<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P0418r2: Fail or succeed: there is no atomic lattice</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;
	}

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

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

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

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   <h1 class="p-name no-ref" id="title">P0418r2<br>Fail or succeed: there is no atomic lattice</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2016-11-09">9 November 2016</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p0418r2">http://wg21.link/p0418r2</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:hboehm@google.com">Hans Boehm</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>SG1, LWG
     <dt>Toggle Diffs:
     <dd>
      <label for="hidedel" id="hidedel-label">Hide deleted text</label>
     <dt>Project:
     <dd><span>ISO JTC1/SC22/WG21: Programming Language C++</span>
     <dt>Source:
     <dd><span><a href="https://github.com/jfbastien/papers/blob/master/source/P0418r2.bs">github.com/jfbastien/papers/blob/master/source/P0418r2.bs</a></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>Try to resolve <a data-link-type="biblio" href="#biblio-lwg2445">[LWG2445]</a>.</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>
     <ol class="toc">
      <li><a href="#issue"><span class="secno">1.1</span> <span class="content">LWG issue #2445</span></a>
      <li><a href="#old-res"><span class="secno">1.2</span> <span class="content">Proposed SG1 resolution from Urbana</span></a>
      <li><a href="#moar"><span class="secno">1.3</span> <span class="content">Further issue</span></a>
     </ol>
    <li><a href="#new-res"><span class="secno">2</span> <span class="content">Updated proposal</span></a>
    <li><a href="#ack"><span class="secno">3</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="bg"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#bg"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-lwg2445">[LWG2445]</a> was discussed and resolved by SG1 in Urbana.</p>
   <p>This revision updates <a data-link-type="biblio" href="#biblio-p0418r1">[P0418r1]</a> with accurate wording for
[util.smartptr.shared.atomic] ¶32, to be deleted from <a data-link-type="biblio" href="#biblio-n4606">[N4606]</a>.</p>
   <h3 class="heading settled" data-level="1.1" id="issue"><span class="secno">1.1. </span><span class="content">LWG issue #2445</span><a class="self-link" href="#issue"></a></h3>
   <blockquote>
    <p>The definitions of compare and exchange in [util.smartptr.shared.atomic]
  ¶32 and [atomics.types.operations.req] ¶21 state:</p>
    <blockquote>
     <p>Requires: The failure argument shall not be <code class="highlight"><span></span><span class="n">memory_order_release</span></code> nor <code class="highlight"><span></span><span class="n">memory_order_acq_rel</span></code>. The failure argument shall be no stronger than the
    success argument.</p>
    </blockquote>
    <p>The term "stronger" isn’t defined by the standard.</p>
    <p>It is hinted at by [atomics.types.operations.req] ¶22:</p>
    <blockquote>
     <p>When only one <code class="highlight"><span></span><span class="n">memory_order</span></code> argument is supplied, the value of <code class="highlight"><span></span><span class="n">success</span></code> is <code class="highlight"><span></span><span class="n">order</span></code>, and the value of <code class="highlight"><span></span><span class="n">failure</span></code> is <code class="highlight"><span></span><span class="n">order</span></code> except that a value of <code class="highlight"><span></span><span class="n">memory_order_acq_rel</span></code> shall be replaced by the value <code class="highlight"><span></span><span class="n">memory_order_acquire</span></code> and a value of <code class="highlight"><span></span><span class="n">memory_order_release</span></code> shall be replaced by the value <code class="highlight"><span></span><span class="n">memory_order_relaxed</span></code>.</p>
    </blockquote>
    <p>Should the standard define a partial ordering for memory orders, where consume
  and acquire are incomparable with release?</p>
   </blockquote>
   <h3 class="heading settled" data-level="1.2" id="old-res"><span class="secno">1.2. </span><span class="content">Proposed SG1 resolution from Urbana</span><a class="self-link" href="#old-res"></a></h3>
   <p>Add the following note:</p>
   <blockquote>
    <ins>
     <p>[ <em>Note:</em> Memory orders have the following relative strengths implied by their
  definitions:</p>
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     <p><em>—end note</em> ]</p>
    </ins>
   </blockquote>
   <h3 class="heading settled" data-level="1.3" id="moar"><span class="secno">1.3. </span><span class="content">Further issue</span><a class="self-link" href="#moar"></a></h3>
   <p>Nonetheless:</p>
   <ul>
    <li data-md="">
     <p>The resolution isn’t on the LWG tracker.</p>
    <li data-md="">
     <p>The proposed note was never moved to the draft Standard.</p>
   </ul>
   <p>Furthermore, the resolution which SG1 came to in Urbana resolves what "stronger"
means by specifying a lattice, but isn’t not clear on what "The failure argument
shall be no stronger than the success argument" means given the lattice.</p>
   <p>There is no relationship, "stronger" or otherwise, between release and
consume/acquire. The current wording says "shall be no stronger" which isn’t the
same as "shall not be stronger" in this context. Is that on purpose? At a
minimum it’s not clear and should be clarified.</p>
   <p>Should the following be valid:</p>
<pre class="highlight"><span></span>  <span class="n">compare_exchange_strong</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">,</span> <span class="n">memory_order_release</span><span class="p">,</span> <span class="n">memory_order_acquire</span><span class="p">);</span></pre>
   <p>Or does the code need to be:</p>
<pre class="highlight"><span></span>  <span class="n">compare_exchange_strong</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">,</span> <span class="n">memory_order_acq_rel</span><span class="p">,</span> <span class="n">memory_order_acquire</span><span class="p">);</span></pre>
   <p>Similar questions can be asked for <code class="highlight"><span></span><span class="n">memory_order_consume</span></code> ordering on <code class="highlight"><span></span><span class="n">failure</span></code>.</p>
   <p>Is there even a point in restricting <code class="highlight"><span></span><span class="n">success</span></code>/<code class="highlight"><span></span><span class="n">failure</span></code> orderings? On
architectures with load-linked/store-conditional instructions the load and store
are distinct instructions which can each have their own memory ordering (with
appropriate leading/trailing fences if required), whereas architectures with
compare-and-exchange already have a limited set of instructions to choose
from. The current limitation (assuming <a data-link-type="biblio" href="#biblio-lwg2445">[LWG2445]</a> is resolved) only seems to
restrict compilers on load-linked/store-conditional architectures.</p>
   <p>The following code could be valid if the stored data didn’t need to be published
nor ordered, whereas any retry needs to read additional data:</p>
<pre class="highlight"><span></span>  <span class="n">compare_exchange_strong</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">,</span> <span class="n">memory_order_relaxed</span><span class="p">,</span> <span class="n">memory_order_acquire</span><span class="p">);</span></pre>
   <p>Even if—for lack of clever instruction—architectures cannot take advantage of
such code, compiler are able to optimize atomics in all sorts of clever ways as
discussed in <a data-link-type="biblio" href="#biblio-n4455">[N4455]</a>.</p>
   <h2 class="heading settled" data-level="2" id="new-res"><span class="secno">2. </span><span class="content">Updated proposal</span><a class="self-link" href="#new-res"></a></h2>
   <p>This paper proposes removing the "stronger" restrictions between
compare-exchange’s <code class="highlight"><span></span><span class="n">success</span></code> and <code class="highlight"><span></span><span class="n">failure</span></code> ordering, and doesn’t add a lattice
to order atomic orderings. The only remaining restriction is that <code class="highlight"><span></span><span class="n">memory_order_release</span></code> and <code class="highlight"><span></span><span class="n">memory_order_acq_rel</span></code> for <code class="highlight"><span></span><span class="n">failure</span></code> are still
disallowed: a failed compare-exchange doesn’t store, the current model is
therefore not sensible with these orderings.</p>
   <p>There have been discussions about <code class="highlight"><span></span><span class="n">memory_order_release</span></code> loads, e.g. for
seqlock. Such potential changes are left up to future papers.</p>
   <p>Modify [util.smartptr.shared.atomic] ¶32 as follows:</p>
   <blockquote>
    <p>
     Requires: 
     <ins>The </ins>
     failure 
     <ins>argument </ins>
     shall not be <code class="highlight"><span></span><span class="n">memory_order_release</span></code>
     <del>,</del>
     <ins> nor</ins>
      <code class="highlight"><span></span><span class="n">memory_order_acq_rel</span></code>
     <del>,
  or stronger than success</del>
     .
    </p>
   </blockquote>
   <p>Modify [atomics.types.operations.req] ¶21 as follows:</p>
   <blockquote>
    <p>
     Requires: The failure argument shall not be <code class="highlight"><span></span><span class="n">memory_order_release</span></code> nor <code class="highlight"><span></span><span class="n">memory_order_acq_rel</span></code>.
     <del> The failure argument shall be no stronger than
  the success argument.</del>
    </p>
   </blockquote>
   <p>Leave [atomics.types.operations.req] ¶22 as-is:</p>
   <blockquote>
    <p>Effects: Atomically, compares the contents of the memory pointed to by <code class="highlight"><span></span><span class="n">object</span></code> or by <code class="highlight"><span></span><span class="k">this</span></code> for equality with that in <code class="highlight"><span></span><span class="n">expected</span></code>, and if <code class="highlight"><span></span><span class="nb">true</span></code>,
  replaces the contents of the memory pointed to by <code class="highlight"><span></span><span class="n">object</span></code> or by <code class="highlight"><span></span><span class="k">this</span></code> with
  that in <code class="highlight"><span></span><span class="n">desired</span></code>, and if <code class="highlight"><span></span><span class="nb">false</span></code>, updates the contents of the memory in <code class="highlight"><span></span><span class="n">expected</span></code> with the contents of the memory pointed to by <code class="highlight"><span></span><span class="n">object</span></code> or by <code class="highlight"><span></span><span class="k">this</span></code>. Further, if the comparison is <code class="highlight"><span></span><span class="nb">true</span></code>, memory is affected according to
  the value of <code class="highlight"><span></span><span class="n">success</span></code>, and if the comparison is <code class="highlight"><span></span><span class="nb">false</span></code>, memory is affected
  according to the value of <code class="highlight"><span></span><span class="n">failure</span></code>.</p>
    <p>When only one <code class="highlight"><span></span><span class="n">memory_order</span></code> argument is supplied, the value of <code class="highlight"><span></span><span class="n">success</span></code> is <code class="highlight"><span></span><span class="n">order</span></code>, and the value of <code class="highlight"><span></span><span class="n">failure</span></code> is <code class="highlight"><span></span><span class="n">order</span></code> except that a value of <code class="highlight"><span></span><span class="n">memory_order_acq_rel</span></code> shall be replaced by the value <code class="highlight"><span></span><span class="n">memory_order_acquire</span></code> and a value of <code class="highlight"><span></span><span class="n">memory_order_release</span></code> shall be replaced by the value <code class="highlight"><span></span><span class="n">memory_order_relaxed</span></code>.</p>
    <p>If the operation returns <code class="highlight"><span></span><span class="nb">true</span></code>, these operations are atomic read-modify-write
  operations (1.10). Otherwise, these operations are atomic load operations.</p>
   </blockquote>
   <h2 class="heading settled" data-level="3" id="ack"><span class="secno">3. </span><span class="content">Acknowledgement</span><a class="self-link" href="#ack"></a></h2>
   <p>Thanks to John McCall for pointing out that the proposed resolution was still
insufficient, and for providing ample feedback.</p>
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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-lwg2445">[LWG2445]
   <dd>JF Bastien. <a href="http://cplusplus.github.io/LWG/lwg-active.html#2445">"Stronger" memory ordering</a>. SG1. URL: <a href="http://cplusplus.github.io/LWG/lwg-active.html#2445">http://cplusplus.github.io/LWG/lwg-active.html#2445</a>
   <dt id="biblio-n4455">[N4455]
   <dd>JF Bastien. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html">No Sane Compiler Would Optimize Atomics</a>. 10 April 2015. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html</a>
   <dt id="biblio-n4606">[N4606]
   <dd>Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf">Working Draft, Standard for Programming Language C++</a>. 12 July 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4606.pdf</a>
   <dt id="biblio-p0418r1">[P0418r1]
   <dd>JF Bastien. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0418r1.html">Fail or succeed: there is no atomic lattice</a>. 2 August 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0418r1.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0418r1.html</a>
  </dl>