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  <title>P3669R2: Non-Blocking Support for `std::execution`</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;
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      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 */
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    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 44bea8ebb, updated Thu Sep 7 16:28:26 2023 -0700" name="generator">
  <link href="http://wg21.link/P3669R2" rel="canonical">
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P3669R2<br>Non-Blocking Support for `std::execution`</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="2025-06-19">2025-06-19</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P3669R2">http://wg21.link/P3669R2</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:dv@vollmann.ch">Detlef Vollmann</a>
     <dt>Audience:
     <dd>SG1, LEWG
     <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><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>execution</c-></code> as currently specified doesn’t provide

        support for non-blocking operations.
        This proposal tries to fix this.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#Revision"><span class="secno">1</span> <span class="content">Revision History</span></a>
    <li><a href="#Introduction"><span class="secno">2</span> <span class="content">Introduction</span></a>
    <li>
     <a href="#Design"><span class="secno">3</span> <span class="content">Design</span></a>
     <ol class="toc">
      <li><a href="#usage-ex"><span class="secno">3.1</span> <span class="content">Usage</span></a>
     </ol>
    <li>
     <a href="#Wording"><span class="secno">4</span> <span class="content">Proposed Wording</span></a>
     <ol class="toc">
      <li>
       <a href="#add-try"><span class="secno">4.1</span> <span class="content"><code class="highlight"><c- n>try_schedule</c-></code> Additions</span></a>
       <ol class="toc">
        <li><a href="#wording-try-scheduler"><span class="secno">4.1.1</span> <span class="content">Try Scheduler Concept</span></a>
        <li><a href="#wording-try-schedule"><span class="secno">4.1.2</span> <span class="content"><code class="highlight"><c- n>execution</c-><c- o>::</c-><c- n>try_schedule</c-></code></span></a>
        <li><a href="#wording-run-loop"><span class="secno">4.1.3</span> <span class="content"><code class="highlight"><c- n>run_loop</c-></code></span></a>
       </ol>
      <li><a href="#mod-conqueue"><span class="secno">4.2</span> <span class="content">Modifications for Concurrent Queues</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="Revision"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#Revision"></a></h2>
   <p>This paper revises P3669R1 - 2025-05-17 as follows:</p>
   <ul>
    <li data-md>
     <p>added more rationale</p>
    <li data-md>
     <p>renamed <code class="highlight"><c- n>concurrent_scheduler</c-></code> to <code class="highlight"><c- n>try_scheduler</c-></code> as a placeholder
based on SG1 feedback</p>
    <li data-md>
     <p>added usage example</p>
   </ul>
   <p>P3669R1 revises P3669R0 - 2025-04-14 as follows:</p>
   <ul>
    <li data-md>
     <p>drops <em>concurrent-op-state</em> and introduces <code class="highlight"><c- n>try_schedule</c-></code></p>
   </ul>
   <h2 class="heading settled" data-level="2" id="Introduction"><span class="secno">2. </span><span class="content">Introduction</span><a class="self-link" href="#Introduction"></a></h2>
   <p>Some execution environments want to make sure that specific operations are
non-blocking.  These can not signal an event using the facilities of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>execution</c-></code> as
currently specified as it doesn’t provide operations
that are guaranteed to be non-blocking.</p>
   <p>E.g. this applies to signal handlers for asynchronous signals.
A framework that claims to support concurrent and asynchronous execution
but doesn’t allow input from asynchronous signal handlers is just broken.</p>
   <p>This became apparent in the proposal for concurrent queues <a data-link-type="biblio" href="https://wg21.link/P0260"><cite>C++ Concurrent Queues (P0260)</cite></a>,
which has support for non-blocking usage, but the problem is far bigger
than the concurrent queue issue.
There are many events that are notified by some kind of signal
(e.g.  in embedded systems essentially all incoming data is notified
using an interrupt, which is a signal in the C++ sense)
that you want to consume using S/R mechanisms.</p>
   <p>The non-blocking operations of concurrent queues got a very weird interface
when interfacing with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>execution</c-></code> due to the lack of non-blocking
signalling to an async waiter.  This interface can be fixed as soon as
there exist a signal-safe mechanism to notify S/R.</p>
   <p>This proposal is completely independent from <a data-link-type="biblio" href="https://wg21.link/P0260"><cite>C++ Concurrent Queues (P0260)</cite></a> as the problem
of signalling an event in a non-blocking way is not specific
to concurrent queues.</p>
   <h2 class="heading settled" data-level="3" id="Design"><span class="secno">3. </span><span class="content">Design</span><a class="self-link" href="#Design"></a></h2>
   <p>In <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>execution</c-></code> the basic operation to signal an event is to schedule
a continuation.
In practice this generally means that a <code class="highlight"><c- n>start</c-></code> operation is called
on an operation state that comes from a sender provided by a scheduler.</p>
   <p>The initial revision of this paper proposed to add a <code class="highlight"><c- b>bool</c-> <c- n>try_start</c-><c- p>()</c-></code> operation to the
operation states that come from a scheduler
and require it to be non-blocking (and to return <code class="highlight">false</code> if it would block).</p>
   <p>After feedback from several people this initial design was dropped.
Instead a new CPO <code class="highlight"><c- n>try_schedule</c-><c- p>()</c-></code> is proposed.</p>
   <p>If a <code class="highlight"><c- n>start</c-><c- p>()</c-></code> on the operation state of a sender returned by <code class="highlight"><c- n>try_schedule</c-><c- p>()</c-></code> would block (e.g. for inserting the operation
into a work queue protected by a mutex),
it immediately calls <code class="highlight"><c- n>set_error</c-><c- p>(</c-><c- n>would_block_t</c-><c- p>())</c-></code> on the connected receiver.</p>
   <p>Unfortunately it is not possible for all kind of schedulers to provide <code class="highlight"><c- n>try_schedule</c-><c- p>()</c-></code>.
Schedulers that are not backed by by an execution context that maintains
a work queue but run the operation right away are an example.
Schedulers that enqueue the work on a different system are another example.</p>
   <p>For this reason, this paper proposes a new concept <code class="highlight"><c- n>try_scheduler</c-></code>. <code class="highlight"><c- n>try_scheduler</c-></code> requires the <code class="highlight"><c- n>try_schedule</c-></code> operation.</p>
   <p>While this proposal is generally independent from <a data-link-type="biblio" href="https://wg21.link/P0260"><cite>C++ Concurrent Queues (P0260)</cite></a>,
we still would like to fix the weird interface in that proposal if possible.
For this the proposed wording contains a part that modifies the wording
for the queue concepts.
The idea here is to make it ill-formed if an async operation is called
for a queue that also implements <code class="highlight"><c- n>concurrent</c-><c- o>-</c-><c- n>queue</c-></code> and its scheduler
does not implement <code class="highlight"><c- n>try_scheduler</c-></code>.</p>
   <h3 class="heading settled" data-level="3.1" id="usage-ex"><span class="secno">3.1. </span><span class="content">Usage</span><a class="self-link" href="#usage-ex"></a></h3>
   <p>Here’s an extract from the implementation of <code class="highlight"><c- n>bounded_queue</c-></code> that
shows how <code class="highlight"><c- n>try_schedule</c-></code> is used there.</p>
   <p><code class="highlight"><c- n>RetrieveOp</c-></code> represents the operation of an <code class="highlight"><c- n>async_pop</c-></code>. <code class="highlight"><c- n>scheduleTryContinuation</c-></code> is called from <code class="highlight"><c- n>try_push</c-></code> if the queue
is currently empty.</p>
<pre class="language-c++ highlight"><c- c1>// helper class to intercept the would_block from try_schedule</c->
<c- n>template</c-> <c- o>&lt;</c-><c- n>execution</c-><c- o>::</c-><c- n>receiver</c-> <c- n>InnerRecv</c-><c- o>></c->
<c- k>struct</c-> <c- nc>PopContReceiver</c->
<c- p>{</c->
  <c- b>void</c-> <c- nf>set_error</c-><c- p>(</c-><c- n>execution</c-><c- o>::</c-><c- n>would_block_t</c-><c- p>)</c-> <c- n>noexcept</c->
  <c- p>{</c->
    <c- n>op</c-><c- o>-></c-><c- n>valid</c-> <c- o>=</c-> false<c- p>;</c->
  <c- p>}</c->

  <c- c1>// all other completions are simply forwarded to r</c->

  <c- n>InnerRecv</c-> <c- o>*</c-><c- n>r</c-><c- p>;</c->
  <c- n>RetrieveOpBase</c-> <c- o>*</c-><c- n>op</c-><c- p>;</c->
<c- p>};</c->

<c- n>template</c-> <c- o>&lt;</c-><c- n>execution</c-><c- o>::</c-><c- n>receiver</c-> <c- n>Recv</c-><c- o>></c->
<c- k>struct</c-> <c- nc>RetrieveOp</c-> <c- o>:</c-> <c- n>RetrieveOpBase</c->
<c- p>{</c->
  <c- n>TryContOpT</c-> <c- n>tryContOp</c-><c- p>;</c->
  <c- n>Recv</c-> <c- n>r</c-><c- p>;</c->
  <c- n>bounded_queue</c-> <c- o>*</c-><c- n>q</c-><c- p>;</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>execution</c-><c- o>::</c-><c- n>receiver</c-> <c- n>Rc</c-><c- o>></c->
  <c- n>RetrieveOp</c-><c- p>(</c-><c- n>Rc</c-> <c- o>&amp;&amp;</c-><c- n>r_</c-><c- p>,</c-> <c- n>bounded_queue</c-> <c- o>*</c-><c- n>q_</c-><c- p>)</c->
    <c- o>:</c-> <c- n>r</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Rc</c-><c- o>></c-><c- p>(</c-><c- n>r_</c-><c- p>))</c->
    <c- p>,</c-> <c- n>tryContOp</c-><c- p>(</c-><c- n>getTryContOp</c-><c- o>&lt;</c-><c- n>Recv</c-><c- o>></c-><c- p>(</c-><c- n>this</c-><c- p>))</c->
    <c- p>,</c-> <c- n>q</c-><c- p>(</c-><c- n>q_</c-><c- p>)</c->
  <c- p>{}</c->

  <c- c1>// this is a helper to get a continuation op with a try scheduler</c->
  <c- n>template</c-> <c- o>&lt;</c-><c- n>execution</c-><c- o>::</c-><c- n>receiver</c-> <c- n>R</c-><c- o>></c->
    <c- n>requires</c-> <c- n>HasTryScheduler</c-><c- o>&lt;</c-><c- n>R</c-><c- o>></c->
  <c- n>constexpr</c-> <c- k>auto</c-> <c- n>getTryContOp</c-><c- p>(</c-><c- n>RetrieveOpBase</c-> <c- o>*</c-><c- n>self</c-><c- p>)</c-> <c- n>noexcept</c->
  <c- p>{</c->
    <c- n>this</c-><c- o>-></c-><c- n>valid</c-> <c- o>=</c-> true<c- p>;</c->
    <c- k>auto</c-> <c- n>sch</c-> <c- o>=</c-> <c- n>execution</c-><c- o>::</c-><c- n>get_scheduler</c-><c- p>(</c-><c- n>execution</c-><c- o>::</c-><c- n>get_env</c-><c- p>(</c-><c- n>r</c-><c- p>));</c->
    <c- k>return</c-> <c- n>execution</c-><c- o>::</c-><c- n>connect</c-><c- p>(</c-><c- n>execution</c-><c- o>::</c-><c- n>try_schedule</c-><c- p>(</c-><c- n>sch</c-><c- p>),</c->
                              <c- n>PopContReceiver</c-><c- o>&lt;</c-><c- n>Recv</c-><c- o>></c-><c- p>(</c-><c- o>&amp;</c-><c- n>r</c-><c- p>,</c-> <c- n>self</c-><c- p>));</c->
  <c- p>}</c->

  <c- b>bool</c-> <c- n>scheduleTryContinuation</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- o>&amp;&amp;</c-><c- n>v</c-><c- p>)</c-> <c- n>override</c->
  <c- p>{</c->
    <c- n>tryContOp</c-><c- p>.</c-><c- n>start</c-><c- p>();</c->      <c- c1>// start on try scheduler</c->

    <c- k>if</c-> <c- p>(</c-><c- n>not</c-> <c- n>this</c-><c- o>-></c-><c- n>valid</c-><c- p>)</c->    <c- c1>// scheduling failed</c->
    <c- p>{</c->
      <c- n>tryContOp</c-> <c- o>=</c-> <c- n>getTryContOp</c-><c- p>(</c-><c- n>this</c-><c- p>);</c-> <c- c1>// reset helper op; we keep this operation</c->
      <c- k>return</c-> false<c- p>;</c->
    <c- p>}</c->

    <c- c1>// we got the continuation scheduled, drop it from our list</c->
    <c- o>--</c-><c- p>(</c-><c- n>q</c-><c- o>-></c-><c- n>asyncPopWaitCnt</c-><c- p>);</c->
    <c- n>q</c-><c- o>-></c-><c- n>asyncPoppers</c-><c- p>.</c-><c- n>pop_front</c-><c- p>();</c->

    <c- n>this</c-><c- o>-></c-><c- n>val</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>v</c-><c- p>);</c->  <c- c1>// only move the value on success</c->
    <c- k>return</c-> true<c- p>;</c->
  <c- p>};</c->
<c- p>};</c->
</pre>
   <h2 class="heading settled" data-level="4" id="Wording"><span class="secno">4. </span><span class="content">Proposed Wording</span><a class="self-link" href="#Wording"></a></h2>
   <p>(Sorry, the wording is still formally horribly incomplete, but I think
substantially it’s fairly complete.)</p>
   <h3 class="heading settled" data-level="4.1" id="add-try"><span class="secno">4.1. </span><span class="content"><code class="highlight"><c- n>try_schedule</c-></code> Additions</span><a class="self-link" href="#add-try"></a></h3>
   <p>Add to synopsis in namespace <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>execution</c-></code>:</p>
<pre class="highlight"><c- k>struct</c-> <c- nc>try_scheduler_t</c-> <c- p>{};</c->

<c- k>struct</c-> <c- nc>would_block_t</c-> <c- p>{};</c->
</pre>
   <h4 class="heading settled" data-level="4.1.1" id="wording-try-scheduler"><span class="secno">4.1.1. </span><span class="content">Try Scheduler Concept</span><a class="self-link" href="#wording-try-scheduler"></a></h4>
   <p>Add to <strong>Schedulers</strong> [exec.sched]:</p>
   <ol>
    <li data-md>
     <p>The concept <code class="highlight"><c- n>try_scheduler</c-></code> defines the requirements of a
scheduler type ([async.ops]) that provides <code class="highlight"><c- n>try_schedule</c-><c- p>()</c-></code>.</p>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-><c- o>::</c-><c- nn>execution</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>S</c-><c- p>,</c-> <c- n>receiver</c-> <c- n>R</c-><c- o>></c->
    <c- k>concept</c-> <c- nc>try_scheduler</c-> <c- o>=</c->
      <c- n>std</c-><c- o>::</c-><c- n>derived_from</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>std</c-><c- o>::</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>_Scheduler</c-><c- o>>::</c-><c- n>try_scheduler_concept</c-><c- p>,</c-> <c- n>try_scheduler_t</c-><c- o>></c-> <c- o>&amp;&amp;</c->
      <c- n>scheduler</c-><c- o>&lt;</c-><c- n>S</c-><c- o>></c-> <c- o>&amp;&amp;</c->
      <c- k>requires</c-> <c- p>(</c-><c- n>S</c-><c- o>&amp;&amp;</c-> <c- n>s</c-><c- p>)</c-> <c- p>{</c->
        <c- p>{</c-> <c- n>try_schedule</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>S</c-><c- o>></c-><c- p>(</c-><c- n>s</c-><c- p>))</c-> <c- p>}</c-> <c- o>-></c-> <c- n>sender</c-><c- p>;</c->
    <c- p>}</c->
<c- p>}</c->
</pre>
   </ol>
   <h4 class="heading settled" data-level="4.1.2" id="wording-try-schedule"><span class="secno">4.1.2. </span><span class="content"><code class="highlight"><c- n>execution</c-><c- o>::</c-><c- n>try_schedule</c-></code></span><a class="self-link" href="#wording-try-schedule"></a></h4>
   <p>Add to <strong>Sender factories</strong> [exec.factories]:</p>
   <ol>
    <li data-md>
     <p><code class="highlight"><c- n>try_schedule</c-></code> obtains a schedule sender ([exec.async.ops])
from a scheduler to potentially start an asynchronous operation
without blocking ([defns.block]).</p>
    <li data-md>
     <p>The name <code class="highlight"><c- n>try_schedule</c-></code> denotes a customization point object.
For a subexpression <code class="highlight"><c- n>sch</c-></code>, the expression <code class="highlight"><c- n>try_schedule</c-><c- p>(</c-><c- n>sch</c-><c- p>)</c-></code> is expression-equivalent to <code class="highlight"><c- n>sch</c-><c- p>.</c-><c- n>try_schedule</c-><c- p>()</c-></code>.</p>
    <li data-md>
     <p><em>Mandates:</em> The type of <code class="highlight"><c- n>sch</c-><c- p>.</c-><c- n>try_schedule</c-><c- p>()</c-></code> satisfies <code class="highlight"><c- n>sender</c-></code>.</p>
    <li data-md>
     <p>If the expression <code class="highlight"><c- n>get_completion_scheduler</c-><c- o>&lt;</c-><c- n>set_value_t</c-><c- o>></c-><c- p>(</c-><c- n>get_env</c-><c- p>(</c-><c- n>sch</c-><c- p>.</c-><c- n>try_schedule</c-><c- p>()))</c-> <c- o>==</c-> <c- n>sch</c-></code> is ill-formed or evaluates to <code class="highlight">false</code>,
the behavior of calling <code class="highlight"><c- n>try_schedule</c-><c- p>(</c-><c- n>sch</c-><c- p>)</c-></code> is undefined.</p>
    <li data-md>
     <p>The expression <code class="highlight"><c- n>sch</c-><c- p>.</c-><c- n>try_schedule</c-><c- p>()</c-></code> has the following semantics:</p>
     <ol>
      <li data-md>
       <p>Let <code class="highlight"><c- n>w</c-></code> be a sender object returned by the expression,
 and <code class="highlight"><c- n>op</c-></code> be an operation state obtained from connecting <code class="highlight"><c- n>w</c-></code> to a receiver <code class="highlight"><c- n>r</c-></code>.</p>
      <li data-md>
       <p><em>Returns:</em> A sender object <code class="highlight"><c- n>w</c-></code> that behaves as follows:</p>
       <ol>
        <li data-md>
         <p><code class="highlight"><c- n>op</c-><c- p>.</c-><c- n>start</c-><c- p>()</c-></code> starts ([async.ops]) the asynchronous operation
 associated with <code class="highlight"><c- n>op</c-></code>, if it can be achieved without blocking.</p>
        <li data-md>
         <p>If starting the asynchronous operation would block,
 it immediately calls <code class="highlight"><c- n>set_error</c-><c- p>(</c-><c- n>r</c-><c- p>,</c-> <c- n>would_block_t</c-><c- p>())</c-></code>.</p>
       </ol>
      <li data-md>
       <p><code class="highlight"><c- n>sch</c-><c- p>.</c-><c- n>try_schedule</c-><c- p>()</c-></code> is signal-safe.</p>
     </ol>
   </ol>
   <h4 class="heading settled" data-level="4.1.3" id="wording-run-loop"><span class="secno">4.1.3. </span><span class="content"><code class="highlight"><c- n>run_loop</c-></code></span><a class="self-link" href="#wording-run-loop"></a></h4>
   <ol>
    <li data-md>
     <p><em>run-loop-scheduler</em> models <code class="highlight"><c- n>try_scheduler</c-></code>.</p>
   </ol>
   <h3 class="heading settled" data-level="4.2" id="mod-conqueue"><span class="secno">4.2. </span><span class="content">Modifications for Concurrent Queues</span><a class="self-link" href="#mod-conqueue"></a></h3>
   <p>Remove <code class="highlight"><c- n>conqueue_errc</c-><c- o>::</c-><c- n>busy_async</c-></code> from the enumeration and the respective
return statements from <code class="highlight"><c- n>try_emplace</c-></code>, <code class="highlight"><c- n>try_push</c-></code> and <code class="highlight"><c- n>try_pop</c-></code> in the <em>concurrent-queue</em> concept.</p>
   <p>In the <em>async-concurrent-queue</em> concept add another bullet to the senders
returned by <code class="highlight"><c- n>async_emplace</c-></code> and <code class="highlight"><c- n>async_pop</c-></code> after bullet 6:</p>
   <ol start="7">
    <li data-md>
     <p>If the scheduler of <code class="highlight"><c- n>r</c-></code> does not model <code class="highlight"><c- n>try_scheduler</c-></code>,
the program is ill-formed.</p>
   </ol>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-p0260">[P0260]
   <dd><a href="https://wg21.link/P0260"><cite>C++ Concurrent Queues (P0260)</cite></a>. URL: <a href="https://wg21.link/P0260">https://wg21.link/P0260</a>
  </dl>