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 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>p0946r0: Towards consistency between &lt;=> and other comparison operators</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

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

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

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

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

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

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

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

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

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

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

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

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

	table.data  thead,
	table.index thead,
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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">p0946r0<br>Towards consistency between <code>&lt;=></code> and other comparison operators</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-02-10">10 February 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p0946r0">http://wg21.link/p0946r0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:richard@metafoo.co.uk">Richard Smith</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: 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>The specification of the <code class="highlight"><span class="o">&lt;=></span></code> operator reexamined the rules underlying comparison operators, and disallowed some cases that have historically been allowed, but error-prone, for the other relational and equality operators. This paper considers options for realigning the rules for these operators, whether by deprecating or removing the corresponding cases for those other operators or by revisiting the rules for the <code class="highlight"><span class="o">&lt;=></span></code> operator. This paper does not provide specific guidance for some of these questions, instead aiming to promote discussion and a search for a better answer.</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="#differences"><span class="secno">1</span> <span class="content">Differences</span></a>
     <ol class="toc">
      <li><a href="#sign-safety"><span class="secno">1.1</span> <span class="content">Sign safety</span></a>
      <li><a href="#enum-safety"><span class="secno">1.2</span> <span class="content">Enum safety</span></a>
      <li><a href="#array-safety"><span class="secno">1.3</span> <span class="content">Array safety</span></a>
      <li><a href="#null-safety"><span class="secno">1.4</span> <span class="content">Null safety</span></a>
      <li><a href="#fp-safety"><span class="secno">1.5</span> <span class="content">Function pointer safety</span></a>
     </ol>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="differences"><span class="secno">1. </span><span class="content">Differences</span><a class="self-link" href="#differences"></a></h2>
   <p>The rules for the built-in <code class="highlight"><span class="o">&lt;=></span></code> operator, introduced in <a data-link-type="biblio" href="#biblio-p0515r3">[P0515R3]</a>, do not match those for the built-in <code class="highlight"><span class="o">&lt;</span></code>, <code class="highlight"><span class="o">></span></code>, <code class="highlight"><span class="o">&lt;=</span></code>, <code class="highlight"><span class="o">>=</span></code>, <code class="highlight"><span class="o">==</span></code>, and <code class="highlight"><span class="o">!=</span></code> operators (herein referred to as <dfn data-dfn-type="dfn" data-noexport="" id="two-way-operators">two-way operators<a class="self-link" href="#two-way-operators"></a></dfn>) in a number of ways:</p>
   <ul>
    <li data-md="">
     <p><a href="#sign-safety">§1.1 Sign safety</a> If one operand is of a signed integral type, but the type in which the comparison would be performed would be an unsigned type, a <code class="highlight"><span class="o">&lt;=></span></code> comparison is rejected unless the signed operand is a constant expression that evaluates to a non-negative value; the other forms of comparison compare the converted value (for example, <code class="highlight"><span class="o">-</span><span class="mi">1</span> <span class="o">&lt;</span> <span class="mi">2u</span></code> evaluates to <code class="highlight">false</code> but <code class="highlight"><span class="o">-</span><span class="mi">1</span> <span class="o">&lt;=></span> <span class="mi">2u</span></code> is ill-formed).</p>
    <li data-md="">
     <p><a href="#enum-safety">§1.2 Enum safety</a> If either operand is of enumeration type, a <code class="highlight"><span class="o">&lt;=></span></code> comparison requires the other operand to have the same type; other forms of comparison merely require the other operand to have arithmetic or enumeration type.</p>
    <li data-md="">
     <p><a href="#array-safety">§1.3 Array safety</a> If both operands are of array type, a <code class="highlight"><span class="o">&lt;=></span></code> comparison between them is ill-formed. The behavior of the two-way operators is unclear in the standard wording, but the widespread implementation consensus is that the array-to-pointer conversion is applied to the operands prior to checking their types, which results in such comparisons being permitted.</p>
    <li data-md="">
     <p><a href="#null-safety">§1.4 Null safety</a> If one operand is a null pointer constant (a <code class="highlight"><span class="mi">0</span></code> literal or <code class="highlight"><span class="k">nullptr</span></code>) and the other is a pointer, the relational <code class="highlight"><span class="o">&lt;</span></code>, <code class="highlight"><span class="o">></span></code>, <code class="highlight"><span class="o">&lt;=</span></code>, and <code class="highlight"><span class="o">>=</span></code> comparisons are ill-formed. But <code class="highlight"><span class="o">&lt;=></span></code> comparisons are permitted, returning a <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">strong_ordering</span></code> with an unspecified value.</p>
    <li data-md="">
     <p><a href="#fp-safety">§1.5 Function pointer safety</a> Relational comparisons between function pointers are permitted, with unspecified results. But <code class="highlight"><span class="o">&lt;=></span></code> produces <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">strong_equality</span></code> for such comparisons, providing no ordering information.</p>
   </ul>
   <p>In most cases, <code class="highlight"><span class="o">&lt;=></span></code> provides superior rules, derived from experience with the existing operators. However, these differences will be the source of confusion and ire from C++ programmers indefinitely if we do nothing to resolve them. These differences will be analyzed in turn below.</p>
   <p>For the sake of exposition, examples below use a simple wrapper type with a defaulted <code class="highlight"><span class="k">operator</span><span class="o">&lt;=></span></code> (this provides a simplified model of types like <code class="highlight"><span class="n">pair</span></code> and <code class="highlight"><span class="n">tuple</span></code>):</p>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">wrapper</span> <span class="p">{</span> <span class="n">T</span> <span class="n">t</span><span class="p">;</span> <span class="p">};</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="n">wrapper</span><span class="p">(</span><span class="n">T</span><span class="p">)</span> <span class="o">-></span> <span class="n">wrapper</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">;</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="p">,</span> <span class="k">typename</span> <span class="n">U</span><span class="o">></span> <span class="k">auto</span> <span class="k">operator</span><span class="o">&lt;=></span><span class="p">(</span><span class="n">wrapper</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">,</span> <span class="n">wrapper</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span><span class="p">)</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
</pre>
   <p>Reasonable user expectation would be that two-way comparisons between <code class="highlight"><span class="n">wrapper</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> and <code class="highlight"><span class="n">wrapper</span><span class="o">&lt;</span><span class="n">U</span><span class="o">></span></code> behave the same as the corresponding two-way comparisons between <code class="highlight"><span class="n">T</span></code> and <code class="highlight"><span class="n">U</span></code>, but as we will see, the above rule differences defy this expectation.</p>
   <h3 class="heading settled" data-level="1.1" id="sign-safety"><span class="secno">1.1. </span><span class="content">Sign safety</span><a class="self-link" href="#sign-safety"></a></h3>
   <div class="example" id="example-4c05608b">
    <a class="self-link" href="#example-4c05608b"></a> 
<pre class="highlight"><span class="k">static_assert</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">></span> <span class="mi">1u</span><span class="p">);</span>          <span class="c1">// assert passes</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">((</span><span class="o">-</span><span class="mi">1</span> <span class="o">&lt;=></span> <span class="mi">1u</span><span class="p">)</span> <span class="o">></span> <span class="mi">0</span><span class="p">);</span>  <span class="c1">// ill-formed</span>
</pre>
   </div>
   <p>The rule used by <code class="highlight"><span class="o">&lt;=></span></code> in this situation avoids a long-standing class of bugs. However, the corresponding case for other comparison operators is likely in use in a significant amount of code; examples such as</p>
<pre class="highlight"><span class="kt">void</span> <span class="nf">f</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">></span> <span class="n">v</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">n</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">n</span> <span class="o">&lt;</span> <span class="n">v</span><span class="p">.</span><span class="n">size</span><span class="p">();</span> <span class="n">n</span><span class="o">++</span><span class="p">)</span>
</pre>
   <p>abound, and implicitly rely on such signed/unsigned comparisons. We cannot realistically reject or even deprecate this code due to the volume of such comparisons in existing code.</p>
   <p>Neither the behavior of the existing operators nor the behavior of <code class="highlight"><span class="o">&lt;=></span></code> is ideal. Designing from a clean slate, one appealing option would be to specify that these comparisons "just work": that is, that they always give the mathematically-correct results, as if no conversion were performed on the operands:</p>
<pre class="highlight"><span class="k">static_assert</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">&lt;</span> <span class="mi">1u</span><span class="p">);</span>          <span class="c1">// OK</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">((</span><span class="o">-</span><span class="mi">1</span> <span class="o">&lt;=></span> <span class="mi">1u</span><span class="p">)</span> <span class="o">&lt;</span> <span class="mi">0</span><span class="p">);</span>  <span class="c1">// OK</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">&lt;</span> <span class="n">UINT_MAX</span><span class="p">);</span>    <span class="c1">// OK</span>
</pre>
   <p>However, we cannot simply give the <code class="highlight"><span class="o">&lt;=></span></code> operator the mathematical meaning without creating a major inconsistency between it and the other comparison operators. Such inconsistency would also leak into other operators, through automatic rewriting of those operators into uses of <code class="highlight"><span class="o">&lt;=></span></code>. Consider:</p>
<pre class="highlight"><span class="k">static_assert</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">&lt;</span> <span class="mi">1u</span><span class="p">);</span>                    <span class="c1">// fails</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">(</span><span class="n">wrapper</span><span class="p">{</span><span class="o">-</span><span class="mi">1</span><span class="p">}</span> <span class="o">&lt;</span> <span class="n">wrapper</span><span class="p">{</span><span class="mi">1u</span><span class="p">});</span>  <span class="c1">// passes!</span>
</pre>
   <p>We also cannot reasonably give the mathematical meaning to both the <code class="highlight"><span class="o">&lt;=></span></code> operator and the two-way operators; existing mixed-signedness comparisons are too prevalent for their meaning to be changed at this stage.</p>
   <p>Our most obvious remaining options are either to remove this narrowing restriction from <code class="highlight"><span class="o">&lt;=></span></code> (allowing these comparisons to continue to produce values different from their mathematical meaning), or to retain the status quo: that <code class="highlight"><span class="o">&lt;=></span></code> has validity rules different from those of two-way operators.</p>
   <p>One non-obvious alternative exists: we could deprecate evaluations of two-way operators whose results are different from their mathematical results, and encourage vendors of static and dynamic analysis tools to diagnose such comparisons. It is unlikely that any deprecation period will suffice to allow us to change the meanings of these operators to their mathematical meaning, but we can at least send a signal that such operations are discouraged.</p>
   <p>In the absence of a clearly best option, this paper makes no recommendation on this issue, but we hope to have motivated a search for a better answer than the status quo.</p>
   <h3 class="heading settled" data-level="1.2" id="enum-safety"><span class="secno">1.2. </span><span class="content">Enum safety</span><a class="self-link" href="#enum-safety"></a></h3>
   <div class="example" id="example-ccdb1785">
    <a class="self-link" href="#example-ccdb1785"></a> 
<pre class="highlight"><span class="k">enum</span> <span class="n">E</span> <span class="p">{</span> <span class="n">a</span> <span class="o">=</span> <span class="mi">3</span> <span class="p">};</span>
<span class="k">static_assert</span><span class="p">(</span><span class="n">a</span> <span class="o">==</span> <span class="mi">3</span><span class="p">);</span>             <span class="c1">// OK</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">((</span><span class="n">a</span> <span class="o">&lt;=></span> <span class="mi">3</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">);</span>     <span class="c1">// ill-formed</span>
<span class="c1"></span>
<span class="k">enum</span> <span class="n">F</span> <span class="p">{</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">3</span> <span class="p">};</span>
<span class="k">static_assert</span><span class="p">(</span><span class="n">a</span> <span class="o">==</span> <span class="n">b</span><span class="p">);</span>             <span class="c1">// OK</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">((</span><span class="n">a</span> <span class="o">&lt;=></span> <span class="n">b</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">);</span>     <span class="c1">// ill-formed</span>
<span class="c1"></span>
<span class="k">static_assert</span><span class="p">(</span><span class="n">a</span> <span class="o">>=</span> <span class="mf">1.34</span><span class="p">);</span>          <span class="c1">// OK</span>
<span class="c1"></span><span class="k">static_assert</span><span class="p">((</span><span class="n">a</span> <span class="o">&lt;=></span> <span class="mf">1.34</span><span class="p">)</span> <span class="o">>=</span> <span class="mi">0</span><span class="p">);</span>  <span class="c1">// ill-formed</span>
</pre>
   </div>
   <p>The <code class="highlight"><span class="o">&lt;=></span></code> operator provides some safety when comparing values of enumeration type. However, this safety comes at the cost of disallowing reasonable comparisons between values of an unscoped enumeration and those of its underlying type. This leads to disallowing cases such as:</p>
<pre class="highlight"><span class="k">enum</span> <span class="n">Bits</span> <span class="p">{</span> <span class="n">Foo</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="n">Bar</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span> <span class="n">Baz</span> <span class="o">=</span> <span class="mi">4</span> <span class="p">};</span>
<span class="kt">bool</span> <span class="nf">containsFooBar</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">set</span><span class="o">&lt;</span><span class="n">Bits</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">less</span><span class="o">&lt;>></span> <span class="o">&amp;</span><span class="n">set</span><span class="p">)</span> <span class="p">{</span>
  <span class="c1">// Note that Foo | Bar is of integral type, not type Bits</span>
<span class="c1"></span>  <span class="k">return</span> <span class="n">set</span><span class="p">.</span><span class="n">count</span><span class="p">(</span><span class="n">Foo</span> <span class="o">|</span> <span class="n">Bar</span><span class="p">);</span>           <span class="c1">// OK</span>
<span class="c1"></span><span class="p">}</span>
<span class="kt">bool</span> <span class="nf">containsFooBar</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">set</span><span class="o">&lt;</span><span class="n">wrapper</span><span class="o">&lt;</span><span class="n">Bits</span><span class="o">></span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">less</span><span class="o">&lt;>></span> <span class="o">&amp;</span><span class="n">set</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">return</span> <span class="n">set</span><span class="p">.</span><span class="n">count</span><span class="p">(</span><span class="n">wrapper</span><span class="p">{</span><span class="n">Foo</span> <span class="o">|</span> <span class="n">Bar</span><span class="p">});</span>  <span class="c1">// ill-formed!</span>
<span class="c1"></span><span class="p">}</span>
</pre>
   <p>We can distinguish (at least) three cases, each of which is currently ill-formed when using <code class="highlight"><span class="o">&lt;=></span></code> but valid under the two-way comparison operators:</p>
   <ul>
    <li data-md="">
     <p>Comparison between a value of one enumeration type and a distinct enumeration type. Such cases are very likely due to bugs, and requiring an explicit cast (for any comparison operator) seems prudent.</p>
    <li data-md="">
     <p>Comparison between a value of an enumeration type and a value of a non-integral (eg, floating-point) type. Likewise, this is probably undesirable, but it is possible that an enumeration type could define enumerators representing values for which comparison against floating-point types is reasonable and intended. (For example: <code class="highlight"><span class="k">enum</span> <span class="n">Brightness</span> <span class="p">{</span> <span class="n">Min</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="n">Max</span> <span class="o">=</span> <span class="mi">255</span> <span class="p">};</span> <span class="kt">double</span> <span class="n">red</span> <span class="o">=</span> <span class="n">Brightness</span><span class="o">::</span><span class="n">Max</span> <span class="o">*</span> <span class="n">std</span><span class="o">::</span><span class="n">exp</span><span class="p">(</span><span class="o">-</span><span class="n">k</span> <span class="o">*</span> <span class="n">t</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">red</span> <span class="o">></span> <span class="n">Brightness</span><span class="o">::</span><span class="n">Max</span><span class="p">)</span> <span class="n">red</span> <span class="o">=</span> <span class="n">Brightness</span><span class="o">::</span><span class="n">Max</span><span class="p">;</span></code>)</p>
    <li data-md="">
     <p>Comparison between a value of an enumeration type and a value of an integral type. Such comparisons are extremely common, and most such cases are probably intended and correct.</p>
   </ul>
   <p>In the first case, the best option would likely be to deprecate or remove the support for such two-way comparisons. In the third case, the rules for the <code class="highlight"><span class="o">&lt;=></span></code> operator seem to prevent a large class of useful programs without a commensurate benefit, and should be revised to permit such comparisons. The middle case seems debatable, but is likely sufficiently rare that requiring an explicit cast is not overly onerous.</p>
   <p>Suggested approach: deprecate two-way comparisons between enumeration types and floating-point types / distinct enumeration types. Permit three-way comparison between unscoped enumeration types and integral types.</p>
   <p>We should also consider whether we wish any such deprecation to occur only for comparison operators, or more generally for any case where the usual arithmetic conversions are applied between an operand of enumeration type and an operand of floating-point or distinct enumeration type (for instance, <code class="highlight"><span class="n">RGB</span><span class="o">::</span><span class="n">Red</span> <span class="o">|</span> <span class="n">HSV</span><span class="o">::</span><span class="n">Blue</span></code> or <code class="highlight"><span class="mf">4.0</span> <span class="o">*</span> <span class="n">SISuffix</span><span class="o">::</span><span class="n">Giga</span></code>).</p>
   <h3 class="heading settled" data-level="1.3" id="array-safety"><span class="secno">1.3. </span><span class="content">Array safety</span><a class="self-link" href="#array-safety"></a></h3>
   <div class="example" id="example-08ef9c5d">
    <a class="self-link" href="#example-08ef9c5d"></a> 
<pre class="highlight"><span class="kt">int</span> <span class="n">a</span><span class="p">[</span><span class="mi">3</span><span class="p">],</span> <span class="n">b</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span>
<span class="kt">int</span> <span class="o">*</span><span class="n">p</span><span class="p">;</span>
<span class="kt">void</span> <span class="nf">f</span><span class="p">()</span> <span class="p">{</span>
  <span class="n">a</span> <span class="o">&lt;</span> <span class="n">b</span><span class="p">;</span>          <span class="c1">// OK (implementation consensus)</span>
<span class="c1"></span>  <span class="n">a</span> <span class="o">&lt;=></span> <span class="n">b</span><span class="p">;</span>        <span class="c1">// ill-formed</span>
<span class="c1"></span>
  <span class="n">a</span> <span class="o">&lt;</span> <span class="n">p</span><span class="p">;</span>          <span class="c1">// OK</span>
<span class="c1"></span>  <span class="n">a</span> <span class="o">&lt;=></span> <span class="n">p</span><span class="p">;</span>        <span class="c1">// OK</span>
<span class="c1"></span>
  <span class="n">a</span> <span class="o">&lt;</span> <span class="k">nullptr</span><span class="p">;</span>    <span class="c1">// ill-formed (by DR 583)</span>
<span class="c1"></span>  <span class="n">a</span> <span class="o">&lt;=></span> <span class="k">nullptr</span><span class="p">;</span>  <span class="c1">// ill-formed</span>
<span class="c1"></span><span class="p">}</span>
</pre>
   </div>
   <p>The validity of a two-way comparison between two array operands is unclear in the current standard text. Current implementations permit it; the chain of reasoning that appears to be used to justify this is:</p>
   <ul>
    <li data-md="">
     <p>Two-way comparison operators require prvalues as their operands, despite this being nowhere stated in the standard text.</p>
    <li data-md="">
     <p>Therefore, operands of array type are decayed to pointers.</p>
    <li data-md="">
     <p>Such decay occurs <em>before</em> the two-way comparison operators perform checks on the types of their operands, so they never see an array operand, only a pointer.</p>
   </ul>
   <p>This reasoning appears to be inspired by the rules of C, where the rules are more explicit and admit only the above interpretation.</p>
   <p>However, the above reasoning breaks down for the <code class="highlight"><span class="o">&lt;=></span></code> operator, because its description explicitly specifies when to apply the array-to-pointer decay, strongly implying that such conversion should not be applied beforehand.</p>
   <p>The rule used by the <code class="highlight"><span class="o">&lt;=></span></code> operator appears to be the more appropriate one in this case. Equality and relational comparisons between two array objects seem highly unlikely to be desirable, and create the false impression of comparing the array contents rather than the decayed addresses. As such, we propose deprecating two-way comparisons where both operands are of array type.</p>
   <h3 class="heading settled" data-level="1.4" id="null-safety"><span class="secno">1.4. </span><span class="content">Null safety</span><a class="self-link" href="#null-safety"></a></h3>
   <div class="example" id="example-24732225">
    <a class="self-link" href="#example-24732225"></a> 
<pre class="highlight"><span class="kt">int</span> <span class="o">*</span><span class="n">p</span><span class="p">;</span>
<span class="kt">void</span> <span class="nf">f</span><span class="p">()</span> <span class="p">{</span>
  <span class="n">p</span> <span class="o">==</span> <span class="k">nullptr</span><span class="p">;</span>   <span class="c1">// OK</span>
<span class="c1"></span>  <span class="n">p</span> <span class="o">&lt;</span> <span class="k">nullptr</span><span class="p">;</span>    <span class="c1">// ill-formed, meaningless</span>
<span class="c1"></span>  <span class="n">p</span> <span class="o">&lt;=></span> <span class="k">nullptr</span><span class="p">;</span>  <span class="c1">// OK, std::strong_ordering,</span>
<span class="c1"></span>                  <span class="c1">// value unspecified if p non-null!</span>
<span class="c1"></span>
  <span class="k">nullptr</span> <span class="o">==</span> <span class="k">nullptr</span><span class="p">;</span>   <span class="c1">// OK, true</span>
<span class="c1"></span>  <span class="k">nullptr</span> <span class="o">&lt;</span> <span class="k">nullptr</span><span class="p">;</span>    <span class="c1">// ill-formed</span>
<span class="c1"></span>  <span class="k">nullptr</span> <span class="o">&lt;=></span> <span class="k">nullptr</span><span class="p">;</span>  <span class="c1">// OK, std::strong_equality::equal</span>
<span class="c1"></span><span class="p">}</span>
</pre>
   </div>
   <p>The resolution of core issue 583 made relational comparisons against null pointer constants ill-formed. Such constructs have always been ill-formed in C, and appear likely to have only ever been valid in C++ due to a wording oversight.</p>
   <p>However, the <code class="highlight"><span class="o">&lt;=></span></code> operator oddly produces a <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">strong_ordering</span></code> result when comparing a null pointer constant against an object pointer, producing <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">strong_ordering</span><span class="o">::</span><span class="n">equal</span></code> when the pointer is null and an unspecified value (which could even be <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">strong_ordering</span><span class="o">::</span><span class="n">equal</span></code>!) otherwise. This seems to also be merely a wording oversight.</p>
   <p>Suggested approach: change the <code class="highlight"><span class="o">&lt;=></span></code> operator to produce <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">strong_equality</span></code>, rather than <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">strong_ordering</span></code>, for comparisons between a null pointer constant and an object pointer.</p>
   <h3 class="heading settled" data-level="1.5" id="fp-safety"><span class="secno">1.5. </span><span class="content">Function pointer safety</span><a class="self-link" href="#fp-safety"></a></h3>
   <div class="example" id="example-d7b7cefd">
    <a class="self-link" href="#example-d7b7cefd"></a> 
<pre class="highlight"><span class="k">using</span> <span class="n">Func</span> <span class="o">=</span> <span class="kt">void</span><span class="p">();</span>
<span class="n">Func</span> <span class="o">*</span><span class="n">p</span><span class="p">;</span>
<span class="n">Func</span> <span class="o">*</span><span class="n">q</span><span class="p">;</span>
<span class="n">Func</span> <span class="o">&amp;</span><span class="n">g</span><span class="p">;</span>
<span class="kt">void</span> <span class="nf">f</span><span class="p">()</span> <span class="p">{</span>
  <span class="n">p</span> <span class="o">==</span> <span class="n">q</span><span class="p">;</span>   <span class="c1">// OK</span>
<span class="c1"></span>  <span class="n">p</span> <span class="o">&lt;</span> <span class="n">q</span><span class="p">;</span>    <span class="c1">// OK?! value unspecified if p != q</span>
<span class="c1"></span>  <span class="n">p</span> <span class="o">&lt;=></span> <span class="n">q</span><span class="p">;</span>  <span class="c1">// OK, std::strong_equality</span>
<span class="c1"></span>
  <span class="n">p</span> <span class="o">==</span> <span class="n">f</span><span class="p">;</span>   <span class="c1">// OK</span>
<span class="c1"></span>  <span class="n">p</span> <span class="o">&lt;</span> <span class="n">f</span><span class="p">;</span>    <span class="c1">// OK (implementation consensus)</span>
<span class="c1"></span>  <span class="n">p</span> <span class="o">&lt;=></span> <span class="n">f</span><span class="p">;</span>  <span class="c1">// OK, std::strong_equality</span>
<span class="c1"></span>
  <span class="n">f</span> <span class="o">==</span> <span class="n">g</span><span class="p">;</span>   <span class="c1">// OK (implementation consensus)</span>
<span class="c1"></span>  <span class="n">f</span> <span class="o">&lt;</span> <span class="n">g</span><span class="p">;</span>    <span class="c1">// OK (implementation consensus)</span>
<span class="c1"></span>  <span class="n">f</span> <span class="o">&lt;=></span> <span class="n">g</span><span class="p">;</span>  <span class="c1">// ill-formed</span>
<span class="c1"></span><span class="p">}</span>
</pre>
   </div>
   <p>The implementation-consensus cases here are analogous to the array cases discussed above; the wording is not completely clear that these cases are valid. However, in the function case, permitting such comparisons seems less harmful: there is little risk of someone believing that the contents of a function rather than its address would be compared. There is a different risk, namely that the user may have intended <code class="highlight"><span class="n">f</span><span class="p">()</span> <span class="o">==</span> <span class="n">g</span><span class="p">()</span></code> instead of <code class="highlight"><span class="n">f</span> <span class="o">==</span> <span class="n">g</span></code>, but such heuristic checks are best left to quality-of-implementation compiler diagnostics.</p>
   <p>The more pressing concern is that of relational comparisons between function pointers. There are two sensible possibilities here: either we should require a useful total ordering over such pointers, exposed by both the relational operators and by the <code class="highlight"><span class="o">&lt;=></span></code> operator, or we should simply disallow such comparisons. We could consider making this choice implementation-defined, as we do for the choice to permit casting between function pointer and object pointer types. But we should not make different decisions for relational ordering operators and the <code class="highlight"><span class="o">&lt;=></span></code> operator.</p>
   <p>Suggested approach: deprecate relational comparisons between function pointers. Clarify that equality comparisons between two functions or references to function is valid.</p>
   <p>It is worth noting that <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">less</span></code> produces a strict total order for function pointers, even though the <code class="highlight"><span class="o">&lt;</span></code> operator does not specify the result for any unequal comparison. Code using, say, <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">set</span><span class="o">&lt;</span><span class="n">Func</span><span class="o">*></span></code> is guaranteed to work today, and it would be reasonable to expect it to continue working in the future. This may require adding a <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">less</span></code> specialization for function pointers if relational function pointer comparisons are ever removed, for implementations that implement <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">less</span></code> in terms of <code class="highlight"><span class="o">&lt;</span></code> today. (However, code using types compounded from function pointers, such as <code class="highlight"><span class="n">std</span><span class="o">::</span><span class="n">set</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">tuple</span><span class="o">&lt;</span><span class="n">Func</span><span class="o">*>></span></code>, would still transition from being valid-but-unspecified to being ill-formed, unless we solve the more general issue that associative container ordering should permit more types than <code class="highlight"><span class="o">&lt;</span></code> does.)</p>
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  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#two-way-operators">two-way operators</a><span>, in §1</span>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-p0515r3">[P0515R3]
   <dd>Herb Sutter, Jens Maurer, Walter E. Brown. <a href="https://wg21.link/p0515r3">Consistent comparison</a>. URL: <a href="https://wg21.link/p0515r3">https://wg21.link/p0515r3</a>
  </dl>
