<!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>P0288R4: any_invocable</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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/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


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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">P0288R4<br>any_invocable</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-06-16">2019-06-16</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p0288r4">http://wg21.link/p0288r4</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Ryan McDougall &lt;mcdougall.ryan@gmail.com></span>
     <dd>
      <dd class="editor p-author h-card vcard"><span class="p-name fn">Matt Calabrese &lt;metaprogrammingtheworld@gmail.com></span>
     <dt>Audience:
     <dd>LWG, LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <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>
    This paper proposes a conservative, move-only equivalent of 
    <tt>std::function</tt>
    .
   </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">Brief History</span></a>
    <li><a href="#Overview"><span class="secno">2</span> <span class="content">Overview</span></a>
    <li><a href="#Specification"><span class="secno">3</span> <span class="content">Specification</span></a>
    <li>
     <a href="#SD2018"><span class="secno">4</span> <span class="content">Polls from LEWG San Diego Review (2018)</span></a>
     <ol class="toc">
      <li><a href="#support-func-func-const-func"><span class="secno">4.1</span> <span class="content">Support func(), func() const, func() &amp;&amp;</span></a>
      <li><a href="#support-func--only"><span class="secno">4.2</span> <span class="content">Support func() &amp;&amp; only</span></a>
      <li><a href="#remove-target-target_type"><span class="secno">4.3</span> <span class="content">Remove target/target_type</span></a>
      <li><a href="#require-more-stuff-noexcept-const"><span class="secno">4.4</span> <span class="content">Require more stuff (noexcept, const&amp;&amp;, ...)</span></a>
      <li><a href="#name-options"><span class="secno">4.5</span> <span class="content">Name Options</span></a>
     </ol>
    <li>
     <a href="#K2019"><span class="secno">5</span> <span class="content">Polls from LEWG Kona Review (2019)</span></a>
     <ol class="toc">
      <li><a href="#we-want-to-spend-time-on-this-now-in-order-to-include-it-in-c20"><span class="secno">5.1</span> <span class="content">We want to spend time on this now in order to include it in C++20</span></a>
      <li><a href="#add-support-for-func-const-and-func"><span class="secno">5.2</span> <span class="content">Add support for func() const&amp; and func()&amp;</span></a>
      <li><a href="#add-support-for-func-noexcept-x-all-of-the-above"><span class="secno">5.3</span> <span class="content">Add support for func() noexcept (x all of the above)</span></a>
      <li><a href="#include-the-option-for-ctad"><span class="secno">5.4</span> <span class="content">Include the option for CTAD</span></a>
      <li><a href="#name-callable-vs-any-invocable"><span class="secno">5.5</span> <span class="content">Name: callable vs any invocable</span></a>
      <li><a href="#any_invocable-vs-invocable"><span class="secno">5.6</span> <span class="content">any_invocable vs invocable</span></a>
      <li><a href="#header-choice"><span class="secno">5.7</span> <span class="content">Header choice</span></a>
      <li><a href="#can-get-stdfunction-from-any_invocable"><span class="secno">5.8</span> <span class="content">Can get std::function from &lt;any_invocable></span></a>
      <li><a href="#can-get-stdfunction-from-invocable"><span class="secno">5.9</span> <span class="content">Can get std::function from &lt;invocable></span></a>
      <li><a href="#remove-the-null-check-in-the-call-operator-and-throwing-of-bad_function_call"><span class="secno">5.10</span> <span class="content">Remove the null-check in the call operator and throwing of bad_function_call</span></a>
      <li><a href="#remove-the-null-check-in-constructors-that-are-not-nullptr_t"><span class="secno">5.11</span> <span class="content">Remove the null-check in constructors that are not nullptr_t</span></a>
      <li><a href="#perfect-forwarding-for-converting-constructor-instead-of-by-value"><span class="secno">5.12</span> <span class="content">Perfect forwarding for converting constructor instead of by-value</span></a>
      <li><a href="#forward-to-lwg-for-c20"><span class="secno">5.13</span> <span class="content">Forward to LWG for C++20</span></a>
     </ol>
    <li><a href="#Impl"><span class="secno">6</span> <span class="content">Implementation Experience</span></a>
    <li><a href="#References"><span class="secno">7</span> <span class="content">References</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="History"><span class="secno">1. </span><span class="content">Brief History</span><a class="self-link" href="#History"></a></h2>
   <p>This paper started as a proposal by David Krauss, N4543[1], from 2015 and there has
been an open issue in the LEWG bugzilla requesting such a facility since 2014[2].</p>
   <p>
    Since then, the paper has gone through 4 revisions and has been considered in
small groups in LEWG multiple times. Gradual feedback has led to the conservative
proposal seen here. The most-recent draft prior to this was a late-paper written
and presented by Ryan McDougall in LEWGI in San Diego[3]. It included multiple
references to implementations of move-only functions and made a strong case for
the importance of a move-only form of 
    <tt>std::function</tt>
    .
   </p>
   <p>Feebdack given was encouragement for targetting C++20.</p>
   <p>An updated version of that paper was presented on Saturday at the end of the
San Diego meeting. Poll results from that presentation are presented after the
overview in this document.</p>
   <p>The revision was presented in Kona, receiving additional polls and feedback, and
was to be forwarded to LWG pending updates reflecting additional poll results. Those
changes have been applied to the wording in this paper. Polls from the LEWG Kona
review are also provided in this document.</p>
   <h2 class="heading settled" data-level="2" id="Overview"><span class="secno">2. </span><span class="content">Overview</span><a class="self-link" href="#Overview"></a></h2>
   <p>
    This conservative 
    <tt>any_invocable</tt>
     is intended to be the same as 
    <tt>std::function</tt>
    ,
with the exceptions of the following:
   </p>
   <ol>
    <li data-md>
     <p>It is move-only.</p>
    <li data-md>
     <p>
      It does not have the const-correctness bug of 
      <tt>std::function</tt>
       detailed in n4348.[4]
     </p>
    <li data-md>
     <p>It provides support for cv/ref/noexcept qualified function types.</p>
    <li data-md>
     <p>
      It does not have the 
      <tt>target_type</tt>
       and 
      <tt>target</tt>
       accessors (direction requested by users and implementors).
     </p>
    <li data-md>
     <p>Invocation has strong preconditions.</p>
   </ol>
   <h2 class="heading settled" data-level="3" id="Specification"><span class="secno">3. </span><span class="content">Specification</span><a class="self-link" href="#Specification"></a></h2>
   <p>The following is relative to N4810.[5]</p>
   <p>Add &lt;any_invocable> to Table 19 — C++ library headers</p>
   <blockquote>
<pre>Header &lt;any_invocable> synopsis [inv.syn]

<tt>namespace std {
  template&lt;class Sig> class any_invocable; // not defined

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...)> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) &amp;> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) &amp;&amp;> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) const> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) const &amp;> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) noexcept> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) &amp; noexcept> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) &amp;&amp; noexcept> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) const noexcept> { <em>see below</em> };

  template&lt;class R, class... ArgTypes>
    class any_invocable&lt;R(ArgTypes...) const &amp; noexcept> { <em>see below</em> };
}</tt>

1 The class definitions for the partial specializations of <tt>any_invocable</tt> mentioned
  above are specified using exposition-only macros <tt>QUAL_OPT</tt>, <tt>QUAL_OPT_REF</tt>,
  and <tt>NOEXCEPT_VAL</tt>, as described below.

2 For each partial specialization of <tt>any_invocable</tt> mentioned above,
  let <tt>QUAL_OPT</tt> be an exposition-only macro defined to be a textual representation
  of the <i>cv</i> and reference qualifiers of the function type parameter of <tt>any_invocable</tt>.

3 [<em>Note:</em>
  For <tt>any_invocable&lt;void() const></tt>
    - <tt>QUAL_OPT</tt> is <tt>const</tt>
    
  For <tt>any_invocable&lt;void() &amp;&amp;></tt>
    - <tt>QUAL_OPT</tt> is <tt>&amp;&amp;</tt>

  For <tt>any_invocable&lt;void()></tt>
    - <tt>QUAL_OPT</tt> is
  ]

4 For each partial specialization of <tt>any_invocable</tt> mentioned above,
let <tt>NOEXCEPT_VAL</tt> be an exposition-only macro defined to be <tt>true</tt> if the function
type parameter of <tt>any_invocable</tt> is noexcept, otherwise <tt>false</tt>.

5 [<em>Note:</em>
  For <tt>any_invocable&lt;void() noexcept></tt>
    - <tt>NOEXCEPT_VAL</tt> is <tt>true</tt>
    
  For <tt>any_invocable&lt;void()></tt>
    - <tt>NOEXCEPT_VAL</tt> is <tt>false</tt>
  ]

6 For each partial specialization of <tt>any_invocable</tt> mentioned above, let
  <tt>QUAL_OPT_REF</tt> be an exposition-only macro defined in the following manner:
      - If the function type parameter of <tt>any_invocable</tt> is reference
        qualified, let <tt>QUAL_OPT_REF</tt> be defined as <tt>QUAL_OPT</tt>.

      - Otherwise, let <tt>QUAL_OPT_REF</tt> be defined as <tt>QUAL_OPT&amp;</tt>.

7 [<em>Note:</em>
  For <tt>any_invocable&lt;void() const></tt>
    - <tt>QUAL_OPT_REF</tt> is <tt>const &amp;</tt>

  For <tt>any_invocable&lt;void() &amp;&amp;></tt>
    - <tt>QUAL_OPT_REF</tt> is <tt>&amp;&amp;</tt>

  For <tt>any_invocable&lt;void()></tt>
    - <tt>QUAL_OPT_REF</tt> is <tt>&amp;</tt>
  ]

8 For each partial specialization of <tt>any_invocable</tt> mentioned above, the following is
  a synopsis of that partial specialization and its associated non-member functions.

<tt>namespace std {

  template&lt;class R, class... ArgTypes>
  class any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)> {
  public:
    using result_type = R;

    // 20.14.16.2.1, construct/copy/destroy
    any_invocable() noexcept;
    any_invocable(nullptr_t) noexcept;
    any_invocable(any_invocable&amp;&amp;) noexcept;
    template&lt;class F> any_invocable(F&amp;&amp;);

    template&lt;class T, class... Args>
      explicit any_invocable(in_place_type_t&lt;T>, Args&amp;&amp;...);
    template&lt;class T, class U, class... Args>
      explicit any_invocable(in_place_type_t&lt;T>, initializer_list&lt;U>, Args&amp;&amp;...);
    
    any_invocable&amp; operator=(any_invocable&amp;&amp;) noexcept;
    any_invocable&amp; operator=(nullptr_t) noexcept;
    template&lt;class F> any_invocable&amp; operator=(F&amp;&amp;);
    template&lt;class F> any_invocable&amp; operator=(reference_wrapper&lt;F>) noexcept;

    ~any_invocable();

    // 20.14.16.2.2, any_invocable modifiers
    void swap(any_invocable&amp;) noexcept;

    // 20.14.16.2.3, any_invocable capacity
    explicit operator bool() const noexcept;

    // 20.14.16.2.4, any_invocable invocation
    R operator()(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL);
  };

  / 20.14.16.2.6, Null pointer comparisons
  template&lt;class R, class... ArgTypes>
    bool operator==(const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp;, nullptr_t) noexcept;

  template&lt;class R, class... ArgTypes>
    bool operator==(nullptr_t, const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp;) noexcept;

  template&lt;class R, class... ArgTypes>
    bool operator!=(const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp;, nullptr_t) noexcept;

  template&lt;class R, class... ArgTypes>
    bool operator!=(nullptr_t, const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp;) noexcept;

  // 20.14.16.2.7, specialized algorithms
  template&lt;class R, class... ArgTypes>
    void swap(any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp;,
        any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp;) noexcept;
}</tt>

9 The <tt>any_invocable</tt> class template provides polymorphic wrappers that generalize
  the notion of a callable object (20.14.2). These wrappers can store, move, and call
  arbitrary callable objects (20.14.2), given a call signature (20.14.2), allowing
  functions to be first-class objects.

10 A callable type (20.14.2) <tt>F</tt> is Callable for argument types <tt>ArgTypes</tt>
   and return type <tt>R</tt> if the expression
   <tt>INVOKE&lt;R>(declval&lt;F>(), declval&lt;ArgTypes>()...)</tt>,
   considered as an unevaluated operand (7.2), is well-formed (20.14.3).

SECTION.1  Constructors and destructor [inv.wrap.func.con]
any_invocable() noexcept;

1 <em>Ensures:</em> !*this.

<tt>any_invocable(nullptr_t) noexcept;</tt>

2 <em>Ensures:</em> <tt>!*this</tt>.

<tt>any_invocable(any_invocable&amp;&amp; f) noexcept;</tt>

3 <em>Ensures:</em> If <tt>!f</tt>, <tt>*this</tt> has no target; otherwise, the target of <tt>*this</tt>
  is equivalent to the target of <tt>f</tt> before the construction, and <tt>f</tt> is in a valid state
  with an unspecified value.

4 [<em>Note:</em> Implementations should avoid the use of dynamically allocated memory for
   small callable objects,for example, wheref’s target is an object holding only a
   pointer or reference to an object and a member function pointer.— <em>end note</em>]

<tt>template&lt;class F> any_invocable(F&amp;&amp; f);</tt>

5 <em>Requires:</em> <tt>decay&lt;F></tt> shall be Cpp17MoveConstructible.

6 <em>Constraints:</em>

(6.1) — <tt>decay_t&lt;F></tt> is not the same type as <tt>any_invocable</tt> and

(6.2) — <tt>decay_t&lt;F></tt> is not a specialization of <tt>in_place_type_t</tt> and

(6.3) — <tt>is_constructible_v&lt;decay_t&lt;F>, F></tt> is <tt>true</tt> and

(6.4) — <tt>is_­move_­constructible_­v&lt;decay_t&lt;F>></tt> is <tt>true</tt> and

(6.5) — <tt>F QUAL_OPT_REF</tt> is Callable (20.14.16.2) for argument types
        <tt>ArgTypes...</tt>, return type <tt>R</tt>, and

(6.6) — <tt>!NOEXCEPT_VAL || noexcept(INVOKE&lt;R>(declval&lt;F QUAL_OPT_REF>(), declval&lt;ArgTypes>()...))</tt> is <tt>true</tt>.

7 <em>Ensures:</em> <tt>!*this</tt> if any of the following hold:

—(7.1) <tt>f</tt> is a null function pointer value.

—(7.2) <tt>f</tt> is a null member pointer value.

—(7.3) <tt>F</tt> is an instance of the <tt>any_invocable</tt> class template, and <tt>!f</tt>. 

8 Otherwise, <tt>*this</tt> targets a copy of <tt>f</tt> initialized with <tt>
  std::forward&lt;F>(f)</tt>. [<em>Note</em>: Implementations should avoid the use of
  dynamically allocated memory for small callable objects, for example, where <tt>f</tt> is
  an object holding only a pointer or reference to an object and a member function
  pointer.— <em>end note</em>]

9 <em>Throws:</em> Shall not throw exceptions when <tt>f</tt> is a function pointer or a
  <tt>reference_wrapper&lt;T></tt> for some <tt>T</tt>. Otherwise, may throw <tt>bad_alloc</tt>
  or any exception thrown by the expression <tt>decay_t&lt;F>(std::forward&lt;F>(f))</tt>.

<tt>template&lt;class T, class... Args>
  explicit any_invocable(in_place_type_t&lt;T>, Args&amp;&amp;... args);</tt>

10 Let <tt>VT</tt> be <tt>decay_t&lt;T></tt>.

11 <em>Requires:</em> <tt>VT</tt> shall be Cpp17MoveConstructible.

12 <em>Constraints:</em>

(12.1) — <tt>is_constructible_v&lt;VT, Args...></tt> is <tt>true</tt> and

(12.2) — <tt>is_­move_­constructible_­v&lt;VT></tt> is <tt>true</tt> and

(12.3) — <tt>VT QUAL_OPT_REF</tt> is Callable (20.14.16.2) for argument types
         <tt>ArgTypes...</tt>, return type <tt>R</tt>, and

(12.4) — <tt>!NOEXCEPT_VAL || noexcept(INVOKE&lt;R>(declval&lt;F QUAL_OPT_REF>(), declval&lt;ArgTypes>()...))</tt> is <tt>true</tt>.

13 <em>Ensures:</em> <tt>*this</tt> targets an object of type <tt>VT</tt> initialized
   with <tt>std::forward&lt;Args>(args)...</tt> by <i>direct-non-list-initialization</i>. [<em>Note</em>:
   Implementations should avoid the use of dynamically allocated memory for small
   callable objects, for example, where <tt>f</tt> is an object holding only a pointer or 
   reference to an object and a member function pointer.— <em>end note</em>]

<tt>template&lt;class T, class U, class... Args>
  explicit any_invocable(in_place_type_t&lt;T>, initializer_list&lt;U> ilist, Args&amp;&amp;... args);</tt>

14 Let VT be decay_t&lt;T>.

15 <em>Requires:</em> <tt>VT</tt> shall be Cpp17MoveConstructible.

16 <em>Constraints:</em>

(16.1) — <tt>is_constructible_v&lt;VT, initializer_list&lt;U>&amp;, Args...></tt> is <tt>true</tt> and

(16.2) — <tt>is_­move_­constructible_­v&lt;VT></tt> is <tt>true</tt> and

(16.3) — <tt>VT QUAL_OPT_REF</tt> is Callable (20.14.16.2) for argument types
         <tt>ArgTypes...</tt>, return type <tt>R</tt>, and

(16.4) — <tt>!NOEXCEPT_VAL || noexcept(INVOKE&lt;R>(declval&lt;F QUAL_OPT_REF>(), declval&lt;ArgTypes>()...))</tt> is <tt>true</tt>.

17 <em>Ensures:</em> <tt>*this</tt> targets an object of type <tt>VT</tt> initialized
   with <tt>ilist, std::forward&lt;ArgTypes>(args)...</tt> by <i>direct-non-list-initialization</i>. [<em>Note</em>:
   Implementations should avoid the use of dynamically allocated memory for small
   callable objects, for example, where <tt>f</tt> is an object holding only a pointer or
   reference to an object and a member function pointer.— <em>end note</em>]

<tt>any_invocable&amp; operator=(any_invocable&amp;&amp; f) noexcept;</tt>

18 <em>Effects:</em> Replaces the target of <tt>*this</tt> with the target of <tt>f</tt>.

19 <em>Returns:</em> <tt>*this</tt>.

<tt>any_invocable&amp; operator=(nullptr_t) noexcept;</tt>

20 <em>Effects:</em> If <tt>*this != nullptr</tt>, destroys the target of <tt>this</tt>.

21 <em>Ensures:</em> <tt>!(*this)</tt>.

22 <em>Returns:</em> <tt>*this</tt>.

<tt>template&lt;class F> any_invocable&amp; operator=(F&amp;&amp; f);</tt>

23 <em>Effects:</em> As if by: <tt>any_invocable(std::forward&lt;F>(f)).swap(*this);</tt>

24 <em>Returns:</em> <tt>*this.</tt>

25 <em>Requires:</em> <tt>decay&lt;F></tt> shall be Cpp17MoveConstructible.

26 <em>Constraints:</em>

(26.1) — <tt>decay_t&lt;F></tt> is not the same type as <tt>any_invocable</tt> and

(26.3) — <tt>is_constructible_v&lt;decay_t&lt;F>, F></tt> is <tt>true</tt> and

(26.4) — <tt>is_­move_­constructible_­v&lt;decay_t&lt;F>></tt> is <tt>true</tt> and

(26.5) — <tt>F QUAL_OPT_REF</tt> is Callable (20.14.16.2) for argument types
         <tt>ArgTypes...</tt>, return type <tt>R</tt>, and

(26.4) — <tt>!NOEXCEPT_VAL || noexcept(INVOKE&lt;R>(declval&lt;F QUAL_OPT_REF>(), declval&lt;ArgTypes>()...))</tt> is <tt>true</tt>.

template&lt;class F> any_invocable&amp; operator=(reference_wrapper&lt;F> f) noexcept;

27 <em>Effects:</em> As if by: <tt>any_invocable(f).swap(*this);</tt>

28 <em>Returns:</em> <tt>*this</tt>.

<tt>~any_invocable()</tt>;

29 <em>Effects:</em> If <tt>*this != nullptr</tt>, destroys the target of <tt>this</tt>.

SECTION.2  Modifiers [inv.wrap.func.mod]
<tt>void swap(any_invocable&amp; other) noexcept;</tt>

1 <em>Effects:</em> Interchanges the targets of <tt>*this</tt> and <tt>other</tt>.

SECTION.3  Capacity [inv.wrap.func.cap]
<tt>explicit operator bool() const noexcept;</tt>

1 <em>Returns:</em> <tt>true</tt> if *<tt>this</tt> has a target, otherwise <tt>false</tt>.

SECTION.4  Invocation [inv.wrap.func.inv]
<tt>R operator()(ArgTypes... args) QUAL_OPT noexcept(NOEXCEPT_VAL);</tt>

1 <em>Returns:</em> <tt>INVOKE&lt;R>(static_cast&lt;decltype(f) QUAL_OPT_REF>(f),
  std​::​forward&lt;ArgTypes>(args)...)</tt> , where <tt>f</tt> is the target object of <tt>*this</tt>,.

2 <em>Requires:</em> <tt>(bool)*this</tt> is <tt>true</tt>.

3 <em>Throws:</em> Any exception thrown by the wrapped callable object.

SECTION.5  Null pointer comparison functions [inv.wrap.func.nullptr]
<tt>template&lt;class R, class... ArgTypes>
  bool operator==(const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp; f, nullptr_t) noexcept;
template&lt;class R, class... ArgTypes>
  bool operator==(nullptr_t, const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp; f) noexcept;</tt>

1 <em>Returns:</em> <tt>!f</tt>.

<tt>template&lt;class R, class... ArgTypes>
  bool operator!=(const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp; f, nullptr_t) noexcept;
template&lt;class R, class... ArgTypes>
  bool operator!=(nullptr_t, const any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp; f) noexcept;</tt>

2 <em>Returns:</em> <tt>(bool)f</tt>.

SECTION.6  Specialized algorithms [inv.wrap.func.alg]
<tt>template&lt;class R, class... ArgTypes>
  void swap(any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp; f1,
      any_invocable&lt;R(ArgTypes...) QUAL_OPT noexcept(NOEXCEPT_VAL)>&amp; f2) noexcept;</tt>

1 Effects:As if by: <tt>f1.swap(f2)</tt>

</pre>
   </blockquote>
   <h2 class="heading settled" data-level="4" id="SD2018"><span class="secno">4. </span><span class="content">Polls from LEWG San Diego Review (2018)</span><a class="self-link" href="#SD2018"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="support-func-func-const-func"><span class="secno">4.1. </span><span class="content">Support func(), func() const, func() &amp;&amp;</span><a class="self-link" href="#support-func-func-const-func"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>6</c->  <c- mi>6</c-> <c- mi>2</c-> <c- mi>1</c-> <c- mi>0</c->
</pre>
   <h3 class="heading settled" data-level="4.2" id="support-func--only"><span class="secno">4.2. </span><span class="content">Support func() &amp;&amp; only</span><a class="self-link" href="#support-func--only"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>2</c->  <c- mi>2</c-> <c- mi>7</c-> <c- mi>1</c-> <c- mi>1</c->
</pre>
   <h3 class="heading settled" data-level="4.3" id="remove-target-target_type"><span class="secno">4.3. </span><span class="content">Remove target/target_type</span><a class="self-link" href="#remove-target-target_type"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>12</c-> <c- mi>5</c-> <c- mi>0</c-> <c- mi>0</c-> <c- mi>0</c->
</pre>
   <h3 class="heading settled" data-level="4.4" id="require-more-stuff-noexcept-const"><span class="secno">4.4. </span><span class="content">Require more stuff (noexcept, const&amp;&amp;, ...)</span><a class="self-link" href="#require-more-stuff-noexcept-const"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>0</c->  <c- mi>1</c-> <c- mi>8</c-> <c- mi>6</c-> <c- mi>0</c->
</pre>
   <p>Note that the final poll (require more stuff) was not due to members being
against the design, but because we could easily add those facilities in a later
standard without any breakage.</p>
   <h3 class="heading settled" data-level="4.5" id="name-options"><span class="secno">4.5. </span><span class="content">Name Options</span><a class="self-link" href="#name-options"></a></h3>
   <p>
    There was one final poll, which brought us to the name 
    <tt>any_invocable</tt>
    .
   </p>
<pre class="highlight"><c- mi>3</c->  <c- n>unique_function</c->
<c- mi>3</c->  <c- n>move_function</c->
<c- mi>2</c->  <c- n>move_only_function</c->
<c- mi>7</c->  <c- n>movable_function</c->
<c- mi>8</c->  <c- n>mfunction</c->
<c- mi>10</c-> <c- n>any_invocable</c->
<c- mi>8</c->  <c- n>mofun</c->
<c- mi>8</c->  <c- n>mofunc</c->
<c- mi>0</c->  <c- n>callback</c->
<c- mi>4</c->  <c- n>mvfunction</c->
<c- mi>2</c->  <c- n>func</c->
<c- mi>0</c->  <c- n>callable</c->
<c- mi>2</c->  <c- n>any_function</c->
</pre>
   <h2 class="heading settled" data-level="5" id="K2019"><span class="secno">5. </span><span class="content">Polls from LEWG Kona Review (2019)</span><a class="self-link" href="#K2019"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="we-want-to-spend-time-on-this-now-in-order-to-include-it-in-c20"><span class="secno">5.1. </span><span class="content">We want to spend time on this now in order to include it in C++20</span><a class="self-link" href="#we-want-to-spend-time-on-this-now-in-order-to-include-it-in-c20"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>8</c->  <c- mi>8</c-> <c- mi>2</c-> <c- mi>0</c-> <c- mi>0</c->
</pre>
   <h3 class="heading settled" data-level="5.2" id="add-support-for-func-const-and-func"><span class="secno">5.2. </span><span class="content">Add support for func() const&amp; and func()&amp;</span><a class="self-link" href="#add-support-for-func-const-and-func"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>0</c->  <c- mi>8</c-> <c- mi>7</c-> <c- mi>0</c-> <c- mi>0</c->
</pre>
   <h3 class="heading settled" data-level="5.3" id="add-support-for-func-noexcept-x-all-of-the-above"><span class="secno">5.3. </span><span class="content">Add support for func() noexcept (x all of the above)</span><a class="self-link" href="#add-support-for-func-noexcept-x-all-of-the-above"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c->  <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>2</c->  <c- mi>12</c-> <c- mi>2</c-> <c- mi>0</c-> <c- mi>0</c->
</pre>
   <h3 class="heading settled" data-level="5.4" id="include-the-option-for-ctad"><span class="secno">5.4. </span><span class="content">Include the option for CTAD</span><a class="self-link" href="#include-the-option-for-ctad"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>0</c->  <c- mi>1</c-> <c- mi>5</c-> <c- mi>9</c-> <c- mi>0</c->
</pre>
   <h3 class="heading settled" data-level="5.5" id="name-callable-vs-any-invocable"><span class="secno">5.5. </span><span class="content">Name: callable vs any invocable</span><a class="self-link" href="#name-callable-vs-any-invocable"></a></h3>
<pre class="highlight"><c- n>SC</c-> <c- n>C</c-> <c- n>N</c-> <c- n>AI</c-> <c- n>SAI</c->
<c- mi>3</c->  <c- mi>2</c-> <c- mi>3</c-> <c- mi>5</c->  <c- mi>6</c->
</pre>
   <h3 class="heading settled" data-level="5.6" id="any_invocable-vs-invocable"><span class="secno">5.6. </span><span class="content">any_invocable vs invocable</span><a class="self-link" href="#any_invocable-vs-invocable"></a></h3>
<pre class="highlight"><c- n>SAI</c-> <c- n>AI</c-> <c- n>N</c-> <c- n>I</c-> <c- n>SI</c->
<c- mi>3</c->   <c- mi>7</c->  <c- mi>2</c-> <c- mi>5</c-> <c- mi>1</c->
</pre>
   <h3 class="heading settled" data-level="5.7" id="header-choice"><span class="secno">5.7. </span><span class="content">Header choice</span><a class="self-link" href="#header-choice"></a></h3>
<pre class="highlight"><c- mi>7</c-> <c- o>&lt;</c-><c- n>functional</c-><c- o>></c->
<c- mi>11</c-> <c- o>&lt;</c-><c- n>any_invocable</c-><c- o>></c->
<c- mi>11</c-> <c- o>&lt;</c-><c- n>invocable</c-><c- o>></c->
<c- mi>3</c-> <c- o>&lt;</c-><c- p>()</c-><c- o>></c->
</pre>
   <h3 class="heading settled" data-level="5.8" id="can-get-stdfunction-from-any_invocable"><span class="secno">5.8. </span><span class="content">Can get std::function from &lt;any_invocable></span><a class="self-link" href="#can-get-stdfunction-from-any_invocable"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>0</c->  <c- mi>1</c-> <c- mi>4</c-> <c- mi>4</c-> <c- mi>7</c->
</pre>
   <h3 class="heading settled" data-level="5.9" id="can-get-stdfunction-from-invocable"><span class="secno">5.9. </span><span class="content">Can get std::function from &lt;invocable></span><a class="self-link" href="#can-get-stdfunction-from-invocable"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>1</c->  <c- mi>3</c-> <c- mi>6</c-> <c- mi>3</c-> <c- mi>2</c->
</pre>
   <p>Decide on &lt;any_invocable>. Unanimous for &lt;functional> to pull it in,
even if in its own header.</p>
   <h3 class="heading settled" data-level="5.10" id="remove-the-null-check-in-the-call-operator-and-throwing-of-bad_function_call"><span class="secno">5.10. </span><span class="content">Remove the null-check in the call operator and throwing of bad_function_call</span><a class="self-link" href="#remove-the-null-check-in-the-call-operator-and-throwing-of-bad_function_call"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>8</c->  <c- mi>2</c-> <c- mi>1</c-> <c- mi>0</c-> <c- mi>0</c->
</pre>
   <h3 class="heading settled" data-level="5.11" id="remove-the-null-check-in-constructors-that-are-not-nullptr_t"><span class="secno">5.11. </span><span class="content">Remove the null-check in constructors that are not nullptr_t</span><a class="self-link" href="#remove-the-null-check-in-constructors-that-are-not-nullptr_t"></a></h3>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>any_callable</c-><voidU00028voidU00029> <c- n>ac</c-> <c- o>=</c-> <c- n>my_ptr_object</c-><c- p>;</c->
<c- k>if</c-><c- p>(</c-><c- n>ac</c-><c- p>)</c->  <c- p>{</c-> <c- d>/* true even if my_ptr is nullptr */</c-> <c- p>}</c->
</voidU00028voidU00029></pre>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>0</c->  <c- mi>2</c-> <c- mi>2</c-> <c- mi>4</c-> <c- mi>3</c->
</pre>
   <h3 class="heading settled" data-level="5.12" id="perfect-forwarding-for-converting-constructor-instead-of-by-value"><span class="secno">5.12. </span><span class="content">Perfect forwarding for converting constructor instead of by-value</span><a class="self-link" href="#perfect-forwarding-for-converting-constructor-instead-of-by-value"></a></h3>
   <p>Unanimous</p>
   <h3 class="heading settled" data-level="5.13" id="forward-to-lwg-for-c20"><span class="secno">5.13. </span><span class="content">Forward to LWG for C++20</span><a class="self-link" href="#forward-to-lwg-for-c20"></a></h3>
<pre class="highlight"><c- n>SF</c-> <c- n>F</c-> <c- n>N</c-> <c- n>A</c-> <c- n>SA</c->
<c- mi>8</c->  <c- mi>5</c-> <c- mi>0</c-> <c- mi>0</c-> <c- mi>0</c->
</pre>
   <h2 class="heading settled" data-level="6" id="Impl"><span class="secno">6. </span><span class="content">Implementation Experience</span><a class="self-link" href="#Impl"></a></h2>
   <p>
    There are many implementations of a move-only 
    <tt>std::function</tt>
     with a
design that is similar to this. What is presented is a conservative subset of
those implementations. The changes suggested in LEWG, though minimal, have not
been used in a large codebase.
   </p>
   <p>Previous revisions of this paper have included publicly accessible move-only
function implementations, notably including implementations in HPX, Folly, and
LLVM.</p>
   <h2 class="heading settled" data-level="7" id="References"><span class="secno">7. </span><span class="content">References</span><a class="self-link" href="#References"></a></h2>
   <p>[1]: David Krauss: N4543 "A polymorphic wrapper for all Callable objects" <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4543.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4543.pdf</a></p>
   <p>[2]: Geoffrey Romer: "Bug 34 - Need type-erased wrappers for move-only callable objects" <a href="https://issues.isocpp.org/show_bug.cgi?id=34">https://issues.isocpp.org/show_bug.cgi?id=34</a></p>
   <p>[3]: Ryan McDougall: P0288R2 "The Need for std::unique_function" <a href="https://wg21.link/p0288r2">https://wg21.link/p0288r2</a></p>
   <p>[4]: Geoffrey Romer: N4348 "Making std::function safe for concurrency" <a href="www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4348.html">www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4348.html</a></p>
   <p>[5]: Richard Smith: N4778 "Working Draft, Standard for Programming Language C++" <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4810.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4810.pdf</a></p>
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