<!doctype html><html lang="en">
 <head>
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  <title>P1102R1: Down with &lt;code>()&lt;/code>!</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

  /* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1102R1<br>Down with <code class="highlight"><c- p>()</c-></code>!</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2020-11-16">2020-11-16</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P1102R1">http://wg21.link/P1102R1</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:achristensen@apple.com">Alex Christensen</a> (<span class="p-org org">Apple</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:cxx@jfbastien.com">JF Bastien</a> (<span class="p-org org">Toyota Research Institute—Advanced Development</span>)
     <dt>Audience:
     <dd>CWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/achristensen07/papers/blob/master/source/P1102r1.bs">https://github.com/achristensen07/papers/blob/master/source/P1102r1.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 proposal for removing unnecessary <code class="highlight"><c- p>()</c-></code>’s from C++ lambdas.</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="#history"><span class="secno">1</span> <span class="content">Revision History</span></a>
    <li><a href="#intro"><span class="secno">2</span> <span class="content">Introduction and motivation</span></a>
    <li><a href="#impact"><span class="secno">3</span> <span class="content">Impact</span></a>
    <li><a href="#word"><span class="secno">4</span> <span class="content">Wording</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>
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  <main>
   <h2 class="heading settled" data-level="1" id="history"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#history"></a></h2>
   <p>EWG discussed this change as <a data-link-type="biblio" href="#biblio-ewg135">[EWG135]</a> in <a href="http://wiki.edg.com/bin/view/Wg21lenexa/EWGIssuesResolutionMinutes">Lenexa</a> and
voted 15 to 1 on forwarding to core. It became <a data-link-type="biblio" href="#biblio-cwg2121">[CWG2121]</a>, discussed in <a href="http://wiki.edg.com/bin/view/Wg21kona2015/CoreWorkingGroup#CWG_2121_More_flexible_lambda_sy">Kona</a> and needed someone to volunteer wording.  This paper presents text that implements that decision, for consideration.</p>
   <p><a data-link-type="biblio" href="#biblio-p1102r0">[P1102R0]</a> was published in June 2018. It was discussed on the EWG reflector in
June 2018, Nina Ranns provided feedback, and EWG chair agreed that the paper
should move to CWG directly given previous polls.</p>
   <p>P1102R1 responds to feedback about ambiguous requires-clauses from Hubert Tong.</p>
   <h2 class="heading settled" data-level="2" id="intro"><span class="secno">2. </span><span class="content">Introduction and motivation</span><a class="self-link" href="#intro"></a></h2>
   <p>Currently, C++ lambdas with no parameters do not require a parameter declaration
clause. The specification even contains this language in [<strong>expr.prim.lambda</strong>]
section 8.4.5 ❡4:</p>
   <blockquote>
    <p>If a lambda-expression does not include a lambda-declarator, it is as if the
lambda-declarator were <code class="highlight"><c- p>()</c-></code>.</p>
   </blockquote>
   <p>This allows us to omit the unused <code class="highlight"><c- p>()</c-></code> in simple lambdas such as this:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>s1</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->
<c- k>auto</c-> <c- n>withParen</c-> <c- o>=</c-> <c- p>[</c-><c- n>s1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>s1</c-><c- p>)]</c-> <c- p>()</c-> <c- p>{</c->
  <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>s1</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c-> 
<c- p>};</c->

<c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>s2</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->
<c- k>auto</c-> <c- n>noSean</c-> <c- o>=</c-> <c- p>[</c-><c- n>s2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>s2</c-><c- p>)]</c-> <c- p>{</c-> <c- c1>// Note no syntax error.</c->
  <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>s2</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c-> 
<c- p>};</c->
</pre>
   <p>These particular lambdas have ownership of the strings, so they ought to be able
to mutate it, but <code class="highlight"><c- n>s1</c-></code> and <code class="highlight"><c- n>s2</c-></code> are const (because the <code class="highlight"><c- k>const</c-></code> operator is
declared <code class="highlight"><c- k>const</c-></code> by default) so we need to add the <code class="highlight"><c- k>mutable</c-></code> keyword:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>s1</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->
<c- k>auto</c-> <c- n>withParen</c-> <c- o>=</c-> <c- p>[</c-><c- n>s1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>s1</c-><c- p>)]</c-> <c- p>()</c-> <c- k>mutable</c-> <c- p>{</c->
  <c- n>s1</c-> <c- o>+=</c-> <c- s>"d"</c-><c- p>;</c->
  <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>s1</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c-> 
<c- p>};</c->

<c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>s2</c-> <c- o>=</c-> <c- s>"abc"</c-><c- p>;</c->
<c- k>auto</c-> <c- n>noSean</c-> <c- o>=</c-> <c- p>[</c-><c- n>s2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>s2</c-><c- p>)]</c-> <c- k>mutable</c-> <c- p>{</c-> <c- c1>// Currently a syntax error.</c->
  <c- n>s2</c-> <c- o>+=</c-> <c- s>"d"</c-><c- p>;</c->
  <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>s2</c-> <c- o>&lt;&lt;</c-> <c- sc>'\n'</c-><c- p>;</c-> 
<c- p>};</c->
</pre>
   <p>Confusingly, the current Standard requires the empty parens when using the <code class="highlight"><c- k>mutable</c-></code> keyword. This rule is unintuitive, causes common syntax errors, and
clutters our code. When compiling with clang, we even get a syntax error that
indicates the compiler knows exactly what is going on:</p>
<pre class="highlight">example.cpp:11:54: error: lambda requires <c- t>'()'</c-> before <c- t>'mutable'</c->
auto <c- g>noSean</c-> <c- o>=</c-> <c- o>[</c-><c- g>s2</c-> <c- o>=</c-> std::move<c- o>(</c->s2<c- o>)]</c-> mutable <c- o>{</c-> // Currently a syntax error.
                                   ^
                                   <c- o>()</c-> 
<c- m>1</c-> error generated.
</pre>
   <p>This proposal would make these parentheses unnecessary like they were before we
added <code class="highlight"><c- k>mutable</c-></code>. This will apply to:</p>
   <ul>
    <li data-md>
     <p>lambda template parameters</p>
    <li data-md>
     <p><code class="highlight"><c- k>constexpr</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- k>mutable</c-></code></p>
    <li data-md>
     <p>Exception specifications and <code class="highlight"><c- k>noexcept</c-></code></p>
    <li data-md>
     <p>attributes</p>
    <li data-md>
     <p>trailing return types</p>
    <li data-md>
     <p><code class="highlight"><c- k>requires</c-></code></p>
   </ul>
   <h2 class="heading settled" data-level="3" id="impact"><span class="secno">3. </span><span class="content">Impact</span><a class="self-link" href="#impact"></a></h2>
   <p>This change will not break existing code.</p>
   <h2 class="heading settled" data-level="4" id="word"><span class="secno">4. </span><span class="content">Wording</span><a class="self-link" href="#word"></a></h2>
   <p>Modify Lambda expressions [<strong>expr.prim.lambda</strong>] as follows:</p>
   <blockquote>
     <i> <indent1>lambda-expression :<br></indent1> <indent2>lambda-introducer <ins>lambda-parameter-list<sub>opt</sub></ins> <del>lambda-declarator<sub>opt</sub></del> <ins>lambda-specifiers</ins><br></indent2> <indent3>compound-statement<br></indent3> <indent2>lambda-introducer &lt; template-parameter-list > requires-clause<sub>opt</sub><br></indent2> <indent3><ins>lambda-parameter-list-and-specifiers<sub>opt</sub></ins><del>lambda-declarator<sub>opt</sub></del> compound-statement<br></indent3> <indent1>lambda-introducer :<br></indent1> <indent2>[ lambda-capture<sub>opt</sub> ]<br></indent2> <indent1><ins>lambda-parameter-list :<br></ins></indent1> <indent2><ins>( parameter-declaration-clause )</ins><br></indent2> <indent1><ins>lambda-parameter-list-and-specifiers :<br></ins></indent1> <indent2><ins>lambda-parameter-list lambda-specifiers</ins><br></indent2> <indent1><del>lambda-declarator</del> <ins>lambda-specifiers</ins> :<br></indent1> <indent2><del>( parameter-declaration-clause )</del> decl-specifier-seq<sub>opt</sub><br></indent2> <indent3>noexcept-specifier<sub>opt</sub> attribute-specifier-seq<sub>opt</sub> trailing-return-type<sub>opt</sub> requires-clause<sub>opt</sub><br></indent3> </i>
    <p><i></i></p>
   </blockquote>
   <p>Because parts of <em>lambda-declarator</em> have been renamed to <em>lambda-parameter-list</em> and <em>lambda-specifiers</em>, each reference to <em>lambda-declarator</em> must be updated.<br></p>
   <p>Modify Function parameter scope [<strong>basic.scope.param</strong>] ❡1:</p>
   <blockquote>
     A function parameter (including one appearing in a 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-parameter-list</em></ins>
    ) or
  function-local predefined variable (9.5) has function parameter scope. [...] 
   </blockquote>
   <p>Modify Lambda expressions [<strong>expr.prim.lambda.general</strong>] ❡3:</p>
   <blockquote>
     In the <em>decl-specifier-seq</em> of the 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-specifiers</em></ins>
    ,
  each <em>decl-specifier</em> shall be one of <code class="highlight"><c- k>mutable</c-></code>, <code class="highlight"><c- k>constexpr</c-></code>, or <code class="highlight"><c- k>consteval</c-></code>. 
   </blockquote>
   <p>Modify Lambda expressions [<strong>expr.prim.lambda.general</strong>] ❡4:</p>
   <blockquote>
     If a <em>lambda-expression</em> does not include a 
    <del><em>lambda-declarator</em></del>
    <ins>lambda-parameter-list</ins>
    ,
  it is as if the 
    <del><em>lambda-declarator</em></del>
    <ins>lambda-parameter-list</ins>
     were <code class="highlight"><c- p>()</c-></code>. The lambda return type is <code class="highlight"><c- k>auto</c-></code>, which is replaced by the type specified by the <em>trailing-return-type</em> if
  provided and/or deduced from <code class="highlight"><c- k>return</c-></code> statements as described in 9.2.9.6. 
   </blockquote>
   <p>Modify Closure types [<strong>expr.prim.lambda.closure</strong>] ❡2 Note 1:</p>
   <blockquote>
     This determines the set of namespaces and classes associated with the closure type (6.5.3).
  The parameter types of a 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-parameter-list</em></ins>
     do not affect these associated namespaces and classes. 
   </blockquote>
   <p>Modify Closure types [<strong>expr.prim.lambda.closure</strong>] ❡3:</p>
   <blockquote>
     [...] The trailing <em>requires-clause</em> of
  the function call operator or operator template is the <em>requires-clause</em> following the 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-parameter-list</em></ins>
    , if any. 
   </blockquote>
   <p>Modify Closure types [<strong>expr.prim.lambda.closure</strong>] ❡4:</p>
   <blockquote>
     [...] An <em>attribute-specifier-seq</em> in a 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-specifiers</em></ins>
     appertains to the type of the corresponding function call operator or operator template. [...] 
   </blockquote>
   <p>Modify Closure types [<strong>expr.prim.lambda.closure</strong>] ❡4 Note 3:</p>
   <blockquote>
     Names referenced in the 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-parameter-list</em></ins>
     are looked up in the context in which the <em>lambda-expression</em> appears. 
   </blockquote>
   <p>Modify Captures [<strong>expr.prim.lambda.capture</strong>] ❡5:</p>
   <blockquote>
     If an <em>identifier</em> in a <em>simple-capture</em> appears as the <em>declarator-id</em> of a parameter of the 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-parameter-list</em></ins>
    ’s <em>parameter-declaration-clause</em>, the program is ill-formed. 
   </blockquote>
   <p>Modify Default arguments [<strong>dcl.fct.default</strong>] ❡3:</p>
   <blockquote>
     A default argument shall be specified only in the <em>parameter-declaration-clause</em> of a function declaration
  or 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-parameter-list</em></ins>
     or in a <em>template-parameter</em> (13.2); in the latter case, the <em>initializer-clause</em> shall be an <em>assignment-expression</em>. [...] 
   </blockquote>
   <p>Modify Name resolution [<strong>temp.res.general</strong>] ❡5.2.5:</p>
   <blockquote>
     <em>parameter-declaration</em> in a 
    <del><em>lambda-declarator</em></del>
    <ins><em>lambda-parameter-list</em></ins>
     or <em>requirement-parameter-list</em>, unless that <em>parameter-declaration</em> appears in a default argument, or 
   </blockquote>
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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-cwg2121">[CWG2121]
   <dd>EWG. <a href="https://wg21.link/cwg2121">More flexible lambda syntax</a>. 6 May 2015. drafting. URL: <a href="https://wg21.link/cwg2121">https://wg21.link/cwg2121</a>
   <dt id="biblio-ewg135">[EWG135]
   <dd>Herb Sutter. <a href="https://wg21.link/ewg135">[tiny] Mutable is part of a lambda-declarator, so when a lambda is mutable, the parentheses aren't optional</a>. New. URL: <a href="https://wg21.link/ewg135">https://wg21.link/ewg135</a>
   <dt id="biblio-p1102r0">[P1102R0]
   <dd>Alex Christensen, JF Bastien. <a href="https://wg21.link/p1102r0">Down with ()!</a>. 20 June 2018. URL: <a href="https://wg21.link/p1102r0">https://wg21.link/p1102r0</a>
  </dl>