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  <title>p1044R0: std::async() in an Executors World</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

  /* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
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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">p1044R0<br>std::async() in an Executors World</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-05-04">4 May 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:trodgers@redhat.com">Thomas Rodgers</a> (<span class="p-org org">RedHat</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:hedwards@nvidia.com">H Carter Edwards</a> (<span class="p-org org">Nvidia</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Mark Hoemmen</span> (<span class="p-org org">Sandia National Laboratories</span>)
     <dt>Audience:
     <dd>SG1, LEWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>We show an evolutionary path for code written to std::async() to leverage Executors</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="#bg"><span class="secno">1</span> <span class="content">Background</span></a>
    <li><a href="#ae"><span class="secno">2</span> <span class="content">An Execution Context for async()</span></a>
    <li><a href="#pw"><span class="secno">3</span> <span class="content">Proposed wording for Standard Async Execution Context and Executor</span></a>
    <li><a href="#ta"><span class="secno">4</span> <span class="content">Transliteration of async()</span></a>
    <li><a href="#oq"><span class="secno">5</span> <span class="content">Open Questions</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="bg"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#bg"></a></h2>
   <p>C++11 introduced async() along with several other basic concurrency primitives. Many regard
async() as fundamentally broken <a data-link-type="biblio" href="#biblio-n3637">[n3637]</a> and in need of deprecation <a data-link-type="biblio" href="#biblio-n3777">[n3777]</a>. Following much discussion
and a passionate plea <a data-link-type="biblio" href="#biblio-n3780">[n3780]</a>, the effort to deprecate async() prior to an executors-based replacement
was abandoned. The guidance from SG1 to the authors of the various Executors proposals, since at least the
2014 Urbana-Champaign SG1 discussion of <a data-link-type="biblio" href="#biblio-n4242">[n4242]</a> (see: <a data-link-type="biblio" href="#biblio-sg1-mins-n4242">[SG1-Mins-n4242]</a>), has been that any Executors
proposal must clearly address the deficiencies of async().</p>
   <p>At the Fall 2017 Albuquerque meeting, a companion paper <a data-link-type="biblio" href="#biblio-p0737">[p0737]</a> to the Executors proposal was presented
that more fully defines the notion of an execution_context. The paper includes a proposal for a formal
definition of an "executor for async()" and its supporting library machinery. This offers an
evolutionary path away from the undesirable, but possibly widely used at this point, semantics
of the current async().</p>
   <p>At the Winter 2018 Jacksonville meeting, there was clear direction to the authors of the Executors
proposal <a data-link-type="biblio" href="#biblio-p0443">[p0443]</a> to provide library usage examples (see: <a data-link-type="biblio" href="#biblio-lewg-mins-p0443">[LEWG-Mins-p0443]</a>). 
This paper intends to address this request by drawing on <a data-link-type="biblio" href="#biblio-p0737">[p0737]</a> as it specifically relates to async().</p>
   <p>The current wording on the behavior of <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> is as follows -</p>
   <blockquote>
    <p>— If launch::async is set in policy, calls <code class="highlight"><span class="n">INVOKE</span><span class="p">(</span><span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">)),</span> <span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">))...)</span></code> (23.14.3, 33.3.2.2) as if in a new thread of execution represented by a thread object with the calls to <code class="highlight"><span class="n">DECAY_COPY</span></code> being
evaluated in the thread that called <code class="highlight"><span class="n">async</span></code>. Any return value is stored as the result in the shared state. Any
exception propagated from the execution of <code class="highlight"><span class="n">INVOKE</span><span class="p">(</span><span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">)),</span> <span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">))...)</span></code> is stored as the exceptional result in the shared state. The thread object is stored in the shared state and affects the behavior
of any asynchronous return objects that reference that state.</p>
    <p>— If launch::deferred is set in policy, stores <code class="highlight"><span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">))</span> <span class="n">and</span> <span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">))...)</span></code> in the shared state. These copies of f and args constitute a deferred function. Invocation of the deferred function evaluates <code class="highlight"><span class="n">INVOKE</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">g</span><span class="p">),</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">xyz</span><span class="p">))</span></code> where g is the stored value of <code class="highlight"><span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">))</span></code> and xyz is
the stored copy of <code class="highlight"><span class="n">DECAY_COPY</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">))...)</span></code>. Any return value is stored as the result in the shared state.
Any exception propagated from the execution of the deferred function is stored as the exceptional result in the shared state.
The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (33.6.5)
on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called
the waiting function. Once evaluation of <code class="highlight"><span class="n">INVOKE</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">g</span><span class="p">),</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">xyz</span><span class="p">))</span></code> begins, the function is no longer considered
deferred. [ Note: If this policy is specified together with other policies, such as when using a policy value of launch::async |
launch::deferred, implementations should defer invocation or the selection of the policy when no more concurrency can be effectively
exploited. — end note ]</p>
    <p>— If no value is set in the launch policy, or a value is set that is neither specified in this document
nor by the implementation, the behavior is undefined.</p>
   </blockquote>
   <p>The language run-time currently provides <em>hidden</em> executor types which fulfills the requirements of
these launch policies. In particular, a launch policy of <code class="highlight"><span class="n">launch</span><span class="o">::</span><span class="n">async</span></code> requires that the run-time
provide a new single thread executor to evaluate the supplied Callable. The policy for <code class="highlight"><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span></code> most resembles the concept of an <em>inline executor</em>, which only evaluates the supplied Callable when the
returned <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">future</span></code> is awaited.</p>
   <p><a data-link-type="biblio" href="#biblio-p0737">[p0737]</a> seeks the formal introduction of an <em>execution context</em> concept,
which <a data-link-type="biblio" href="#biblio-p0443">[p0443]</a> currently leaves unspecified. <a data-link-type="biblio" href="#biblio-p0737">[p0737]</a> defines the ExecutionContext as follows -</p>
   <blockquote>
     A concurrency and parallelism <strong>execution context</strong> manages a set of <strong>execution agents</strong> on a set of <strong>execution resources</strong> of a given <strong>execution architecture</strong>. 
    <p>These execution agents execute work, implemented by a callable,
that is submitted to the execution context by an <strong>executor</strong>.</p>
    <p>One or more types of executors may submit work to the same
execution context.
Work submitted to an execution context is <strong>incomplete</strong> until
(1) it is invoked and exits execution by return or exception or 
(2) its submission for execution is canceled.</p>
    <p class="note" role="note"><span>Note:</span> The <em>execution context</em> terminology used here
    and in the Networking TS <a data-link-type="biblio" href="#biblio-n4656">[n4656]</a> deviate from the 
    traditional <em>context of execution</em> usage that refers
    to the state of a single executing callable; <em>e.g.</em>,
    program counter, registers, stack frame.</p>
   </blockquote>
   <p>This ExecutionContext concept is specified as follows -</p>
<pre class="highlight"><span class="k">class</span> <span class="nc">ExecutionContext</span> <span class="cm">/* exposition only */</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>

  <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">ExecutionContextProperty</span><span class="o">></span>
    <span class="cm">/* exposition only */</span> <span class="n">detail</span><span class="o">::</span><span class="n">query_t</span><span class="o">&lt;</span> <span class="n">ExecutionContext</span> <span class="p">,</span> <span class="n">ExecutionContextProperty</span> <span class="o">></span>
  <span class="n">query</span><span class="p">(</span><span class="n">ExecutionContextProperty</span> <span class="n">p</span><span class="p">)</span> <span class="k">const</span> <span class="p">;</span>

  <span class="o">~</span><span class="n">ExecutionContext</span><span class="p">();</span>

  <span class="c1">// Not copyable or moveable</span>
<span class="c1"></span>  <span class="n">ExecutionContext</span><span class="p">(</span> <span class="n">ExecutionContext</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>
  <span class="n">ExecutionContext</span><span class="p">(</span> <span class="n">ExecutionContext</span> <span class="o">&amp;&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>
  <span class="n">ExecutionContext</span> <span class="o">&amp;</span> <span class="k">operator</span> <span class="o">=</span> <span class="p">(</span> <span class="n">ExecutionContext</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>
  <span class="n">ExecutionContext</span> <span class="o">&amp;</span> <span class="k">operator</span> <span class="o">=</span> <span class="p">(</span> <span class="n">ExecutionContext</span> <span class="o">&amp;&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>

  <span class="c1">// Execution resource</span>
<span class="c1"></span>  <span class="k">using</span> <span class="n">execution_resource_t</span> <span class="o">=</span> <span class="cm">/* implementation defined */</span> <span class="p">;</span>

  <span class="n">execution_resource_t</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="n">execution_resource</span><span class="p">()</span> <span class="k">const</span> <span class="k">noexcept</span> <span class="p">;</span>

  <span class="c1">// Executor generator</span>
<span class="c1"></span>  <span class="k">template</span><span class="o">&lt;</span> <span class="k">class</span> ... <span class="nc">ExecutorProperties</span> <span class="o">></span>
    <span class="cm">/* exposition only */</span> <span class="n">detail</span><span class="o">::</span><span class="n">executor_t</span><span class="o">&lt;</span> <span class="n">ExecutionContext</span> <span class="p">,</span> <span class="n">ExecutorProperties</span><span class="p">...</span> <span class="o">></span>
  <span class="n">executor</span><span class="p">(</span> <span class="n">ExecutorProperties</span><span class="p">...</span> <span class="p">);</span>

  <span class="c1">// Waiting functions:</span>
<span class="c1"></span>  <span class="kt">void</span> <span class="nf">wait</span><span class="p">();</span>
  <span class="k">template</span><span class="o">&lt;</span> <span class="k">class</span> <span class="nc">Clock</span> <span class="p">,</span> <span class="k">class</span> <span class="nc">Duration</span> <span class="o">></span>
  <span class="kt">bool</span> <span class="n">wait_until</span><span class="p">(</span> <span class="n">chrono</span><span class="o">::</span><span class="n">time_point</span><span class="o">&lt;</span><span class="n">Clock</span><span class="p">,</span><span class="n">Duration</span><span class="o">></span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">);</span>
  <span class="k">template</span><span class="o">&lt;</span> <span class="k">class</span> <span class="nc">Rep</span> <span class="p">,</span> <span class="k">class</span> <span class="nc">Period</span> <span class="o">></span>
  <span class="kt">bool</span> <span class="n">wait_for</span><span class="p">(</span> <span class="n">chrono</span><span class="o">::</span><span class="n">duration</span><span class="o">&lt;</span><span class="n">Rep</span><span class="p">,</span><span class="n">Period</span><span class="o">></span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">);</span>
<span class="p">};</span>

<span class="kt">bool</span> <span class="k">operator</span> <span class="o">==</span> <span class="p">(</span> <span class="n">ExecutionContext</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">,</span> <span class="n">ExecutionContext</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">);</span>
<span class="kt">bool</span> <span class="k">operator</span> <span class="o">!=</span> <span class="p">(</span> <span class="n">ExecutionContext</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">,</span> <span class="n">ExecutionContext</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">);</span>

<span class="c1">// Execution context properties:</span>
<span class="c1"></span>
<span class="k">constexpr</span> <span class="k">struct</span> <span class="n">reject_on_destruction_t</span> <span class="p">{}</span> <span class="n">reject_on_destruction</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="k">struct</span> <span class="n">abandon_on_destruction_t</span> <span class="p">{}</span> <span class="n">abandon_on_destruction</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="k">struct</span> <span class="n">abort_on_destruction_t</span> <span class="p">{}</span> <span class="n">abort_on_destruction</span><span class="p">;</span>
<span class="k">constexpr</span> <span class="k">struct</span> <span class="n">wait_on_destruction_t</span> <span class="p">{}</span> <span class="n">wait_on_destruction</span><span class="p">;</span>
</pre>
   <h2 class="heading settled" data-level="2" id="ae"><span class="secno">2. </span><span class="content">An Execution Context for async()</span><a class="self-link" href="#ae"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0737">[p0737]</a> Proposes that there exists in the underlying runtime, a <em>hidden</em> execution context, into which <strong>33.6.9 Function template async</strong> launches asynchronous work, and proposes that this
execution context be exposed along with the <em>hidden</em> executors fulfilling <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> launch policies.
Exposing this execution context and these executor types enables current use of <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> to be
transliterated to the executor model as follows.</p>
<pre class="highlight"><span class="c1">// Equivalent without- and with-executor async statements without launch policy</span>
<span class="c1"></span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"anonymous way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">async_execution_context</span><span class="p">.</span><span class="n">executor</span><span class="p">()</span> <span class="p">,</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"executor way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>

<span class="c1">// Equivalent without- and with-executor async statements with launch policy</span>
<span class="c1"></span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span> <span class="p">,</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"anonymous way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">async_execution_context</span><span class="p">.</span><span class="n">executor</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span> <span class="p">)</span> <span class="p">,</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"executor way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>
</pre>
   <p>Asynchronous execution behavior is unchanged following such a well-defined transliteration.
The use of <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> is now prepared, at the developers' discretion, to replace the leading executor
argument with a different executor.</p>
   <h2 class="heading settled" data-level="3" id="pw"><span class="secno">3. </span><span class="content">Proposed wording for Standard Async Execution Context and Executor</span><a class="self-link" href="#pw"></a></h2>
   <p>The proposed wording is extracted from <a data-link-type="biblio" href="#biblio-p0737">[p0737]</a> with modifications
for a minimal proposal and to align with executors proposal <a data-link-type="biblio" href="#biblio-p0443">[p0443]</a>.</p>
<pre class="highlight"><span class="k">namespace</span> <span class="n">std</span> <span class="p">{</span>
<span class="k">namespace</span> <span class="n">experimental</span> <span class="p">{</span>
<span class="kr">inline</span> <span class="k">namespace</span> <span class="n">executors_v1</span> <span class="p">{</span>
<span class="k">namespace</span> <span class="n">execution</span> <span class="p">{</span>

  <span class="k">namespace</span> <span class="n">launch</span> <span class="p">{</span>
    <span class="k">struct</span> <span class="n">async_t</span> <span class="p">{</span>
      <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_requirable</span> <span class="o">=</span> true<span class="p">;</span>
      <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_preferable</span> <span class="o">=</span> true<span class="p">;</span>
    <span class="p">};</span>

    <span class="k">struct</span> <span class="n">deferred_t</span> <span class="p">{</span>
      <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_requirable</span> <span class="o">=</span> true<span class="p">;</span>
      <span class="k">static</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_preferable</span> <span class="o">=</span> true<span class="p">;</span>
    <span class="p">};</span>

    <span class="k">constexpr</span> <span class="n">async_t</span> <span class="n">async</span><span class="p">;</span> <span class="c1">// executor property</span>
<span class="c1"></span>    <span class="k">constexpr</span> <span class="n">deferred_t</span> <span class="n">deferred</span><span class="p">;</span> <span class="c1">// executor property</span>
<span class="c1"></span>  <span class="p">}</span> <span class="c1">// namespace launch</span>
<span class="c1"></span>
  <span class="k">class</span> <span class="nc">async_executor_t</span> <span class="p">;</span> <span class="c1">// implementation defined</span>
<span class="c1"></span>
  <span class="k">class</span> <span class="nc">async_execution_context_t</span> <span class="p">{</span>
  <span class="k">public</span><span class="o">:</span>

    <span class="c1">// Not copyable or moveable</span>
<span class="c1"></span>    <span class="n">async_execution_context_t</span><span class="p">(</span> <span class="n">async_execution_context_t</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>
    <span class="n">async_execution_context_t</span><span class="p">(</span> <span class="n">async_execution_context_t</span> <span class="o">&amp;&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>
    <span class="n">async_execution_context_t</span> <span class="o">&amp;</span> <span class="k">operator</span> <span class="o">=</span> <span class="p">(</span> <span class="n">async_execution_context_t</span> <span class="k">const</span> <span class="o">&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>
    <span class="n">async_execution_context_t</span> <span class="o">&amp;</span> <span class="k">operator</span> <span class="o">=</span> <span class="p">(</span> <span class="n">async_execution_context_t</span> <span class="o">&amp;&amp;</span> <span class="p">)</span> <span class="o">=</span> <span class="k">delete</span> <span class="p">;</span>

    <span class="c1">// Executor generator</span>
<span class="c1"></span>    <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> ... <span class="nc">ExecutorProperties</span><span class="o">></span>
      <span class="cm">/* exposition only */</span> <span class="n">detail</span><span class="o">::</span><span class="n">executor_t</span><span class="o">&lt;</span> <span class="n">ExecutionContext</span> <span class="p">,</span> <span class="n">ExecutorProperties</span><span class="p">...</span> <span class="o">></span>
    <span class="n">executor</span><span class="p">(</span> <span class="n">ExecutorProperties</span> <span class="p">...</span> <span class="n">p</span> <span class="p">);</span>

    <span class="c1">// Execution resource - deferred</span>
<span class="c1"></span>    <span class="c1">// Waiting - deferred</span>
<span class="c1"></span>  <span class="p">};</span>

  <span class="c1">// Execution context into which std::async launches work</span>
<span class="c1"></span>  <span class="k">extern</span> <span class="n">async_execution_context_t</span> <span class="n">async_execution_context</span><span class="p">;</span>

<span class="p">}</span> <span class="c1">// namespace execution</span>
<span class="c1"></span><span class="p">}</span> <span class="c1">// inline namespace executors_v1</span>
<span class="c1"></span><span class="p">}</span> <span class="c1">// namespace experimental</span>
<span class="c1"></span><span class="p">}</span> <span class="c1">// namespace std</span>
<span class="c1"></span>
<span class="k">namespace</span> <span class="n">std</span> <span class="p">{</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">Function</span> <span class="p">,</span> <span class="k">class</span> ... <span class="nc">Args</span><span class="o">></span>
  <span class="n">future</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">result_of</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">Function</span><span class="o">></span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">...)</span><span class="o">>></span>
  <span class="n">async</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">async_executor_t</span> <span class="n">exec</span> <span class="p">,</span> <span class="n">Function</span> <span class="o">&amp;&amp;</span> <span class="n">f</span> <span class="p">,</span> <span class="n">Args</span> <span class="o">&amp;&amp;</span> <span class="p">...</span> <span class="n">args</span><span class="p">);</span>
<span class="p">}</span> <span class="c1">// namespace std</span>
</pre>
<pre class="highlight"><span class="n">experimental</span><span class="o">::</span><span class="n">execute</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">async</span><span class="p">;</span>
<span class="n">experimental</span><span class="o">::</span><span class="n">execute</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span><span class="p">;</span>
</pre>
   <p>Executor properties isomorphic with <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">launch</span></code> launch policies of the same name.</p>
   <p><code class="highlight"><span class="k">class</span> <span class="nc">async_executor_t</span><span class="p">;</span></code></p>
   <p>An implementation-defined type satisfying <code class="highlight"><span class="n">TwoWayExecutor</span></code> requirements <a data-link-type="biblio" href="#biblio-p0443">[p0443]</a>.</p>
   <p><code class="highlight"><span class="k">class</span> <span class="nc">async_execution_context_t</span><span class="p">;</span></code></p>
   <p>An implementation-defined type partially satisfying execution context requirements <a data-link-type="biblio" href="#biblio-p0737">[p0737]</a>.
  Specification of execution context properties, construction, destruction, and wait functionality 
  is deferred.</p>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span> <span class="k">class</span> ... <span class="nc">ExecutorProperties</span> <span class="o">></span>
  <span class="cm">/* exposition only */</span> <span class="n">detail</span><span class="o">::</span><span class="n">executor_t</span><span class="o">&lt;</span> <span class="n">ExecutionContext</span> <span class="p">,</span> <span class="n">ExecutorProperties</span><span class="p">...</span> <span class="o">></span>
<span class="n">async_execution_context_t</span><span class="o">::</span><span class="n">executor</span><span class="p">(</span> <span class="n">ExecutorProperties</span> <span class="p">...</span> <span class="n">p</span> <span class="p">);</span>
</pre>
   <p>Returns:
  A <em>executor</em> with <code class="highlight"><span class="o">*</span><span class="k">this</span></code> <em>execution context</em> and execution properties <code class="highlight"><span class="n">p</span></code>.
  If <code class="highlight"><span class="n">p</span></code> is empty, is <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">async</span></code>, or is <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span></code> the <em>executor</em> type is <code class="highlight"><span class="n">async_executor_t</span></code>.</p>
   <p><code class="highlight"><span class="k">extern</span> <span class="n">async_execution_context_t</span> <span class="n">async_execution_context</span><span class="p">;</span></code></p>
   <p>Global execution context object enabling the equivalent invocation of callables
  through the with-async_executor_t <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> and without-executor <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code>.
  Guaranteed to be initialized during or before the first use.</p>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span> <span class="k">class</span> <span class="nc">Function</span> <span class="p">,</span> <span class="k">class</span> ... <span class="nc">Args</span> <span class="o">></span>
<span class="n">future</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">result_of</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">Function</span><span class="o">></span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">...)</span><span class="o">>></span>
<span class="n">async</span><span class="p">(</span> <span class="n">async_executor_t</span> <span class="n">exec</span> <span class="p">,</span> <span class="n">Function</span> <span class="o">&amp;&amp;</span> <span class="n">f</span> <span class="p">,</span> <span class="n">Args</span> <span class="o">&amp;&amp;</span> <span class="p">...</span> <span class="n">args</span> <span class="p">);</span>
</pre>
   <p>Effects:
  If <code class="highlight"><span class="n">exec</span></code> has an <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">launch</span></code> property
  then equivalent to invoking <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span></code> <em>policy</em> <code class="highlight"><span class="p">,</span> <span class="n">f</span> <span class="p">,</span> <span class="n">args</span><span class="p">...</span> <span class="p">);</span></code> with <em>policy</em> of the same name;
  otherwise equivalent to invoking <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="n">f</span> <span class="p">,</span> <span class="n">args</span><span class="p">...</span> <span class="p">);</span></code> Equivalency is symmetric with respect to the current, non-executor <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> functions.</p>
   <h2 class="heading settled" data-level="4" id="ta"><span class="secno">4. </span><span class="content">Transliteration of async()</span><a class="self-link" href="#ta"></a></h2>
   <p>With this proposal, transliteration of current <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> usage to the executor model is as follows.</p>
<pre class="highlight"><span class="c1">// Equivalent without- and with-executor async statements without launch policy</span>
<span class="c1"></span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"anonymous way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">async_execution_context</span><span class="p">.</span><span class="n">executor</span><span class="p">()</span> <span class="p">,</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"executor way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>

<span class="c1">// Equivalent without- and with-executor async statements with launch policy</span>
<span class="c1"></span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span> <span class="p">,</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"anonymous way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>
<span class="k">auto</span> <span class="n">f</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">async_execution_context</span><span class="p">.</span><span class="n">executor</span><span class="p">(</span> <span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span> <span class="p">)</span> <span class="p">,</span> <span class="p">[]{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"executor way</span><span class="se">\n</span><span class="s">"</span><span class="p">}</span> <span class="p">);</span>
</pre>
   <h2 class="heading settled" data-level="5" id="oq"><span class="secno">5. </span><span class="content">Open Questions</span><a class="self-link" href="#oq"></a></h2>
   <ol>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-p0737">[p0737]</a> does not propose a mechanism to migrate the executor aware <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">async</span></code> template from <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execute</span><span class="o">::</span><span class="n">async_executor_t</span></code> to other executors types.</p>
    <li data-md="">
     <p><strong>33.6.2</strong> currently specifies <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">async</span></code> and <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span></code> as:</p>
<pre class="highlight"><span class="k">enum</span> <span class="k">class</span> <span class="nc">launch</span> <span class="o">:</span> <span class="n">unspecified</span> <span class="p">{</span>
  <span class="n">async</span> <span class="o">=</span> <span class="n">unspecified</span> <span class="p">,</span>
  <span class="n">deferred</span> <span class="o">=</span> <span class="n">unspecified</span> <span class="p">,</span>
  <span class="n">implementation</span><span class="o">-</span><span class="n">defined</span>
<span class="p">};</span>
</pre>
     <p><strong>33.6.2</strong> further constrains <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">launch</span></code> to be a <em>bitmask</em> with the following note -
 [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched
   by async() applicable to a corresponding subset of available launch policies. Implementations can extend
   the behavior of the first overload of async() by adding their extensions to the launch policy under the “as if”
   rule.
   -- end note]</p>
     <p>In order to preserve backward compatibility with existing usage of <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">launch</span></code> this would imply that the <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execute</span><span class="o">::</span><span class="n">launch</span></code> property types would also need to behave as-if they were a bitmask. 
 This implies that <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">async</span> <span class="o">|</span> <span class="n">std</span><span class="o">::</span><span class="n">experimental</span><span class="o">::</span><span class="n">execution</span><span class="o">::</span><span class="n">launch</span><span class="o">::</span><span class="n">deferred</span></code> return another viable property type that represents the union of these two launch properties.
 Another option is for <a data-link-type="biblio" href="#biblio-p0443">[p0443]</a> to be revised to allow executor properties to be implemented as <code class="highlight"><span class="k">enum</span> <span class="k">class</span></code> or <code class="highlight"><span class="k">constexpr</span></code> values potentially of the same type.</p>
    <li data-md="">
     <p><a data-link-type="biblio" href="#biblio-n4406">[n4406]</a> and <a data-link-type="biblio" href="#biblio-p0761">[p0761]</a> suggest a <code class="highlight"><span class="p">.</span><span class="n">on</span><span class="p">()</span></code> member taking an executor to allow parallel algorithm launch policies to
 compose with executors. Do the policies of <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">launch</span></code> differ enough, conceptually, from the launch policies
 specified in <strong>23.19.2</strong> to warrant a different mechanism for composition with executors?</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-lewg-mins-p0443">[LEWG-Mins-p0443]
   <dd>Minutes of LEWG discussion of [p0443] at 2018 Jacksonville WG21 meeting. 
   <dt id="biblio-n3637">[N3637]
   <dd>Herb Sutter; Chandler Carruth; Niklas Gustafsson. <a href="https://wg21.link/n3637">async and ~future (Revision 3)</a>. April 17, 2013. URL: <a href="https://wg21.link/n3637">https://wg21.link/n3637</a>
   <dt id="biblio-n3777">[N3777]
   <dd>Herb Sutter. <a href="https://wg21.link/n3777">Wording for deprecating async</a>. September 23, 2013. URL: <a href="https://wg21.link/n3777">https://wg21.link/n3777</a>
   <dt id="biblio-n3780">[N3780]
   <dd>Nicolai Josuttis. <a href="https://wg21.link/n3780">Why Deprecating async() is the Worst of all Options</a>. September 26, 2013. URL: <a href="https://wg21.link/n3780">https://wg21.link/n3780</a>
   <dt id="biblio-n4242">[N4242]
   <dd>Chris Kohlhoff &lt;chris@kohlhoff.com>. <a href="https://wg21.link/n4242">Executors and Asynchronous Operations</a>. October 13, 2014. URL: <a href="https://wg21.link/n4242">https://wg21.link/n4242</a>
   <dt id="biblio-n4406">[N4406]
   <dd>Jared Hoberock; Michael Garland; Olivier Girioux. <a href="https://wg21.link/n4406">Parallel Algorithms Need Executors</a>. April 10, 2015. URL: <a href="https://wg21.link/n4406">https://wg21.link/n4406</a>
   <dt id="biblio-n4656">[N4656]
   <dd><a href="https://wg21.link/n4656">Working Draft, C++ Extensions for Networking</a>. 2017-03-17. URL: <a href="https://wg21.link/n4656">https://wg21.link/n4656</a>
   <dt id="biblio-p0443">[P0443]
   <dd>Jared Hoberock; et al. <a href="https://wg21.link/p0443">A Unified Executors Proposal for C++</a>. February 12, 2018. URL: <a href="https://wg21.link/p0443">https://wg21.link/p0443</a>
   <dt id="biblio-p0737">[P0737]
   <dd>H. Carter Edwards; et al. <a href="https://wg21.link/p0737">Execution Context of Execution Agents</a>. URL: <a href="https://wg21.link/p0737">https://wg21.link/p0737</a>
   <dt id="biblio-p0761">[P0761]
   <dd>Jared Hoberock; et al. <a href="https://wg21.link/p0761">Executors Design Document</a>. February 12, 2018. URL: <a href="https://wg21.link/p0761">https://wg21.link/p0761</a>
   <dt id="biblio-sg1-mins-n4242">[SG1-Mins-n4242]
   <dd>Minutes of SG1 discussion of [n4242] at 2014 Urbana-Champaign WG21 meeting. 
  </dl>