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  <title>P1175R0: a simple and practical optional reference for C++</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,
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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">P1175R0<br>a simple and practical optional reference for C++</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-10-06">2018-10-06</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:phdofthehouse@gmail.com">JeanHeyd Meneide</a>
     <dt>Audience:
     <dd>LEWG, LWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Current Source:
     <dd><a href="https://github.com/ThePhD/future_cxx/blob/master/papers/source/d1175.bs">github.com/ThePhD/future_cxx/blob/master/papers/source/d1175.bs</a>
     <dt>Current:
     <dd><a href="https://rawgit.com/ThePhD/future_cxx/master/papers/d1175.html">https://rawgit.com/ThePhD/future_cxx/papers/d1175.html</a>
     <dt>Reply To:
     <dd><a href="mailto:phdofthehouse@gmail.com">JeanHeyd Meneide</a>, <a href="https://twitter.com/thephantomderp">@thephantomderp</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>optional has seen much contention over its semantics and other points related to Regularity and Consistency, but not being able to answer these questions has delayed important advances in expected, variant, optional and other vocabulary types. This proposal demonstrates a safe and simple subset of optional that can gain consensus and still leaves the door open for when questions of consistency and regularity can be answered with confidence.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#changelog"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#changelog-r0"><span class="secno">1.1</span> <span class="content">Revision 0</span></a>
     </ol>
    <li>
     <a href="#overview"><span class="secno">2</span> <span class="content">Overview</span></a>
     <ol class="toc">
      <li><a href="#overview-behavior"><span class="secno">2.1</span> <span class="content">The Great Big Table of Behaviors</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">3</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#design"><span class="secno">4</span> <span class="content">Design Considerations</span></a>
     <ol class="toc">
      <li><a href="#design-conservative"><span class="secno">4.1</span> <span class="content">The Solution</span></a>
     </ol>
    <li><a href="#experience"><span class="secno">5</span> <span class="content">Implementation Experience</span></a>
    <li>
     <a href="#wording"><span class="secno">6</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#wording-intent"><span class="secno">6.1</span> <span class="content">Intent</span></a>
      <li><a href="#wording-feature"><span class="secno">6.2</span> <span class="content">Feature Test Macro</span></a>
      <li><a href="#wording-library"><span class="secno">6.3</span> <span class="content">Proposed Wording</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">7</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="changelog-r0"><span class="secno">1.1. </span><span class="content">Revision 0</span><a class="self-link" href="#changelog-r0"></a></h3>
   <p>Initial release.</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>
   <h3 class="heading settled" data-level="2.1" id="overview-behavior"><span class="secno">2.1. </span><span class="content">The Great Big Table of Behaviors</span><a class="self-link" href="#overview-behavior"></a></h3>
   <p>Below is a succint synopsis of the options presented in this paper and their comparison with known solutions and alternative implementations. It does not include the totality of the optional API surface, but has the most exemplary pieces. A key for the symbols:</p>
   <p>✔️ - Succeeds</p>
   <p>🚫 - Compile-Time Error</p>
   <p>❌ - Runtime Error</p>
   <p>❓ - Implementation Inconsistency (between engaged/unengaged states, runtime behaviors, etc.)</p>
   <table>
    <tbody>
     <tr>
      <th colspan="6">optional behaviors
     <tr>
      <th>Operation
      <th>T
      <th>std::reference_wrapper&lt;T>
      <th><em>Proposed:</em><br><a href="#design-conservative">T&amp; conservative</a>
     <tr>
      <td>exemplary implementation(s)
      <td>✔️<br>std::optional<br><a data-link-type="biblio" href="#biblio-martinmoene-optional">nonstd::optional</a><br><a data-link-type="biblio" href="#biblio-llvm-optional">llvm::Optional</a><br><a data-link-type="biblio" href="#biblio-folly-optional">folly::Optional</a><br><a data-link-type="biblio" href="#biblio-mnmlstc-optional">core::Optional</a>
      <td>✔️<br>std::optional<br><a data-link-type="biblio" href="#biblio-martinmoene-optional">nonstd::optional</a><br><a data-link-type="biblio" href="#biblio-llvm-optional">llvm::Optional</a><br><a data-link-type="biblio" href="#biblio-folly-optional">folly::Optional</a><br><a data-link-type="biblio" href="#biblio-mnmlstc-optional">core::Optional</a>
      <td>✔️<br><a data-link-type="biblio" href="#biblio-akrzemi-optional">std::experimental::optional</a><br><a data-link-type="biblio" href="#biblio-sol2">sol::optional</a>
     <tr>
      <td><code class="highlight"><c- n>optional</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&amp;</c-><c- p>)</c-></code>
      <td>✔️<br>copy constructs <code class="highlight"><c- n>T</c-></code> (disengaged: nothing)
      <td>✔️<br>binds reference (disengaged: nothing)
      <td>✔️<br>binds reference (disengaged: nothing)
     <tr>
      <td><code class="highlight"><c- n>optional</c-><c- p>(</c-><c- n>optional</c-><c- o>&amp;&amp;</c-><c- p>)</c-></code>
      <td>✔️<br>move constructs <code class="highlight"><c- n>T</c-></code> (disengaged: nothing)
      <td>✔️<br>binds reference (disengaged: nothing)
      <td>✔️<br>binds reference (disengaged: nothing)
     <tr>
      <td><code class="highlight"><c- n>optional</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code>
      <td>✔️<br>(copy) constructs <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>binds reference
      <td>✔️<br>binds reference
     <tr>
      <td><code class="highlight"><c- n>optional</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;&amp;</c-><c- p>)</c-></code>
      <td>✔️<br>(move) constructs <code class="highlight"><c- n>T</c-></code>
      <td>🚫<br>compile-time error
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code><br><em>engaged</em>
      <td>✔️<br>overwrites <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>rebinds data
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code><br><em>disengaged</em>
      <td>️✔️<br>overwrites data
      <td>✔️<br>rebinds data (overwrites reference wrapper)
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;&amp;</c-><c- p>)</c-></code><br><em>engaged</em>
      <td>✔️<br>move-assigns <code class="highlight"><c- n>T</c-></code>
      <td>🚫<br>compile-time error
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;&amp;</c-><c- p>)</c-></code><br><em>disengaged</em>
      <td>✔️<br>constructs <code class="highlight"><c- n>T</c-></code>
      <td>🚫<br>compile-time error
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code><br><em>engaged</em>
      <td>✔️<br>overwrites <code class="highlight"><c- n>T</c-></code>
      <td>🚫<br>compile-time error
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-><c- p>)</c-></code><br><em>disengaged</em>
      <td>️✔️<br>overwrites data
      <td>✔️<br>overwrites data
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;&amp;</c-><c- p>)</c-></code><br><em>engaged;</em><br><em>arg engaged</em>
      <td>✔️<br>move assign <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>rebind data
      <td>✔️<br>rebind data
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;&amp;</c-><c- p>)</c-></code><br><em>disengaged;</em><br><em>arg engaged</em>
      <td>✔️<br>move construct <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>rebind data
      <td>✔️<br>rebind data
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;&amp;</c-><c- p>)</c-></code><br><em>engaged;</em><br><em>arg disengaged</em>
      <td>✔️<br>disengage <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>disengage <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>disengage <code class="highlight"><c- n>T</c-></code>
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;&amp;</c-><c- p>)</c-></code><br><em>disengaged;</em><br><em>arg disengaged</em>
      <td>✔️<br>nothing
      <td>✔️<br>nothing
      <td>✔️<br>nothing
     <tr>
      <td><code class="highlight"><c- o>*</c-><c- n>my_op</c-> <c- o>=</c-> <c- n>value</c-></code><br><em>engaged</em>
      <td>✔️<br>copy assigns <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>copy assigns <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>copy assigns <code class="highlight"><c- n>T</c-></code>
     <tr>
      <td><code class="highlight"><c- o>*</c-><c- n>my_op</c-> <c- o>=</c-> <c- n>value</c-></code><br><em>disengaged</em>
      <td>❌<br>runtime error
      <td>❌<br>runtime error
      <td>❌<br>runtime error
     <tr>
      <td><code class="highlight"><c- o>*</c-><c- n>my_op</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>value</c-><c- p>)</c-></code><br><em>engaged</em>
      <td>✔️<br>move assigns <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>move assigns <code class="highlight"><c- n>T</c-></code>
      <td>✔️<br>move assigns <code class="highlight"><c- n>T</c-></code>
     <tr>
      <td><code class="highlight"><c- o>*</c-><c- n>my_op</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>value</c-><c- p>)</c-></code><br><em>disengaged</em>
      <td>❌<br>runtime error
      <td>❌<br>runtime error
      <td>❌<br>runtime error
     <tr>
      <td><code class="highlight"><c- p>(</c-><c- o>*</c-><c- n>my_op</c-><c- p>).</c-><c- n>some_member</c-><c- p>()</c-></code><br><em>engaged</em>
      <td>✔️<br>calls <code class="highlight"><c- n>some_member</c-><c- p>()</c-></code>
      <td>🚫<br>compile-time error
      <td>✔️<br>calls <code class="highlight"><c- n>some_member</c-><c- p>()</c-></code>
     <tr>
      <td><code class="highlight"><c- p>(</c-><c- o>*</c-><c- n>my_op</c-><c- p>).</c-><c- n>some_member</c-><c- p>()</c-></code><br><em>disengaged</em>
      <td>❌<br>runtime error
      <td>❌<br>runtime error
      <td>❌<br>runtime error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&amp;</c-><c- p>)</c-></code>
      <td>✔️<br>compares values if both engaged, returns true if both disengaged, returns false otherwise
      <td>✔️<br>compares values if both engaged, returns true if both disengaged, returns false otherwise
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>&lt;</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&amp;</c-><c- p>)</c-></code>
      <td>✔️<br>compares values if both engaged, returns true if both disengaged, returns false otherwise
      <td>✔️<br>compares values if both engaged, returns true if both disengaged, returns false otherwise
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code><br><em>engaged</em>
      <td>✔️<br>compares values
      <td>✔️<br>compares values
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>&lt;</c-><c- p>(</c-><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code><br><em>engaged</em>
      <td>✔️<br>compares values
      <td>✔️<br>compares values
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code><br><em>disengaged</em>
      <td>✔️<br>returns false
      <td>✔️<br>returns false
      <td>🚫<br>compile-time error
     <tr>
      <td><code class="highlight"><c- k>operator</c-><c- o>&lt;</c-><c- p>(</c-><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-><c- p>)</c-></code><br><em>disengaged</em>
      <td>✔️<br>returns false
      <td>✔️<br>returns false
      <td>🚫<br>compile-time error
   </table>
   <h2 class="heading settled" data-level="3" id="motivation"><span class="secno">3. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Originally, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> -- where <code class="highlight"><c- n>T</c-></code> denotes the name of a type -- contained a specialization to work with regular references, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;></c-></code>. When some of the semantics for references were called into question with respect to assign-through semantics  (assign into the value or rebind the optional) and how comparisons would be performed, the debate stopped early and no full consensus was reached. Rather than remove just the operator or modify comparison operators, the entirety of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;></c-></code> was removed entirely.</p>
   <p>This left many codebases in an interesting limbo: previous implementations and external implementations handled references without a problem. Transitioning to pointers created both a problem of unclear API (pointers are an exceedingly overloaded construct used for way too many things) and had serious implications for individuals who wanted to use temporaries as part of their function calls.</p>
   <p>As Library Evolution Working Group Chair Titus Winters has frequently stated and demonstrated, having multiple vocabulary types inhibits growth of the C++ ecosystem and fragments libraries and their developers. This comes at an especially high cost for <code class="highlight"><c- n>optional</c-></code>, <code class="highlight"><c- n>variant</c-></code>, <code class="highlight"><c- n>any</c-></code>, <code class="highlight"><c- n>expected</c-></code> and more. There are at least 6 different <code class="highlight"><c- n>optional</c-></code>s in the wild with very slightly differing semantics, a handful more <code class="highlight"><c- n>variant</c-></code>s, a few <code class="highlight"><c- n>expected</c-></code> types, and more (not including the ones from <code class="highlight"><c- n>boost</c-><c- o>::</c-></code>). Of note is that many optionals have been created and are being nurtured to this day without the need to take care of legacy code, which greatly inhibits interopability between code bases and general sharing.</p>
   <h2 class="heading settled" data-level="4" id="design"><span class="secno">4. </span><span class="content">Design Considerations</span><a class="self-link" href="#design"></a></h2>
   <p>This solution is the simplest cross-section that enables behavior without encroaching upon a future where the to be posed in the yet-to-be-released p1129r0 will reach an answer to move the C++ community forward. Care has been taken to only approach the most useful subsection, while keeping everything else deleted. This will enable developers to use optional for the 80% use cases, while users handle the 20% use case of rebinding, assigning through, or comparing values / location by choosing much more explicit syntax that will not be deprecated.</p>
   <h3 class="heading settled" data-level="4.1" id="design-conservative"><span class="secno">4.1. </span><span class="content">The Solution</span><a class="self-link" href="#design-conservative"></a></h3>
   <p>This baseline version is the version that has seen adoption from hundreds of companies and users: an optional where <code class="highlight"><c- k>operator</c-><c- o>=</c-></code> is not allowed, comparison operators are nuked, rebinding is done with an explicit wrapping of <code class="highlight"><c- n>my_op</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;></c-><c- p>(</c-> <c- n>my_lvalue</c-> <c- p>)</c-></code> and assign-through is performed using <code class="highlight"><c- o>*</c-><c- n>my_op</c-> <c- o>=</c-> <c- n>my_l_or_rvalue</c-></code>. This keeps <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code> as a delay-constructed type, allows usage in all of the places a programmer might want to put it trivially, allows it to be used as a parameter, and allows it to be used as a return type.</p>
   <p>It forces the user to choose assign-through by explicitly dereferencing the optional as in <code class="highlight"><c- o>*</c-><c- n>my_op</c-> <c- o>=</c-> <c- n>some_value</c-></code>, and forces rebind by making the user specify <code class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;</c-><c- n>amp</c-><c- p>;</c-><c- o>></c-><c- p>(</c-><c- n>some_lvalue</c-><c- p>)</c-></code>. It is safe, but penalizes the user for this safety with verbosity (and, arguably, disappointment). It also prevents users of <code class="highlight"><c- n>boost</c-><c- o>::</c-><c- n>optional</c-></code>, <code class="highlight"><c- n>tl</c-><c- o>::</c-><c- n>optional</c-></code>, <code class="highlight"><c- n>ts</c-><c- o>::</c-><c- n>optional_ref</c-></code> and others from migrating painlessly to the standard version, but still allows many of the high-priority uses of such classes with references to transition to using the standard library version.</p>
   <p>Another notable feature of adding optional references and using <code class="highlight"><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-></code> is the ability to transition codebases that use temporary values (r-values) passed to functions, this solution will work for individuals without requiring a full rewrite of the code. For example, the function <code class="highlight"><c- b>void</c-> <c- nf>old_foo</c-><c- p>(</c-> <c- b>int</c-> <c- n>arg</c-><c- p>,</c-> <c- k>const</c-> <c- n>options</c-><c- o>&amp;</c-> <c- n>opts</c-><c- p>);</c-></code> can be transitioned to <code class="highlight"><c- b>void</c-> <c- nf>old_foo</c-><c- p>(</c-> <c- b>int</c-> <c- n>arg</c-><c- p>,</c-> <c- n>optional</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>options</c-><c- o>&amp;></c-> <c- n>opts</c-><c- p>);</c-></code> and work for both l-values and r-values passed to the type. This is safe thanks to C++'s lifetime rules around temporary objects, when they bind to references, and when they are lifetime extended; see [class.temporary]/6 for applicable lifetime extension clauses of temporaries, including temporaries that bind to stored references.</p>
   <h2 class="heading settled" data-level="5" id="experience"><span class="secno">5. </span><span class="content">Implementation Experience</span><a class="self-link" href="#experience"></a></h2>
   <p>This "simple", baseline version is featured in <a data-link-type="biblio" href="#biblio-akrzemi-optional">akrzemi/optional</a>, <a data-link-type="biblio" href="#biblio-sol2">[sol2]</a>, and the "portable" version of <a data-link-type="biblio" href="#biblio-boost-optional">[boost-optional]</a> (following Boost’s advice to avoid the use of the assignment operator in select cases for compilers with degenerate behavior). It is the least offensive, tasteless, hazard-proof, odorless, non-toxic, biodegradable, organic, and politically correct choice™; it can also be expanded upon at a later date.</p>
   <p>This specific version has seen experience for about 8+ years. It is known to be safe and easy to use and covers a good portion of user’s use cases without having to invoke the problem of figuring out assign-through or rebind semantics.</p>
   <p>The comparison operators do not exist, which make it a subset of <code class="highlight"><c- n>boost</c-></code> and other optional implementations. Additional work can be done later once the Committee and its constituents .</p>
   <h2 class="heading settled" data-level="6" id="wording"><span class="secno">6. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>All wording is relative to <a data-link-type="biblio" href="#biblio-n4762">[n4762]</a>.</p>
   <h3 class="heading settled" data-level="6.1" id="wording-intent"><span class="secno">6.1. </span><span class="content">Intent</span><a class="self-link" href="#wording-intent"></a></h3>
   <p>The intent of this proposal is to provide a l-value reference optional. The comparison and equality operators will not be provided. The assignment operator from an l-value reference or from an l-value reference of its base will not be provided. The copy-assignment from two optionals will rebind the optional.</p>
   <p>Comparison to <code class="highlight"><c- n>nullopt</c-></code> with equality will provided.</p>
   <h3 class="heading settled" data-level="6.2" id="wording-feature"><span class="secno">6.2. </span><span class="content">Feature Test Macro</span><a class="self-link" href="#wording-feature"></a></h3>
   <p>The proposed feature test macro is <code class="highlight"><c- n>__cpp_lib_optional_ref</c-></code>.</p>
   <h3 class="heading settled" data-level="6.3" id="wording-library"><span class="secno">6.3. </span><span class="content">Proposed Wording</span><a class="self-link" href="#wording-library"></a></h3>
   <p>Append to §16.3.1 General [<strong>support.limits.general</strong>]'s <strong>Table 35</strong> one additional entry:</p>
   <blockquote>
    <table>
     <tbody>
      <tr>
       <th>Macro name
       <th>Value
      <tr>
       <td>
        <ins>__cpp_lib_optional_ref</ins>
       <td>
        <ins>201811L</ins>
    </table>
   </blockquote>
   <p>Add additional class template specialization to §19.6.2 Header <code class="highlight"><c- o>&lt;</c-><c- n>optional</c-><c- o>></c-></code> synopsis [<strong>optional.syn</strong>]:</p>
   <blockquote>
<pre class="highlight"><c- c1>// 19.6.3, class template optional</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>optional</c-><c- p>;</c->
</pre>
    <ins>
<pre class="highlight"><c- c1>// 19.6.4, class template optional for lvalue reference types</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;></c-><c- p>;</c->
</pre>
    </ins>
   </blockquote>
   <p>Insert §19.6.4 [<strong>optional.lref</strong>] after §19.6.3 Class template <code class="highlight"><c- n>optional</c-></code> [<strong>optional.mod</strong>]:</p>
   <blockquote>
    <ins>19.6.4 Class template optional for lvalue reference types [<strong>optional.lref</strong>]</ins>
    <ins>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->

  <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- k>class</c-> <c- nc>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;></c-> <c- p>{</c->
  <c- k>public</c-><c- o>:</c->
    <c- k>typedef</c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>value_type</c-><c- p>;</c->

    <c- c1>// 19.6.4.1, construction/destruction</c->
    <c- k>constexpr</c-> <c- nf>optional</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- nf>optional</c-><c- p>(</c-><c- n>nullopt_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- nf>optional</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- n>optional</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- nf>optional</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- n>optional</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>U</c-><c- o>&amp;>&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- o>~</c-><c- n>optional</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

    <c- c1>// 19.6.4.2, mutation</c->
    <c- k>constexpr</c-> <c- n>optional</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>nullopt_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- n>optional</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>optional</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
    <c- n>optional</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->

    <c- c1>// 19.6.4.3, observers</c->
    <c- k>constexpr</c-> <c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- n>T</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
    <c- k>constexpr</c-> <c- k>explicit</c-> <c- k>operator</c-> <c- nf>bool</c-><c- p>()</c-> <c- k>const</c-> <c- k>noexcept</c-><c- p>;</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- k>constexpr</c-> <c- n>T</c-> <c- n>value_or</c-><c- p>(</c-><c- n>U</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- k>const</c-><c- o>&amp;</c-><c- p>;</c->
 
    <c- c1>// 19.6.4.4, modifiers</c->
    <c- b>void</c-> <c- nf>reset</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->

  <c- k>private</c-><c- o>:</c->
    <c- n>T</c-><c- o>*</c-> <c- n>ref</c-><c- p>;</c->  <c- c1>// exposition only</c->
  <c- p>};</c->

<c- p>}</c-> <c- c1>// namespace std</c->
</pre>
    </ins>
    <p>
     <ins><sup>1</sup> Engaged instances of <code class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> where <code class="highlight"><c- n>T</c-></code> is of lvalue reference type, refer to objects of type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>remove_reference_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, but their life-time is not connected to the life-time of the referred to object. Destroying or disengageing the optional object does not affect the state of the referred to object.</ins>
    </p>
    <p>
     <ins><sup>2</sup> Member <code class="highlight"><c- n>ref</c-></code> is provided for exposition only. Implementations need not provide this member. If <code class="highlight"><c- n>ref</c-> <c- o>==</c-> <c- k>nullptr</c-></code>, optional object is disengaged; otherwise <code class="highlight"><c- n>ref</c-></code> points to a valid object.</ins>
    </p>
    <p>
     <ins><strong>19.6.4.1</strong> Construction and destruction [<strong>optional.lref.ctor</strong>]</ins>
    </p>
    <ins>
<pre class="highlight"><c- k>constexpr</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- n>optional</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>constexpr</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- n>optional</c-><c- p>(</c-><c- n>nullopt_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>1</sup> Effects: Constructs a disengaged optional object by initializing ref with nullptr.</ins>
    </p>
    <p>
     <ins><sup>2</sup> Ensures: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> false<c- p>.</c-></code></ins>
    </p>
    <p>
     <ins><sup>3</sup> Remarks: For every object type <code class="highlight"><c- n>T</c-></code> these constructors shall be constexpr constructors.</ins>
    </p>
    <ins>
<pre class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- n>optional</c-><c- p>(</c-><c- n>T</c-><c- o>&amp;</c-> <c- n>v</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>4</sup> Effects: Constructs an engaged optional object by initializing ref with <code class="highlight"><c- n>addressof</c-><c- p>(</c-><c- n>v</c-><c- p>)</c-></code>.</ins>
    </p>
    <p>
     <ins><sup>5</sup> Ensures: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> true <c- o>&amp;&amp;</c-> <c- n>addressof</c-><c- p>(</c-><c- o>*</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>))</c-> <c- o>==</c-> <c- n>addressof</c-><c- p>(</c-><c- n>v</c-><c- p>)</c-></code>.</ins>
    </p>
    <ins>
<pre class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- n>optional</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- n>optional</c-><c- p>(</c-><c- k>const</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>U</c-><c- o>&amp;>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>6</sup> Constraints: <code class="highlight"><c- n>is_base_of</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>>::</c-><c- n>value</c-> <c- o>==</c-> true</code>, and <code class="highlight"><c- n>is_convertible</c-><c- o>&lt;</c-><c- n>U</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>T</c-><c- o>&amp;>::</c-><c- n>value</c-></code> is true.</ins>
    </p>
    <p>
     <ins><sup>7</sup> Effects: If rhs is disengaged, initializes ref with <code class="highlight"><c- k>nullptr</c-></code>; otherwise, constructs an engaged object by initializing ref with <code class="highlight"><c- n>addressof</c-><c- p>(</c-><c- o>*</c-><c- n>rhs</c-><c- p>)</c-></code>.</ins>
    </p>
    <ins>
<pre class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::~</c-><c- n>optional</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>9</sup> Effects: No effect. This destructor shall be a trivial destructor.</ins>
    </p>
    <p>
     <ins><strong>19.6.4.2</strong> Mutation [<strong>optional.lref.mutate</strong>]</ins>
    </p>
    <ins>
<pre class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>&amp;</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>nullopt_t</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>1</sup> Effects: Assigns ref with a value of <code class="highlight"><c- k>nullptr</c-></code>. If ref was non-null initially, the object it referred to is unaffected.</ins>
    </p>
    <p>
     <ins><sup>2</sup> Returns: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</ins>
    </p>
    <p>
     <ins><sup>3</sup> Ensures: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> false</code>.</ins>
    </p>
    <p>
     <ins><strong>19.6.4.3</strong> Observers [<strong>optional.lref.observe</strong>]</ins>
    </p>
    <ins>
<pre class="highlight"><c- n>T</c-><c- o>*</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>1</sup> Requires: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> true</code>.</ins>
    </p>
    <p>
     <ins><sup>2</sup> Returns: ref.</ins>
    </p>
    <p>
     <ins><sup>3</sup> Throws: nothing.</ins>
    </p>
    <ins>
<pre class="highlight"><c- n>T</c-><c- o>&amp;</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>4</sup> Requires: <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-> <c- o>==</c-> true</code>.</ins>
    </p>
    <p>
     <ins><sup>5</sup> Returns: <code class="highlight"><c- o>*</c-><c- n>ref</c-></code></ins>
    </p>
    <p>
     <ins><sup>6</sup> Throws: nothing.</ins>
    </p>
    <ins>
<pre class="highlight"><c- k>explicit</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- k>operator</c-> <c- b>bool</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>7</sup> Returns: <code class="highlight"><c- n>ref</c-> <c- o>!=</c-> <c- k>nullptr</c-></code></ins>
    </p>
    <ins>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- n>value_or</c-><c- p>(</c-><c- n>U</c-><c- o>&amp;&amp;</c-> <c- n>u</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>8</sup> Returns: <code class="highlight"><c- p>.</c-><c- n>value</c-><c- p>()</c-></code> when <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-></code> is true, otherwise <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)</c-></code>.</ins>
    </p>
    <p>
     <ins><strong>19.6.4.4</strong> Observers [<strong>optional.lref.modifiers</strong>]</ins>
    </p>
    <ins>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- o>></c-> <c- n>T</c-><c- o>&amp;</c-> <c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>&amp;>::</c-><c- n>value_or</c-><c- p>(</c-><c- n>U</c-><c- o>&amp;&amp;</c-> <c- n>u</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
</pre>
    </ins>
    <p>
     <ins><sup>12</sup> Returns: <code class="highlight"><c- p>.</c-><c- n>value</c-><c- p>()</c-></code> when <code class="highlight"><c- b>bool</c-><c- p>(</c-><c- o>*</c-><c- k>this</c-><c- p>)</c-></code> is true, otherwise <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>(</c-><c- n>u</c-><c- p>)</c-></code>.</ins>
    </p>
   </blockquote>
   <p>Modify §19.6.6 Relational operators [<strong>optional.relops</strong>] to include the following top-level clause:</p>
   <blockquote>
    <ins> <sup>1</sup> None of the comparisons in this subsection participate in overload resolutions if <code class="highlight"><c- n>T</c-></code> or <code class="highlight"><c- n>U</c-></code> in <code class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> or <code class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-></code> are an l-value reference. </ins>
   </blockquote>
   <p>Modify §19.6.8 Comparison with <code class="highlight"><c- n>T</c-></code> [<strong>optional.comp_with_t</strong>] to include the following top-level clause:</p>
   <blockquote>
    <ins> <sup>1</sup> None of the comparisons in this subsection participate in overload resolutions if <code class="highlight"><c- n>T</c-></code> or <code class="highlight"><c- n>U</c-></code> in <code class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> or <code class="highlight"><c- n>optional</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-></code> are an l-value reference. </ins>
   </blockquote>
   <h2 class="heading settled" data-level="7" id="acknowledgements"><span class="secno">7. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thank you to sol2 users for encouraging me to fix this in the standard. Thank you to Lisa Lippincott for encouraging me to make this and one other proposal after seeing my C++Now 2018 presentation. Thank you to Matt Calabrese, R. Martinho Fernandes and Michał Dominiak for the advice on how to write and handle a paper of this magnitude.</p>
   <p>Thank you to Tim Song and Walter Brown for reviewing one of my papers, and thus allowing me to improve all of them.</p>
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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-akrzemi-optional">[AKRZEMI-OPTIONAL]
   <dd>Andrzej Krzemieński. <a href="https://github.com/akrzemi1/Optional">Optional (nullable) objects for C++14</a>. April 23rd, 2018. URL: <a href="https://github.com/akrzemi1/Optional">https://github.com/akrzemi1/Optional</a>
   <dt id="biblio-boost-optional">[BOOST-OPTIONAL]
   <dd>Fernando Luis Cacciola Carballal; Andrzej Krzemieński. <a href="https://www.boost.org/doc/libs/1_67_0/libs/optional/doc/html/index.html">Boost.Optional</a>. July 24th, 2018. URL: <a href="https://www.boost.org/doc/libs/1_67_0/libs/optional/doc/html/index.html">https://www.boost.org/doc/libs/1_67_0/libs/optional/doc/html/index.html</a>
   <dt id="biblio-folly-optional">[FOLLY-OPTIONAL]
   <dd>Facebook. <a href="https://github.com/facebook/folly">folly/Optional</a>. August 11th, 2018. URL: <a href="https://github.com/facebook/folly">https://github.com/facebook/folly</a>
   <dt id="biblio-llvm-optional">[LLVM-OPTIONAL]
   <dd>LLVM Developer Group. <a href="http://llvm.org/doxygen/Optional_8h_source.html">Optional.h</a>. July 4th, 2018. URL: <a href="http://llvm.org/doxygen/Optional_8h_source.html">http://llvm.org/doxygen/Optional_8h_source.html</a>
   <dt id="biblio-martinmoene-optional">[MARTINMOENE-OPTIONAL]
   <dd>Martin Moene. <a href="https://github.com/martinmoene/optional-lite">Optional Lite</a>. June 21st, 2018. URL: <a href="https://github.com/martinmoene/optional-lite">https://github.com/martinmoene/optional-lite</a>
   <dt id="biblio-mnmlstc-optional">[MNMLSTC-OPTIONAL]
   <dd>Isabella Muerte. <a href="https://mnmlstc.github.io/core/optional.html">core::optional</a>. February 26th, 2018. URL: <a href="https://mnmlstc.github.io/core/optional.html">https://mnmlstc.github.io/core/optional.html</a>
   <dt id="biblio-n4762">[N4762]
   <dd>ISO/IEC JTC1/SC22/WG21 - The C++ Standards Committee; Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4750.pdf">n4762 - Working Draft, Standard for Programming Language C++</a>. May 7th, 2018. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4750.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4750.pdf</a>
   <dt id="biblio-sol2">[SOL2]
   <dd>ThePhD. <a href="https://github.com/ThePhD/sol2">sol2: C++ &lt;-> Lua Binding Framework</a>. July 3rd, 2018. URL: <a href="https://github.com/ThePhD/sol2">https://github.com/ThePhD/sol2</a>
  </dl>