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  <title>P2752R2: Static storage for braced initializers</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;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom: 3px solid #054572;
			border-bottom: 3px solid var(--toclink-underline);
			margin-bottom: -3px;
		}
		#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 3383f409d, updated Fri Apr 28 15:49:22 2023 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
hilite  {background-color: #FFFF00; font-weight: bold;}
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* 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">P2752R2<br>Static storage for braced initializers</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="2023-05-14">2023-05-14</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:arthur.j.odwyer@gmail.com">Arthur O'Dwyer</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Draft Revision:
     <dd>8
     <dt>Current Source:
     <dd><a href="https://github.com/Quuxplusone/draft/blob/gh-pages/d2752-static-storage-for-braced-initializers.bs">github.com/Quuxplusone/draft/blob/gh-pages/d2752-static-storage-for-braced-initializers.bs</a>
     <dt>Current:
     <dd><a href="https://rawgit.com/Quuxplusone/draft/gh-pages/d2752-static-storage-for-braced-initializers.html">rawgit.com/Quuxplusone/draft/gh-pages/d2752-static-storage-for-braced-initializers.html</a>
    </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>Initializing a vector from a <i>braced-initializer-list</i> like <code class="highlight"><c- p>{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>}</c-></code> copies the data from static storage to a backing array on the stack,
  and thence into the vector. This wastes CPU cycles, and it wastes stack space.
  Eliminate the waste by letting a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>’s
  backing array occupy static storage.</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="#background"><span class="secno">2</span> <span class="content">Background</span></a>
     <ol class="toc">
      <li><a href="#workarounds"><span class="secno">2.1</span> <span class="content">Workarounds</span></a>
     </ol>
    <li><a href="#solution"><span class="secno">3</span> <span class="content">Solution</span></a>
    <li><a href="#constexpr-evaluation"><span class="secno">4</span> <span class="content">Constexpr evaluation</span></a>
    <li>
     <a href="#experience"><span class="secno">5</span> <span class="content">Implementation experience</span></a>
     <ol class="toc">
      <li><a href="#fmerge-all-constants"><span class="secno">5.1</span> <span class="content">GCC <code class="highlight"><c- o>-</c-><c- n>fmerge</c-><c- o>-</c-><c- n>all</c-><c- o>-</c-><c- n>constants</c-></code></span></a>
     </ol>
    <li>
     <a href="#wording"><span class="secno">6</span> <span class="content">Proposed wording relative to the current C++23 draft</span></a>
     <ol class="toc">
      <li><a href="#wording-annexc"><span class="secno">6.1</span> <span class="content">Addition to Annex C</span></a>
     </ol>
    <li><a href="#acknowledgments"><span class="secno">7</span> <span class="content">Acknowledgments</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>R2 (pre-Varna 2023):</p>
     <ul>
      <li data-md>
       <p>Add discussion of unspecified behavior in constexpr evaluation.</p>
     </ul>
    <li data-md>
     <p>R1 (post-Issaquah 2023):</p>
     <ul>
      <li data-md>
       <p>Remove discussion of mangling; it’s not a problem. Add discussion of GCC’s <code class="highlight"><c- o>-</c-><c- n>fmerge</c-><c- o>-</c-><c- n>all</c-><c- o>-</c-><c- n>constants</c-></code>.</p>
      <li data-md>
       <p>Proposed wording reviewed by Jens Maurer.</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="background"><span class="secno">2. </span><span class="content">Background</span><a class="self-link" href="#background"></a></h2>
   <p><a href="https://eel.is/c++draft/dcl.init.list#5">[dcl.init.list]/5–6</a> says:</p>
   <p><small></small></p>
   <blockquote><small> An object of type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> is constructed from an initializer list
as if the implementation generated and materialized a prvalue of type “array of <em>N</em> <code class="highlight"><c- k>const</c-> <c- n>E</c-></code>”,
where <em>N</em> is the number of elements in the initializer list. Each element of that array
is copy-initialized with the corresponding element of the initializer list,
and the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> object is constructed to refer to that array.<p></p> <p>[<i>Example 12:</i><small></small></p><pre class="highlight"><small>    <c- k>struct</c-> <c- nc>X</c-> <c- p>{</c->
      <c- n>X</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-> <c- n>v</c-><c- p>);</c->
    <c- p>};</c->
    <c- n>X</c-> <c- n>x</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-><p></p>
</small></pre> The initialization will be implemented in a way roughly equivalent to this: <small><pre class="highlight">    <c- k>const</c-> <c- b>double</c-> <c- n>__a</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- o>=</c-> <c- p>{</c-><c- b>double</c-><c- p>{</c-><c- mi>1</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- mi>2</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- mi>3</c-><c- p>}};</c->
    <c- n>X</c-> <c- nf>x</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-><c- p>(</c-><c- n>__a</c-><c- p>,</c-> <c- n>__a</c-><c- o>+</c-><c- mi>3</c-><c- p>));</c->
</pre></small> assuming that the implementation can construct an <code class="highlight"><c- n>initializer_list</c-></code> object with a pair of pointers. <i>—end example</i>] [...] <p><b>[<i>Note 6: The implementation is free to allocate the array in read-only memory
if an explicit array with the same initializer can be so allocated. —end note</i>]</b></p> </small></blockquote>
   <p>In December 2022, Jason Merrill observed (<a data-link-type="biblio" href="#biblio-corereflector" title="[isocpp-core] initializer_list and read-only memory">[CoreReflector]</a>) that this note isn’t saying much.
Consider the following translation unit:</p>
<pre class="language-c++ highlight"><c- b>void</c-> <c- nf>f</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>il</c-><c- p>);</c->

<c- b>void</c-> <c- nf>g</c-><c- p>()</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- p>}</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c-> <c- n>g</c-><c- p>();</c-> <c- p>}</c->
</pre>
   <p>Can the backing array for <code class="highlight"><c- p>{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>}</c-></code> be allocated in static storage?
No, because <code class="highlight"><c- n>f</c-></code>'s implementation might look like this:</p>
<pre class="language-c++ highlight"><c- b>void</c-> <c- nf>g</c-><c- p>();</c->

<c- b>void</c-> <c- nf>f</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>il</c-><c- p>)</c-> <c- p>{</c->
    <c- k>static</c-> <c- k>const</c-> <c- b>int</c-> <c- o>*</c-><c- n>ptr</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->
    <c- k>if</c-> <c- p>(</c-><c- n>ptr</c-> <c- o>==</c-> <c- n>nullptr</c-><c- p>)</c-> <c- p>{</c->
        <c- n>ptr</c-> <c- o>=</c-> <c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>();</c->
        <c- n>g</c-><c- p>();</c->
    <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
        <c- n>assert</c-><c- p>(</c-><c- n>ptr</c-> <c- o>!=</c-> <c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>());</c->
    <c- p>}</c->
<c- p>}</c->
</pre>
   <p>A conforming C++23 implementation must compile this code in such a way that the
two temporary backing arrays — the one pointed to by <code class="highlight"><c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-></code> during the first
recursive call to <code class="highlight"><c- n>f</c-></code>, and the one pointed to by <code class="highlight"><c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-></code> during the second recursive call
to <code class="highlight"><c- n>f</c-></code> — have distinct addresses, because that would also be true of "an explicit array [variable]"
in the scope of <code class="highlight"><c- n>g</c-></code>.</p>
   <p>All three of GCC, Clang, and MSVC compile this code in a conforming manner.
(<a href="https://godbolt.org/z/KExEG4a6v">Godbolt.</a>)</p>
   <p>Expert programmers tend to understand that when they write</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>v</c-> <c- o>=</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- mi>4</c-><c- p>,</c-><c- mi>5</c-><c- p>,</c-><c- mi>6</c-><c- p>,</c-><c- mi>7</c-><c- p>,</c-><c- mi>8</c-><c- p>,</c-><c- mi>9</c-><c- p>,</c-><c- mi>10</c-><c- p>};</c->
</pre>
   they’re getting a copy of the data from its original storage into the heap-allocated
STL container: obviously the data has to get from point A to point B somehow.
But even expert programmers are surprised to learn that there are actually <em>two</em> copies happening here — one from static storage onto the stack, and another from
stack to heap! 
   <p>Worse: <a data-link-type="biblio" href="#biblio-p1967r9" title="#embed - a scannable, tooling-friendly binary resource inclusion mechanism">[P1967R9]</a> (adopted for C++26) allows programmers to write</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>char</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- p>{</c->
    <c- cp>#embed "2mb-image.png"</c->
<c- p>};</c->
</pre>
   Suppose "2mb-image.png" contains 2MB of data. Then the function that initializes <code class="highlight"><c- n>v</c-></code> here will create a temporary backing array of size 2MB. That is, we’re adding 2MB to
the stack frame of that function.
Suddenly your function’s stack frame is 2MB larger than you expected!
This applies even if the "function" in question is the compiler-generated
initialization routine for a dynamically initialized global variable <code class="highlight"><c- n>v</c-></code>.
You might not even control the function whose stack frame is blowing up. 
   <p>This kind of thing was always possible pre-P1967, but was hard to hit in human-generated code,
because braced initializer lists were generally short. But post-P1967, this is easy
to hit. I think this stack-frame blowup will become a well-known problem unless we solve it first.</p>
   <h3 class="heading settled" data-level="2.1" id="workarounds"><span class="secno">2.1. </span><span class="content">Workarounds</span><a class="self-link" href="#workarounds"></a></h3>
   <p>This code creates a 2MB backing array on the stack frame of the function that initializes <code class="highlight"><c- n>v</c-></code>:</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>char</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- p>{</c->
    <c- cp>#embed "2mb-image.png"</c->
<c- p>};</c->
</pre>
   This code does not: 
<pre class="language-c++ highlight"><c- k>static</c-> <c- k>const</c-> <c- b>char</c-> <c- n>backing</c-><c- p>[]</c-> <c- o>=</c-> <c- p>{</c->
    <c- cp>#embed "2mb-image.png"</c->
<c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>char</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>char</c-><c- o>></c-><c- p>(</c-><c- n>backing</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>end</c-><c- p>(</c-><c- n>backing</c-><c- p>));</c->
</pre>
   So the latter is a workaround. But it shouldn’t be necessary to work around this issue;
we should fix it instead. 
   <p>JeanHeyd Meneide points out that C’s "compound literals" have the same problem, and they
chose to permit the programmer to work around it in C23 by adding storage class annotations
directly to their literals, like this:</p>
<pre class="language-c++ highlight"><c- b>int</c-> <c- nf>f</c-><c- p>(</c-><c- k>const</c-> <c- b>int</c-> <c- o>*</c-><c- n>p</c-><c- p>);</c->
<c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- n>f</c-><c- p>((</c-><c- k>static</c-> <c- k>const</c-> <c- b>int</c-><c- p>[]){</c-> <c- c1>// C23 syntax</c->
    <c- cp>#embed "2mb-image.png"</c->
<c- p>});</c->
</pre>
   Zhihao Yuan’s <a data-link-type="biblio" href="#biblio-p2174r1" title="Compound Literals">[P2174R1]</a> proposes adding C-compatible compound literals to C++, but does not propose storage-class
annotations. Anyway, that’s not a solution to the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-></code> problem, because <code class="highlight"><c- n>initializer_list</c-></code> syntax is lightweight by design. We don’t need new syntax; we just need the existing syntax to
Do The Right Thing by default. 
   <h2 class="heading settled" data-level="3" id="solution"><span class="secno">3. </span><span class="content">Solution</span><a class="self-link" href="#solution"></a></h2>
   <p>Essentially we want the semantics of braced initializer lists to match the semantics
of string literals (<a href="https://timsong-cpp.github.io/cppwp/n4861/lex.string#9">[lex.string]/9</a>).</p>
   <p>We want to encourage tomorrow’s compilers to avoid taking up any stack space for
constant-initialized initializer lists. If possible I’d like to <em>mandate</em> they not take
any stack space, but I don’t currently know how to specify that. Quality implementations
will take advantage of this permission anyway.</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>i1</c-> <c- o>=</c-> <c- p>{</c->
    <c- cp>#embed "very-large-file.png"  </c-><c- c1>// OK</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>f2</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>ia</c-><c- p>,</c->
        <c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>ib</c-><c- p>)</c-> <c- p>{</c->
    <c- n>PERMIT</c-><c- p>(</c-><c- n>ia</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-> <c- o>==</c-> <c- n>ib</c-><c- p>.</c-><c- n>begin</c-><c- p>());</c->
<c- p>}</c->
<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- n>f2</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- p>{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>});</c->
<c- p>}</c->
</pre>
   <p>We want to permit tomorrow’s compilers to share backing arrays with elements in common, just like
today’s compilers can share string literals. High-quality implementations might take advantage
of this permission; mainstream compilers probably won’t bother.</p>
<pre class="language-c++ highlight"><c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- n>p1</c-> <c- o>=</c-> <c- s>"hello world"</c-><c- p>;</c->
<c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- n>p2</c-> <c- o>=</c-> <c- s>"world"</c-><c- p>;</c->
<c- n>PERMIT</c-><c- p>(</c-><c- n>p2</c-> <c- o>==</c-> <c- n>p1</c-> <c- o>+</c-> <c- mi>6</c-><c- p>);</c->  <c- c1>// today’s behavior</c->

<c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>i1</c-> <c- o>=</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- mi>4</c-><c- p>,</c-><c- mi>5</c-><c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>i2</c-> <c- o>=</c-> <c- p>{</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>,</c-><c- mi>4</c-><c- p>};</c->
<c- n>PERMIT</c-><c- p>(</c-><c- n>i1</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-> <c- o>==</c-> <c- n>i2</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-> <c- o>+</c-> <c- mi>1</c-><c- p>);</c->  <c- c1>// tomorrow’s proposed behavior</c->
</pre>
   <p>We even intend to permit tomorrow’s compilers to share backing arrays between static
and dynamic initializer lists, if they can prove it’s safe.</p>
   <p>The lifetime of the backing array remains tied to its original <code class="highlight"><c- n>initializer_list</c-></code>.
Accessing the backing array outside its lifetime (as in <code class="highlight"><c- n>f4</c-></code>) remains UB. <a href="https://eel.is/c++draft/class.base.init#11">[class.base.init]/11</a>,
which makes it ill-formed to bind a reference member to a temporary (as in <code class="highlight"><c- n>C5</c-></code>), is also unaffected.
(As of December 2022, Clang diagnoses <code class="highlight"><c- n>C5</c-></code>; GCC doesn’t; MSVC gives a possibly unrelated error message.)</p>
<pre class="language-c++ highlight"><c- k>const</c-> <c- b>int</c-> <c- o>*</c-><c- nf>f4</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>i4</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>i4</c-><c- p>.</c-><c- n>begin</c-><c- p>();</c->
<c- p>}</c->
<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- k>const</c-> <c- b>int</c-> <c- o>*</c-><c- n>p</c-> <c- o>=</c-> <c- n>f4</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- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- o>*</c-><c- n>p</c-><c- p>;</c->  <c- c1>// still UB, not OK</c->
<c- p>}</c->

<c- k>struct</c-> <c- nc>C5</c-> <c- p>{</c->
    <c- n>C5</c-><c- p>()</c-> <c- o>:</c-> <c- n>i5</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- p>{}</c->  <c- c1>// still ill-formed, not OK</c->
    <c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>i5</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>We do not permit deferring, combining, or omitting the side effects of
constructors or destructors related to the backing array.
In practice, compilers will "static-fy" backing arrays of
types that are constinit-constructible and trivially destructible,
and not "static-fy" anything else.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>C6</c-> <c- p>{</c->
    <c- n>constexpr</c-> <c- nf>C6</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- p>{}</c->
    <c- o>~</c-><c- n>C6</c-><c- p>()</c-> <c- p>{</c-> <c- n>printf</c-><c- p>(</c-><c- s>" X"</c-><c- p>);</c-> <c- p>}</c->
<c- p>};</c->
<c- b>void</c-> <c- nf>f6</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>C6</c-><c- o>></c-><c- p>)</c-> <c- p>{}</c->
<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- n>f6</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>// must still print X X X</c->
    <c- n>f6</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>// must still print X X X</c->
<c- p>}</c->
</pre>
   <h2 class="heading settled" data-level="4" id="constexpr-evaluation"><span class="secno">4. </span><span class="content">Constexpr evaluation</span><a class="self-link" href="#constexpr-evaluation"></a></h2>
   <p class="note" role="note"><span class="marker">Note:</span> This section is new in R2. Thanks to Lénárd Szolnoki for the tip.</p>
   <p>Lénárd Szolnoki points out that P2752 introduces a new way for a pointer comparison
to be "unspecified." This is fine, but it is also observable.</p>
<pre class="language-c++ highlight"><c- n>constexpr</c-> <c- b>bool</c-> <c- n>f9</c-><c- p>(</c-><c- k>const</c-> <c- b>int</c-> <c- o>*</c-><c- n>p</c-><c- p>)</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>il</c-> <c- o>=</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- k>return</c-> <c- n>p</c-> <c- o>?</c-> <c- p>(</c-><c- n>p</c-> <c- o>==</c-> <c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>())</c-> <c- o>:</c-> <c- n>f9</c-><c- p>(</c-><c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>());</c->
<c- p>}</c->

<c- kr>inline</c-> <c- n>constexpr</c-> <c- b>bool</c-> <c- n>b9</c-> <c- o>=</c-> <c- n>f9</c-><c- p>(</c-><c- n>nullptr</c-><c- p>);</c->
</pre>
   <p>Here the compile-time value of <code class="highlight"><c- n>b9</c-></code> depends on whether <code class="highlight"><c- p>{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>}</c-></code>'s backing array
is on the stack or not. Today, this program is well-formed and <code class="highlight"><c- n>b9</c-></code> is <code class="highlight">false</code>.
P2752 makes this program ill-formed, because we make the result of <code class="highlight"><c- n>p</c-> <c- o>==</c-> <c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-></code> unspecified, and therefore
(<a href="https://eel.is/c++draft/expr.const#5.24">[expr.const]/5.24</a>)
not a core constant expression.</p>
   <p>Remove the <code class="highlight"><c- k>constexpr</c-></code> from that program, and it becomes well-formed, but the
programmer might notice that the initialization of inline variable <code class="highlight"><c- n>b9</c-></code> is no
longer being done statically; instead it’s initialized dynamically, so that we
can ensure that every TU gets the same value for it.</p>
   <p>Here’s one more example:</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c->
<c- n>constexpr</c-> <c- b>bool</c-> <c- n>f10</c-><c- p>(</c-><c- n>T</c-> <c- n>s</c-><c- p>,</c-> <c- n>T</c-> <c- n>t</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>s</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-> <c- o>==</c-> <c- n>t</c-><c- p>.</c-><c- n>begin</c-><c- p>();</c->
<c- p>}</c->

<c- n>constexpr</c-> <c- b>bool</c-> <c- n>b10a</c-> <c- o>=</c-> <c- n>f10</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-><c- o>></c-><c- p>(</c-><c- s>"abc"</c-><c- p>,</c-> <c- s>"abc"</c-><c- p>);</c->
    <c- c1>// Today: Ill-formed because of unspecified comparison</c->
    <c- c1>// Clang rejects; GCC and MSVC say true</c->

<c- n>constexpr</c-> <c- b>bool</c-> <c- n>b10b</c-> <c- o>=</c-> <c- n>f10</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-><c- o>></c-><c- p>(</c-><c- s>"abc"</c-><c- p>,</c-> <c- s>"def"</c-><c- p>);</c->
    <c- c1>// Today: Well-formed false</c->
    <c- c1>// Clang rejects; GCC and MSVC say false</c->

<c- n>constexpr</c-> <c- b>bool</c-> <c- n>b10c</c-> <c- o>=</c-> <c- n>f10</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></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- 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>// Today: Well-formed false (all vendors agree)</c->
    <c- c1>// Tomorrow: Ill-formed because of unspecified comparison</c->
    <c- c1>//   (but expect vendor divergence as above)</c->

<c- n>constexpr</c-> <c- b>bool</c-> <c- n>b10d</c-> <c- o>=</c-> <c- n>f10</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></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- p>{</c-><c- mi>4</c-><c- p>,</c-><c- mi>5</c-><c- p>,</c-><c- mi>6</c-><c- p>});</c->
    <c- c1>// Today: Well-formed false (all vendors agree)</c->
    <c- c1>// Tomorrow: Well-formed false</c->
    <c- c1>//   (but expect vendor divergence as above)</c->
</pre>
   <h2 class="heading settled" data-level="5" id="experience"><span class="secno">5. </span><span class="content">Implementation experience</span><a class="self-link" href="#experience"></a></h2>
   <p>I have an experimental patch against Clang trunk; see <a data-link-type="biblio" href="#biblio-patch" title="Implement P2752R0 Static storage for braced initializers">[Patch]</a>.
It is incomplete, and (as of this writing) buggy; it has not received attention from anyone but myself.
You can experiment with it <a href="https://godbolt.org/z/cxvdWcKbd">on Godbolt Compiler Explorer</a>;
just use the P1144 branch of Clang, with the <code class="highlight"><c- o>-</c-><c- n>fstatic</c-><c- o>-</c-><c- n>init</c-><c- o>-</c-><c- n>lists</c-></code> command-line switch.</p>
   <h3 class="heading settled" data-level="5.1" id="fmerge-all-constants"><span class="secno">5.1. </span><span class="content">GCC <code class="highlight"><c- o>-</c-><c- n>fmerge</c-><c- o>-</c-><c- n>all</c-><c- o>-</c-><c- n>constants</c-></code></span><a class="self-link" href="#fmerge-all-constants"></a></h3>
   <p>See <a href="https://quuxplusone.github.io/blog/2023/02/17/issaquah-status/#static-storage-for-braced-initializers">this blog post</a>:
GCC has for a long time implemented a non-conforming mode <code class="highlight"><c- o>-</c-><c- n>fmerge</c-><c- o>-</c-><c- n>all</c-><c- o>-</c-><c- n>constants</c-></code> in which all constants
are merged, even constants that C++ requires to have different addresses, such as</p>
<pre class="language-c++ highlight"><c- k>const</c-> <c- b>int</c-> <c- n>a</c-><c- p>[]</c-> <c- o>=</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- k>const</c-> <c- b>int</c-> <c- n>b</c-><c- p>[]</c-> <c- o>=</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>
   This is a superset of P2752’s proposed behavior. When P2752 is adopted, GCC can probably reuse
some of that machinery to enable the optimization specifically for initializer lists (while leaving
the rest disabled by default). 
   <h2 class="heading settled" data-level="6" id="wording"><span class="secno">6. </span><span class="content">Proposed wording relative to the current C++23 draft</span><a class="self-link" href="#wording"></a></h2>
   <p>Modify <a href="https://timsong-cpp.github.io/cppwp/n4861/dcl.init.list#5">[dcl.init.list]/5–6</a> as follows:</p>
   <p><small></small></p>
   <blockquote><small> An object of type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> is constructed from an initializer list
as if the implementation generated <del>and materialized a prvalue</del> <ins>an object</ins> of type “array of <em>N</em> <code class="highlight"><c- k>const</c-> <c- n>E</c-></code>”,
where <em>N</em> is the number of elements in the initializer list<ins>; this is called the initializer list’s <em>backing array</em></ins>.
Each element of <del>that array</del> <ins>the backing array</ins> is copy-initialized with the corresponding element of the initializer list,
and the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> object is constructed to refer to that array.<p></p> <p> <del>[<em>Example 12:</em></del> <small></small></p><pre class="highlight"><small>    <del><c- k>struct</c-> <c- nc>X</c-> <c- p>{</c-></del>
      <del><c- n>X</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-> <c- n>v</c-><c- p>);</c-></del>
    <del><c- p>};</c-></del>
    <del><c- n>X</c-> <c- n>x</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-></del>
</small></pre> <del>The initialization will be implemented in a way roughly equivalent to this:</del> <small><pre class="highlight">    <del><c- k>const</c-> <c- b>double</c-> <c- n>__a</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- o>=</c-> <c- p>{</c-><c- b>double</c-><c- p>{</c-><c- mi>1</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- mi>2</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- mi>3</c-><c- p>}};</c-></del>
    <del><c- n>X</c-> <c- nf>x</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-><c- p>(</c-><c- n>__a</c-><c- p>,</c-> <c- n>__a</c-><c- o>+</c-><c- mi>3</c-><c- p>));</c-></del>
</pre></small> <del>assuming that the implementation can construct an <code class="highlight"><c- n>initializer_list</c-></code> object with a pair of pointers. <em>—end example</em>]</del> <p> <ins>Whether two backing arrays with the same contents are distinct (that is,
are stored in nonoverlapping objects) is unspecified.</ins> </p><p>The <ins>backing</ins> array has the same lifetime as <del>any other</del> <ins>a</ins> temporary object, except that initializing
an <code class="highlight"><c- n>initializer_list</c-></code> object from the array extends the lifetime of the array exactly like
binding a reference to a temporary.</p> <ins>[<em>Example 12:</em></ins> <small><pre class="highlight">    <ins><c- b>void</c-> <c- nf>f</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-> <c- n>il</c-><c- p>);</c-></ins>
    <ins><c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>float</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-></ins>
      <ins><c- n>f</c-><c- p>({</c-><c- mi>1</c-><c- p>,</c-> <c- n>x</c-><c- p>,</c-> <c- mi>3</c-><c- p>});</c-></ins>
    <ins><c- p>}</c-></ins>
    <ins><c- b>void</c-> <c- nf>h</c-><c- p>()</c-> <c- p>{</c-></ins>
      <ins><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-></ins>
    <ins><c- p>}</c-></ins>
</pre></small> <ins>The initialization will be implemented in a way roughly equivalent to this:</ins> <small><pre class="highlight">    <ins><c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>float</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-></ins>
      <ins><c- k>const</c-> <c- b>double</c-> <c- n>__a</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- o>=</c-> <c- p>{</c-><c- b>double</c-><c- p>{</c-><c- mi>1</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- n>x</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- mi>3</c-><c- p>}};</c-> <c- c1>// backing array</c-></ins>
      <ins><c- n>f</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-><c- p>(</c-><c- n>__a</c-><c- p>,</c-> <c- n>__a</c-><c- o>+</c-><c- mi>3</c-><c- p>));</c-></ins>
    <ins><c- p>}</c-></ins>
    <ins><c- b>void</c-> <c- nf>h</c-><c- p>()</c-> <c- p>{</c-></ins>
      <ins><c- k>static</c-> <c- k>constexpr</c-> <c- b>double</c-> <c- n>__b</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-> <c- o>=</c-> <c- p>{</c-><c- b>double</c-><c- p>{</c-><c- mi>1</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- mi>2</c-><c- p>},</c-> <c- b>double</c-><c- p>{</c-><c- mi>3</c-><c- p>}};</c-> <c- c1>// backing array</c-></ins>
      <ins><c- n>f</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-><c- p>(</c-><c- n>__b</c-><c- p>,</c-> <c- n>__b</c-><c- o>+</c-><c- mi>3</c-><c- p>));</c-></ins>
    <ins><c- p>}</c-></ins>
</pre></small> <ins>assuming that the implementation can construct an <code class="highlight"><c- n>initializer_list</c-></code> object with a pair of pointers. <em>—end example</em>]</ins> <p>[<em>Example 13:</em> <small></small></p><pre class="highlight"><small>    <c- k>typedef</c-> <c- n>std</c-><c- o>::</c-><c- n>complex</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-> <c- n>cmplx</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>cmplx</c-><c- o>></c-> <c- n>v1</c-> <c- o>=</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- b>void</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c->
      <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>cmplx</c-><c- o>></c-> <c- n>v2</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- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>i3</c-> <c- o>=</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- p>}</c->
    <c- k>struct</c-> <c- nc>A</c-> <c- p>{</c->
      <c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>i4</c-><c- p>;</c->
      <c- n>A</c-><c- p>()</c-> <c- o>:</c-> <c- n>i4</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- p>{}</c->            <c- c1>// ill-formed, would create a dangling reference</c->
    <c- p>};</c-><p></p>
</small></pre> For <code class="highlight"><c- n>v1</c-></code> and <code class="highlight"><c- n>v2</c-></code>, the <code class="highlight"><c- n>initializer_list</c-></code> object is a parameter in a function call,
so the array created for <code class="highlight"><c- p>{</c-> <c- mi>1</c-><c- p>,</c-> <c- mi>2</c-><c- p>,</c-> <c- mi>3</c-> <c- p>}</c-></code> has full-expression lifetime. For <code class="highlight"><c- n>i3</c-></code>, the <code class="highlight"><c- n>initializer_list</c-></code> object is a variable, so the array persists for the lifetime of
the variable. For <code class="highlight"><c- n>i4</c-></code>, the <code class="highlight"><c- n>initializer_list</c-></code> object is initialized in the constructor’s <i>ctor-initializer</i> as if by binding a temporary array to a reference member, so the
program is ill-formed. <em>—end example</em>] <p> <del>[<em>Note 6: The implementation is free to allocate the array in read-only memory
if an explicit array with the same initializer can be so allocated. —end note</em>]</del> </p></small></blockquote>
   <h3 class="heading settled" data-level="6.1" id="wording-annexc"><span class="secno">6.1. </span><span class="content">Addition to Annex C</span><a class="self-link" href="#wording-annexc"></a></h3>
   <p>We might want to add something to Annex C <a href="https://eel.is/c++draft/diff.cpp20.expr">[diff.cpp20.expr]</a>,
since technically this is a breaking change; but on the other hand we don’t actually
expect anyone to notice. Their code should just silently get faster.</p>
   <h2 class="heading settled" data-level="7" id="acknowledgments"><span class="secno">7. </span><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
   <ul>
    <li data-md>
     <p>Thanks to Jason Merrill for the original issue, and to Andrew Tomazos for recommending Arthur write this paper.</p>
    <li data-md>
     <p>Thanks to Jens Maurer for reviewing P2752R1’s proposed wording.</p>
    <li data-md>
     <p>Thanks to Lénárd Szolnoki for pointing out the ramifications for constexpr.</p>
   </ul>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-corereflector">[CoreReflector]
   <dd>Jason Merrill. <a href="https://lists.isocpp.org/core/2022/12/13625.php"><cite>[isocpp-core] initializer_list and read-only memory</cite></a>. December 2022. URL: <a href="https://lists.isocpp.org/core/2022/12/13625.php">https://lists.isocpp.org/core/2022/12/13625.php</a>
   <dt id="biblio-p1967r9">[P1967R9]
   <dd>JeanHeyd Meneide. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1967r9.html"><cite>#embed - a scannable, tooling-friendly binary resource inclusion mechanism</cite></a>. October 2022. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1967r9.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1967r9.html</a>
   <dt id="biblio-p2174r1">[P2174R1]
   <dd>Zhihao Yuan. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2174r1.html"><cite>Compound Literals</cite></a>. April 2022. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2174r1.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2174r1.html</a>
   <dt id="biblio-patch">[Patch]
   <dd>Arthur O'Dwyer. <a href="https://github.com/Quuxplusone/llvm-project/tree/p2752"><cite>Implement P2752R0 Static storage for braced initializers</cite></a>. January 2023. URL: <a href="https://github.com/Quuxplusone/llvm-project/tree/p2752">https://github.com/Quuxplusone/llvm-project/tree/p2752</a>
  </dl>