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  <title>p0323r4: std::expected</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: normal;
	}
	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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  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">p0323r4<br>std::expected</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2017-11-26">26 November 2017</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0323r4">http://wg21.link/P0323r4</a>
     <dt>Issue Tracking:
     <dd><a href="#issues-index">Inline In Spec</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:vicente.botet@nokia.com">Vicente Botet</a> (<span class="p-org org">Nokia</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dt>Audience:
     <dd>LWG
     <dt>Project:
     <dd><span>ISO JTC1/SC22/WG21: Programming Language C++</span>
     <dt>Source:
     <dd><span><a href="https://github.com/jfbastien/papers/blob/master/source/P0323r4.bs">github.com/jfbastien/papers/blob/master/source/P0323r4.bs</a></span>
    </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>Utility class to represent expected object: wording and open questions.</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="#word"><span class="secno">1</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#unexpected"><span class="secno">1.1</span> <span class="content">�.� Unexpected objects [<em>unexpected</em>]</span></a>
      <li><a href="#unexpected.general"><span class="secno">1.2</span> <span class="content">�.�.1 General [<em>unexpected.general</em>]</span></a>
      <li><a href="#unexpected.synop"><span class="secno">1.3</span> <span class="content">�.�.2 Header <code class="highlight"><span class="o">&lt;</span><span class="n">experimental</span><span class="o">/</span><span class="n">unexpected</span><span class="o">></span></code> synopsis [<em>unexpected.synop</em>]</span></a>
      <li><a href="#unexpected.object"><span class="secno">1.4</span> <span class="content">�.�.3 Unexpected object type [<em>unexpected.object</em>]</span></a>
      <li><a href="#unexpected.relational_op"><span class="secno">1.5</span> <span class="content">�.�.4 Unexpected relational operators [<em>unexpected.relational_op</em>]</span></a>
      <li><a href="#expected"><span class="secno">1.6</span> <span class="content">�.� Expected objects [expected]</span></a>
      <li><a href="#expected.general"><span class="secno">1.7</span> <span class="content">�.�.1 In general [<em>expected.general</em>]</span></a>
      <li><a href="#expected.synop"><span class="secno">1.8</span> <span class="content">�.�.2 Header <code class="highlight"><span class="o">&lt;</span><span class="n">experimental</span><span class="o">/</span><span class="n">expected</span><span class="o">></span></code> synopsis [<em>expected.synop</em>]</span></a>
      <li><a href="#expected.defs"><span class="secno">1.9</span> <span class="content">�.�.3 Definitions [<em>expected.defs</em>]</span></a>
      <li><a href="#expected.object"><span class="secno">1.10</span> <span class="content">�.�.4 expected for object types [<em>expected.object</em>]</span></a>
      <li><a href="#expected.object.ctor"><span class="secno">1.11</span> <span class="content">�.�.4.1 Constructors [<em>expected.object.ctor</em>]</span></a>
      <li><a href="#expected.object.dtor"><span class="secno">1.12</span> <span class="content">�.�.4.2 Destructor [<em>expected.object.dtor</em>]</span></a>
      <li><a href="#expected.object.assign"><span class="secno">1.13</span> <span class="content">�.�.4.3 Assignment [<em>expected.object.assign</em>]</span></a>
      <li><a href="#expected.object.swap"><span class="secno">1.14</span> <span class="content">�.�.4.4 Swap [<em>expected.object.swap</em>]</span></a>
      <li><a href="#expected.object.observe"><span class="secno">1.15</span> <span class="content">�.�.4.5 Observers [<em>expected.object.observe</em>]</span></a>
      <li><a href="#expected.unexpect"><span class="secno">1.16</span> <span class="content">�.�.6 <code class="highlight"><span class="n">unexpect</span></code> tag [<em>expected.unexpect</em>]</span></a>
      <li><a href="#expected.bad_expected_access"><span class="secno">1.17</span> <span class="content">�.�.7 Template Class <code class="highlight"><span class="n">bad_expected_access</span></code> [<em>expected.bad_expected_access</em>]</span></a>
      <li><a href="#expected.bad_expected_access_base"><span class="secno">1.18</span> <span class="content">�.�.7 Template Class <code class="highlight"><span class="n">bad_expected_access</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">></span></code> [<em>expected.bad_expected_access_base</em>]</span></a>
      <li><a href="#expected.relational_op"><span class="secno">1.19</span> <span class="content">�.�.8 Expected Relational operators [<em>expected.relational_op</em>]</span></a>
      <li><a href="#expected.comparison_T"><span class="secno">1.20</span> <span class="content">�.�.9 Comparison with <code class="highlight"><span class="n">T</span></code> [<em>expected.comparison_T</em>]</span></a>
      <li><a href="#expected.comparison_unexpected_E"><span class="secno">1.21</span> <span class="content">�.�.10 Comparison with <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> [<em>expected.comparison_unexpected_E</em>]</span></a>
      <li><a href="#expected.specalg"><span class="secno">1.22</span> <span class="content">�.�.11 Specialized algorithms [<em>expected.specalg</em>]</span></a>
     </ol>
    <li>
     <a href="#qq"><span class="secno">2</span> <span class="content">Open Questions</span></a>
     <ol class="toc">
      <li><a href="#ergo"><span class="secno">2.1</span> <span class="content">Ergonomics</span></a>
      <li><a href="#dis"><span class="secno">2.2</span> <span class="content">Disappointment</span></a>
      <li><a href="#stl"><span class="secno">2.3</span> <span class="content">STL Usage</span></a>
     </ol>
    <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>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </ol>
  </nav>
  <main>
   <p>This paper revises <a data-link-type="biblio" href="#biblio-p0323r3">[P0323r3]</a> by applying feedback obtained from LEWG and
EWG. The previous paper contains motivation, design rationale, implementability
information, sample usage, history, alternative designs, and related types. This
update only contains wording and open questions because its purpose is twofold:</p>
   <ul>
    <li data-md="">
     <p>Present appropriate wording for including in the Library Fundamentals TS v3.</p>
    <li data-md="">
     <p>List open questions which the TS should aim to answer.</p>
   </ul>
   <h2 class="heading settled" data-level="1" id="word"><span class="secno">1. </span><span class="content">Wording</span><a class="self-link" href="#word"></a></h2>
   <p>Below, substitute the <code class="highlight">�</code> character with a number or name the editor finds
appropriate for the sub-section.</p>
   <h3 class="heading settled" data-level="1.1" id="unexpected"><span class="secno">1.1. </span><span class="content">�.� Unexpected objects [<em>unexpected</em>]</span><a class="self-link" href="#unexpected"></a></h3>
   <h3 class="heading settled" data-level="1.2" id="unexpected.general"><span class="secno">1.2. </span><span class="content">�.�.1 General [<em>unexpected.general</em>]</span><a class="self-link" href="#unexpected.general"></a></h3>
   <p>This subclause describes class template <code class="highlight"><span class="n">unexpected</span></code> that contain objects
representing an unexpected outcome.</p>
   <h3 class="heading settled" data-level="1.3" id="unexpected.synop"><span class="secno">1.3. </span><span class="content">�.�.2 Header <code class="highlight"><span class="o">&lt;</span><span class="n">experimental</span><span class="o">/</span><span class="n">unexpected</span><span class="o">></span></code> synopsis [<em>unexpected.synop</em>]</span><a class="self-link" href="#unexpected.synop"></a></h3>
<pre class="language-c++ highlight"><span class="n">namespace</span> <span class="n">std</span> <span class="p">{</span>
<span class="n">namespace</span> <span class="n">experimental</span> <span class="p">{</span>
<span class="kr">inline</span> <span class="n">namespace</span> <span class="n">fundamentals_v3</span> <span class="p">{</span>
    <span class="c1">// �.�.3, Unexpected object type
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">class</span> <span class="n">unexpected</span><span class="p">;</span>

    <span class="c1">// �.�.4, Unexpected relational operators
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span>
        <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span>
        <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>

<span class="p">}</span><span class="p">}</span><span class="p">}</span>
</pre>
   <p>A program that needs the instantiation of template <code class="highlight"><span class="n">unexpected</span></code> for a reference
type or <code class="highlight"><span class="kt">void</span></code> is ill-formed.</p>
   <h3 class="heading settled" data-level="1.4" id="unexpected.object"><span class="secno">1.4. </span><span class="content">�.�.3 Unexpected object type [<em>unexpected.object</em>]</span><a class="self-link" href="#unexpected.object"></a></h3>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
<span class="n">class</span> <span class="n">unexpected</span> <span class="p">{</span>
<span class="nl">public</span><span class="p">:</span>
    <span class="n">unexpected</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span> <span class="n">delete</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">explicit</span> <span class="nf">unexpected</span><span class="p">(</span><span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">explicit</span> <span class="nf">unexpected</span><span class="p">(</span><span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">E</span> <span class="k">const</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
<span class="nl">private</span><span class="p">:</span>
    <span class="n">E</span> <span class="n">val</span><span class="p">;</span> <span class="c1">// exposition only
</span><span class="p">}</span><span class="p">;</span>
</pre>
   <p>If <code class="highlight"><span class="n">E</span></code> is void the program is ill formed.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="n">explicit</span> <span class="nf">unexpected</span><span class="p">(</span><span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Build an <code class="highlight"><span class="n">unexpected</span></code> by copying the parameter to the internal storage <code class="highlight"><span class="n">val</span></code>.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="n">explicit</span> <span class="nf">unexpected</span><span class="p">(</span><span class="n">E</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Build an <code class="highlight"><span class="n">unexpected</span></code> by moving the parameter to the internal storage <code class="highlight"><span class="n">val</span></code>.</p>
<pre class="language-c++ highlight">    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Returns</em>: <code class="highlight"><span class="n">val</span></code>.</p>
<pre class="language-c++ highlight">    <span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">E</span> <span class="k">const</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Returns</em>: <code class="highlight"><span class="n">move</span><span class="p">(</span><span class="n">val</span><span class="p">)</span></code>.</p>
   <h3 class="heading settled" data-level="1.5" id="unexpected.relational_op"><span class="secno">1.5. </span><span class="content">�.�.4 Unexpected relational operators [<em>unexpected.relational_op</em>]</span><a class="self-link" href="#unexpected.relational_op"></a></h3>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">y</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="n">E</span></code> shall meet the requirements of <em>EqualityComparable</em>.</p>
   <p><em>Returns</em>: <code class="highlight"><span class="n">x</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">y</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: Specializations of this function template, for which <code class="highlight"><span class="n">x</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">y</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span></code> is a core constant expression, shall be constexpr functions.</p>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">y</span><span class="p">)</span><span class="p">;</span>
</pre>
   <em>Requires</em>: <code class="highlight"><span class="n">E</span></code> shall meet the requirements of <em>EqualityComparable</em>. 
   <p><em>Returns</em>: <code class="highlight"><span class="n">x</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="n">y</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: Specializations of this function template, for which <code class="highlight"><span class="n">x</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="n">y</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span></code> is a core constant expression, shall be constexpr functions.</p>
   <h3 class="heading settled" data-level="1.6" id="expected"><span class="secno">1.6. </span><span class="content">�.� Expected objects [expected]</span><a class="self-link" href="#expected"></a></h3>
   <h3 class="heading settled" data-level="1.7" id="expected.general"><span class="secno">1.7. </span><span class="content">�.�.1 In general [<em>expected.general</em>]</span><a class="self-link" href="#expected.general"></a></h3>
   <p>This subclause describes class template expected that represents expected
objects. An <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> object is an object that contains the storage for
another object and manages the lifetime of this contained object <code class="highlight"><span class="n">T</span></code>,
alternatively it could contain the storage for another unexpected object <code class="highlight"><span class="n">E</span></code>. The contained object may not be initialized after the expected object has
been initialized, and may not be destroyed before the expected object has been
destroyed. The initialization state of the contained object is tracked by the
expected object.</p>
   <h3 class="heading settled" data-level="1.8" id="expected.synop"><span class="secno">1.8. </span><span class="content">�.�.2 Header <code class="highlight"><span class="o">&lt;</span><span class="n">experimental</span><span class="o">/</span><span class="n">expected</span><span class="o">></span></code> synopsis [<em>expected.synop</em>]</span><a class="self-link" href="#expected.synop"></a></h3>
<pre class="language-c++ highlight"><span class="n">namespace</span> <span class="n">std</span> <span class="p">{</span>
<span class="n">namespace</span> <span class="n">experimental</span> <span class="p">{</span>
<span class="kr">inline</span> <span class="n">namespace</span> <span class="n">fundamentals_v3</span> <span class="p">{</span>
    <span class="c1">// �.�.4, Expected for object types
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">class</span> <span class="n">expected</span><span class="p">;</span>

    <span class="c1">// �.�.5, Expected specialization for void
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">class</span> <span class="n">expected</span><span class="o">&lt;</span><span class="kt">void</span><span class="p">,</span><span class="n">E</span><span class="o">></span><span class="p">;</span>

    <span class="c1">// �.�.6, unexpect tag
</span>    <span class="k">struct</span> <span class="n">unexpect_t</span> <span class="p">{</span>
       <span class="n">unexpect_t</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
    <span class="p">}</span><span class="p">;</span>
    <span class="kr">inline</span> <span class="n">constexpr</span> <span class="n">unexpect_t</span> <span class="n">unexpect</span><span class="p">{</span><span class="p">}</span><span class="p">;</span>

    <span class="c1">// �.�.7, class bad_expected_access
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">E</span><span class="o">></span>
       <span class="n">class</span> <span class="n">bad_expected_access</span><span class="p">;</span>

    <span class="c1">// �.�.8, Specialization for void.
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="o">></span>
       <span class="n">class</span> <span class="n">bad_expected_access</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">></span><span class="p">;</span>

    <span class="c1">// �.�.9, Expected relational operators
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>

    <span class="c1">// �.�.10, Comparison with T
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>

    <span class="c1">// �.�.10, Comparison with unexpected&lt;E>
</span>    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
      <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>

    <span class="c1">// �.�.11, Specialized algorithms
</span>    <span class="kt">void</span> <span class="nf">swap</span><span class="p">(</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>

<span class="p">}</span><span class="p">}</span><span class="p">}</span>
</pre>
   <p>A program that necessitates the instantiation of template <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">T</span></code> for a reference type or for possibly cv-qualified types <code class="highlight"><span class="n">in_place_t</span></code>, <code class="highlight"><span class="n">unexpect_t</span></code> or <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> or <code class="highlight"><span class="n">E</span></code> for a reference type or <code class="highlight"><span class="kt">void</span></code> is ill-formed.</p>
   <h3 class="heading settled" data-level="1.9" id="expected.defs"><span class="secno">1.9. </span><span class="content">�.�.3 Definitions [<em>expected.defs</em>]</span><a class="self-link" href="#expected.defs"></a></h3>
   <p>An instance of <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> is said to be valued if it contains a value of
type <code class="highlight"><span class="n">T</span></code>. An instance of <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> is said to be unexpected if it
contains an object of type <code class="highlight"><span class="n">E</span></code>.</p>
   <h3 class="heading settled" data-level="1.10" id="expected.object"><span class="secno">1.10. </span><span class="content">�.�.4 expected for object types [<em>expected.object</em>]</span><a class="self-link" href="#expected.object"></a></h3>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
<span class="n">class</span> <span class="n">expected</span>
<span class="p">{</span>
<span class="nl">public</span><span class="p">:</span>
    <span class="k">typedef</span> <span class="n">T</span> <span class="n">value_type</span><span class="p">;</span>
    <span class="k">typedef</span> <span class="n">E</span> <span class="n">error_type</span><span class="p">;</span>
    <span class="k">typedef</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="n">unexpected_type</span><span class="p">;</span>

    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="o">></span>
        <span class="k">struct</span> <span class="n">rebind</span> <span class="p">{</span>
        <span class="n">using</span> <span class="n">type</span> <span class="o">=</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span> <span class="n">error_type</span><span class="o">></span><span class="p">;</span>
      <span class="p">}</span><span class="p">;</span>

    <span class="c1">// �.�.4.1, constructors
</span>    <span class="n">constexpr</span> <span class="nf">expected</span><span class="p">(</span><span class="p">)</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="nf">expected</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="nf">expected</span><span class="p">(</span><span class="n">expected</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span> <span class="n">G</span><span class="o">></span>
        <span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span> <span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span> <span class="n">G</span><span class="o">></span>
        <span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span> <span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>

    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span> <span class="o">=</span> <span class="n">T</span><span class="o">></span>
        <span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">)</span><span class="p">;</span>

    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">in_place_t</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">in_place_t</span><span class="p">,</span> <span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">G</span> <span class="o">=</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">G</span><span class="o">></span> <span class="k">const</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">G</span> <span class="o">=</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">G</span><span class="o">></span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpect_t</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpect_t</span><span class="p">,</span> <span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span><span class="p">;</span>

    <span class="c1">// �.�.4.2, destructor
</span>    <span class="o">~</span><span class="n">expected</span><span class="p">(</span><span class="p">)</span><span class="p">;</span>

    <span class="c1">// �.�.4.3, assignment
</span>    <span class="n">expected</span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">expected</span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="n">expected</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span> <span class="o">=</span> <span class="n">T</span><span class="o">></span> <span class="n">expected</span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">G</span> <span class="o">=</span> <span class="n">E</span><span class="o">></span>
        <span class="n">expected</span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">G</span> <span class="o">=</span> <span class="n">E</span><span class="o">></span>
        <span class="n">expected</span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>

    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
        <span class="kt">void</span> <span class="n">emplace</span><span class="p">(</span><span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
        <span class="kt">void</span> <span class="n">emplace</span><span class="p">(</span><span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="p">)</span><span class="p">;</span>

    <span class="c1">// �.�.4.4, swap
</span>    <span class="kt">void</span> <span class="nf">swap</span><span class="p">(</span><span class="n">expected</span><span class="o">&amp;</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>

    <span class="c1">// �.�.4.5, observers
</span>    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">*</span> <span class="n">operator</span> <span class="o">-</span><span class="o">></span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">T</span><span class="o">*</span> <span class="n">operator</span> <span class="o">-</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">operator</span> <span class="nf">bool</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="n">noexcept</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="kt">bool</span> <span class="nf">has_value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="n">noexcept</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="n">T</span> <span class="n">value_or</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="o">></span>
        <span class="n">T</span> <span class="n">value_or</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>

<span class="nl">private</span><span class="p">:</span>
    <span class="kt">bool</span> <span class="n">has_val</span><span class="p">;</span> <span class="c1">// exposition only
</span>    <span class="k">union</span>
    <span class="p">{</span>
        <span class="n">value_type</span> <span class="n">val</span><span class="p">;</span> <span class="c1">// exposition only
</span>        <span class="n">unexpected_type</span> <span class="n">unexpect</span><span class="p">;</span> <span class="c1">// exposition only
</span>    <span class="p">}</span><span class="p">;</span>
<span class="p">}</span><span class="p">;</span>
</pre>
   <p>Valued instances of <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> where <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">E</span></code> are of object type shall
contain a value of type <code class="highlight"><span class="n">T</span></code> or a value of type <code class="highlight"><span class="n">E</span></code> within its own storage. These
values are referred to as the contained or the unexpected value of the <code class="highlight"><span class="n">expected</span></code> object. Implementations are not permitted to use additional storage,
such as dynamic memory, to allocate its contained or unexpected value. The
contained or unexpected value shall be allocated in a region of the <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> storage suitably aligned for the type <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>. Members <code class="highlight"><span class="n">has_val</span></code>, <code class="highlight"><span class="n">val</span></code> and <code class="highlight"><span class="n">unexpect</span></code> are provided for exposition
only. Implementations need not provide those members. <code class="highlight"><span class="n">has_val</span></code> indicates
whether the expected object’s contained value has been initialized (and not yet
destroyed); when <code class="highlight"><span class="n">has_val</span></code> is true <code class="highlight"><span class="n">val</span></code> points to the contained value, and when
it is false <code class="highlight"><span class="n">unexpect</span></code> points to the erroneous value.</p>
   <p><code class="highlight"><span class="n">T</span></code> must be <code class="highlight"><span class="kt">void</span></code> or shall be object type and shall satisfy the requirements of <code class="highlight"><span class="n">Destructible</span></code> (Table 27).</p>
   <p><code class="highlight"><span class="n">E</span></code> shall be object type and shall satisfy the requirements of <code class="highlight"><span class="n">Destructible</span></code> (Table 27).</p>
   <h3 class="heading settled" data-level="1.11" id="expected.object.ctor"><span class="secno">1.11. </span><span class="content">�.�.4.1 Constructors [<em>expected.object.ctor</em>]</span><a class="self-link" href="#expected.object.ctor"></a></h3>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="nf">expected</span><span class="p">(</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the contained value as if direct-non-list-initializing an
object of type <code class="highlight"><span class="n">T</span></code> with the expression <code class="highlight"><span class="n">T</span><span class="p">{</span><span class="p">}</span></code> (if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>).</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: Any exception thrown by the default constructor of <code class="highlight"><span class="n">T</span></code> (nothing if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code>).</p>
   <p><em>Remarks</em>: If value-initialization of <code class="highlight"><span class="n">T</span></code> is a constexpr constructor or <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> this constructor shall be constexpr. This constructor shall be defined as
deleted unless <code class="highlight"><span class="n">is_default_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> or <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code>.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="nf">expected</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&amp;</span> <span class="n">rhs</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: If <code class="highlight"><span class="n">rhs</span></code> contains a value, initializes the contained value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with the expression <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code> (if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>).</p>
   <p>If <code class="highlight"><span class="n">rhs</span></code> does not contain a value initializes the unexpected value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with the
expression <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code>.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">T</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> or by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.</p>
   <p><em>Remarks</em>: This constructor shall be defined as deleted unless <code class="highlight"><span class="n">is_copy_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> or <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_copy_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>. If <code class="highlight"><span class="n">is_trivially_copy_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> is <code class="highlight">true</code> or <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_trivially_copy_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> is <code class="highlight">true</code>,
this constructor shall be a constexpr constructor.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="nf">expected</span><span class="p">(</span><span class="n">expected</span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">rhs</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: If <code class="highlight"><span class="n">rhs</span></code> contains a value initializes the contained value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with the expression <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="o">*</span><span class="n">rhs</span><span class="p">)</span></code> (if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>).</p>
   <p>If <code class="highlight"><span class="n">rhs</span></code> does not contain a value initializes the unexpected value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with the
expression <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">)</span></code>.</p>
   <p><code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span></code> is unchanged.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code>.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">T</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> or by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.</p>
   <p><em>Remarks</em>: The expression inside <code class="highlight"><span class="k">noexcept</span></code> is equivalent to: <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">or</span></code>T <code class="highlight"><span class="n">is</span> ``<span class="kt">void</span></code> and <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.  This constructor shall not participate in
overload resolution unless <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">and</span> <span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.  If <code class="highlight"><span class="n">is_trivially_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> is <code class="highlight">true</code> or <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_trivially_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> is <code class="highlight">true</code>,
this constructor shall be a constexpr constructor.</p>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span> <span class="n">G</span><span class="o">></span>
    <span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span> <span class="n">rhs</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: If <code class="highlight"><span class="n">rhs</span></code> contains a value initializes the contained value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with the expression <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code> (if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>).</p>
   <p>If <code class="highlight"><span class="n">rhs</span></code> does not contain a value initializes the unexpected value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with the
expression <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code>.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">T</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> or by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.</p>
   <p><em>Remarks</em>: This constructor shall not participate in overload resolution unless:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="k">const</span> <span class="n">U</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">true</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="p">,</span> <span class="k">const</span> <span class="n">G</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">true</code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>, and</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>.</p>
   </ul>
   <p>The constructor is explicit if and only if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">U</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is false or <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">G</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> is
false.</p>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span> <span class="n">G</span><span class="o">></span>
    <span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">rhs</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: If <code class="highlight"><span class="n">rhs</span></code> contains a value initializes the contained value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with the expression <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="o">*</span><span class="n">rhs</span><span class="p">)</span></code> or nothing if <code class="highlight"><span class="n">T</span></code>is <code class="highlight"><span class="kt">void</span></code>.</p>
   <p>If <code class="highlight"><span class="n">rhs</span></code> does not contain a value initializes the unexpected value as if
direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with the
expression <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">)</span></code>. <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span></code> is unchanged</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code>.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">T</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> or by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.</p>
   <p><em>Remarks</em>: This constructor shall not participate in overload resolution unless:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">true</code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="p">,</span> <span class="n">G</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">true</code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>, and</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">U</span><span class="p">,</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is <code class="highlight">false</code> or <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code> are <code class="highlight"><span class="kt">void</span></code>.</p>
   </ul>
   <p>The constructor is explicit if and only if <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is false
or <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">G</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> is false.</p>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span> <span class="o">=</span> <span class="n">T</span><span class="o">></span>
    <span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the contained value as if direct-non-list-initializing an
object of type <code class="highlight"><span class="n">T</span></code> with the expression <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">(</span><span class="n">v</span><span class="p">)</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">T</span></code>.</p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">T</span></code>'s selected constructor is a constexpr constructor, this
constructor shall be a constexpr constructor.  This constructor shall not
participate in overload resolution unless <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code> is <code class="highlight">true</code>, <code class="highlight"><span class="n">is_same_v</span><span class="o">&lt;</span><span class="n">decay_t</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">,</span> <span class="n">in_place_t</span><span class="o">></span></code> is <code class="highlight">false</code>, <code class="highlight"><span class="n">is_same_v</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="p">,</span> <span class="n">decay_t</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="o">></span></code> is false, and <code class="highlight"><span class="n">is_same_v</span><span class="o">&lt;</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">,</span> <span class="n">decay_t</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="o">></span></code> is false. The constructor is explicit if
and only if <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is false.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
  <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">in_place_t</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">args</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the contained value as if direct-non-list-initializing an
object of type <code class="highlight"><span class="n">T</span></code> with the arguments <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">T</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>.</p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">T</span></code>'s constructor selected for the initialization is a constexpr
constructor, this constructor shall be a constexpr constructor.  This
constructor shall not participate in overload resolution unless <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="k">sizeof</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="n">Args</span><span class="p">)</span><span class="o">=</span><span class="o">=</span><span class="mi">0</span></code> or <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span></code>.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
  <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">in_place_t</span><span class="p">,</span> <span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span> <span class="n">il</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">args</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the contained value as if direct-non-list-initializing an
object of type <code class="highlight"><span class="n">T</span></code> with the arguments <code class="highlight"><span class="n">il</span><span class="p">,</span> <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">T</span></code>.</p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">T</span></code>'s constructor selected for the initialization is a constexpr
constructor, this constructor shall be a constexpr constructor.  This
constructor shall not participate in overload resolution unless <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_constructible</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">initializer_list_v</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span></code>.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">G</span> <span class="o">=</span> <span class="n">E</span><span class="o">></span>
<span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">G</span><span class="o">></span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">e</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the unexpected value as if direct-non-list-initializing
an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with the expression <code class="highlight"><span class="n">e</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
   <p><em>Remark</em>: If <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>'s selected constructor is a constexpr constructor,
this constructor shall be a constexpr constructor. This constructor shall not
participate in overload resolution unless <code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="p">,</span> <span class="k">const</span> <span class="n">G</span><span class="o">&amp;</span><span class="o">></span></code>. The
constructor is explicit if and only if <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">G</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> is false.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">G</span> <span class="o">=</span> <span class="n">E</span><span class="o">></span>
<span class="n">EXPLICIT</span> <span class="n">constexpr</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">G</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">e</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the unexpected value as if direct-non-list-initializing
an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with the expression <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="n">e</span><span class="p">)</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
   <p><em>Remark</em>: If <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>'s selected constructor is a constexpr constructor,
this constructor shall be a constexpr constructor.  The expression inside <code class="highlight"><span class="k">noexcept</span></code> is equivalent to: <code class="highlight"><span class="n">is_nothrow_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="p">,</span> <span class="n">G</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code>.  This
constructor shall not participate in overload resolution unless <code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="p">,</span> <span class="n">G</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code>. The constructor is explicit if and only if <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">G</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> is false.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
  <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpect_t</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">args</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the unexpected value as if direct-non-list-initializing
an object of type<code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>with the arguments <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>'s constructor selected for the initialization is a
constexpr constructor, this constructor shall be a constexpr constructor. This
constructor shall not participate in overload resolution unless <code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span></code>.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
  <span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">expected</span><span class="p">(</span><span class="n">unexpect_t</span><span class="p">,</span> <span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span> <span class="n">il</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">args</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Initializes the unexpected value as if direct-non-list-initializing
an object of type<code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with the arguments <code class="highlight"><span class="n">il</span><span class="p">,</span> <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected constructor of <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.</p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>'s constructor selected for the initialization is a
constexpr constructor, this constructor shall be a constexpr constructor.  This
constructor shall not participate in overload resolution unless <code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="p">,</span> <span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span></code>.</p>
   <h3 class="heading settled" data-level="1.12" id="expected.object.dtor"><span class="secno">1.12. </span><span class="content">�.�.4.2 Destructor [<em>expected.object.dtor</em>]</span><a class="self-link" href="#expected.object.dtor"></a></h3>
<pre class="language-c++ highlight"><span class="o">~</span><span class="n">expected</span><span class="p">(</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: If <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_trivially_destructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="o">!</span><span class="o">=</span> true</code> and <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value, calls <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>.  If <code class="highlight"><span class="n">is_trivially_destructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="o">!</span><span class="o">=</span> true</code> and <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value, calls <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> or <code class="highlight"><span class="n">is_trivially_destructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">and</span> <span class="n">is_trivially_destructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> is <code class="highlight">true</code> then this destructor shall be a
trivial destructor.</p>
   <h3 class="heading settled" data-level="1.13" id="expected.object.assign"><span class="secno">1.13. </span><span class="content">�.�.4.3 Assignment [<em>expected.object.assign</em>]</span><a class="self-link" href="#expected.object.assign"></a></h3>
<pre class="language-c++ highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">rhs</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>:</p>
   <p>If <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value and <code class="highlight"><span class="n">rhs</span></code> contains a value,</p>
   <ul>
    <li data-md="">
     <p>assigns <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code> to the contained value <code class="highlight"><span class="n">val</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>;</p>
   </ul>
   <p>otherwise, if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value and <code class="highlight"><span class="n">rhs</span></code> does not contain a
value,</p>
   <ul>
    <li data-md="">
     <p>assigns <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> to the unexpected value <code class="highlight"><span class="n">unexpect</span></code>;</p>
   </ul>
   <p>otherwise, if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value and <code class="highlight"><span class="n">rhs</span></code> does not contain a value,</p>
   <ul>
    <li data-md="">
     <p>if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code></p>
     <ul>
      <li data-md="">
       <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>. Either</p>
      <li data-md="">
       <p>The didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> (which can’t throw), or</p>
      <li data-md="">
       <p>the constructor did throw, an nothing was changed.</p>
     </ul>
    <li data-md="">
     <p>otherwise <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>, if <code class="highlight"><span class="n">is_nothrow_copy_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>destroys the contained value by calling <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="n">T</span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
     </ul>
    <li data-md="">
     <p>otherwise if <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>constructs a new <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code> (this can throw),</p>
      <li data-md="">
       <p>destroys the contained value by calling <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="n">T</span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
     </ul>
   </ul>
   <p>otherwise as <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
   <ul>
    <li data-md="">
     <p>constructs a new <code class="highlight"><span class="n">T</span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="o">*</span><span class="k">this</span></code> (this can throw),</p>
    <li data-md="">
     <p>destroys the contained value by calling <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="n">T</span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>,</p>
    <li data-md="">
     <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>. Either,</p>
     <ul>
      <li data-md="">
       <p>the last constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> (which can’t throw), or</p>
      <li data-md="">
       <p>the last constructor did throw, so move-construct the <code class="highlight"><span class="n">T</span></code> from the stack <code class="highlight"><span class="n">tmp</span></code> back into the expected storage (which can’t throw as <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> is <code class="highlight">true</code>), and rethrow the exception.</p>
     </ul>
   </ul>
   <p>otherwise <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value and <code class="highlight"><span class="n">rhs</span></code> contains a value</p>
   <ul>
    <li data-md="">
     <p>if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code></p>
    <li data-md="">
     <p>otherwise <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>, if <code class="highlight"><span class="n">is_nothrow_copy_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code></p>
      <li data-md="">
       <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code>;</p>
     </ul>
    <li data-md="">
     <p>otherwise <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>, if <code class="highlight"><span class="n">is_nothrow_copy_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code></p>
      <li data-md="">
       <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="n">move</span><span class="p">(</span><span class="o">*</span><span class="n">rhs</span><span class="p">)</span></code>;</p>
     </ul>
    <li data-md="">
     <p>otherwise if <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>constructs a new <code class="highlight"><span class="n">T</span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code> (this can throw),</p>
      <li data-md="">
       <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code></p>
      <li data-md="">
       <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="n">move</span><span class="p">(</span><span class="n">tmp</span><span class="p">)</span></code>;</p>
     </ul>
    <li data-md="">
     <p>otherwise as <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>constructs a new <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="k">this</span><span class="o">-</span><span class="o">></span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> (which can throw),</p>
      <li data-md="">
       <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code>. Either,</p>
      <li data-md="">
       <p>the constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">T</span></code> (which can’t throw), or</p>
      <li data-md="">
       <p>the constructor did throw, so move-construct the <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> from the stack <code class="highlight"><span class="n">tmp</span></code> back into the expected storage (which can’t throw as <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> is <code class="highlight">true</code>), and rethrow the exception.</p>
     </ul>
   </ul>
   <p><em>Returns</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code>.</p>
   <p><em>Throws</em>: any exception throw by the selected operations.</p>
   <p><em>Remarks</em>: If any exception is thrown, the values of <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span> <span class="n">and</span> <span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span></code> remain unchanged.</p>
   <p>If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s or <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>'s copy
constructor, no effect. If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s or <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>'s copy assignment, the state of its contained value is as defined
by the exception safety guarantee of <code class="highlight"><span class="n">T</span></code>'s or <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>'s copy assignment.</p>
   <p>This operator shall be defined as deleted unless</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_copy_assignable_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_copy_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> or</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_copy_assignable_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_copy_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_copy_assignable_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_copy_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> and (<code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> or <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code>).</p>
<pre class="language-c++ highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">rhs</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>
</pre>
   </ul>
   <p><em>Effects</em>:</p>
   <p>If <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value and <code class="highlight"><span class="n">rhs</span></code> contains a value,</p>
   <ul>
    <li data-md="">
     <p>move assign <code class="highlight"><span class="o">*</span><span class="n">rhs</span></code> to the contained value <code class="highlight"><span class="n">val</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>;</p>
   </ul>
   <p>otherwise, if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value and <code class="highlight"><span class="n">rhs</span></code> does not contain a
value,</p>
   <ul>
    <li data-md="">
     <p>move assign <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> to the unexpected value <code class="highlight"><span class="n">unexpect</span></code>;</p>
   </ul>
   <p>otherwise, if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value and <code class="highlight"><span class="n">rhs</span></code> does not contain a value,</p>
   <ul>
    <li data-md="">
     <p>if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code></p>
     <ul>
      <li data-md="">
       <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>. Either</p>
      <li data-md="">
       <p>the constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> (which can’t throw), or</p>
      <li data-md="">
       <p>the constructor did throw, and nothing was changed.</p>
     </ul>
    <li data-md="">
     <p>otherwise if <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>destroys the contained value by calling <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="n">T</span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">)</span></code>;</p>
     </ul>
    <li data-md="">
     <p>otherwise as <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>move constructs a new <code class="highlight"><span class="n">T</span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="o">*</span><span class="k">this</span></code> (which can’t throw as <code class="highlight"><span class="n">T</span></code> is nothrow-move-constructible),</p>
      <li data-md="">
       <p>destroys the contained value by calling <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="n">T</span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected_type</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">)</span></code>. Either,</p>
      <li data-md="">
       <p>The constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">unexpected_type</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> (which can’t throw), or</p>
      <li data-md="">
       <p>The constructor did throw, so move-construct the <code class="highlight"><span class="n">T</span></code> from the stack <code class="highlight"><span class="n">tmp</span></code> back into the expected storage (which can’t throw as <code class="highlight"><span class="n">T</span></code> is nothrow-move-constructible), and rethrow the exception.</p>
     </ul>
   </ul>
   <p>otherwise <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value and <code class="highlight"><span class="n">rhs</span></code> contains a value,</p>
   <ul>
    <li data-md="">
     <p>if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code></p>
    <li data-md="">
     <p>otherwise, if <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="o">*</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span></code>;</p>
     </ul>
    <li data-md="">
     <p>otherwise as <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
     <ul>
      <li data-md="">
       <p>move constructs a new <code class="highlight"><span class="n">unpepected_type</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="k">this</span><span class="o">-</span><span class="o">></span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> (which can’t throw as <code class="highlight"><span class="n">E</span></code> is nothrow-move-constructible),</p>
      <li data-md="">
       <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="o">*</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span></code>. Either,</p>
      <li data-md="">
       <p>The constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">T</span></code> (which can’t throw), or</p>
      <li data-md="">
       <p>The constructor did throw, so move-construct the <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> from the stack <code class="highlight"><span class="n">tmp</span></code> back into the expected storage (which can’t throw as <code class="highlight"><span class="n">E</span></code> is nothrow-move-constructible), and rethrow the exception.</p>
     </ul>
   </ul>
   <p><em>Returns</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: The expression inside noexcept is equivalent to: <code class="highlight"><span class="n">is_nothrow_move_assignable_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code>.</p>
   <p>If any exception is thrown, the values of <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span> <span class="n">and</span> <span class="kt">bool</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span></code> remain
unchanged. If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s copy constructor,
no effect. If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s copy assignment,
the state of its contained value is as defined by the exception safety guarantee
of <code class="highlight"><span class="n">T</span></code>'s copy assignment. If an exception is thrown during the call to <code class="highlight"><span class="n">E</span></code>'s
copy assignment, the state of its contained unexpected value is as defined by
the exception safety guarantee of <code class="highlight"><span class="n">E</span></code>'s copy assignment.</p>
   <p>This operator shall be defined as deleted unless <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_move_assignable_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_nothrow_move_assignable_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="o">></span>
  <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>:</p>
   <p>If <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value, assigns <code class="highlight"><span class="n">forward</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">(</span><span class="n">v</span><span class="p">)</span></code> to the contained value <code class="highlight"><span class="n">val</span></code>;</p>
   <p>otherwise, if <code class="highlight"><span class="n">is_nothrow_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="o">></span></code></p>
   <ul>
    <li data-md="">
     <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
    <li data-md="">
     <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="n">forward</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">(</span><span class="n">v</span><span class="p">)</span></code> and</p>
    <li data-md="">
     <p>set <code class="highlight"><span class="n">has_val</span></code>to <code class="highlight">true</code>;</p>
   </ul>
   <p>otherwise as <code class="highlight"><span class="n">is_nothrow_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
   <ul>
    <li data-md="">
     <p>move constructs a new <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="k">this</span><span class="o">-</span><span class="o">></span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> (which can’t throw as <code class="highlight"><span class="n">E</span></code> is nothrow-move-constructible),</p>
    <li data-md="">
     <p>destroys the contained value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
    <li data-md="">
     <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="n">forward</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">(</span><span class="n">v</span><span class="p">)</span></code>. Either,</p>
     <ul>
      <li data-md="">
       <p>the constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">T</span></code> (which can’t throw), that is set <code class="highlight"><span class="n">has_val</span></code> to <code class="highlight">true</code>, or</p>
      <li data-md="">
       <p>the constructor did throw, so move construct the <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> from the stack <code class="highlight"><span class="n">tmp</span></code> back into the expected storage (which can’t throw as <code class="highlight"><span class="n">E</span></code> is nothrow-move-constructible), and re-throw the exception.</p>
     </ul>
   </ul>
   <p><em>Returns</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Remarks</em>: If any exception is thrown, the value of <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code> remains
 unchanged. If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s constructor, no
 effect. If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s copy assignment, the
 state of its contained value is as defined by the exception safety guarantee of <code class="highlight"><span class="n">T</span></code>'s copy assignment.</p>
   <p>This function shall not participate in overload resolution unless:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_same_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span><span class="kt">void</span><span class="o">></span></code> is <code class="highlight">false</code> and</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_same_v</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span><span class="n">E</span><span class="o">></span><span class="p">,</span> <span class="n">decay_t</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="o">></span></code> is <code class="highlight">false</code> and</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">conjunction_v</span><span class="o">&lt;</span><span class="n">is_scalar</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">,</span> <span class="n">is_same</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">decay_t</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="o">></span><span class="o">></span></code> is <code class="highlight">false</code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">U</span><span class="o">></span></code> is <code class="highlight">true</code>,</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_assignable_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">U</span><span class="o">></span></code> is <code class="highlight">true</code> and</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> is <code class="highlight">true</code>.</p>
<pre class="language-c++ highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">e</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>
</pre>
   </ul>
   <p><em>Effects</em>:</p>
   <p>If <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value, assigns <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> to the
unexpected value <code class="highlight"><span class="n">unexpect</span></code>;</p>
   <p>otherwise,</p>
   <ul>
    <li data-md="">
     <p>destroys the contained value by calling <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">></span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span><span class="p">)</span></code> and set <code class="highlight"><span class="n">has_val</span></code> to <code class="highlight">false</code>.</p>
   </ul>
   <p><em>Returns</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Remarks</em>: If any exception is thrown, value of valued remains unchanged.</p>
   <p>This signature shall not participate in overload resolution unless <code class="highlight"><span class="n">is_nothrow_copy_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_assignable_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code>.</p>
<pre class="language-c++ highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">operator</span><span class="o">=</span><span class="p">(</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">e</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>:</p>
   <p>If <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value, move assign <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> to
the unexpected value <code class="highlight"><span class="n">unexpect</span></code>;</p>
   <p>otherwise,</p>
   <ul>
    <li data-md="">
     <p>destroys the contained value by calling <code class="highlight"><span class="n">val</span><span class="p">.</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code> if <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>,</p>
    <li data-md="">
     <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">move</span><span class="p">(</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">></span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span><span class="p">)</span><span class="p">)</span></code> and set <code class="highlight"><span class="n">has_val</span></code> to <code class="highlight">false</code>.</p>
   </ul>
   <p><em>Returns</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Remarks</em>: If any exception is thrown, value of valued remains unchanged.</p>
   <p>This signature shall not participate in overload resolution unless <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_move_assignable_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code>.</p>
<pre class="language-c++ highlight">  <span class="kt">void</span> <span class="n">expected</span><span class="o">&lt;</span><span class="kt">void</span><span class="p">,</span><span class="n">E</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">emplace</span><span class="p">(</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>:</p>
   <p>If <code class="highlight"><span class="o">*</span><span class="k">this</span></code> doesn’t contains a value</p>
   <ul>
    <li data-md="">
     <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
    <li data-md="">
     <p>set <code class="highlight"><span class="n">has_val</span></code>to <code class="highlight">true</code></p>
   </ul>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: nothing</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
  <span class="kt">void</span> <span class="n">emplace</span><span class="p">(</span><span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">args</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>:</p>
   <p>If <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value, assigns the contained value <code class="highlight"><span class="n">val</span></code> as if
constructing an object of type <code class="highlight"><span class="n">T</span></code> with the arguments <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code></p>
   <p>otherwise, if <code class="highlight"><span class="n">is_nothrow_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span></code></p>
   <ul>
    <li data-md="">
     <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
    <li data-md="">
     <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code> and</p>
    <li data-md="">
     <p>set <code class="highlight"><span class="n">has_val</span></code> to <code class="highlight">true</code>;</p>
   </ul>
   <p>otherwise if <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code></p>
   <ul>
    <li data-md="">
     <p>constructs a new <code class="highlight"><span class="n">T</span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code> (which can throw),</p>
    <li data-md="">
     <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
    <li data-md="">
     <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="n">move</span><span class="p">(</span><span class="n">tmp</span><span class="p">)</span></code> (which can not throw) and</p>
    <li data-md="">
     <p>set <code class="highlight"><span class="n">has_val</span></code> to <code class="highlight">true</code>;</p>
   </ul>
   <p>otherwise as <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code></p>
   <ul>
    <li data-md="">
     <p>move constructs a new <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="n">tmp</span></code> on the stack from <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="k">this</span><span class="o">-</span><span class="o">></span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> (which can’t throw),</p>
    <li data-md="">
     <p>destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
    <li data-md="">
     <p>initializes the contained value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">T</span></code> with <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code>. Either,</p>
     <ul>
      <li data-md="">
       <p>the constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">T</span></code> (which can’t throw), that is set <code class="highlight"><span class="n">has_val</span></code> to <code class="highlight">true</code>, or</p>
      <li data-md="">
       <p>the constructor did throw, so move-construct the <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> from the stack <code class="highlight"><span class="n">tmp</span></code> back into the expected storage (which can’t throw as <code class="highlight"><span class="n">E</span></code> is nothrow-move-constructible), and re-throw the exception.</p>
     </ul>
   </ul>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected assignment of <code class="highlight"><span class="n">T</span></code>.</p>
   <p><em>Remarks</em>: If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s assignment,
nothing changes.</p>
   <p>This signature shall not participate in overload resolution unless <code class="highlight"><span class="n">is_no_throw_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span></code>.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="p">,</span> <span class="n">class</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">Args</span><span class="o">></span>
  <span class="kt">void</span> <span class="n">emplace</span><span class="p">(</span><span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span> <span class="n">il</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span> <span class="n">args</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value, assigns the contained value <code class="highlight"><span class="n">val</span></code> as if
constructing an object of type <code class="highlight"><span class="n">T</span></code> with the arguments <code class="highlight"><span class="n">il</span><span class="p">,</span> <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code>, otherwise destroys the unexpected value by calling <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code> and initializes the contained value as if
constructing an object of type <code class="highlight"><span class="n">T</span></code> with the arguments <code class="highlight"><span class="n">il</span><span class="p">,</span> <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="p">.</span><span class="p">.</span><span class="p">.</span></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: Any exception thrown by the selected assignment of <code class="highlight"><span class="n">T</span></code>.</p>
   <p><em>Remarks</em>: If an exception is thrown during the call to <code class="highlight"><span class="n">T</span></code>'s assignment nothing
changes.</p>
   <p>The function shall not participate in overload resolution unless: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and <code class="highlight"><span class="n">is_no_throw_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">initializer_list</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="o">&amp;</span><span class="p">,</span> <span class="n">Args</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">.</span><span class="p">.</span><span class="p">.</span><span class="o">></span></code>.</p>
   <h3 class="heading settled" data-level="1.14" id="expected.object.swap"><span class="secno">1.14. </span><span class="content">�.�.4.4 Swap [<em>expected.object.swap</em>]</span><a class="self-link" href="#expected.object.swap"></a></h3>
<pre class="language-c++ highlight"><span class="kt">void</span> <span class="nf">swap</span><span class="p">(</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">rhs</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">see</span> <span class="n">below</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p class="issue" id="issue-5d614af5"><a class="self-link" href="#issue-5d614af5"></a> Check <code class="highlight"><span class="n">swap</span></code> Effects.</p>
   <p><em>Effects</em>: if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value and <code class="highlight"><span class="n">rhs</span></code> contains a value,</p>
   <ul>
    <li data-md="">
     <p>calls <code class="highlight"><span class="k">using</span> <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">swap</span><span class="p">;</span> <span class="n">swap</span><span class="p">(</span><span class="n">val</span><span class="p">,</span> <span class="n">rhs</span><span class="p">.</span><span class="n">val</span><span class="p">)</span></code>,</p>
   </ul>
   <p>otherwise if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value and <code class="highlight"><span class="n">rhs</span></code> does not contain a value,</p>
   <ul>
    <li data-md="">
     <p>calls <code class="highlight"><span class="k">using</span> <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">swap</span><span class="p">;</span> <span class="n">swap</span><span class="p">(</span><span class="n">err</span><span class="p">,</span> <span class="n">rhs</span><span class="p">.</span><span class="n">err</span><span class="p">)</span></code>,</p>
   </ul>
   <p>otherwise if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contains a value and <code class="highlight"><span class="n">rhs</span></code> contains a value,</p>
   <ul>
    <li data-md="">
     <p>calls <code class="highlight"><span class="n">rhs</span><span class="p">.</span><span class="n">swap</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code>,</p>
   </ul>
   <p>otherwise, <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value and <code class="highlight"><span class="n">rhs</span></code> does not contains a value,</p>
   <ul>
    <li data-md="">
     <p>if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code></p>
     <ul>
      <li data-md="">
       <p>initializes the unexpected value as if direct-non-list-initializing an object of type <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> with <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">)</span><span class="p">)</span></code>. Either</p>
      <li data-md="">
       <p>the constructor didn’t throw, so mark the expected as holding a <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>,  destroys the unexpected value by calling <code class="highlight"><span class="n">rhs</span><span class="p">.</span><span class="n">unexpect</span><span class="p">.</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code> set <code class="highlight"><span class="n">rhs</span><span class="p">.</span><span class="n">has_val</span></code> to <code class="highlight">true</code>.</p>
      <li data-md="">
       <p>the constructor did throw, rethrow the exception.</p>
     </ul>
    <li data-md="">
     <p>otherwise <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>, if <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code>,</p>
     <ul>
      <li data-md="">
       <p>the unexpected value of <code class="highlight"><span class="n">rhs</span></code> is moved to a temporary variable <code class="highlight"><span class="n">tmp</span></code> of type <code class="highlight"><span class="n">unpected_type</span></code>,</p>
      <li data-md="">
       <p>followed by destruction of the unexpected value as if by <code class="highlight"><span class="n">rhs</span><span class="p">.</span><span class="n">unexpect</span><span class="p">.</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>the contained value of <code class="highlight"><span class="n">rhs</span></code> is direct-initialized from <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="o">*</span><span class="n">other</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>followed by destruction of the contained value as if by <code class="highlight"><span class="n">rhs</span><span class="p">.</span><span class="n">val</span><span class="o">-</span><span class="o">></span><span class="n">T</span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>the unexpected value of <code class="highlight"><span class="k">this</span></code> is direct-initialized from <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="n">tmp</span><span class="p">)</span></code>, after this, <code class="highlight"><span class="k">this</span></code> does not contain a value; and <code class="highlight"><span class="n">rhs</span></code> contains a value.</p>
     </ul>
    <li data-md="">
     <p>otherwise if <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code>,</p>
     <ul>
      <li data-md="">
       <p>the contained value of <code class="highlight"><span class="n">rhs</span></code> is moved to a temporary variable <code class="highlight"><span class="n">tmp</span></code> of type <code class="highlight"><span class="n">T</span></code>,</p>
      <li data-md="">
       <p>followed by destruction of the contained value as if by <code class="highlight"><span class="n">rhs</span><span class="p">.</span><span class="n">val</span><span class="p">.</span><span class="n">T</span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">T</span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>the unexpected value of <code class="highlight"><span class="n">rhs</span></code> is direct-initialized from <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="n">other</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>followed by destruction of the unexpected value as if by <code class="highlight"><span class="n">rhs</span><span class="p">.</span><span class="n">unexpect</span><span class="o">-</span><span class="o">></span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="o">~</span><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="p">(</span><span class="p">)</span></code>,</p>
      <li data-md="">
       <p>the contained value of <code class="highlight"><span class="k">this</span></code> is direct-initialized from <code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="n">tmp</span><span class="p">)</span></code>, after this, <code class="highlight"><span class="k">this</span></code> does not contain a value; and <code class="highlight"><span class="n">rhs</span></code> contains a value.</p>
     </ul>
   </ul>
   <p><em>Throws</em>: Any exceptions that the expressions in the Effects clause throw.</p>
   <p class="issue" id="issue-a083188f"><a class="self-link" href="#issue-a083188f"></a> Adapt <code class="highlight"><span class="n">swap</span></code> Remarks once Effects are good.</p>
   <p><em>Remarks</em>: The expression inside noexcept is equivalent to: <code class="highlight"><span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span> <span class="n">and</span> <span class="k">noexcept</span><span class="p">(</span><span class="n">swap</span><span class="p">(</span><span class="n">declval</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&amp;</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">,</span> <span class="n">declval</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&amp;</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">)</span> <span class="n">and</span> <span class="n">is_nothrow_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span> <span class="n">and</span> <span class="k">noexcept</span><span class="p">(</span><span class="n">swap</span><span class="p">(</span><span class="n">declval</span><span class="o">&lt;</span><span class="n">E</span><span class="o">&amp;</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">,</span> <span class="n">declval</span><span class="o">&lt;</span><span class="n">E</span><span class="o">&amp;</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">)</span></code>.  The function shall not
participate in overload resolution unless: LValues of type <code class="highlight"><span class="n">T</span></code> shall be <code class="highlight"><span class="n">Swappable</span></code>, LValues of type <code class="highlight"><span class="n">E</span></code> shall be <code class="highlight"><span class="n">Swappable</span></code> and <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> or <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code>.</p>
   <h3 class="heading settled" data-level="1.15" id="expected.object.observe"><span class="secno">1.15. </span><span class="content">�.�.4.5 Observers [<em>expected.object.observe</em>]</span><a class="self-link" href="#expected.object.observe"></a></h3>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">*</span> <span class="n">operator</span><span class="o">-</span><span class="o">></span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
<span class="n">T</span><span class="o">*</span> <span class="n">operator</span><span class="o">-</span><span class="o">></span><span class="p">(</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Returns</em>: <code class="highlight"><span class="o">&amp;</span><span class="n">val</span></code>.</p>
   <p><em>Remarks</em>: Unless <code class="highlight"><span class="n">T</span></code> is a user-defined type with overloaded unary <code class="highlight"><span class="k">operator</span><span class="o">&amp;</span></code>,
the first operator shall be a constexpr function. 
The operator shall not participate in overload resolution unless: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
<span class="n">T</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Returns</em>: <code class="highlight"><span class="n">val</span></code>.</p>
   <p><em>Remarks</em>: The first operator shall be a constexpr function.
The operator shall not participate in overload resolution unless: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
<span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">operator</span> <span class="o">*</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Returns</em>: <code class="highlight"><span class="n">move</span><span class="p">(</span><span class="n">val</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: This operator shall be a constexpr function.
The operator shall not participate in overload resolution unless: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="n">explicit</span> <span class="n">operator</span> <span class="nf">bool</span><span class="p">(</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">;</span>
</pre>
   <p><em>Returns</em>: <code class="highlight"><span class="n">has_val</span></code>.</p>
   <p><em>Remarks</em>: This operator shall be a constexpr function.</p>
<pre class="language-c++ highlight">    <span class="n">constexpr</span> <span class="kt">bool</span> <span class="nf">has_value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="n">noexcept</span><span class="p">;</span>
</pre>
   <p><em>Returns</em>: <code class="highlight"><span class="n">has_val</span></code>.</p>
   <p><em>Remarks</em>: This function shall be a constexpr function.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="kt">void</span> <span class="n">expected</span><span class="o">&lt;</span><span class="kt">void</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
</pre>
   <p><em>Throws</em>: <code class="highlight"><span class="n">bad_expected_access</span><span class="p">(</span><span class="n">err</span><span class="p">)</span></code> if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">expected</span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
<span class="n">constexpr</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">expected</span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Returns</em>: <code class="highlight"><span class="n">val</span></code>, if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: <code class="highlight"><span class="n">bad_expected_access</span><span class="p">(</span><span class="n">err</span><span class="p">)</span></code> if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Remarks</em>: These functions shall be constexpr functions.
The operator shall not participate in overload resolution unless: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">expected</span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
<span class="n">constexpr</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">expected</span><span class="o">:</span><span class="o">:</span><span class="n">value</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Returns</em>: <code class="highlight"><span class="n">move</span><span class="p">(</span><span class="n">val</span><span class="p">)</span></code>, if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> contains a value.</p>
   <p><em>Throws</em>: <code class="highlight"><span class="n">bad_expected_access</span><span class="p">(</span><span class="n">err</span><span class="p">)</span></code> if <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Remarks</em>: These functions shall be constexpr functions.
The operator shall not participate in overload resolution unless: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code>.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
<span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Returns</em>: <code class="highlight"><span class="n">unexpect</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: The first function shall be a constexpr function.</p>
<pre class="language-c++ highlight"><span class="n">constexpr</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
<span class="n">constexpr</span> <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="o">*</span><span class="k">this</span></code> does not contain a value.</p>
   <p><em>Returns</em>: <code class="highlight"><span class="n">move</span><span class="p">(</span><span class="n">unexpect</span><span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: The first function shall be a constexpr function.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="o">></span>
  <span class="n">constexpr</span> <span class="n">T</span> <span class="n">value_or</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span> <span class="o">?</span> <span class="o">*</span><span class="o">*</span><span class="k">this</span> <span class="o">:</span> <span class="k">static_cast</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">(</span><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">(</span><span class="n">v</span><span class="p">)</span><span class="p">)</span><span class="p">;</span></code>.</p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">is_copy_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is
 false the program is ill-formed.</p>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">U</span><span class="o">></span>
  <span class="n">T</span> <span class="n">value_or</span><span class="p">(</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Equivalent to <code class="highlight"><span class="k">return</span> <span class="nf">bool</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span> <span class="o">?</span> <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">move</span><span class="p">(</span><span class="o">*</span><span class="o">*</span><span class="k">this</span><span class="p">)</span> <span class="o">:</span> <span class="k">static_cast</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">(</span><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">forward</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">(</span><span class="n">v</span><span class="p">)</span><span class="p">)</span><span class="p">;</span></code>.</p>
   <p><em>Remarks</em>: If <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> and <code class="highlight"><span class="n">is_convertible_v</span><span class="o">&lt;</span><span class="n">U</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">,</span> <span class="n">T</span><span class="o">></span></code> is
 false the program is ill-formed.</p>
   <h3 class="heading settled" data-level="1.16" id="expected.unexpect"><span class="secno">1.16. </span><span class="content">�.�.6 <code class="highlight"><span class="n">unexpect</span></code> tag [<em>expected.unexpect</em>]</span><a class="self-link" href="#expected.unexpect"></a></h3>
<pre class="language-c++ highlight"><span class="k">struct</span> <span class="n">unexpect_t</span> <span class="p">{</span>
    <span class="n">explicit</span> <span class="n">unexpect_t</span><span class="p">(</span><span class="p">)</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
<span class="p">}</span><span class="p">;</span>
<span class="kr">inline</span> <span class="n">constexpr</span> <span class="n">unexpect_t</span> <span class="n">unexpect</span><span class="p">{</span><span class="p">}</span><span class="p">;</span>
</pre>
   <h3 class="heading settled" data-level="1.17" id="expected.bad_expected_access"><span class="secno">1.17. </span><span class="content">�.�.7 Template Class <code class="highlight"><span class="n">bad_expected_access</span></code> [<em>expected.bad_expected_access</em>]</span><a class="self-link" href="#expected.bad_expected_access"></a></h3>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">E</span><span class="o">></span>
<span class="k">class</span> <span class="nc">bad_expected_access</span> <span class="o">:</span> <span class="k">public</span> <span class="n">bad_expected_access</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">></span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
    <span class="k">explicit</span> <span class="n">bad_expected_access</span><span class="p">(</span><span class="n">E</span><span class="p">)</span><span class="p">;</span>
    <span class="k">virtual</span> <span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="nf">what</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="k">noexcept</span> <span class="k">override</span><span class="p">;</span>
    <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
    <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span>  <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
<span class="k">private</span><span class="o">:</span>
    <span class="n">E</span> <span class="n">val</span><span class="p">;</span> <span class="c1">// exposition only
</span><span class="p">}</span><span class="p">;</span>
</pre>
   <p class="issue" id="issue-d50f9df2"><a class="self-link" href="#issue-d50f9df2"></a> Wondering if we just need an <code class="highlight"><span class="k">const</span> <span class="o">&amp;</span></code> overload as we do for <code class="highlight"><span class="n">system_error</span></code>.</p>
   <p>The template class <code class="highlight"><span class="n">bad_expected_access</span></code> defines the type of objects thrown as
exceptions to report the situation where an attempt is made to access the value
of a unexpected expected object.</p>
<pre class="language-c++ highlight"><span class="n">bad_expected_access</span><span class="o">:</span><span class="o">:</span><span class="n">bad_expected_access</span><span class="p">(</span><span class="n">E</span> <span class="n">e</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Constructs an object of class <code class="highlight"><span class="n">bad_expected_access</span></code> storing the
 parameter.</p>
   <p><em>Postconditions</em>: <code class="highlight"><span class="n">what</span><span class="p">(</span><span class="p">)</span></code> returns an implementation-defined NTBS.</p>
<pre class="language-c++ highlight">    <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="n">E</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><span class="k">return</span> <span class="n">val</span><span class="p">;</span></code></p>
<pre class="language-c++ highlight">    <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
    <span class="k">const</span> <span class="n">E</span><span class="o">&amp;</span><span class="o">&amp;</span> <span class="n">error</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="o">&amp;</span><span class="o">&amp;</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><span class="k">return</span> <span class="nf">move</span><span class="p">(</span><span class="n">val</span><span class="p">)</span><span class="p">;</span></code></p>
<pre class="language-c++ highlight"><span class="n">virtual</span> <span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="nf">what</span><span class="p">(</span><span class="p">)</span> <span class="k">const</span> <span class="n">noexcept</span> <span class="n">override</span><span class="p">;</span>
</pre>
   <p><em>Returns</em>: An implementation-defined NTBS.</p>
   <h3 class="heading settled" data-level="1.18" id="expected.bad_expected_access_base"><span class="secno">1.18. </span><span class="content">�.�.7 Template Class <code class="highlight"><span class="n">bad_expected_access</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">></span></code> [<em>expected.bad_expected_access_base</em>]</span><a class="self-link" href="#expected.bad_expected_access_base"></a></h3>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="o">></span>
<span class="k">class</span> <span class="nc">bad_expected_access</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">></span> <span class="o">:</span> <span class="k">public</span> <span class="n">exception</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
    <span class="k">explicit</span> <span class="n">bad_expected_access</span><span class="p">(</span><span class="p">)</span><span class="p">;</span>
<span class="p">}</span><span class="p">;</span>
</pre>
   <p>The template class <code class="highlight"><span class="n">bad_expected_access</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">></span></code> defines the type of objects
thrown as exceptions to report the situation where an attempt is made to access
the value of a unexpected expected object.</p>
   <h3 class="heading settled" data-level="1.19" id="expected.relational_op"><span class="secno">1.19. </span><span class="content">�.�.8 Expected Relational operators [<em>expected.relational_op</em>]</span><a class="self-link" href="#expected.relational_op"></a></h3>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">y</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="n">T</span></code> (if not <code class="highlight"><span class="kt">void</span></code>) and <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> shall meet the requirements of <em>EqualityComparable</em>.</p>
   <p><em>Returns</em>: If <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="n">y</span><span class="p">)</span></code>, <code class="highlight">false</code>; otherwise if <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> false</code>, <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">unexpected</span><span class="p">(</span><span class="n">y</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>; otherwise <code class="highlight">true</code> if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> or <code class="highlight"><span class="o">*</span><span class="n">x</span> <span class="o">=</span><span class="o">=</span> <span class="o">*</span><span class="n">y</span></code> otherwise.</p>
   <p><em>Remarks</em>: Specializations of this function template, for which <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> or <code class="highlight"><span class="o">*</span><span class="n">x</span> <span class="o">=</span><span class="o">=</span> <span class="o">*</span><span class="n">y</span></code> and <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">unexpected</span><span class="p">(</span><span class="n">y</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> are core
constant expression, shall be constexpr functions.</p>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
        <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span>    <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">y</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="n">T</span></code> (if not <code class="highlight"><span class="kt">void</span></code>) and <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> shall meet the requirements
of <em>EqualityComparable</em>.</p>
   <p><em>Returns</em>: If <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="kt">bool</span><span class="p">(</span><span class="n">y</span><span class="p">)</span></code>, <code class="highlight">true</code>; otherwise if <code class="highlight"><span class="kt">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> false</code>, <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="n">unexpected</span><span class="p">(</span><span class="n">y</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code>; otherwise <code class="highlight">true</code> if <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> or <code class="highlight"><span class="o">*</span><span class="n">x</span> <span class="o">!</span><span class="o">=</span> <span class="o">*</span><span class="n">y</span></code>.</p>
   <p><em>Remarks</em>: Specializations of this function template, for which <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> or <code class="highlight"><span class="o">*</span><span class="n">x</span> <span class="o">!</span><span class="o">=</span> <span class="o">*</span><span class="n">y</span></code> and <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="n">unexpected</span><span class="p">(</span><span class="n">y</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span></code> are core
constant expression, shall be constexpr functions.</p>
   <h3 class="heading settled" data-level="1.20" id="expected.comparison_T"><span class="secno">1.20. </span><span class="content">�.�.9 Comparison with <code class="highlight"><span class="n">T</span></code> [<em>expected.comparison_T</em>]</span><a class="self-link" href="#expected.comparison_T"></a></h3>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and the expression <code class="highlight"><span class="o">*</span><span class="n">x</span> <span class="o">=</span><span class="o">=</span> <span class="n">v</span></code> shall be well-formed
and its result shall be convertible to <code class="highlight"><span class="kt">bool</span></code>. [ Note: <code class="highlight"><span class="n">T</span></code> need not be <em>EqualityComparable</em>. - end note]</p>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><span class="k">return</span> <span class="nf">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">?</span> <span class="o">*</span><span class="n">x</span> <span class="o">=</span><span class="o">=</span> <span class="nl">v</span> <span class="p">:</span> false<span class="p">;</span></code>.</p>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: <code class="highlight"><span class="n">T</span></code> is not <code class="highlight"><span class="kt">void</span></code> and the expression <code class="highlight"><span class="o">*</span><span class="n">x</span> <span class="o">=</span><span class="o">=</span> <span class="n">v</span></code> shall be well-formed
and its result shall be convertible to <code class="highlight"><span class="kt">bool</span></code>. [ Note: <code class="highlight"><span class="n">T</span></code> need not be <em>EqualityComparable</em>. - end note]</p>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><span class="k">return</span> <span class="nf">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">?</span> <span class="o">*</span><span class="n">x</span> <span class="o">!</span><span class="o">=</span> <span class="nl">v</span> <span class="p">:</span> false<span class="p">;</span></code>.</p>
   <h3 class="heading settled" data-level="1.21" id="expected.comparison_unexpected_E"><span class="secno">1.21. </span><span class="content">�.�.10 Comparison with <code class="highlight"><span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> [<em>expected.comparison_unexpected_E</em>]</span><a class="self-link" href="#expected.comparison_unexpected_E"></a></h3>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">e</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">=</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">e</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: The expression <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">e</span></code> shall be well-formed and
its result shall be convertible to <code class="highlight"><span class="kt">bool</span></code>. [ Note: <code class="highlight"><span class="n">E</span></code> need not be <em>EqualityComparable</em>. - end note]</p>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><span class="k">return</span> <span class="nf">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">?</span> true <span class="o">:</span> <span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">=</span><span class="o">=</span> <span class="n">e</span><span class="p">;</span></code>.</p>
<pre class="language-c++ highlight">    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">e</span><span class="p">)</span><span class="p">;</span>
    <span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span> <span class="n">constexpr</span> <span class="kt">bool</span> <span class="n">operator</span><span class="o">!</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">unexpected</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">e</span><span class="p">,</span> <span class="k">const</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Requires</em>: The expression <code class="highlight"><span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="n">e</span></code> shall be well-formed and
its result shall be convertible to <code class="highlight"><span class="kt">bool</span></code>. [ Note: <code class="highlight"><span class="n">E</span></code> need not be <em>EqualityComparable</em>. - end note]</p>
   <p><em>Effects</em>: Equivalent to: <code class="highlight"><span class="k">return</span> <span class="nf">bool</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">?</span> false <span class="o">:</span> <span class="n">unexpected</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="p">)</span><span class="p">)</span> <span class="o">!</span><span class="o">=</span> <span class="n">e</span><span class="p">;</span></code>.</p>
   <h3 class="heading settled" data-level="1.22" id="expected.specalg"><span class="secno">1.22. </span><span class="content">�.�.11 Specialized algorithms [<em>expected.specalg</em>]</span><a class="self-link" href="#expected.specalg"></a></h3>
<pre class="language-c++ highlight"><span class="n">template</span> <span class="o">&lt;</span><span class="n">class</span> <span class="n">T</span><span class="p">,</span> <span class="n">class</span> <span class="n">E</span><span class="o">></span>
<span class="kt">void</span> <span class="n">swap</span><span class="p">(</span><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span><span class="o">&amp;</span> <span class="n">y</span><span class="p">)</span> <span class="n">noexcept</span><span class="p">(</span><span class="n">noexcept</span><span class="p">(</span><span class="n">x</span><span class="p">.</span><span class="n">swap</span><span class="p">(</span><span class="n">y</span><span class="p">)</span><span class="p">)</span><span class="p">)</span><span class="p">;</span>
</pre>
   <p><em>Effects</em>: Calls <code class="highlight"><span class="n">x</span><span class="p">.</span><span class="n">swap</span><span class="p">(</span><span class="n">y</span><span class="p">)</span></code>.</p>
   <p><em>Remarks</em>: This function shall not participate in overload resolution unless <code class="highlight"><span class="n">T</span></code> is <code class="highlight"><span class="kt">void</span></code> or <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> is <code class="highlight">true</code>, <code class="highlight"><span class="n">is_swappable_v</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> is <code class="highlight">true</code> and <code class="highlight"><span class="n">is_move_constructible_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> is <code class="highlight">true</code> and <code class="highlight"><span class="n">is_swappable_v</span><span class="o">&lt;</span><span class="n">E</span><span class="o">></span></code> is <code class="highlight">true</code></p>
   <h2 class="heading settled" data-level="2" id="qq"><span class="secno">2. </span><span class="content">Open Questions</span><a class="self-link" href="#qq"></a></h2>
   <p><code class="highlight"><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">expected</span></code> is a vocabulary type with an opinionated design and a proven
record under varied forms in a multitude of codebases. Its current form has
undergone multiple revisions and received substantial feedback, falling roughly
in the following categories:</p>
   <ol>
    <li data-md="">
     <p><strong>Ergonomics</strong>: is this <em>the right way</em> to expose such functionality?</p>
    <li data-md="">
     <p><strong>Disappointment</strong>: should we expose this in the Standard, given C++'s
 existing error handling mechanisms?</p>
    <li data-md="">
     <p><strong>STL usage</strong>: should the Standard Template Library adopt this class, at
 which pace, and where?</p>
   </ol>
   <p>LEWG and EWG have nonetheless reached consensus that a class of this general
approach is probably desirable, and the only way to truly answer these questions
is to try it out in a TS and ask for explicit feedback from developers. The
authors hope that developers will provide new information which they’ll be able
to communicate to the Committee.</p>
   <p>Here are open questions, and questions which the Committee thinks are settled
and which new information can justify revisiting.</p>
   <h3 class="heading settled" data-level="2.1" id="ergo"><span class="secno">2.1. </span><span class="content">Ergonomics</span><a class="self-link" href="#ergo"></a></h3>
   <ol>
    <li data-md="">
     <p>Name:</p>
     <ul>
      <li data-md="">
       <p>Is <code class="highlight"><span class="n">expected</span></code> the right name?</p>
      <li data-md="">
       <p>Does it express intent both as a consumer and a producer?</p>
     </ul>
    <li data-md="">
     <p>Is <code class="highlight"><span class="n">E</span></code> a salient property of <code class="highlight"><span class="n">expected</span></code>?</p>
    <li data-md="">
     <p>Is <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="kt">void</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> clear on what it expresses as a return type?</p>
    <li data-md="">
     <p>Would it make sense for <code class="highlight"><span class="n">expected</span></code> to support containing <em>both</em> <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">E</span></code> (in some designs, either one of them being optional), or is this usecase better handled by a separate proposal?</p>
    <li data-md="">
     <p>Is the order of parameters <code class="highlight"><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> appropriate?</p>
    <li data-md="">
     <p>Is usage of <code class="highlight"><span class="n">expected</span></code> "viral" in a codebase, or can it be adopted incrementally?</p>
    <li data-md="">
     <p>Comparisons:</p>
     <ul>
      <li data-md="">
       <p>Are <code class="highlight"><span class="o">=</span><span class="o">=</span></code> and <code class="highlight"><span class="o">!</span><span class="o">=</span></code> useful?</p>
      <li data-md="">
       <p>Should other comparisons be provided?</p>
      <li data-md="">
       <p>What usages of <code class="highlight"><span class="n">expected</span></code> mandate putting instances in a <code class="highlight"><span class="n">map</span></code>, or other such container?</p>
      <li data-md="">
       <p>Should <code class="highlight"><span class="n">hash</span></code> be provided?</p>
      <li data-md="">
       <p>What usages of <code class="highlight"><span class="n">expected</span></code> mandate putting instances in an <code class="highlight"><span class="n">unordered_map</span></code>, or other such container?</p>
      <li data-md="">
       <p>Should <code class="highlight"><span class="n">expected</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">></span></code> always be comparable if <code class="highlight"><span class="n">T</span></code> is comparable, even if <code class="highlight"><span class="n">E</span></code> is not comparable?</p>
     </ul>
    <li data-md="">
     <p>Error type <code class="highlight"><span class="n">E</span></code>:</p>
     <ul>
      <li data-md="">
       <p><code class="highlight"><span class="n">E</span></code> has no default. Should it?</p>
      <li data-md="">
       <p>Should <code class="highlight"><span class="n">expected</span></code> be specialized for particular <code class="highlight"><span class="n">E</span></code> types such as <code class="highlight"><span class="n">exception_ptr</span></code>, and how?</p>
      <li data-md="">
       <p>Should <code class="highlight"><span class="n">expected</span></code> handle <code class="highlight"><span class="n">E</span></code> types with a built-in "success" value any differently, and how?</p>
      <li data-md="">
       <p><code class="highlight"><span class="n">expected</span></code> is not implicitly constructible from an <code class="highlight"><span class="n">E</span></code>, even when unambiguous from <code class="highlight"><span class="n">T</span></code>, because as a vocabulary type it wants unexpected error construction to be verbose, and require hopping through an <code class="highlight"><span class="n">unexpected</span></code>. Is the verbosity extraneous?</p>
     </ul>
    <li data-md="">
     <p>Does usage of this class cause a meaningful performance impact compared to using error codes?</p>
    <li data-md="">
     <p>The accessor design offers a terse unchecked dereference operator (expected to be used alongside the implicit <code class="highlight"><span class="kt">bool</span></code> conversion), as well as <code class="highlight"><span class="n">value</span><span class="p">(</span><span class="p">)</span></code> and <code class="highlight"><span class="n">error</span><span class="p">(</span><span class="p">)</span></code> accessors which are checked. Is that a gotcha, or is it similar enough to classes such as <code class="highlight"><span class="n">optional</span></code> to be unsurprising?</p>
    <li data-md="">
     <p>Is <code class="highlight"><span class="n">bad_expected_access</span></code> the right thing to throw?</p>
    <li data-md="">
     <p>Should some members be <code class="highlight"><span class="p">[</span><span class="p">[</span><span class="n">nodiscard</span><span class="p">]</span><span class="p">]</span></code>?</p>
   </ol>
   <h3 class="heading settled" data-level="2.2" id="dis"><span class="secno">2.2. </span><span class="content">Disappointment</span><a class="self-link" href="#dis"></a></h3>
   <p>C++ already supports exceptions and error codes, <code class="highlight"><span class="n">expected</span></code> would be a third
kind of error handling.</p>
   <ol>
    <li data-md="">
     <p>where does <code class="highlight"><span class="n">expected</span></code> work better than either exceptions or error handling?</p>
    <li data-md="">
     <p><code class="highlight"><span class="n">expected</span></code> was designed to be particularly well suited to APIs which require their immediate caller to consider an error scenario. Do it succeed in that purpose?</p>
    <li data-md="">
     <p>Do codebases successfully compose these three types of error handling?</p>
    <li data-md="">
     <p>Is debuggability any harder?</p>
    <li data-md="">
     <p>Is it easy to teach C++ as a whole with a third type of error handling?</p>
   </ol>
   <h3 class="heading settled" data-level="2.3" id="stl"><span class="secno">2.3. </span><span class="content">STL Usage</span><a class="self-link" href="#stl"></a></h3>
   <ol>
    <li data-md="">
     <p>Should <code class="highlight"><span class="n">expected</span></code> be used in the STL at the same time as it gets standardized?</p>
    <li data-md="">
     <p>Where, considering <code class="highlight"><span class="n">std2</span></code> may be a good place to change APIs?</p>
   </ol>
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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-p0323r3">[P0323r3]
   <dd>Vicente J. Botet Escriba. <a href="https://wg21.link/p0323r3">Utility class to represent expected object</a>. URL: <a href="https://wg21.link/p0323r3">https://wg21.link/p0323r3</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> Check <code class="highlight"><span class="n">swap</span></code> Effects.<a href="#issue-5d614af5"> ↵ </a></div>
   <div class="issue"> Adapt <code class="highlight"><span class="n">swap</span></code> Remarks once Effects are good.<a href="#issue-a083188f"> ↵ </a></div>
   <div class="issue"> Wondering if we just need an <code class="highlight"><span class="k">const</span> <span class="o">&amp;</span></code> overload as we do for <code class="highlight"><span class="n">system_error</span></code>.<a href="#issue-d50f9df2"> ↵ </a></div>
  </div>