>](</c-><c- n>std</c-><c- o>::</c-><c- n>fs</c-><c- o>::</c-><c- n>path</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>fname</c-><c- p>)</c-> <c- o>-></c-> <c- b>bool</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>grep_file</c-><c- p>(</c-><c- n>fname</c-><c- p>,</c-> <c- n>needle</c-><c- p>);</c->
  <c- p>});</c->
</pre>
   <h2 class="heading settled" id="apxB"><span class="content">Appendix B (Prototype)</span><a class="self-link" href="#apxB"></a></h2>
   <p>Prototype implementation based on <a data-link-type="biblio" href="#biblio-exec26" title="beman.execution: Building Block For Asynchronous Programs">[exec26]</a>:</p>
<pre class="language-c++ highlight"><c- cp>#include</c-> &lt;beman/execution26/execution.hpp>
<c- cp>#include</c-> &lt;beman/execution26/detail/queryable.hpp>
<c- cp>#include</c-> &lt;optional>
<c- cp>#include</c-> &lt;print>


<c- n>namespace</c-> <c- n>stdexec</c-> <c- o>=</c-> <c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- p>;</c->

<c- c1>//------------------------------------------------------------------------------------------------------------------------------</c->
<c- c1>// try out different options</c->

<c- c1>//#define OPTION1(...)  __VA_ARGS__</c->
<c- cp>#define OPTION1(...)</c->

<c- cp>#define OPTION2b(...)  __VA_ARGS__</c->
<c- c1>//#define OPTION2b(...)</c->

<c- c1>//------------------------------------------------------------------------------------------------------------------------------</c->
<c- c1>// Optional Sender adaptor (taken from P3552-task branch)</c->
<c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-><c- p>...</c-><c- o>></c-> <c- k>struct</c-> <c- nc>type_list</c-> <c- p>{};</c->

<c- kr>inline</c-> <c- n>constexpr</c-> <c- k>struct</c-> <c- nc>into_optional_t</c-> <c- o>:</c-> <c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>sender_adaptor_closure</c-><c- o>&lt;</c-><c- n>into_optional_t</c-><c- o>></c-> <c- p>{</c->
    <c- n>template</c-> <c- o>&lt;</c-><c- n>stdexec</c-><c- o>::</c-><c- n>sender</c-> <c- n>Upstream</c-><c- o>></c->
    <c- k>struct</c-> <c- nc>sender</c-> <c- p>{</c->
        <c- n>using</c-> <c- n>upstream_t</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Upstream</c-><c- o>></c-><c- p>;</c->
        <c- n>using</c-> <c- n>sender_concept</c-> <c- o>=</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>sender_t</c-><c- p>;</c->
        <c- n>upstream_t</c-> <c- n>upstream</c-><c- p>;</c->

        <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>static</c-> <c- k>auto</c-> <c- n>find_type</c-><c- p>(</c-><c- n>type_list</c-><c- o>&lt;</c-><c- n>type_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>)</c->                <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>{};</c-> <c- p>}</c->
        <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>static</c-> <c- k>auto</c-> <c- n>find_type</c-><c- p>(</c-><c- n>type_list</c-><c- o>&lt;</c-><c- n>type_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>type_list</c-><c- o>&lt;>></c-><c- p>)</c->   <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>{};</c-> <c- p>}</c->
        <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>static</c-> <c- k>auto</c-> <c- n>find_type</c-><c- p>(</c-><c- n>type_list</c-><c- o>&lt;</c-><c- n>type_list</c-><c- o>&lt;></c-><c- p>,</c-> <c- n>type_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c-><c- p>)</c->   <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>{};</c-> <c- p>}</c->

        <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>Env</c-><c- o>></c-> <c- k>auto</c-> <c- n>get_type</c-><c- p>(</c-><c- n>Env</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>decltype</c-><c- p>(</c-><c- n>find_type</c-><c- p>(</c-><c- n>stdexec</c-><c- o>::</c-><c- n>value_types_of_t</c-><c- o>&lt;</c-><c- n>Upstream</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Env</c-><c- o>></c-><c- p>,</c-> <c- n>type_list</c-><c- p>,</c-> <c- n>type_list</c-><c- o>></c-><c- p>())){};</c->
        <c- p>}</c->
        <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-><c- p>...</c-> <c- n>E</c-><c- p>,</c-> <c- n>typename</c-><c- p>...</c-> <c- n>S</c-><c- o>></c->
        <c- n>constexpr</c-> <c- k>auto</c-> <c- n>make_signatures</c-><c- p>(</c-><c- k>auto</c-><c- o>&amp;&amp;</c-> <c- n>env</c-><c- p>,</c-> <c- n>type_list</c-><c- o>&lt;</c-><c- n>E</c-><c- p>...</c-><c- o>></c-><c- p>,</c-> <c- n>type_list</c-><c- o>&lt;</c-><c- n>S</c-><c- p>...</c-><c- o>></c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>completion_signatures</c-><c- o>&lt;</c->
                <c- n>stdexec</c-><c- o>::</c-><c- n>set_value_t</c-><c- p>(</c-><c- n>decltype</c-><c- p>(</c-><c- n>this</c-><c- o>-></c-><c- n>get_type</c-><c- p>(</c-><c- n>env</c-><c- p>))),</c->
                <c- n>stdexec</c-><c- o>::</c-><c- n>set_error_t</c-><c- p>(</c-><c- n>E</c-><c- p>)...,</c->
                <c- n>S</c-><c- p>...</c->
                <c- o>></c-><c- p>();</c->
        <c- p>}</c->
        <c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>Env</c-><c- o>></c->
        <c- k>auto</c-> <c- n>get_completion_signatures</c-><c- p>(</c-><c- n>Env</c-><c- o>&amp;&amp;</c-> <c- n>env</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>make_signatures</c-><c- p>(</c-><c- n>env</c-><c- p>,</c->
                                   <c- n>stdexec</c-><c- o>::</c-><c- n>error_types_of_t</c-><c- o>&lt;</c-><c- n>Upstream</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Env</c-><c- o>></c-><c- p>,</c-> <c- n>type_list</c-><c- o>></c-><c- p>{},</c->
                                   <c- n>std</c-><c- o>::</c-><c- n>conditional_t</c-><c- o>&lt;</c->
                                        <c- n>stdexec</c-><c- o>::</c-><c- n>sends_stopped</c-><c- o>&lt;</c-><c- n>Upstream</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Env</c-><c- o>>></c-><c- p>,</c->
                                        <c- n>type_list</c-><c- o>&lt;</c-><c- n>stdexec</c-><c- o>::</c-><c- n>set_stopped_t</c-><c- p>()</c-><c- o>></c-><c- p>,</c->
                                        <c- n>type_list</c-><c- o>&lt;>></c-><c- p>{}</c->
                                   <c- p>);</c->
        <c- p>}</c->

        <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>Receiver</c-><c- o>></c->
        <c- k>auto</c-> <c- n>connect</c-><c- p>(</c-><c- n>Receiver</c-><c- o>&amp;&amp;</c-> <c- n>receiver</c-><c- p>)</c-> <c- o>&amp;&amp;</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>connect</c-><c- p>(</c->
                <c- n>stdexec</c-><c- o>::</c-><c- n>then</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>this</c-><c- o>-></c-><c- n>upstream</c-><c- p>),</c->
                    <c- p>[]</c-><c- o>&lt;</c-><c- n>typename</c-><c- p>...</c-><c- n>A</c-><c- o>></c-><c- p>(</c-><c- n>A</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>a</c-><c- p>)</c-><c- o>-></c-><c- n>decltype</c-><c- p>(</c-><c- n>get_type</c-><c- p>(</c-><c- n>stdexec</c-><c- o>::</c-><c- n>get_env</c-><c- p>(</c-><c- n>receiver</c-><c- p>)))</c-> <c- p>{</c->
                        <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- k>sizeof</c-><c- p>...(</c-><c- n>A</c-><c- p>)</c-> <c- o>==</c-> <c- mi>0u</c-><c- p>)</c->
                            <c- k>return</c-> <c- p>{};</c->
                        <c- k>else</c->
                            <c- k>return</c-> <c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>A</c-><c- o>></c-><c- p>(</c-><c- n>a</c-><c- p>)...};</c->
                <c- p>}),</c->
                <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Receiver</c-><c- o>></c-><c- p>(</c-><c- n>receiver</c-><c- p>)</c->
            <c- p>);</c->
        <c- p>}</c->
    <c- p>};</c->

    <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>Upstream</c-><c- o>></c->
    <c- k>auto</c-> <c- n>operator</c-><c- p>()(</c-><c- n>Upstream</c-><c- o>&amp;&amp;</c-> <c- n>upstream</c-><c- p>)</c-> <c- k>const</c-> <c- o>-></c-> <c- n>sender</c-><c- o>&lt;</c-><c- n>Upstream</c-><c- o>></c-> <c- p>{</c-> <c- k>return</c-> <c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Upstream</c-><c- o>></c-><c- p>(</c-><c- n>upstream</c-><c- p>)};</c-> <c- p>}</c->
<c- p>}</c-> <c- n>into_optional</c-><c- p>{};</c->


<c- n>namespace</c-> <c- n>P3570_detail</c-> <c- p>{</c->
<c- c1>//------------------------------------------------------------------------------------------------------------------------------</c->
<c- c1>// this type would become part of the standard</c->
<c- k>struct</c-> <c- nc>get_await_completion_adaptor_t</c-> <c- p>{</c->
    <c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>T</c-><c- o>></c->
    <c- k>auto</c-> <c- n>operator</c-><c- p>()(</c-><c- n>T</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>env</c-><c- p>)</c-> <c- k>const</c-> <c- n>noexcept</c-> 
        <c- n>requires</c-> <c- n>requires</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;&amp;</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c-> <c- p>{</c-> <c- n>t</c-><c- p>.</c-><c- n>query</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>get_await_completion_adaptor_t</c-><c- o>></c-><c- p>())</c-> <c- p>};</c-> <c- p>}</c->
    <c- p>{</c->
        <c- k>return</c-> <c- n>env</c-><c- p>.</c-><c- n>query</c-><c- p>(</c-><c- o>*</c-><c- n>this</c-><c- p>);</c->
    <c- p>}</c->
<c- p>};</c->
<c- kr>inline</c-> <c- n>constexpr</c-> <c- n>get_await_completion_adaptor_t</c-> <c- n>get_await_completion_adaptor</c-><c- p>;</c->
<c- c1>//------------------------------------------------------------------------------------------------------------------------------</c->
<c- c1>// implementation detail, can needs to be cleaned up</c->
<c- k>struct</c-> <c- nc>no_tranform_t</c-><c- p>{</c-> <c- c1>// implementation only tag type, no need to standardize, other strategies would be possible</c->
    <c- c1>// member is not necessary, only there to </c->
    <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>Expr</c-><c- o>></c-> <c- k>auto</c-> <c- n>operator</c-><c- p>()(</c-><c- n>Expr</c-><c- o>&amp;&amp;</c-> <c- n>expr</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>);</c-> <c- p>}</c->
<c- p>};</c->
<c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>Sender</c-><c- o>></c-> <c- n>concept</c-> <c- n>_has_env</c-> <c- o>=</c-> <c- n>requires</c-><c- p>(</c-><c- n>Sender</c-><c- o>&amp;&amp;</c-> <c- n>sender</c-><c- p>)</c-> <c- p>{</c-> 
                            <c- p>{</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>get_env</c-><c- p>(</c-><c- n>sender</c-><c- p>)</c-> <c- p>};</c->
                        <c- p>};</c->

<c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>Sender</c-><c- p>,</c-> <c- n>typename</c-> <c- n>Env</c-> <c- o>=</c-> <c- n>Sender</c-><c- o>></c->
<c- n>concept</c-> <c- n>has_p3570_opt_in</c-> <c- o>=</c-> <c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>sender</c-><c- o>&lt;</c-><c- n>Sender</c-><c- o>></c-> 
                        <c- o>&amp;&amp;</c-> <c- n>_has_env</c-><c- o>&lt;</c-><c- n>Sender</c-><c- o>></c->
                        <c- o>&amp;&amp;</c-> <c- n>requires</c-><c- p>(</c-><c- n>Sender</c-><c- o>&amp;&amp;</c-> <c- n>sender</c-><c- p>,</c-> <c- n>Env</c-><c- o>&amp;&amp;</c-> <c- n>env</c-><c- p>)</c-> <c- p>{</c-> 
                            <c- p>{</c->
                                <c- n>P3570_detail</c-><c- o>::</c-><c- n>get_await_completion_adaptor</c-><c- p>(</c-><c- n>stdexec</c-><c- o>::</c-><c- n>get_env</c-><c- p>(</c-><c- n>env</c-><c- p>))</c->
                            <c- p>};</c->
                        <c- p>};</c->  

<c- c1>// Exposition only trait type mentioned in the paper</c->
<c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>Sender</c-><c- o>></c-> <c- k>struct</c-> <c- nc>_transform_trait</c-> <c- p>{</c-> <c- n>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>no_tranform_t</c-><c- p>;</c-> <c- p>};</c->
<c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>Sender</c-><c- o>></c-> <c- n>requires</c-> <c- n>has_p3570_opt_in</c-><c- o>&lt;</c-><c- n>Sender</c-><c- o>></c->
<c- k>struct</c-> <c- nc>_transform_trait</c-><c- o>&lt;</c-><c- n>Sender</c-><c- o>></c-> <c- p>{</c->
    <c- n>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>decltype</c-><c- p>(</c-><c- n>P3570_detail</c-><c- o>::</c-><c- n>get_await_completion_adaptor</c-><c- p>(</c-><c- n>stdexec</c-><c- o>::</c-><c- n>get_env</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>Sender</c-><c- o>></c-><c- p>())));</c->
<c- p>};</c->
<c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>Sender</c-><c- o>></c-> <c- n>using</c-> <c- n>_transform_trait_t</c-> <c- o>=</c-> <c- n>typename</c-> <c- n>_transform_trait</c-><c- o>&lt;</c-><c- n>Sender</c-><c- o>>::</c-><c- n>type</c-><c- p>;</c->

<c- c1>// I used an additional parameter to implement the transformation, but that is not necessary.</c->
<c- c1>// the trait could just be used internally as well.</c->
<c- k>struct</c-> <c- nc>as_awaitable_t</c-> <c- p>{</c->
    <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>Expr</c-><c- p>,</c-> <c- n>typename</c-> <c- n>Promise</c-><c- p>,</c-> <c- n>typename</c-> <c- n>Transform</c-> <c- o>=</c-> <c- n>_transform_trait_t</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>>></c->
    <c- k>auto</c-> <c- n>operator</c-><c- p>()(</c-><c- n>Expr</c-><c- o>&amp;&amp;</c-> <c- n>expr</c-><c- p>,</c-> <c- n>Promise</c-><c- o>&amp;</c-> <c- n>promise</c-><c- p>,</c-> <c- n>Transform</c-> <c- n>transform</c-> <c- o>=</c-> <c- n>Transform</c-><c- p>{})</c-> <c- k>const</c-> <c- p>{</c->
        <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- n>requires</c-> <c- p>{</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>).</c-><c- n>as_awaitable</c-><c- p>(</c-><c- n>promise</c-><c- p>);</c-> <c- p>})</c-> <c- p>{</c->
            <c- k>static_assert</c-><c- p>(</c->
                <c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>detail</c-><c- o>::</c-><c- n>is_awaitable</c-><c- o>&lt;</c-><c- n>decltype</c-><c- p>(</c-><c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>).</c-><c- n>as_awaitable</c-><c- p>(</c-><c- n>promise</c-><c- p>)),</c->
                                                           <c- n>Promise</c-><c- o>></c-><c- p>,</c->
                <c- s>"as_awaitable must return an awaitable"</c-><c- p>);</c->
            <c- k>return</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>).</c-><c- n>as_awaitable</c-><c- p>(</c-><c- n>promise</c-><c- p>);</c->
        <c- p>}</c-> <c- k>else</c-> <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-><c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>detail</c-><c- o>::</c->
                                 <c- n>is_awaitable</c-><c- o>&lt;</c-><c- n>Expr</c-><c- p>,</c-> <c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>detail</c-><c- o>::</c-><c- n>unspecified_promise</c-><c- o>></c-> <c- o>||</c->
                             <c- n>not</c-> <c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>detail</c-><c- o>::</c-><c- n>awaitable_sender</c-><c- o>&lt;</c-><c- n>Expr</c-><c- p>,</c-> <c- n>Promise</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
            <c- n>using</c-> <c- n>TExpr</c-> <c- o>=</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>decltype</c-><c- p>(</c-><c- n>transform</c-><c- p>(</c-><c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>)))</c-><c- o>></c-><c- p>;</c->        
            <c- k>if</c-> <c- n>constexpr</c-> <c- p>(</c-> <c- n>std</c-><c- o>::</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>Transform</c-><c- o>></c-><c- p>,</c-> <c- n>no_tranform_t</c-><c- o>></c-> 
                        <c- o>||</c-> <c- n>not</c-> <c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>detail</c-><c- o>::</c-><c- n>awaitable_sender</c-><c- o>&lt;</c-><c- n>TExpr</c-><c- p>,</c-> <c- n>Promise</c-><c- o>></c-><c- p>)</c-> <c- p>{</c->
                <c- k>return</c-> <c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>);</c->
            <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
                <c- k>return</c-> <c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>detail</c-><c- o>::</c-><c- n>sender_awaitable</c-><c- o>&lt;</c-><c- n>TExpr</c-><c- p>,</c-> <c- n>Promise</c-><c- o>></c-><c- p>{</c-><c- n>transform</c-><c- p>(</c-><c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>)),</c-> <c- n>promise</c-> <c- p>};</c->
            <c- p>}</c->
        <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
            <c- k>return</c-> <c- o>::</c-><c- n>beman</c-><c- o>::</c-><c- n>execution26</c-><c- o>::</c-><c- n>detail</c-><c- o>::</c-><c- n>sender_awaitable</c-><c- o>&lt;</c-><c- n>Expr</c-><c- p>,</c-> <c- n>Promise</c-><c- o>></c-><c- p>{</c-><c- o>::</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Expr</c-><c- o>></c-><c- p>(</c-><c- n>expr</c-><c- p>),</c-> <c- n>promise</c-><c- p>};</c->
        <c- p>}</c->
    <c- p>}</c->
<c- p>};</c->
<c- kr>inline</c-> <c- n>constexpr</c-> <c- o>::</c-><c- n>P3570_detail</c-><c- o>::</c-><c- n>as_awaitable_t</c-> <c- n>as_awaitable</c-><c- p>{};</c->
<c- p>}</c->

<c- c1>//------------------------------------------------------------------------------------------------------------------------------</c->
<c- c1>// Fake concurrent queue to illustrate the mechanism</c->

<c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>T_</c-><c- o>></c->
<c- k>struct</c-> <c- nc>coqueue</c-><c- p>{</c->
    <c- n>using</c-> <c- n>T</c-> <c- o>=</c-> <c- b>int</c-><c- p>;</c-> <c- c1>// I just want to demonstrate the S/R and Coroutine mechanism, </c->
                   <c- c1>// and make the fake queue as simple as prossible </c->

    <c- k>struct</c-> <c- nc>pop_sender</c-> <c- p>{</c->
        <c- n>using</c-> <c- n>sender_concept</c->        <c- o>=</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>sender_t</c-><c- p>;</c-> 
        <c- n>using</c-> <c- n>completion_signatures</c-> <c- o>=</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>completion_signatures</c-><c- o>&lt;</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>set_value_t</c-><c- p>()</c->
                                                                    <c- p>,</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>set_value_t</c-><c- p>(</c-><c- n>T</c-><c- p>)</c->
                                                                    <c- o>></c-><c- p>;</c->

        <c- n>OPTION1</c-><c- p>(</c->
        <c- c1>// Option 1: custom awaiter for pop_sender, works in the simple case </c->
        <c- c1>//            (if somefuturestd::task does not adapt the sender)</c->
        <c- c1>//           + No need to change any S/R machinery, works out of the box</c->
        <c- c1>//           - any Sender adaption will break the special casing.</c->
        <c- c1>//           - some complexity from implementing S/R operation state completion will be</c->
        <c- c1>//             duplicated in awaiter/await_resume</c->

        <c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>U</c-><c- p>,</c-> <c- n>typename</c-> <c- n>promise_t</c-><c- o>></c->
        <c- k>struct</c-> <c- nc>awaiter</c-> <c- p>{</c->
            <c- k>auto</c-> <c- n>await_ready</c-><c- p>()</c-> <c- o>-></c-> <c- b>bool</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c->
            <c- k>auto</c-> <c- n>await_suspend</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;></c-> <c- n>p</c-><c- p>)</c-> <c- o>-></c-> <c- n>std</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;></c-> <c- p>{</c-> <c- k>return</c-> <c- n>p</c-><c- p>;</c-> <c- p>}</c->
            <c- k>auto</c-> <c- n>await_resume</c-><c- p>()</c-> <c- o>-></c-> <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-> <c- p>{</c-> 
                <c- k>return</c-> <c- n>queue_</c-><c- p>.</c-><c- n>empty</c-><c- p>()</c-> <c- o>?</c-> <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>{}</c-> 
                                      <c- o>:</c-> <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>{</c-><c- n>queue_</c-><c- p>.</c-><c- n>internal_pop_</c-><c- p>()};</c-> 
            <c- p>}</c->

            <c- n>promise_t</c-> <c- n>promise_</c-><c- p>;</c->
            <c- n>coqueue</c-><c- o>&lt;</c-><c- n>U</c-><c- o>>&amp;</c-> <c- n>queue_</c-><c- p>;</c->
        <c- p>};</c->

        <c- n>template</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>promise_t</c-><c- o>></c->
        <c- k>auto</c-> <c- n>as_awaitable</c-><c- p>(</c-><c- n>promise_t</c-><c- o>&amp;&amp;</c-> <c- n>promise</c-><c- p>)</c-> <c- p>{</c-> 
            <c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"Using option 1"</c-><c- p>);</c->
            <c- k>return</c-> <c- n>awaiter</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>promise_t</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>promise_t</c-><c- o>></c-><c- p>(</c-><c- n>promise</c-><c- p>),</c-> <c- n>queue_</c-><c- p>};</c-> 
        <c- p>}</c->
        <c- p>)</c-> <c- c1>// /OPTION1</c->

        <c- c1>// this is the S/R mechanism for the "queue" (slideware version), </c->
        <c- c1>// Option 1 bypasses this part of the machinery in the coroutine case</c->
        <c- n>template</c-><c- o>&lt;</c-><c- n>stdexec</c-><c- o>::</c-><c- n>receiver</c-> <c- n>rcvr</c-><c- o>></c->
        <c- k>struct</c-> <c- nc>state</c-> <c- p>{</c->
            <c- n>using</c-> <c- n>_result_t</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->
            <c- n>using</c-> <c- n>_complete_t</c-> <c- o>=</c-> <c- b>void</c-> <c- p>(</c-><c- n>state</c-><c- o>&amp;</c-><c- p>);</c->

            <c- n>rcvr</c-> <c- n>rcvr_</c-><c- p>;</c->
            <c- n>_complete_t</c-><c- o>*</c-> <c- n>complete_</c-><c- p>;</c->
            <c- n>coqueue</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>queue_</c-><c- p>;</c->
            
            <c- n>explicit</c-> <c- nf>state</c-><c- p>(</c-><c- n>rcvr</c-><c- o>&amp;&amp;</c-> <c- n>r</c-><c- p>,</c-> <c- n>coqueue</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>queue</c-><c- p>)</c-> 
                <c- o>:</c-> <c- n>rcvr_</c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>r</c-><c- p>)}</c->
                <c- p>,</c-> <c- n>complete_</c-><c- p>{[](</c-><c- n>state</c-><c- o>&amp;</c-> <c- n>self</c-><c- p>)</c-> <c- p>{</c-> 
                    <c- c1>// here be synchronization </c->
                    <c- k>if</c-> <c- p>(</c-> <c- n>not</c-> <c- n>self</c-><c- p>.</c-><c- n>queue_</c-><c- p>.</c-><c- n>empty</c-><c- p>())</c-> <c- p>{</c-> <c- c1>// this would check error conditions in the proper impl</c->
                        <c- n>stdexec</c-><c- o>::</c-><c- n>set_value</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>self</c-><c- p>.</c-><c- n>rcvr_</c-><c- p>),</c-> <c- n>self</c-><c- p>.</c-><c- n>queue_</c-><c- p>.</c-><c- n>internal_pop_</c-><c- p>());</c->  
                    <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
                        <c- n>stdexec</c-><c- o>::</c-><c- n>set_value</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>self</c-><c- p>.</c-><c- n>rcvr_</c-><c- p>));</c-> 
                    <c- p>}</c->
                <c- p>}}</c->
                <c- p>,</c-> <c- n>queue_</c-><c- p>{</c-><c- n>queue</c-><c- p>}</c->
            <c- p>{</c->
                <c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"Using option 2 or S/R directly"</c-><c- p>);</c->
            <c- p>}</c->


            <c- n>using</c-> <c- n>operation_state_concept</c-> <c- o>=</c-> <c- n>stdexec</c-><c- o>::</c-><c- n>operation_state_t</c-><c- p>;</c->
            <c- k>auto</c-> <c- n>start</c-><c- p>()</c-> <c- n>noexcept</c-> <c- o>-></c-> <c- b>void</c-> <c- p>{</c-> 
                <c- p>(</c-><c- o>*</c-><c- n>complete_</c-><c- p>)(</c-><c- o>*</c-><c- n>this</c-><c- p>);</c-> 
            <c- p>}</c->

        <c- c1>// Option 2:</c->
        <c- c1>// this needs modification in stdexec::as_awatiable, so that it can detect that the sender</c->
        <c- c1>// should result in an awaiter that returns `std::optional`s. </c->
        <c- c1>// In the beman::execution26 impl this would mean one additonal exposition only class and concept</c->
        
        <c- c1>// Option 2.a1</c->
        <c- c1>//    any sender that has a completion signature of `stdexec::set_value_t()` and `stdexec::set_value_t(T)`</c->
        <c- c1>//    is automatically considered an awatiter that returns `std::optional`.</c->
        <c- c1>// Option 2.a2</c->
        <c- c1>//    same as 2.a1, plus a mechanism to opt out (via environments or a type tag?)</c->
        <c- c1>// Option 2.b</c->
        <c- c1>//    senders have to explicitly opt in. (via environments or a type tag?)</c->

        <c- p>};</c->

        <c- c1>// Opt-In (Option 2.b)</c->
        <c- n>OPTION2b</c-><c- p>(</c->
        <c- k>struct</c-> <c- nc>env</c-> <c- p>{</c-> <c- k>auto</c-> <c- n>query</c-><c- p>(</c-><c- n>P3570_detail</c-><c- o>::</c-><c- n>get_await_completion_adaptor_t</c-><c- p>)</c-> <c- k>const</c-> <c- o>-></c-> <c- n>into_optional_t</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c-> <c- p>};</c->
        <c- k>auto</c-> <c- n>get_env</c-><c- p>()</c-> <c- k>const</c-> <c- n>noexcept</c-> <c- o>-></c-> <c- n>env</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c->
        <c- p>)</c-> <c- c1>// /OPTION2b</c->

        <c- c1>// common impl</c->
        <c- n>template</c-><c- o>&lt;</c-><c- n>stdexec</c-><c- o>::</c-><c- n>receiver</c-> <c- n>rcvr</c-><c- o>></c-> <c- k>auto</c-> <c- n>connect</c-><c- p>(</c-><c- n>rcvr</c-> <c- n>r</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>state</c-><c- o>&lt;</c-><c- n>rcvr</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>r</c-><c- p>),</c-> <c- n>queue_</c-><c- p>};</c-> <c- p>}</c->

        <c- n>coqueue</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-> <c- n>queue_</c-><c- p>;</c->
    <c- p>};</c->

    <c- k>auto</c-> <c- n>async_pop</c-><c- p>()</c-> <c- o>-></c-> <c- n>pop_sender</c-> <c- p>{</c-> <c- k>return</c-> <c- n>pop_sender</c-><c- p>{</c-><c- o>*</c-><c- n>this</c-><c- p>};</c-> <c- p>}</c->

    <c- c1>// Fake queue - returns count ints in decending order queue turns empty when count is 0</c->
    <c- k>auto</c-> <c- n>empty</c-><c- p>()</c-> <c- o>-></c-> <c- b>bool</c-> <c- p>{</c-> <c- k>return</c-> <c- n>count_</c-> <c- o>==</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c->

    <c- k>auto</c-> <c- n>internal_pop_</c-><c- p>()</c-> <c- o>-></c-> <c- n>T</c-> <c- p>{</c-> <c- k>return</c-> <c- n>count_</c-><c- o>--</c-><c- p>;</c-> <c- p>}</c->
    <c- b>int</c-> <c- n>count_</c-><c- p>;</c->
<c- p>};</c->

<c- c1>//------------------------------------------------------------------------------------------------------------------------------</c->
<c- c1>// Task type, ducttape version (P3552 lazy)</c->
<c- k>struct</c-> <c- nc>task</c-><c- p>{</c->
    <c- k>struct</c-> <c- nc>promise_type</c-> <c- p>{</c->
        <c- k>auto</c-> <c- n>initial_suspend</c-><c- p>()</c->                  <c- o>-></c-> <c- n>std</c-><c- o>::</c-><c- n>suspend_never</c->       <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c->
        <c- k>auto</c-> <c- n>final_suspend</c-><c- p>()</c->        <c- n>noexcept</c->    <c- o>-></c-> <c- n>std</c-><c- o>::</c-><c- n>suspend_always</c->      <c- p>{</c-> <c- k>return</c-> <c- p>{};</c-> <c- p>}</c->
        <c- p>[[</c-><c- k>noreturn</c-><c- p>]]</c-> <c- k>auto</c-> <c- n>unhandled_exception</c-><c- p>()</c-> <c- o>-></c-> <c- b>void</c->                     <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>terminate</c-><c- p>();</c-> <c- p>}</c->
        <c- k>auto</c-> <c- n>unhandled_stopped</c-><c- p>()</c->                <c- o>-></c-> <c- n>std</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;></c->  <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>noop_coroutine</c-><c- p>();</c-> <c- p>}</c->

        <c- k>auto</c-> <c- n>get_return_object</c-><c- p>()</c->                <c- o>-></c-> <c- n>task</c->                     <c- p>{</c-> <c- k>return</c-> <c- n>task</c-><c- p>{};</c-> <c- p>}</c->
        <c- k>auto</c-> <c- n>return_void</c-><c- p>()</c->                      <c- o>-></c-> <c- b>void</c->                     <c- p>{</c-> <c- k>return</c-><c- p>;</c-> <c- p>}</c->

        <c- n>template</c-> <c- o>&lt;</c-><c- n>stdexec</c-><c- o>::</c-><c- n>sender</c-> <c- n>Awaitable</c-><c- o>></c->
        <c- k>auto</c-> <c- n>await_transform</c-><c- p>(</c-><c- n>Awaitable</c-><c- o>&amp;&amp;</c-> <c- n>awaitable</c-><c- p>)</c-> <c- n>noexcept</c-> <c- o>-></c-> <c- n>decltype</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- p>{</c->
            <c- k>return</c-> <c- n>P3570_detail</c-><c- o>::</c-><c- n>as_awaitable</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Awaitable</c-><c- o>></c-><c- p>(</c-><c- n>awaitable</c-><c- p>),</c-> <c- o>*</c-><c- n>this</c-><c- p>);</c->
        <c- p>}</c->

        <c- n>private</c-><c- o>:</c->
            <c- k>auto</c-> <c- n>tag</c-><c- p>()</c-> <c- o>-></c-> <c- b>char</c-> <c- k>const</c-><c- o>*</c-> <c- p>{</c-> <c- k>return</c-> <c- s>""</c-><c- p>;</c-> <c- p>}</c->
    <c- p>};</c->
<c- p>};</c->


<c- c1>//------------------------------------------------------------------------------------------------------------------------------</c->
<c- c1>// Example -> usage code in the coroutine</c->

<c- k>auto</c-> <c- n>use</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- o>-></c-> <c- b>void</c-> <c- p>{</c->   <c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>"used #{}"</c-><c- p>,</c-> <c- n>i</c-><c- p>);</c-> <c- p>}</c->

<c- k>auto</c-> <c- n>coro</c-><c- p>(</c-><c- n>coqueue</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>&amp;</c-> <c- n>data</c-><c- p>)</c-> <c- o>-></c-> <c- n>task</c-> <c- p>{</c->

    <c- k>while</c-><c- p>(</c-><c- k>auto</c-> <c- n>item</c-> <c- o>=</c-> <c- n>co_await</c-> <c- n>data</c-><c- p>.</c-><c- n>async_pop</c-><c- p>())</c-> <c- p>{</c->
        <c- n>use</c-><c- p>(</c-><c- o>*</c-><c- n>item</c-><c- p>);</c->
    <c- p>}</c->
    
    <c- n>co_return</c-><c- p>;</c->
<c- p>}</c->

<c- k>auto</c-> <c- n>main</c-><c- p>()</c-> <c- o>-></c-> <c- b>int</c-> <c- p>{</c->
    <c- n>coqueue</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>queue</c-><c- p>{</c-><c- mi>3</c-><c- p>};</c->

    <c- k>auto</c-> <c- n>_</c-> <c- o>=</c-> <c- n>coro</c-><c- p>(</c-><c- n>queue</c-><c- p>);</c->
    <c- n>std</c-><c- o>::</c-><c- n>println</c-><c- p>(</c-><c- s>" ---[done]------ "</c-><c- p>);</c->

    <c- k>return</c-> <c- n>EXIT_SUCCESS</c-><c- p>;</c->
<c- p>}</c->
</pre>
  </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="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-execasawaitable">[EXEC.AS.AWAITABLE]
   <dd><a href="https://eel.is/c++draft/exec.as.awaitable"><cite>[exec.as.awaitable]</cite></a>. 2025-02-06. URL: <a href="https://eel.is/c++draft/exec.as.awaitable">https://eel.is/c++draft/exec.as.awaitable</a>
   <dt id="biblio-p0260r13">[P0260R13]
   <dd>Lawrence Crowl; et al. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p0260r13.html"><cite>C++ Concurrent Queues</cite></a>. 2024-12-10. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p0260r13.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p0260r13.html</a>
  </dl>
  <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-exec26">[EXEC26]
   <dd>Dietmar Kühl. <a href="https://github.com/bemanproject/execution"><cite>beman.execution: Building Block For Asynchronous Programs</cite></a>. 2025-01-25. URL: <a href="https://github.com/bemanproject/execution">https://github.com/bemanproject/execution</a>
   <dt id="biblio-p3552r0">[P3552R0]
   <dd>Dietmar Kühl; Maikel Nadolski. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3552r0.pdf"><cite>Add a Coroutine Lazy Type</cite></a>. 2025-01-13. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3552r0.pdf">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3552r0.pdf</a>
  </dl>