<!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>P1110R0: A placeholder with no name</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

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

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

  /* Do something nice. */

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

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

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

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

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

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

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

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

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

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

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

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

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

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

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

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		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;
		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;
		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  {
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	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
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	table.data  tbody td[rowspan]:not([rowspan='1']),
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	table.data  tbody th[rowspan]:first-child,
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	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-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
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		border-color: #3980B5;
		border-bottom-width: 3px !important;
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		/* Allow colors to cascade through from link styling */
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		line-height: 1.1rem; /* consistent spacing */
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	.toc > li li li li li .secno { font-size: 100%; }

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		.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 */
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			width: 4rem;
			white-space: nowrap;
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		.toc > li li li .secno       { margin-left: -7rem; }
		.toc > li li li li .secno    { margin-left: -9rem; }
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		@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; }
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		/* Use #toc over .toc to override non-@supports rules. */
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			align-content: start;
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			grid-column: 1 / -1;
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			/* The spans of the top-level list,
			   comprising the first items of each top-level section. */
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			grid-column: 1;
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		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
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		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


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

	/* Index Lists: Layout */
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/** 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;
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	table.index td,
	table.index th {
		padding: 0.4em;
	}

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		background: #f7f8f9;
	}

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

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

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		/* Pages have their own margins. */
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			margin: 0;
		}
		/* Serif for print. */
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			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



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

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		/* in case figure is overlarge, limit caption to 50em */
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		margin-left: auto;
		margin-right: auto;
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			/* See Lea Verou's explanation background-attachment:
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			background-repeat: no-repeat;
			*/
		}
	}
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<style type="text/css">
    table, th, td {
      border: 1px solid black;
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      border-right: none;
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    }
  </style>
  <meta content="Bikeshed version 6abc9f98f620481a5031cd9b44944d60af9b79b9" name="generator">
  <link href="http://wg21.link/P1110R0" rel="canonical">
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  <meta content="8044448d5b0656fb5fa345bdd201cc479e35f834" name="document-revision">
<style>/* style-md-lists */

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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1110R0<br>A placeholder with no name</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-06-07">7 June 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P1110R0">http://wg21.link/P1110R0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jyasskin@google.com">Jeffrey Yasskin</a> (<span class="p-org org">Google</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
     <dt>Source:
     <dd><a href="https://github.com/jyasskin/cxx-unnamed-placeholder/blob/master/unnamed.bs">https://github.com/jyasskin/cxx-unnamed-placeholder/blob/master/unnamed.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>A C++ token for locations that would normally introduce a name, that says not to introduce the name.</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="#intro"><span class="secno">1</span> <span class="content">Introduction and motivation</span></a>
    <li>
     <a href="#examples"><span class="secno">2</span> <span class="content">Examples</span></a>
     <ol class="toc">
      <li><a href="#ex-variables"><span class="secno">2.1</span> <span class="content">Variables</span></a>
      <li><a href="#ex-structured-binding"><span class="secno">2.2</span> <span class="content">Structured Bindings</span></a>
      <li><a href="#ex-enumerator"><span class="secno">2.3</span> <span class="content">Enumerators</span></a>
      <li><a href="#ex-concept"><span class="secno">2.4</span> <span class="content">Speculative: Concept-constrained declarations</span></a>
      <li>
       <a href="#less-useful-ex"><span class="secno">2.5</span> <span class="content">Less useful examples</span></a>
       <ol class="toc">
        <li><a href="#ex-class"><span class="secno">2.5.1</span> <span class="content">Class, Struct, Enum</span></a>
        <li><a href="#ex-unnamed-param"><span class="secno">2.5.2</span> <span class="content">Unnamed parameters</span></a>
        <li><a href="#ex-function"><span class="secno">2.5.3</span> <span class="content">Function names</span></a>
        <li><a href="#ex-using"><span class="secno">2.5.4</span> <span class="content">Using</span></a>
        <li><a href="#ex-concept-intro"><span class="secno">2.5.5</span> <span class="content">Concept introduction</span></a>
       </ol>
      <li>
       <a href="#discouraged-ex"><span class="secno">2.6</span> <span class="content">Discouraged examples</span></a>
       <ol class="toc">
        <li><a href="#ex-anon-namespace"><span class="secno">2.6.1</span> <span class="content">Anonymous Namespaces</span></a>
        <li><a href="#ex-anon-union"><span class="secno">2.6.2</span> <span class="content">Anonymous Unions</span></a>
       </ol>
     </ol>
    <li>
     <a href="#prior"><span class="secno">3</span> <span class="content">Prior Art</span></a>
     <ol class="toc">
      <li><a href="#prior-haskell"><span class="secno">3.1</span> <span class="content">Haskell</span></a>
      <li><a href="#prior-python"><span class="secno">3.2</span> <span class="content">Python</span></a>
      <li>
       <a href="#prior-scala"><span class="secno">3.3</span> <span class="content">Scala</span></a>
       <ol class="toc">
        <li><a href="#scala-patterns"><span class="secno">3.3.1</span> <span class="content"><span>Pattern matching</span></span></a>
        <li><a href="#scala-defaults"><span class="secno">3.3.2</span> <span class="content"><span>Defaulted definitions</span></span></a>
        <li><a href="#scala-imports"><span class="secno">3.3.3</span> <span class="content"><span>Imports</span></span></a>
        <li><a href="#scala-type-wildcards"><span class="secno">3.3.4</span> <span class="content">Wildcards</span></a>
       </ol>
      <li><a href="#prior-rust"><span class="secno">3.4</span> <span class="content">Rust</span></a>
      <li><a href="#prior-csharp"><span class="secno">3.5</span> <span class="content">C#</span></a>
      <li><a href="#prior-java"><span class="secno">3.6</span> <span class="content">Java</span></a>
      <li><a href="#prior-googlemock"><span class="secno">3.7</span> <span class="content">Googlemock</span></a>
      <li><a href="#prior-halide"><span class="secno">3.8</span> <span class="content">Halide</span></a>
      <li><a href="#prior-others"><span class="secno">3.9</span> <span class="content">Others</span></a>
     </ol>
    <li>
     <a href="#spelling"><span class="secno">4</span> <span class="content">Spelling</span></a>
     <ol class="toc">
      <li><a href="#spell-underscore"><span class="secno">4.1</span> <span class="content"><code class="highlight"><span class="n">_</span></code></span></a>
      <li><a href="#spell-double-under"><span class="secno">4.2</span> <span class="content"><code class="highlight"><span class="n">__</span></code></span></a>
      <li><a href="#spell-question"><span class="secno">4.3</span> <span class="content"><code class="highlight"><span class="o">?</span></code></span></a>
      <li><a href="#spell-rep-question"><span class="secno">4.4</span> <span class="content">Repeated <code class="highlight"><span class="o">?</span></code></span></a>
      <li><a href="#spell-auto"><span class="secno">4.5</span> <span class="content"><code class="highlight"><span class="k">auto</span></code></span></a>
      <li><a href="#spell-omit"><span class="secno">4.6</span> <span class="content">Omit the name</span></a>
      <li><a href="#💩"><span class="secno">4.7</span> <span class="content">Various Unicode characters</span></a>
     </ol>
    <li>
     <a href="#semantic-qs"><span class="secno">5</span> <span class="content">Semantic questions</span></a>
     <ol class="toc">
      <li><a href="#qs-second-use"><span class="secno">5.1</span> <span class="content">Only on second use</span></a>
     </ol>
    <li><a href="#wording"><span class="secno">6</span> <span class="content">Wording</span></a>
    <li><a href="#ack"><span class="secno">7</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <blockquote>
    <p>❝I’ve been through the compiler on a var with no name<br> It felt good to be out of <code class="highlight"><span class="n">main</span></code><br> In the compiler you can remember your name<br> 'Cause there ain’t no diagnostic for to give you no pain❞</p>
    <p>— A Var with no Name</p>
   </blockquote>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction and motivation</span><a class="self-link" href="#intro"></a></h2>
   <p>We have several locations in C++ where a name is expected, but programmers may
not need to use that name ever again. Right now, they have to pick a different
name for each location, and readers can’t immediately tell that the name isn’t
used without an attribute or a naming convention. It would be nice to have a
clear, enforceable way to state that a name is ignored.</p>
   <p>To enforce that the name is actually ignored, and that people reading the code
don’t think that an unused variable is actually used, using it in an expression
should be an error instead of having a tag type like <code class="highlight"><span class="n">nullptr_t</span></code>.</p>
   <p>One of the authors of this paper filed <a data-link-type="biblio" href="#biblio-ewg35">[EWG35]</a> on this subject years ago and
then forgot about it. Unlike that issue, this paper does not expect the
semantics of the placeholder name to be defined in terms of a generated unique
name.</p>
   <h2 class="heading settled" data-level="2" id="examples"><span class="secno">2. </span><span class="content">Examples</span><a class="self-link" href="#examples"></a></h2>
   <p>The goal of this section is to provide an example of using the placeholder in
all declaration contexts where it makes sense. These examples use <code class="highlight"><span class="n">__</span></code> as the
placeholder, but see <a href="#spelling">§4 Spelling</a> for an analysis of several options.</p>
   <h3 class="heading settled" data-level="2.1" id="ex-variables"><span class="secno">2.1. </span><span class="content">Variables</span><a class="self-link" href="#ex-variables"></a></h3>
   <p>To lock a mutex for a scope, without worrying that the variable name is already
taken:</p>
<pre class="language-c++ highlight"><span class="n">std</span><span class="o">::</span><span class="n">lock_guard</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">mutex</span><span class="o">></span> <span class="n">__</span><span class="p">(</span><span class="n">a_mutex</span><span class="p">);</span>
<span class="n">std</span><span class="o">::</span><span class="n">lock_guard</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">mutex</span><span class="o">></span> <span class="n">__</span><span class="p">(</span><span class="n">a_second_mutex</span><span class="p">);</span>
</pre>
   <p>To hold some data alive that’s used via internal pointers:</p>
<pre class="language-c++ highlight"><span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">ptr</span> <span class="o">=</span> <span class="p">...;</span>
<span class="k">auto</span><span class="o">&amp;</span> <span class="n">field1</span> <span class="o">=</span> <span class="n">ptr</span><span class="o">-></span><span class="n">field1</span><span class="p">;</span>
<span class="k">auto</span><span class="o">&amp;</span> <span class="n">field2</span> <span class="o">=</span> <span class="n">ptr</span><span class="o">-></span><span class="n">field2</span>
<span class="p">[</span><span class="n">__</span><span class="o">=</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">ptr</span><span class="p">),</span> <span class="o">&amp;</span><span class="n">field1</span><span class="o">=</span><span class="n">field1</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">field2</span><span class="o">=</span><span class="n">field2</span><span class="p">](){...};</span>
</pre>
   <p>Note that the lambda capture case syntactically allows <code class="highlight"><span class="p">[</span><span class="o">&amp;</span><span class="n">__</span><span class="o">=</span><span class="n">init</span><span class="p">,</span> <span class="p">...](){}</span></code>, but
this is probably not useful.</p>
   <p>Classes can also have data members named <code class="highlight"><span class="n">__</span></code>, which have the marginal use of
reserving space for the future addition of new ABI-compatible fields. These
fields cannot be used for the same purpose as in lambda captures because they
can’t be initialized.</p>
<pre class="language-c++ highlight"><span class="n">class</span> <span class="n">Foo</span> <span class="p">{</span>
  <span class="n">Foo</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">ptr</span><span class="p">)</span>
    <span class="o">:</span> <span class="n">__</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">ptr</span><span class="p">))</span> <span class="c1">// Doesn’t work.</span>
<span class="c1"></span>    <span class="p">{}</span>

  <span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">__</span><span class="p">;</span> <span class="c1">// Not effective.</span>
<span class="c1"></span>  <span class="kt">int32_t</span> <span class="n">__</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span> <span class="c1">// Reserve 4 words of space for future fields.</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   <h3 class="heading settled" data-level="2.2" id="ex-structured-binding"><span class="secno">2.2. </span><span class="content">Structured Bindings</span><a class="self-link" href="#ex-structured-binding"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p0144r2">[P0144R2]</a> §3.8 mentions that structured bindings could potentially use a
syntax to ignore some fields from a bound structure:</p>
   <blockquote>
<pre class="language-c++ highlight"><span class="n">tuple</span><span class="o">&lt;</span><span class="n">T1</span><span class="p">,</span><span class="n">T2</span><span class="p">,</span><span class="n">T3</span><span class="o">></span> <span class="n">f</span><span class="p">();</span>
<span class="k">auto</span> <span class="p">[</span><span class="n">x</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">ignore</span><span class="p">,</span> <span class="n">z</span><span class="p">]</span> <span class="o">=</span> <span class="n">f</span><span class="p">();</span> <span class="c1">// NOT proposed: ignore second element</span>
</pre>
   </blockquote>
   <p><a data-link-type="biblio" href="#biblio-p0144r2">[P0144R2]</a> suggests waiting until a pattern matching proposal, at which point
the right token will fall out. I suggest that this proposal also provides a
reasonable token, making this:</p>
<pre class="language-c++ highlight"><span class="n">tuple</span><span class="o">&lt;</span><span class="n">T1</span><span class="p">,</span><span class="n">T2</span><span class="p">,</span><span class="n">T3</span><span class="o">></span> <span class="n">f</span><span class="p">();</span>
<span class="k">auto</span> <span class="p">[</span><span class="n">x</span><span class="p">,</span> <span class="n">__</span><span class="p">,</span> <span class="n">z</span><span class="p">]</span> <span class="o">=</span> <span class="n">f</span><span class="p">();</span> <span class="c1">// Ignore second element.</span>
</pre>
   <h3 class="heading settled" data-level="2.3" id="ex-enumerator"><span class="secno">2.3. </span><span class="content">Enumerators</span><a class="self-link" href="#ex-enumerator"></a></h3>
   <p>This makes it very slightly easier to skip a value:</p>
<pre class="language-c++ highlight"><span class="k">enum</span> <span class="n">MmapBits</span> <span class="p">{</span>
  <span class="n">Shared</span><span class="p">,</span>
  <span class="n">Private</span><span class="p">,</span>
  <span class="n">__</span><span class="p">,</span>
  <span class="n">__</span><span class="p">,</span>
  <span class="n">Fixed</span><span class="p">,</span>
  <span class="n">Rename</span><span class="p">,</span>
  <span class="p">...</span>
<span class="p">};</span>
</pre>
   <p>I suspect that this is less clear than explicitly assigning the values, so it
would make sense to not support this.</p>
   <h3 class="heading settled" data-level="2.4" id="ex-concept"><span class="secno">2.4. </span><span class="content">Speculative: Concept-constrained declarations</span><a class="self-link" href="#ex-concept"></a></h3>
   <p>To define a deduced-type non-type template parameter where the type matches a
concept but the type isn’t given a name in the body of the class or function:</p>
<pre class="language-c++ highlight"><span class="n">template</span><span class="o">&lt;</span><span class="n">Integral</span> <span class="n">__</span> <span class="n">N</span><span class="o">></span> <span class="n">class</span> <span class="n">integral_constant</span> <span class="p">{</span> <span class="p">...</span> <span class="p">};</span>
</pre>
   <p>To declare a deduced-type parameter or local variable whose type is constrained
by a concept:</p>
<pre class="language-c++ highlight"><span class="n">Numeric</span> <span class="nf">multiplyAdd</span><span class="p">(</span><span class="n">Numeric</span> <span class="n">__</span> <span class="n">x</span><span class="p">,</span> <span class="n">Numeric</span> <span class="n">__</span> <span class="n">y</span><span class="p">,</span> <span class="n">Numeric</span> <span class="n">__</span> <span class="n">z</span><span class="p">)</span> <span class="p">{</span>
  <span class="n">Numeric</span> <span class="n">__</span> <span class="n">multiplied</span> <span class="o">=</span> <span class="n">x</span> <span class="o">*</span> <span class="n">y</span><span class="p">;</span>
  <span class="k">return</span> <span class="n">multiplied</span> <span class="o">+</span> <span class="n">z</span><span class="p">;</span>
<span class="p">}</span>
</pre>
   <h3 class="heading settled" data-level="2.5" id="less-useful-ex"><span class="secno">2.5. </span><span class="content">Less useful examples</span><a class="self-link" href="#less-useful-ex"></a></h3>
   <h4 class="heading settled" data-level="2.5.1" id="ex-class"><span class="secno">2.5.1. </span><span class="content">Class, Struct, Enum</span><a class="self-link" href="#ex-class"></a></h4>
   <p>These aren’t necessary because developers can simply omit the name, but might
still be useful to simplify the declaration syntax:</p>
<pre class="language-c++ highlight"><span class="n">class</span> <span class="nl">__</span> <span class="p">:</span> <span class="n">Base</span> <span class="p">{</span> <span class="p">...</span> <span class="p">};</span>
<span class="k">struct</span> <span class="nl">__</span> <span class="p">:</span> <span class="n">Base</span> <span class="p">{</span> <span class="p">...</span> <span class="p">};</span>
<span class="k">enum</span> <span class="n">__</span> <span class="p">{</span> <span class="n">Enumerator</span> <span class="p">};</span>
</pre>
   <p>We couldn’t actually require a name there for several more versions of C++, if
ever, but local style guides could require it in order to simplify what they
need to teach new C++ developers.</p>
   <h4 class="heading settled" data-level="2.5.2" id="ex-unnamed-param"><span class="secno">2.5.2. </span><span class="content">Unnamed parameters</span><a class="self-link" href="#ex-unnamed-param"></a></h4>
   <p>Like unused class names, parameter names can simply be omitted instead of
needing a placeholder. Again, requiring an explicitly-unused name, instead of
allowing it to be omitted, simplifies the declaration grammar.</p>
<pre class="language-c++ highlight"><span class="n">class</span> <span class="nl">C</span> <span class="p">:</span> <span class="n">public</span> <span class="n">Base</span> <span class="p">{</span>
  <span class="kt">void</span> <span class="n">Func</span><span class="p">(</span><span class="kt">int</span> <span class="n">i</span><span class="p">,</span> <span class="kt">long</span> <span class="n">__</span><span class="p">)</span> <span class="n">override</span><span class="p">;</span>
<span class="p">};</span>

<span class="n">template</span><span class="o">&lt;</span><span class="kr">typename</span> <span class="n">__</span><span class="o">></span>
<span class="n">class</span> <span class="n">UsedInTemplateTemplateParam</span> <span class="p">{</span> <span class="p">...</span> <span class="p">};</span>
</pre>
   <h4 class="heading settled" data-level="2.5.3" id="ex-function"><span class="secno">2.5.3. </span><span class="content">Function names</span><a class="self-link" href="#ex-function"></a></h4>
   <p>Using <code class="highlight"><span class="n">__</span></code> to name a function is a similar case to declaring a variable, except
that function definitions don’t have the kind of side-effects that variable
definitions have.</p>
<pre class="language-c++ highlight"><span class="kt">void</span> <span class="nf">__</span><span class="p">(</span><span class="kt">int</span> <span class="n">arg</span><span class="p">)</span> <span class="p">{</span> <span class="p">...</span> <span class="p">}</span>
<span class="kt">void</span> <span class="nf">__</span><span class="p">(</span><span class="kt">long</span> <span class="n">arg</span><span class="p">)</span> <span class="p">{</span> <span class="p">...</span> <span class="p">}</span>
</pre>
   <p>Since the use of <code class="highlight"><span class="n">__</span></code> prevents code from referring to these functions after
they’re declared, the above code creates two independent functions rather than
an overload set (although I can’t think of a way to distinguish the two).</p>
   <p>Virtual functions can also be named <code class="highlight"><span class="n">__</span></code>. Having an unnamed virtual function
take up a vtable slot could be useful to preserve ABI compatibility when a
future function is added, but that’s a matter for platform ABIs rather than the
standard. This kind of virtual function cannot be overridden, of course, since
it can’t be named again after its declaration.</p>
   <h4 class="heading settled" data-level="2.5.4" id="ex-using"><span class="secno">2.5.4. </span><span class="content">Using</span><a class="self-link" href="#ex-using"></a></h4>
   <p>To use a type name, without being able to use the alias. I don’t think using a
type name has side-effects, so there’s probably not much point.</p>
<pre class="language-c++ highlight"><span class="n">using</span> <span class="n">__</span> <span class="o">=</span> <span class="kr">typename</span> <span class="n">Base</span><span class="o">::</span><span class="n">value_type</span><span class="p">;</span>
</pre>
   <p>Similarly non-useful:</p>
<pre class="language-c++ highlight"><span class="k">typedef</span> <span class="kr">typename</span> <span class="n">Base</span><span class="o">::</span><span class="n">value_type</span> <span class="n">__</span><span class="p">;</span>
<span class="n">namespace</span> <span class="n">__</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">ranges</span><span class="p">;</span>
</pre>
   <h4 class="heading settled" data-level="2.5.5" id="ex-concept-intro"><span class="secno">2.5.5. </span><span class="content">Concept introduction</span><a class="self-link" href="#ex-concept-intro"></a></h4>
   <p>Like name aliases, a <code class="highlight"><span class="n">requires</span></code> clause doesn’t have side-effects, so doesn’t
need to be assigned to an unnamed concept.</p>
<pre class="language-c++ highlight"><span class="n">concept</span> <span class="n">__</span> <span class="o">=</span> <span class="n">requires</span> <span class="p">{...};</span>
</pre>
   <h3 class="heading settled" data-level="2.6" id="discouraged-ex"><span class="secno">2.6. </span><span class="content">Discouraged examples</span><a class="self-link" href="#discouraged-ex"></a></h3>
   <h4 class="heading settled" data-level="2.6.1" id="ex-anon-namespace"><span class="secno">2.6.1. </span><span class="content">Anonymous Namespaces</span><a class="self-link" href="#ex-anon-namespace"></a></h4>
   <p>It would be possible to use <code class="highlight"><span class="n">__</span></code> as the name of an unnamed namespace:</p>
<pre class="language-c++ highlight"><span class="n">namespace</span> <span class="n">__</span> <span class="p">{</span> <span class="p">...</span> <span class="p">};</span>
</pre>
   <p>However, the meaning of an anonymous namespace isn’t just the meaning of a
namespace with a name you never use again. Instead, [<strong>namespace.unnamed</strong>]
says,</p>
   <blockquote>
    <p>An <em>unnamed-namespace-definition</em> behaves as if it were replaced by</p>
<pre class="highlight"><span class="n">inline</span><sub><i><span class="n">opt</span></i></sub> <span class="k">namespace</span> <i><span class="n">unique</span></i> <span class="p">{</span> <span class="cm">/* empty body */</span> <span class="p">}</span>
<span class="k">using</span> <span class="k">namespace</span> <i><span class="n">unique</span></i> <span class="p">;</span>
<span class="k">namespace</span> <i><span class="n">unique</span></i> <span class="p">{</span> <i><span class="k">namespace</span><span class="o">-</span><span class="n">body</span></i> <span class="p">}</span></pre>
   </blockquote>
   <p>Because of this divergence in meaning from the other places <code class="highlight"><span class="n">__</span></code> can appear, I
recommend disallowing it as a namespace name.</p>
   <h4 class="heading settled" data-level="2.6.2" id="ex-anon-union"><span class="secno">2.6.2. </span><span class="content">Anonymous Unions</span><a class="self-link" href="#ex-anon-union"></a></h4>
   <p>[<strong>class.union.anon</strong>] says that</p>
<pre class="language-c++ highlight"><span class="k">struct</span> <span class="n">MyStruct</span> <span class="p">{</span>
    <span class="k">union</span> <span class="p">{</span>
        <span class="kt">int</span> <span class="n">i</span><span class="p">;</span>
        <span class="kt">double</span> <span class="n">d</span><span class="p">;</span>
    <span class="p">};</span>
<span class="p">};</span>
</pre>
   <p>Makes the union’s members' names available as direct members of <code class="highlight"><span class="n">MyStruct</span></code>.
Because of that extra behavior, I also don’t think we should allow</p>
<pre class="language-c++ highlight"><span class="k">struct</span> <span class="n">MyStruct</span> <span class="p">{</span>
    <span class="k">union</span> <span class="n">__</span> <span class="p">{</span>
        <span class="kt">int</span> <span class="n">i</span><span class="p">;</span>
        <span class="kt">double</span> <span class="n">d</span><span class="p">;</span>
    <span class="p">}</span> <span class="n">__</span><span class="p">;</span>
<span class="p">};</span>
</pre>
   <p>We could say that naming either the union or the variable allows <code class="highlight"><span class="n">__</span></code> in the
other position, or disallow <code class="highlight"><span class="n">__</span></code> in either position.</p>
   <h2 class="heading settled" data-level="3" id="prior"><span class="secno">3. </span><span class="content">Prior Art</span><a class="self-link" href="#prior"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="prior-haskell"><span class="secno">3.1. </span><span class="content">Haskell</span><a class="self-link" href="#prior-haskell"></a></h3>
   <p>Haskell uses a <code class="highlight"><span class="n">_</span></code> token to indicate unused things:</p>
<pre class="language-haskell highlight"><span class="nf">head</span> <span class="p">(</span><span class="n">x</span><span class="kt">:</span><span class="kr">_</span><span class="p">)</span>  <span class="ow">=</span> <span class="n">x</span>
<span class="nf">tail</span> <span class="p">(</span><span class="kr">_</span><span class="kt">:</span><span class="n">xs</span><span class="p">)</span> <span class="ow">=</span> <span class="n">xs</span>
</pre>
   <p>Note that the _ can be "initialized" multiple times, unlike a named but unused
variable:</p>
<pre class="language-haskell highlight"><span class="kr">data</span> <span class="kt">Colour</span> <span class="ow">=</span> <span class="kt">Colour</span> <span class="p">{</span> <span class="n">red</span><span class="ow">::</span><span class="kt">Int</span><span class="p">,</span> <span class="n">green</span><span class="ow">::</span><span class="kt">Int</span><span class="p">,</span> <span class="n">blue</span><span class="ow">::</span><span class="kt">Int</span><span class="p">,</span> <span class="n">opacity</span><span class="ow">::</span><span class="kt">Int</span><span class="p">}</span>

<span class="nf">isOpaqueColour</span> <span class="ow">::</span> <span class="kt">Colour</span> <span class="ow">-></span> <span class="kt">Bool</span>
<span class="nf">isOpaqueColour</span> <span class="p">(</span><span class="kt">Colour</span> <span class="kr">_</span> <span class="kr">_</span> <span class="kr">_</span> <span class="n">opacity</span><span class="p">)</span> <span class="ow">=</span> <span class="n">opacity</span> <span class="o">==</span> <span class="mi">255</span>
</pre>
   <h3 class="heading settled" data-level="3.2" id="prior-python"><span class="secno">3.2. </span><span class="content">Python</span><a class="self-link" href="#prior-python"></a></h3>
   <p><code class="highlight"><span class="n">_</span></code> is conventionally used to name unused variables. For example:</p>
<pre class="language-python highlight"><span class="n">label</span><span class="p">,</span> <span class="n">has_label</span><span class="p">,</span> <span class="n">_</span> <span class="o">=</span> <span class="n">text</span><span class="o">.</span><span class="n">partition</span><span class="p">(</span><span class="s1">':'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> range<span class="p">(</span><span class="mi">10</span><span class="p">):</span>
  <span class="n">call_ten_times</span><span class="p">()</span>
</pre>
   <p>https://stackoverflow.com/a/5893946/943619 describes this use, but there is no
mention in the official Python documentation.</p>
   <h3 class="heading settled" data-level="3.3" id="prior-scala"><span class="secno">3.3. </span><span class="content">Scala</span><a class="self-link" href="#prior-scala"></a></h3>
   <p>Scala uses <code class="highlight"><span class="n">_</span></code> for many kinds of placeholders:</p>
   <h4 class="heading settled" data-level="3.3.1" id="scala-patterns"><span class="secno">3.3.1. </span><span class="content"><a href="https://docs.scala-lang.org/tour/pattern-matching.html#pattern-guards">Pattern matching</a></span><a class="self-link" href="#scala-patterns"></a></h4>
<pre class="language-scala highlight"><span class="k">def</span> <span class="n">showImportantNotification</span><span class="o">(</span><span class="n">notification</span><span class="k">:</span> <span class="kt">Notification</span><span class="o">,</span> <span class="n">importantPeopleInfo</span><span class="k">:</span> <span class="kt">Seq</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span><span class="k">:</span> <span class="kt">String</span> <span class="o">=</span> <span class="o">{</span>
  <span class="n">notification</span> <span class="k">match</span> <span class="o">{</span>
    <span class="k">case</span> <span class="nc">Email</span><span class="o">(</span><span class="n">email</span><span class="o">,</span> <span class="k">_</span><span class="o">,</span> <span class="k">_</span><span class="o">)</span> <span class="k">if</span> <span class="n">importantPeopleInfo</span><span class="o">.</span><span class="n">contains</span><span class="o">(</span><span class="n">email</span><span class="o">)</span> <span class="k">=></span>
      <span class="s">"You got an email from special someone!"</span>
    <span class="k">case</span> <span class="nc">SMS</span><span class="o">(</span><span class="n">number</span><span class="o">,</span> <span class="k">_</span><span class="o">)</span> <span class="k">if</span> <span class="n">importantPeopleInfo</span><span class="o">.</span><span class="n">contains</span><span class="o">(</span><span class="n">number</span><span class="o">)</span> <span class="k">=></span>
      <span class="s">"You got an SMS from special someone!"</span>
    <span class="k">case</span> <span class="n">other</span> <span class="k">=></span>
      <span class="n">showNotification</span><span class="o">(</span><span class="n">other</span><span class="o">)</span> <span class="c1">// nothing special, delegate to our original showNotification function</span>
<span class="c1"></span>  <span class="o">}</span>
<span class="o">}</span>
</pre>
   <p>Variable declarations can be patterns as well, which has the side-effect that</p>
<pre class="language-scala highlight"><span class="k">var</span> <span class="k">_</span> <span class="k">=</span> <span class="o">...</span>
</pre>
   <p>defines an un-named variable like the subject of this proposal. This is less
useful in Scala than in C++ because Scala doesn’t have destrutors.</p>
   <h4 class="heading settled" data-level="3.3.2" id="scala-defaults"><span class="secno">3.3.2. </span><span class="content"><a href="https://www.scala-lang.org/files/archive/spec/2.12/04-basic-declarations-and-definitions.html#variable-declarations-and-definitions">Defaulted definitions</a></span><a class="self-link" href="#scala-defaults"></a></h4>
   <p>The following initializes <code class="highlight"><span class="n">x</span></code> to a default value depending on its type. For
example, integral types get <code class="highlight"><span class="mi">0</span></code>, and reference types get <code class="highlight"><span class="n">null</span></code>.</p>
<pre class="language-scala highlight"><span class="k">var</span> <span class="n">x</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span> <span class="k">_</span>
</pre>
   <h4 class="heading settled" data-level="3.3.3" id="scala-imports"><span class="secno">3.3.3. </span><span class="content"><a href="https://www.scala-lang.org/files/archive/spec/2.12/04-basic-declarations-and-definitions.html#import-clauses">Imports</a></span><a class="self-link" href="#scala-imports"></a></h4>
   <blockquote>
    <p>The import clause <code class="highlight"><span class="n">import</span> <span class="n">p</span><span class="p">.</span><span class="n">_</span></code> … makes available without qualification all
members of <code class="highlight"><span class="n">p</span></code> (this is analogous to <code class="highlight"><span class="n">import</span> <span class="n">p</span><span class="p">.</span><span class="o">*</span></code> in Java).</p>
   </blockquote>
   <h4 class="heading settled" data-level="3.3.4" id="scala-type-wildcards"><span class="secno">3.3.4. </span><span class="content">Wildcards</span><a class="self-link" href="#scala-type-wildcards"></a></h4>
   <p>Scala uses the <code class="highlight"><span class="n">_</span></code> for wildcards in both types and functions. It’s possible to
write an anonymous function <a href="https://www.scala-lang.org/files/archive/spec/2.12/06-expressions.html#placeholder-syntax-for-anonymous-functions">just by using an _ in an
expression</a>.
The first column in each of the rows in the following table is equivalent to the
second.</p>
   <table>
    <tbody>
     <tr>
      <td><code class="highlight"><span class="k">_</span> <span class="o">+</span> <span class="mi">1</span></code>
      <td><code class="highlight"><span class="n">x</span> <span class="k">=></span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span></code>
     <tr>
      <td><code class="highlight"><span class="k">_</span> <span class="o">*</span> <span class="k">_</span></code>
      <td><code class="highlight"><span class="o">(</span><span class="n">x1</span><span class="o">,</span> <span class="n">x2</span><span class="o">)</span> <span class="k">=></span> <span class="n">x1</span> <span class="o">*</span> <span class="n">x2</span></code>
     <tr>
      <td><code class="highlight"><span class="o">(</span><span class="k">_:</span> <span class="kt">Int</span><span class="o">)</span> <span class="o">*</span> <span class="mi">2</span></code>
      <td><code class="highlight"><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="k">=></span> <span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="o">*</span> <span class="mi">2</span></code>
     <tr>
      <td><code class="highlight"><span class="k">if</span> <span class="o">(</span><span class="k">_</span><span class="o">)</span> <span class="n">x</span> <span class="k">else</span> <span class="n">y</span></code>
      <td><code class="highlight"><span class="n">z</span> <span class="k">=></span> <span class="k">if</span> <span class="o">(</span><span class="n">z</span><span class="o">)</span> <span class="n">x</span> <span class="k">else</span> <span class="n">y</span></code>
     <tr>
      <td><code class="highlight"><span class="k">_</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">f</span><span class="o">)</span></code>
      <td><code class="highlight"><span class="n">x</span> <span class="k">=></span> <span class="n">x</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">f</span><span class="o">)</span></code>
     <tr>
      <td><code class="highlight"><span class="k">_</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="k">_</span> <span class="o">+</span> <span class="mi">1</span><span class="o">)</span></code>
      <td><code class="highlight"><span class="n">x</span> <span class="k">=></span> <span class="n">x</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">y</span> <span class="k">=></span> <span class="n">y</span> <span class="o">+</span> <span class="mi">1</span><span class="o">)</span></code>
   </table>
   <p>Similarly, <a href="https://www.scala-lang.org/files/archive/spec/2.12/03-types.html#placeholder-syntax-for-existential-types">the following two types are equivalent</a>:</p>
<pre class="language-scala highlight"><span class="nc">Ref</span><span class="o">[</span><span class="k">_</span> <span class="k">&lt;:</span> <span class="kt">java.lang.Number</span><span class="o">]</span>
<span class="nc">Ref</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span> <span class="k">forSome</span> <span class="o">{</span> <span class="k">type</span> <span class="kt">T</span> <span class="k">&lt;:</span> <span class="kt">java.lang.Number</span> <span class="o">}</span>
</pre>
   <h3 class="heading settled" data-level="3.4" id="prior-rust"><span class="secno">3.4. </span><span class="content">Rust</span><a class="self-link" href="#prior-rust"></a></h3>
   <p>Rust provides a couple ways to <a href="https://doc.rust-lang.org/book/second-edition/ch18-03-pattern-syntax.html#ignoring-values-in-a-pattern">ignore values in
patterns</a>,
which include variable and function parameter declarations:</p>
   <p><code class="highlight"><span class="n">_</span></code>:</p>
<pre class="language-rust highlight"><span class="k">fn</span> <span class="nf">foo</span><span class="p">(</span><span class="n">_</span>: <span class="kt">i32</span><span class="p">,</span> <span class="n">y</span>: <span class="kt">i32</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">println</span><span class="o">!</span><span class="p">(</span><span class="s">"This code only uses the y parameter: {}"</span><span class="p">,</span> <span class="n">y</span><span class="p">);</span>
<span class="p">}</span>
</pre>
   <p>Variables prefixed with <code class="highlight"><span class="n">_</span></code>:</p>
   <p>The compiler will usually give an error for an unused variable, but if it’s
prefixed with <code class="highlight"><span class="n">_</span></code>, the error is suppressed:</p>
<pre class="language-rust highlight"><span class="kd">let</span> <span class="n">s</span> <span class="o">=</span> Some<span class="p">(</span>String::<span class="n">from</span><span class="p">(</span><span class="s">"Hello!"</span><span class="p">));</span>

<span class="k">if</span> <span class="kd">let</span> Some<span class="p">(</span><span class="n">_s</span><span class="p">)</span> <span class="o">=</span> <span class="n">s</span> <span class="p">{</span>
    <span class="n">println</span><span class="o">!</span><span class="p">(</span><span class="s">"found a string"</span><span class="p">);</span>
<span class="p">}</span>
</pre>
   <p>However, because of Rust’s borrow checker, this is semantically different from a
simple <code class="highlight"><span class="n">_</span></code>: the <code class="highlight"><span class="n">_s</span></code> takes ownership of the object it’s bound to, where a <code class="highlight"><span class="n">_</span></code> wouldn’t.</p>
   <p><code class="highlight"><span class="p">..</span></code> to ignore several parts of a value:</p>
<pre class="language-rust highlight"><span class="kd">let</span> <span class="n">numbers</span> <span class="o">=</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">16</span><span class="p">,</span> <span class="mi">32</span><span class="p">);</span>
<span class="k">match</span> <span class="n">numbers</span> <span class="p">{</span>
    <span class="p">(</span><span class="n">first</span><span class="p">,</span> <span class="p">..,</span> <span class="n">last</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
        <span class="n">println</span><span class="o">!</span><span class="p">(</span><span class="s">"Some numbers: {}, {}"</span><span class="p">,</span> <span class="n">first</span><span class="p">,</span> <span class="n">last</span><span class="p">);</span>
    <span class="p">},</span>
<span class="p">}</span>
</pre>
   <h3 class="heading settled" data-level="3.5" id="prior-csharp"><span class="secno">3.5. </span><span class="content">C#</span><a class="self-link" href="#prior-csharp"></a></h3>
   <p>C# defines the <code class="highlight"><span class="n">_</span></code> variable as a <a href="https://docs.microsoft.com/en-us/dotnet/csharp/discards">"discard"</a>, and allows
their use in assignments in the following contexts:</p>
   <ul>
    <li data-md="">
     <p>Tuple and object deconstruction.</p>
    <li data-md="">
     <p>Pattern matching with is and switch.</p>
    <li data-md="">
     <p>Calls to methods with <code class="highlight"><span class="n">out</span></code> parameters.</p>
    <li data-md="">
     <p>A standalone <code class="highlight"><span class="n">_</span></code> when no <code class="highlight"><span class="n">_</span></code> is in scope.</p>
   </ul>
   <p>C# had to deal with <code class="highlight"><span class="n">_</span></code> being a pre-existing valid variable name, which causes <a href="https://docs.microsoft.com/en-us/dotnet/csharp/discards#a-standalone-discard">certain uses of _ to be errors or
bugs</a> if a <code class="highlight"><span class="n">_</span></code> variable already exists in the same scope.</p>
   <h3 class="heading settled" data-level="3.6" id="prior-java"><span class="secno">3.6. </span><span class="content">Java</span><a class="self-link" href="#prior-java"></a></h3>
   <p>Java uses a <code class="highlight"><span class="o">?</span></code> token to represent a "wildcard" generic argument.</p>
<pre class="language-java highlight"><span class="kt">void</span> <span class="nf">printCollection</span><span class="o">(</span><span class="n">Collection</span><span class="o">&lt;?></span> <span class="n">c</span><span class="o">)</span> <span class="o">{</span>
    <span class="k">for</span> <span class="o">(</span><span class="n">Object</span> <span class="n">e</span> <span class="o">:</span> <span class="n">c</span><span class="o">)</span> <span class="o">{</span>
        <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="n">e</span><span class="o">);</span>
    <span class="o">}</span>
<span class="o">}</span>
</pre>
   <p>Java generic arguments usually need to be declared, but wildcards imply that a
function is generic:</p>
<pre class="language-java highlight"><span class="kd">class</span> <span class="nc">Collections</span> <span class="o">{</span>
    <span class="kd">public</span> <span class="kd">static</span> <span class="o">&lt;</span><span class="n">T</span><span class="o">,</span> <span class="n">S</span> <span class="kd">extends</span> <span class="n">T</span><span class="o">></span> <span class="kt">void</span> <span class="nf">copy</span><span class="o">(</span><span class="n">List</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">dest</span><span class="o">,</span> <span class="n">List</span><span class="o">&lt;</span><span class="n">S</span><span class="o">></span> <span class="n">src</span><span class="o">)</span> <span class="o">{</span>
    <span class="o">...</span>
<span class="o">}</span>

<span class="kd">class</span> <span class="nc">Collections</span> <span class="o">{</span>
    <span class="kd">public</span> <span class="kd">static</span> <span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="kt">void</span> <span class="nf">copy</span><span class="o">(</span><span class="n">List</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">dest</span><span class="o">,</span> <span class="n">List</span><span class="o">&lt;?</span> <span class="kd">extends</span> <span class="n">T</span><span class="o">></span> <span class="n">src</span><span class="o">)</span> <span class="o">{</span>
    <span class="o">...</span>
<span class="o">}</span>
</pre>
   <h3 class="heading settled" data-level="3.7" id="prior-googlemock"><span class="secno">3.7. </span><span class="content">Googlemock</span><a class="self-link" href="#prior-googlemock"></a></h3>
   <p>Googlemock defines <a href="https://github.com/google/googletest/blob/master/googlemock/docs/CheatSheet.md#wildcard">_</a> to match anything. It doesn’t use it for declarations, but it’s still an "ignore
this" token.</p>
<pre class="language-c++ highlight"><span class="c1">// Expects the turtle to move forward by 100 units.</span>
<span class="c1"></span><span class="n">EXPECT_CALL</span><span class="p">(</span><span class="n">turtle</span><span class="p">,</span> <span class="n">Forward</span><span class="p">(</span><span class="mi">100</span><span class="p">));</span>

<span class="c1">// Expects the turtle to move forward.</span>
<span class="c1"></span><span class="n">EXPECT_CALL</span><span class="p">(</span><span class="n">turtle</span><span class="p">,</span> <span class="n">Forward</span><span class="p">(</span><span class="n">_</span><span class="p">));</span>
</pre>
   <h3 class="heading settled" data-level="3.8" id="prior-halide"><span class="secno">3.8. </span><span class="content">Halide</span><a class="self-link" href="#prior-halide"></a></h3>
   <p>From Halide’s <a href="http://halide-lang.org/docs/class_halide_1_1_var.html#a333e72cf9af6339530cb3544b7fe1324">documentation</a>:</p>
   <blockquote>
    <p>For example, consider the definition:</p>
<pre class="language-c++ highlight"><span class="n">Func</span> <span class="n">f</span><span class="p">,</span> <span class="n">g</span><span class="p">;</span>
<span class="n">Var</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">;</span>
<span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="o">=</span> <span class="mi">3</span><span class="p">;</span>
</pre>
    <p>A call to f with the placeholder symbol _ will have implicit arguments injected automatically, so f(2, _) is equivalent to f(2, _0), where _0 = Var::implicit(0), and f(_) (and indeed f when cast to an Expr) is equivalent to f(_0, _1). The following definitions are all equivalent, differing only in the variable names.</p>
<pre class="language-c++ highlight"><span class="n">g</span><span class="p">(</span><span class="n">_</span><span class="p">)</span> <span class="o">=</span> <span class="n">f</span><span class="o">*</span><span class="mi">3</span><span class="p">;</span>
<span class="n">g</span><span class="p">(</span><span class="n">_</span><span class="p">)</span> <span class="o">=</span> <span class="n">f</span><span class="p">(</span><span class="n">_</span><span class="p">)</span><span class="o">*</span><span class="mi">3</span><span class="p">;</span>
<span class="n">g</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">_</span><span class="p">)</span> <span class="o">=</span> <span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">_</span><span class="p">)</span><span class="o">*</span><span class="mi">3</span><span class="p">;</span>
<span class="n">g</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="o">=</span> <span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span><span class="o">*</span><span class="mi">3</span><span class="p">;</span>
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="3.9" id="prior-others"><span class="secno">3.9. </span><span class="content">Others</span><a class="self-link" href="#prior-others"></a></h3>
   <p>I’m told that Swift, Erlang, Elixir, Prolog, OCaml, and F# also use the <code class="highlight"><span class="n">_</span></code> for purposes related to ignoring variables, but I ran out of time to investigate
and describe them all.</p>
   <h2 class="heading settled" data-level="4" id="spelling"><span class="secno">4. </span><span class="content">Spelling</span><a class="self-link" href="#spelling"></a></h2>
   <p>How should we spell the token?</p>
   <h3 class="heading settled" data-level="4.1" id="spell-underscore"><span class="secno">4.1. </span><span class="content"><code class="highlight"><span class="n">_</span></code></span><a class="self-link" href="#spell-underscore"></a></h3>
   <p>I believe <code class="highlight"><span class="n">_</span></code> is the ideal spelling, but [<strong>lex.name</strong>] ¶3.2 only reserves it in
the global namespace, and it’s now used by libraries:</p>
   <ul>
    <li data-md="">
     <p><a href="https://github.com/google/googletest/blob/master/googlemock/docs/CheatSheet.md#wildcard">testing::_ in
Googlemock</a></p>
    <li data-md="">
     <p><a href="http://halide-lang.org/docs/_var_8h.html">Halide::_ in Halide</a></p>
    <li data-md="">
     <p><a href="https://www.gnu.org/software/gettext/manual/html_node/Mark-Keywords.html">The _() macro in Gnu
Gettext</a></p>
   </ul>
   <p>To avoid making these existing uses of <code class="highlight"><span class="n">_</span></code> ill-formed, see <a href="#qs-second-use">§5.1 Only on second use</a>.</p>
   <h3 class="heading settled" data-level="4.2" id="spell-double-under"><span class="secno">4.2. </span><span class="content"><code class="highlight"><span class="n">__</span></code></span><a class="self-link" href="#spell-double-under"></a></h3>
   <p>This is already a reserved identifier per [<strong>lex.name</strong>] ¶3.1.</p>
   <blockquote>
    <p>Each identifier that contains a double underscore <code class="highlight"><span class="n">__</span></code> or begins with an
underscore followed by an uppercase letter is reserved to the implementation
for any use.</p>
   </blockquote>
   <p>It takes a little longer to type than <code class="highlight"><span class="n">_</span></code>, and in many fonts the difference
between <code class="highlight"><span class="n">__</span></code> and <code class="highlight"><span class="n">_</span></code> may be unclear.</p>
   <p>This paper uses <code class="highlight"><span class="n">__</span></code> in the <a href="#examples">examples</a>.</p>
   <p>The <code class="highlight"><span class="n">__</span></code> identifier is used in some codebases, such as <a href="https://github.com/v8/v8/search?q=%22define+__%22&amp;unscoped_q=%22define+__%22_">the V8 JavaScript
engine</a>.
Using a macro named <code class="highlight"><span class="n">__</span></code> as V8 does means that breakage will only occur if that
codebase also attempts to use this new feature.</p>
   <h3 class="heading settled" data-level="4.3" id="spell-question"><span class="secno">4.3. </span><span class="content"><code class="highlight"><span class="o">?</span></code></span><a class="self-link" href="#spell-question"></a></h3>
   <p><code class="highlight"><span class="o">?</span></code> might be confusable with the ternary <code class="highlight"><span class="o">?:</span></code> operator, but developers can
distinguish based on it appearing in a declaration rather than expression
context.</p>
   <p><code class="highlight"><span class="o">?</span></code> may also be confused with a wildcard like <code class="highlight"><span class="o">*</span></code>, to mean "union all the
possible values in this position", for example in a <code class="highlight"><span class="n">nested</span><span class="o">::</span><span class="n">name</span><span class="o">::</span><span class="n">specifier</span></code>.
Fortunately, I don’t see a reason for <code class="highlight"><span class="o">?</span></code> to be valid in contexts where that
interpretation is plausible.</p>
   <p>Using <code class="highlight"><span class="o">?</span></code> for the placeholder identifier might prevent its use in the syntax for
a forwarding reference. e.g. <code class="highlight"><span class="n">ForwardingRef</span><span class="o">&amp;&amp;?</span></code>.</p>
   <h3 class="heading settled" data-level="4.4" id="spell-rep-question"><span class="secno">4.4. </span><span class="content">Repeated <code class="highlight"><span class="o">?</span></code></span><a class="self-link" href="#spell-rep-question"></a></h3>
   <p>Multiple <code class="highlight"><span class="o">?</span></code> could be used to disambiguate some of the issues with a single <code class="highlight"><span class="o">?</span></code>.
Using <code class="highlight"><span class="o">??</span></code> or <code class="highlight"><span class="o">???</span></code> is more typing but is fairly obvious, though it would likely
be better to make this a token so whitespace cannot be used between question marks.</p>
   <h3 class="heading settled" data-level="4.5" id="spell-auto"><span class="secno">4.5. </span><span class="content"><code class="highlight"><span class="k">auto</span></code></span><a class="self-link" href="#spell-auto"></a></h3>
   <p>The "attribute" proposal for concept declarations uses <code class="highlight"><span class="k">auto</span></code> in a place a type
might appear, which one might consider precedent for using it as a more general
placeholder.</p>
   <h3 class="heading settled" data-level="4.6" id="spell-omit"><span class="secno">4.6. </span><span class="content">Omit the name</span><a class="self-link" href="#spell-omit"></a></h3>
   <p>We could just allow omitted names in more places:</p>
<pre class="language-c++ highlight"><span class="k">auto</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">lock_guard</span><span class="p">(</span><span class="n">mx</span><span class="p">);</span>
</pre>
   <p>This option «mov[es] closer to "all syntax is valid and means something, but
maybe not what you meant".» — <a href="http://lists.isocpp.org/ext/2018/06/4518.php">Tony Van
Eerd</a></p>
   <h3 class="heading settled" data-level="4.7" id="💩"><span class="secno">4.7. </span><span class="content">Various Unicode characters</span><a class="self-link" href="#%ed%a0%bd%ed%b2%a9"></a></h3>
   <p>We could use various Unicode characters, such as the replacement character <code class="highlight">�</code>, the empty set <code class="highlight">∅</code>, the interrobang <code class="highlight">‽</code>, a trash can <code class="highlight">🗑</code>, one of the
recycle signs <code class="highlight">♻️</code>, and countless other characters.
These all have the shortcoming that not all toolchains and editors support
them well or uniformly.</p>
   <h2 class="heading settled" data-level="5" id="semantic-qs"><span class="secno">5. </span><span class="content">Semantic questions</span><a class="self-link" href="#semantic-qs"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="qs-second-use"><span class="secno">5.1. </span><span class="content">Only on second use</span><a class="self-link" href="#qs-second-use"></a></h3>
   <p>We could declare that our placeholder makes variables anonymous only once it’s
used for the second time in a scope. That is:</p>
<pre class="language-c++ highlight"><span class="kt">int</span> <span class="n">_</span> <span class="o">=</span> <span class="mi">10</span><span class="p">;</span> <span class="c1">// fine</span>
<span class="c1"></span><span class="n">_</span> <span class="o">=</span> <span class="mi">11</span><span class="p">;</span> <span class="c1">// fine, can use it</span>
<span class="c1"></span><span class="kt">int</span> <span class="n">_</span> <span class="o">=</span> <span class="mi">12</span><span class="p">;</span> <span class="c1">// another _, fine</span>
<span class="c1">// at this point, both _ variables exist, but can no longer be accessed:</span>
<span class="c1"></span><span class="n">_</span> <span class="o">=</span> <span class="mi">13</span><span class="p">;</span> <span class="c1">// error - which one?</span>
</pre>
   <p>Within this option, we could decide whether or not a second declaration is
ill-formed if <code class="highlight"><span class="n">_</span></code> has already been used within the scope.</p>
   <p>Allowing this option at namespace scope, as <a href="#prior-googlemock">Googlemock</a> would need, seems to make ODR violations more likely. A file with only 1 unused
variable (perhaps used to register a static initializer) would be likely to
forget that variable’s <code class="highlight"><span class="k">static</span></code> and so generate an external symbol, which would
collide with a similar symbol from another file.</p>
   <h2 class="heading settled" data-level="6" id="wording"><span class="secno">6. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>TBD</p>
   <h2 class="heading settled" data-level="7" id="ack"><span class="secno">7. </span><span class="content">Acknowledgements</span><a class="self-link" href="#ack"></a></h2>
   <p>Thanks to:</p>
   <ul>
    <li data-md="">
     <p>Daveed Vandevoorde for suggesting using <code class="highlight"><span class="o">?</span></code>.</p>
    <li data-md="">
     <p>Thomas Köppe for suggesting <code class="highlight"><span class="k">auto</span></code>.</p>
    <li data-md="">
     <p>Mathias Stearn for suggesting omitting the name.</p>
    <li data-md="">
     <p>Tony Van Eerd for providing an argument against omitting the name, and for
suggesting that <code class="highlight"><span class="n">_</span></code> only become magic when it’s re-used.</p>
    <li data-md="">
     <p>Richard Smith for pointing out Halide.</p>
    <li data-md="">
     <p>Michael Park for a long list of languages that use <code class="highlight"><span class="n">_</span></code>.</p>
   </ul>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-ewg35">[EWG35]
   <dd>Jeffrey Yasskin. <a href="https://wg21.link/ewg35">[tiny] Some concise way to generate a unique, unused variable name</a>. Open. URL: <a href="https://wg21.link/ewg35">https://wg21.link/ewg35</a>
   <dt id="biblio-p0144r2">[P0144R2]
   <dd>Herb Sutter. <a href="https://wg21.link/p0144r2">Structured Bindings</a>. 16 March 2016. URL: <a href="https://wg21.link/p0144r2">https://wg21.link/p0144r2</a>
  </dl>