<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P2733R0: Fix handling of empty specifiers in std::format</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 span:not(.dfn-paneled) {
			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 type="text/css">
    table, th, td {
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      border-collapse: collapse;
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      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 */
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    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 */
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    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 */
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    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 3d6a6fcbb, updated Wed Jun 16 16:19:39 2021 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
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	ins { text-decoration: none; }
  ins code { background: #cfc !important; }
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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">P2733R0<br>Fix handling of empty specifiers in std::format</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="2022-11-26">2022-11-26</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:victor.zverovich@gmail.com">Victor Zverovich</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </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="#proposal"><span class="secno">1</span> <span class="content">Proposal</span></a>
    <li><a href="#poll-results"><span class="secno">2</span> <span class="content">LEWG Poll Results</span></a>
    <li><a href="#wording"><span class="secno">3</span> <span class="content">Wording</span></a>
    <li><a href="#ack"><span class="secno">4</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="proposal"><span class="secno">1. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-lwg3776">[LWG3776]</a> "Avoid parsing format-spec if it is not present or empty" proposed
omitting the call to <code class="highlight"><c- n>formatter</c-><c- o>::</c-><c- n>parse</c-></code> for empty format specifiers
(<em>format-spec</em> in <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=794">[format.string.general</a>] of <a data-link-type="biblio" href="#biblio-n4917">[N4917]</a>).</p>
   <p>Consider the following example:</p>
<pre class="highlight"><c- k>struct</c-> <c- nc>S</c-> <c- p>{};</c->

<c- k>template</c-> <c- o>&lt;></c->
<c- k>struct</c-> <c- nc>std</c-><c- o>::</c-><c- n>formatter</c-><c- o>&lt;</c-><c- n>S</c-><c- o>></c-> <c- p>{</c->
  <c- k>auto</c-> <c- nf>parse</c-><c- p>(</c-><c- n>format_parse_context</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>ctx</c-><c- p>.</c-><c- n>begin</c-><c- p>();</c-> <c- p>}</c->
  <c- k>auto</c-> <c- nf>format</c-><c- p>(</c-><c- n>S</c-><c- p>,</c-> <c- n>format_context</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- n>ctx</c-><c- p>.</c-><c- n>out</c-><c- p>();</c-> <c- p>}</c->
<c- p>};</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
  <c- k>auto</c-> <c- n>s1</c-> <c- o>=</c-> <c- n>fmt</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"{}"</c-><c- p>,</c-> <c- n>S</c-><c- p>());</c->  <c- c1>// (1) no format-spec</c->
  <c- k>auto</c-> <c- n>s2</c-> <c- o>=</c-> <c- n>fmt</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"{:}"</c-><c- p>,</c-> <c- n>S</c-><c- p>());</c-> <c- c1>// (2) empty format-spec</c->
<c- p>}</c->
</pre>
   <p>In (1) <em>format-spec</em> is not present and in (2) it is present but empty.
There is nothing to parse in both of these cases and therefore requiring 
implementations to call <code class="highlight"><c- n>formatter</c-><c- o>::</c-><c- n>parse</c-></code> doesn’t make a lot of sense.
It only adds unnecessary overhead for the common case which is what <a data-link-type="biblio" href="#biblio-lwg3776">[LWG3776]</a> was proposing to eliminate. Implementation experience in {fmt} showed that
requiring the call to <code class="highlight"><c- n>parse</c-></code> has negative impact on formatting of ranges where
we had to unnecessarily call this function from multiple places. The same issue
may exist in other contexts such as format string compilation.</p>
   <p>Additionally <a data-link-type="biblio" href="#biblio-lwg3776">[LWG3776]</a> made a drive-by fix, clarifying that the two cases are
equivalent which was not obvious from existing wording. This is arguably even
more important than omitting <code class="highlight"><c- n>parse</c-></code>, particularly because formatting of ranges
(<a data-link-type="biblio" href="#biblio-p2286">[P2286]</a>) doesn’t allow distinguishing between the two forms for nested
specifiers, e.g.</p>
<pre class="highlight"><c- k>auto</c-> <c- n>s</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"{::}"</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>S</c-><c- o>></c-><c- p>(</c-><c- mi>2</c-><c- p>));</c->
<c- c1>//                       ^ empty format-spec for S</c->
</pre>
   <p>Having the two cases equivalent is also more intuitive and consistent with all
existing standard formatters.</p>
   <p>Library Evolution Working Group (LEWG) reviewed <a data-link-type="biblio" href="#biblio-lwg3776">[LWG3776]</a> in Kona and
approved it with the amendment that implementations are allowed but not required
to omit the call to <code class="highlight"><c- n>formatter</c-><c- o>::</c-><c- n>parse</c-></code> for empty <em>format-spec</em>.</p>
   <p>Barry Revzin pointed out an existing limitation of the formatting ranges design
that requires calling <code class="highlight"><c- n>set_debug_format</c-></code> from the <code class="highlight"><c- n>parse</c-></code> function. However,
as discovered by Mark de Wever while implementing ranges formatting in libc++,
the <code class="highlight"><c- n>formatter</c-></code> specialization for tuples already omits the call to <code class="highlight"><c- n>parse</c-></code> for the underlying type so we need to fix this anyway. The following example
illustrates the fix:</p>
<pre class="highlight"><c- k>auto</c-> <c- n>s</c-> <c- o>=</c-> <c- n>fmt</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"{}"</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>make_tuple</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>make_tuple</c-><c- p>(</c-><c- sc>'a'</c-><c- p>)));</c->
</pre>
   <table class="poll">
    <tbody>
     <tr>
      <th>Before 
      <th>After 
     <tr>
      <td><code class="highlight"><c- n>s</c-> <c- o>==</c-> <c- p>((</c-><c- n>a</c-><c- p>))</c-></code> 
      <td><code class="highlight"><c- n>s</c-> <c- o>==</c-> <c- p>((</c-><c- sc>'a'</c-><c- p>))</c-></code> 
   </table>
   <p>This paper amends the proposed resolution of <a data-link-type="biblio" href="#biblio-lwg3776">[LWG3776]</a> per LEWG feedback and
makes the necessary changes to the <code class="highlight"><c- n>set_debug_format</c-></code> API both to enable the
proposed resolution and to fix tuple formatting. It also fixes a specification
bug in <code class="highlight"><c- n>range_formatter</c-></code> that doesn’t mention calling <code class="highlight"><c- n>parse</c-></code> for the
underlying formatter. This proposal has been implemented in <a data-link-type="biblio" href="#biblio-fmt">[FMT]</a> and in a
branch of <a data-link-type="biblio" href="#biblio-libcxx">[LIBCXX]</a>.</p>
   <h2 class="heading settled" data-level="2" id="poll-results"><span class="secno">2. </span><span class="content">LEWG Poll Results</span><a class="self-link" href="#poll-results"></a></h2>
   <p><strong>POLL</strong>: Relax the requirements table 74 and 75 to make the optimization allowed
by the issue resolution of LWG3776 a QoI issue with additional changes to the
handle class removed</p>
   <table class="poll">
    <tbody>
     <tr>
      <th>SF 
      <th>F 
      <th>N 
      <th>A 
      <th>SA 
     <tr>
      <td>1 
      <td>9 
      <td>2 
      <td>1 
      <td>1 
   </table>
   <p><strong>Outcome</strong>: consensus in favour</p>
   <p><strong>POLL</strong>: Adopt the amended proposed resolution of LWG3776 "Avoid parsing
format-spec if it is not present or empty". Return the issue to LWG for C++23
(to be confirmed by electronic polling)</p>
   <table class="poll">
    <tbody>
     <tr>
      <th>SF 
      <th>F 
      <th>N 
      <th>A 
      <th>SA 
     <tr>
      <td>2 
      <td>6 
      <td>1 
      <td>2 
      <td>1 
   </table>
   <p><strong>Outcome</strong>: weak consensus in favour</p>
   <h2 class="heading settled" data-level="3" id="wording"><span class="secno">3. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>This wording is relative to <a data-link-type="biblio" href="#biblio-n4917">[N4917]</a>.</p>
   <p>Modify 22.14.6.1 <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=802">[formatter.requirements</a>]
as indicated:</p>
   <p>-3- Given character type <code class="highlight"><c- n>charT</c-></code>, output iterator type <code class="highlight"><c- n>Out</c-></code>, and formatting
argument type <code class="highlight"><c- n>T</c-></code>, in Table 74 and Table 75:</p>
   <p>...</p>
   <p>
    <code class="highlight"><c- n>pc</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-></code> points to the beginning of the <em>format-spec</em> (22.14.2
[format.string]) of the replacement field being formatted in the format string.
If <em>format-spec</em> is 
    <ins>not present or</ins>
     empty then either <code class="highlight"><c- n>pc</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-> <c- o>==</c-> <c- n>pc</c-><c- p>.</c-><c- n>end</c-><c- p>()</c-></code> or <code class="highlight"><c- o>*</c-><c- n>pc</c-><c- p>.</c-><c- n>begin</c-><c- p>()</c-> <c- o>==</c-> <c- sc>'}'</c-></code>.
   </p>
   <p>Modify <em>BasicFormatter</em> requirements <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=803">[tab:formatter.basic</a>]
as indicated:</p>
   <table>
    <tbody>
     <tr>
      <th>Expression 
      <th>Return type 
      <th>Requirement 
     <tr>
      <td><code class="highlight"><c- n>f</c-><c- p>.</c-><c- n>format</c-><c- p>(</c-><c- n>u</c-><c- p>,</c-> <c- n>fc</c-><c- p>)</c-></code> 
      <td><code class="highlight"><c- n>FC</c-><c- o>::</c-><c- n>iterator</c-></code> 
      <td>
       Formats <code class="highlight"><c- n>u</c-></code> according to the specifiers stored in <code class="highlight"><c- o>*</c-><c- k>this</c-></code>, writes the output to <code class="highlight"><c- n>fc</c-><c- p>.</c-><c- n>out</c-><c- p>()</c-></code>, and returns
      an iterator past the end of the output range.
      The output shall only depend on <code class="highlight"><c- n>u</c-></code>, <code class="highlight"><c- n>fc</c-><c- p>.</c-><c- n>locale</c-><c- p>()</c-></code>, <code class="highlight"><c- n>fc</c-><c- p>.</c-><c- n>arg</c-><c- p>(</c-><c- n>n</c-><c- p>)</c-></code> for any value <code class="highlight"><c- n>n</c-></code> of type <code class="highlight"><c- b>size_t</c-></code>, and
      the range <code class="highlight"><c- p>[</c-><c- n>pc</c-><c- p>.</c-><c- n>begin</c-><c- p>(),</c-> <c- n>pc</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-></code> from the last
      call to <code class="highlight"><c- n>f</c-><c- p>.</c-><c- n>parse</c-><c- p>(</c-><c- n>pc</c-><c- p>)</c-></code>. 
       <ins>When the <em>format-spec</em> (22.14.2 [format.string]) is not present or empty
      the call to <code class="highlight"><c- n>f</c-><c- p>.</c-><c- n>parse</c-><c- p>(</c-><c- n>pc</c-><c- p>)</c-></code> may be omitted.</ins>
   </table>
   <p>Modify <em>Formatter</em> requirements <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=803">[tab:formatter</a>]
as indicated:</p>
   <table>
    <tbody>
     <tr>
      <th>Expression 
      <th>Return type 
      <th>Requirement 
     <tr>
      <td><code class="highlight"><c- n>f</c-><c- p>.</c-><c- n>format</c-><c- p>(</c-><c- n>t</c-><c- p>,</c-> <c- n>fc</c-><c- p>)</c-></code> 
      <td><code class="highlight"><c- n>FC</c-><c- o>::</c-><c- n>iterator</c-></code> 
      <td>
       Formats <code class="highlight"><c- n>t</c-></code> according to the specifiers stored in <code class="highlight"><c- o>*</c-><c- k>this</c-></code>, writes the output to <code class="highlight"><c- n>fc</c-><c- p>.</c-><c- n>out</c-><c- p>()</c-></code>, and returns
      an iterator past the end of the output range.
      The output shall only depend on <code class="highlight"><c- n>t</c-></code>, <code class="highlight"><c- n>fc</c-><c- p>.</c-><c- n>locale</c-><c- p>()</c-></code>, <code class="highlight"><c- n>fc</c-><c- p>.</c-><c- n>arg</c-><c- p>(</c-><c- n>n</c-><c- p>)</c-></code> for any value <code class="highlight"><c- n>n</c-></code> of type <code class="highlight"><c- b>size_t</c-></code>, and
      the range <code class="highlight"><c- p>[</c-><c- n>pc</c-><c- p>.</c-><c- n>begin</c-><c- p>(),</c-> <c- n>pc</c-><c- p>.</c-><c- n>end</c-><c- p>())</c-></code> from the last
      call to <code class="highlight"><c- n>f</c-><c- p>.</c-><c- n>parse</c-><c- p>(</c-><c- n>pc</c-><c- p>)</c-></code>. 
       <ins>When the <em>format-spec</em> (22.14.2 [format.string]) is not present or empty
      the call to <code class="highlight"><c- n>f</c-><c- p>.</c-><c- n>parse</c-><c- p>(</c-><c- n>pc</c-><c- p>)</c-></code> may be omitted.</ins>
   </table>
   <p>In <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=803">[format.formatter.spec</a>]:</p>
   <p>
    -2- Let <code class="highlight"><c- n>charT</c-></code> be either <code class="highlight"><c- b>char</c-></code> or <code class="highlight"><c- n>wchar_</c-></code>. Each specialization of <code class="highlight"><c- n>formatter</c-></code> is either enabled or disabled, as described below. A <em>debug-enabled</em> specialization of <code class="highlight"><c- n>formatter</c-></code> additionally provides a public, constexpr,
non-static member function <code class="highlight"><c- n>set_debug_format</c-><c- p>(</c-><ins><c- b>bool</c-> <c- n>set</c-></ins><c- p>)</c-></code> which modifies the state of the <code class="highlight"><c- n>formatter</c-></code> to be as if the type of the <em>std-format-spec</em> parsed by the last call to <code class="highlight"><c- n>parse</c-></code> were <code class="highlight"><c- o>?</c-></code> 
    <ins>if <code class="highlight"><c- n>set</c-></code> is <code class="highlight">true</code> and none otherwise</ins>
    . Each header that declares the template <code class="highlight"><c- n>formatter</c-></code> provides the following enabled specializations:
   </p>
   <p>...</p>
   <p>In <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=809">[format.range.formatter</a>]</p>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>charT</c-> <c- o>=</c-> <c- b>char</c-><c- o>></c->
    <c- k>requires</c-> <c- n>same_as</c-><c- o>&lt;</c-><c- n>remove_cvref_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>T</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>formattable</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>charT</c-><c- o>></c->
  <c- k>class</c-> <c- nc>range_formatter</c-> <c- p>{</c->
    <c- p>...</c->
    <c- k>constexpr</c-> <c- k>const</c-> <c- n>formatter</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>charT</c-><c- o>>&amp;</c-> <c- n>underlying</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <i><c- n>underlying_</c-></i><c- p>;</c-> <c- p>}</c->

    <ins><c- k>constexpr</c-> <c- n>range_formatter</c-><c- p>();</c-></ins>

    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
      <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
        <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->
  <c- p>};</c->
<c- p>}</c->
</pre>
   <p>...</p>
<pre class="highlight"><c- k>constexpr</c-> <c- b>void</c-> <c- n>set_brackets</c-><c- p>(</c-><c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-> <c- n>opening</c-><c- p>,</c-> <c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-> <c- n>closing</c-><c- p>);</c->
</pre>
   <em>Effects</em>: Equivalent to: 
<pre class="highlight"><i><c- n>opening</c-><c- o>-</c-><c- n>bracket_</c-></i> <c- o>=</c-> <c- n>opening</c-><c- p>;</c->
<i><c- n>closing</c-><c- o>-</c-><c- n>bracket_</c-></i> <c- o>=</c-> <c- n>closing</c-><c- p>;</c->
</pre>
<pre class="highlight"><ins><c- k>constexpr</c-> <c- nf>range_formatter</c-><c- p>();</c->
</ins></pre>
   <ins> <em>Effects</em>: Calls <code class="highlight"><i><c- n>underlying_</c-></i><c- p>.</c-><c- n>set_debug_format</c-><c- p>(</c->true<c- p>)</c-></code> if it is a valid
expression. </ins>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
  <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
    <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->
</pre>
   <p>
    <em>Effects</em>: Parses the format specifier
    <ins>s</ins>
     as a <em>range-format-spec</em> and stores the parsed specifiers in <code class="highlight"><c- o>*</c-><c- k>this</c-></code>. The values of <code class="highlight"><i><c- n>opening</c-><c- o>-</c-><c- n>bracket_</c-></i></code>, <code class="highlight"><i><c- n>closing</c-><c- o>-</c-><c- n>bracket_</c-></i></code>, and <code class="highlight"><i><c- n>separator_</c-></i></code> are
modified if and only if required by the <em>range-type</em> or the <code class="highlight"><c- n>n</c-></code> option, if
present. 
    <del>If:</del>
   </p>
   <ul>
    <li data-md>
     <del>the range-type is neither <code class="highlight"><c- n>s</c-></code> nor <code class="highlight"><c- o>?</c-><c- n>s</c-></code>, </del>
    <li data-md>
     <del><code class="highlight"><i><c- n>underlying_</c-></i><c- p>.</c-><c- n>set_debug_format</c-><c- p>()</c-></code> is a valid expression, and </del>
    <li data-md>
     <del>there is no <em>range-underlying-spec</em>, </del>
   </ul>
   <del>then calls <code class="highlight"><i><c- n>underlying_</c-></i><c- n>underlying_</c-><c- p>.</c-><c- n>set_debug_format</c-><c- p>()</c-></code>.</del>
   <p>
    <ins>If there is <em>range-underlying-spec</em>: </ins>
   </p>
   <ul>
    <li data-md>
     <ins>calls <code class="highlight"><c- n>underlying_</c-><c- p>.</c-><c- n>set_debug_format</c-><c- p>(</c->false<c- p>)</c-></code> if it is a valid expression,</ins>
    <li data-md>
     <ins>calls <code class="highlight"><c- n>underlying_</c-><c- p>.</c-><c- n>parse</c-></code> to parse <em>range-underlying-spec</em>.</ins>
   </ul>
   <p>In <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=813">[format.range.fmtstr</a>]:</p>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- n>range_format</c-> <c- n>K</c-><c- p>,</c-> <c- n>ranges</c-><c- o>::</c-><c- n>input_</c->­<c- n>range</c-> <c- n>R</c-><c- p>,</c-> <c- k>class</c-> <c- nc>charT</c-><c- o>></c->
    <c- k>requires</c-> <c- p>(</c-><c- n>K</c-> <c- o>==</c-> <c- n>range_format</c-><c- o>::</c-><c- n>string</c-> <c- o>||</c-> <c- n>K</c-> <c- o>==</c-> <c- n>range_format</c-><c- o>::</c-><c- n>debug_string</c-><c- p>)</c->
  <c- k>struct</c-> <i><c- nc>range</c-><c- o>-</c-><c- k>default</c-><c- o>-</c-><c- n>formatter</c-></i><c- o>&lt;</c-><c- n>K</c-><c- p>,</c-> <c- n>R</c-><c- p>,</c-> <c- n>charT</c-><c- o>></c-> <c- p>{</c->

    <c- p>...</c->

  <c- k>public</c-><c- o>:</c->
    <ins><i><c- k>constexpr</c-> <c- n>range</c-><c- o>-</c-><c- k>default</c-><c- o>-</c-><c- n>formatter</c-></i><c- p>();</c-></ins>
    
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
      <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
        <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->

    <c- p>...</c->
  <c- p>};</c->
<c- p>}</c->
</pre>
<pre class="highlight"><ins><c- k>constexpr</c-> <i><c- n>range</c-><c- o>-</c-><c- k>default</c-><c- o>-</c-><c- n>formatter</c-></i><c- p>();</c-></ins>
</pre>
   <ins> <em>Effects</em>: Calls <code class="highlight"><i><c- n>underlying_</c-></i><c- p>.</c-><c- n>set_debug_format</c-><c- p>(</c->true<c- p>)</c-></code> if it is a valid
expression and <code class="highlight"><c- n>K</c-> <c- o>==</c-> <c- n>range_format</c-><c- o>::</c-><c- n>debug_string</c-></code>. </ins>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
  <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
    <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->
</pre>
   <p>-2- <em>Effects</em>: Equivalent to:</p>
<pre class="highlight"><del><c- k>auto</c-> <c- n>i</c-> <c- o>=</c-> <i><c- n>underlying_</c-><c- p>.</c-></i><c- n>parse</c-><c- p>(</c-><c- n>ctx</c-><c- p>);</c->
<c- k>if</c-> <c- nf>constexpr</c-> <c- p>(</c-><c- n>K</c-> <c- o>==</c-> <c- n>range_format</c-><c- o>::</c-><c- n>debug_string</c-><c- p>)</c-> <c- p>{</c->
  <i><c- n>underlying_</c-><c- p>.</c-></i><c- n>set_debug_format</c-><c- p>(</c->true<c- p>);</c->
<c- p>}</c->
<c- k>return</c-> <c- n>i</c-><c- p>;</c->
</del><ins><c- k>if</c-> <c- nf>constexpr</c-> <c- p>(</c-><c- n>K</c-> <c- o>==</c-> <c- n>range_format</c-><c- o>::</c-><c- n>debug_string</c-><c- p>)</c-> <c- p>{</c->
  <i><c- n>underlying_</c-><c- p>.</c-></i><c- n>set_debug_format</c-><c- p>(</c->false<c- p>);</c->
<c- p>}</c->
<c- k>return</c-> <i><c- n>underlying_</c-><c- p>.</c-></i><c- n>parse</c-><c- p>(</c-><c- n>ctx</c-><c- p>);</c->
</ins></pre>
   <p>In <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf#page=816">[format.tuple</a>]:</p>
   <p>-1- For each of <code class="highlight"><c- n>pair</c-></code> and <code class="highlight"><c- n>tuple</c-></code>, the library provides the following formatter
specialization where <em>pair-or-tuple</em> is the name of the template:</p>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>charT</c-><c- p>,</c-> <c- n>formattable</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
  <c- k>struct</c-> <c- nc>formatter</c-><c- o>&lt;</c-><c- n>pair</c-><c- o>-</c-><c- n>or</c-><c- o>-</c-><c- n>tuple</c-><c- o>&lt;</c-><c- n>Ts</c-><c- p>...</c-><c- o>></c-><c- p>,</c-> <c- n>charT</c-><c- o>></c-> <c- p>{</c->

  <c- p>...</c->

  <c- k>constexpr</c-> <c- b>void</c-> <c- n>set_brackets</c-><c- p>(</c-><c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-> <c- n>opening</c-><c- p>,</c->
                              <c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-> <c- n>closing</c-><c- p>);</c->

  <ins><c- k>constexpr</c-> <c- nf>formatter</c-><c- p>();</c-></ins>
                              
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
      <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->
  <c- p>};</c->
<c- p>}</c->
</pre>
   <p>...</p>
<pre class="highlight"><c- k>constexpr</c-> <c- b>void</c-> <c- n>set_brackets</c-><c- p>(</c-><c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-> <c- n>opening</c-><c- p>,</c-> <c- n>basic_string_view</c-><c- o>&lt;</c-><c- n>charT</c-><c- o>></c-> <c- n>closing</c-><c- p>);</c->
</pre>
   <p>-6- <em>Effects</em>: Equivalent to:</p>
<pre class="highlight"><i><c- n>opening</c-><c- o>-</c-><c- n>bracket_</c-></i> <c- o>=</c-> <c- n>opening</c-><c- p>;</c->
<i><c- n>closing</c-><c- o>-</c-><c- n>bracket_</c-></i> <c- o>=</c-> <c- n>closing</c-><c- p>;</c->
</pre>
<pre class="highlight"><ins><c- k>constexpr</c-> <c- nf>formatter</c-><c- p>();</c->
</ins></pre>
   <ins><em>Effects</em>: For each
element <code class="highlight"><i><c- n>e</c-></i></code> in <code class="highlight"><i><c- n>underlying_</c-></i></code>, if <code class="highlight"><i><c- n>e</c-></i><c- p>.</c-><c- n>set_debug_format</c-><c- p>(</c->true<c- p>)</c-></code> is a valid expression, calls <code class="highlight"><i><c- n>e</c-></i><c- p>.</c-><c- n>set_debug_format</c-><c- p>(</c->true<c- p>)</c-></code>.</ins>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>ParseContext</c-><c- o>></c->
  <c- k>constexpr</c-> <c- k>typename</c-> <c- nc>ParseContext</c-><c- o>::</c-><c- n>iterator</c->
    <c- n>parse</c-><c- p>(</c-><c- n>ParseContext</c-><c- o>&amp;</c-> <c- n>ctx</c-><c- p>);</c->
</pre>
   <p>
    -7- <em>Effects</em>: Parses the format specifier
    <ins>s</ins>
     as a <em>tuple-format-spec</em> and stores the parsed specifiers in <code class="highlight"><c- o>*</c-><c- k>this</c-></code>. The values of <code class="highlight"><i><c- n>opening</c-><c- o>-</c-><c- n>bracket_</c-></i></code>, <code class="highlight"><i><c- n>closing</c-><c- o>-</c-><c- n>bracket_</c-></i></code>, and <code class="highlight"><i><c- n>separator_</c-></i></code> are
modified if and only if required by the <em>tuple-type</em>, if present. 
    <del>For each
element <code class="highlight"><i><c- n>e</c-></i></code> in <code class="highlight"><i><c- n>underlying_</c-></i></code>, if <code class="highlight"><i><c- n>e</c-></i><c- p>.</c-><c- n>set_debug_format</c-><c- p>()</c-></code> is a valid expression, calls <code class="highlight"><i><c- n>e</c-></i><c- p>.</c-><c- n>set_debug_format</c-><c- p>()</c-></code>.</del>
   </p>
   <p>-8- <em>Returns</em>: An iterator past the end of the <em>tuple-format-spec</em>.</p>
   <h2 class="heading settled" data-level="4" id="ack"><span class="secno">4. </span><span class="content">Acknowledgements</span><a class="self-link" href="#ack"></a></h2>
   <p>Thanks to Barry Revzin and Mark de Wever for pointing out issues with debug
formatting of ranges and tuples.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-fmt">[FMT]
   <dd>Victor Zverovich; et al. <a href="https://github.com/fmtlib/fmt">The fmt library</a>. URL: <a href="https://github.com/fmtlib/fmt">https://github.com/fmtlib/fmt</a>
   <dt id="biblio-libcxx">[LIBCXX]
   <dd><a href="https://libcxx.llvm.org/">“libc++” C++ Standard Library</a>. URL: <a href="https://libcxx.llvm.org/">https://libcxx.llvm.org/</a>
   <dt id="biblio-lwg3776">[LWG3776]
   <dd>Mark de Wever. <a href="https://cplusplus.github.io/LWG/issue3776">Avoid parsing format-spec if it is not present or empty</a>. URL: <a href="https://cplusplus.github.io/LWG/issue3776">https://cplusplus.github.io/LWG/issue3776</a>
   <dt id="biblio-n4917">[N4917]
   <dd>Thomas Köppe; et al. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf">Working Draft, Standard for Programming Language C++</a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4917.pdf</a>
   <dt id="biblio-p2286">[P2286]
   <dd>Barry Revzin. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2286r8.html">Formatting Ranges</a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2286r8.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2286r8.html</a>
  </dl>