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  <title>P0451r0: Future-Proofing Parallel Algorithms Exception Handling</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 0;
	}
	[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: normal;
	}
	dfn var {
		font-style: normal;
	}

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

	del { color: red;  background-color: #FFA0A0; text-decoration: line-through; }
	ins { color: #080; background-color: #A0FFA0; text-decoration: none;         }

	del code, del pre, code del, pre del { background-color: #FFA0A0 !important; text-decoration: line-through; }
	ins code, ins pre, code ins, pre ins { background-color: #A0FFA0 !important; text-decoration: none;         }

/** 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;
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   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0451r0<br>Future-Proofing Parallel Algorithms Exception Handling</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2016-10-14">14 October 2016</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0451r0">http://wg21.link/P0451r0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:balelbach@lbl.gov">Bryce Adelstein Lelbach</a> (<span class="p-org org">Lawrence Berkeley National Laboratory</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:alisdairm@me.com">Alisdair Meredith</a> (<span class="p-org org">Bloomberg</span>)
     <dt>Audience:
     <dd>SG1, LEWG, LWG
     <dt>Project:
     <dd><span>ISO JTC1/SC22/WG21: Programming Language C++</span>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
  <div class="p-summary" data-fill-with="abstract">
   <p>Addressing US NB comments on the <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code>-on-uncaught-exception behavior introduced by <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> and ensuring future extensibility of parallel algorithm exception handling.</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="#back"><span class="secno">1</span> <span class="content">Background</span></a>
    <li><a href="#feedp0394"><span class="secno">2</span> <span class="content">Feedback on P0394r4</span></a>
    <li><a href="#reso"><span class="secno">3</span> <span class="content">Proposed Resolutions</span></a>
    <li><a href="#ack"><span class="secno">4</span> <span class="content">Acknowledgement</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="back"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#back"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a>, which was adopted for C++17 at the 2016 Oulu meeting, simplified
the exception handling behavior of parallel algorithms. Now, if the invocation
of an element access function (operations on the iterators, operations on
sequence elements, user-provided function objects and operations on those
function objects) exits via uncaught exception during the execution of a
parallel algorithm, <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code> is called. This change removes the previous
inconsistency in exception handling behavior between execution policies. It also
removed the need for <code class="highlight"><span class="n">exception_list</span></code>s, which SG1 and LEWG felt had not
received sufficient implementation experience and were underspecified, lacking a
mechanism for users to modify and construct them.</p>
   <p>A user-constructible and modifiable <code class="highlight"><span class="n">exception_list</span></code> would need to be a
concurrent data structure due to the possibility of concurrent access via ABI
functions. The <code class="highlight"><span class="n">exception_list</span></code> in the Parallelism TS v1 would have no
such requirement. Design of a user-constructible and modifiable <code class="highlight"><span class="n">exception_list</span></code> and a C++ library interface for the kind of concurrent data
structure required to implement it (a persistent container) is still in the
early stages, and we have no implementation experience whatsoever with such an <code class="highlight"><span class="n">exception_list</span></code>. Because <code class="highlight"><span class="n">exception_list</span></code> is an exception type, extending its
interface in the future would radically change how it can be implemented (today
it can be implemented with a <code class="highlight"><span class="n">vector</span><span class="o">&lt;</span><span class="n">exception_ptr</span><span class="o">></span></code>; in the future it would
need to be a persistent container of some type) and thus would likely require
ABI-breaking backwards-incompatible changes.</p>
   <p>The authors of <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> and this paper feel that the adoption of <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> is a step in the right direction. We believe the re-introduction of <code class="highlight"><span class="n">exception_list</span></code> or inconsistencies in exception handling between different
execution policies to the C++17 would decrease the quality of the standard in
general and the parallel algorithms library in particular.</p>
   <p><a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> does not preclude the addition of a better exception handling
mechanism in the future. In the context of the executors proposal (<a data-link-type="biblio" href="#biblio-p0443r0">[P0443r0]</a>),
there is an implicit "default" executor that is currently used with all
execution policies.  One backwards-compatible extension approach would be to
make the <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code>-on-uncaught-exception behavior a property of this
implicit "default" executor. Alternatively, an executor agnostic approach could
be taken by introducing new execution policies with a different exception
handling mechanism (e.g. <code class="highlight"><span class="n">seq_except</span></code>, <code class="highlight"><span class="n">par_except</span></code> and <code class="highlight"><span class="n">par_unseq_except</span></code>).</p>
   <p>However, since the 2016 Oulu meeting, a number of individuals (including the
authors) have suggested changes and improvements to <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> via US national
ballot comments. This paper addresses those suggestions and proposes a few
possible resolutions for those comments that will ensure that <code class="highlight"><span class="n">exception_list</span></code> and other parallel exception handling mechanisms can be added in the future
with the introduction of executors.</p>
   <h2 class="heading settled" data-level="2" id="feedp0394"><span class="secno">2. </span><span class="content">Feedback on P0394r4</span><a class="self-link" href="#feedp0394"></a></h2>
   <p>After Oulu, Alisdair sent the following mail:</p>
   <blockquote>
    <p> The key take-away I got from the SG1 session is that we might extend the
parallel policies in the future, with throwing policies, when we better
understand the domain. The main problem with this is that will mean rewriting
each algorithm for each new policy, essentially for only error handling. </p>
    <p> I think we could add customizable error handling to the current scheme by
making the policy type incorporate the named tag, and a static <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> function - where all of the current policies <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> by simply calling <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code>. This would allow us to compose into the future with a policy
that instead <code class="highlight"><span class="n">fail</span></code>ed by throwing an exception, etc. </p>
    <p> The main problem with this (other than being late) is that we don’t have any
context of the failure captured with a simple <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> call taking no
arguments. I still think that would be better than leaving no customization
point to embed in the existing algorithm implementations. </p>
   </blockquote>
   <p>Bryce submitted a US national ballot comment about this issue:</p>
   <blockquote> The current wording does not leave the door open for executors (a feature
under development by SG1) to modify the exception-handling behaviour of
parallel algorithms in the future without breaking backwards compatibility. </blockquote>
   <p>The proposed resolution for this comment is to add a customization point in the <code class="highlight"><span class="n">execution</span></code> namespace instead of adding a method to the execution policy types.</p>
   <p>However, the authors now concur that a <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> method is a better approach,
although a non-static member may be a better choice than a static member.
While the execution policies are currently just tag types, the executor paper
(<a data-link-type="biblio" href="#biblio-p0443r0">[P0443r0]</a>) proposes adding an interface to execution policies to allow them to
compose with executors. Since execution policies are likely to have an
interface in the future anyways, adding a customization point would cause
inconsistent. Such a <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> function would likely need specific wording
(similar to <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code>) allowing making it implementation-defined whether or
not the stack is unwound before <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> is called.</p>
   <p>Another ballot comment was submitted suggesting that the <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code>-on-uncaught-exception behavior might become a pitfall:</p>
   <blockquote> Calling <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code> when an element access function exits via an
uncaught exception effectively disables the normal means of C++ error
handling and propagation when using the parallel algorithms. This will be
both confusing to users and a common source of bugs. Furthermore, by
defining this behavior we are essentially preventing further solutions to
this problem. </blockquote>
   <p>The authors agree. This behavior is not ideal and we must make sure that the
changes in <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> do not prevent future improvements.</p>
   <p>The comment suggests the following possible solutions:</p>
   <blockquote> #1. Make it undefined behavior when an element access function exits via an
uncaught exception. This will allow for a future solution to this problem
that is backwards compatible. </blockquote>
   <p>The authors believe this approach would work. In fact, when <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> was
written, the general consensus among us was that undefined behavior was probably
a better approach, but the caveats and implications of calling <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code> were
better understood. We are open to seeing if there is consensus on the committee
for changing to undefined behavior here.</p>
   <p>This option would allow an implementation to implement the exception handling
behavior described in the Parallelism TS v1 and throw an <code class="highlight"><span class="n">exception_list</span></code>. Said <code class="highlight"><span class="n">exception_list</span></code> might not be compatible with a future standardized <code class="highlight"><span class="n">exception_list</span></code>, but we do not believe this is a substantial concern. Based on
conversations with implementers, we think most implementations would not take
this approach and would just call <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code> if we switched to undefined
behavior.</p>
   <blockquote> #2. When an element access function exits via an
uncaught exception, throw a <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">exception_list</span></code> which represents a collection of exceptions that
were thrown in parallel. </blockquote>
   <p>We still believe this is not a viable option because of the issues with <code class="highlight"><span class="n">exception_list</span></code> described in <a href="#back">§1 Background</a> and <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a>.</p>
   <blockquote> #3. When an element access function exits via an
uncaught exception, throw an unspecified <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">exception</span></code>. </blockquote>
   <p>This approach will not work because it would re-introduce inconsistencies between
different execution policies. The potential for exceptions - any exceptions, not
just <code class="highlight"><span class="n">exception_list</span></code>s - escaping from element access functions introduces control
flow divergence. This divergence prevents significant hurdles on vector
hardware. On such platforms that also support shared libraries, it is very difficult
for a compiler to prove that this divergence is not possible even if none of the
element access functions could possible throw. The implementation must assume
that any external function in a shared library which is not marked <code class="highlight"><span class="k">noexcept</span></code> might
cause control flow divergence due to different exit points, even if the external
function actually does not throw an exception. We believe that inconsistencies in
exception handling behavior between the different kinds of execution policies
(<code class="highlight"><span class="n">seq</span></code>, <code class="highlight"><span class="n">par</span></code> and <code class="highlight"><span class="n">par_unseq</span></code>) are undesirable as they will introduce pitfalls
and force users to learn caveats.</p>
   <blockquote> #4. Rename the parallel algorithms to clarify that exception throwing code
will result in a call to <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">terminate</span></code>. For example <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">execution</span><span class="o">:</span><span class="o">:</span><span class="n">parallel_policy</span></code> would be renamed to <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">execution</span><span class="o">:</span><span class="o">:</span><span class="n">parallel_policy_noexcept</span></code> and <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">execution</span><span class="o">:</span><span class="o">:</span><span class="n">par</span></code> would
be renamed to <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">execution</span><span class="o">:</span><span class="o">:</span><span class="n">par_noexcept</span></code>. </blockquote>
   <p>The authors believe this approach would decrease the usability and elegance of
the parallel algorithms interface. Users of the parallel algorithms library
will be writing the execution policy tags (<code class="highlight"><span class="n">seq</span></code>, <code class="highlight"><span class="n">par</span></code> and <code class="highlight"><span class="n">par_unseq</span></code>)
frequently, so shorter identifiers are desirable. Renaming the execution policy
types would be acceptable, but confusing if we later add <code class="highlight"><span class="n">seq_except</span></code>, <code class="highlight"><span class="n">par_except</span></code> and <code class="highlight"><span class="n">par_unseq_except</span></code>.</p>
   <p>Other comments suggest that <code class="highlight"><span class="n">exception_list</span></code> should be re-introduced. We agree
that it would be nice to have, but we disagree that it should be in C++17 for
the reasons outlined in <a href="#back">§1 Background</a> and <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a>.</p>
   <h2 class="heading settled" data-level="3" id="reso"><span class="secno">3. </span><span class="content">Proposed Resolutions</span><a class="self-link" href="#reso"></a></h2>
   <p>We suggest that the committee select one of the two following resolutions to
the aforementioned ballot comments:</p>
   <ol>
    <li data-md="">
     <p>Make it undefined behavior for an element access function to exit via an
uncaught exception.</p>
    <li data-md="">
     <p>Add a <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> method which calls <code class="highlight"><span class="n">terminate</span><span class="p">(</span><span class="p">)</span></code> to the three execution
policies and have the <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> method be called. SG1 and LEWG should decide
if a static or non-static method would be better.</p>
    <li data-md="">
     <p>Do #2 and also make it implementation-defined whether or
not the stack is unwound before <code class="highlight"><span class="n">fail</span><span class="p">(</span><span class="p">)</span></code> is called.</p>
   </ol>
   <h2 class="heading settled" data-level="4" id="ack"><span class="secno">4. </span><span class="content">Acknowledgement</span><a class="self-link" href="#ack"></a></h2>
   <p>Thanks to:</p>
   <ul>
    <li data-md="">
     <p>David Sankel for his invaluable and thoughtful feedback and interest in resolving this issue.</p>
    <li data-md="">
     <p>JF Bastien for co-authoring <a data-link-type="biblio" href="#biblio-p0394r4">[P0394r4]</a> and providing feedback on this paper.</p>
   </ul>
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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-p0394r4">[P0394r4]
   <dd>JF Bastien, Bryce Adelstein Lelbach. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0394r4.html">Hotel Parallelifornia: terminate() for Parallel Algorithms Exception Handling</a>. 23 June 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0394r4.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0394r4.html</a>
   <dt id="biblio-p0443r0">[P0443r0]
   <dd>Jared Hoberock, Michael Garland, Chris Kohlhoff, Chris Mysen, Carter Edwards. <a href="http://wg21.link/P0443">A Unified Executors Proposal for C++</a>. 17 October 2016. URL: <a href="http://wg21.link/P0443">http://wg21.link/P0443</a>
  </dl>