<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P2893R3: Variadic friends</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;			  /* limit line length to 50em for readability   */
		margin: 0 auto;				/* center text within page                    */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag    */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em; }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small; }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;	/* Inherit the font family. */
		line-height: 1.2;		/* Keep wrapped headings compact */
		hyphens: manual;		/* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#profile-and-date {
		/* #profile-and-date is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	border-left: 0.5em solid #DEF;
	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 border-radius: .5em;
	 margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	 margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	 margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	margin-left: -2em;
	}
	dl.switch {
	padding-left: 2em;
	}
	dl.switch > dt {
	text-indent: -1.5em;
	margin-top: 1em;
	}
	dl.switch > dt + dt {
	margin-top: 0;
	}
	dl.switch > dt::before {
	content: '\21AA';
	padding: 0 0.5em 0 0;
	display: inline-block;
	width: 1em;
	text-align: right;
	line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: inherit;
	}
	dfn var {
		font-style: normal;
	}

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

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

	/* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

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

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

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

	/* For autogen numbers, add
	  .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,	.sidefigure img,	figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
		height: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em;
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em; }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote,
	.amendment, .correction, .addition {
		margin: 1em auto;
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

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

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

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

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	  or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #e0cb52;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	  or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52e052;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

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

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

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

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
		border-bottom: 1px solid var(--defrow-border);
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def td.footnote {
		padding-top: 0.6em;
	}
	table.def td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		<table class="data"> highlights structural relationships in a table
		when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		Use class="complex data" for particularly complicated tables --
		(This will draw more lines: busier, but clearer.)

		Use class="long" on table cells with paragraph-like contents
		(This will adjust text alignment accordingly.)
		Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-color: var(--datacell-border);
		border-top-style: solid;
	}

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		border-top: 1px solid var(--datacell-border);
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version d765c696b, updated Fri Mar 8 15:58:52 2024 -0800" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="dark light" name="color-scheme">
<style>
hilite  {background-color: #FFFF00; font-weight: bold;}
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* Boilerplate: style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}

@media (prefers-color-scheme: dark) {
    :root {
        --selflink-text: black;
        --selflink-bg: silver;
        --selflink-hover-text: white;
    }
}
</style>
<style>/* Boilerplate: style-colors */
/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
:root {
    color-scheme: light dark;

    --text: black;
    --bg: white;

    --unofficial-watermark: url(https://www.w3.org/StyleSheets/TR/2016/logos/UD-watermark);

    --logo-bg: #1a5e9a;
    --logo-active-bg: #c00;
    --logo-text: white;

    --tocnav-normal-text: #707070;
    --tocnav-normal-bg: var(--bg);
    --tocnav-hover-text: var(--tocnav-normal-text);
    --tocnav-hover-bg: #f8f8f8;
    --tocnav-active-text: #c00;
    --tocnav-active-bg: var(--tocnav-normal-bg);

    --tocsidebar-text: var(--text);
    --tocsidebar-bg: #f7f8f9;
    --tocsidebar-shadow: rgba(0,0,0,.1);
    --tocsidebar-heading-text: hsla(203,20%,40%,.7);

    --toclink-text: var(--text);
    --toclink-underline: #3980b5;
    --toclink-visited-text: var(--toclink-text);
    --toclink-visited-underline: #054572;

    --heading-text: #005a9c;

    --hr-text: var(--text);

    --algo-border: #def;

    --del-text: red;
    --del-bg: transparent;
    --ins-text: #080;
    --ins-bg: transparent;

    --a-normal-text: #034575;
    --a-normal-underline: #bbb;
    --a-visited-text: var(--a-normal-text);
    --a-visited-underline: #707070;
    --a-hover-bg: rgba(75%, 75%, 75%, .25);
    --a-active-text: #c00;
    --a-active-underline: #c00;

    --blockquote-border: silver;
    --blockquote-bg: transparent;
    --blockquote-text: currentcolor;

    --issue-border: #e05252;
    --issue-bg: #fbe9e9;
    --issue-text: var(--text);
    --issueheading-text: #831616;

    --example-border: #e0cb52;
    --example-bg: #fcfaee;
    --example-text: var(--text);
    --exampleheading-text: #574b0f;

    --note-border: #52e052;
    --note-bg: #e9fbe9;
    --note-text: var(--text);
    --noteheading-text: hsl(120, 70%, 30%);
    --notesummary-underline: silver;

    --assertion-border: #aaa;
    --assertion-bg: #eee;
    --assertion-text: black;

    --advisement-border: orange;
    --advisement-bg: #fec;
    --advisement-text: var(--text);
    --advisementheading-text: #b35f00;

    --warning-border: red;
    --warning-bg: hsla(40,100%,50%,0.95);
    --warning-text: var(--text);

    --amendment-border: #330099;
    --amendment-bg: #F5F0FF;
    --amendment-text: var(--text);
    --amendmentheading-text: #220066;

    --def-border: #8ccbf2;
    --def-bg: #def;
    --def-text: var(--text);
    --defrow-border: #bbd7e9;

    --datacell-border: silver;

    --indexinfo-text: #707070;

    --indextable-hover-text: black;
    --indextable-hover-bg: #f7f8f9;

    --outdatedspec-bg: rgba(0, 0, 0, .5);
    --outdatedspec-text: black;
    --outdated-bg: maroon;
    --outdated-text: white;
    --outdated-shadow: red;

    --editedrec-bg: darkorange;
}

@media (prefers-color-scheme: dark) {
    :root {
        --text: #ddd;
        --bg: black;

        --unofficial-watermark: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='400' height='400'%3E%3Cg fill='%23100808' transform='translate(200 200) rotate(-45) translate(-200 -200)' stroke='%23100808' stroke-width='3'%3E%3Ctext x='50%25' y='220' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EUNOFFICIAL%3C/text%3E%3Ctext x='50%25' y='305' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EDRAFT%3C/text%3E%3C/g%3E%3C/svg%3E");

        --logo-bg: #1a5e9a;
        --logo-active-bg: #c00;
        --logo-text: white;

        --tocnav-normal-text: #999;
        --tocnav-normal-bg: var(--bg);
        --tocnav-hover-text: var(--tocnav-normal-text);
        --tocnav-hover-bg: #080808;
        --tocnav-active-text: #f44;
        --tocnav-active-bg: var(--tocnav-normal-bg);

        --tocsidebar-text: var(--text);
        --tocsidebar-bg: #080808;
        --tocsidebar-shadow: rgba(255,255,255,.1);
        --tocsidebar-heading-text: hsla(203,20%,40%,.7);

        --toclink-text: var(--text);
        --toclink-underline: #6af;
        --toclink-visited-text: var(--toclink-text);
        --toclink-visited-underline: #054572;

        --heading-text: #8af;

        --hr-text: var(--text);

        --algo-border: #456;

        --del-text: #f44;
        --del-bg: transparent;
        --ins-text: #4a4;
        --ins-bg: transparent;

        --a-normal-text: #6af;
        --a-normal-underline: #555;
        --a-visited-text: var(--a-normal-text);
        --a-visited-underline: var(--a-normal-underline);
        --a-hover-bg: rgba(25%, 25%, 25%, .2);
        --a-active-text: #f44;
        --a-active-underline: var(--a-active-text);

        --borderedblock-bg: rgba(255, 255, 255, .05);

        --blockquote-border: silver;
        --blockquote-bg: var(--borderedblock-bg);
        --blockquote-text: currentcolor;

        --issue-border: #e05252;
        --issue-bg: var(--borderedblock-bg);
        --issue-text: var(--text);
        --issueheading-text: hsl(0deg, 70%, 70%);

        --example-border: hsl(50deg, 90%, 60%);
        --example-bg: var(--borderedblock-bg);
        --example-text: var(--text);
        --exampleheading-text: hsl(50deg, 70%, 70%);

        --note-border: hsl(120deg, 100%, 35%);
        --note-bg: var(--borderedblock-bg);
        --note-text: var(--text);
        --noteheading-text: hsl(120, 70%, 70%);
        --notesummary-underline: silver;

        --assertion-border: #444;
        --assertion-bg: var(--borderedblock-bg);
        --assertion-text: var(--text);

        --advisement-border: orange;
        --advisement-bg: #222218;
        --advisement-text: var(--text);
        --advisementheading-text: #f84;

        --warning-border: red;
        --warning-bg: hsla(40,100%,20%,0.95);
        --warning-text: var(--text);

        --amendment-border: #330099;
        --amendment-bg: #080010;
        --amendment-text: var(--text);
        --amendmentheading-text: #cc00ff;

        --def-border: #8ccbf2;
        --def-bg: #080818;
        --def-text: var(--text);
        --defrow-border: #136;

        --datacell-border: silver;

        --indexinfo-text: #aaa;

        --indextable-hover-text: var(--text);
        --indextable-hover-bg: #181818;

        --outdatedspec-bg: rgba(255, 255, 255, .5);
        --outdatedspec-text: black;
        --outdated-bg: maroon;
        --outdated-text: white;
        --outdated-shadow: red;

        --editedrec-bg: darkorange;
    }
    /* In case a transparent-bg image doesn't expect to be on a dark bg,
       which is quite common in practice... */
    img { background: white; }
}
</style>
<style>/* Boilerplate: style-counters */
body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}
</style>
<style>/* Boilerplate: style-issues */
a[href].issue-return {
    float: right;
    float: inline-end;
    color: var(--issueheading-text);
    font-weight: bold;
    text-decoration: none;
}
</style>
<style>/* Boilerplate: style-md-lists */
/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}
</style>
<style>/* Boilerplate: style-selflinks */
:root {
    --selflink-text: white;
    --selflink-bg: gray;
    --selflink-hover-text: black;
}
.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
.example > a.self-link,
.note > a.self-link,
.issue > a.self-link {
    /* These blocks are overflow:auto, so positioning outside
       doesn't work. */
    left: auto;
    right: 0;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: var(--selflink-bg);
    color: var(--selflink-text);
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: var(--selflink-hover-text);
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }
</style>
<style>/* Boilerplate: style-syntax-highlighting */
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }

.highlight:not(.idl) { background: rgba(0, 0, 0, .03); }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */

@media (prefers-color-scheme: dark) {
    .highlight:not(.idl) { background: rgba(255, 255, 255, .05); }

    c-[a] { color: #d33682 } /* Keyword.Declaration */
    c-[b] { color: #d33682 } /* Keyword.Type */
    c-[c] { color: #2aa198 } /* Comment */
    c-[d] { color: #2aa198 } /* Comment.Multiline */
    c-[e] { color: #268bd2 } /* Name.Attribute */
    c-[f] { color: #b58900 } /* Name.Tag */
    c-[g] { color: #cb4b16 } /* Name.Variable */
    c-[k] { color: #d33682 } /* Keyword */
    c-[l] { color: #657b83 } /* Literal */
    c-[m] { color: #657b83 } /* Literal.Number */
    c-[n] { color: #268bd2 } /* Name */
    c-[o] { color: #657b83 } /* Operator */
    c-[p] { color: #657b83 } /* Punctuation */
    c-[s] { color: #6c71c4 } /* Literal.String */
    c-[t] { color: #6c71c4 } /* Literal.String.Single */
    c-[u] { color: #6c71c4 } /* Literal.String.Double */
    c-[ch] { color: #2aa198 } /* Comment.Hashbang */
    c-[cp] { color: #2aa198 } /* Comment.Preproc */
    c-[cpf] { color: #2aa198 } /* Comment.PreprocFile */
    c-[c1] { color: #2aa198 } /* Comment.Single */
    c-[cs] { color: #2aa198 } /* Comment.Special */
    c-[kc] { color: #d33682 } /* Keyword.Constant */
    c-[kn] { color: #d33682 } /* Keyword.Namespace */
    c-[kp] { color: #d33682 } /* Keyword.Pseudo */
    c-[kr] { color: #d33682 } /* Keyword.Reserved */
    c-[ld] { color: #657b83 } /* Literal.Date */
    c-[nc] { color: #268bd2 } /* Name.Class */
    c-[no] { color: #268bd2 } /* Name.Constant */
    c-[nd] { color: #268bd2 } /* Name.Decorator */
    c-[ni] { color: #268bd2 } /* Name.Entity */
    c-[ne] { color: #268bd2 } /* Name.Exception */
    c-[nf] { color: #268bd2 } /* Name.Function */
    c-[nl] { color: #268bd2 } /* Name.Label */
    c-[nn] { color: #268bd2 } /* Name.Namespace */
    c-[py] { color: #268bd2 } /* Name.Property */
    c-[ow] { color: #657b83 } /* Operator.Word */
    c-[mb] { color: #657b83 } /* Literal.Number.Bin */
    c-[mf] { color: #657b83 } /* Literal.Number.Float */
    c-[mh] { color: #657b83 } /* Literal.Number.Hex */
    c-[mi] { color: #657b83 } /* Literal.Number.Integer */
    c-[mo] { color: #657b83 } /* Literal.Number.Oct */
    c-[sa] { color: #6c71c4 } /* Literal.String.Affix */
    c-[sb] { color: #6c71c4 } /* Literal.String.Backtick */
    c-[sc] { color: #6c71c4 } /* Literal.String.Char */
    c-[dl] { color: #6c71c4 } /* Literal.String.Delimiter */
    c-[sd] { color: #6c71c4 } /* Literal.String.Doc */
    c-[se] { color: #6c71c4 } /* Literal.String.Escape */
    c-[sh] { color: #6c71c4 } /* Literal.String.Heredoc */
    c-[si] { color: #6c71c4 } /* Literal.String.Interpol */
    c-[sx] { color: #6c71c4 } /* Literal.String.Other */
    c-[sr] { color: #6c71c4 } /* Literal.String.Regex */
    c-[ss] { color: #6c71c4 } /* Literal.String.Symbol */
    c-[fm] { color: #268bd2 } /* Name.Function.Magic */
    c-[vc] { color: #cb4b16 } /* Name.Variable.Class */
    c-[vg] { color: #cb4b16 } /* Name.Variable.Global */
    c-[vi] { color: #cb4b16 } /* Name.Variable.Instance */
    c-[vm] { color: #cb4b16 } /* Name.Variable.Magic */
    c-[il] { color: #657b83 } /* Literal.Number.Integer.Long */
}
</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2893R3<br>Variadic friends</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2024-03-22">2024-03-22</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Authors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:coachhagins@gmail.com">Jody Hagins</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:arthur.j.odwyer@gmail.com">Arthur O'Dwyer</a>
     <dt>Audience:
     <dd>CWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Support <code class="highlight"><c- k>friend</c-> <c- n>Ts</c-><c- p>...</c-></code> in C++26.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#changelog"><span class="secno">1</span> <span class="content">Changelog</span></a>
    <li>
     <a href="#introduction"><span class="secno">2</span> <span class="content">Introduction</span></a>
     <ol class="toc">
      <li><a href="#passkey-idiom"><span class="secno">2.1</span> <span class="content">Passkey idiom</span></a>
      <li><a href="#crtp-access"><span class="secno">2.2</span> <span class="content">CRTP access to derived</span></a>
     </ol>
    <li>
     <a href="#confusing"><span class="secno">3</span> <span class="content">Confusing grammatical examples</span></a>
     <ol class="toc">
      <li><a href="#not-for-stamping"><span class="secno">3.1</span> <span class="content"><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c-> <c- k>friend</c-> <c- n>Ts</c-><c- p>...</c-></code></span></a>
      <li><a href="#confusing-elab"><span class="secno">3.2</span> <span class="content"><code class="highlight"><c- k>friend</c-> <c- k>class</c-> <c- nc>Ts</c-><c- p>...</c-></code></span></a>
     </ol>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Proposed wording</span></a>
     <ol class="toc">
      <li>
       <a href="#wording1"><span class="secno">4.1</span> <span class="content">Option 1 (proposed for C++26)</span></a>
       <ol class="toc">
        <li><a href="#wording1-cpp.predefined"><span class="secno">4.1.1</span> <span class="content">[cpp.predefined]</span></a>
        <li><a href="#wording1-class.mem.general"><span class="secno">4.1.2</span> <span class="content">[class.mem.general]</span></a>
        <li><a href="#wording1-dcl.pre"><span class="secno">4.1.3</span> <span class="content">[dcl.pre]</span></a>
        <li><a href="#wording1-temp.pre"><span class="secno">4.1.4</span> <span class="content">[temp.pre]</span></a>
        <li><a href="#wording1-class.friend"><span class="secno">4.1.5</span> <span class="content">[class.friend]</span></a>
        <li><a href="#wording1-dcl.type.elab"><span class="secno">4.1.6</span> <span class="content">[dcl.type.elab]</span></a>
        <li><a href="#wording1-temp.res.general"><span class="secno">4.1.7</span> <span class="content">[temp.res.general]</span></a>
        <li><a href="#wording1-temp.variadic"><span class="secno">4.1.8</span> <span class="content">[temp.variadic]</span></a>
       </ol>
     </ol>
    <li><a href="#acknowledgments"><span class="secno">5</span> <span class="content">Acknowledgments</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Changelog</span><a class="self-link" href="#changelog"></a></h2>
   <ul>
    <li data-md>
     <p>R3 (post-CWG at Tokyo 2024):</p>
     <ul>
      <li data-md>
       <p>Minor wording updates</p>
      <li data-md>
       <p>Remove the non-proposed "Option 2"; see <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2893r2.html">R2</a> for that comparison.</p>
     </ul>
    <li data-md>
     <p>R2 (post-Kona 2023):</p>
     <ul>
      <li data-md>
       <p>New and improved wording after CWG feedback (<a href="https://wiki.edg.com/bin/view/Wg21kona2023/CoreWorkingGroup#P2893R1">minutes</a>)</p>
      <li data-md>
       <p>Add feature-test macro <code class="highlight"><c- n>__cpp_variadic_friend</c-></code></p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="introduction"><span class="secno">2. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
   <p>This paper proposes support for granting friendship to all classes in a parameter pack. Several existing idioms
are implemented by providing friendship to a class via template parameter. However, these patterns can only
be used with a single template parameter, because friendship cannot be currently granted to a pack of types.</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>=</c-><c- b>void</c-><c- p>,</c->
         <c- k>class</c-> <c- nc>U</c-><c- o>=</c-><c- b>void</c-><c- o>></c->
<c- k>class</c-> <c- nc>Foo</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>T</c-><c- p>;</c->
  <c- k>friend</c-> <c- n>U</c-><c- p>;</c->
<c- p>};</c->
</pre>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>class</c-> <c- nc>Foo</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>Ts</c-><c- p>...;</c->
<c- p>};</c->
</pre>
   </table>
   <h3 class="heading settled" data-level="2.1" id="passkey-idiom"><span class="secno">2.1. </span><span class="content">Passkey idiom</span><a class="self-link" href="#passkey-idiom"></a></h3>
   <p>The Passkey idiom allows granting access to individual member functions
on a per-function basis. In the example below, <code class="highlight"><c- n>C</c-></code> grants friendship to <code class="highlight"><c- n>A</c-></code>,
meaning that <code class="highlight"><c- n>A</c-><c- o>::</c-><c- n>m</c-></code> can access all of <code class="highlight"><c- n>C</c-></code>’s internals. But <code class="highlight"><c- n>C</c-></code> also grants
access to <code class="highlight"><c- n>intentionalA</c-></code> and <code class="highlight"><c- n>intentionalB</c-></code> using the Passkey idiom.
Class <code class="highlight"><c- n>Passkey</c-><c- o>&lt;</c-><c- n>B</c-><c- o>></c-></code> has a private constructor accessible only to its friend <code class="highlight"><c- n>B</c-></code>,
so nobody but <code class="highlight"><c- n>B</c-></code> can construct instances of <code class="highlight"><c- n>Passkey</c-><c- o>&lt;</c-><c- n>B</c-><c- o>></c-></code>. You can’t call <code class="highlight"><c- n>intentionalB</c-></code> without an instance of <code class="highlight"><c- n>Passkey</c-><c- o>&lt;</c-><c- n>B</c-><c- o>></c-></code> as the first argument.
So, even though <code class="highlight"><c- n>intentionalB</c-></code> is public, it is callable only from <code class="highlight"><c- n>B</c-></code>.</p>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>Passkey</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>T</c-><c- p>;</c->
  <c- n>Passkey</c-><c- p>()</c-> <c- p>{}</c->
<c- p>};</c->

<c- k>class</c-> <c- nc>A</c-><c- p>;</c->
<c- k>class</c-> <c- nc>B</c-><c- p>;</c->

<c- k>class</c-> <c- nc>C</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>A</c-><c- p>;</c->
<c- k>private</c-><c- o>:</c->
  <c- b>void</c-> <c- n>internal</c-><c- p>();</c->
<c- k>public</c-><c- o>:</c->
  <c- b>void</c-> <c- n>intentionalA</c-><c- p>(</c-><c- n>Passkey</c-><c- o>&lt;</c-><c- n>A</c-><c- o>></c-><c- p>);</c->
  <c- b>void</c-> <c- nf>intentionalB</c-><c- p>(</c-><c- n>Passkey</c-><c- o>&lt;</c-><c- n>B</c-><c- o>></c-><c- p>);</c->
<c- p>};</c->

<c- k>class</c-> <c- nc>A</c-> <c- p>{</c->
  <c- b>void</c-> <c- nf>m</c-><c- p>(</c-><c- n>C</c-><c- o>&amp;</c-> <c- n>c</c-><c- p>)</c-> <c- p>{</c->
    <c- n>c</c-><c- p>.</c-><c- n>internal</c-><c- p>();</c-> <c- c1>// OK</c->
    <c- n>c</c-><c- p>.</c-><c- n>intentionalA</c-><c- p>({});</c-> <c- c1>// OK</c->
    <c- n>c</c-><c- p>.</c-><c- n>intentionalB</c-><c- p>({});</c-> <c- c1>// Error, Passkey&lt;B>'s ctor is inaccessible</c->
  <c- p>}</c->
<c- p>};</c->

<c- k>class</c-> <c- nc>B</c-> <c- p>{</c->
  <c- b>void</c-> <c- nf>m</c-><c- p>(</c-><c- n>C</c-><c- o>&amp;</c-> <c- n>c</c-><c- p>)</c-> <c- p>{</c->
    <c- n>c</c-><c- p>.</c-><c- n>intentionalB</c-><c- p>({});</c-> <c- c1>// OK</c->
  <c- p>}</c->
<c- p>};</c->
</pre>
   <p>We would like to expand this idiom, and grant access to <code class="highlight"><c- n>Foo</c-><c- o>::</c-><c- n>intentional</c-></code> from multiple classes.</p>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>class</c-> <c- nc>Passkey</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>Ts</c-><c- p>...;</c-> <c- c1>// Today: Error. Proposed: OK</c->
  <c- n>Passkey</c-><c- p>()</c-> <c- p>{}</c->
<c- p>};</c->

<c- k>class</c-> <c- nc>C</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
  <c- c1>// Only callable from Blarg, Blip, and Baz</c->
  <c- b>void</c-> <c- n>intentional</c-><c- p>(</c-><c- n>Passkey</c-><c- o>&lt;</c-><c- n>Blarg</c-><c- p>,</c-> <c- n>Blip</c-><c- p>,</c-> <c- n>Baz</c-><c- o>></c-><c- p>);</c->
<c- p>};</c->
</pre>
   <h3 class="heading settled" data-level="2.2" id="crtp-access"><span class="secno">2.2. </span><span class="content">CRTP access to derived</span><a class="self-link" href="#crtp-access"></a></h3>
   <p>Another common pattern is to inherit from some class template, passing the
type of the derived class as a template parameter to the base class.</p>
   <p>There may be parts of the derived class API which are needed in the base class,
but only the base class, so they are private, and friendship is granted to
the base class.</p>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Crtp</c-><c- p>,</c-> <c- k>class</c-> <c- nc>MsgT</c-><c- o>></c->
<c- k>class</c-> <c- nc>Receiver</c-> <c- p>{</c->
  <c- b>void</c-> <c- nf>receive</c-><c- p>(</c-><c- n>MsgT</c-><c- p>)</c-> <c- p>{</c->
    <c- k>static_cast</c-><c- o>&lt;</c-><c- n>Crtp</c-><c- o>*></c-><c- p>(</c-><c- k>this</c-><c- p>)</c-><c- o>-></c-><c- n>private_</c-> <c- o>+=</c-> <c- mi>1</c-><c- p>;</c->
  <c- p>}</c->
<c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>MsgT</c-><c- o>></c->
<c- k>struct</c-> <c- nc>Dispatcher</c-> <c- o>:</c->
  <c- k>public</c-> <c- n>Receiver</c-><c- o>&lt;</c-><c- n>Dispatcher</c-><c- o>&lt;</c-><c- n>MsgT</c-><c- o>></c-><c- p>,</c-> <c- n>MsgT</c-><c- o>></c->
<c- p>{</c->
  <c- k>using</c-> <c- n>Receiver</c-><c- o>&lt;</c-><c- n>Dispatcher</c-><c- p>,</c-> <c- n>MsgT</c-><c- o>>::</c-><c- n>Receiver</c-><c- p>;</c->
  <c- k>friend</c-> <c- n>Receiver</c-><c- o>&lt;</c-><c- n>Dispatcher</c-><c- p>,</c-> <c- n>MsgT</c-><c- o>></c-><c- p>;</c->

<c- k>private</c-><c- o>:</c->
  <c- b>int</c-> <c- n>private_</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>To support multiple base classes, we would like to make <code class="highlight"><c- n>Dispatcher</c-></code> variadic:</p>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Crtp</c-><c- p>,</c-> <c- k>class</c-> <c- nc>MsgT</c-><c- o>></c->
<c- k>class</c-> <c- nc>Receiver</c-> <c- p>{</c->
  <c- b>void</c-> <c- nf>receive</c-><c- p>(</c-><c- n>MsgT</c-><c- p>)</c-> <c- p>{</c->
    <c- k>static_cast</c-><c- o>&lt;</c-><c- n>Crtp</c-><c- o>*></c-><c- p>(</c-><c- k>this</c-><c- p>)</c-><c- o>-></c-><c- n>private_</c-> <c- o>+=</c-> <c- mi>1</c-><c- p>;</c->
  <c- p>}</c->
<c- p>};</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>MsgTs</c-><c- o>></c->
<c- k>struct</c-> <c- nc>Dispatcher</c-> <c- o>:</c->
  <c- k>public</c-> <c- n>Receiver</c-><c- o>&lt;</c-><c- n>Dispatcher</c-><c- o>&lt;</c-><c- n>MsgTs</c-><c- p>...</c-><c- o>></c-><c- p>,</c-> <c- n>MsgTs</c-><c- o>></c-><c- p>...</c-> <c- c1>// OK</c->
<c- p>{</c->
  <c- k>using</c-> <c- n>Receiver</c-><c- o>&lt;</c-><c- n>Dispatcher</c-><c- p>,</c-> <c- n>MsgTs</c-><c- o>>::</c-><c- n>Receiver</c-><c- p>...;</c->  <c- c1>// OK</c->
  <c- k>friend</c-> <c- n>Receiver</c-><c- o>&lt;</c-><c- n>Dispatcher</c-><c- p>,</c-> <c- n>MsgTs</c-><c- o>></c-><c- p>...;</c-> <c- c1>// Today: Error. Proposed: OK</c->

<c- k>private</c-><c- o>:</c->
  <c- b>int</c-> <c- n>private_</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>Note that both inheritance and <code class="highlight"><c- k>using</c-></code> support pack-expansion. Only <code class="highlight"><c- k>friend</c-></code> does not.</p>
   <h2 class="heading settled" data-level="3" id="confusing"><span class="secno">3. </span><span class="content">Confusing grammatical examples</span><a class="self-link" href="#confusing"></a></h2>
   <p>The following two subsections merely record some design notes
in case we want to refer back to them during CWG discussion.
You can skip reading these subsections for now.</p>
   <h3 class="heading settled" data-level="3.1" id="not-for-stamping"><span class="secno">3.1. </span><span class="content"><code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c-> <c- k>friend</c-> <c- n>Ts</c-><c- p>...</c-></code></span><a class="self-link" href="#not-for-stamping"></a></h3>
   <p>A declaration <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>friend</c-> <c- o>~~~</c-></code> is not a template for stamping out friends;
it is a request to befriend a specific template.</p>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>::</c-><c- n>Nested</c-><c- p>;</c->
</pre>
   <p>declares that <code class="highlight"><c- n>C</c-></code> has a member named <code class="highlight"><c- n>Nested</c-></code> which itself is a class template, and we’re
befriending that template. That is:</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>C</c-> <c- p>{</c-> <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>class</c-> <c- nc>Nested</c-><c- p>;</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>::</c-><c- n>Nested</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>Therefore it is never well-formed to use the parameters of the friend declaration’s own <em>template-head</em> within the declarator itself. That is, these are all ill-formed, and replacing today’s <code class="highlight"><c- n>U</c-></code> with tomorrow’s <code class="highlight"><c- n>Us</c-><c- p>...</c-></code> won’t change anything:</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- n>U</c-><c- p>;</c-> <c- c1>// ill-formed</c->
   
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>U</c-><c- p>;</c-> <c- c1>// ill-formed</c->
   
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>;</c-> <c- c1>// ill-formed</c->
   
   
   
   
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>U</c-><c- o>>::</c-><c- n>Nested</c-><c- p>;</c-> <c- c1>// ill-formed</c->
<c- p>};</c->
</pre>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>struct</c-> <c- nc>VS</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Us</c-><c- o>></c->
  <c- k>friend</c-> <c- n>Us</c-><c- p>...;</c-> <c- c1>// ill-formed</c->
   
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Us</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>Us</c-><c- p>...;</c-> <c- c1>// ill-formed</c->
   
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>...;</c-> <c- c1>// ill-formed</c->
   
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Us</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>Ts</c-><c- p>...</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- n>Us</c-><c- o>></c-><c- p>...;</c-> <c- c1>// ill-formed</c->
   
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Us</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>Us</c-><c- o>>::</c-><c- n>Nested</c-><c- p>...;</c-> <c- c1>// ill-formed</c->
<c- p>};</c->
</pre>
   </table>
   <p>On the other hand, this usage (to befriend the member template <code class="highlight"><c- n>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Nested</c-></code>) is well-formed,
and remains well-formed after replacing today’s <code class="highlight"><c- n>T</c-></code> with tomorrow’s <code class="highlight"><c- n>Ts</c-><c- p>...</c-></code>:</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>C</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- k>struct</c-> <c- nc>Nested</c-><c- p>;</c->
<c- p>};</c->
 
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Nested</c-><c- p>;</c-> <c- c1>// OK</c->
<c- p>};</c->
 
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
<c- k>struct</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Nested</c-> <c- p>{</c->
  <c- b>int</c-> <c- nf>m</c-><c- p>(</c-><c- n>S</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>&amp;</c-> <c- n>s</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>s</c-><c- p>.</c-><c- n>private_</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- n>S</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>s</c-><c- p>;</c->
  <c- n>C</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-><c- p>().</c-><c- n>m</c-><c- p>(</c-><c- n>s</c-><c- p>);</c-> <c- c1>// OK</c->
   
  <c- n>C</c-><c- o>&lt;</c-><c- b>float</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-><c- p>().</c-><c- n>m</c-><c- p>(</c-><c- n>s</c-><c- p>);</c->
    <c- c1>// error, inaccessible</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>C</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- k>struct</c-> <c- nc>Nested</c-><c- p>;</c->
<c- p>};</c->
 
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>struct</c-> <c- nc>VS</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>>::</c-><c- n>Nested</c-><c- p>...;</c-> <c- c1>// OK</c->
<c- p>};</c->
 
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
<c- k>struct</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Nested</c-> <c- p>{</c->
  <c- b>int</c-> <c- nf>m</c-><c- p>(</c-><c- n>VS</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>char</c-><c- o>>&amp;</c-> <c- n>vs</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>vs</c-><c- p>.</c-><c- n>private_</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- n>VS</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>char</c-><c- o>></c-> <c- n>vs</c-><c- p>;</c->
  <c- n>C</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-><c- p>().</c-><c- n>m</c-><c- p>(</c-><c- n>vs</c-><c- p>);</c-> <c- c1>// OK</c->
  <c- n>C</c-><c- o>&lt;</c-><c- b>char</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-><c- p>().</c-><c- n>m</c-><c- p>(</c-><c- n>vs</c-><c- p>);</c-> <c- c1>// OK</c->
  <c- n>C</c-><c- o>&lt;</c-><c- b>float</c-><c- o>>::</c-><c- n>Nested</c-><c- o>&lt;</c-><c- b>float</c-><c- o>></c-><c- p>().</c-><c- n>m</c-><c- p>(</c-><c- n>vs</c-><c- p>);</c->
    <c- c1>// error, inaccessible</c->
<c- p>}</c->
</pre>
   </table>
   <h3 class="heading settled" data-level="3.2" id="confusing-elab"><span class="secno">3.2. </span><span class="content"><code class="highlight"><c- k>friend</c-> <c- k>class</c-> <c- nc>Ts</c-><c- p>...</c-></code></span><a class="self-link" href="#confusing-elab"></a></h3>
   <p>C++ already disallows <code class="highlight"><c- k>friend</c-> <c- k>class</c-> <c- nc>T</c-></code> when it stands alone (<a href="https://godbolt.org/z/zMefooj6h">Godbolt</a>):</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>T</c-><c- p>;</c-> <c- c1>// Error</c->
<c- p>};</c->
</pre>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>Ts</c-><c- p>...;</c-> <c- c1>// Error</c->
<c- p>};</c->
</pre>
   </table>
    but permits these ways to befriend a specific class instantiated from a class template <code class="highlight"><c- n>C</c-></code>,
or to befriend a member class template: 
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>N</c-><c- o>::</c-><c- n>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c->
<c- p>};</c->
</pre>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>></c-><c- p>...;</c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>N</c-><c- o>::</c-><c- n>C</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>></c-><c- p>...;</c->
<c- p>};</c->
</pre>
     <tr>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Nested</c-><c- p>;</c->
<c- p>};</c->
</pre>
      <td>
<pre class="highlight">
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- o>></c->
<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>C</c-><c- o>&lt;</c-><c- n>Ts</c-><c- o>>::</c-><c- n>Nested</c-><c- p>...;</c->
<c- p>};</c->
</pre>
   </table>
   <p>
    The existing wording for this is in <a href="https://eel.is/c++draft/dcl.type.elab#4">[dcl.type.elab]/4</a>, which we modify <a href="#wording1-dcl.type.elab">below</a> by adding an 
    <ins><code class="highlight"><c- p>...</c-></code><sub><i>opt</i></sub></ins>
    .
   </p>
   <h2 class="heading settled" data-level="4" id="wording"><span class="secno">4. </span><span class="content">Proposed wording</span><a class="self-link" href="#wording"></a></h2>
   <p>In <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2893r2.html">P2893R2</a> we presented two possible wording options: our preferred way, and a second option for comparison.</p>
   <table>
    <tbody>
     <tr>
      <th>Option 1
      <th>Option 2
     <tr>
      <td><code class="highlight"><c- k>friend</c-> <c- n>Ts</c-><c- p>...</c-></code> expands to <code class="highlight"><c- k>friend</c-> <c- n>T1</c-><c- p>,</c-> <c- n>T2</c-></code>
      <td><code class="highlight"><c- k>friend</c-> <c- n>Ts</c-><c- p>...</c-></code> expands to <code class="highlight"><c- k>friend</c-> <c- n>T1</c-><c- p>;</c-> <c- k>friend</c-> <c- n>T2</c-></code>
     <tr>
      <td><code class="highlight"><c- k>friend</c-> <c- n>T1</c-><c- p>,</c-> <c- n>T2</c-><c- p>;</c-></code> is OK
      <td><code class="highlight"><c- k>friend</c-> <c- n>T1</c-><c- p>,</c-> <c- n>T2</c-><c- p>;</c-></code> remains ill-formed
     <tr>
      <td>Consistent with <code class="highlight"><c- k>using</c-> <c- n>T1</c-><c- o>::</c-><c- n>T1</c-><c- p>,</c-> <c- n>T2</c-><c- o>::</c-><c- n>T2</c-><c- p>;</c-></code>
      <td>—
     <tr>
      <td>Does the work up front
      <td>We expect future proposals for <code class="highlight"><c- k>friend</c-> <c- n>T1</c-><c- p>,</c-> <c- n>T2</c-><c- p>;</c-></code> anyway
     <tr>
      <td>Adds <i>friend-type-declaration</i>
      <td>Adds no new nonterminals
     <tr>
      <td>Fails to disentangle the whole <code class="highlight"><c- k>friend</c-></code> grammar
      <td>Doesn’t even <i>try</i> to disentangle the <code class="highlight"><c- k>friend</c-></code> grammar
     <tr>
      <td>Wording seems OK
      <td>Wording apparently permits <code class="highlight"><c- b>void</c-> <c- nf>f</c-><c- p>(</c-><c- b>int</c-><c- p>...</c-> <c- p>...</c-><c- n>xs</c-><c- p>);</c-></code> and <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- b>int</c-><c- p>...)</c-></code>
     <tr>
      <td><b>We prefer Option 1.</b>
      <td>Provided for comparison only.
   </table>
   <p>P2893R0 presented Option 1, but with incomplete wording. At Kona, EWG was concerned
that the wording for <code class="highlight"><c- k>friend</c-> <c- n>T1</c-><c- p>,</c-> <c- n>T2</c-></code> might be difficult, and so R1’s presentation
drifted toward Option 2. R2 presented Option 1 and Option 2 side-by-side for comparison.
CWG reviewed the wording for Option 1 (our preferred option), and now in R3 we present only that
finalized wording.</p>
   <p>Relevant paragraphs not modified here include:</p>
   <ul>
    <li data-md>
     <p><a href="https://eel.is/c++draft/dcl.meaning.general#2">[dcl.meaning.general]/2</a> "If the declaration is a friend declaration, the declarator does not bind a name..."</p>
    <li data-md>
     <p><a href="https://eel.is/c++draft/temp.friend">[temp.friend]</a> "A friend template may be declared within a class or class template ... A friend template shall not be declared in a local class ..."</p>
   </ul>
   <p>Precedent for "If a <em>member-declaration</em> matches..." is found in <a href="https://eel.is/c++draft/dcl.pre#10">[dcl.pre]/10</a> "If a <em>static-assert-message</em> matches..."</p>
   <p>CWG discussion in Kona (<a href="https://wiki.edg.com/bin/view/Wg21kona2023/CoreWorkingGroup#P2893R1">minutes</a>)
was uncomfortable with <code class="highlight"><c- k>friend</c-> <c- k>class</c-> <c- nc>A</c-><c- p>,</c-> <c- k>class</c-> <c- nc>B</c-><c- p>;</c-></code>.
However, as shown above, we need to support <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- o>></c-> <c- k>friend</c-> <c- k>class</c-> <c- nc>Ts</c-><c- o>::</c-><c- n>Nested</c-><c- p>...;</c-></code> which
means that <em>friend-type-specifier</em> must be producible from <em>elaborated-type-specifier</em>; and <a href="#wording1-dcl.type.elab">[dcl.type.elab]/4</a> applies both before and after pack-expansion,
so any attempt to forbid <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- o>></c-> <c- k>friend</c-> <c- k>class</c-> <c- nc>T1</c-><c- o>::</c-><c- n>Nested</c-><c- p>,</c-> <c- k>class</c-> <c- nc>T2</c-><c- o>::</c-><c- n>Nested</c-><c- p>;</c-></code> would also forbid <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- o>></c-> <c- k>friend</c-> <c- k>class</c-> <c- nc>Ts</c-><c- o>::</c-><c- n>Nested</c-><c- p>...;</c-></code> which would be bad.</p>
   <p>CWG discussion in Tokyo established that the intent of <a href="#wording1-dcl.type.elab">[dcl.type.elab]/4</a> was really just to prevent <code class="highlight"><c- k>friend</c-> <c- k>enum</c-> <c- nc>E</c-><c- p>;</c-></code> and <code class="highlight"><c- k>friend</c-> <c- k>class</c-> <c- p>[[</c-><c- n>nodiscard</c-><c- p>]]</c-> <c- n>C</c-><c- p>;</c-></code> — that is, enums and
attributes. There doesn’t seem to be any concrete historic reason to disallow <code class="highlight"><c- k>friend</c-> <c- k>class</c-> <c- nc>A</c-><c- p>,</c-> <c- k>class</c-> <c- nc>B</c-><c- p>;</c-></code> or even <code class="highlight"><c- k>friend</c-> <c- n>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>C</c-><c- p>,</c-> <c- n>U</c-><c- p>;</c-></code> so CWG’s suggestion is simply to allow the programmer to write those lines.
The programmer is expected to use this freedom responsibly.</p>
   <p>Today <code class="highlight"><c- k>friend</c-> <c- k>typename</c-> <c- nc>X</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Y</c-><c- p>;</c-></code> is legal, as is <code class="highlight"><c- k>friend</c-> <c- n>X</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>Y</c-><c- p>;</c-></code>. The identifier there is in
a type-only context (<a href="#wording1-temp.res.general">[temp.res.general]</a>)
because it’s a <em>decl-specifier</em> of the <em>decl-specifier-seq</em> of a <em>member-declaration</em>.
After our patch, it’s in a type-only context because it’s the <em>friend-type-specifier</em> of a <em>friend-type-declaration</em>.</p>
   <p>To befriend a non-template, you use a "friend declaration" which is a <em>member-declaration</em>.
To befriend a template, you use a <em>template-declaration</em>, which consists of a <em>template-head</em> followed by a <em>declaration</em>; the "friend declaration" here is the <em>declaration</em> (<a href="#wording1-class.friend">[class.friend]/3</a>).
Therefore, <em>friend-type-declaration</em> must appear in the list
of alternatives for both <i><a href="#wording1-class.mem.general">member-declaration</a></i> and <i><a href="#wording1-dcl.pre">declaration</a></i>.
Friend declarations are already forbidden to appear outside of classes, although that wording is
elusive (maybe it doesn’t exist).</p>
   <h3 class="heading settled" data-level="4.1" id="wording1"><span class="secno">4.1. </span><span class="content">Option 1 (proposed for C++26)</span><a class="self-link" href="#wording1"></a></h3>
   <h4 class="heading settled" data-level="4.1.1" id="wording1-cpp.predefined"><span class="secno">4.1.1. </span><span class="content">[cpp.predefined]</span><a class="self-link" href="#wording1-cpp.predefined"></a></h4>
   <p>Add a feature-test macro to the table in <a href="https://eel.is/c++draft/cpp.predefined#tab:cpp.predefined.ft">[cpp.predefined]</a>:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- n>__cpp_variable_templates</c->  <c- mf>201304L</c->
<ins><c- n>__cpp_variadic_friend</c->     <c- n>YYYYMML</c-></ins>
<c- n>__cpp_variadic_templates</c->  <c- mf>200704L</c->
<c- n>__cpp_variadic_using</c->      <c- mf>201611L</c->
</pre></small>
   </blockquote>
   <h4 class="heading settled" data-level="4.1.2" id="wording1-class.mem.general"><span class="secno">4.1.2. </span><span class="content">[class.mem.general]</span><a class="self-link" href="#wording1-class.mem.general"></a></h4>
   <p>Modify <a href="https://eel.is/c++draft/class.mem.general#nt:member-declaration">[class.mem.general]</a> as follows:</p>
   <p><small></small></p>
   <blockquote><small> <i>member-declaration</i>:<br>   <i>attribute-specifier-seq</i><sub>opt</sub> <i>decl-specifier-seq</i><sub>opt</sub> <i>member-declarator-list</i><sub>opt</sub> <code class="highlight"><c- p>;</c-></code><br>   <i>function-definition</i><br>   <ins><i>friend-type-declaration</i></ins><br>   <i>using-declaration</i><br>   <i>using-enum-declaration</i><br>   <i>static_assert-declaration</i><br>   <i>template-declaration</i><br>   <i>explicit-specialization</i><br>   <i>deduction-guide</i><br>   <i>alias-declaration</i><br>   <i>opaque-enum-declaration</i><br>   <i>empty-declaration</i><br><p></p> <ins><i>friend-type-declaration</i>:</ins><br>   <ins><code class="highlight"><c- k>friend</c-></code> <i>friend-type-specifier-list</i> <code class="highlight"><c- p>;</c-></code></ins><br> <ins><i>friend-type-specifier-list</i>:</ins><br>   <ins><i>friend-type-specifier</i> <code class="highlight"><c- p>...</c-></code><sub>opt</sub></ins><br>   <ins><i>friend-type-specifier-list</i> <code class="highlight"><c- p>,</c-></code> <i>friend-type-specifier</i> <code class="highlight"><c- p>...</c-></code><sub>opt</sub></ins><br> <ins><i>friend-type-specifier</i>:</ins><br>   <ins><i>simple-type-specifier</i></ins><br>   <ins><i>elaborated-type-specifier</i></ins><br>   <ins><i>typename-specifier</i></ins><br> <p><i>member-declarator-list</i>:<br>   <i>member-declarator</i><br>   <i>member-declarator-list</i> <code class="highlight"><c- p>,</c-></code> <i>member-declarator</i><br></p> <p>[...]</p> <p>2. A <em>member-declaration</em> does not declare new members of the class if it is </p><ul><li data-md><p>a friend declaration ([class.friend]),</p> </li><li data-md><p>a <em>deduction-guide</em> ([temp.deduct.guide]),</p> </li><li data-md><p>a <em>template-declaration</em> whose declaration is one of the above,</p> </li><li data-md><p>a <em>static_assert-declaration</em>,</p> </li><li data-md><p>a <em>using-declaration</em> ([namespace.udecl]), or</p> </li><li data-md><p>an <em>empty-declaration</em>.</p> </li></ul> <p>For any other <em>member-declaration</em>, each declared entity that is not an unnamed bit-field
is a member of the class, and each such <em>member-declaration</em> shall either
declare at least one member name of the class or declare at least one unnamed bit-field. </p><p>[...] </p><p>8. A class <code class="highlight"><c- n>C</c-></code> is complete at a program point <em>P</em> if the definition of <code class="highlight"><c- n>C</c-></code> is reachable from <em>P</em> ([module.reach])
or if <em>P</em> is in a complete-class context of <code class="highlight"><c- n>C</c-></code>. Otherwise, <code class="highlight"><c- n>C</c-></code> is incomplete at <em>P</em>. </p><p><ins>x. If a <em>member-declaration</em> matches the syntactic requirements of <em>friend-type-declaration</em>, it is a <em>friend-type-declaration</em>.</ins> </p><p>9. In a <em>member-declarator</em>, an <code class="highlight"><c- o>=</c-></code> immediately following the <em>declarator</em> is interpreted as
introducing a <em>pure-specifier</em> if the <em>declarator-id</em> has function type;
otherwise it is interpreted as introducing a <em>brace-or-equal-initializer</em>. [...] </p><p>10. In a <em>member-declarator</em> for a bit-field, the <em>constant-expression</em> is parsed as
the longest sequence of tokens that could syntactically form a <em>constant-expression</em>. [...] </p></small></blockquote>
   <h4 class="heading settled" data-level="4.1.3" id="wording1-dcl.pre"><span class="secno">4.1.3. </span><span class="content">[dcl.pre]</span><a class="self-link" href="#wording1-dcl.pre"></a></h4>
   <p>Modify <a href="https://eel.is/c++draft/dcl.pre">[dcl.pre]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>1. Declarations generally specify how names are to be interpreted. Declarations have the form </p><p><i>declaration-seq</i>:<br>   <i>declaration</i><br>   <i>declaration-seq declaration</i><br></p> <p><i>declaration</i>:<br>   <i>name-declaration</i><br>   <i>special-declaration</i><br></p> <p><i>name-declaration</i>:<br>   <i>block-declaration</i><br>   <i>nodeclspec-function-declaration</i><br>   <i>function-definition</i><br>   <ins><i><a href="#wording1-class.mem.general">friend-type-declaration</a></i></ins><br>   <i>template-declaration</i><br>   <i>deduction-guide</i><br>   <i>linkage-specification</i><br>   <i>namespace-definition</i><br>   <i>empty-declaration</i><br>   <i>attribute-declaration</i><br>   <i>module-import-declaration</i><br></p> <p><i>special-declaration</i>:<br>   <i>explicit-instantiation</i><br>   <i>explicit-specialization</i><br>   <i>export-declaration</i><br></p> <p>[...]</p> <p>2. Certain declarations contain one or more scopes ([basic.scope.scope]). Unless otherwise stated,
utterances in [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof
refer only to those components of the declaration that are not nested within scopes nested within the declaration. </p><p><ins>x. If a <em>name-declaration</em> matches the syntactic requirements of <em>friend-type-declaration</em>, it is a <em>friend-type-declaration</em>.</ins> </p><p>3. A <em>simple-declaration</em> or <em>nodeclspec-function-declaration</em> of the form [...] </p></small>
   </blockquote>
   <h4 class="heading settled" data-level="4.1.4" id="wording1-temp.pre"><span class="secno">4.1.4. </span><span class="content">[temp.pre]</span><a class="self-link" href="#wording1-temp.pre"></a></h4>
   <p>Modify <a href="https://eel.is/c++draft/temp.pre">[temp.pre]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>1. A <em>template</em> defines a family of classes, functions, or variables, an alias for a family of types, or a concept. </p><p><i>template-declaration</i>:<br>   <i>template-head declaration</i><br>   <i>template-head concept-definition</i></p> <p>[...]</p> <p>2. The <em>declaration</em> in a <em>template-declaration</em> (if any) shall </p><ul><li data-md><p>declare or define a function, a class, or a variable, or</p> </li><li data-md><p>define a member function, a member class, a member enumeration, or a static data member of a class template or of a class nested within a class template, or</p> </li><li data-md><p>define a member template of a class or class template, or</p> </li><li data-md><ins>be a <i><a href="#wording1-class.mem.general">friend-type-declaration</a></i>, or</ins> </li><li data-md><p>be a <em>deduction-guide</em>, or</p> </li><li data-md><p>be an <em>alias-declaration</em>.</p> </li></ul> </small>
   </blockquote>
   <h4 class="heading settled" data-level="4.1.5" id="wording1-class.friend"><span class="secno">4.1.5. </span><span class="content">[class.friend]</span><a class="self-link" href="#wording1-class.friend"></a></h4>
   <p>Modify <a href="https://eel.is/c++draft/class.friend">[class.friend]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>3. A friend declaration that does not declare a function shall <ins>be
a <i><a href="#wording1-class.mem.general">friend-type-declaration</a></i>.</ins> <del>have one of the following forms:</del> </p><p>  <del><code class="highlight"><c- k>friend</c-></code> <em>elaborated-type-specifier</em> <code class="highlight"><c- p>;</c-></code></del><br>   <del><code class="highlight"><c- k>friend</c-></code> <em>simple-type-specifier</em> <code class="highlight"><c- p>;</c-></code></del><br>   <del><code class="highlight"><c- k>friend</c-></code> <em>typename-specifier</em> <code class="highlight"><c- p>;</c-></code></del><br></p> <p>[<em>Note:</em> A friend declaration can be the <em>declaration</em> in a <em>template-declaration</em> (<a href="#wording1-temp.pre">[temp.pre]</a>, [temp.friend]). — <em>end note</em>]</p> <p>If <del>the type specifier</del> <ins>a <i><a href="#wording1-class.mem.general">friend-type-specifier</a></i></ins> in
a friend declaration designates a (possibly cv-qualified) class type,
that class is declared as a friend; otherwise, <del>the friend declaration</del> <ins>the <i>friend-type-specifier</i></ins> is ignored.</p> <p>[<em>Example 4:</em></p> <pre class="highlight"><c- k>class</c-> <c- nc>C</c-><c- p>;</c->
<c- k>typedef</c-> <c- n>C</c-> <c- n>Ct</c-><c- p>;</c->
<ins><c- k>struct</c-> <c- nc>E</c-><c- p>;</c-></ins>

<c- k>class</c-> <c- nc>X1</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>C</c-><c- p>;</c->                     <c- c1>// OK, class C is a friend</c->
<c- p>};</c->

<c- k>class</c-> <c- nc>X2</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>Ct</c-><c- p>;</c->                    <c- c1>// OK, class C is a friend</c->
  <c- k>friend</c-> <c- n>D</c-><c- p>;</c->                     <c- c1>// error: D not found</c->
  <c- k>friend</c-> <c- k>class</c-> <c- nc>D</c-><c- p>;</c->               <c- c1>// OK, elaborated-type-specifier declares new class</c->
<c- p>};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-><ins><c- p>...</c-></ins> <c- n>T</c-><ins><c- n>s</c-></ins><c- o>></c-> <c- k>class</c-> <c- nc>R</c-> <c- p>{</c->
  <c- k>friend</c-> <c- n>T</c-><ins><c- n>s</c-><c- p>...</c-></ins><c- p>;</c->
<c- p>};</c->

<ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Ts</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Us</c-><c- o>></c-></ins>
<ins><c- k>class</c-> <c- nc>R</c-><c- o>&lt;</c-><c- n>R</c-><c- o>&lt;</c-><c- n>Ts</c-><c- p>...</c-><c- o>></c-><c- p>,</c-> <c- n>R</c-><c- o>&lt;</c-><c- n>Us</c-><c- p>...</c-><c- o>>></c-> <c- p>{</c-></ins>
<ins>  <c- k>friend</c-> <c- n>Ts</c-><c- o>::</c-><c- n>Nested</c-><c- p>...,</c-> <c- n>Us</c-><c- p>...;</c-></ins>
<ins><c- p>};</c-></ins>

<c- n>R</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-> <c- n>rc</c-><c- p>;</c->                        <c- c1>// class C is a friend of R&lt;C></c->
<ins><c- n>R</c-><c- o>&lt;</c-><c- n>C</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-> <c- n>rce</c-><c- p>;</c->                    <c- c1>// classes C and E are friends of R&lt;C, E></c-></ins>
<c- n>R</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>Ri</c-><c- p>;</c->                      <c- c1>// OK, "friend int;" is ignored</c->

<ins><c- k>struct</c-> <c- nc>E</c-> <c- p>{</c-> <c- k>struct</c-> <c- nc>Nested</c-><c- p>;</c-> <c- p>};</c-></ins>

<ins><c- n>R</c-><c- o>&lt;</c-><c- n>R</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-><c- p>,</c-> <c- n>R</c-><c- o>&lt;</c-><c- n>C</c-><c- p>,</c-> <c- b>int</c-><c- o>>></c-> <c- n>rr</c-><c- p>;</c->    <c- c1>// E::Nested and C are friends of R&lt;R&lt;E>, R&lt;C, int>></c-></ins>
</pre><p>— <em>end example</em>]</p> </small>
   </blockquote>
   <h4 class="heading settled" data-level="4.1.6" id="wording1-dcl.type.elab"><span class="secno">4.1.6. </span><span class="content">[dcl.type.elab]</span><a class="self-link" href="#wording1-dcl.type.elab"></a></h4>
   <p>Modify <a href="https://eel.is/c++draft/dcl.type.elab#4">[dcl.type.elab]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>4. <del>If an <em>elaborated-type-specifier</em> appears with the <code class="highlight"><c- k>friend</c-></code> specifier
as an entire <em>member-declaration</em>, the <em>member-declaration</em></del> <ins>A <i><a href="#wording1-class.mem.general">friend-type-specifier</a></i> that is an <em>elaborated-type-specifier</em></ins> shall have one of the following forms: </p><ul><li data-md><del><code class="highlight"><c- k>friend</c-></code></del> <em>class-key nested-name-specifier</em><sub>opt</sub> <em>identifier</em> <del><code class="highlight"><c- p>;</c-></code></del> </li><li data-md><del><code class="highlight"><c- k>friend</c-></code></del> <em>class-key simple-template-id</em> <del><code class="highlight"><c- p>;</c-></code></del> </li><li data-md><del><code class="highlight"><c- k>friend</c-></code></del> <em>class-key nested-name-specifier</em> <code class="highlight"><c- k>template</c-></code><sub>opt</sub> <em>simple-template-id</em> <del><code class="highlight"><c- p>;</c-></code></del> </li></ul> <p>Any unqualified lookup for the <em>identifier</em> (in the first case) does not consider scopes that contain the target scope; no name is bound.</p> <p>[<em>Note:</em> A using-directive in the target scope is ignored if it refers to a namespace not contained by that scope.
[basic.lookup.elab] describes how name lookup proceeds in an elaborated-type-specifier. — <em>end note</em>] </p><p>[<em>Note:</em> An <em>elaborated-type-specifier</em> can be used to refer to a previously declared <em>class-name</em> or <em>enum-name</em> even if the name has been hidden by a non-type declaration. — <em>end note</em>] </p><p>5. If the <em>identifier</em> or <em>simple-template-id</em> resolves to a <em>class-name</em> or <em>enum-name</em>, the <em>elaborated-type-specifier</em> introduces it
into the declaration the same way a <em>simple-type-specifier</em> introduces its <em>type-name</em> ([dcl.type.simple]).
If the <em>identifier</em> or <em>simple-template-id</em> resolves to a <em>typedef-name</em> ([dcl.typedef], [temp.names]), the <em>elaborated-type-specifier</em> is ill-formed. </p><p>[<em>Note:</em> This implies that, within a class template with a template <em>type-parameter</em> <code class="highlight"><c- n>T</c-></code>, the declaration <code class="highlight"><c- k>friend</c-> <c- k>class</c-> <c- nc>T</c-><c- p>;</c-></code> is ill-formed. However, the similar declaration <code class="highlight"><c- k>friend</c-> <c- n>T</c-><c- p>;</c-></code> is well-formed ([class.friend]). — <em>end note</em>] </p></small>
   </blockquote>
   <h4 class="heading settled" data-level="4.1.7" id="wording1-temp.res.general"><span class="secno">4.1.7. </span><span class="content">[temp.res.general]</span><a class="self-link" href="#wording1-temp.res.general"></a></h4>
   <p>Modify <a href="https://eel.is/c++draft/temp.res.general">[temp.res.general]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>4. A qualified or unqualified name is said to be in a <em>type-only context</em> if it is the terminal name of </p><ul><li data-md><p>a <em>typename-specifier</em>, <em>nested-name-specifier</em>, <em>elaborated-type-specifier</em>, <em>class-or-decltype</em>, or</p> </li><li data-md><ins>a <em>simple-type-specifier</em> of a <i><a href="#wording1-class.mem.general">friend-type-specifier</a></i>, or</ins> </li><li data-md><p>a <em>type-specifier</em> of a</p> <ul><li data-md><p><em>new-type-id</em>,</p> </li><li data-md><p><em>defining-type-id</em>,</p> </li><li data-md><p><em>conversion-type-id</em>,</p> </li><li data-md><p><em>trailing-return-type</em>,</p> </li><li data-md><p>default argument of a <em>type-parameter</em>, or</p> </li><li data-md><p><em>type-id</em> of a <code class="highlight"><c- k>static_cast</c-></code>, <code class="highlight"><c- k>const_cast</c-></code>, <code class="highlight"><c- k>reinterpret_cast</c-></code>, or <code class="highlight"><c- k>dynamic_cast</c-></code>, or</p> </li></ul></li><li data-md><p>a <em>decl-specifier</em> of the <em>decl-specifier-seq</em> of a</p> <ul><li data-md><p><em>simple-declaration</em> or <em>function-definition</em> in namespace scope,</p> </li><li data-md><p><em>member-declaration</em>,</p> </li><li data-md><p><em>parameter-declaration</em> in a <em>member-declaration</em>, unless that <em>parameter-declaration</em> appears in a default argument,</p> </li><li data-md><p><em>parameter-declaration</em> in a declarator of a function or function template declaration whose <em>declarator-id</em> is qualified, unless that <em>parameter-declaration</em> appears in a default argument,</p> </li><li data-md><p><em>parameter-declaration</em> in a <em>lambda-declarator</em> or <em>requirement-parameter-list</em>, unless that <em>parameter-declaration</em> appears in a default argument, or</p> </li><li data-md><p><em>parameter-declaration</em> of a (non-type) <em>template-parameter</em>.</p> </li></ul></li></ul> </small>
   </blockquote>
   <h4 class="heading settled" data-level="4.1.8" id="wording1-temp.variadic"><span class="secno">4.1.8. </span><span class="content">[temp.variadic]</span><a class="self-link" href="#wording1-temp.variadic"></a></h4>
   <p>Modify <a href="https://eel.is/c++draft/temp.variadic">[temp.variadic]</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>5. A <em>pack expansion</em> consists of a <em>pattern</em> and an <em>ellipsis</em>, the instantiation of which produces
zero or more instantiations of the pattern in a list (described below). The form of the pattern depends
on the context in which the expansion occurs. Pack expansions can occur in the following contexts: </p><ul><li data-md><p>In a function parameter pack ([dcl.fct]); the pattern is the <em>parameter-declaration</em> without the ellipsis.</p> </li><li data-md><p>In a <em>using-declaration</em> ([namespace.udecl]); the pattern is a <em>using-declarator</em>.</p> </li><li data-md><ins>In a <em>friend-type-declaration</em> (<a href="#wording1-class.mem.general">[class.mem.general]</a>); the pattern is a <em>friend-type-specifier</em>.</ins> </li><li data-md><p>In a template parameter pack that is a pack expansion ([temp.param]):</p> </li><li data-md><p>if the template parameter pack is a <em>parameter-declaration</em>; the pattern is the <em>parameter-declaration</em> without the ellipsis;</p> </li><li data-md><p>if the template parameter pack is a <em>type-parameter</em>; the pattern is the corresponding <em>type-parameter</em> without the ellipsis.</p> </li><li data-md><p>In an <em>initializer-list</em> ([dcl.init]); the pattern is an <em>initializer-clause</em>.</p> </li><li data-md><p>In a <em>base-specifier-list</em> ([class.derived]); the pattern is a <em>base-specifier</em>.</p> </li><li data-md><p>In a <em>mem-initializer-list</em> ([class.base.init]) for a <em>mem-initializer</em> whose <em>mem-initializer-id</em> denotes a base class; the pattern is the <em>mem-initializer</em>.</p> </li><li data-md><p>In a <em>template-argument-list</em> ([temp.arg]); the pattern is a <em>template-argument</em>.</p> </li><li data-md><p>In an <em>attribute-list</em> ([dcl.attr.grammar]); the pattern is an <em>attribute</em>.</p> </li><li data-md><p>In an <em>alignment-specifier</em> ([dcl.align]); the pattern is the <em>alignment-specifier</em> without the ellipsis.</p> </li><li data-md><p>In a <em>capture-list</em> ([expr.prim.lambda.capture]); the pattern is the <em>capture</em> without the ellipsis.</p> </li><li data-md><p>In a <code class="highlight"><c- k>sizeof</c-><c- p>...</c-></code> expression; the pattern is an <em>identifier</em>.</p> </li><li data-md><p>In a <em>fold-expression</em> ([expr.prim.fold]); the pattern is the <em>cast-expression</em> that contains an unexpanded pack.</p> </li></ul> </small>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="acknowledgments"><span class="secno">5. </span><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
   <p>I had been sitting on this since posting to the mailing list about it in January 2020. This paper was finally
written during C++Now 2023, Feature in a Week. Thus, it is very appropriate to say that it would not exist
without that program. Thanks to Jeff Garland, Marshall Clow, Barry Revzin, and JF Bastien for running that
program, and all the attendees who helped discuss and review the proposal. Special thanks to Barry Revzin for
lots of help with the document itself.</p>
   <p>Thanks to Brian Bi and Krystian Stasiowski for further review of the R1 wording.</p>
  </main>
<script>
(function() {
  "use strict";
  var collapseSidebarText = '<span aria-hidden="true">←</span> '
                          + '<span>Collapse Sidebar</span>';
  var expandSidebarText   = '<span aria-hidden="true">→</span> '
                          + '<span>Pop Out Sidebar</span>';
  var tocJumpText         = '<span aria-hidden="true">↑</span> '
                          + '<span>Jump to Table of Contents</span>';

  var sidebarMedia = window.matchMedia('screen and (min-width: 78em)');
  var autoToggle   = function(e){ toggleSidebar(e.matches) };
  if(sidebarMedia.addListener) {
    sidebarMedia.addListener(autoToggle);
  }

  function toggleSidebar(on) {
    if (on == undefined) {
      on = !document.body.classList.contains('toc-sidebar');
    }

    /* Don't scroll to compensate for the ToC if we're above it already. */
    var headY = 0;
    var head = document.querySelector('.head');
    if (head) {
      // terrible approx of "top of ToC"
      headY += head.offsetTop + head.offsetHeight;
    }
    var skipScroll = window.scrollY < headY;

    var toggle = document.getElementById('toc-toggle');
    var tocNav = document.getElementById('toc');
    if (on) {
      var tocHeight = tocNav.offsetHeight;
      document.body.classList.add('toc-sidebar');
      document.body.classList.remove('toc-inline');
      toggle.innerHTML = collapseSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, 0 - tocHeight);
      }
      tocNav.focus();
      sidebarMedia.addListener(autoToggle); // auto-collapse when out of room
    }
    else {
      document.body.classList.add('toc-inline');
      document.body.classList.remove('toc-sidebar');
      toggle.innerHTML = expandSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, tocNav.offsetHeight);
      }
      if (toggle.matches(':hover')) {
        /* Unfocus button when not using keyboard navigation,
           because I don't know where else to send the focus. */
        toggle.blur();
      }
    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn't exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn't standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don't have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we're at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can't find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>