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  <title>P3093R0: 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 */
.css.css, .property.property, .descriptor.descriptor {
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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">P3093R0<br>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="2024-01-19">2024-01-19</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:giuseppe.dangelo@kdab.com">Giuseppe D'Angelo</a>
     <dt>Audience:
     <dd>EWG, SG22
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>We propose 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="#usecases"><span class="secno">2.1</span> <span class="content">Use cases</span></a>
      <li><a href="#history"><span class="secno">2.2</span> <span class="content">Why doesn’t C++ already support attributes on expressions?</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="#production"><span class="secno">3.1</span> <span class="content">How to support expression attributes in the C++ grammar</span></a>
      <li>
       <a href="#rejectedapproaches"><span class="secno">3.2</span> <span class="content">Rejected approaches</span></a>
       <ol class="toc">
        <li><a href="#problems"><span class="secno">3.2.1</span> <span class="content">Problems</span></a>
       </ol>
     </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. This paper has been split from <a data-link-type="biblio" href="#biblio-p2992r0" title="Attribute [[discard]] and attributes on expressions">[P2992R0]</a>.</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>The C++ grammar does not allow for attributes on arbitrary expressions.</p>
   <p>For instance, a snippet like this:</p>
<pre class="highlight"><c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- p>([[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-><c- p>));</c->
</pre>
   <p>is ill-formed and rejected (with "interesting" error messages) by GCC,
Clang and MSVC.</p>
   <p>The following code is instead well-formed:</p>
<pre class="highlight"><c- p>[[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-><c- p>);</c->
</pre>
   <p>The reason why this code is legal (and the previous is not) is that
here the attribute appertains to the <strong>statement</strong>, not to the <strong>expression</strong>.</p>
   <p>The grammar productions that are relevant are the <em>statement</em> and <em>expression-statement</em> productions (<a href="https://wg21.link/stmt.pre">[stmt.pre]</a>, <a href="https://wg21.link/stmt.expr">[stmt.expr]</a>):</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>and the production for <em>expression</em> itself (<a href="https://wg21.link/expr.comma">[expr.comma]</a>):</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>There are no other productions for expressions that allow for an
attribute to be present, and this explains why the first code was illegal.</p>
   <p>Here are some more examples of illegal placement of attributes on expressions:</p>
<pre class="highlight"><c- c1>// All currently ill-formed:</c->

<c- c1>// parenthesized version of the above:</c->
<c- p>([[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-><c- p>));</c->

<c- c1>// attribute on function argument:</c->
<c- n>process</c-><c- p>(([[</c-><c- n>lock</c-><c- p>]]</c-> <c- n>g</c-><c- p>()),</c-> <c- mi>42</c-><c- p>);</c->

<c- c1>// in a comma expression:</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>,</c-> <c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>()))</c->
    <c- n>doSomething</c-><c- p>(</c-><c- n>i</c-><c- p>);</c->

<c- c1>// in a member initialization list:</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>debug_only</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- b>int</c-> <c- n>m_i</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p><strong>This paper proposes to allow attributes on expressions.</strong></p>
   <h3 class="heading settled" data-level="2.1" id="usecases"><span class="secno">2.1. </span><span class="content">Use cases</span><a class="self-link" href="#usecases"></a></h3>
   <p>This paper is a spin-off of <a data-link-type="biblio" href="#biblio-p2992r0" title="Attribute [[discard]] and attributes on expressions">[P2992R0]</a>, which is proposing the
addition of the <code class="highlight"><c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- n>reason</c-><c- p>)]]</c-></code> attribute as a more expressive
version of a cast to <code class="highlight"><c- b>void</c-></code>.</p>
   <p>Such an attribute is meant to be used in places where a programmer
deliberately wants to discard the result of a <code class="highlight"><c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-></code> function call,
suppressing the warning that the implementation would otherwise raise.</p>
   <p>As such, the attribute should be placed wherever a function call can
appear, which is (in the general case) a sub-expression:</p>
<pre class="highlight"><c- c1>// returns an error code to be checked</c->
<c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- b>int</c-> <c- n>f</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>);</c->

<c- c1>// attribute on statement, already possible:</c->
<c- p>[[</c-><c- n>discard</c-><c- p>(</c-><c- s>"f always succeeds for 42"</c-><c- p>)]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>42</c-><c- p>);</c->

<c- c1>// attribute on expression, not currently possible:</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>,</c-> <c- p>([[</c-><c- n>discard</c-><c- p>(</c-><c- s>"f succeeds for inputs >= 0"</c-><c- p>)]]</c-> <c- n>f</c-><c- p>(</c-><c- n>i</c-><c- p>)))</c->
    <c- n>doSomething</c-><c- p>(</c-><c- n>i</c-><c- p>);</c->
</pre>
   <p>One can concoct other similar situations:
in <a data-link-type="biblio" href="#biblio-guardnounorverb" title="Guard nouns or guard verbs?">this blog post</a> Arthur O’Dwyer makes an example
of using <a data-link-type="biblio" href="#biblio-p2946r1" title="A flexible solution to the problems of `noexcept`">[P2946R1]</a>'s <code class="highlight"><c- p>[[</c-><c- n>throws_nothing</c-><c- p>]]</c-></code> attribute as a
statement/expression attribute, as a way to make the compiler aware
that a function to a non-<code class="highlight"><c- k>noexcept</c-></code> function will in fact never throw
an exception, and thus the compiler can do a better job at optimizing
the call:</p>
<pre class="highlight"><c- c1>// as statement attribute:</c->
<c- p>[[</c-><c- n>throws_nothing</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>42</c-><c- p>);</c->

<c- c1>// as expression attribute:</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>throws_nothing</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- n>i</c-><c- p>)))</c-> <c- p>{}</c->

    <c- b>int</c-> <c- n>m_i</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <h3 class="heading settled" data-level="2.2" id="history"><span class="secno">2.2. </span><span class="content">Why doesn’t C++ already support attributes on expressions?</span><a class="self-link" href="#history"></a></h3>
   <p>A possible reason for this is offered by <a data-link-type="biblio" href="#biblio-n2761" title="Towards support for attributes in C++ (Revision 6)">[N2761]</a> ("Towards support
for attributes in C++"), where in Chapter 7 it is argued that a feature
"used in expressions as opposed to declarations" should "use/reuse a
keyword" instead.</p>
   <p>Adding a keyword has however a very high barrier and cost for the
language and ecosystem.</p>
   <p>A keyword is also fundamentally different from an attribute: a keyword
is not ignorable, while an attribute can be ignored. A vendor cannot
add vendor-specific keywords without forking the language, but they can
add vendor-specific attributes. With the current rules on attribute
ignorability (cf. <a data-link-type="biblio" href="#biblio-p2552r3" title="On the ignorability of standard attributes">[P2552R3]</a>), standard attributes have "optional
semantics", while any other attribute is either picked up by the
implementation or it must be ignored (<a href="https://eel.is/c++draft/dcl.attr.grammar#6.sentence-1">[dcl.attr.grammar]/6</a>).</p>
   <p>For this reason we think that attributes should be supported on
expressions.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design Decisions</span><a class="self-link" href="#design"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="production"><span class="secno">3.1. </span><span class="content">How to support expression attributes in the C++ grammar</span><a class="self-link" href="#production"></a></h3>
   <p>An "obvious" modification of the <em>expression</em> 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>This change however clashes with the <em>statement</em> 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>

<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>resulting in an ambiguity for a statement like this:</p>
<pre class="highlight"><c- p>[[</c-><c- n>attribute</c-><c- p>]]</c-> <c- n>x</c-> <c- o>=</c-> <c- mi>42</c-><c- p>;</c-> <c- c1>// is this a statement attribute or an expression attribute?</c->
</pre>
   <p>Changing the meaning of the snippet above would be a
source-incompatible break, because it could alter the semantics of the
attribute and/or make the code ill-formed (in case the attribute can
only appertain to statements). This is something that we do not want to
do.</p>
   <p>We also do not want to complicate the grammar and/or the semantics of
attributes, for instance by:</p>
   <ul>
    <li data-md>
     <p>having each attribute "state" somehow if it should apply to statements or expressions;</p>
    <li data-md>
     <p>adding normative wording to disambiguate the above case in favor of the status quo, that is, make the attribute always appertain to the statement. This would still leave us with the problem of how to apply an attribute to the expression in the snippet.</p>
   </ul>
   <hr>
   <p>Instead, we are going to propose a different change in the grammar:
allow attributes only on <strong>parenthesized expressions</strong>. In this case
there’s a token (the open parenthesis) that separates the expression
from anything preceding it, avoiding the clash.</p>
   <p>The extra verbosity of having to use parenthesis is justified by the
fact that attributes are rarely used anyhow.</p>
   <p>This is the grammar change that we are proposing:</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>We also also going to special-case the semantics of parenthesized
expressions, so that their attribute applies to the inner expression.</p>
   <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- 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>3</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>4</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- 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- p>(</c-><c- n>x</c-> <c- o>+</c-> <c- n>y</c-><c- p>));</c->  <c- c1>// ditto, parenthesized sub-expression</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- 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-> <c- p>);</c->
</pre>
   <p>The previous examples would all become well-formed:</p>
<pre class="highlight"><c- c1>// OK, applies to the entire expression</c->
<c- p>([[</c-><c- n>attr</c-><c- p>]]</c-> <c- n>f</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-><c- p>));</c->

<c- c1>// OK, applies to `g()`</c->
<c- n>process</c-><c- p>(([[</c-><c- n>lock</c-><c- p>]]</c-> <c- n>g</c-><c- p>()),</c-> <c- mi>42</c-><c- p>);</c->

<c- c1>// OK, applies to `f()`</c->
<c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>,</c-> <c- p>([[</c-><c- n>discard</c-><c- p>]]</c-> <c- n>f</c-><c- p>()))</c->
    <c- n>doSomething</c-><c- p>(</c-><c- n>i</c-><c- p>);</c->

<c- c1>// OK, applies to `i`</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>debug_only</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- b>int</c-> <c- n>m_i</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>Despite the extra verbosity, we strongly believe that by using
parenthesis, it becomes very clear to which sub-expression an attribute
appertains to.</p>
   <p>We are also confident that this grammar change does not result in any
ambiguity or conflicts. (If it did, such conflicts would already exist
with the grammar for statements.)</p>
   <h3 class="heading settled" data-level="3.2" id="rejectedapproaches"><span class="secno">3.2. </span><span class="content">Rejected approaches</span><a class="self-link" href="#rejectedapproaches"></a></h3>
   <p>Given the grammar clash described above, if we do not want users to
have to add parenthesis to every expression they want to tag with an
attribute, we could decide to allow attributes on the <b>right hand
side</b> of an <i>expression</i>.</p>
   <p>We could 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>  <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>Here are some examples of what this approach would look like:</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>This approach has a number of shortcomings.</p>
   <p>The biggest one is purely esthetical: having attributes on the right
hand side of the entity they appertain to feels very unnatural, an
impedance mismatch with the rest of the language. In this snippet:</p>
<pre class="highlight"><c- n>result</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->
</pre>
   <p>it’s not obvious at all that the attribute is being applied to the
entire expression (and not just to <code class="highlight"><c- n>y</c-></code> or to <code class="highlight"><c- n>x</c-> <c- o>+</c-> <c- n>y</c-></code>).</p>
   <p>A second limitation is due to the fact that, by changing only the <em>expression</em> grammar production, we would not actually allow attributes
on <strong>all</strong> possible kinds of sub-expressions. For instance, this would
be ill-formed:</p>
<pre class="highlight"><c- n>result</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>// still illegal with the grammar change</c->
</pre>
   <p>because <code class="highlight"><c- n>x</c-></code> isn’t a result of the <em>expression</em> production.</p>
   <p>Complicating the grammar to allow for attributes "everywhere" is likely
not worth the effort, because one can always wrap a subexpression in
parenthesis in order to apply an attribute to it. Still, the above code
could be surprising.</p>
   <p>Finally, this approach also 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? A possible solution could be to simply
enshrine that, in case of an ambiguity, the tie is resolved in favour
of the status-quo. If instead grammar changes for these productions are
wanted, unfortunately we are unable to evaluate the real-world breakage
that could result.</p>
   <p>We do not feel comfortable at introducing breaking changes, so, once
more, we are not pursuing this approach.</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 changes to the C++ grammar to allow attributes on expressions.</p>
   <p>No changes are required in the Standard Library.</p>
   <h2 class="heading settled" data-level="5" id="technicalspecifications"><span class="secno">5. </span><span class="content">Technical Specifications</span><a class="self-link" href="#technicalspecifications"></a></h2>
   <p>All the proposed changes are relative to <a data-link-type="biblio" href="#biblio-n4971" title="Working Draft, Programming Languages — C++">[N4971]</a>.</p>
   <h3 class="heading settled" data-level="5.1" id="proposedwording"><span class="secno">5.1. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposedwording"></a></h3>
   <p>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
       <del> or</del>
       <ins>,</ins>
        statement
       <ins> or expression</ins>
       , identified by the syntactic context where it appears ([stmt.stmt], [dcl.dcl], [dcl.decl]
       <ins>, [<b>expr.prim</b>]</ins>
       ). If an <i>attribute-specifier-seq</i> that appertains to some entity
       <del> or</del>
       <ins>,</ins>
        statement 
       <ins>or expression</ins>
        contains an attribute or alignment-specifier that is not allowed to apply to that entity
       <del> or</del>
       <ins>,</ins>
        statement
       <ins> or expression</ins>
       , the program is ill-formed. If an <i>attribute-specifier-seq</i> appertains to a friend declaration ([class.friend]), that declaration shall be a definition.
      </p>
    </ol>
   </blockquote>
   <hr>
   <p>Modify the "Feature-test macros" table in <a href="https://eel.is/c++draft/tab:cpp.predefined.ft">[tab:cpp.predefined.ft]</a>, by adding a new row as shown:</p>
   <table>
    <thead>
     <tr>
      <th>Macro name
      <th>Value
    <tbody>
     <tr>
      <td>
       <ins><code class="highlight"><c- n>__cpp_expression_attributes</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>
   <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-guardnounorverb">[GUARDNOUNORVERB]
   <dd>Arthur O'Dwyer. <a href="https://quuxplusone.github.io/blog/2023/11/11/guard-nouns-or-guard-verbs/"><cite>Guard nouns or guard verbs?</cite></a>. URL: <a href="https://quuxplusone.github.io/blog/2023/11/11/guard-nouns-or-guard-verbs/">https://quuxplusone.github.io/blog/2023/11/11/guard-nouns-or-guard-verbs/</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-n4971">[N4971]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4971"><cite>Working Draft, Programming Languages — C++</cite></a>. 18 December 2023. URL: <a href="https://wg21.link/n4971">https://wg21.link/n4971</a>
   <dt id="biblio-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-p2552r3">[P2552R3]
   <dd>Timur Doumler. <a href="https://wg21.link/p2552r3"><cite>On the ignorability of standard attributes</cite></a>. 14 June 2023. URL: <a href="https://wg21.link/p2552r3">https://wg21.link/p2552r3</a>
   <dt id="biblio-p2946r1">[P2946R1]
   <dd>Pablo Halpern. <a href="https://wg21.link/p2946r1"><cite>A flexible solution to the problems of `noexcept`</cite></a>. 16 January 2024. URL: <a href="https://wg21.link/p2946r1">https://wg21.link/p2946r1</a>
   <dt id="biblio-p2992r0">[P2992R0]
   <dd>Giuseppe D'Angelo. <a href="https://wg21.link/p2992r0"><cite>Attribute [[discard]] and attributes on expressions</cite></a>. 10 October 2023. URL: <a href="https://wg21.link/p2992r0">https://wg21.link/p2992r0</a>
  </dl>