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  <title>P2216R0: std::format improvements</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
 *
 * 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)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * 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
 *
 * 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)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	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;

		/* Colors */
		color: black;
		background: white 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-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		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;
		border-color: #c00;
	}

	/* 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: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* 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-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** 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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}
		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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}

		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;
		background: transparent;
	}

	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,
	#subtitle {
		/* #subtitle 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) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		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;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; 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: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** 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;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	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;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* 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;
	}
	.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 {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* 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;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* 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;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** 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: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    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;
	}

	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-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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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">P2216R0<br>std::format improvements</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2020-09-06">2020-09-06</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://fmt.dev/papers/p2216r0.html">http://fmt.dev/papers/p2216r0.html</a>
     <dt>Author:
     <dd>
      <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="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#checks"><span class="secno">2</span> <span class="content">Compile-time checks</span></a>
    <li><a href="#binary"><span class="secno">3</span> <span class="content">Binary size</span></a>
    <li><a href="#impact"><span class="secno">4</span> <span class="content">Impact on existing code</span></a>
    <li><a href="#wording"><span class="secno">5</span> <span class="content">Wording</span></a>
    <li><a href="#ack"><span class="secno">6</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <p style="text-align: right"> "Safety doesn’t happen by accident." <br> ― unknown </p>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>This paper proposes the following improvements to the C++20 formatting facility:</p>
   <ul>
    <li data-md>
     <p>Improving safety via compile-time format string checks</p>
    <li data-md>
     <p>Reducing binary code size of <code class="highlight"><c- n>format_to</c-></code></p>
   </ul>
   <h2 class="heading settled" data-level="2" id="checks"><span class="secno">2. </span><span class="content">Compile-time checks</span><a class="self-link" href="#checks"></a></h2>
   <p>Consider the following example:</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>s</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- s>"{:d}"</c-><c- p>,</c-> <c- s>"I am not a number"</c-><c- p>);</c->
</pre>
   <p>In C++20 (<a data-link-type="biblio" href="#biblio-n4861">[N4861]</a>) it throws <code class="highlight"><c- n>format_error</c-></code> because <code class="highlight"><c- n>d</c-></code> is not a valid format
specifier for a null-terminated character string.</p>
   <p>We propose making it ill-formed so that given a proper language facility
(<a data-link-type="biblio" href="#biblio-p1045">[P1045]</a>, <a data-link-type="biblio" href="#biblio-p1221">[P1221]</a> or similar) this results in a compile-time rather than a
runtime error. This will significantly improve safety of the formatting API and
bring it on par with other languages such as D (<a data-link-type="biblio" href="#biblio-d-format">[D-FORMAT]</a>) and Rust
(<a data-link-type="biblio" href="#biblio-rust-fmt">[RUST-FMT]</a>).</p>
   <p>This proposal has been shown to work on a version of clang that implements <a data-link-type="biblio" href="#biblio-p1221">[P1221]</a>: <a href="https://godbolt.org/z/hcnxfY">https://godbolt.org/z/hcnxfY</a>.
Format string parsing in C++20 has been designed with such checks in mind
(<a data-link-type="biblio" href="#biblio-p0645">[P0645]</a>) and is already <code class="highlight"><c- k>constexpr</c-></code>.</p>
   <p>Without a language or implementation support it’s only possible to emulate the
desired behavior by passing format strings wrapped in a <code class="highlight"><c- k>consteval</c-></code> function, a
user-defined literal, a macro or as a template parameter, for example:</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>s</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>format</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>static_string</c-><c- p>(</c-><c- s>"{:d}"</c-><c- p>),</c-> <c- s>"I am not a number"</c-><c- p>);</c->
</pre>
   <p>This is clearly not a satisfactory solution because it doesn’t improve safety
of the existing API (another wrong default). Template parameters additionally
introduce confusing API that interacts poorly with argument indexing.</p>
   <p>From the extensive usage experience in the {fmt} library (<a data-link-type="biblio" href="#biblio-fmt">[FMT]</a>) that provides
compile-time checks as an opt-in we’ve found that users expect errors in literal
format strings to be diagnosed at compile time by default. One of the reasons is
that such diagnostic is commonly done in <code class="highlight"><c- n>printf</c-></code>, for example:</p>
<pre class="language-c++ highlight"><c- n>printf</c-><c- p>(</c-><c- s>"%d"</c-><c- p>,</c-> <c- s>"I am not a number"</c-><c- p>);</c->
</pre>
   <p>gives a warning both in GCC and clang:</p>
<pre class="highlight"><c- nl>warning</c-><c- p>:</c-> <c- n>format</c-> <c- n>specifies</c-> <c- n>type</c-> '<c- b>int</c->' <c- n>but</c-> <c- n>the</c-> <c- n>argument</c-> <c- n>has</c-> <c- n>type</c-> '<c- k>const</c-> <c- b>char</c-> <c- o>*</c->' <c- p>[</c-><c- o>-</c-><c- n>Wformat</c-><c- p>]</c->
</pre>
   so users expect the same or better level of diagnostics from a similar C++
facility. 
   <h2 class="heading settled" data-level="3" id="binary"><span class="secno">3. </span><span class="content">Binary size</span><a class="self-link" href="#binary"></a></h2>
   <p>The <code class="highlight"><c- n>vformat_to</c-></code> functions take format args parameterized on the output
iterator via the formatting context:</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>Out</c-><c- p>,</c-> <c- n>class</c-> <c- n>charT</c-><c- o>></c->
    <c- n>using</c-> <c- n>format_args_t</c-> <c- o>=</c-> <c- n>basic_format_args</c-><c- o>&lt;</c-><c- n>basic_format_context</c-><c- o>&lt;</c-><c- n>Out</c-><c- p>,</c-> <c- n>charT</c-><c- o>>></c-><c- p>;</c->

<c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c->
                 <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>char</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
</pre>
   <p>Unfortunately it may result in significant code bloat because formatting code
will have to be instantiated for every iterator type used with <code class="highlight"><c- n>format_to</c-></code> or <code class="highlight"><c- n>vformat_to</c-></code>. This happens even for argument types that are not formatted,
clearly violating "you don’t pay for what you don’t use" principle. Also this is
unnecessary because the iterator type can be erased via the internal buffer as
it is done in <code class="highlight"><c- n>format</c-></code> and <code class="highlight"><c- n>vformat</c-></code>. Therefore we propose using <code class="highlight"><c- n>format_args</c-></code> and <code class="highlight"><c- n>wformat_args</c-></code> instead of <code class="highlight"><c- n>format_args_t</c-></code> in these overloads:</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>format_args</c-> <c- n>args</c-><c- p>);</c->
</pre>
   <p><code class="highlight"><c- n>formatter</c-></code> specializations will continue to support output iterators so this
only affects type-erased API and not the one with compiled format strings that
will be proposed separately. The latter will not be affected by the code bloat
issue because instantiations will be limited only to used argument types.</p>
   <p>This proposal has been successfully implemented in the {fmt} library (<a data-link-type="biblio" href="#biblio-fmt">[FMT]</a>).</p>
   <h2 class="heading settled" data-level="4" id="impact"><span class="secno">4. </span><span class="content">Impact on existing code</span><a class="self-link" href="#impact"></a></h2>
   <p>Making invalid format strings ill-formed and removing problematic <code class="highlight"><c- n>vformat_to</c-></code> overloads are breaking changes although at the time of writing none of the
standard libraries implements the С++20 formatting facility and therefore there
is no code using it.</p>
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">5. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>All wording is relative to the C++ working draft <a data-link-type="biblio" href="#biblio-n4861">[N4861]</a>.</p>
   <p>Update the value of the feature-testing macro <code class="highlight"><c- n>__cpp_lib_format</c-></code> to the date of
adoption in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf#page=493">[version.syn]</a>:</p>
   <p>Change in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf#page=736">[format.err.report]</a>:</p>
   <p>Formatting functions throw <code class="highlight"><c- n>format_error</c-></code> if an argument <code class="highlight"><c- n>fmt</c-></code> is passed that is
not a format string for <code class="highlight"><c- n>args</c-></code>. They propagate exceptions thrown by operations
of formatter specializations and iterators. Failure to allocate storage is
reported by throwing an exception as described in [res.on.exception.handling].</p>
   <ins> Passing an argument <code class="highlight"><c- n>fmt</c-></code> that is not a format string for parameter pack <code class="highlight"><c- n>args</c-></code> is ill-formed with no diagnostic required. </ins>
   <p>Change in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf#page=729">[format.syn]</a>:</p>
<pre class="highlight"><del>  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- p>,</c-> <c- k>class</c-> <c- nc>charT</c-><c- o>></c->
    <c- k>using</c-> <c- n>format_args_t</c-> <c- o>=</c-> <c- n>basic_format_args</c-><c- o>&lt;</c-><c- n>basic_format_context</c-><c- o>&lt;</c-><c- n>Out</c-><c- p>,</c-> <c- n>charT</c-><c- o>>></c-><c- p>;</c->
</del></pre>
   <p>...</p>
<pre class="highlight"><del>  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c->
                   <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>char</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c->
                   <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>wchar_t</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c->
                   <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>char</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c->
                   <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>wchar_t</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
</del></pre>
<pre class="highlight"><ins>  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>format_args</c-> <c- n>args</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>wformat_args</c-> <c- n>args</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c->
                   <c- n>format_args</c-> <c- n>args</c-><c- p>);</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
    <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c->
                   <c- n>wformat_args</c-> <c- n>args</c-><c- p>);</c->
</ins></pre>
   <p>Change in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf#page=736">[format.functions]</a>:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- p>,</c-> <c- n>class</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
  <c- n>Out</c-> <c- n>format_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- k>const</c-> <c- n>Args</c-><c- o>&amp;</c-><c- n>amp</c-><c- p>;...</c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- p>,</c-> <c- n>class</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
  <c- n>Out</c-> <c- n>format_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- k>const</c-> <c- n>Args</c-><c- o>&amp;</c-><c- n>amp</c-><c- p>;...</c-> <c- n>args</c-><c- p>);</c->
</pre>
   <p><em>Effects:</em> Equivalent to:</p>
<pre class="highlight"><del><c- k>using</c-> <c- n>context</c-> <c- o>=</c-> <c- n>basic_format_context</c-><c- o>&lt;</c-><c- n>Out</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>fmt</c-><c- p>)</c-><c- o>::</c-><c- n>value_type</c-><c- o>></c-><c- p>;</c->
<c- k>return</c-> <c- nf>vformat_to</c-><c- p>(</c-><c- n>out</c-><c- p>,</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>make_format_args</c-><c- o>&lt;</c-><c- n>context</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>...));</c->
</del></pre>
<pre class="highlight"><ins><c- k>return</c-> <c- nf>vformat_to</c-><c- p>(</c-><c- n>out</c-><c- p>,</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>make_format_args</c-><c- p>(</c-><c- n>args</c-><c- p>...));</c->
</ins></pre>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- p>,</c-> <c- n>class</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
  <c- n>Out</c-> <c- n>format_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- k>const</c-> <c- n>Args</c-><c- o>&amp;</c-><c- p>...</c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- p>,</c-> <c- n>class</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
  <c- n>Out</c-> <c- n>format_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- k>const</c-> <c- n>Args</c-><c- o>&amp;</c-><c- p>...</c-> <c- n>args</c-><c- p>);</c->
</pre>
   <p><em>Effects:</em> Equivalent to:</p>
<pre class="highlight"><del><c- k>using</c-> <c- n>context</c-> <c- o>=</c-> <c- n>basic_format_context</c-><c- o>&lt;</c-><c- n>Out</c-><c- p>,</c-> <c- k>decltype</c-><c- p>(</c-><c- n>fmt</c-><c- p>)</c-><c- o>::</c-><c- n>value_type</c-><c- o>></c-><c- p>;</c->
<c- k>return</c-> <c- nf>vformat_to</c-><c- p>(</c-><c- n>out</c-><c- p>,</c-> <c- n>loc</c-><c- p>,</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>make_format_args</c-><c- o>&lt;</c-><c- n>context</c-><c- o>></c-><c- p>(</c-><c- n>args</c-><c- p>...));</c->
</del></pre>
<pre class="highlight"><ins><c- k>return</c-> <c- nf>vformat_to</c-><c- p>(</c-><c- n>out</c-><c- p>,</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>make_wformat_args</c-><c- p>(</c-><c- n>args</c-><c- p>...));</c->
</ins></pre>
<pre class="highlight"><del><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c->
                 <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>char</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c->
                 <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>wchar_t</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c->
                 <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>char</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c->
                 <c- n>format_args_t</c-><c- o>&lt;</c-><c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Out</c-><c- o>></c-><c- p>,</c-> <c- b>wchar_t</c-><c- o>></c-> <c- n>args</c-><c- p>);</c->
</del></pre>
<pre class="highlight"><ins><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>format_args</c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c-> <c- n>wformat_args</c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>string_view</c-> <c- n>fmt</c-><c- p>,</c->
                 <c- n>format_args</c-> <c- n>args</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Out</c-><c- o>></c->
  <c- n>Out</c-> <c- n>vformat_to</c-><c- p>(</c-><c- n>Out</c-> <c- n>out</c-><c- p>,</c-> <c- k>const</c-> <c- n>locale</c-><c- o>&amp;</c-> <c- n>loc</c-><c- p>,</c-> <c- n>wstring_view</c-> <c- n>fmt</c-><c- p>,</c->
                 <c- n>wformat_args</c-> <c- n>args</c-><c- p>);</c->
</ins></pre>
   <h2 class="heading settled" data-level="6" id="ack"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#ack"></a></h2>
   <p>Thanks to Hana Dusíková for demonstrating that the optimal formatting API can be
implemented with P1221.</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-d-format">[D-FORMAT]
   <dd><a href="https://dlang.org/phobos/std_format.html">D Library Reference, std.format</a>. URL: <a href="https://dlang.org/phobos/std_format.html">https://dlang.org/phobos/std_format.html</a>
   <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-n4861">[N4861]
   <dd>Richard Smith; et al. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf">Working Draft, Standard for Programming Language C++</a>. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf</a>
   <dt id="biblio-p0645">[P0645]
   <dd>Victor Zverovich. <a href="https://wg21.link/p0645">Text Formatting</a>. URL: <a href="https://wg21.link/p0645">https://wg21.link/p0645</a>
   <dt id="biblio-p1045">[P1045]
   <dd>David Stone. <a href="http://wg21.link/p1045">constexpr Function Parameters</a>. URL: <a href="http://wg21.link/p1045">http://wg21.link/p1045</a>
   <dt id="biblio-p1221">[P1221]
   <dd>Jason Rice. <a href="http://wg21.link/p1221">Parametric Expressions</a>. URL: <a href="http://wg21.link/p1221">http://wg21.link/p1221</a>
   <dt id="biblio-rust-fmt">[RUST-FMT]
   <dd><a href="https://doc.rust-lang.org/std/fmt/">The Rust Standard Library, Module std::fmt</a>. URL: <a href="https://doc.rust-lang.org/std/fmt/">https://doc.rust-lang.org/std/fmt/</a>
  </dl>