<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P2468R0: The Equality Operator You Are Looking For</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 span:not(.dfn-paneled) {
			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 type="text/css">
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      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 */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    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 */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    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 d7036035b, updated Fri Oct 8 17:07:11 2021 -0700" name="generator">
  <link href="http://wg21.link/p2468r0" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
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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">P2468R0<br>The Equality Operator You Are Looking For</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="2020-06-18">2020-06-18</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p2468r0">http://wg21.link/p2468r0</a>
     <dt class="editor">Authors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:barry.revzin@gmail.com">Barry Revzin</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:bjarne@stroustrup.com">Bjarne Stroustrup</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:cadacama@microsoft.com">Cameron DaCamara</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:daveed@edg.com">Daveed Vandevoorde</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:gdr@microsoft.com">Gabriel Dos Reis</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:hsutter@microsoft.com">Herb Sutter</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jason@redhat.com">Jason Merrill</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:joncaves@microsoft.com">Jonathan Caves</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:richardsmith@google.com">Richard Smith</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ville.voutilainen@gmail.com">Ville Voutilainen</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper details some changes to make rewriting equality in expressions less of a breaking change</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#rewritten-equality-inequality-in-expressions"><span class="secno">1</span> <span class="content">Rewritten equality/inequality in expressions</span></a>
     <ol class="toc">
      <li><a href="#compiler-disagreements"><span class="secno">1.1</span> <span class="content">Compiler disagreements</span></a>
      <li>
       <a href="#existing-code-impact"><span class="secno">1.2</span> <span class="content">Existing code impact</span></a>
       <ol class="toc">
        <li><a href="#open-source-sampling"><span class="secno">1.2.1</span> <span class="content">Open source sampling</span></a>
       </ol>
      <li>
       <a href="#proposed-resolution"><span class="secno">1.3</span> <span class="content">Proposed resolution</span></a>
       <ol class="toc">
        <li><a href="#first-resolution"><span class="secno">1.3.1</span> <span class="content">First resolution</span></a>
        <li><a href="#second-resolution"><span class="secno">1.3.2</span> <span class="content">Second resolution</span></a>
       </ol>
     </ol>
    <li><a href="#wording"><span class="secno">2</span> <span class="content">Wording</span></a>
    <li>
     <a href="#appendix"><span class="secno">3</span> <span class="content">Appendix</span></a>
     <ol class="toc">
      <li><a href="#code-patterns-which-fail-to-compile-even-under-new-rules"><span class="secno">3.1</span> <span class="content">Code patterns which fail to compile even under new rules</span></a>
      <li><a href="#code-patterns-which-fail-at-runtime"><span class="secno">3.2</span> <span class="content">Code patterns which fail at runtime</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="rewritten-equality-inequality-in-expressions"><span class="secno">1. </span><span class="content">Rewritten equality/inequality in expressions</span><a class="self-link" href="#rewritten-equality-inequality-in-expressions"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="compiler-disagreements"><span class="secno">1.1. </span><span class="content">Compiler disagreements</span><a class="self-link" href="#compiler-disagreements"></a></h3>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> true<c- p>;</c-> <c- p>}</c-> <c- c1>// The non-const member is important here.</c->
  <c- b>bool</c-> <c- n>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> false<c- p>;</c-> <c- p>}</c-> <c- c1>// The non-const member is important here.</c->
<c- p>};</c->
<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>S</c-><c- p>{}</c-> <c- o>!=</c-> <c- n>S</c-><c- p>{};</c->
</pre>
   <p>GCC accepts this (despite overload resolution being ambiguous under C++20).</p>
   <p>Clang accepts with a warning indicating the rewrite resulting in ambiguous overload resolution.</p>
<pre class="highlight"><c- o>&lt;</c-><c- n>source</c-><c- o>>:</c-><c- mi>5</c-><c- o>:</c-><c- mi>14</c-><c- o>:</c-> <c- nl>warning</c-><c- p>:</c-> <c- n>ISO</c-> <c- n>C</c-><c- o>++</c-><c- mi>20</c-> <c- n>considers</c-> <c- n>use</c-> <c- n>of</c-> <c- n>overloaded</c-> <c- k>operator</c-> '<c- o>!=</c->' <c- p>(</c-><c- n>with</c-> <c- n>operand</c-> <c- n>types</c-> <c- sc>'S'</c-> <c- n>and</c-> <c- sc>'S'</c-><c- p>)</c-> <c- n>to</c-> <c- n>be</c-> <c- n>ambiguous</c-> <c- n>despite</c-> <c- n>there</c-> <c- n>being</c-> <c- n>a</c-> <c- n>unique</c-> <c- n>best</c-> <c- n>viable</c-> <c- n>function</c-> <c- n>with</c-> <c- n>non</c-><c- o>-</c-><c- n>reversed</c-> <c- n>arguments</c-> <c- p>[</c-><c- o>-</c-><c- n>Wambiguous</c-><c- o>-</c-><c- n>reversed</c-><c- o>-</c-><c- k>operator</c-><c- p>]</c->
<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>S</c-><c- p>{}</c-> <c- o>!=</c-> <c- n>S</c-><c- p>{};</c->
</pre>
   <p>MSVC accepts this by eagerly dropping rewritten candidates.</p>
   <p>The reason this sample should result in an ambiguity and does not get to the
intended tiebreaker step <a href="http://eel.is/c%2B%2Bdraft/over.match#general-2.8"><code class="highlight"><c- p>[</c-><c- n>over</c-><c- p>.</c-><c- n>match</c-><c- p>.</c-><c- n>general</c-><c- p>]</c-><c- o>/</c-><c- mf>2.8</c-></code></a> is because ambiguity is actually the result of finding best conversions.  During
the best conversions step we discover that the candidate <code class="highlight"><c- k>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-><c- p>)</c-></code> and
rewritten candidate <code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-><c- p>)</c-></code> are equally as good as each other.  To
the compiler, it is as if the user had written:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- n>friend</c-> <c- b>bool</c-> <c- n>f</c-><c- p>(</c-><c- n>S</c-><c- p>,</c-> <c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-><c- p>)</c-> <c- p>{</c-> <c- p>...</c-> <c- p>}</c->
  <c- n>friend</c-> <c- b>bool</c-> <c- n>f</c-><c- p>(</c-><c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>S</c-><c- p>)</c-> <c- p>{</c-> <c- p>...</c-> <c- p>}</c->
<c- p>};</c->
<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>f</c-><c- p>(</c-><c- n>S</c-><c- p>{},</c-> <c- n>S</c-><c- p>{});</c->
</pre>
   <p>Which is a clear ambiguity during the comparison of best conversions.</p>
   <h3 class="heading settled" data-level="1.2" id="existing-code-impact"><span class="secno">1.2. </span><span class="content">Existing code impact</span><a class="self-link" href="#existing-code-impact"></a></h3>
   <p>It is clear to the author(s) of this paper, since the compiler vendors feel the need
to bypass the overload resolution rules defined in the standard that there must be
some valid code patterns for which the new overload resolution rules unintentionally break.</p>
   <p>Using MSVC we implemented the strict rules defined by <code class="highlight"><c- p>[</c-><c- n>over</c-><c- p>.</c-><c- n>match</c-><c- p>.</c-><c- n>general</c-><c- p>]</c-></code> and ran the compiler
against a series of open source projects to gather data on how impactful the strict application
of the new overload resolution rules can be.</p>
   <h4 class="heading settled" data-level="1.2.1" id="open-source-sampling"><span class="secno">1.2.1. </span><span class="content">Open source sampling</span><a class="self-link" href="#open-source-sampling"></a></h4>
   <p>The results of applying the strict rules are as follows:</p>
   <table>
    <tbody>
     <tr>
      <th>Total Projects 
      <th>Failed Projects 
     <tr>
      <td>59 
      <td>20 
   </table>
   <p>Many of the failures are caused by the first code sample mentioned.  Other failures include:</p>
<pre class="language-c++ highlight"><c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>T</c-><c- o>></c->
<c- k>struct</c-> <c- nc>Base</c-> <c- p>{</c->
  <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>Derived</c-> <c- o>:</c-> <c- n>Base</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->

<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>Derived</c-><c- p>{}</c-> <c- o>==</c-> <c- n>Derived</c-><c- p>{};</c->
</pre>
   <p>In this case the user intended to use some common base class to implement all of the comparison
operators.  Because the new operator rewriting rules will also add the synthesized candidate to
the overload set the result becomes ambiguous when trying to compare best conversions.  Both the
regular candidate and the synthesized candidate contain a derived-to-base conversion which makes
one not strictly better than the other.</p>
   <p>We feel that the code above should be <strong>accepted</strong>.</p>
<pre class="language-c++ highlight"><c- n>template</c-> <c- o>&lt;</c-><c- b>bool</c-><c- o>></c->
<c- k>struct</c-> <c- nc>GenericIterator</c-> <c- p>{</c->
  <c- n>using</c-> <c- n>ConstIterator</c-> <c- o>=</c-> <c- n>GenericIterator</c-><c- o>&lt;</c->true<c- o>></c-><c- p>;</c->
  <c- n>using</c-> <c- n>NonConstIterator</c-> <c- o>=</c-> <c- n>GenericIterator</c-><c- o>&lt;</c->false<c- o>></c-><c- p>;</c->
  <c- n>GenericIterator</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
  <c- n>GenericIterator</c-><c- p>(</c-><c- k>const</c-> <c- n>NonConstIterator</c-><c- o>&amp;</c-><c- p>);</c->

  <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- n>ConstIterator</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<c- p>};</c->
<c- n>using</c-> <c- n>Iterator</c-> <c- o>=</c-> <c- n>GenericIterator</c-><c- o>&lt;</c->false<c- o>></c-><c- p>;</c->

<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>Iterator</c-><c- p>{}</c-> <c- o>==</c-> <c- n>Iterator</c-><c- p>{};</c->
</pre>
   <p>This is a scenario where the user is depending on implicit conversions to get the desired effect of
comparisons being compared as a <code class="highlight"><c- n>ConstIterator</c-></code>.  The issue is that the converting constructor enables
conversions on both sides which are no better than the other when considering the reversed candidate.</p>
   <p>We feel that the code above should be <strong>accepted</strong>.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>Iterator</c-> <c- p>{</c->
  <c- n>Iterator</c-><c- p>();</c->
  <c- n>Iterator</c-><c- p>(</c-><c- b>int</c-><c- o>*</c-><c- p>);</c->
  <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>Iterator</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
  <c- n>operator</c-> <c- b>int</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
<c- p>};</c->

<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>nullptr</c-> <c- o>!=</c-> <c- n>Iterator</c-><c- p>{};</c->
</pre>
   <p>This code was relying on the fact that an implicit conversion gives the user <code class="highlight"><c- o>!=</c-></code> for free.  The issue
is that this implicit conversion creates a scenario where the user-defined conversion operator creates
a case where it is not a better choice than the reversed parameter candidate rewrite where a temporary <code class="highlight"><c- n>Iterator</c-></code> type is constructed from <code class="highlight"><c- k>nullptr</c-></code>.</p>
   <p>We feel that this is a case where C++20 helps the user identify possible semantic problems.  It is a
case we should <strong>reject</strong> despite being observed in two different projects.</p>
<pre class="language-c++ highlight"><c- n>using</c-> <c- n>ubool</c-> <c- o>=</c-> <c- b>unsigned</c-> <c- b>char</c-><c- p>;</c->

<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- n>operator</c-> <c- nf>bool</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
<c- p>};</c->
<c- n>ubool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- n>S</c-><c- p>,</c-> <c- n>S</c-><c- p>);</c->

<c- n>ubool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>S</c-><c- p>{}</c-> <c- o>!=</c-> <c- n>S</c-><c- p>{};</c->
</pre>
   <p>Based on the merge of <a href="https://wg21.link/p1630r1">P1630R1</a> there is a case added for the
rewritten candidate chosen must return <em>cv</em>-<code class="highlight"><c- b>bool</c-></code> and if it does not the candidate is rejected, but
because that condition happens after overload resolution the code author ends up seeing:</p>
<pre class="highlight"><c- n>error</c-> <c- nl>C2088</c-><c- p>:</c-> '<c- o>!=</c->'<c- o>:</c-> <c- n>illegal</c-> <c- k>for</c-> <c- k>struct</c->
</pre>
   <p>While this paper does not want to tackle that wording, we feel that the code above should remain <strong>rejected</strong> until a future update.</p>
   <h3 class="heading settled" data-level="1.3" id="proposed-resolution"><span class="secno">1.3. </span><span class="content">Proposed resolution</span><a class="self-link" href="#proposed-resolution"></a></h3>
   <h4 class="heading settled" data-level="1.3.1" id="first-resolution"><span class="secno">1.3.1. </span><span class="content">First resolution</span><a class="self-link" href="#first-resolution"></a></h4>
   <p>To help address the issues mentioned above we gathered input from the following individuals
regarding how GCC and Clang implements overload resolution:</p>
   <ul>
    <li data-md>
     <p>Jason Merril</p>
    <li data-md>
     <p>Richard Smith</p>
   </ul>
   <p>The implementation approach taken by GCC appeared to be the most permissive and applied seemingly
reasonable rules which allow most of the above samples to compile.  The GCC approach is generalized
as:</p>
   <blockquote>
    <p>Before comparing candidates for best conversion sequences, compare each candidate to each other
candidate (or function template they are specializations of) and if the parameter types match and
one is a rewritten candidate the rewritten candidate is not considered for later tiebreakers.</p>
   </blockquote>
   <p>After implementing the rule above in MSVC we obtained the following results from running that
compiler against open source projects:</p>
   <table>
    <tbody>
     <tr>
      <th>Total Projects 
      <th>Failed Projects 
     <tr>
      <td>59 
      <td>10 <em>(down from 20)</em> 
   </table>
   <p>It is immediately clear that the GCC implementation was on the correct path to a good solution.  The
GCC approach ticked all the boxes for code we wanted to compile (mentioned above) while maintaining
the spirit of the original operator rewriting proposal <a href="https://wg21.link/p0515r3">P0515R3</a> by allowing heterogenous comparisons.</p>
   <h4 class="heading settled" data-level="1.3.2" id="second-resolution"><span class="secno">1.3.2. </span><span class="content">Second resolution</span><a class="self-link" href="#second-resolution"></a></h4>
   <p>After discussing the GCC rule Richard Smith proposed a new one:</p>
   <blockquote>
    <p>what if we do not consider rewriting to an <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> if there is an <code class="highlight"><c- k>operator</c-><c- o>!=</c-></code> with the same
signature declared in the same scope? One could argue that all reasonable C++&lt;=17 overload sets
will declare <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> and <code class="highlight"><c- k>operator</c-><c- o>!=</c-></code> together, with the same signatures, and all code
intending to use the C++20 rules will declare only <code class="highlight"><c- k>operator</c-><c- o>==</c-></code>. Then the way you turn off
rewriting is by doing exactly what you did before C++20: you write an <code class="highlight"><c- k>operator</c-><c- o>!=</c-></code> with your <code class="highlight"><c- k>operator</c-><c- o>==</c-></code>.</p>
   </blockquote>
   <p>Note that the above proposal is nearly identical to the GCC approach with the exception of the
'declared in the same scope' rule.  After implementing the above rule in MSVC (but applying to any
rewritten operator not just <code class="highlight"><c- o>==</c-></code>) we obtained the following results:</p>
   <table>
    <tbody>
     <tr>
      <th>Total Projects 
      <th>Failed Projects 
     <tr>
      <td>59 
      <td>10 
   </table>
   <p>The results are identical to that of the GCC rule which implies that we neither regressed behavior
nor improved it.  The good thing about the rule as proposed by Richard is that it offers more
freedom for the implementation to blindly rewrite <code class="highlight"><c- o>!=</c-></code> using <code class="highlight"><c- k>operator</c-><c- o>==</c-></code>.  This proposal still
checks all the boxes above of code patterns we want to enable while also maintaining strong rules
around comparing viable candidates for rewriting.</p>
   <p>Based on the results and the principled approach of the latter rule we propose this resolution for
standardization.</p>
   <h2 class="heading settled" data-level="2" id="wording"><span class="secno">2. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>Change in 12.2.2.3 [over.match.oper] paragraph 3:</p>
   <blockquote>
    <p>The rewritten candidate set is determined as follows:</p>
    <ul>
     <li data-md>
      <p>For the relational ([expr.rel]) operators, the rewritten candidates include all non-rewritten candidates for the expression x &lt;=> y.</p>
     <li data-md>
      <p>For the relational ([expr.rel]) and three-way comparison ([expr.spaceship]) operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y &lt;=> x.</p>
     <li data-md>
      <p>For the != operator ([expr.eq]), the rewritten candidates include all non-rewritten candidates for the expression x == y.</p>
     <li data-md>
      <p>For the equality operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y == x.</p>
     <li data-md>
      <ins>For any rewritten operator candidate, if there exists another candidate with the same signature declared in the same scope and that candidate is a non-rewritten candidate then the rewritten candiate is removed from the candidate set.</ins>
     <li data-md>
      <p>For all other operators, the rewritten candidate set is empty.</p>
    </ul>
   </blockquote>
   <blockquote>
    <p>[Note 2: A candidate synthesized from a member candidate has its implicit object parameter as the second parameter, thus implicit conversions are considered for the first, but not for the second, parameter. — end note]</p>
   </blockquote>
   <h2 class="heading settled" data-level="3" id="appendix"><span class="secno">3. </span><span class="content">Appendix</span><a class="self-link" href="#appendix"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="code-patterns-which-fail-to-compile-even-under-new-rules"><span class="secno">3.1. </span><span class="content">Code patterns which fail to compile even under new rules</span><a class="self-link" href="#code-patterns-which-fail-to-compile-even-under-new-rules"></a></h3>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>Derived</c-><c- o>></c->
<c- k>struct</c-> <c- nc>Base</c-> <c- p>{</c->
  <c- b>int</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- b>double</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
  <c- n>friend</c-> <c- kr>inline</c-> <c- b>int</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- b>double</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Derived</c-><c- o>&amp;</c-><c- p>);</c->
  <c- b>int</c-> <c- n>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- b>double</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
  <c- n>friend</c-> <c- kr>inline</c-> <c- b>int</c-> <c- n>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- b>double</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Derived</c-><c- o>&amp;</c-><c- p>);</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>X</c-> <c- o>:</c-> <c- n>Base</c-><c- o>&lt;</c-><c- n>X</c-><c- o>></c-> <c- p>{</c-> <c- p>};</c->

<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>X</c-><c- p>{}</c-> <c- o>==</c-> <c- mf>0.</c-><c- p>;</c->
</pre>
   <p>Fails due to requirements of rewritten operator returning <em>cv</em>-<code class="highlight"><c- b>bool</c-></code>.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>Base</c-> <c- p>{</c->
    <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>Base</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
    <c- b>bool</c-> <c- n>operator</c-><c- o>!=</c-><c- p>(</c-><c- k>const</c-> <c- n>Base</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>Derived</c-> <c- o>:</c-> <c- n>Base</c-> <c- p>{</c->
    <c- n>Derived</c-><c- p>(</c-><c- k>const</c-> <c- n>Base</c-><c- o>&amp;</c-><c- p>);</c->
    <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>Derived</c-><c- o>&amp;</c-> <c- n>rhs</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>static_cast</c-><c- o>&lt;</c-><c- k>const</c-> <c- n>Base</c-><c- o>&amp;></c-><c- p>(</c-><c- o>*</c-><c- n>this</c-><c- p>)</c-> <c- o>==</c-> <c- n>rhs</c-><c- p>;</c->
    <c- p>}</c->
<c- p>};</c->
</pre>
   <p>The code above fails due to relying on a derived-to-base conversion on both sides of the comparison
(once the synthesized candidate is taken into account).  If one imagines a similar scenario:</p>
<pre class="language-c++ highlight"><c- b>bool</c-> <c- n>b1</c-> <c- o>=</c-> <c- n>Derived</c-><c- p>{}</c-> <c- o>==</c-> <c- n>Base</c-><c- p>{};</c->
<c- b>bool</c-> <c- n>b2</c-> <c- o>=</c-> <c- n>Base</c-><c- p>{}</c-> <c- o>==</c-> <c- n>Derived</c-><c- p>{};</c->
</pre>
   <p>This would also be ambiguous even without the definition of <code class="highlight"><c- n>Derived</c-><c- o>::</c-><c- k>operator</c-><c- o>==</c-></code> and in our view is
considered a bugfix in C++20 which forces the user to carefully consider how objects are being
compared.</p>
   <h3 class="heading settled" data-level="3.2" id="code-patterns-which-fail-at-runtime"><span class="secno">3.2. </span><span class="content">Code patterns which fail at runtime</span><a class="self-link" href="#code-patterns-which-fail-at-runtime"></a></h3>
   <p>In C++20, because of how candidate sets are expanded there are a few scenarios where a new candidate
introduced is then selected as the best candidate without any code change.  In many cases this can
be fine but there are a few cases we identified as being potentially problematic for code authors.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>iterator</c-><c- p>;</c->
<c- k>struct</c-> <c- nc>const_iterator</c-> <c- p>{</c->
  <c- n>const_iterator</c-><c- p>(</c-><c- k>const</c-> <c- n>iterator</c-><c- o>&amp;</c-><c- p>);</c->
  <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>const_iterator</c-> <c- o>&amp;</c-><c- n>ci</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>iterator</c-> <c- p>{</c->
  <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>const_iterator</c-> <c- o>&amp;</c-><c- n>ci</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- n>ci</c-> <c- o>==</c-> <c- o>*</c-><c- n>this</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->
</pre>
   <p>In C++17 the sample above would compile and the function selected for the comparison <code class="highlight"><c- n>ci</c-> <c- o>==</c-> <c- o>*</c-><c- k>this</c-></code> would be <code class="highlight"><c- n>const_iterator</c-><c- o>::</c-><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>const_iterator</c-><c- o>&amp;</c-><c- p>)</c-></code>.  In C++20 the function chosen for the
same comparison is the (rewritten) function <code class="highlight"><c- n>iterator</c-><c- o>::</c-><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>const_iterator</c-><c- o>&amp;</c-><c- p>)</c-></code> using the
reversed parameter order rule.  The chosen function in C++20 causes a new runtime failure where the
comparison will cause infinite recursion where there wasn’t one before.</p>
   <p>Another example:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>U</c-> <c- p>{</c-> <c- p>};</c->

<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
  <c- n>template</c-> <c- o>&lt;</c-><c- n>typename</c-> <c- n>T</c-><c- o>></c->
  <c- n>friend</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>T</c-><c- o>&amp;</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> true<c- p>;</c-> <c- p>}</c->

  <c- n>friend</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>U</c-><c- o>&amp;</c-> <c- n>u</c-><c- p>,</c-> <c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-> <c- n>s</c-><c- p>)</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>s</c-> <c- o>==</c-> <c- n>u</c-><c- p>;</c->
  <c- p>}</c->
<c- p>};</c->

<c- b>bool</c-> <c- n>b</c-> <c- o>=</c-> <c- n>U</c-><c- p>{}</c-> <c- o>==</c-> <c- n>S</c-><c- p>{};</c->
</pre>
   <p>In C++17 the user intended for comparisons to be dispatched to the templated <code class="highlight"><c- k>operator</c-><c- o>==</c-></code>.  In C++20
the templated <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> is considered to be a worse match based on <a href="http://eel.is/c%2B%2Bdraft/over.match#best.general-2.4"><code class="highlight"><c- p>[</c-><c- n>over</c-><c- p>.</c-><c- n>match</c-><c- p>.</c-><c- n>best</c-><c- p>.</c-><c- n>general</c-><c- p>]</c-><c- o>/</c-><c- mf>2.4</c-></code></a> which is a tiebreaker before rewritten candidate tiebreakers, which makes <code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>U</c-><c- o>&amp;</c-> <c- n>u</c-><c- p>,</c-> <c- k>const</c-> <c- n>S</c-><c- o>&amp;</c-> <c- n>s</c-><c- p>)</c-></code> the best match for the comparison <code class="highlight"><c- n>s</c-> <c- o>==</c-> <c- n>u</c-></code>.</p>
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