<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P2992R0: Attribute [[discard]] and attributes on expressions</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">P2992R0<br>Attribute [[discard]] and attributes on expressions</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="2023-10-09">2023-10-09</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>SG17, 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-></code>, to express the explicit intent of discarding the result of an expression.

    We also propose to allow attributes on expressions.</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="#existing"><span class="secno">2.1</span> <span class="content">Existing discarding strategies</span></a>
       <ol class="toc">
        <li><a href="#voidcast"><span class="secno">2.1.1</span> <span class="content">Cast to <code class="highlight"><c- b>void</c-></code></span></a>
        <li><a href="#assigntostdignore"><span class="secno">2.1.2</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">2.2</span> <span class="content">A new attribute</span></a>
     </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="#expressionattributes1"><span class="secno">3.2</span> <span class="content">Attributes on expressions: approach 1</span></a>
       <ol class="toc">
        <li><a href="#problems"><span class="secno">3.2.1</span> <span class="content">Problems</span></a>
       </ol>
      <li><a href="#expressionattributes2"><span class="secno">3.3</span> <span class="content">Attributes on expressions: approach 2</span></a>
      <li><a href="#discardvoid"><span class="secno">3.4</span> <span class="content">Can <code class="highlight"><c- b>void</c-></code> be discarded?</span></a>
      <li><a href="#discardonnondiscardedvalue"><span class="secno">3.5</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 expression which isn’t a discarded-value expression?</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>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>
   <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>
   <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>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>
    <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.</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>
   </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="existing"><span class="secno">2.1. </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="2.1.1" id="voidcast"><span class="secno">2.1.1. </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- n>f</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- c1>// no warning</c->
<c- b>void</c-><c- p>(</c-><c- n>f</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 leeway that <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> leaves, 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/impossible 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.</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="2.1.2" id="assigntostdignore"><span class="secno">2.1.2. </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- n>std</c-><c- o>::</c-><c- n>ignore</c-> <c- o>=</c-> <c- n>f</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-p2968r0" title="Make std::ignore a first-class object">[P2968R0]</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="2.2" id="attributediscard"><span class="secno">2.2. </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- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>int</c-> <c- n>f</c-><c- p>();</c->

<c- n>f</c-><c- p>();</c->                              <c- c1>// warning here</c->
<c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"just testing"</c-><c- p>)]]</c-> <c- n>f</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-></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-></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 moderately more verbose than the cast to <code class="highlight"><c- b>void</c-></code>.</p>
   </ul>
   <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>Since <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> is really meant to be applied on function calls, which are expressions, it sounds natural that it should be described as an attribute that appertains to expressions.
In principle this would also allow for it to be applied on subexpressions and have a "localized" effect:</p>
<pre class="highlight"><c- c1>//  do not warn for discarding the result of f() nor g(),</c->
<c- c1>//  but warn for discarding a() and/or b():</c->

<c- n>a</c-><c- p>(),</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-> <c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>g</c-><c- p>(),</c-> <c- mi>2</c-><c- p>),</c-> <c- mi>3</c-><c- p>),</c-> <c- n>b</c-><c- p>();</c->
</pre>
   <hr>
   <p>Now, attributes on expressions are a novelty, because C++'s grammar does not yet support them. The grammar production for <em>expression</em> (<a href="https://wg21.link/expr.comma">[expr.comma]</a>) is:</p>
<pre class="highlight"><i><c- nl>expression</c-><c- p>:</c-></i>
    <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i>
    <i><c- n>expression</c-></i> <c- p>,</c-> <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i>
</pre>
   <p>The "obvious" modification of this production to introduce attributes could look like this:</p>
<pre class="highlight"><i><c- nl>expression</c-><c- p>:</c-></i>
    <ins><i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i></ins> <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i>  <c- c1>// not proposed!</c->
    <i><c- n>expression</c-></i> <c- p>,</c-> <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i>
</pre>
   <p>However <strong>this modification clashes</strong> with the position where we can declare attributes on <em>expression-statement</em>s productions (<a href="https://wg21.link/stmt.pre">[stmt.pre]</a>, <a href="https://wg21.link/stmt.expr">[stmt.expr]</a>). This is the relevant production:</p>
<pre class="highlight"><i><c- nl>statement</c-></i><c- p>:</c->
    <i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i> <i><c- n>expression</c-><c- o>-</c-><c- n>statement</c-></i>
</pre>
   <p>with</p>
<pre class="highlight"><i><c- n>expression</c-><c- o>-</c-><c- n>statement</c-></i><c- o>:</c->
    <i><c- n>expression</c-><sub><c- n>opt</c-></sub></i> <c- p>;</c->     
</pre>
   <p>In other words, in code like:</p>
<pre class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>();</c->  <c- c1>// the *statement* is discarded!</c->
</pre>
   <p>the grammar is already ruling that the attribute appertains to the <strong>statement</strong>, not to the <strong>expression</strong>! We do not want to modify the <em>statement</em> productions, as that would be a source incompatible change (for instance, attributes that can only appertain to statements would now be rejected).</p>
   <p>On the other hand, the snippet above shows what is going to be the most common usage of <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code>: applied to a statement that contains a function call. <b>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>.</b></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>This leaves us with the more general case of discarding just an expression. This is a somehow "secondary" goal, that we are pursuing for completeness' sake, because the only practical applications of such a discarding mechanism that we can find exists in the context of using the builtin comma operator. 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><strong>Nonetheless, we propose to introduce attributes on expressions.</strong> To this end, in this paper we are exploring two different approaches.</p>
   <h3 class="heading settled" data-level="3.2" id="expressionattributes1"><span class="secno">3.2. </span><span class="content">Attributes on expressions: approach 1</span><a class="self-link" href="#expressionattributes1"></a></h3>
   <p>Given the grammar clash described above, the only "room" we have left to attach an attribute to a general expression is on the <b>right hand side</b> of an <i>expression</i>.
We can modify the <i>expression</i> production as follows:</p>
<pre class="highlight"><i><c- nl>expression</c-><c- p>:</c-></i>
    <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i> <ins><i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i></ins>
    <i><c- n>expression</c-></i> <c- p>,</c-> <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i>
</pre>
   <p>We can also special-case parenthesized expressions, so that their attribute applies to the inner expression.</p>
   <p>With <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code>, this would allow for instance this:</p>
<pre class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>();</c->  <c- c1>// no warning, attribute on statement</c->
<c- n>f</c-><c- p>()</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]];</c->  <c- c1>// ditto, equivalent, attribute on the expression</c->

<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- c1>// no warnings, attribute on statement</c->
<c- n>a</c-><c- p>(),</c-> <c- n>b</c-><c- p>()</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]];</c->  <c- c1>// no warnings, attribute on the entire expression</c->
<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- c1>// no warning for a(), possible warning for b()</c->

<c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- p>(</c-><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- c1>// no warning; suppressed for a(), and b() isn’t discarded</c->

<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
    <c- n>S</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> 
        <c- o>:</c-> <c- n>m_i</c-><c- p>((</c-><c- n>check</c-><c- p>(</c-><c- n>i</c-><c- p>)</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]],</c-> <c- n>i</c-><c- p>))</c-> <c- p>{}</c->   <c- c1>// no warning</c->

    <c- b>int</c-> <c- n>m_i</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>Note that, for the moment being, in this approach we are proposing attributes on the <em>expression</em> grammar production, and not attributes on <strong>all</strong> possible kinds of sub-expressions. We believe that complicating the grammar to allow for attributes "everywhere" is not worth the effort, because one can always wrap a subexpression in parenthesis in order to apply an attribute to it. However, we also believe that this approach does not impede such an extension in the future.</p>
   <p>Here are some more examples:</p>
<pre class="highlight"><c- b>int</c-> <c- n>a</c-><c- p>[</c-><c- mi>10</c-><c- p>];</c-> 

<c- n>a</c-><c- p>[</c-><c- mi>1</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]];</c->    <c- c1>// attr applies to `a[1] = x + y`</c->
<c- n>a</c-><c- p>[</c-><c- mi>2</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-> <c- o>+</c-> <c- p>(</c-><c- n>y</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]);</c->  <c- c1>// attr applies to `y`</c->
<c- n>a</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- o>=</c-> <c- p>((</c-><c- n>x</c-><c- o>+</c-><c- n>y</c-><c- p>)</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]);</c->  <c- c1>// attr applies to `x+y`</c->
<c- n>a</c-><c- p>[</c-><c- mi>4</c-><c- p>]</c-> <c- o>=</c-> <c- p>(</c-><c- n>x</c-><c- o>+</c-><c- n>y</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]);</c->    <c- c1>// attr applies to `x+y`</c->

<c- c1>// Attributes can only be applied on expressions, and not (unparenthesized)</c->
<c- c1>//   assignment-expressions, primary-expressions, etc.:</c->
<c- n>a</c-><c- p>[</c-><c- mi>5</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c->    <c- c1>// ill-formed</c->
<c- n>a</c-><c- p>[</c-><c- n>i</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- p>]</c-> <c- o>=</c-> <c- mi>42</c-><c- p>;</c->      <c- c1>// ill-formed</c->
<c- n>a</c-><c- p>[</c-><c- mi>6</c-><c- p>]</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- o>=</c-> <c- mi>123</c-><c- p>;</c->      <c- c1>// ill-formed</c->

<c- n>x</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- o>=</c-> <c- mi>-1</c-><c- p>;</c->          <c- c1>// ill-formed</c->

<c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>f</c-><c- p>();</c->     <c- c1>// ill-formed</c->
<c- b>int</c-> <c- n>y</c-> <c- o>=</c-> <c- n>f</c-><c- p>()</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]];</c->     <c- c1>// ill-formed (the initializer wants an assignment-expression, not an arbitrary expression)</c->
<c- b>int</c-> <c- n>z</c-> <c- o>=</c-> <c- p>(</c-><c- n>f</c-><c- p>()</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]);</c->   <c- c1>// OK: attr applies to `f()`</c->


<c- c1>// We can apply attributes to arbitrary sub-expressions by parenthesizing them:</c->
<c- c1>// attr1 applies to `x`</c->
<c- c1>// attr2 applies to `y+2`</c->
<c- c1>// attr3 applies to the whole expression</c->
<c- p>(</c-><c- n>x</c-> <c- p>[[</c-><c- n>attr1</c-><c- p>]])</c-> <c- o>=</c-> <c- p>(</c-><c- n>y</c-><c- o>+</c-><c- mi>2</c-> <c- p>[[</c-><c- n>attr2</c-><c- p>]])</c-> <c- p>[[</c-><c- n>attr3</c-><c- p>]];</c->


<c- c1>// attr1 applies to `c.foo()`</c->
<c- c1>// attr2 applies to `*c`</c->
<c- c1>// attr3 applies to the whole requires-expression</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c-> 
<c- k>concept</c-> <c- nc>C</c-> <c- o>=</c-> <c- p>(</c-><c- k>requires</c-> <c- p>(</c-><c- n>C</c-> <c- n>c</c-><c- p>)</c-> <c- p>{</c->
    <c- n>c</c-><c- p>.</c-><c- n>foo</c-><c- p>()</c-> <c- p>[[</c-><c- n>attr1</c-><c- p>]];</c->
    <c- p>{</c-> <c- p>(</c-><c- o>*</c-><c- n>c</c-><c- p>)</c-> <c- p>[[</c-><c- n>attr2</c-><c- p>]]</c-> <c- p>}</c-> <c- o>-></c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-><c- p>;</c->
<c- p>}</c-> <c- p>[[</c-><c- n>attr3</c-><c- p>]]);</c->


<c- c1>// attr1 applies to the statement</c->
<c- c1>// attr2 applies to the closure’s function call operator</c->
<c- c1>// attr3 applies to the closure’s function call operator’s type</c->
<c- c1>// attr4 applies to the overall expression</c->
<c- p>[[</c-><c- n>attr1</c-><c- p>]]</c-> <c- p>[]</c-> <c- p>[[</c-><c- n>attr2</c-><c- p>]]</c-> <c- p>()</c-> <c- p>[[</c-><c- n>attr3</c-><c- p>]]</c-> <c- p>{}</c-> <c- p>()</c-> <c- p>[[</c-><c- n>attr4</c-><c- p>]];</c->

<c- c1>// attr applies to the closure’s function call operator, and not</c->
<c- c1>// to the requires-expression in the requires-clause, as per</c->
<c- c1>// [expr.prim.lambda.general]/3</c->
<c- p>[]</c-><c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c-> <c- k>requires</c-> 
    <c- k>requires</c-> <c- p>(</c-><c- n>T</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c-> <c- o>*</c-><c- n>t</c-><c- p>;</c-> <c- p>}</c->
        <c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- p>()</c-> <c- p>{};</c->
</pre>
   <h4 class="heading settled" data-level="3.2.1" id="problems"><span class="secno">3.2.1. </span><span class="content">Problems</span><a class="self-link" href="#problems"></a></h4>
   <p>The proposed change conflicts with some existing grammar productions. We are aware of at least two.</p>
   <ol>
    <li data-md>
     <p>The production(s) for new expressions for arrays, added by <a data-link-type="biblio" href="#biblio-n3033" title="Core issue 951: Various Attribute Issues">[N3033]</a> as resolution of <a data-link-type="biblio" href="#biblio-cwg951" title="Problems with attribute-specifiers">[CWG951]</a>. In <a href="https://wg21.link/gram.expr">[expr.new]</a> there are the following productions:</p>
<pre class="highlight"><i><c- n>noptr</c-><c- o>-</c-><c- k>new</c-><c- o>-</c-><c- n>declarator</c-><c- o>:</c-></i>
    <c- p>[</c-> <i><c- n>expression</c-><sub><c- n>opt</c-></sub></i> <c- p>]</c-> <i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i>
    <i><c- n>noptr</c-><c- o>-</c-><c- k>new</c-><c- o>-</c-><c- n>declarator</c-></i> <c- p>[</c-> <i><c- n>constant</c-><c- o>-</c-><c- n>expression</c-></i> <c- p>]</c-> <i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i>
</pre>
     <p>with the attribute appertaining to the associated array type. This means that <code class="highlight"><c- k>auto</c-> <c- n>ptr</c-> <c- o>=</c-> <c- p>(</c-><c- k>new</c-> <c- n>T</c-><c- p>[</c-><c- mi>123</c-><c- p>]</c-> <c- p>[[</c-><c- n>someattribute</c-><c- p>]]);</c-></code> is legitimate code today.</p>
     <p>We are unsure about a use case for allowing attributes specifically on new expressions for arrays. (Rather than applying an attribute on the array type right into the new expression, can’t the same intent be better expressed by having an attribute on e.g. a type alias to the array type, while allowing the attribute in <code class="highlight"><c- k>new</c-></code> to appertain to the expression?)</p>
    <li data-md>
     <p>The production(s) for conversion functions in <a href="https://wg21.link/class.conv.fct">[class.conv.fct]</a>, added by <a data-link-type="biblio" href="#biblio-n2761" title="Towards support for attributes in C++ (Revision 6)">[N2761]</a>. A <em>primary-expression</em> can contain a <em>conversion-function-id</em> as subexpression, and the associated grammar allows attributes at the end:</p>
<pre class="highlight"><i><c- n>ptr</c-><c- o>-</c-><c- n>declarator</c-></i> <c- p>(</c-> <i><c- n>parameter</c-><c- o>-</c-><c- n>declaration</c-><c- o>-</c-><c- n>clause</c-></i> <c- p>)</c-> <i><c- n>cv</c-><c- o>-</c-><c- n>qualifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i>
  <i><c- n>ref</c-><c- o>-</c-><c- n>qualifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i> <i><c- k>noexcept</c-><c- o>-</c-><c- n>specifier</c-><sub><c- n>opt</c-></sub></i> <i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i>
</pre>
     <p>Here the attribute appertains to the function type (<a href="https://wg21.link/dcl.fct#1.sentence-2">[dcl.fct]/1</a>). For instance, this code is legitimate:</p>
<pre class="highlight"><c- k>struct</c-> <c- nc>S</c-> <c- p>{</c-> <c- k>operator</c-> <c- b>int</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>ptr</c-> <c- o>=</c-> <c- p>(</c-><c- o>&amp;</c-><c- n>S</c-><c- o>::</c-><c- k>operator</c-> <c- b>int</c-> <c- p>[[</c-><c- n>attribute</c-><c- p>]]);</c->
</pre>
     <p>A similar example is available in <a data-link-type="biblio" href="#biblio-p2173r1" title="Attributes on Lambda-Expressions">[P2173R1]</a>.</p>
     <p>An implementation-specific attribute can, in principle, be used to select a specific overload (since they apply to the type):</p>
<pre class="highlight"><c- c1>// example and explanation courtesy of Richard Smith</c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c-> 
  <c- k>operator</c-> <c- b>int</c-><c- p>()</c-> <c- p>[[</c-><c- n>vendor</c-><c- o>::</c-><c- n>attr1</c-><c- p>]]</c-> <c- k>const</c-><c- p>;</c->  <c- c1>// #1</c->
  <c- k>operator</c-> <c- b>int</c-><c- p>()</c-> <c- p>[[</c-><c- n>vendor</c-><c- o>::</c-><c- n>attr2</c-><c- p>]]</c-> <c- k>const</c-><c- p>;</c->  <c- c1>// #2</c->
<c- p>};</c->

<c- k>auto</c-> <c- n>ptr</c-> <c- o>=</c-> <c- p>(</c-><c- o>&amp;</c-><c- n>S</c-><c- o>::</c-><c- k>operator</c-> <c- b>int</c-> <c- p>[[</c-><c- n>vendor</c-><c- o>::</c-><c- n>attr2</c-><c- p>]]);</c-> <c- c1>// select #2</c->
</pre>
   </ol>
   <p>How to solve these cases? <strong>Here we seek EWGI and EWG guidance.</strong></p>
   <p>A possible solution could be to simply enshrine that, in case of an ambiguity, the tie is resolved in favour of the status-quo.</p>
   <p>If instead grammar changes for these productions are wanted, unfortunately we are unable to evaluate the real-world breakage that could result.</p>
   <h3 class="heading settled" data-level="3.3" id="expressionattributes2"><span class="secno">3.3. </span><span class="content">Attributes on expressions: approach 2</span><a class="self-link" href="#expressionattributes2"></a></h3>
   <p>Instead of allowing attributes on any expression, we may just introduce them on <strong>parenthesized expressions</strong>. In this case we would have room on the <strong>left-hand side</strong>, as there’s a token (the open parenthesis) that separates the expression from anything preceding it.</p>
   <p>This is the grammar change that is required:</p>
<pre class="highlight"><i><c- n>primary</c-><c- o>-</c-><c- n>expression</c-></i><c- o>:</c->
    <i><c- n>literal</c-></i>
    <c- k>this</c->
    <c- p>(</c-> <ins><i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i></ins> <i><c- n>expression</c-></i> <c- p>)</c->
    <i><c- n>id</c-><c- o>-</c-><c- n>expression</c-></i>
    <i><c- n>lambda</c-><c- o>-</c-><c- n>expression</c-></i>
    <i><c- n>fold</c-><c- o>-</c-><c- n>expression</c-></i>
    <i><c- k>requires</c-><c- o>-</c-><c- n>expression</c-></i>
</pre>
   <p>Here’s some examples of attributes on expressions that this approach allows for:</p>
<pre class="highlight"><c- b>int</c-> <c- n>a</c-><c- p>[</c-><c- mi>10</c-><c- p>];</c->

<c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>a</c-><c- p>[</c-><c- mi>0</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c->    <c- c1>// attr applies to the statement</c->
<c- p>([[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>a</c-><c- p>[</c-><c- mi>1</c-><c- p>])</c-> <c- o>=</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c->  <c- c1>// attr applies to `a[1]`</c->
<c- n>a</c-><c- p>[</c-><c- mi>2</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-> <c- p>[[</c-><c- n>attr</c-><c- p>];</c->     <c- c1>// ill-formed</c->
<c- n>a</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- o>=</c-> <c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c->    <c- c1>// ill-formed</c->
<c- n>a</c-><c- p>[</c-><c- mi>4</c-><c- p>]</c-> <c- o>=</c-> <c- p>([[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>x</c-><c- p>)</c-> <c- o>+</c-> <c- n>y</c-><c- p>;</c->  <c- c1>// attr applies to `x`</c->
<c- n>a</c-><c- p>[</c-><c- mi>5</c-><c- p>]</c-> <c- o>=</c-> <c- p>([[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>);</c->  <c- c1>// attr applies to `x + y`</c->
<c- p>([[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>a</c-><c- p>[</c-><c- mi>6</c-><c- p>]</c-> <c- o>=</c-> <c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>);</c->  <c- c1>// attr applies to `a[6] = x + y`</c->


<c- c1>// attr1 applies to the whole requires-expression</c->
<c- c1>// attr2 applies to `c.foo()`</c->
<c- c1>// attr3 applies to `*c`</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- o>></c-> 
<c- k>concept</c-> <c- nc>C</c-> <c- o>=</c-> <c- p>([[</c-><c- n>attr1</c-><c- p>]]</c-> <c- k>requires</c-> <c- p>(</c-><c- n>C</c-> <c- n>c</c-><c- p>)</c-> <c- p>{</c->
    <c- p>([[</c-><c- n>attr2</c-><c- p>]]</c-> <c- n>c</c-><c- p>.</c-><c- n>foo</c-><c- p>());</c->
    <c- p>{</c-> <c- p>([[</c-><c- n>attr3</c-><c- p>]]</c-> <c- o>*</c-><c- n>c</c-><c- p>)</c-> <c- p>}</c-> <c- o>-></c-> <c- n>convertible_to</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-><c- p>;</c->
<c- p>});</c->


<c- c1>// attr1 applies to the statement</c->
<c- c1>// attr2 applies to the overall expression</c->
<c- c1>// attr3 applies to the closure’s function call operator</c->
<c- c1>// attr4 applies to the closure’s function call operator’s type</c->
<c- p>[[</c-><c- n>attr1</c-><c- p>]]</c-> <c- p>([[</c-><c- n>attr2</c-><c- p>]]</c-> <c- p>[]</c-> <c- p>[[</c-><c- n>attr3</c-><c- p>]]</c-> <c- p>()</c-> <c- p>[[</c-><c- n>attr4</c-><c- p>]]</c-> <c- p>{}</c-> <c- p>());</c->
</pre>
   <p>Specifically for attribute <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code>, this approach leads to this syntax/semantics:</p>
<pre class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>();</c->   <c- c1>// no warning, attribute on statement</c->
<c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>());</c-> <c- c1>// no warning, attribute on expression</c->
<c- n>f</c-><c- p>()</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]];</c->   <c- c1>// ill-formed</c->

<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- c1>// no warnings, attribute on statement</c->
<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- c1>// no warnings, attribute on the entire expression</c->
<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- c1>// no warning for a(), possible warning for b()</c->

<c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <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- c1>// no warning; suppressed for a(), and b() isn’t discarded</c->
<c- b>int</c-> <c- n>y</c-> <c- o>=</c-> <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- c1>// no warning; ditto</c->

<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
    <c- n>S</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> 
        <c- o>:</c-> <c- n>m_i</c-><c- p>(([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>check</c-><c- p>(</c-><c- n>i</c-><c- p>)),</c-> <c- n>i</c-><c- p>))</c-> <c- p>{}</c->   <c- c1>// no warning</c->

    <c- b>int</c-> <c- n>m_i</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>From purely an aesthetic point of view, having the attribute on the left-hand of the expression that it appertains to may feel more more "natural"; expressions are usually read left-to-right, and having the attribute at the very end (like in approach 1) may be surprising.</p>
   <p>On the other hand, this approach requires parenthesizing all the expressions that we want to mark with an attribute. Given the relatively rarity of such expressions, the trade-off of the extra syntax can be very acceptable. Again, compared with approach 1, it makes it more clear which sub-expressions are affected by the attribute.</p>
   <p><strong>Again, we seek EWGI and EWG guidance here.</strong></p>
   <p>We are not aware of any conflicts in the grammar for this approach (if there were, they would already by conflicting with the grammar for statements).</p>
   <h3 class="heading settled" data-level="3.4" id="discardvoid"><span class="secno">3.4. </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="3.5" id="discardonnondiscardedvalue"><span class="secno">3.5. </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 expression which isn’t a discarded-value expression?</span><a class="self-link" href="#discardonnondiscardedvalue"></a></h3>
   <p>(Or, similarly, applied to an <em>expression-statement</em> whose expression isn’t discarded-value.)</p>
   <p>For example, using the syntax from approach 1:</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- c1>// not *actually* discarding:</c->
<c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- p>(</c-><c- n>f</c-><c- p>()</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]]);</c->
</pre>
   <p>Should we accept or forbid these usages, as the attribute is meaningless (at best) or misleading (at worst)? For the moment being, 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. Implementations can still diagnose these usages as QoI.</p>
   <hr>
   <p>There is also the broader issue of expressions marked as <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> that contain discarded-value subexpressions:</p>
<pre class="highlight"><c- c1>// f()'s return value is discarded, but g()'s is not</c->
<c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- p>((</c-><c- n>f</c-><c- p>(),</c-> <c- n>g</c-><c- p>())</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]]);</c->
</pre>
   <p>In the above example the attribute applies to the entire comma expression. Should the <code class="highlight"><c- n>f</c-><c- p>()</c-></code> discarded-value subexpression be diagnosed? We propose that the warning should be suppressed even in this case, as the expression is part of a broader one marked as <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>]]</c-></code> (in other words, that the user was OK with the idea of discarding values somewhere <em>inside</em> that expression).</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:</p>
   <ul>
    <li data-md>
     <p>changes to the C++ grammar to allow attributes on expressions;</p>
    <li data-md>
     <p>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 expressions or statements whose result we want to expressly discard.</p>
   </ul>
   <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-n4958" title="Working Draft, Programming Languages — C++">[N4958]</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>There are some slight changes in wording depending on the approach chosen.</p>
   <hr>
   <p><strong>Approach 1 only</strong>: modify the grammar productions for <em>expression</em> in <a href="https://wg21.link/expr.comma">[expr.comma]</a> and in <a href="https://wg21.link/gram.expr">[gram.expr]</a> as shown:</p>
   <blockquote>
<pre class="highlight"><i><c- nl>expression</c-><c- p>:</c-></i>
    <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i> <ins><i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i></ins>
    <i><c- n>expression</c-></i> <c- p>,</c-> <i><c- n>assignment</c-><c- o>-</c-><c- n>expression</c-></i>
</pre>
   </blockquote>
   <p>In <a href="https://wg21.link/expr.comma">[expr.comma]</a>, add a new paragraph after paragraph 1:</p>
   <blockquote>
    <ins>2. The optional <i>attribute-specifier-seq</i> appertains to the expression, unless the expression is a parenthesized expression ([expr.prim.paren]), in which case it appertains to the expression between the parentheses.</ins>
   </blockquote>
   <hr>
   <p><strong>Approach 2 only</strong>: modify the grammar productions for <em>primary-expression</em> in <a href="https://wg21.link/expr.prim">[expr.prim]</a> and in <a href="https://wg21.link/gram.expr">[gram.expr]</a> as shown:</p>
   <blockquote>
<pre class="highlight"><i><c- n>primary</c-><c- o>-</c-><c- n>expression</c-></i><c- o>:</c->
    <i><c- n>literal</c-></i>
    <c- k>this</c->
    <c- p>(</c-> <ins><i><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></i></ins> <i><c- n>expression</c-></i> <c- p>)</c->
    <i><c- n>id</c-><c- o>-</c-><c- n>expression</c-></i>
    <i><c- n>lambda</c-><c- o>-</c-><c- n>expression</c-></i>
    <i><c- n>fold</c-><c- o>-</c-><c- n>expression</c-></i>
    <i><c- k>requires</c-><c- o>-</c-><c- n>expression</c-></i>
</pre>
   </blockquote>
   <p>In <a href="https://wg21.link/expr.prim.paren">[expr.prim.paren]</a>, append a new paragraph:</p>
   <blockquote>
    <ins>2. The optional <i>attribute-specifier-seq</i> appertains to the expression, unless the expression is itself a parenthesized expression, in which case it appertains to the expression between the parentheses.</ins>
   </blockquote>
   <hr>
   <p>Modify <a href="https://wg21.link/dcl.attr.grammar#5.sentence-1">[dcl.attr.grammar]/5</a> as shown:</p>
   <blockquote>
    <ol start="5">
     <li data-md>
      <p>
       Each <i>attribute-specifier-seq</i> is said to <i>appertain</i> to some entity
       <ins>, expression</ins>
        or statement, identified by the syntactic context where it appears ([stmt.stmt], [dcl.dcl], [dcl.decl]
       <ins>, [<b>LINK</b>]</ins>
       ).
      </p>
    </ol>
   </blockquote>
   <p>With <em>LINK</em> being "expr.comma" for approach 1 and "expr.prim" for approach 2.</p>
   <hr>
   <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 <em>expression</em> (or subexpression thereof) marked with the <code class="highlight"><c- n>discard</c-></code> attribute ([dcl.attr.discard]), 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>: Appearance of a 
    <del>nodiscard call as a potentially-evaluated discarded-value expression ([expr.prop])</del>
    <ins>discouraged nodiscard call</ins>
     is discouraged 
    <del>unless explicitly cast to <code class="highlight"><c- b>void</c-></code></del>
    . Implementations should issue a warning in such cases. [...] 
   </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"</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->
  
  <c- c1>// approach 1:  </c->
  <c- n>my_scopeguard</c-><c- p>()</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]],</c->
    <c- n>launch_missiles</c-><c- p>();</c->                     <c- c1>// warning not encouraged</c->
    
  <c- n>my_scopeguard</c-><c- p>()</c-> <c- p>[[</c-><c- n>discard</c-><c- p>]],</c->             <c- c1>// warning not encouraged</c->
    <c- n>my_unique</c-><c- p>(</c-><c- mi>42</c-><c- p>);</c->                         <c- c1>// warning encouraged </c->
    
  <c- c1>// approach 2:  </c->
  <c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>my_scopeguard</c-><c- p>()),</c->
    <c- n>launch_missiles</c-><c- p>();</c->                     <c- c1>// warning not encouraged</c->
    
  <c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>my_scopeguard</c-><c- p>()),</c->           <c- c1>// warning not encouraged</c->
    <c- n>my_unique</c-><c- p>(</c-><c- mi>42</c-><c- p>);</c->                         <c- c1>// warning 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</em> or 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 <em>expression</em> (or subexpression thereof) marked as <code class="highlight"><c- n>discard</c-></code>, or 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 discard 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-cwg951">[CWG951]
   <dd>Sean Hunt. <a href="https://wg21.link/cwg951"><cite>Problems with attribute-specifiers</cite></a>. 5 August 2009. CD2. URL: <a href="https://wg21.link/cwg951">https://wg21.link/cwg951</a>
   <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-n2761">[N2761]
   <dd>J. Maurer, M. Wong. <a href="https://wg21.link/n2761"><cite>Towards support for attributes in C++ (Revision 6)</cite></a>. 18 September 2008. URL: <a href="https://wg21.link/n2761">https://wg21.link/n2761</a>
   <dt id="biblio-n3033">[N3033]
   <dd>Daveed Vandevoorde. <a href="https://wg21.link/n3033"><cite>Core issue 951: Various Attribute Issues</cite></a>. 5 February 2010. URL: <a href="https://wg21.link/n3033">https://wg21.link/n3033</a>
   <dt id="biblio-n4958">[N4958]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4958"><cite>Working Draft, Programming Languages — C++</cite></a>. 14 August 2023. URL: <a href="https://wg21.link/n4958">https://wg21.link/n4958</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-p2173r1">[P2173R1]
   <dd>Daveed Vandevoorde, Inbal Levi, Ville Voutilainen. <a href="https://wg21.link/p2173r1"><cite>Attributes on Lambda-Expressions</cite></a>. 9 December 2021. URL: <a href="https://wg21.link/p2173r1">https://wg21.link/p2173r1</a>
   <dt id="biblio-p2968r0">[P2968R0]
   <dd>Peter Sommerlad. <a href="https://wg21.link/p2968r0"><cite>Make std::ignore a first-class object</cite></a>. 7 September 2023. URL: <a href="https://wg21.link/p2968r0">https://wg21.link/p2968r0</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>