<!DOCTYPE html>
<!-- saved from url=(0031)https://api.csswg.org/bikeshed/ -->
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  <title>P1091R3: Extending structured bindings to be more like variable declarations</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,
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/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


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/******************************************************************************/
/*                                    Print                                   */
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/******************************************************************************/
/*                                    Legacy                                  */
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 </head><body class="h-entry toc-inline"><p id="toc-nav"><a id="toc-jump" href="https://api.csswg.org/bikeshed/#toc"><span aria-hidden="true">↑</span> <span>Jump to Table of Contents</span></a><a id="toc-toggle" href="https://api.csswg.org/bikeshed/#toc"><span aria-hidden="true">→</span> <span>Pop Out Sidebar</span></a></p>
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1091R3<br>Extending structured bindings to be more like variable declarations</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-02-18">2019-02-18</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     </dt><dd><a class="u-url" href="https://wg21.link/p1091r3">https://wg21.link/p1091r3</a>
     </dd><dt>Author:
     </dt><dd>
      </dd><dd class="editor p-author h-card vcard"><span class="p-name fn">Nicolas Lesser (blitzrakete[at]gmail[dot][com])</span>
     </dd><dt>Audience:
     </dt><dd>CWG
     </dd><dt>Project:
     </dt><dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dd></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>Making structured bindings more like first-class-citizen.</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="https://api.csswg.org/bikeshed/#this-revision"><span class="secno">1</span> <span class="content">This revision</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#changes"><span class="secno">2</span> <span class="content">Changes</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#introduction"><span class="secno">3</span> <span class="content">Introduction</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#motivation"><span class="secno">4</span> <span class="content">Motivation</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#linkage"><span class="secno">5</span> <span class="content">Linkage</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#extern"><span class="secno">6</span> <span class="content">Extern</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#static-thread_local"><span class="secno">7</span> <span class="content">Static and thread_local</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#inline"><span class="secno">8</span> <span class="content">Inline</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#constexpr"><span class="secno">9</span> <span class="content">Constexpr</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#lambda-captures"><span class="secno">10</span> <span class="content">Lambda captures</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#maybe-unused"><span class="secno">11</span> <span class="content">maybe-unused</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#template"><span class="secno">12</span> <span class="content">Template</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#impact"><span class="secno">13</span> <span class="content">Impact</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#wording"><span class="secno">14</span> <span class="content">Proposed wording</span></a>
    </li><li><a href="https://api.csswg.org/bikeshed/#acknowledgements"><span class="secno">15</span> <span class="content">Acknowledgements</span></a>
   </li></ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="this-revision"><span class="secno">1. </span><span class="content">This revision</span><a class="self-link" href="https://api.csswg.org/bikeshed/#this-revision"></a></h2>
   <p>This revision contains only wording for the changes approved by EWG:</p>
   <ul>
    <li data-md="">
     <p>By value capture of structured bindings</p>
    </li><li data-md="">
     <p>static and thread_local for structured binding declarations</p>
   </li></ul>
   <p>and various proposed wording fixes by Core.</p>
   <h2 class="heading settled" data-level="2" id="changes"><span class="secno">2. </span><span class="content">Changes</span><a class="self-link" href="https://api.csswg.org/bikeshed/#changes"></a></h2>
   <p>r0:</p>
   <ul>
    <li data-md="">
     <p>Initial revision</p>
   </li></ul>
   <p>r1:</p>
   <ul>
    <li data-md="">
     <p>Better wording for Core</p>
    </li><li data-md="">
     <p>Lambda captures</p>
    </li><li data-md="">
     <p><code class="highlight"><c- p="">[[</c-><c- n="">maybe_unused</c-><c- p="">]]</c-></code></p>
    </li><li data-md="">
     <p>SB templates</p>
   </li></ul>
   <p>r2:</p>
   <ul>
    <li data-md="">
     <p>Just wording</p>
   </li></ul>
   <p>r3:</p>
   <p>Wording as reviewed by Core for what EWG approved:</p>
   <ul>
    <li data-md="">
     <p>by-value capture of SBs</p>
    </li><li data-md="">
     <p>static and thread_local</p>
    </li><li data-md="">
     <p>original motivation</p>
   </li></ul>
   <h2 class="heading settled" data-level="3" id="introduction"><span class="secno">3. </span><span class="content">Introduction</span><a class="self-link" href="https://api.csswg.org/bikeshed/#introduction"></a></h2>
   <p>There are a lot of restriction on structured bindings compared to variable declarations, like not being able to mark them <code class="highlight"><c- k="">static</c-></code>, <code class="highlight"><c- k="">constexpr</c-></code> or them not having unclear linkage. This proposal’s aim is to fix this by for example making the underlying structured binding object (and tuple binding variables) have external linkage and by allowing various specifiers (<code class="highlight"><c- k="">static</c-></code>, <code class="highlight"><c- k="">thread_local</c-></code>, <code class="highlight"><c- k="">constexpr</c-></code>, <code class="highlight"><c- kr="">inline</c-></code>, <code class="highlight"><c- k="">extern</c-></code>) on structured bindings.</p>
   <h2 class="heading settled" data-level="4" id="motivation"><span class="secno">4. </span><span class="content">Motivation</span><a class="self-link" href="https://api.csswg.org/bikeshed/#motivation"></a></h2>
   <p>Structured bindings, although very useful, are actually pretty magical. They don’t introduce variables in the normal sense for each binding, rather, they are names that refer to specific objects. As such, there are problems including what it means for a binding to be <code class="highlight"><c- k="">static</c-></code> and how it would work and what linkage do those bindings even have.</p>
   <p>Those problems could not be resolved during the discussion of the paper and afterwards, a paper was requested to analyse the possible design impact that such additions to structured bindings would have after two NB comments proposing this were rejected. This paper attempts to do so.</p>
   <p>One motivation to do so is to bring structured bindings closer to actual variable declarations, so consistency. This will also make structured bindings more useful, as they are currently lacking for example <code class="highlight"><c- k="">constexpr</c-></code>, which is becoming every more important for various features of the language.</p>
   <p>As a consequence this paper also fixes some DRs that were filed and are under consideration or going to be eventually by either Evolution or Core.</p>
   <h2 class="heading settled" data-level="5" id="linkage"><span class="secno">5. </span><span class="content">Linkage</span><a class="self-link" href="https://api.csswg.org/bikeshed/#linkage"></a></h2>
   <p>As per [basic.link]p8, bindings do not have any linkage because they’re just names. Currently, in both gcc and clang, tuple binding have linkage, as they were specified as variable declarations in the standard. This is no longer the case though due to the resolution of DR2313<sup>1</sup>.</p>
   <p>But the underlying structured binding object is an actual variable, which can have either internal or external linkage depending on the declaration of the structured binding, as gcc and clang do it. There is no way to refer to that object though without making the program IF-NDR as the object has a name like <code class="highlight"><c- n="">_ZDC1a2bbE</c-></code> for <code class="highlight"><c- k="">auto</c-><c- p="">[</c-><c- n="">a</c-><c- p="">,</c-> <c- n="">bb</c-><c- p="">]</c-></code>.</p>
   <p>[dcl.struct.bind]p1 has the following to say about the object:</p>
   <blockquote>
    <p>First, a variable with a unique name <em>e</em> is introduced.</p>
   </blockquote>
   <p>It follows that the variable cannot be referenced in a conforming program anyways, and as such, it doesn’t make much sense to give it external linkage. Nonetheless, to be consistent with the rest of the declarations and being able to use just <code class="highlight"><c- kr="">inline</c-></code> (see below) without an extra <code class="highlight"><c- k="">extern</c-></code> to give the structured binding external linkage, the underlying object should have external linkage.</p>
   <h2 class="heading settled" data-level="6" id="extern"><span class="secno">6. </span><span class="content">Extern</span><a class="self-link" href="https://api.csswg.org/bikeshed/#extern"></a></h2>
   <p>As discussed in the previous section, there is no way to reference either the bindings or the underlying object within a conforming program, so allowing <code class="highlight"><c- k="">extern</c-></code> on such a structured binding would not make much sense.</p>
   <p>However, this would prohibit <code class="highlight"><c- kr="">inline</c-></code> on structured bindings that have been declared <code class="highlight"><c- k="">const</c-></code> and <code class="highlight"><c- kr="">inline</c-></code>, which might be desirable in some cases. For this reason, it should be allowed. Note that <code class="highlight"><c- k="">extern</c-></code> would have no effect on the individual bindings, except for the tuple case.</p>
   <h2 class="heading settled" data-level="7" id="static-thread_local"><span class="secno">7. </span><span class="content">Static and thread_local</span><a class="self-link" href="https://api.csswg.org/bikeshed/#static-thread_local"></a></h2>
   <p><code class="highlight"><c- k="">static</c-></code> and <code class="highlight"><c- k="">thread_local</c-></code> on a structured binding make sense and are actually useful. The way to make this work nicely in the standard is to only apply them on the underlying object, and not on the bindings (which wouldn’t make sense and can’t work today without major changes to the specification of bindings anyways). For the tuple case on the additional variable declarations too.</p>
   <p>Because bindings refer to certain objects (depending on the initializer of the structured binding), they would implicitly get the desired semantics of <code class="highlight"><c- k="">static</c-></code> and <code class="highlight"><c- k="">thread_local</c-></code>, as they refer to either objects within the underlying object or to separate variables (in the tuple case) which are marked with the desired specifiers.</p>
   <h2 class="heading settled" data-level="8" id="inline"><span class="secno">8. </span><span class="content">Inline</span><a class="self-link" href="https://api.csswg.org/bikeshed/#inline"></a></h2>
   <p><code class="highlight"><c- kr="">inline</c-></code> is also useful and will be consistent with inline variables. It will work just as with <code class="highlight"><c- k="">static</c-></code> and <code class="highlight"><c- k="">thread_local</c-></code>: The underlying object is marked <code class="highlight"><c- kr="">inline</c-></code> and any additional variables introduced as part of tuple bindings.</p>
   <h2 class="heading settled" data-level="9" id="constexpr"><span class="secno">9. </span><span class="content">Constexpr</span><a class="self-link" href="https://api.csswg.org/bikeshed/#constexpr"></a></h2>
   <p>If <code class="highlight"><c- k="">constexpr</c-></code> were applied just like <code class="highlight"><c- k="">static</c-></code> and co. are, then there would be a problem, because the current language rules make the following code ill-formed:</p>
<pre class="highlight"><code class="c++ language-c++ highlight"><c- c1=""><c- c1="">// at block scope</c-></c->
<c- k=""><c- n="">constexpr</c-></c-> <c- k=""><c- k="">auto</c-></c-><c- p=""><c- p="">[</c-></c-><c- n=""><c- n="">a</c-></c-><c- p=""><c- p="">]</c-></c-> <c- o=""><c- o="">=</c-></c-> <c- n=""><c- n="">std</c-></c-><c- o=""><c- o="">::</c-></c-><c- n=""><c- n="">tuple</c-></c-><c- o=""><c- o="">&lt;</c-></c-><c- b=""><c- b="">int</c-></c-><c- o=""><c- o="">&gt;</c-></c-><c- p=""><c- p="">(</c-></c-><c- mi=""><c- mi="">1</c-></c-><c- p=""><c- p="">);</c-></c->

<c- c1=""><c- c1="">// "equivalent" to</c-></c->

<c- k=""><c- n="">constexpr</c-></c-> <c- k=""><c- k="">auto</c-></c-> <c- n=""><c- n="">__sb</c-></c-> <c- o=""><c- o="">=</c-></c-> <c- n=""><c- n="">std</c-></c-><c- o=""><c- o="">::</c-></c-><c- n=""><c- n="">tuple</c-></c-><c- o=""><c- o="">&lt;</c-></c-><c- b=""><c- b="">int</c-></c-><c- o=""><c- o="">&gt;</c-></c-><c- p=""><c- p="">(</c-></c-><c- mi=""><c- mi="">1</c-></c-><c- p=""><c- p="">);</c-></c->

<c- k=""><c- n="">constexpr</c-></c-> <c- k=""><c- k="">const</c-></c-> <c- b=""><c- b="">int</c-></c-><c- o=""><c- o="">&amp;</c-></c-> <c- n=""><c- n="">__a</c-></c-> <c- o=""><c- o="">=</c-></c-> <c- n=""><c- n="">std</c-></c-><c- o=""><c- o="">::</c-></c-><c- n=""><c- n="">get</c-></c-><c- o=""><c- o="">&lt;</c-></c-><c- mi=""><c- mi="">0</c-></c-><c- o=""><c- o="">&gt;</c-></c-><c- p=""><c- p="">(</c-></c-><c- n=""><c- n="">__sb</c-></c-><c- p=""><c- p="">);</c-></c-> <c- c1=""><c- c1="">// ill-formed today</c-></c->
</code></pre>
   <p>A reference must be initialized by a constant expression to be a core constant expression ([expr.const]p2.11], but <code class="highlight"><c- n="">std</c-><c- o="">::</c-><c- n="">get</c-><c- o="">&lt;</c-><c- mi="">0</c-><c- o="">&gt;</c-><c- p="">(</c-><c- n="">__sb</c-><c- p="">)</c-></code> is not a constant expression due to [expr.const]p6.</p>
   <p>Richard Smith on the core reflector<sup>2</sup> suggested to relax the restriction on what constitutes a core constant expression of a reference by just requiring that the reference must be initialized by a <em>core</em> constant expression instead (see below to what this change entails).</p>
   <p>This would mean that to make structured bindings <code class="highlight"><c- k="">constexpr</c-></code>, it is necessary to apply <code class="highlight"><c- k="">constexpr</c-></code> to the underlying object, and apply <code class="highlight"><c- k="">const</c-></code> to any other variable introduced by tuple bindings.</p>
   <p>Of course, one thing to note is that it is important to guarantee that the call to <code class="highlight"><c- n="">get</c-></code> is a constant expression, because or else <code class="highlight"><c- k="">constexpr</c-></code> will act like <code class="highlight"><c- k="">const</c-></code>, which is only maybe a core constant expression.</p>
   <h2 class="heading settled" data-level="10" id="lambda-captures"><span class="secno">10. </span><span class="content">Lambda captures</span><a class="self-link" href="https://api.csswg.org/bikeshed/#lambda-captures"></a></h2>
   <p>Lambda captures aren’t currently allowed to refer to a structured binding. There seems to be no technical reason to disallow this, and indeed, the wording for allowing this just removes the restriction on capturing structured bindings.</p>
   <h2 class="heading settled" data-level="11" id="maybe-unused"><span class="secno">11. </span><span class="content">maybe-unused</span><a class="self-link" href="https://api.csswg.org/bikeshed/#maybe-unused"></a></h2>
   <p>Currently, <code class="highlight"><c- p="">[[</c-><c- n="">maybe_unused</c-><c- p="">]]</c-></code> cannot be applied to a structured binding declaration. There doesn’t seem to be a good reason to disallow this; EDG, clang and gcc all already support <code class="highlight"><c- p="">[[</c-><c- n="">maybe_unused</c-><c- p="">]]</c-></code> on structured bindings.</p>
   <h2 class="heading settled" data-level="12" id="template"><span class="secno">12. </span><span class="content">Template</span><a class="self-link" href="https://api.csswg.org/bikeshed/#template"></a></h2>
   <p>The following code is ill-formed as of C++20:</p>
<pre class="highlight"><code class="c++ language-c++ highlight"><c- k=""><c- n="">template</c-></c-> <c- o=""><c- o="">&lt;</c-></c-><c- k=""><c- k="">auto</c-></c-> <c- n=""><c- n="">Var</c-></c-><c- o=""><c- o="">&gt;</c-></c-><c- k=""><c- n="">constexpr</c-></c-> <c- k=""><c- k="">auto</c-></c-><c- p=""><c- p="">[</c-></c-><c- n=""><c- n="">X</c-></c-><c- p=""><c- p="">,</c-></c-> <c- n=""><c- n="">Y</c-></c-><c- p=""><c- p="">]</c-></c-> <c- o=""><c- o="">=</c-></c-> <c- n=""><c- n="">Var</c-></c-><c- p=""><c- p="">;</c-></c->
</code></pre>
   <p>As it uses 1) a structured binding declaration template and 2) it is constexpr. The latter is already handled. Should a structured binding be a valid <em>template-declaration</em>? The author argues that yes, it should be. It allows for code that can decompose any non-type template parameter (which now can be any class type, thanks to P0732r2).</p>
<pre class="highlight"><code class="c++ language-c++ highlight"><c- k=""><c- n="">constexpr</c-></c-> <c- n=""><c- n="">std</c-></c-><c- o=""><c- o="">::</c-></c-><c- n=""><c- n="">pair</c-></c-> <c- n=""><c- n="">Position</c-></c-><c- p=""><c- p="">(</c-></c-><c- mi=""><c- mi="">1</c-></c-><c- p=""><c- p="">,</c-></c-> <c- mi=""><c- mi="">2</c-></c-><c- p=""><c- p="">);</c-></c-><c- k=""><c- n="">constexpr</c-></c-> <c- n=""><c- n="">std</c-></c-><c- o=""><c- o="">::</c-></c-><c- n=""><c- n="">pair</c-></c-> <c- n=""><c- n="">Flag</c-></c-><c- p=""><c- p="">(</c-></c-><c- mi=""><c- mi="">4</c-></c-><c- p=""><c- p="">,</c-></c-> <c- mi=""><c- mi="">5</c-></c-><c- p=""><c- p="">);</c-></c->

<c- k=""><c- k="">if</c-></c-> <c- p=""><c- p="">(</c-></c-><c- n=""><c- n="">X</c-></c-><c- o=""><c- o="">&lt;</c-></c-><c- n=""><c- n="">Position</c-></c-><c- o=""><c- o="">&gt;</c-></c-> <c- o=""><c- o="">==</c-></c-> <c- n=""><c- n="">Y</c-></c-><c- o=""><c- o="">&lt;</c-></c-><c- n=""><c- n="">Flag</c-></c-><c- o=""><c- o="">&gt;</c-></c-><c- p=""><c- p="">)</c-></c->
 <c- p=""><c- p="">;</c-></c-> <c- c1=""><c- c1="">// almost there!</c-></c->
</code></pre>
   <p>Note: The author doesn’t know how to change the standard to allow this completely.</p>
   <h2 class="heading settled" data-level="13" id="impact"><span class="secno">13. </span><span class="content">Impact</span><a class="self-link" href="https://api.csswg.org/bikeshed/#impact"></a></h2>
   <p>This proposal only makes ill-formed or code with unspecified behavior well-formed in relation to structured bindings.</p>
   <p>Due to <code class="highlight"><c- k="">constexpr</c-></code> tuple binding variables requiring a change to what constitutes a core constant expression, ill-formed code today will become well-formed:</p>
<pre class="highlight"><code class="c++ language-c++ highlight"><c- c1=""><c- c1="">// at block scope</c-></c->
<c- k=""><c- k="">const</c-></c-> <c- b=""><c- b="">int</c-></c-> <c- n=""><c- n="">var</c-></c-> <c- o=""><c- o="">=</c-></c-> <c- mi=""><c- mi="">1</c-></c-><c- p=""><c- p="">;</c-></c->

<c- k=""><c- k="">const</c-></c-> <c- b=""><c- b="">int</c-></c-><c- o=""><c- o="">&amp;</c-></c-> <c- n=""><c- n="">ref</c-></c-> <c- o=""><c- o="">=</c-></c-> <c- n=""><c- n="">var</c-></c-><c- p=""><c- p="">;</c-></c->

<c- k=""><c- n="">static_assert</c-></c-><c- p=""><c- p="">(</c-></c-><c- n=""><c- n="">ref</c-></c-> <c- o=""><c- o="">==</c-></c-> <c- mi=""><c- mi="">1</c-></c-><c- p=""><c- p="">);</c-></c-> <c- c1=""><c- c1="">// ill-formed today, well-formed with this proposal</c-></c->
</code></pre>
   <h2 class="heading settled" data-level="14" id="wording"><span class="secno">14. </span><span class="content">Proposed wording</span><a class="self-link" href="https://api.csswg.org/bikeshed/#wording"></a></h2>
   <p>Change [expr.prim.lambda.capture]p8 (7.5.5.2) as follows:</p>
   <blockquote>
     If a <em>lambda-expression</em> explicitly captures an entity that is not odr-usable 
    <del>or captures a structured binding (explicitly or implicitly)</del>
    , the program is ill-formed. 
   </blockquote>
   <p>Change [expr.prim.lambda.capture]p12 (7.5.5.2) as follows:</p>
   <blockquote>
     A bit-field
    <ins>, a structured binding,</ins>
     or a member of an anonymous union shall not be captured by reference. 
   </blockquote>
   <p>Change [dcl.struct.bind]p1 (9.5) as follows:</p>
   <blockquote>
     Let <em>cv</em> denote the <em>cv-qualifiers</em> in the <em>decl-specifier-seq</em> 
    <ins>and <em>S</em> consist of the <em>storage-class-specifier</em>s of the <em>decl-specifier-seq</em> (if any)</ins>
    . [...]
If the <em>assignment-expression</em> in the <em>initializer</em> has array type <code class="highlight"><c- n="">A</c-></code> and no <em>ref-qualifier</em> is present, <code class="highlight"><c- n="">e</c-></code> 
    <del>has type <em>cv</em> <code class="highlight"><c- n="">A</c-></code></del>
    <ins>is defined by</ins>
    <p style="text-indent: 50px">
     <ins><em>attribute-specifier-seq</em><sub>opt</sub> <em>S</em> <em>cv</em> <code class="highlight"><c- n="">A</c-></code> <em>e</em> ;</ins>
    </p>
    <p>and each element is copy-initialized or direct-initialized from the corresponding element of the <em>assignment-expression</em> as specified by the form of the <em>initializer</em>.</p>
   </blockquote>
   <p>Change [dcl.struct.bind]p4 (9.5) as follows:</p>
   <blockquote>
     Given the type T<sub>i</sub> designated by <code class="highlight"><c- n="">std</c-><c- o="">::</c-><c- n="">tuple_element</c-><c- o="">&lt;</c-><c- n="">i</c-><c- p="">,</c-> <c- n="">E</c-><c- o="">&gt;::</c-><c- n="">type</c-></code> 
    <ins>
     and the type U<sub>i</sub> designated by either T<sub>i</sub>&amp; or T<sub>i</sub>&amp;&amp;, where 
     <ins>U<sub>i</sub></ins>
      is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise
    </ins>
    , variables are introduced with unique names r<sub>i</sub> 
    <ins>as follows:</ins>
    <p style="text-indent: 50px">
     <ins><em>S</em> U<sub>i</sub> r<sub>i</sub> = <em>initializer</em>; </ins>
    </p>
    <del>of type "reference to T<sub>i</sub>" initialized with the initializer, where the referernce is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise.</del>
   </blockquote>
   <p>Change [dcl.dcl]p8 (10) completely as follows:</p>
   <blockquote>
    <ins>If</ins>
     the <em>decl-specifier-seq</em> 
    <del>shall</del>
     contain
    <ins>s any <em>decl-specifier</em> other than static, thread_local,</ins>
    <del>only the <em>type-specifier</em></del>
     <code class="highlight"><c- k="">auto</c-></code>
    <ins>,</ins>
    <del>and</del>
    <ins>or</ins>
     <em>cv-qualifier</em>s
    <ins>, the program is ill-formed.</ins>
   </blockquote>
   <p>Change [dcl.stc]p3 (10.1.1) as follows:</p>
   <blockquote>
     The <code class="highlight"><c- k="">thread_local</c-></code> specifier indicates that the named entity has thread storage duration. It shall be applied only to the names of variables of namespace or block scope
    <ins>, to structured binding declarations ([dcl.struct.bind]),</ins>
     and to the names of static data members. 
   </blockquote>
   <p>Change [dcl.stc]p4 (10.1.1) as follows:</p>
   <blockquote>
     The <code class="highlight"><c- k="">static</c-></code> specifier can be applied only to names of variables and functions
    <ins>, to structured binding declarations ([dcl.struct.bind]),</ins>
     and to anonymous unions. 
   </blockquote>
   <h2 class="heading settled" data-level="15" id="acknowledgements"><span class="secno">15. </span><span class="content">Acknowledgements</span><a class="self-link" href="https://api.csswg.org/bikeshed/#acknowledgements"></a></h2>
   <p>Thanks to Jens for reviewing the first draft of the proposed wording.</p>
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  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></body></html>