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  <title>P2447R3: &lt;code>std::span&lt;/code> over an initializer list</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 d9f9c3533, updated Fri Mar 10 11:31:29 2023 -0800" 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;}
td {font-size: smaller;}
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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">P2447R3<br><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-></code> over an initializer list</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="2023-03-11">2023-03-11</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Authors:
     <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>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:federico.kircheis@gmail.com">Federico Kircheis</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Draft Revision:
     <dd>4
     <dt>Current Source:
     <dd><a href="https://github.com/Quuxplusone/draft/blob/gh-pages/d2447-std-span-over-initializer-list.bs">github.com/Quuxplusone/draft/blob/gh-pages/d2447-std-span-over-initializer-list.bs</a>
     <dt>Current:
     <dd><a href="https://rawgit.com/Quuxplusone/draft/gh-pages/d2447-std-span-over-initializer-list.html">rawgit.com/Quuxplusone/draft/gh-pages/d2447-std-span-over-initializer-list.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><code class="highlight"><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-></code> can be a lightweight drop-in replacement

  for <code class="highlight"><c- k>const</c-> <c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>&amp;</c-></code> in the same way that <code class="highlight"><c- n>string_view</c-></code> can replace <code class="highlight"><c- k>const</c-> <c- n>string</c-><c- o>&amp;</c-></code>. While <code class="highlight"><c- s>"abc"</c-></code> binds to a <code class="highlight"><c- n>string_view</c-></code> function parameter, <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> fails to bind to a <code class="highlight"><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-></code> function parameter.
  We show why this gap is undesirable, and propose to close it, ideally as a DR.</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>
    <li>
     <a href="#solution"><span class="secno">3</span> <span class="content">Solution</span></a>
     <ol class="toc">
      <li><a href="#experience"><span class="secno">3.1</span> <span class="content">Implementation experience</span></a>
      <li><a href="#dangling"><span class="secno">3.2</span> <span class="content">What about dangling?</span></a>
      <li><a href="#p2752"><span class="secno">3.3</span> <span class="content">Relation to P2752 Static storage for braced initializers</span></a>
      <li><a href="#braces"><span class="secno">3.4</span> <span class="content">Why not just double the braces?</span></a>
     </ol>
    <li>
     <a href="#annex-c"><span class="secno">4</span> <span class="content">Annex C examples</span></a>
     <ol class="toc">
      <li><a href="#annex-c-1"><span class="secno">4.1</span> <span class="content">Overload resolution is affected</span></a>
      <li><a href="#annex-c-2"><span class="secno">4.2</span> <span class="content">The <code class="highlight"><c- n>initializer_list</c-></code> ctor has high precedence</span></a>
      <li><a href="#annex-c-3"><span class="secno">4.3</span> <span class="content">Implicit two-argument construction with a highly convertible <code class="highlight"><c- n>value_type</c-></code></span></a>
     </ol>
    <li><a href="#wording"><span class="secno">5</span> <span class="content">Proposed wording</span></a>
    <li><a href="#acknowledgments"><span class="secno">6</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>R3:</p>
     <ul>
      <li data-md>
       <p>Changed primary authorship from Federico Kircheis to Arthur O’Dwyer.</p>
      <li data-md>
       <p>Removed R2’s feature macro <code class="highlight"><c- n>__cpp_lib_span_init</c-></code>; it didn’t seem motivated.</p>
     </ul>
    <li data-md>
     <p>R2:</p>
     <ul>
      <li data-md>
       <p>Discussed in LEWG telecon, <a href="https://wiki.edg.com/bin/view/Wg21telecons2022/P2447#Library-Evolution-2022-07-26">2022-07-26</a></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>C++17 added <code class="highlight"><c- n>string_view</c-></code> as a "view" over constant string data. Its main purpose
is as a lightweight drop-in replacement for <code class="highlight"><c- k>const</c-> <c- n>string</c-><c- o>&amp;</c-></code> function parameters.</p>
   <table>
    <tbody>
     <tr>
      <td>C++14 <code class="highlight"><c- n>string</c-></code>
      <td>C++17 <code class="highlight"><c- n>string_view</c-></code>
     <tr>
      <td>
<pre class="highlight"><c- b>int</c-> <c- nf>take</c-><c- p>(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>string</c-><c- o>&amp;</c-> <c- n>s</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>s</c-><c- p>[</c-><c- mi>0</c-><c- p>]</c-> <c- o>+</c-> <c- n>s</c-><c- p>.</c-><c- n>size</c-><c- p>();</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- b>int</c-> <c- nf>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>sv</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>s</c-><c- p>[</c-><c- mi>0</c-><c- p>]</c-> <c- o>+</c-> <c- n>s</c-><c- p>.</c-><c- n>size</c-><c- p>();</c->
<c- p>}</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>abc</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->
<c- n>take</c-><c- p>(</c-><c- n>abc</c-><c- p>);</c->
</pre>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>abc</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->
<c- n>take</c-><c- p>(</c-><c- n>abc</c-><c- p>);</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- n>take</c-><c- p>(</c-><c- s>"abc"</c-><c- p>);</c->
</pre>
      <td>
<pre class="highlight"><c- n>take</c-><c- p>(</c-><c- s>"abc"</c-><c- p>);</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- n>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- p>(</c-><c- s>"abc"</c-><c- p>));</c->
</pre>
      <td>
<pre class="highlight"><c- n>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-><c- p>(</c-><c- s>"abc"</c-><c- p>));</c->
<c- n>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-><c- p>(</c-><c- s>"abc"</c-><c- p>));</c->
</pre>
   </table>
   <p>C++20 added <code class="highlight"><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>T</c-><c- o>></c-></code> as a "view" over constant contiguous data of type <code class="highlight"><c- n>T</c-></code> (such as
arrays and vectors). One of its main purposes (although not its only one) is as a
lightweight drop-in replacement for <code class="highlight"><c- k>const</c-> <c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-></code> function parameters.</p>
   <table>
    <tbody>
     <tr>
      <td>C++17 <code class="highlight"><c- n>vector</c-></code>
      <td>C++20 <code class="highlight"><c- n>span</c-></code>
     <tr>
      <td>
<pre class="highlight"><c- b>int</c-> <c- nf>take</c-><c- p>(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>&amp;</c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>v</c-><c- p>[</c-><c- mi>0</c-><c- p>]</c-> <c- o>+</c-> <c- n>v</c-><c- p>.</c-><c- n>size</c-><c- p>();</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- b>int</c-> <c- nf>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>v</c-><c- p>[</c-><c- mi>0</c-><c- p>]</c-> <c- o>+</c-> <c- n>v</c-><c- p>.</c-><c- n>size</c-><c- p>();</c->
<c- p>}</c->
</pre>
     <tr>
      <td>
<pre class="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>abc</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- n>take</c-><c- p>(</c-><c- n>abc</c-><c- p>);</c->
</pre>
      <td>
<pre class="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>abc</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- n>take</c-><c- p>(</c-><c- n>abc</c-><c- p>);</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- n>take</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>
      <td>
     <tr>
      <td>
<pre class="highlight"><c- n>take</c-><c- p>({});</c-> <c- c1>// size=0</c->
<c- n>take</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>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>vector</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>take</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- p>{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>,</c-><c- mi>3</c-><c- p>});</c->
</pre>
      <td>
<pre class="highlight"><c- n>take</c-><c- p>({});</c-> <c- c1>// size=0</c->
<c- n>take</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>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>vector</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>take</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- 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>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</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->
</pre>
   </table>
   <p>This table has a conspicuous gap. The singly-braced initializer list <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> is implicitly convertible to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>, but not to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-></code>.</p>
   <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>We propose simply that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>T</c-><c- o>></c-></code> should be convertible from an appropriate <em>braced-initializer-list</em>. In practice this means adding a constructor from <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-></code>.</p>
   <h3 class="heading settled" data-level="3.1" id="experience"><span class="secno">3.1. </span><span class="content">Implementation experience</span><a class="self-link" href="#experience"></a></h3>
   <p>This proposal has been implemented in Arthur’s fork of libc++ since October 2021.
See <a href="https://quuxplusone.github.io/blog/2021/10/03/p2447-span-from-initializer-list/">"<code class="highlight"><c- n>span</c-></code> should have a converting constructor from <code class="highlight"><c- n>initializer_list</c-></code>"</a> (2021-10-03) and <a data-link-type="biblio" href="#biblio-patch" title="Implement P2447 std::span convertible from std::initializer_list">[Patch]</a>.</p>
   <h3 class="heading settled" data-level="3.2" id="dangling"><span class="secno">3.2. </span><span class="content">What about dangling?</span><a class="self-link" href="#dangling"></a></h3>
   <p><code class="highlight"><c- n>span</c-></code>, like <code class="highlight"><c- n>string_view</c-></code>, is specifically designed to bind to rvalues as well as lvalues.
This is what lets us write useful code like:</p>
<pre class="language-c++ highlight"><c- b>int</c-> <c- nf>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>s</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- nf>give_string</c-><c- p>();</c->
<c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- n>take</c-><c- p>(</c-><c- n>give_string</c-><c- p>());</c->

<c- b>int</c-> <c- nf>take</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-> <c- n>v</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>give_vector</c-><c- p>();</c->
<c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- n>take</c-><c- p>(</c-><c- n>give_vector</c-><c- p>());</c->
</pre>
   <p>Careless misuse of <code class="highlight"><c- n>string_view</c-></code> and <code class="highlight"><c- n>span</c-></code> outside a function parameter list can dangle:</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>s</c-> <c- o>=</c-> <c- n>give_string</c-><c- p>();</c-> <c- c1>// dangles</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- n>give_vector</c-><c- p>();</c-> <c- c1>// dangles</c->
</pre>
   <p>P2447 doesn’t propose to increase the risk in this area; dangling is already likely when <code class="highlight"><c- n>span</c-></code> or <code class="highlight"><c- n>string_view</c-></code> is carelessly misused. We simply propose to close the ergonomic syntax gap between <code class="highlight"><c- n>span</c-></code> and <code class="highlight"><c- n>string_view</c-></code>.</p>
   <table>
    <tbody>
     <tr>
      <td>Before
      <td>After P2447
     <tr>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string_view</c->      <c- n>s</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->  <c- c1>// OK</c->
<c- n>std</c-><c- o>::</c-><c- n>string_view</c->      <c- n>s</c-> <c- o>=</c-> <c- s>"abc"</c-><c- n>s</c-><c- p>;</c-> <c- c1>// dangles</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>char</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->  <c- c1>// OK</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>char</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- s>"abc"</c-><c- n>s</c-><c- p>;</c-> <c- c1>// dangles</c->
</pre>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string_view</c->      <c- n>s</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->  <c- c1>// OK</c->
<c- n>std</c-><c- o>::</c-><c- n>string_view</c->      <c- n>s</c-> <c- o>=</c-> <c- s>"abc"</c-><c- n>s</c-><c- p>;</c-> <c- c1>// dangles</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>char</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->  <c- c1>// OK</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>char</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- s>"abc"</c-><c- n>s</c-><c- p>;</c-> <c- c1>// dangles</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</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>// dangles</c->
<c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</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- c1>// dangles</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</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- c1>// dangles</c->
</pre>
      <td>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</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>// dangles</c->
<c- k>auto</c-> <c- n>v</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</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- c1>// dangles</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</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- c1>// dangles</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</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- c1>// dangles</c->
</pre>
   </table>
   <h3 class="heading settled" data-level="3.3" id="p2752"><span class="secno">3.3. </span><span class="content">Relation to P2752 Static storage for braced initializers</span><a class="self-link" href="#p2752"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p2752r1" title="Static storage for braced initializers">[P2752R1]</a> (<a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2752r0.html">R0</a>)
proposes to permit a constant <code class="highlight"><c- n>initializer_list</c-></code> 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> to refer to
a backing array in static storage, rather than forcing all backing arrays onto the stack.
At least for now, this doesn’t change anything dangling-wise: referring to the backing array
of an <code class="highlight"><c- n>initializer_list</c-></code> outside the lifetime of that <code class="highlight"><c- n>initializer_list</c-></code> remains undefined behavior.</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>s</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->            <c- c1>// OK, no dangling</c->
<c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</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- c1>// dangles, even after P2752</c->
</pre>
   <h3 class="heading settled" data-level="3.4" id="braces"><span class="secno">3.4. </span><span class="content">Why not just double the braces?</span><a class="self-link" href="#braces"></a></h3>
   <p>Since we can already write</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</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- c1>// dangles</c->
</pre>
   then why not call that "good enough"? Why do we need to be able to use a single set of braces? 
   <p>Well, a single set of braces is good enough for <code class="highlight"><c- n>vector</c-></code>, and we want <code class="highlight"><c- n>span</c-></code> to be a drop-in replacement
for <code class="highlight"><c- n>vector</c-></code> in function parameter lists, so we need to support the syntax <code class="highlight"><c- n>vector</c-></code> does.
There was a period right after C++11 where some people were writing</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->
</pre>
   but by C++14 we had settled firmly on "one set of braces" as the preferred style (matching the
preferred style for C arrays, pairs, tuples, etc.) 
   <p>So I prefer to turn the question around and say: Since we can already implicitly treat <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> as a <code class="highlight"><c- n>span</c-></code>, how could there be any additional harm in treating <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> as a <code class="highlight"><c- n>span</c-></code>?</p>
   <p>This also relates to <a data-link-type="biblio" href="#biblio-p2752r1" title="Static storage for braced initializers">[P2752R1]</a>: Today, <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> converts to <code class="highlight"><c- n>span</c-></code> by materializing
a temporary of type <code class="highlight"><c- k>const</c-> <c- b>int</c-><c- p>[</c-><c- mi>3</c-><c- p>]</c-></code> on the stack. Tomorrow, if P2447 is adopted, <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> will convert to <code class="highlight"><c- n>span</c-></code> via an <code class="highlight"><c- n>initializer_list</c-></code> that refers to a backing array also allocated on the stack. <a data-link-type="biblio" href="#biblio-p2752r1" title="Static storage for braced initializers">[P2752R1]</a> proposes to performance-optimize the latter case (permitting the backing array to occupy
static storage) but not the former case (keeping the status quo for materialized array temporaries).
In other words, tomorrow’s (single-braced or double-braced) <code class="highlight"><c- n>initializer_list</c-></code> constructor is
"more optimizable" than today’s double-braced array-temporary constructor.</p>
   <p>The following program (<a href="https://godbolt.org/z/7a5zGnx5s">Godbolt</a>) shows how
P2447 lets us benefit from P2752’s optimization:</p>
<pre class="language-c++ highlight"><c- b>int</c-> <c- nf>perf</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-><c- p>);</c->

<c- b>int</c-> <c- nf>test</c-><c- p>()</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>perf</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>
   <table>
    <tbody>
     <tr>
      <td>
      <td><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>
      <td><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>
     <tr>
      <th>Today
      <td>Array on stack
      <td>Ill-formed
     <tr>
      <th>P2752 only
      <td>Array on stack
      <td>Ill-formed
     <tr>
      <th>P2447 only
      <td>IL on stack
      <td>IL on stack
     <tr>
      <th>P2447+P2752
      <td>IL in rodata, tail-call
      <td>IL in rodata, tail-call
   </table>
   <h2 class="heading settled" data-level="4" id="annex-c"><span class="secno">4. </span><span class="content">Annex C examples</span><a class="self-link" href="#annex-c"></a></h2>
   <p>This change will, of course, break some code (most of it pathological).
We might want to add some of these examples to Annex C.</p>
   <p>However, <em>any</em> change to overload sets (particularly the addition of new
non-<code class="highlight"><c- k>explicit</c-></code> constructors) can break code.
For example, there was nothing wrong with C++23’s adopting <a data-link-type="biblio" href="#biblio-p1425" title="Iterator-pair constructors for stack and queue">[P1425]</a> "Iterator-pair constructors for <code class="highlight"><c- n>stack</c-></code> and <code class="highlight"><c- n>queue</c-></code>"
with no change to Annex C, despite its breaking code like this:</p>
<pre class="language-c++ highlight"><c- b>void</c-> <c- nf>zero</c-><c- p>(</c-><c- n>queue</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>);</c->
<c- b>void</c-> <c- nf>zero</c-><c- p>(</c-><c- n>pair</c-><c- o>&lt;</c-><c- b>int</c-><c- o>*</c-><c- p>,</c-><c- b>int</c-><c- o>*></c-><c- p>);</c->
<c- b>int</c-> <c- n>a</c-><c- p>[</c-><c- mi>10</c-><c- p>];</c->
<c- b>void</c-> <c- nf>test</c-><c- p>()</c-> <c- p>{</c-> <c- n>zero</c-><c- p>({</c-><c- n>a</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>10</c-><c- p>});</c-> <c- p>}</c->
</pre>
   <b>Before:</b> Calls <code class="highlight"><c- n>zero</c-><c- p>(</c-><c- n>pair</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-><c- p>)</c-></code>.<br> <b>After P1425:</b> Ambiguous.<br> <b>To fix:</b> Eliminate the ambiguous overloading, or cast the argument to <code class="highlight"><c- n>pair</c-></code>. 
   <p>We can simply agree that such examples are sufficiently unlikely in practice,
and sufficiently easy to fix, that the benefits of the changed overload set
outweigh the costs of running into these examples.</p>
   <h3 class="heading settled" data-level="4.1" id="annex-c-1"><span class="secno">4.1. </span><span class="content">Overload resolution is affected</span><a class="self-link" href="#annex-c-1"></a></h3>
<pre class="language-c++ highlight"><c- b>void</c-> <c- nf>one</c-><c- p>(</c-><c- n>pair</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-><c- p>);</c->
<c- b>void</c-> <c- nf>one</c-><c- p>(</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-><c- p>);</c->
<c- b>void</c-> <c- nf>test</c-><c- p>()</c-> <c- p>{</c-> <c- n>one</c-><c- p>({</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>});</c-> <c- p>}</c->
</pre>
   <b>Before:</b> Calls <code class="highlight"><c- n>one</c-><c- p>(</c-><c- n>pair</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-><c- p>)</c-></code>.<br> <b>After P2447:</b> Ambiguous.<br> <b>To fix:</b> Eliminate the ambiguous overloading, or cast the argument to <code class="highlight"><c- n>pair</c-></code>. 
   <h3 class="heading settled" data-level="4.2" id="annex-c-2"><span class="secno">4.2. </span><span class="content">The <code class="highlight"><c- n>initializer_list</c-></code> ctor has high precedence</span><a class="self-link" href="#annex-c-2"></a></h3>
<pre class="language-c++ highlight"><c- b>void</c-> <c- nf>two</c-><c- p>(</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- p>,</c-> <c- mi>2</c-><c- o>></c-><c- p>);</c->
<c- b>void</c-> <c- nf>test</c-><c- p>()</c-> <c- p>{</c-> <c- n>two</c-><c- p>({{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>}});</c-> <c- p>}</c->
</pre>
   <b>Before:</b> Selects <code class="highlight"><c- n>span</c-><c- p>(</c-><c- k>const</c-> <c- b>int</c-><c- p>(</c-><c- o>&amp;</c-><c- p>)[</c-><c- mi>2</c-><c- p>])</c-></code>, which is non-<code class="highlight"><c- k>explicit</c-></code>; success.<br> <b>After P2447:</b> Selects <code class="highlight"><c- n>span</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>)</c-></code>, which is <code class="highlight"><c- k>explicit</c-></code> for <code class="highlight"><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- p>,</c-> <c- mi>2</c-><c- o>></c-></code>; failure.<br> <b>To fix:</b> Replace <code class="highlight"><c- p>{{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>}}</c-></code> with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>array</c-><c- p>{</c-><c- mi>1</c-><c- p>,</c-><c- mi>2</c-><c- p>}</c-></code>; or, replace <code class="highlight"><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- p>,</c-> <c- mi>2</c-><c- o>></c-></code> with <code class="highlight"><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>></c-></code>.<br> 
   <h3 class="heading settled" data-level="4.3" id="annex-c-3"><span class="secno">4.3. </span><span class="content">Implicit two-argument construction with a highly convertible <code class="highlight"><c- n>value_type</c-></code></span><a class="self-link" href="#annex-c-3"></a></h3>
   <p>In these two highly contrived examples, the caller deliberately constructs
a <code class="highlight"><c- n>span</c-></code> via its iterator-pair constructor <em>implicitly, from a braced initializer of two elements,</em> and furthermore <code class="highlight"><c- n>value_type</c-></code> is implicitly convertible from the iterator type.
These examples strike me as highly contrived: both conditions are unlikely, and their
conjunction is unlikelier still.</p>
<pre class="language-c++ highlight"><c- b>int</c-> <c- nf>three</c-><c- p>(</c-><c- n>span</c-><c- o>&lt;</c-><c- b>void</c-><c- o>*</c-> <c- k>const</c-><c- o>></c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>v</c-><c- p>.</c-><c- n>size</c-><c- p>();</c-> <c- p>}</c->
<c- b>void</c-> <c- o>*</c-><c- n>a</c-><c- p>[</c-><c- mi>10</c-><c- p>];</c->
<c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- n>three</c-><c- p>({</c-><c- n>a</c-><c- p>,</c-> <c- mi>0</c-><c- p>});</c->
</pre>
   <b>Before:</b> Selects <code class="highlight"><c- n>span</c-><c- p>(</c-><c- b>void</c-><c- o>**</c-><c- p>,</c-> <c- b>int</c-><c- p>)</c-></code>; <code class="highlight"><c- n>x</c-></code> is 0.<br> <b>After P2447:</b> Selects <code class="highlight"><c- n>span</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>void</c-><c- o>*></c-><c- p>)</c-></code>; <code class="highlight"><c- n>x</c-></code> is 2.<br> <b>To fix:</b> Replace <code class="highlight"><c- p>{</c-><c- n>a</c-><c- p>,</c-> <c- mi>0</c-><c- p>}</c-></code> with <code class="highlight"><c- n>span</c-><c- p>(</c-><c- n>a</c-><c- p>,</c-> <c- mi>0</c-><c- p>)</c-></code>. 
<pre class="language-c++ highlight"><c- b>int</c-> <c- nf>four</c-><c- p>(</c-><c- n>span</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>any</c-><c- o>></c-> <c- n>v</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>v</c-><c- p>.</c-><c- n>size</c-><c- p>();</c-> <c- p>}</c->
<c- n>any</c-> <c- n>a</c-><c- p>[</c-><c- mi>10</c-><c- p>];</c->
<c- b>int</c-> <c- n>y</c-> <c- o>=</c-> <c- n>four</c-><c- p>({</c-><c- n>a</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>10</c-><c- p>});</c->
</pre>
   <b>Before:</b> Selects <code class="highlight"><c- n>span</c-><c- p>(</c-><c- n>any</c-><c- o>*</c-><c- p>,</c-> <c- n>any</c-><c- o>*</c-><c- p>)</c-></code>; <code class="highlight"><c- n>y</c-></code> is 10.<br> <b>After P2447:</b> Selects <code class="highlight"><c- n>span</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>any</c-><c- o>></c-><c- p>)</c-></code>; <code class="highlight"><c- n>y</c-></code> is 2.<br> <b>To fix:</b> Replace <code class="highlight"><c- p>{</c-><c- n>a</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>10</c-><c- p>}</c-></code> with <code class="highlight"><c- n>span</c-><c- p>(</c-><c- n>a</c-><c- p>,</c-> <c- n>a</c-><c- o>+</c-><c- mi>10</c-><c- p>)</c-></code>. 
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">5. </span><span class="content">Proposed wording</span><a class="self-link" href="#wording"></a></h2>
   <p>Modify <a href="https://eel.is/c++draft/span.syn">[span.syn]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
<pre class="highlight"><small><c- cp>#include</c-> &lt;initializer_list><c- c1>     </c-><i><c- c1>// see [initializer.list.syn]</c-></i></small></pre>
   </blockquote>
   <p></p>
   <p>Modify <a href="https://eel.is/c++draft/span.overview">[span.overview]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
<pre class="highlight"><small>  <c- k>template</c-><c- o>&lt;</c-><c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>span</c-><c- p>(</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>element_type</c-><c- o>></c-> <c- p>(</c-><c- o>&amp;</c-><c- n>arr</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>span</c-><c- p>(</c-><c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>>&amp;</c-> <c- n>arr</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>N</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>span</c-><c- p>(</c-><c- k>const</c-> <c- n>array</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>N</c-><c- o>>&amp;</c-> <c- n>arr</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>R</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>extent</c-> <c- o>!=</c-> <c- n>dynamic_extent</c-><c- p>)</c-> <c- n>span</c-><c- p>(</c-><c- n>R</c-><c- o>&amp;&amp;</c-> <c- n>r</c-><c- p>);</c-><p></p>
  <ins><c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>extent</c-> <c- o>!=</c-> <c- n>dynamic_extent</c-><c- p>)</c-> <c- n>span</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><value_type> <c- n>il</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c-></value_type></ins>
  <c- k>constexpr</c-> <c- n>span</c-><c- p>(</c-><c- k>const</c-> <c- n>span</c-><c- o>&amp;</c-> <c- n>other</c-><c- p>)</c-> <c- k>noexcept</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>OtherElementType</c-><c- p>,</c-> <c- b>size_t</c-> <c- n>OtherExtent</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><i><c- n>see</c-> <c- n>below</c-></i><c- p>)</c-> <c- n>span</c-><c- p>(</c-><c- k>const</c-> <c- n>span</c-><c- o>&lt;</c-><c- n>OtherElementType</c-><c- p>,</c-> <c- n>OtherExtent</c-><c- o>>&amp;</c-> <c- n>s</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</small></pre>
   </blockquote>
   <p>Modify <a href="https://eel.is/c++draft/span.cons">[span.cons]</a> as follows:</p>
   <blockquote>
    <small><pre class="highlight">   <ins><c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- n>extent</c-> <c- o>!=</c-> <c- n>dynamic_extent</c-><c- p>)</c-> <c- n>span</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>initializer_list</c-><value_type> <c- n>il</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c-></value_type></ins>
</pre></small> <br>
    <ins><i>Constraints:</i> <code class="highlight"><c- n>is_const_v</c-><c- o>&lt;</c-><c- n>element_type</c-><c- o>></c-></code> is <code class="highlight">true</code>.</ins>
    <p>
     <ins><i>Preconditions:</i> If <code class="highlight"><c- n>extent</c-></code> is not equal to <code class="highlight"><c- n>dynamic_extent</c-></code>, then <code class="highlight"><c- n>il</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code> is equal to <code class="highlight"><c- n>extent</c-></code>.</ins>
    </p>
    <p>
     <ins><i>Effects:</i> Initializes <i><code class="highlight"><c- n>data_</c-></code></i> with <code class="highlight"><c- n>il</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-></code> and <i><code class="highlight"><c- n>size_</c-></code></i> with <code class="highlight"><c- n>il</c-><c- p>.</c-><c- n>size</c-><c- p>()</c-></code>.</ins>
    </p>
   </blockquote>
   <h2 class="heading settled" data-level="6" id="acknowledgments"><span class="secno">6. </span><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
   <ul>
    <li data-md>
     <p>Thanks to Federico Kircheis for writing the first drafts of this paper.</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-p1425">[P1425]
   <dd>Corentin Jabot. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1425r4.pdf"><cite>Iterator-pair constructors for stack and queue</cite></a>. March 2021. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1425r4.pdf">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1425r4.pdf</a>
   <dt id="biblio-p2752r1">[P2752R1]
   <dd>Arthur O'Dwyer. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2752r1.html"><cite>Static storage for braced initializers</cite></a>. March 2023. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2752r1.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2752r1.html</a>
   <dt id="biblio-patch">[Patch]
   <dd>Arthur O'Dwyer. <a href="https://github.com/Quuxplusone/llvm-project/commit/d0d11ae5f2146d2ac76680bd1ddaf1f011f96ef4"><cite>Implement P2447 std::span convertible from std::initializer_list</cite></a>. October 2021. URL: <a href="https://github.com/Quuxplusone/llvm-project/commit/d0d11ae5f2146d2ac76680bd1ddaf1f011f96ef4">https://github.com/Quuxplusone/llvm-project/commit/d0d11ae5f2146d2ac76680bd1ddaf1f011f96ef4</a>
  </dl>