<!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>P2992R1: Attribute [[discard("reason")]]</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

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

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

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

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

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

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

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

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

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

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

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

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

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

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

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

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

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

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

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

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

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

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

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

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

	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
	}

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

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

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

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

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

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	pre code,
	code code {
		font-size: 100%;
	}

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

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

	/* Do something nice. */

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

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

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

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

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

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

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

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

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

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

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

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

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

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

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

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	  or use class="marker" to mark up the issue number in source. */

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

	.example {
		border-color: #e0cb52;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	  or use class="marker" to mark up the example number in source. */

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

	.note {
		border-color: #52e052;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

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

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

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

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

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

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

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 82ce88815, updated Thu Sep 7 16:33:55 2023 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* Boilerplate: style-autolinks */
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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">P2992R1<br>Attribute [[discard("reason")]]</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-01-19">2024-01-19</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:giuseppe.dangelo@kdab.com">Giuseppe D'Angelo</a>
     <dt>Audience:
     <dd>EWG, SG22
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>We propose a new standard attribute, <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"reason"</c-><c- p>)]]</c-></code>, to express the explicit intent of discarding the result of an expression.</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="#changelog"><span class="secno">1</span> <span class="content">Changelog</span></a>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivation and Scope</span></a>
     <ol class="toc">
      <li><a href="#discardedvalue"><span class="secno">2.1</span> <span class="content">What’s a discarded return value?</span></a>
      <li>
       <a href="#existing"><span class="secno">2.2</span> <span class="content">Existing discarding strategies</span></a>
       <ol class="toc">
        <li><a href="#voidcast"><span class="secno">2.2.1</span> <span class="content">Cast to <code class="highlight"><c- b>void</c-></code></span></a>
        <li><a href="#assigntostdignore"><span class="secno">2.2.2</span> <span class="content">Assignment to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ignore</c-></code></span></a>
       </ol>
      <li><a href="#attributediscard"><span class="secno">2.3</span> <span class="content">A new attribute</span></a>
      <li>
       <a href="#codesamples"><span class="secno">2.4</span> <span class="content">Sampling existing codebases</span></a>
       <ol class="toc">
        <li><a href="#googletest"><span class="secno">2.4.1</span> <span class="content">Google Test</span></a>
        <li><a href="#libfmt"><span class="secno">2.4.2</span> <span class="content">libfmt</span></a>
        <li><a href="#qt"><span class="secno">2.4.3</span> <span class="content">Qt</span></a>
        <li><a href="#boostasio"><span class="secno">2.4.4</span> <span class="content">Boost.Asio</span></a>
       </ol>
     </ol>
    <li>
     <a href="#design"><span class="secno">3</span> <span class="content">Design Decisions</span></a>
     <ol class="toc">
      <li><a href="#discardappertainstowhat"><span class="secno">3.1</span> <span class="content">Is <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> an attribute on expressions or statements?</span></a>
      <li><a href="#discardvoid"><span class="secno">3.2</span> <span class="content">Can <code class="highlight"><c- b>void</c-></code> be discarded?</span></a>
      <li><a href="#discardonnondiscardedvalue"><span class="secno">3.3</span> <span class="content">What should happen if <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> is applied to an <em>expression-statement</em> whose expression isn’t discarded-value?</span></a>
      <li><a href="#naming"><span class="secno">3.4</span> <span class="content">Bikeshedding: naming</span></a>
     </ol>
    <li><a href="#impact"><span class="secno">4</span> <span class="content">Impact on the Standard</span></a>
    <li>
     <a href="#technicalspecifications"><span class="secno">5</span> <span class="content">Technical Specifications</span></a>
     <ol class="toc">
      <li><a href="#proposedwording"><span class="secno">5.1</span> <span class="content">Proposed wording</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">6</span> <span class="content">Acknowledgements</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="changelog"><span class="secno">1. </span><span class="content">Changelog</span><a class="self-link" href="#changelog"></a></h2>
   <ul>
    <li data-md>
     <p>R1</p>
     <ul>
      <li data-md>
       <p>Split the proposal of the new attribute (this paper) from proposing attributes on expressions.</p>
      <li data-md>
       <p>Change audience from EWGI to EWG.</p>
     </ul>
    <li data-md>
     <p>R0</p>
     <ul>
      <li data-md>
       <p>First submission</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation and Scope</span><a class="self-link" href="#motivation"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0068r0" title="Proposal of [[unused]], [[nodiscard]] and [[fallthrough]] attributes">[P0068R0]</a> (and its subsequent <a data-link-type="biblio" href="#biblio-p0189r1" title="Wording for [[nodiscard]] attribute">[P0189R1]</a> and <a data-link-type="biblio" href="#biblio-p1301r4" title="[[nodiscard(&quot;should have a reason&quot;)]]">[P1301R4]</a>) introduced the <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> attribute in C++17; <a data-link-type="biblio" href="#biblio-wg14-n2267" title="The nodiscard attribute">[WG14-N2267]</a> introduced the same attribute in C23.</p>
   <p>The main use case for the <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> attribute is to mark functions (and types returned by functions) whose return value is deemed important, and discarding it can have important consequences.</p>
   <p>Examples of such functions include:</p>
   <ul>
    <li data-md>
     <p><strong>Pure functions</strong>, that is, functions with no side effects, that are just called for their return value (example: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>::</c-><c- n>empty</c-><c- p>()</c-></code>). If the return value is not examined, it does not make sense to call the function at all.<br> This is not merely a performance issue, but also a correctness one: functions and types that are poorly named (again, <code class="highlight"><c- n>empty</c-><c- p>()</c-></code>) may mislead the user into thinking that the function actually does something (it <em>empties</em> the vector).</p>
    <li data-md>
     <p><strong>Functions that return error codes</strong> or similar values that <em>must be examined</em> (and therefore used) for the calling code to be correct.<br> In this case, discarding the return value constitutes a logical bug in user code, and encouraging a warning from the implementation should make the user aware of their mistake.</p>
    <li data-md>
     <p><strong>Functions that return unmanaged objects</strong> that must be reclaimed in a special way (example: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>allocator</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>allocate</c-></code>, that returns a pointer allocated with <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code>).<br> For these functions, accidentally discarding the return value incurs in resource leaks.</p>
    <li data-md>
     <p><strong>Functions that return manager objects</strong> that should be kept alive; for instance, RAII managers that need to be kept alive in the block in which the function is invoked.<br> Discarding the return value does not constitute any resource leak <em>per se</em>, but code "close" to the function call (e.g. in the same block) may operate under the false assumption that the manager (and thus the resource) is still alive/acquired.<br> Examples of these functions are (after <a data-link-type="biblio" href="#biblio-p1771r1" title="[[nodiscard]] for constructors">[P1771R1]</a>) constructors of lock managers, where the constructed object itself should not get discarded, otherwise the following code would operate without the necessary mutex protection.</p>
   </ul>
   <hr>
   <p>Still: in some scenarios it is useful to expressely discard the result of an expression, without having the implementation generating a warning in that case. Examples include:</p>
   <ul>
    <li data-md>
     <p><strong>Partial domains</strong>. A nodiscard function that returns an error code may also document that it will never fail under certain conditions (for instance, when certain values are passed as parameters). If the user is already checking that the parameters are in the non-failing domain, there is no need to use the return value; it can be safely discarded.<br> One could argue that the code could still consume the error code, and for instance <code class="highlight"><c- n>assert</c-></code> on it; but, in general, we may simply not be interested in <em>enforcing a contract</em> on the function. This is akin to testing the function itself; in other words, to verify that the function matches its documented semantics. This is not the job of the user of a function, but of the implementor of the function.<br></p>
    <li data-md>
     <p><strong>Legacy</strong>. A function may return a status code to indicate success or failure; then, later, its implementation evolves in a way such that it never fails (example: <code class="highlight"><c- n>pthread_once</c-></code> on Linux). Still, the function may decide to keep returning the status code to preserve API and ABI compatibility. The return value can always be safely discarded as it’s meaningless.</p>
    <li data-md>
     <p><strong>Testing</strong>. We may want to call a nodiscard function in order to e.g. perform smoke testing, make sure that wide-contract functions don’t crash, and similar. In these cases we are simply not interested in the return value, yet we do not want a warning.</p>
   </ul>
   <p><strong>This paper proposes a standard way to express the user’s intent to discard the value of an expression</strong>. Such a way is currently missing, and workarounds are employed instead.</p>
   <h3 class="heading settled" data-level="2.1" id="discardedvalue"><span class="secno">2.1. </span><span class="content">What’s a discarded return value?</span><a class="self-link" href="#discardedvalue"></a></h3>
   <p>A "discarded return value" is formally defined as a <em>discarded-value expression</em> in <a href="https://wg21.link/expr.context#def:discarded-value_expression">[expr.context]/2</a>. They are:</p>
   <ul>
    <li data-md>
     <p>expressions appearing as <em>expression-statement</em> productions (<a href="https://wg21.link/stmt.expr#1.sentence-2">[stmt.expr]/1</a>);</p>
    <li data-md>
     <p>the left-side expression of the builtin comma operator (<a href="https://wg21.link/expr.comma#1.sentence-2">[expr.comma]/1</a>);</p>
    <li data-md>
     <p>expressions explicitly converted to (cv-)void (<a href="https://wg21.link/expr.static.cast#6.sentence-1">[expr.static.cast]/6</a>).</p>
   </ul>
   <p>
    As per <a href="https://wg21.link/dcl.attr.nodiscard#4.sentence-1">[dcl.attr.nodiscard]/4</a>,
implementations are encouraged to emit warnings when a <i>nodiscard
call</i> (a call to a function marked <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> or returning an
object of a type marked <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code>) appears in the code and the
result is not cast to 
    <tt>void</tt>
    .
   </p>
   <h3 class="heading settled" data-level="2.2" id="existing"><span class="secno">2.2. </span><span class="content">Existing discarding strategies</span><a class="self-link" href="#existing"></a></h3>
   <p>There are currently two main strategies for discarding a value returned from a <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> function without producing a warning by the implementation.</p>
   <h4 class="heading settled" data-level="2.2.1" id="voidcast"><span class="secno">2.2.1. </span><span class="content">Cast to <code class="highlight"><c- b>void</c-></code></span><a class="self-link" href="#voidcast"></a></h4>
   <p><a href="https://wg21.link/dcl.attr.nodiscard#4.sentence-1">[dcl.attr.nodiscard]/4</a> states:</p>
   <blockquote> Appearance of a nodiscard call as a potentially-evaluated discarded-value expression (<a href="https://wg21.link/expr.prop">[expr.prop]</a>) is discouraged unless explicitly cast to <code class="highlight"><c- b>void</c-></code>. Implementations should issue a warning in such cases. </blockquote>
   <p>This means that an explicit cast to <code class="highlight"><c- b>void</c-></code> can be used to suppress the nodiscard warning:</p>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>int</c-> <c- n>f</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>);</c->

<c- n>f</c-><c- p>(</c-><c- mi>123</c-><c- p>);</c->         <c- c1>// warning here</c->
<c- p>(</c-><c- b>void</c-><c- p>)</c-> <c- n>f</c-><c- p>(</c-><c- mi>123</c-><c- p>);</c->  <c- c1>// no warning</c->
<c- b>void</c-><c- p>(</c-><c- n>f</c-><c- p>(</c-><c- mi>123</c-><c- p>));</c->   <c- c1>// no warning</c->
</pre>
   <p>Using a cast to suppress the warning has several major shortcomings:</p>
   <ul>
    <li data-md>
     <p>It is a total abuse of notation. A cast notation is used, although one does not intend perform any type conversion; the code is just using an "arcane" language rule (any expression can be converted to <em>cv</em> <code class="highlight"><c- b>void</c-></code>, [expr.static.cast]/6), as well as the explicit "concession" in the semantics of the <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> attribute, in order to suppress the warning.</p>
    <li data-md>
     <p>It is hard to explain to language novices why the cast works and what it means.</p>
    <li data-md>
     <p>It is hard to grep for in a codebase. Also, different codebases use slightly different spellings for the cast (<code class="highlight"><c- b>void</c-><c- p>(</c-><c- n>x</c-><c- p>)</c-></code>, <code class="highlight"><c- p>(</c-><c- b>void</c-><c- p>)</c-><c- n>x</c-></code>, etc.). As a workaround, some codebases hide the cast behind a macro, to better express the intent and make it greppable again.</p>
    <li data-md>
     <p>The very same notation is also used for <code class="highlight"><c- p>[[</c-><c- n>maybe_unused</c-><c- p>]]</c-></code> semantics, that is, "this object may be unused in this scope". Again, some codebases hide this <em>different</em> intent with a different macro. Therefore, grepping may yield false positives.</p>
    <li data-md>
     <p>The rationale for the discard can only be provided via comments, not in code, and therefore it’s hard/impossible to enforce rules that mandate such a rationale in a codebase.</p>
   </ul>
   <h4 class="heading settled" data-level="2.2.2" id="assigntostdignore"><span class="secno">2.2.2. </span><span class="content">Assignment to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ignore</c-></code></span><a class="self-link" href="#assigntostdignore"></a></h4>
   <p>It is possible to utilize <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ignore</c-></code> as a "sink" object:</p>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>int</c-> <c- n>f</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>);</c->

<c- n>std</c-><c- o>::</c-><c- n>ignore</c-> <c- o>=</c-> <c- n>f</c-><c- p>(</c-><c- mi>123</c-><c- p>);</c-> <c- c1>// return value isn’t discarded => no warning</c->
</pre>
   <p>Nitpicking, at the moment <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ignore</c-></code> is a facility related to tuples and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>tie</c-></code> (see <a href="https://wg21.link/tuple.creation">[tuple.creation]</a>); the code above is not <em>guaranteed</em> to work, although it does work on all major implementations. <a data-link-type="biblio" href="#biblio-p2968r2" title="Make std::ignore a first-class object">[P2968R2]</a> aims at standardizing the current behavior, and therefore give precise semantics to the code above.</p>
   <p>This solution is also "blessed" by the C++ Core Guidelines (<a data-link-type="biblio" href="#biblio-es48" title="C++ Core Guidelines, ES.48: Avoid casts">[ES.48]</a>), which favor it over the cast to <code class="highlight"><c- b>void</c-></code>.</p>
   <p>Still, we claim that this solution has a number of shortcomings:</p>
   <ul>
    <li data-md>
     <p>It is not usable by generic code, since it requires the right-hand side of the assignment to yield a value. If the right-hand side expression evaluates to <code class="highlight"><c- b>void</c-></code>, the code becomes ill-formed.</p>
    <li data-md>
     <p>It is not compatible with C.</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>ignore</c-></code> is a library solution. We believe a language solution to the problem would be better suited for what’s exquisitely a language issue.</p>
    <li data-md>
     <p>It is relatively verbose compared to the cast to <code class="highlight"><c- b>void</c-></code>.</p>
    <li data-md>
     <p>Again, the rationale as of why we want to discard the result value can only be provided in comments.</p>
   </ul>
   <h3 class="heading settled" data-level="2.3" id="attributediscard"><span class="secno">2.3. </span><span class="content">A new attribute</span><a class="self-link" href="#attributediscard"></a></h3>
   <p>We find both strategies suboptimal, and therefore in this paper we are proposing a novel one: the <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> attribute, that complements <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code>:</p>
<pre class="highlight"><c- c1>// returns an error code</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>int</c-> <c- n>f</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>);</c->

<c- n>f</c-><c- p>(</c-><c- mi>123</c-><c- p>);</c->                                                        <c- c1>// warning here</c->
<c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"f always succeeds with positive inputs"</c-><c- p>)]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>123</c-><c- p>);</c->  <c- c1>// no warning</c->
</pre>
   <p>Compared with the previous solutions:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> is symmetrical to <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> in usage and syntax: <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>(</c-><c- s>"reason"</c-><c- p>)]]</c-></code> is placed on the function/type declaration that we do not want callers to ignore; <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"reason"</c-><c- p>)]]</c-></code> is placed on the expression whose result one expressly want to discard. This makes it straightforward to learn and understand.</p>
    <li data-md>
     <p>It supports a reason, expressed right in the attribute. While a C++ compiler would not use the reason itself (because it <strong>won’t</strong> generate a warning), having a reason available is useful for users and other tooling. Specifically:</p>
     <ul>
      <li data-md>
       <p>since the whole <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> mechanism is opt-in, if a function or a type is marked as such, it means that someone went the extra mile to warn us about a possible mistake we are making (possibly with a message,  as in <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>(</c-><c- s>"reason"</c-><c- p>)]]</c-></code>). Therefore, we would have a <em>structured</em> way to justify why we think that discarding the result is appropriate;</p>
      <li data-md>
       <p>having a reason can also be enforced by tooling (for instance, code checkers could ban the usage of <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> <em>without</em> a reason).</p>
     </ul>
    <li data-md>
     <p>It works with generic code (one can discard <code class="highlight"><c- b>void</c-></code>).</p>
    <li data-md>
     <p>It does not require to pull components of the Standard Library for suppressing a language warning.</p>
    <li data-md>
     <p>It is perfectly compatible with C, and we would welcome its adoption by WG14 as well.</p>
    <li data-md>
     <p>It is only moderately more verbose than the cast to <code class="highlight"><c- b>void</c-></code>.</p>
   </ul>
   <h3 class="heading settled" data-level="2.4" id="codesamples"><span class="secno">2.4. </span><span class="content">Sampling existing codebases</span><a class="self-link" href="#codesamples"></a></h3>
   <p>A major compelling motivation for the <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> attribute is to be
able to express (and possibly enforce) <em>the reason</em> why the result of
an expression is discarded.</p>
   <p>Existing, widely used codebases feature cases where results are
discarded without offering a motivation (in comments or commit
messages). This makes it hard to understand "after the fact" if the
code is correct, and what the intentions of the authors were.</p>
   <p>Here’s a few examples found in the wild where we think using <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> would improve the existing code.</p>
   <h4 class="heading settled" data-level="2.4.1" id="googletest"><span class="secno">2.4.1. </span><span class="content">Google Test</span><a class="self-link" href="#googletest"></a></h4>
   <p>In <code class="highlight"><c- n>gtest</c-><c- p>.</c-><c- n>cc</c-></code> there is this snippet:</p>
<pre class="highlight"><c- c1>// In debug mode, the Windows CRT can crash with an assertion over invalid</c->
<c- c1>// input (e.g. passing an invalid file descriptor).  The default handling</c->
<c- c1>// for these assertions is to pop up a dialog and wait for user input.</c->
<c- c1>// Instead ask the CRT to dump such assertions to stderr non-interactively.</c->
<c- k>if</c-> <c- p>(</c-><c- o>!</c-><c- n>IsDebuggerPresent</c-><c- p>())</c-> <c- p>{</c->
  <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- n>_CrtSetReportMode</c-><c- p>(</c-><c- n>_CRT_ASSERT</c-><c- p>,</c->
                          <c- n>_CRTDBG_MODE_FILE</c-> <c- o>|</c-> <c- n>_CRTDBG_MODE_DEBUG</c-><c- p>);</c->
  <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- n>_CrtSetReportFile</c-><c- p>(</c-><c- n>_CRT_ASSERT</c-><c- p>,</c-> <c- n>_CRTDBG_FILE_STDERR</c-><c- p>);</c->
<c- p>}</c->
</pre>
   <p>It is not clear at all why the return values are being explictly
discarded; the functions themselves are not marked <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> (they are C functions, part of Windows' CRT), and removing the casts to
void compiles without any warnings. Maybe some static analysis tooling
raised a warning about the discarded return value? Maybe it’s an
internal code convention? We cannot know for sure; <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> with
an explanation would help.</p>
   <h4 class="heading settled" data-level="2.4.2" id="libfmt"><span class="secno">2.4.2. </span><span class="content">libfmt</span><a class="self-link" href="#libfmt"></a></h4>
   <p>The only occurences of discarded values in libfmt are in testing code,
where <code class="highlight"><c- n>fmt</c-><c- o>::</c-><c- n>format</c-></code> itself (a <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> function) is being
tested:</p>
<pre class="highlight"><c- n>EXPECT_THROW_MSG</c-><c- p>(</c->
    <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- n>fmt</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- n>runtime</c-><c- p>(</c-><c- s>"{0:+}"</c-><c- p>),</c-> <c- k>reinterpret_cast</c-><c- o>&lt;</c-><c- b>void</c-><c- o>*></c-><c- p>(</c-><c- mh>0x42</c-><c- p>)),</c->
    <c- n>format_error</c-><c- p>,</c-> <c- s>"invalid format specifier"</c-><c- p>);</c->
</pre>
   <p>Here we want to discard the return value anyhow because we want to test
that the call throws a given exception. The cast could be replaced by
something like <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"testing exceptions"</c-><c- p>)]]</c-></code>.</p>
   <h4 class="heading settled" data-level="2.4.3" id="qt"><span class="secno">2.4.3. </span><span class="content">Qt</span><a class="self-link" href="#qt"></a></h4>
<pre class="highlight"><c- n>QDuplicateTracker</c-><c- o>&lt;</c-><c- n>QString</c-><c- o>></c-> <c- n>known</c-><c- p>;</c->
<c- p>(</c-><c- b>void</c-><c- p>)</c-><c- n>known</c-><c- p>.</c-><c- n>hasSeen</c-><c- p>(</c-><c- n>path</c-><c- p>);</c->
<c- k>do</c-> <c- p>{</c->
    <c- c1>// ...</c->
    <c- c1>// more code that uses `known`</c->
    <c- c1>// ...</c->
<c- p>}</c-> <c- k>while</c-> <c- p>(</c-><c- n>separatorPos</c-> <c- o>!=</c-> <c- mi>-1</c-><c- p>);</c->
</pre>
   <p>This code is discarding the result of a <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> call, because
it knows its outcome (since <code class="highlight"><c- n>known</c-></code> has just been built, <code class="highlight"><c- n>hasSeen</c-></code> will
return <code class="highlight">false</code>). What a casual reader may fail to realize is that
*<code class="highlight"><c- n>known</c-><c- p>.</c-><c- n>hasSeen</c-><c- p>(</c-><c- n>path</c-><c- p>)</c-></code> actually modifies <code class="highlight"><c- n>known</c-></code>* by making it remember
that it has seen <code class="highlight"><c- n>path</c-></code> (in other words, the code is somehow
initializing <code class="highlight"><c- n>known</c-></code> before using it in the loop).</p>
   <p>The alternative formulation is much more expressive:</p>
<pre class="highlight"><c- n>QDuplicateTracker</c-><c- o>&lt;</c-><c- n>QString</c-><c- o>></c-> <c- n>known</c-><c- p>;</c->

<c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"Loop invariant: the path is known by the tracker"</c-><c- p>)]]</c->
    <c- n>known</c-><c- p>.</c-><c- n>hasSeen</c-><c- p>(</c-><c- n>path</c-><c- p>);</c->
</pre>
   <p>This is another example:</p>
<pre class="highlight"><c- n>Qt</c-><c- o>::</c-><c- n>Orientation</c-> <c- nf>QGridLayoutEngine::constraintOrientation</c-><c- p>()</c-> <c- k>const</c->
<c- p>{</c->
    <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- n>ensureDynamicConstraint</c-><c- p>();</c->
    <c- k>return</c-> <c- p>(</c-><c- n>Qt</c-><c- o>::</c-><c- n>Orientation</c-><c- p>)</c-><c- n>q_cachedConstraintOrientation</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p><code class="highlight"><c- n>ensureDynamicConstraint</c-></code> does some calculations, and returns a <code class="highlight"><c- b>bool</c-></code> indicating whether it has succeded. The result of these calculations is
stored (cached) in member variables. So why is this code discarding the
success flag? The code doesn’t say; maybe this function has
preconditions, and one must first check that the calculations can
succeed, and then one can call <code class="highlight"><c- n>constraintOrientation</c-><c- p>()</c-></code>. But then why
not asserting on the result of calling <code class="highlight"><c- n>ensureDynamicConstraint</c-><c- p>()</c-></code>? We
can only speculate; this code has been written over 15 years ago.</p>
   <h4 class="heading settled" data-level="2.4.4" id="boostasio"><span class="secno">2.4.4. </span><span class="content">Boost.Asio</span><a class="self-link" href="#boostasio"></a></h4>
   <p>The <code class="highlight"><c- n>posix_mutex</c-></code> class has this code in its body:</p>
<pre class="highlight"><c- k>class</c-> <c- nc>posix_mutex</c-> <c- p>{</c->
    <c- c1>// [...]</c->

    <c- c1>// Destructor.</c->
    <c- o>~</c-><c- n>posix_mutex</c-><c- p>()</c->
    <c- p>{</c->
      <c- o>::</c-><c- n>pthread_mutex_destroy</c-><c- p>(</c-><c- o>&amp;</c-><c- n>mutex_</c-><c- p>);</c-> <c- c1>// Ignore EBUSY.</c->
    <c- p>}</c->

    <c- c1>// Lock the mutex.</c->
    <c- b>void</c-> <c- n>lock</c-><c- p>()</c->
    <c- p>{</c->
      <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- o>::</c-><c- n>pthread_mutex_lock</c-><c- p>(</c-><c- o>&amp;</c-><c- n>mutex_</c-><c- p>);</c-> <c- c1>// Ignore EINVAL.</c->
    <c- p>}</c->

    <c- c1>// Unlock the mutex.</c->
    <c- b>void</c-> <c- n>unlock</c-><c- p>()</c->
    <c- p>{</c->
      <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- o>::</c-><c- n>pthread_mutex_unlock</c-><c- p>(</c-><c- o>&amp;</c-><c- n>mutex_</c-><c- p>);</c-> <c- c1>// Ignore EINVAL.</c->
    <c- p>}</c->
<c- p>};</c->
</pre>
   <p>What do the comments refer to? One may assume they refer to the reason
behind discarding the return value of <code class="highlight"><c- n>pthread_mutex_lock</c-></code> and <code class="highlight"><c- n>pthread_mutex_unlock</c-></code>; for whatever reason, this class does not care
for handling the error.</p>
   <p>But then, why isn’t the destructor also discarding the return value of <code class="highlight"><c- n>pthread_mutex_destroy</c-></code>?</p>
   <p>This raises the suspicion that the comments may not refer to the reason
of the discarding, but to a some more general property of the
operations: the error <em>cannot</em> be generated given the class'
invariants. This would be something better expressed via
assertions/contracts.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design Decisions</span><a class="self-link" href="#design"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="discardappertainstowhat"><span class="secno">3.1. </span><span class="content">Is <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> an attribute on expressions or statements?</span><a class="self-link" href="#discardappertainstowhat"></a></h3>
   <p>Based on the current usages of casting to <code class="highlight"><c- b>void</c-></code> that we have sampled,
the most natural use of <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> is going to look like this:</p>
<pre class="highlight"><c- c1>// function call whose result we want to discard</c->
<c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"f never fails with positive numbers"</c-><c- p>)]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>42</c-><c- p>);</c->
</pre>
   <p>In this case the C++ grammar is already ruling that the attribute
appertains to the <strong>statement</strong>. For this reason we are proposing that
the <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> attribute should first and foremost be applicable to
an <i>expression-statement</i>.</p>
   <p>If that expression contains multiple discarded-value expressions (by
means of operator comma), the attribute will apply to them all,
suppressing all their possible warnings:</p>
<pre class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>a</c-><c- p>(),</c-> <c- n>b</c-><c- p>(),</c-> <c- n>c</c-><c- p>();</c->   <c- c1>// don’t generate discarding warnings</c->
</pre>
   <hr>
   <p>What about applying <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> to individual sub-expressions?
Unfortunately at the moment the C++ grammar does not allow for
attributes on expressions, and therefore we are unable to provide
support for those cases.</p>
   <p>We are authoring a different proposal (P3093) that would enable
attributes on expressions; with that proposal, in principle one could
write:</p>
<pre class="highlight"><c- n>a</c-><c- p>(),</c-> <c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>b</c-><c- p>()),</c-> <c- n>c</c-><c- p>();</c->
</pre>
   <p>and only suppress discarding warnings for the call to <code class="highlight"><c- n>b</c-></code>.</p>
   <p>Supporting <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> on expressions is somehow a secondary goal,
because the only practical applications of such a discarding mechanism
exists in the context of using the builtin comma operator (at least, to
the best of our knowledge). We strongly believe that usages of the
builtin comma operator should be frowned upon, except in corner cases
where it’s unpractical to use alternatives.</p>
   <p>Still, some scenarios where <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> on expressions would be nice
to have are:</p>
   <ul>
    <li data-md>
     <p>in a constructor’s member initialization list;</p>
    <li data-md>
     <p>as the increment (third) parameter of a <code class="highlight"><c- k>for</c-></code> loop.</p>
   </ul>
   <p>For instance:</p>
<pre class="highlight"><c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>,</c-> <c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- n>i</c-><c- p>)))</c->
    <c- n>doSomething</c-><c- p>(</c-><c- n>i</c-><c- p>);</c->
</pre>
   <p>We therefore plan to add support for discarding expressions, should
attributes on expressions become possible.</p>
   <h3 class="heading settled" data-level="3.2" id="discardvoid"><span class="secno">3.2. </span><span class="content">Can <code class="highlight"><c- b>void</c-></code> be discarded?</span><a class="self-link" href="#discardvoid"></a></h3>
   <p>Yes. We believe that such a situation can happen in practice, for
instance in generic code, and such a restriction sounds therefore
unnecessary and vexing.</p>
   <h3 class="heading settled" data-level="3.3" id="discardonnondiscardedvalue"><span class="secno">3.3. </span><span class="content">What should happen if <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> is applied to an <em>expression-statement</em> whose expression isn’t discarded-value?</span><a class="self-link" href="#discardonnondiscardedvalue"></a></h3>
   <p>Or, more in general, if it is applied to a statement that does not contain a nodiscard call?</p>
   <p>For example:</p>
<pre class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>int</c-> <c- n>f</c-><c- p>();</c->

<c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>a</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// This is not *actually* discarding. Should it be legal?</c->


<c- b>bool</c-> <c- nf>g</c-><c- p>();</c->        <c- c1>// This is not marked as [[nodiscard]]</c->
<c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>g</c-><c- p>();</c-> <c- c1>// Legal?</c->
</pre>
   <p>Should we accept or forbid these usages, as the attribute is
meaningless (at best) or misleading (at worst)?
We are proposing to <strong>accept</strong> the code, under the rationale that the
attribute serves to suppress a <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> warning. Since the
warning would not be generated, there is nothing to suppress.</p>
   <p><code class="highlight"><c- n>g</c-><c- p>()</c-></code>’s example is actually very compelling: there’s a number of
reasons why <code class="highlight"><c- n>g</c-><c- p>()</c-></code> might not have been marked as <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> (legacy
code, C APIs, 3rd parties that we can’t modify ourselves, etc.); yet
it’s "normally" important to examine its return value. By allowing the
attribute, one can still document why we are not examining it in that
particular circumstance.</p>
   <p>Of course implementations can still diagnose "questionable" usages as QoI.</p>
   <h3 class="heading settled" data-level="3.4" id="naming"><span class="secno">3.4. </span><span class="content">Bikeshedding: naming</span><a class="self-link" href="#naming"></a></h3>
   <p>The most obvious name for the attribute that we are proposing is <code class="highlight"><c- n>discard</c-></code>, to mirror the existing <code class="highlight"><c- n>nodiscard</c-></code> attribute.</p>
   <p>There is a possible objection to this name: this attribute does not
actually "force" the value of an expression to be discarded:</p>
<pre class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>result</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// nothing is discarded here</c->
</pre>
   <p>One could therefore prefer a more nuanced form, such as <code class="highlight"><c- n>may_discard</c-></code> (along the lines of <code class="highlight"><c- n>maybe_unused</c-></code>):</p>
<pre class="highlight"><c- p>[[</c-><c- n>may_discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>();</c->          <c- c1>// actually discarding</c->
<c- p>[[</c-><c- n>may_discard</c-><c- p>]]</c-> <c- n>result</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// not discarding</c->
</pre>
   <p><strong>We would like to request a poll to EWG.</strong></p>
   <h2 class="heading settled" data-level="4" id="impact"><span class="secno">4. </span><span class="content">Impact on the Standard</span><a class="self-link" href="#impact"></a></h2>
   <p>This proposal is a core language extension. It proposes a new standard
attribute, spelled <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> or <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"with reason"</c-><c- p>)]]</c-></code>, to
mark <em>expression-statement</em>s whose expressions' results we want to
expressly discard.</p>
   <p>No changes are required in the Standard Library.</p>
   <h2 class="heading settled" data-level="5" id="technicalspecifications"><span class="secno">5. </span><span class="content">Technical Specifications</span><a class="self-link" href="#technicalspecifications"></a></h2>
   <p>All the proposed changes are relative to <a data-link-type="biblio" href="#biblio-n4971" title="Working Draft, Programming Languages — C++">[N4971]</a>.</p>
   <h3 class="heading settled" data-level="5.1" id="proposedwording"><span class="secno">5.1. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposedwording"></a></h3>
   <p>In <a href="https://wg21.link/cpp.cond">[cpp.cond]</a> append a new row to Table 21 (<a href="https://wg21.link/tab:cpp.cond.ha">[tab:cpp.cond.ha]</a>):</p>
   <table>
    <thead>
     <tr>
      <th>Attribute
      <th>Value
    <tbody>
     <tr>
      <td>
       <ins><code class="highlight"><c- n>discard</c-></code></ins>
      <td>
       <ins><code class="highlight"><c- n>YYYYMML</c-></code></ins>
   </table>
   <p>with <code class="highlight"><c- n>YYYYMML</c-></code> determined as usual.</p>
   <hr>
   <p>In <a href="https://wg21.link/dcl.attr.nodiscard">[dcl.attr.nodiscard]</a> insert a new paragraph after 3:</p>
   <blockquote>
    <ins>4. A potentially-evaluated discarded-value expression ([expr.prop]) which is a nodiscard call and which is neither</ins>
    <ins>
     <ul>
      <li data-md>
       <p>explicitly cast to <code class="highlight"><c- b>void</c-></code> ([expr.static.cast]), or</p>
      <li data-md>
       <p>an expression (or subexpression thereof) of an <em>expression-statement</em> marked with the <code class="highlight"><c- n>discard</c-></code> attribute ([dcl.attr.discard])</p>
     </ul>
    </ins>
    <ins>is a <em>discouraged nodiscard call</em>.</ins>
   </blockquote>
   <p>Renumber and modify the existing paragraph 4 as shown:</p>
   <blockquote>
    <del>4.</del>
    <ins>5.</ins>
     <em>Recommended practice</em>: 
    <del>Appearance of a nodiscard call as a potentially-evaluated discarded-value expression ([expr.prop]) is discouraged unless explicitly cast to <code class="highlight"><c- b>void</c-></code></del>
    . Implementations should issue a warning 
    <del>in such cases</del>
    <ins>for discouraged nodiscard calls</ins>
    . [...] 
   </blockquote>
   <p>And renumber the rest of the paragraphs in <a href="https://wg21.link/dcl.attr.nodiscard">[dcl.attr.nodiscard]</a>.</p>
   <p>Modify the Example 1 as shown (including only the lines for the chosen approach):</p>
   <blockquote>
<pre class="highlight"><c- k>struct</c-> <c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>my_scopeguard</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>my_unique</c-> <c- p>{</c->
  <c- n>my_unique</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->                                <c- c1>// does not acquire resource</c->
  <c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>my_unique</c-><c- p>(</c-><c- b>int</c-> <c- n>fd</c-><c- p>)</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c->         <c- c1>// acquires resource</c->
  <c- o>~</c-><c- n>my_unique</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c->                   <c- c1>// releases resource, if any</c->
  <c- d>/* ... */</c->
<c- p>};</c->
<c- k>struct</c-> <c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>error_info</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>};</c->
<c- n>error_info</c-> <c- nf>enable_missile_safety_mode</c-><c- p>();</c->
<c- b>void</c-> <c- nf>launch_missiles</c-><c- p>();</c->
<c- b>void</c-> <c- nf>test_missiles</c-><c- p>()</c-> <c- p>{</c->
  <c- n>my_scopeguard</c-><c- p>();</c->              <c- c1>// warning encouraged</c->
  <c- p>(</c-><c- b>void</c-><c- p>)</c-><c- n>my_scopeguard</c-><c- p>(),</c->        <c- c1>// warning not encouraged, cast to void</c->
    <c- n>launch_missiles</c-><c- p>();</c->          <c- c1>// comma operator, statement continues</c->
  <c- n>my_unique</c-><c- p>(</c-><c- mi>42</c-><c- p>);</c->                <c- c1>// warning encouraged</c->
  <c- n>my_unique</c-><c- p>();</c->                  <c- c1>// warning not encouraged</c->

  <c- n>enable_missile_safety_mode</c-><c- p>();</c-> <c- c1>// warning encouraged</c->
  <c- n>launch_missiles</c-><c- p>();</c->

<ins>
  <c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>my_unique</c-><c- p>(</c-><c- mi>123</c-><c- p>);</c->         <c- c1>// warning not encouraged</c->
  <c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"testing illegal fds"</c-><c- p>)]]</c->
              <c- n>my_unique</c-><c- p>(</c-><c- mi>-1</c-><c- p>);</c->          <c- c1>// warning not encouraged</c->
  <c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>my_unique</c-><c- p>();</c->            <c- c1>// warning not encouraged</c->
  
  <c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>my_unique</c-><c- p>(),</c-> 
    <c- n>enable_missile_safety_mode</c-><c- p>();</c->     <c- c1>// warning not encouraged</c->
</ins>  
<c- p>}</c->
<c- n>error_info</c-> <c- o>&amp;</c-><c- nf>foo</c-><c- p>();</c->
<c- b>void</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- n>foo</c-><c- p>();</c-> <c- p>}</c->             <c- c1>// warning not encouraged: not a nodiscard call, because neither</c->
                                <c- c1>// the (reference) return type nor the function is declared nodiscard</c->
</pre>
   </blockquote>
   <hr>
   <p>Add a new subclause at the end of <a href="https://wg21.link/dcl.attr">[dcl.attr]</a>, with the following content:</p>
   <blockquote>
     <strong>??? Discard attribute [dcl.attr.discard]</strong> 
    <ol>
     <li data-md>
      <p>The <em>attribute-token</em> <code class="highlight"><c- n>discard</c-></code> may be applied to an <em>expression-statement</em>. An <em>attribute-argument-clause</em> may be present and, if present, shall have the form:</p>
    </ol>
    <blockquote> <code class="highlight"><c- p>(</c-></code> <em>unevaluated-string</em> <code class="highlight"><c- p>)</c-></code> </blockquote>
    <ol start="2">
     <li data-md>
      <p><em>Recommended practice</em>: Implementations should suppress the warning associated with a nodiscard call ([dcl.attr.nodiscard]) if such a call is an expression (or subexpression thereof) of an <em>expression-statement</em> marked as <code class="highlight"><c- n>discard</c-></code>. The value of a <em>has-attribute-expression</em> for the <code class="highlight"><c- n>discard</c-></code> attribute should be <code class="highlight"><c- mi>0</c-></code> unless the implementation can suppress such warnings.<br><br> The <em>unevaluated-string</em> in a discard <em>attribute-argument-clause</em> is ignored.<br><br> [<em>Note</em> 1: the string is meant to be used in code reviews, by static analyzers and in similar scenarios. — <em>end note</em>]</p>
    </ol>
   </blockquote>
   <h2 class="heading settled" data-level="6" id="acknowledgements"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to KDAB for supporting this work.</p>
   <p>All remaining errors are ours and ours only.</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-es48">[ES.48]
   <dd>Bjarne Stroustrup; Herb Sutter. <a href="https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts"><cite>C++ Core Guidelines, ES.48: Avoid casts</cite></a>. URL: <a href="https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts">https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts</a>
   <dt id="biblio-n4971">[N4971]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4971"><cite>Working Draft, Programming Languages — C++</cite></a>. 18 December 2023. URL: <a href="https://wg21.link/n4971">https://wg21.link/n4971</a>
   <dt id="biblio-p0068r0">[P0068R0]
   <dd>Andrew Tomazos. <a href="https://wg21.link/p0068r0"><cite>Proposal of [[unused]], [[nodiscard]] and [[fallthrough]] attributes</cite></a>. 3 September 2015. URL: <a href="https://wg21.link/p0068r0">https://wg21.link/p0068r0</a>
   <dt id="biblio-p0189r1">[P0189R1]
   <dd>Andrew Tomazos. <a href="https://wg21.link/p0189r1"><cite>Wording for [[nodiscard]] attribute</cite></a>. 29 February 2016. URL: <a href="https://wg21.link/p0189r1">https://wg21.link/p0189r1</a>
   <dt id="biblio-p1301r4">[P1301R4]
   <dd>JeanHeyd Meneide, Isabella Muerte. <a href="https://wg21.link/p1301r4"><cite>[[nodiscard("should have a reason")]]</cite></a>. 5 August 2019. URL: <a href="https://wg21.link/p1301r4">https://wg21.link/p1301r4</a>
   <dt id="biblio-p1771r1">[P1771R1]
   <dd>Peter Sommerlad. <a href="https://wg21.link/p1771r1"><cite>[[nodiscard]] for constructors</cite></a>. 19 July 2019. URL: <a href="https://wg21.link/p1771r1">https://wg21.link/p1771r1</a>
   <dt id="biblio-p2968r2">[P2968R2]
   <dd>Peter Sommerlad. <a href="https://wg21.link/p2968r2"><cite>Make std::ignore a first-class object</cite></a>. 13 December 2023. URL: <a href="https://wg21.link/p2968r2">https://wg21.link/p2968r2</a>
   <dt id="biblio-wg14-n2267">[WG14-N2267]
   <dd>Aaron Ballman. <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2267.pdf"><cite>The nodiscard attribute</cite></a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2267.pdf">https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2267.pdf</a>
  </dl>