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 <head>
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  <title>P2549R0: std::unexpected&lt;E> should have error() as member accessor</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
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * 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)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * 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
 *
 * Outdated warning for old specs
 *
 * 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)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	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;

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) 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: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		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;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

	/* 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: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		/* 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 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);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) 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);
			color: var(--tocsidebar-heading-text);
		}
		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);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) 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);
			color: var(--tocsidebar-heading-text);
		}

		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;
		color: var(--heading-text);
	}

	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,
	#profile-and-date {
		/* #profile-and-date 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) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		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;
	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 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: inherit;
	}
	dfn var {
		font-style: normal;
	}

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

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

/** 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;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	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;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

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

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

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

	/* 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;
		height: 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,
	.amendment, .correction, .addition {
		margin: 1em auto;
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

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

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

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

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* 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;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* 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;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

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

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

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

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** 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: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    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;
		border-bottom: 1px solid var(--defrow-border);
	}

	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-color: var(--datacell-border);
		border-top-style: solid;
	}

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		border-top: 1px solid var(--datacell-border);
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom: 3px solid #054572;
			border-bottom: 3px solid var(--toclink-underline);
			margin-bottom: -3px;
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
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      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
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    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
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    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
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    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
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    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 2e3c4c68b, updated Fri Jan 14 14:49:00 2022 -0800" name="generator">
  <link href="https://wg21.link/P2549R0" rel="canonical">
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<style>
table, th, td { border:1px solid grey; }
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2549R0<br>std::unexpected&lt;E> should have error() as member accessor</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2022-02-13">2022-02-13</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P2549R0">https://wg21.link/P2549R0</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:winmikedows@hotmail.com">Yihe Li</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Target:
     <dd>C++23
     <dt>Source:
     <dd><a href="https://github.com/Mick235711/wg21-papers/blob/main/P2549R0.bs">github.com/Mick235711/wg21-papers/blob/main/P2549R0.bs</a>
     <dt>Issue Tracking:
     <dd><a href="https://github.com/Mick235711/wg21-papers/issues">GitHub Mick235711/wg21-papers</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </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="#revisions"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r0"><span class="secno">1.1</span> <span class="content">R0</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#design"><span class="secno">3</span> <span class="content">Design</span></a>
     <ol class="toc">
      <li>
       <a href="#alt-design"><span class="secno">3.1</span> <span class="content">Alternative Design</span></a>
       <ol class="toc">
        <li><a href="#badly-inconsistent"><span class="secno">3.1.1</span> <span class="content">But we are already badly inconsistent!</span></a>
        <li><a href="#conversion-operator"><span class="secno">3.1.2</span> <span class="content">Conversion operator</span></a>
        <li><a href="#no-member-accessor"><span class="secno">3.1.3</span> <span class="content">No member accessor</span></a>
       </ol>
      <li><a href="#target-vehicle"><span class="secno">3.2</span> <span class="content">Target Vehicle</span></a>
      <li><a href="#feature-test-macro"><span class="secno">3.3</span> <span class="content">Feature Test Macro</span></a>
     </ol>
    <li>
     <a href="#implementation"><span class="secno">4</span> <span class="content">Implementation &amp; Usage Experience</span></a>
     <ol class="toc">
      <li><a href="#outcome-v2"><span class="secno">4.1</span> <span class="content">Outcome v2</span></a>
      <li><a href="#leaf"><span class="secno">4.2</span> <span class="content">Boost.LEAF</span></a>
     </ol>
    <li>
     <a href="#wording"><span class="secno">5</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li>
       <a href="#expected.un.object"><span class="secno">5.1</span> <span class="content">20.9.3.2 Class template <code class="highlight"><c- n>unexpected</c-></code> [expected.un.object]</span></a>
       <ol class="toc">
        <li><a href="#expected.un.object.general"><span class="secno">5.1.1</span> <span class="content">20.9.3.2.1 General [expected.un.object.general]</span></a>
        <li><a href="#expected.un.obs"><span class="secno">5.1.2</span> <span class="content">20.9.3.2.3 Observers [expected.un.obs]</span></a>
        <li><a href="#expected.un.eq"><span class="secno">5.1.3</span> <span class="content">20.9.3.2.5 Equality operator [expected.un.eq]</span></a>
       </ol>
      <li>
       <a href="#expected.expected"><span class="secno">5.2</span> <span class="content">20.9.6 Class template <code class="highlight"><c- n>expected</c-></code> [expected.expected]</span></a>
       <ol class="toc">
        <li><a href="#expected.object.ctor"><span class="secno">5.2.1</span> <span class="content">20.9.6.2 Constructors [expected.object.ctor]</span></a>
        <li><a href="#expected.object.assign"><span class="secno">5.2.2</span> <span class="content">20.9.6.4 Assignment [expected.object.assign]</span></a>
        <li><a href="#expected.object.eq"><span class="secno">5.2.3</span> <span class="content">20.9.6.7 Equality operators [expected.object.eq]</span></a>
       </ol>
      <li>
       <a href="#expected.void"><span class="secno">5.3</span> <span class="content">20.9.7 Partial specialization of <code class="highlight"><c- n>expected</c-></code> for <code class="highlight"><c- b>void</c-></code> types [expected.void]</span></a>
       <ol class="toc">
        <li><a href="#expected.void.ctor"><span class="secno">5.3.1</span> <span class="content">20.9.7.2 Constructors [expected.void.ctor]</span></a>
        <li><a href="#expected.void.assign"><span class="secno">5.3.2</span> <span class="content">20.9.7.4 Assignment [expected.void.assign]</span></a>
        <li><a href="#expected.void.eq"><span class="secno">5.3.3</span> <span class="content">20.9.7.7 Equality operators [expected.void.eq]</span></a>
       </ol>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <p><a data-link-type="biblio" href="#biblio-p0323r11">[P0323R11]</a> (will be update to R12 once that revision published) introduced class template <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>,
which is a vocabulary type that contains an expected value of type <code class="highlight"><c- n>T</c-></code> or an error <code class="highlight"><c- n>E</c-></code>.
Similar to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> provided member function <code class="highlight"><c- n>value</c-><c- p>()</c-></code> and <code class="highlight"><c- n>error</c-><c- p>()</c-></code> in order to allow access to the contained value or error type.
The proposal also includes the auxillary type <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> to wrap the error type, to both disambiguating between value and error types, and also introduce
explicit marker for returning error (unexpected outcome) types. The introduction of the wrapper type allows both <code class="highlight"><c- n>T</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> to be
implicitly convertible to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>, and thus allows the following usage:</p>
<pre class="language-cpp highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>errc</c-><c- o>></c-> <c- n>svtoi</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>string_view</c-> <c- n>sv</c-><c- p>)</c->
<c- p>{</c->
    <c- b>int</c-> <c- n>value</c-><c- p>{</c-><c- mi>0</c-><c- p>};</c->
    <c- k>auto</c-> <c- p>[</c-><c- n>ptr</c-><c- p>,</c-> <c- n>ec</c-><c- p>]</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>from_chars</c-><c- p>(</c-><c- n>sv</c-><c- p>.</c-><c- n>begin</c-><c- p>(),</c-> <c- n>sv</c-><c- p>.</c-><c- n>end</c-><c- p>(),</c-> <c- n>value</c-><c- p>);</c->
    <c- k>if</c-> <c- p>(</c-><c- n>ec</c-> <c- o>==</c-> <c- n>std</c-><c- o>::</c-><c- n>errc</c-><c- p>{})</c->
    <c- p>{</c->
        <c- k>return</c-> <c- n>value</c-><c- p>;</c->
    <c- p>}</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- p>(</c-><c- n>ec</c-><c- p>);</c->
<c- p>}</c->
</pre>
   However, even though <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> is simply a wrapper over <code class="highlight"><c- n>E</c-></code>, we need to use its member method <code class="highlight"><c- n>value</c-><c- p>()</c-></code> to access the contained error value.
The name of the member method is inconsistent with the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> usage and intuition, so this proposal seeks to correct the name of the member access method to <code class="highlight"><c- n>error</c-><c- p>()</c-></code>. 
   <p><a data-link-type="biblio" href="#biblio-p0323r11">[P0323R11]</a> is adopted for C++23 at the Feburary 2022 WG21 Plenary, so this proposal also targets C++23 to fix this.</p>
   <h2 class="heading settled" data-level="1" id="revisions"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#revisions"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="r0"><span class="secno">1.1. </span><span class="content">R0</span><a class="self-link" href="#r0"></a></h3>
    Initial revision. 
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Consistency among library vocabulary types is important and makes user interaction intuitive, and since <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> is specifically based on and extends <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> <a data-link-type="biblio" href="#biblio-n3793">[N3793]</a>, it is especially important to maintain a similar interface between <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>, and also within the <code class="highlight"><c- n>expected</c-></code> design.
By this way, users will not be surprised if they switch between different sum types.</p>
   <p>We can have a comparison on the various member access method of the <code class="highlight"><c- n>optional</c-></code> and <code class="highlight"><c- n>expected</c-></code> interface:</p>
   <table>
    <thead>
     <tr>
      <td>Member
      <td>Return Type
      <td><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-></code><br><code class="highlight"><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>
      <td><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-></code><br><code class="highlight"><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>
      <td><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-></code><br><code class="highlight"><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code>
      <td><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bad_expected</c-></code><br><code class="highlight"><c- n>_access</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code>
    <tbody>
     <tr>
      <td>(Normal) Value
      <td><code class="highlight"><c- n>T</c-></code>
      <td><code class="highlight"><c- n>value</c-><c- p>()</c-></code><br><code class="highlight"><c- k>operator</c-><c- o>*</c-></code><br><code class="highlight"><c- k>operator</c-><c- o>-></c-></code>
      <td><code class="highlight"><c- n>value</c-><c- p>()</c-></code><br><code class="highlight"><c- k>operator</c-><c- o>*</c-></code><br><code class="highlight"><c- k>operator</c-><c- o>-></c-></code>
      <td>N/A
      <td>N/A
     <tr>
      <td>Unexpected Outcome<br>(Error)
      <td><code class="highlight"><c- n>E</c-></code> (<code class="highlight"><c- n>std</c-><c- o>::</c-></code><br><code class="highlight"><c- n>nullopt_t</c-></code>)
      <td>N/A
      <td><code class="highlight"><c- n>error</c-><c- p>()</c-></code>
      <td><code class="highlight"><c- n>value</c-><c- p>()</c-></code>
      <td><code class="highlight"><c- n>error</c-><c- p>()</c-></code>
   </table>
   <p>We can see that the only outlier in this table is <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>>::</c-><c- n>value</c-><c- p>()</c-></code>, which is both inconsistent with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bad_expected_access</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> that also (possibly) holds an error value, and also inconsistent with other standard library types providing <code class="highlight"><c- n>value</c-><c- p>()</c-></code>, including <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>.
These types all provide <code class="highlight"><c- n>value</c-><c- p>()</c-></code> to access the normal value they hold, and often have preconditions (or throw exceptions if violated) that they holds a value instead of an error.</p>
   <p>Provide <code class="highlight"><c- n>error</c-><c- p>()</c-></code> instead of <code class="highlight"><c- n>value</c-><c- p>()</c-></code> for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> has several benefits:</p>
   <ol>
    <li data-md>
     <p><strong>Consistency:</strong> Both consistent with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bad_expected_access</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> to provide <code class="highlight"><c- n>error</c-><c- p>()</c-></code> to return error value, and also reduce inconsistency
with other <code class="highlight"><c- n>value</c-><c- p>()</c-></code>-providing types that have different preconditions.</p>
    <li data-md>
     <p><strong>Generic:</strong> Same name means more generic code is allowed. For example, generic code can do <code class="highlight"><c- n>e</c-><c- p>.</c-><c- n>error</c-><c- p>()</c-></code> on any (potentially) error-wrapping types to retrieve the error, this
includes <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>bad_expected_access</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code>, and possible further error-handling types like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>status_code</c-></code> <a data-link-type="biblio" href="#biblio-p1028r3">[P1028R3]</a> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>error</c-></code> <a data-link-type="biblio" href="#biblio-p0709r4">[P0709R4]</a>.</p>
    <li data-md>
     <p><strong>Safety &amp; Inituitive:</strong> Other <code class="highlight"><c- n>value</c-><c- p>()</c-></code> type often have different preconditions, for example throwing when the type does not holds a normal value,
or (worse) have narrow contract and UB on abnormal call. Passing the current <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code>-wrapped type to interface expecting the normal <code class="highlight"><c- n>value</c-><c- p>()</c-></code> semantics
can be surprising when leading to runtime exception or (worse) UB.</p>
   </ol>
   <p>Side note: you can even smell the inconsistency when many of the wording of equality operator between <code class="highlight"><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> and <code class="highlight"><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> in <a data-link-type="biblio" href="#biblio-p0323r11">[P0323R11]</a> contains clause such as <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>error</c-><c- p>()</c-> <c- o>==</c-> <c- n>e</c-><c- p>.</c-><c- n>value</c-><c- p>()</c-></code>.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design</span><a class="self-link" href="#design"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="alt-design"><span class="secno">3.1. </span><span class="content">Alternative Design</span><a class="self-link" href="#alt-design"></a></h3>
    This section lists the alternative choices and possible arguments against this proposal that has been considered. 
   <h4 class="heading settled" data-level="3.1.1" id="badly-inconsistent"><span class="secno">3.1.1. </span><span class="content">But we are already badly inconsistent!</span><a class="self-link" href="#badly-inconsistent"></a></h4>
    Some may argue that the intensive use of <code class="highlight"><c- n>value</c-><c- p>()</c-></code> across the library is already inconsistent, and we do not need to keep it consistent. 
   <p>I would argue that most use of <code class="highlight"><c- n>value</c-><c- p>()</c-></code> member function across the standard library adhere to the tradition, aka return the normal "value" that the type is holding.
Functions like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>leap_second</c-><c- o>::</c-><c- n>value</c-><c- p>()</c-></code> can be thought as an extended definition of "value": leap second can hold either <code class="highlight"><c- o>+</c-><c- mi>1</c-><c- n>s</c-></code> or <code class="highlight"><c- mi>-1</c-><c- n>s</c-></code> as value.
The only case that I agree are related is the C++11 <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>error_code</c-></code>/<code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>error_condition</c-></code> pair and their <code class="highlight"><c- n>value</c-><c- p>()</c-></code> member that returns the error code,
which seems to return an error(-related) value. However, I want to point out that <code class="highlight"><c- n>value</c-><c- p>()</c-></code> is not really the "error value" or "unexpected outcome" of these types,
since this is the expected outcome (or "normal value") or an <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>error_code</c-></code>. Furthermore, <code class="highlight"><c- n>value</c-><c- p>()</c-></code> is not really the whole "error" contained in these types,
since these two types consists of <code class="highlight"><c- n>value</c-><c- p>()</c-></code> plus <code class="highlight"><c- n>category</c-><c- p>()</c-></code>. Only <code class="highlight"><c- n>value</c-><c- p>()</c-></code> cannot represent a unique error, and should not be taken as the "error representation".</p>
   <h4 class="heading settled" data-level="3.1.2" id="conversion-operator"><span class="secno">3.1.2. </span><span class="content">Conversion operator</span><a class="self-link" href="#conversion-operator"></a></h4>
    The other standard library wrapper type, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>reference_wrapper</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, provided an (implicit) conversion operator to <code class="highlight"><c- n>T</c-><c- o>&amp;</c-></code>, its wrapped value. This leads to thoughts on
whether <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> should simply provide an (implicit or explicit) conversion operator to <code class="highlight"><c- n>E</c-></code> as its member access method. 
   <p>A similar choice had been facing the designer of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, and their decision (later inherited by <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>) is to reject: (<a data-link-type="biblio" href="#biblio-n3672">[N3672]</a>, 7.9)</p>
   <blockquote>
    <p>We do not think that providing an implicit conversion to T would be a good choice. First, it would require different way of checking for the empty state; and second, such implicit conversion is not perfect and still requires other means of accessing the contained value if we want to call a member function on it.</p>
   </blockquote>
   <p>I think that this reasoning also applies here. Even if it is implicit, conversion operator is not perfect (call member functions), and we still need <code class="highlight"><c- k>static_cast</c-></code> or other member accessors to do that. Also, there seems to be no benefit in providing such conversion (besides, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> is just intended as a "trampoline"
for constructing an <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>, it is not intended to be used extensively/on its own). Therefore I rejected this option.</p>
   <h4 class="heading settled" data-level="3.1.3" id="no-member-accessor"><span class="secno">3.1.3. </span><span class="content">No member accessor</span><a class="self-link" href="#no-member-accessor"></a></h4>
    The above discussion leads to the consideration: since <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> is just meant as a "trampoline", does it need a member accessor at all?
Besides, the intended usage is just <code class="highlight"><c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>unexpected</c-><c- p>(</c-><c- n>some_error</c-><c- p>);</c-></code>, provide a member accessor does not seems to help this use case at all. 
   <p>This is an interesting point. Also, one of <a data-link-type="biblio" href="#biblio-p0323r11">[P0323R11]</a>'s referenced implementation <a data-link-type="biblio" href="#biblio-viboes-expected">[viboes-expected]</a> does this: its <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>fundamental_v3</c-><c- o>::</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> type
have no accessor at all. However, providing an accessor does not seems to do any harm, and may have interesting use cases that I’m not aware of. Therefore
I do not propose this, but will not against changing the proposal to this direction if L(E)WG favor this.</p>
   <h3 class="heading settled" data-level="3.2" id="target-vehicle"><span class="secno">3.2. </span><span class="content">Target Vehicle</span><a class="self-link" href="#target-vehicle"></a></h3>
    This proposal targets C++23. I’m aware that the design freeze deadline of C++23 is already passed, but I think this can be classified as an improvement/fix over the defect
in <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>. Furthermore, this proposal will be a huge breaking change (that makes it simply unviable to propose) after C++23. 
   <h3 class="heading settled" data-level="3.3" id="feature-test-macro"><span class="secno">3.3. </span><span class="content">Feature Test Macro</span><a class="self-link" href="#feature-test-macro"></a></h3>
    As long as the proposal lands in C++23, I don’t think there is a need to change any feature test macro. However, if L(E)WG feels there is a need, then I
suggest bumping <code class="highlight"><c- n>__cpp_lib_expected</c-></code> to the date of adoption (definitely larger than <code class="highlight"><c- mf>202202L</c-></code>). 
   <h2 class="heading settled" data-level="4" id="implementation"><span class="secno">4. </span><span class="content">Implementation &amp; Usage Experience</span><a class="self-link" href="#implementation"></a></h2>
   <p>The referenced implementation of <a data-link-type="biblio" href="#biblio-p0323r11">[P0323R11]</a> all implement the interface of original proposal (except <a data-link-type="biblio" href="#biblio-viboes-expected">[viboes-expected]</a> mentioned above).
This section thus investigated several similar implementations.</p>
   <h3 class="heading settled" data-level="4.1" id="outcome-v2"><span class="secno">4.1. </span><span class="content">Outcome v2</span><a class="self-link" href="#outcome-v2"></a></h3>
    <a data-link-type="biblio" href="#biblio-outcome-v2">[outcome-v2]</a> is a popular library invested in a set of tools for reporting and handling function failures in contexts where <em>directly</em> using C++ exception
handling is unsuitable. It is both provided as Boost.Outcome and the standalone GitHub repository, and also having an experimental branch that is the
basis of <a data-link-type="biblio" href="#biblio-p1095r0">[P1095R0]</a> and <a data-link-type="biblio" href="#biblio-p1028r3">[P1028R3]</a> <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>status_code</c-></code>. The library provided <code class="highlight"><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- p>,</c-> <c- n>Policy</c-><c- o>></c-></code> and <code class="highlight"><c- n>outcome</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>EC</c-><c- p>,</c-> <c- n>EP</c-><c- p>,</c-> <c- n>Policy</c-><c- o>></c-></code> types that represents value/error
duo type, just like <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>, with the difference in interface and also <code class="highlight"><c- n>outcome</c-></code> can hold both <code class="highlight"><c- n>EC</c-></code> and <code class="highlight"><c- n>EP</c-></code> (error code and exception (pointer)).
The design of <code class="highlight"><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> also deeply influences <a data-link-type="biblio" href="#biblio-p0323r11">[P0323R11]</a>, and the final adopted design of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> is very similar to what <code class="highlight"><c- n>outcome</c-><c- o>::</c-><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> provides. 
   <p>One of the main design difference is that <code class="highlight"><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> can be implicitly constructed from both <code class="highlight"><c- n>T</c-></code> and <code class="highlight"><c- n>E</c-></code>, while <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> can only be implicitly constructed from the former.
For this reason, <code class="highlight"><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> does not allow for <code class="highlight"><c- n>T</c-></code> and <code class="highlight"><c- n>E</c-></code> to be the same, and also does not provide <code class="highlight"><c- k>operator</c-><c- o>*</c-></code> and <code class="highlight"><c- k>operator</c-><c- o>-></c-></code> accessor.
Thus, there are wrapper for both success and failure value for construction, and <code class="highlight"><c- n>success_type</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> wrap a success <code class="highlight"><c- n>T</c-></code>, while <code class="highlight"><c- n>failure_type</c-><c- o>&lt;</c-><c- n>EC</c-><c- p>,</c-> <c- n>EP</c-><c- o>></c-></code> wraps an
unexpected <code class="highlight"><c- n>E</c-></code> (or <code class="highlight"><c- n>EC</c-></code> and <code class="highlight"><c- n>EP</c-></code>). Their accessors are: (the <code class="highlight"><c- n>assume_</c-><c- o>*</c-></code> narrow-contract accessors and <code class="highlight"><c- n>failure</c-><c- p>()</c-></code> are not shown)</p>
   <table>
    <thead>
     <tr>
      <td>Member
      <td>Return Type
      <td><code class="highlight"><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>
      <td><code class="highlight"><c- n>outcome</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>EC</c-><c- p>,</c-> <c- n>EP</c-><c- o>></c-></code>
      <td><code class="highlight"><c- n>success_type</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>
      <td><code class="highlight"><c- n>failure_type</c-><c- o>&lt;</c-><c- n>EC</c-><c- p>,</c-> <c- n>EP</c-><c- o>></c-></code>
    <tbody>
     <tr>
      <td>(Normal) Value
      <td><code class="highlight"><c- n>T</c-></code>
      <td><code class="highlight"><c- n>value</c-><c- p>()</c-></code>
      <td><code class="highlight"><c- n>value</c-><c- p>()</c-></code>
      <td><code class="highlight"><c- n>value</c-><c- p>()</c-></code>
      <td>N/A
     <tr>
      <td>Unexpected Outcome<br>(Error)
      <td><code class="highlight"><c- n>E</c-></code> (or <code class="highlight"><c- n>EC</c-></code> and <code class="highlight"><c- n>EP</c-></code>)
      <td><code class="highlight"><c- n>error</c-><c- p>()</c-></code>
      <td><code class="highlight"><c- n>error</c-><c- p>()</c-></code><br><code class="highlight"><c- n>exception</c-><c- p>()</c-></code>
      <td>N/A
      <td><code class="highlight"><c- n>error</c-><c- p>()</c-></code><br><code class="highlight"><c- n>exception</c-><c- p>()</c-></code>
   </table>
   <p>We can see that Outcome v2 is pretty consistent in member accessor, and especially its <code class="highlight"><c- n>failure_type</c-><c- o>&lt;</c-><c- n>EC</c-><c- p>,</c-> <c- n>EP</c-><c- o>></c-></code> provides <code class="highlight"><c- n>error</c-><c- p>()</c-></code> and <code class="highlight"><c- n>exception</c-><c- p>()</c-></code>,
not <code class="highlight"><c- n>value</c-><c- p>()</c-></code>. Also note that the default exception being thrown, <code class="highlight"><c- n>bad_result_access</c-></code> and <code class="highlight"><c- n>bad_outcome_access</c-></code>, does not holds the error/exception value at all.
There is a <code class="highlight"><c- n>bad_result_access_with</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> for consistency with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>.</p>
   <h3 class="heading settled" data-level="4.2" id="leaf"><span class="secno">4.2. </span><span class="content">Boost.LEAF</span><a class="self-link" href="#leaf"></a></h3>
    Lightweight Error Augmentation Framework (LEAF), or <a data-link-type="biblio" href="#biblio-boostleaf">[Boost.LEAF]</a>, is a lightweight error handling library for C++11. It is intended to be an improved version of Outcome,
by eliminating branchy code and remove error type from <code class="highlight"><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> signature. The author describe it as 
   <blockquote>
    <p>LEAF is designed with a strong bias towards the common use case where callers of functions which may fail check for success and forward errors up the call stack but do not handle them. In this case, only a trivial success-or-failure discriminant is transported. Actual error objects are communicated directly to the error handling scope, skipping the intermediate check-only frames altogether.</p>
   </blockquote>
   <p>The main type for LEAF is <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, which is again a counterpart of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>expected</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code> and <code class="highlight"><c- n>outcome</c-><c- o>::</c-><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-></code>, but with <code class="highlight"><c- n>E</c-></code> eliminated from signature.
Unexpected results are produced by <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>new_error</c-><c- p>(</c-><c- n>some_error</c-><c- p>)</c-></code>, which returns an <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>error_id</c-></code> object that can be converted to an unexpected <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>.
There is also a <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>error_info</c-></code> that is used as the generic error type receiver for functions such as <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>try_catch</c-></code>.
The member accessor is:</p>
   <table>
    <thead>
     <tr>
      <td>Member
      <td>Return Type
      <td><code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>result</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>
      <td><code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>error_info</c-></code>
    <tbody>
     <tr>
      <td>(Normal) Value
      <td><code class="highlight"><c- n>T</c-></code>
      <td><code class="highlight"><c- n>value</c-><c- p>()</c-></code>
      <td>N/A
     <tr>
      <td>Unexpected Outcome<br>(Error)
      <td><code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>error_id</c-></code>
      <td><code class="highlight"><c- n>error</c-><c- p>()</c-></code>
      <td><code class="highlight"><c- n>error</c-><c- p>()</c-></code>
   </table>
    (Notice that <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>error_id</c-></code> is the final error (unexpected outcome) type, its <code class="highlight"><c- n>value</c-><c- p>()</c-></code> is similar to that of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>error_code</c-></code>, which does not return an "unexpected outcome",
but instead return an error ID for the alternative description of <code class="highlight"><c- n>leaf</c-><c- o>::</c-><c- n>error_id</c-></code>, which actually fits into my reasoning of returning "value".) Again we
can see a consistency here. 
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">5. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>The wording below is based on <a data-link-type="biblio" href="#biblio-n4901">[N4901]</a> plus the editorial PR of P0323R12. This will be rebased onto post-Feburary Working Draft in the next revision, with hopefully no change of section name.</p>
   <h3 class="heading settled" data-level="5.1" id="expected.un.object"><span class="secno">5.1. </span><span class="content">20.9.3.2 Class template <code class="highlight"><c- n>unexpected</c-></code> [expected.un.object]</span><a class="self-link" href="#expected.un.object"></a></h3>
   <h4 class="heading settled" data-level="5.1.1" id="expected.un.object.general"><span class="secno">5.1.1. </span><span class="content">20.9.3.2.1 General [expected.un.object.general]</span><a class="self-link" href="#expected.un.object.general"></a></h4>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>E</c-><c- o>></c->
    <c- k>class</c-> <c- nc>unexpected</c-> <c- p>{</c->
    <c- k>public</c-><c- o>:</c->
        <c- k>constexpr</c-> <c- n>unexpected</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- nf>unexpected</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
        <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
            <c- k>constexpr</c-> <c- k>explicit</c-> <c- n>unexpected</c-><c- p>(</c-><c- n>in_place_t</c-><c- p>,</c-> <c- n>Args</c-><c- o>&amp;&amp;</c-><c- p>...);</c->
        <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>U</c-><c- p>,</c-> <c- k>class</c-><c- p>...</c-> <c- n>Args</c-><c- o>></c->
            <c- k>constexpr</c-> <c- k>explicit</c-> <c- n>unexpected</c-><c- p>(</c-><c- n>in_place_t</c-><c- p>,</c-> <c- n>initializer_list</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-><c- p>,</c-> <c- n>Args</c-><c- o>&amp;&amp;</c-><c- p>...);</c->
        <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Err</c-> <c- o>=</c-> <c- n>E</c-><c- o>></c->
            <c- k>constexpr</c-> <c- k>explicit</c-> <c- n>unexpected</c-><c- p>(</c-><c- n>Err</c-><c- o>&amp;&amp;</c-><c- p>);</c->

        <c- k>constexpr</c-> <c- n>unexpected</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- n>unexpected</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->

        <c- k>constexpr</c-> <c- k>const</c-> <c- n>E</c-><c- o>&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- k>const</c-><c- o>&amp;</c-> <c- k>noexcept</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- n>E</c-><c- o>&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- o>&amp;</c-> <c- k>noexcept</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- k>const</c-> <c- n>E</c-><c- o>&amp;&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- k>const</c-><c- o>&amp;&amp;</c-> <c- k>noexcept</c-><c- p>;</c->
        <c- k>constexpr</c-> <c- n>E</c-><c- o>&amp;&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- k>noexcept</c-><c- p>;</c->

        <c- k>constexpr</c-> <c- b>void</c-> <c- n>swap</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&amp;</c-> <c- n>other</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>(</c-><i><c- n>see</c-> <c- n>below</c-></i><c- p>);</c->

        <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>E2</c-><c- o>></c->
            <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>E2</c-><c- o>>&amp;</c-><c- p>);</c->

        <c- k>friend</c-> <c- k>constexpr</c-> <c- b>void</c-> <c- n>swap</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- n>unexpected</c-><c- o>&amp;</c-> <c- n>y</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>(</c-><c- k>noexcept</c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><c- n>swap</c-><c- p>(</c-><c- n>y</c-><c- p>)));</c->

    <c- k>private</c-><c- o>:</c->
        <c- n>E</c-> <i><c- n>val</c-></i><c- p>;</c-> <c- c1>// </c-><i><c- c1>exposition only</c-></i>
    <c- p>};</c->

    <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>E</c-><c- o>></c-> <c- n>unexpected</c-><c- p>(</c-><c- n>E</c-><c- p>)</c-> <c- o>-></c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-><c- p>;</c->
<c- p>}</c->
</pre>
   <h4 class="heading settled" data-level="5.1.2" id="expected.un.obs"><span class="secno">5.1.2. </span><span class="content">20.9.3.2.3 Observers [expected.un.obs]</span><a class="self-link" href="#expected.un.obs"></a></h4>
<pre class="highlight"><c- k>constexpr</c-> <c- k>const</c-> <c- n>E</c-><c- o>&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- k>const</c-><c- o>&amp;</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>constexpr</c-> <c- n>E</c-><c- o>&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- o>&amp;</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <p><em>Returns</em>: <code class="highlight"><i><c- n>val</c-></i></code>.</p>
<pre class="highlight"><c- k>constexpr</c-> <c- n>E</c-><c- o>&amp;&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- k>noexcept</c-><c- p>;</c->
<c- k>constexpr</c-> <c- k>const</c-> <c- n>E</c-><c- o>&amp;&amp;</c-> <del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- k>const</c-><c- o>&amp;&amp;</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
   <p><em>Returns</em>: <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><i><c- n>val</c-></i><c- p>)</c-></code>.</p>
   <h4 class="heading settled" data-level="5.1.3" id="expected.un.eq"><span class="secno">5.1.3. </span><span class="content">20.9.3.2.5 Equality operator [expected.un.eq]</span><a class="self-link" href="#expected.un.eq"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>E2</c-><c- o>></c->
    <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>E2</c-><c- o>>&amp;</c-> <c- n>y</c-><c- p>);</c->
</pre>
   <p><em>Mandates</em>: The expression <code class="highlight"><c- n>x</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- o>==</c-> <c- n>y</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-></code> is well-formed and its result
is convertible to <code class="highlight"><c- b>bool</c-></code>.</p>
   <p><em>Returns</em>: <code class="highlight"><c- n>x</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-> <c- o>==</c-> <c- n>y</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-></code>.</p>
   <h3 class="heading settled" data-level="5.2" id="expected.expected"><span class="secno">5.2. </span><span class="content">20.9.6 Class template <code class="highlight"><c- n>expected</c-></code> [expected.expected]</span><a class="self-link" href="#expected.expected"></a></h3>
   <h4 class="heading settled" data-level="5.2.1" id="expected.object.ctor"><span class="secno">5.2.1. </span><span class="content">20.9.6.2 Constructors [expected.object.ctor]</span><a class="self-link" href="#expected.object.ctor"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- o>!</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>G</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-><c- p>)</c-> <c- n>expected</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;</c-> <c- n>e</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- o>!</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>G</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-><c- p>)</c-> <c- n>expected</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;&amp;</c-> <c- n>e</c-><c- p>);</c->
</pre>
   <p>Let <code class="highlight"><c- n>GF</c-></code> be <code class="highlight"><c- k>const</c-> <c- n>G</c-><c- o>&amp;</c-></code> for the first overload and <code class="highlight"><c- n>G</c-></code> for the second overload.</p>
   <p><em>Constraints</em>: <code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>E</c-><c- p>,</c-> <c- n>GF</c-><c- o>></c-></code> is <code class="highlight">true</code>.</p>
   <p><em>Effects</em>: Direct-non-list-initializes <code class="highlight"><i><c- n>unex</c-></i></code> with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>GF</c-><c- o>></c-><c- p>(</c-><c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>())</c-></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><c- n>has_value</c-><c- p>()</c-></code> is <code class="highlight">false</code>.</p>
   <p><em>Throws</em>: Any exception thrown by the initialization of <code class="highlight"><i><c- n>unex</c-></i></code>.</p>
   <h4 class="heading settled" data-level="5.2.2" id="expected.object.assign"><span class="secno">5.2.2. </span><span class="content">20.9.6.4 Assignment [expected.object.assign]</span><a class="self-link" href="#expected.object.assign"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>expected</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;</c-> <c- n>e</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>expected</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;&amp;</c-> <c- n>e</c-><c- p>);</c->
</pre>
   <p>Let <code class="highlight"><c- n>GF</c-></code> be <code class="highlight"><c- k>const</c-> <c- n>G</c-><c- o>&amp;</c-></code> for the first overload and <code class="highlight"><c- n>G</c-></code> for the second overload.</p>
   <p><em>Constraints</em>:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>E</c-><c- p>,</c-> <c- n>GF</c-><c- o>></c-></code> is <code class="highlight">true</code>; and</p>
    <li data-md>
     <p><code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- n>E</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>GF</c-><c- o>></c-></code> is <code class="highlight">true</code>; and</p>
    <li data-md>
     <p><code class="highlight"><c- n>is_nothrow_constructible_v</c-><c- o>&lt;</c-><c- n>E</c-><c- p>,</c-> <c- n>GF</c-><c- o>></c-> <c- o>||</c-> <c- n>is_nothrow_move_constructible_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- o>||</c-> <c- n>is_nothrow_move_constructible_v</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-></code> is <code class="highlight">true</code>.</p>
   </ul>
   <p><em>Effects</em>:</p>
   <ul>
    <li data-md>
     <p>If <code class="highlight"><c- n>has_value</c-><c- p>()</c-></code> is <code class="highlight">true</code>, equivalent to:</p>
<pre class="highlight"><i><c- n>reinit</c-><c- o>-</c-><c- n>expected</c-></i><c- p>(</c-><i><c- n>unex</c-></i><c- p>,</c-> <i><c- n>val</c-></i><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>GF</c-><c- o>></c-><c- p>(</c-><c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()));</c->
<i><c- n>has_val</c-></i> <c- o>=</c-> false<c- p>;</c->
</pre>
    <li data-md>
     <p>Otherwise, equivalent to: <code class="highlight"><i><c- n>unex</c-></i> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>GF</c-><c- o>></c-><c- p>(</c-><c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>());</c-></code></p>
   </ul>
   <p><em>Returns</em>: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
   <h4 class="heading settled" data-level="5.2.3" id="expected.object.eq"><span class="secno">5.2.3. </span><span class="content">20.9.6.7 Equality operators [expected.object.eq]</span><a class="self-link" href="#expected.object.eq"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>E2</c-><c- o>></c-> <c- k>friend</c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>expected</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>E2</c-><c- o>>&amp;</c-> <c- n>e</c-><c- p>);</c->
</pre>
   <p><em>Mandates</em>: The expression <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>error</c-><c- p>()</c-> <c- o>==</c-> <c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-></code> is well-formed and
its result is convertible to <code class="highlight"><c- b>bool</c-></code>.</p>
   <p><em>Returns</em>: <code class="highlight"><c- o>!</c-><c- n>x</c-><c- p>.</c-><c- n>has_value</c-><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><c- n>error</c-><c- p>()</c-> <c- o>==</c-> <c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>())</c-></code>.</p>
   <h3 class="heading settled" data-level="5.3" id="expected.void"><span class="secno">5.3. </span><span class="content">20.9.7 Partial specialization of <code class="highlight"><c- n>expected</c-></code> for <code class="highlight"><c- b>void</c-></code> types [expected.void]</span><a class="self-link" href="#expected.void"></a></h3>
   <h4 class="heading settled" data-level="5.3.1" id="expected.void.ctor"><span class="secno">5.3.1. </span><span class="content">20.9.7.2 Constructors [expected.void.ctor]</span><a class="self-link" href="#expected.void.ctor"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- o>!</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>G</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-><c- p>)</c-> <c- n>expected</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;</c-> <c- n>e</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>explicit</c-><c- p>(</c-><c- o>!</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>G</c-><c- p>,</c-> <c- n>E</c-><c- o>></c-><c- p>)</c-> <c- n>expected</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;&amp;</c-> <c- n>e</c-><c- p>);</c->
</pre>
   <p>Let <code class="highlight"><c- n>GF</c-></code> be <code class="highlight"><c- k>const</c-> <c- n>G</c-><c- o>&amp;</c-></code> for the first overload and <code class="highlight"><c- n>G</c-></code> for the second overload.</p>
   <p><em>Constraints</em>: <code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>E</c-><c- p>,</c-> <c- n>GF</c-><c- o>></c-></code> is <code class="highlight">true</code>.</p>
   <p><em>Effects</em>: Direct-non-list-initializes <code class="highlight"><i><c- n>unex</c-></i></code> with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>GF</c-><c- o>></c-><c- p>(</c-><c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>())</c-></code>.</p>
   <p><em>Postconditions</em>: <code class="highlight"><c- n>has_value</c-><c- p>()</c-></code> is <code class="highlight">false</code>.</p>
   <p><em>Throws</em>: Any exception thrown by the initialization of <code class="highlight"><i><c- n>unex</c-></i></code>.</p>
   <h4 class="heading settled" data-level="5.3.2" id="expected.void.assign"><span class="secno">5.3.2. </span><span class="content">20.9.7.4 Assignment [expected.void.assign]</span><a class="self-link" href="#expected.void.assign"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>expected</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;</c-> <c- n>e</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>G</c-><c- o>></c->
    <c- k>constexpr</c-> <c- n>expected</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>unexpected</c-><c- o>&lt;</c-><c- n>G</c-><c- o>>&amp;&amp;</c-> <c- n>e</c-><c- p>);</c->
</pre>
   <p>Let <code class="highlight"><c- n>GF</c-></code> be <code class="highlight"><c- k>const</c-> <c- n>G</c-><c- o>&amp;</c-></code> for the first overload and <code class="highlight"><c- n>G</c-></code> for the second overload.</p>
   <p><em>Constraints</em>: <code class="highlight"><c- n>is_constructible_v</c-><c- o>&lt;</c-><c- n>E</c-><c- p>,</c-> <c- n>GF</c-><c- o>></c-></code> is <code class="highlight">true</code> and <code class="highlight"><c- n>is_assignable_v</c-><c- o>&lt;</c-><c- n>E</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>GF</c-><c- o>></c-></code> is <code class="highlight">true</code>.</p>
   <p><em>Effects</em>:</p>
   <ul>
    <li data-md>
     <p>If <code class="highlight"><c- n>has_value</c-><c- p>()</c-></code> is <code class="highlight">true</code>, equivalent to:</p>
<pre class="highlight"><c- n>construct_at</c-><c- p>(</c-><c- n>addressof</c-><c- p>(</c-><i><c- n>unex</c-></i><c- p>),</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>GF</c-><c- o>></c-><c- p>(</c-><c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()));</c->
<i><c- n>has_val</c-></i> <c- o>=</c-> false<c- p>;</c->
</pre>
    <li data-md>
     <p>Otherwise, equivalent to: <code class="highlight"><i><c- n>unex</c-></i> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>GF</c-><c- o>></c-><c- p>(</c-><c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>());</c-></code></p>
   </ul>
   <p><em>Returns</em>: <code class="highlight"><c- o>*</c-><c- k>this</c-></code>.</p>
   <h4 class="heading settled" data-level="5.3.3" id="expected.void.eq"><span class="secno">5.3.3. </span><span class="content">20.9.7.7 Equality operators [expected.void.eq]</span><a class="self-link" href="#expected.void.eq"></a></h4>
<pre class="language-cpp highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>E2</c-><c- o>></c->
    <c- k>constexpr</c-> <c- b>bool</c-> <c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>expected</c-><c- o>&amp;</c-> <c- n>x</c-><c- p>,</c-> <c- k>const</c-> <c- n>unexpected</c-><c- o>&lt;</c-><c- n>E2</c-><c- o>>&amp;</c-> <c- n>e</c-><c- p>);</c->
</pre>
   <p><em>Mandates</em>: The expression <code class="highlight"><c- n>x</c-><c- p>.</c-><c- n>error</c-><c- p>()</c-> <c- o>==</c-> <c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>()</c-></code> is well-formed and
its result is convertible to <code class="highlight"><c- b>bool</c-></code>.</p>
   <p><em>Returns</em>: <code class="highlight"><c- o>!</c-><c- n>x</c-><c- p>.</c-><c- n>has_value</c-><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-><c- p>(</c-><c- n>x</c-><c- p>.</c-><c- n>error</c-><c- p>()</c-> <c- o>==</c-> <c- n>e</c-><c- p>.</c-><del><c- n>value</c-></del><ins><c- n>error</c-></ins><c- p>())</c-></code>.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-p0323r11">[P0323R11]
   <dd>JF Bastien, Jonathan Wakely, Vicente Botet. <a href="https://wg21.link/p0323r11"><cite>std::expected</cite></a>. 16 November 2021. URL: <a href="https://wg21.link/p0323r11">https://wg21.link/p0323r11</a>
  </dl>
  <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-boostleaf">[Boost.LEAF]
   <dd>Emil Dotchevski. <a href="https://boostorg.github.io/leaf/"><cite>Lightweight Error Augmentation Framework written in C++11</cite></a>. URL: <a href="https://boostorg.github.io/leaf/">https://boostorg.github.io/leaf/</a>
   <dt id="biblio-n3672">[N3672]
   <dd>F. Cacciola, A. Krzemieński. <a href="https://wg21.link/n3672"><cite>A proposal to add a utility class to represent optional objects (Revision 4)</cite></a>. 19 April 2013. URL: <a href="https://wg21.link/n3672">https://wg21.link/n3672</a>
   <dt id="biblio-n3793">[N3793]
   <dd>F. Cacciola, A. Krzemieński. <a href="https://wg21.link/n3793"><cite>A proposal to add a utility class to represent optional objects (Revision 5)</cite></a>. 3 October 2013. URL: <a href="https://wg21.link/n3793">https://wg21.link/n3793</a>
   <dt id="biblio-n4901">[N4901]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4901"><cite>Working Draft, Standard for Programming Language C++</cite></a>. 22 October 2021. URL: <a href="https://wg21.link/n4901">https://wg21.link/n4901</a>
   <dt id="biblio-outcome-v2">[OUTCOME-V2]
   <dd>Niall Douglas. <a href="https://github.com/ned14/outcome"><cite>Standalone Outcome v2: Lightweight Error Handling Framework</cite></a>. URL: <a href="https://github.com/ned14/outcome">https://github.com/ned14/outcome</a>
   <dt id="biblio-p0709r4">[P0709R4]
   <dd>Herb Sutter. <a href="https://wg21.link/p0709r4"><cite>Zero-overhead deterministic exceptions: Throwing values</cite></a>. 4 August 2019. URL: <a href="https://wg21.link/p0709r4">https://wg21.link/p0709r4</a>
   <dt id="biblio-p1028r3">[P1028R3]
   <dd>Niall Douglas. <a href="https://wg21.link/p1028r3"><cite>SG14 status_code and standard error object</cite></a>. 12 January 2020. URL: <a href="https://wg21.link/p1028r3">https://wg21.link/p1028r3</a>
   <dt id="biblio-p1095r0">[P1095R0]
   <dd>Niall Douglas. <a href="https://wg21.link/p1095r0"><cite>Zero overhead deterministic failure - A unified mechanism for C and C++</cite></a>. 29 August 2018. URL: <a href="https://wg21.link/p1095r0">https://wg21.link/p1095r0</a>
   <dt id="biblio-viboes-expected">[VIBOES-EXPECTED]
   <dd>Vicente J. Botet Escriba. <a href="https://github.com/viboes/std-make/blob/master/include/experimental/fundamental/v3/expected2/expected.hpp"><cite>viboes's Implementation of LFTSv3 std::expected&lt;T, E></cite></a>. URL: <a href="https://github.com/viboes/std-make/blob/master/include/experimental/fundamental/v3/expected2/expected.hpp">https://github.com/viboes/std-make/blob/master/include/experimental/fundamental/v3/expected2/expected.hpp</a>
  </dl>