<!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>P2592R1: Hashing support for std::chrono value classes</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
	}


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

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

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

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

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

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

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

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

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

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

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

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

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

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

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
	}

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	pre code,
	code code {
		font-size: 100%;
	}

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

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

	/* Do something nice. */

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

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

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	  or use class="marker" to mark up the issue number in source. */

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

	.example {
		border-color: #e0cb52;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	  or use class="marker" to mark up the example number in source. */

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

	.note {
		border-color: #52e052;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

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

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

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

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

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

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

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

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

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

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      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 66c4449, updated Wed Jun 29 14:21:13 2022 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: 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">P2592R1<br>Hashing support for std::chrono value classes</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2022-06-30">2022-06-30</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:giuseppe.dangelo@kdab.com">Giuseppe D'Angelo</a>
     <dt>Audience:
     <dd>LEWG
     <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>We propose to add <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>hash</c-></code> specializations for the

    value classes in the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-></code> namespace.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#changelog"><span class="secno">1</span> <span class="content">Changelog</span></a>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation and Scope</span></a>
    <li><a href="#impact"><span class="secno">3</span> <span class="content">Impact On The Standard</span></a>
    <li>
     <a href="#designdecisions"><span class="secno">4</span> <span class="content">Design Decisions</span></a>
     <ol class="toc">
      <li><a href="#what-about-time_zone"><span class="secno">4.1</span> <span class="content">What about <code class="highlight"><c- n>time_zone</c-></code>?</span></a>
      <li>
       <a href="#support-for-heterogeneous-hashing"><span class="secno">4.2</span> <span class="content">Support for heterogeneous hashing</span></a>
       <ol class="toc">
        <li><a href="#implicit-conversions"><span class="secno">4.2.1</span> <span class="content">Implicit conversions</span></a>
       </ol>
      <li><a href="#handling-the-unspecified-state-of-the-calendaring-classes"><span class="secno">4.3</span> <span class="content">Handling the unspecified state of the calendaring classes</span></a>
     </ol>
    <li>
     <a href="#technicalspecifications"><span class="secno">5</span> <span class="content">Technical Specifications</span></a>
     <ol class="toc">
      <li><a href="#feature-testing-macro"><span class="secno">5.1</span> <span class="content">Feature testing macro</span></a>
      <li><a href="#proposed-wording"><span class="secno">5.2</span> <span class="content">Proposed wording</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">6</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Changelog</span><a class="self-link" href="#changelog"></a></h2>
   <ul>
    <li data-md>
     <p>R1</p>
     <ul>
      <li data-md>
       <p>Incorporated the feedback from a LEWG teleconference.</p>
      <li data-md>
       <p>Added a note to the specializations for the civil calendar classes, highlighting the fact that it’s possible to build objects of those classes with unspecified values. If so, hashing them also yields unspecified values.</p>
      <li data-md>
       <p>Added notes about support for heterogeneous hashing (or lack thereof).</p>
     </ul>
    <li data-md>
     <p>R0</p>
     <ul>
      <li data-md>
       <p>First submission</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation and Scope</span><a class="self-link" href="#motivation"></a></h2>
   <p>
    The time library defines several value classes and class templates in
the 
    <tt>std::chrono</tt>
     namespace. All of the following are
copyable and comparable for equality (and some are also default
constructible, therefore satisfying <code class="highlight"><c- n>regular</c-></code>):
   </p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>duration</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>time_point</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>day</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>month</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>year</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>weekday</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>weekday_indexed</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>weekday_last</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>month_day</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>month_day_last</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>month_weekday</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>month_weekday_last</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>year_month</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>year_month_day</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>year_month_day_last</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>year_month_weekday</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>year_month_weekday_last</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>zoned_time</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>leap_second</c-></code></p>
   </ul>
   <p>Unfortunately, <strong>none</strong> of these classes comes with an enabled <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>hash</c-></code> specialization. This means that for instance it’s not
possible to store them in the Standard Library unordered associative
containers:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unordered_set</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>milliseconds</c-><c- o>></c-> <c- n>unique_measurements</c-><c- p>;</c-> <c- c1>// ERROR</c->
</pre>
   <p>Of course users can work around this limitation by providing a custom
hash, for instance something like this:</p>
<pre class="highlight"><c- k>struct</c-> <c- nc>duration_hash</c->
<c- p>{</c->
    <c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>Rep</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Period</c-><c- o>></c->
    <c- k>constexpr</c-> <c- k>auto</c-> <c- k>operator</c-><c- p>()(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>duration</c-><c- o>&lt;</c-><c- n>Rep</c-><c- p>,</c-> <c- n>Period</c-><c- o>></c-> <c- o>&amp;</c-><c- n>d</c-><c- p>)</c-> <c- k>const</c->
        <c- k>noexcept</c-><c- p>(</c-><c- d>/* ... */</c-><c- p>)</c->
    <c- p>{</c->
        <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>Rep</c-><c- o>></c-> <c- n>h</c-><c- p>;</c->
        <c- k>return</c-> <c- n>h</c-><c- p>(</c-><c- n>d</c-><c- p>.</c-><c- n>count</c-><c- p>());</c->
    <c- p>}</c->
<c- p>};</c->

<c- n>std</c-><c- o>::</c-><c- n>unordered_set</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>milliseconds</c-><c- p>,</c-> <c- n>duration_hash</c-><c- o>></c-> <c- n>unique_measurements</c-><c- p>;</c-> <c- c1>// OK</c->
</pre>
   <p>This limitation seems however to be unnecessarily vexing. Most of the
types listed above can actually have a straightforward implementation
for their hashing function, and there’s little justification for the
Standard Library itself not to provide it.</p>
   <p>(On the contrary: we argue that it’s the Standard Library responsability
to provide hashing for all of its movable types that can be compared
for equality.)</p>
   <p>The only case where a slightly more challenging implementation may be
required is <code class="highlight"><c- n>zoned_time</c-></code>, because it may need to mix the hash for
the time zone pointer with the hash for the time point. This is not
however different from "prior art"; for instance, the
implementation of <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>error_code</c-><c- o>></c-></code> has to solve the same problem.</p>
   <h2 class="heading settled" data-level="3" id="impact"><span class="secno">3. </span><span class="content">Impact On The Standard</span><a class="self-link" href="#impact"></a></h2>
   <p>This proposal is a pure library addition.</p>
   <p>This proposal does not depend on any other library extensions.</p>
   <p>This proposal does not require any changes in the core language.</p>
   <h2 class="heading settled" data-level="4" id="designdecisions"><span class="secno">4. </span><span class="content">Design Decisions</span><a class="self-link" href="#designdecisions"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="what-about-time_zone"><span class="secno">4.1. </span><span class="content">What about <code class="highlight"><c- n>time_zone</c-></code>?</span><a class="self-link" href="#what-about-time_zone"></a></h3>
   <p>While <code class="highlight"><c- n>time_zone</c-></code> is comparable for equality, it is not a regular type
because it is not copyable.
Users that need to store <code class="highlight"><c- n>time_zone</c-></code> objects typically use <em>pointers</em> to them. As such, we do not see the need to add a <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>time_zone</c-><c- o>></c-></code> specialization, and we are not proposing it here.</p>
   <p>The same reasoning applies to <code class="highlight"><c- n>time_zone_link</c-></code>.</p>
   <h3 class="heading settled" data-level="4.2" id="support-for-heterogeneous-hashing"><span class="secno">4.2. </span><span class="content">Support for heterogeneous hashing</span><a class="self-link" href="#support-for-heterogeneous-hashing"></a></h3>
   <p>The <code class="highlight"><c- n>hash</c-></code> specializations that we are proposing do not directly
support heterogeneous hashing, as they lack the <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c->​<c- n>is_</c->­<c- n>transparent</c-></code> type ([unord.req.general/10.7]) which is
needed to enable the heterogeneous overloads of <code class="highlight"><c- n>find</c-></code>, <code class="highlight"><c- n>erase</c-></code>, and so
on. This is consistent with all the other <code class="highlight"><c- n>hash</c-></code> specializations
available in the Standard Library, which are not enabled for
heterogeneous hashing.</p>
   <p>This is an important safety net, as hashing objects of different
datatypes may yield results that would violate the hashing requirements.
For instance, given</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>milliseconds</c-> <c- n>a</c-> <c- o>=</c-> <c- mi>1000</c-><c- n>ms</c-><c- p>;</c->
<c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>seconds</c->      <c- n>b</c-> <c- o>=</c-> <c- mi>1</c-><c- n>s</c-><c- p>;</c->
</pre>
   <p>then <code class="highlight"><c- n>a</c-></code> and <code class="highlight"><c- n>b</c-></code> do compare equal, but it is not guaranteed that
hashing the two objects would produce the same value (on any
"reasonable" implementation they would actually produce different
values).</p>
   <h4 class="heading settled" data-level="4.2.1" id="implicit-conversions"><span class="secno">4.2.1. </span><span class="content">Implicit conversions</span><a class="self-link" href="#implicit-conversions"></a></h4>
   <p>The lack of direct support for heterogeneous hashing does not
mean that one cannot simply build on top of the existing implicit
conversions for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-></code> classes. Consider this example:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unordered_set</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>milliseconds</c-><c- o>></c-> <c- n>unique_measurements</c-><c- p>;</c->
<c- n>unique_measurements</c-><c- p>.</c-><c- n>insert</c-><c- p>(</c-><c- mi>1000</c-><c- n>ms</c-><c- p>);</c->
<c- n>assert</c-><c- p>(</c-><c- n>unique_measurements</c-><c- p>.</c-><c- n>contains</c-><c- p>(</c-><c- mi>1</c-><c- n>s</c-><c- p>));</c-> <c- c1>// OK</c->
</pre>
   <p>With the proposed wording, the example compiles and works as expected;
the <code class="highlight"><c- n>assert</c-></code> does not fire. What is happening here is that the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>second</c-></code> object passed to <code class="highlight"><c- n>contains</c-></code> is being implictly
converted to a <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>milliseconds</c-></code> object, which is then used
by <code class="highlight"><c- n>contains</c-></code>.</p>
   <p>Having implicit conversions between different specializations is is a
major design choice of <code class="highlight"><c- n>chrono</c-></code> class templates, specifically for <code class="highlight"><c- n>duration</c-></code>, <code class="highlight"><c- n>time_point</c-></code> and <code class="highlight"><c- n>zoned_time</c-></code> (the last two are indeed
built on top of <code class="highlight"><c- n>duration</c-></code>); the converting constructors are constrained
to work only under precise conditions. The same constraints would apply
in code like the one exemplified above.</p>
   <h3 class="heading settled" data-level="4.3" id="handling-the-unspecified-state-of-the-calendaring-classes"><span class="secno">4.3. </span><span class="content">Handling the unspecified state of the calendaring classes</span><a class="self-link" href="#handling-the-unspecified-state-of-the-calendaring-classes"></a></h3>
   <p>The calendaring classes introduced in C++20 can have an unspecified
state if built with out-of-range values. For instance:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-> <c- nf>d</c-><c- p>(</c-><c- mi>0u</c-><c- p>);</c-> <c- c1>// unspecified; not undefined behavior</c->
</pre>
   <p>What should happen if one tries to hash <code class="highlight"><c- n>d</c-></code>? We claim that the
operation should simply give an unspecified result, and <em>not</em> undefined
behavior.</p>
   <p>The <code class="highlight"><c- n>chrono</c-></code> facilities deliberately avoid triggering undefined
behavior in many places, preferring to return unspecified values
instead. Let’s extend the example:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-> <c- nf>d</c-><c- p>(</c-><c- mi>0u</c-><c- p>);</c->                       <c- c1>// unspecified</c->
<c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_day</c-> <c- n>md</c-> <c- o>=</c-> <c- n>April</c-> <c- o>/</c-> <c- n>d</c-><c- p>;</c->        <c- c1>// unspecified because of 'd'</c->
<c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_day</c-> <c- n>ymd</c-> <c- o>=</c-> <c- mi>2022</c-> <c- o>/</c-> <c- n>md</c-><c- p>;</c->  <c- c1>// unspecified because of 'md'</c->

<c- c1>// conversion, [time.cal.ymd.members]</c->
<c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>sys_days</c-> <c- n>sd</c-> <c- o>=</c-> <c- n>ymd</c-><c- p>;</c->               <c- c1>// unspecified</c->
</pre>
   <p>We believe that the proposed <code class="highlight"><c- n>hash</c-></code> specializations should follow this
precedent and also return unspecified values.</p>
   <p>However it’s important to remark that this does not mean that an
implementation is allowed to return "anything": hashing objects that
compare equal must still yield the same value. In other words, the
hashing <em>requirements</em> must still be fulfilled, even in the presence
of unspecified inputs.</p>
   <p>Therefore:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-> <c- nf>d</c-><c- p>(</c-><c- mi>0u</c-><c- p>);</c-> <c- c1>// Unspecified...</c->

<c- n>assert</c-><c- p>(</c-><c- n>d</c-> <c- o>==</c-> <c- n>d</c-><c- p>);</c->         <c- c1>// but whatever the value contained inside 'd' is, this holds;</c->

<c- k>auto</c-> <c- n>h1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-><c- o>></c-><c- p>()(</c-><c- n>d</c-><c- p>);</c->
<c- k>auto</c-> <c- n>h2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-><c- o>></c-><c- p>()(</c-><c- n>d</c-><c- p>);</c->

<c- n>assert</c-><c- p>(</c-><c- n>h1</c-> <c- o>==</c-> <c- n>h2</c-><c- p>);</c->       <c- c1>// and therefore this must hold as well.</c->
</pre>
   <p>or generalizing:</p>
<pre class="highlight"><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-> <c- nf>d1</c-><c- p>(</c-><c- mi>0u</c-><c- p>);</c->
<c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-> <c- nf>d2</c-><c- p>(</c-><c- mi>1234u</c-><c- p>);</c->

<c- k>auto</c-> <c- n>h1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-><c- o>></c-><c- p>()(</c-><c- n>d1</c-><c- p>);</c->
<c- k>auto</c-> <c- n>h2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-><c- o>></c-><c- p>()(</c-><c- n>d2</c-><c- p>);</c->

<c- c1>// d1, d2, h1, h2 are all unspecified.</c->
<c- c1>// 'd1' is not guaranteed to be equal to (or different from) 'd2';</c->
<c- c1>// but if it is equal, then the hashes are equal:</c->
<c- k>if</c-> <c- p>(</c-><c- n>d1</c-> <c- o>==</c-> <c- n>d2</c-><c- p>)</c->
  <c- n>assert</c-><c- p>(</c-><c- n>h1</c-> <c- o>==</c-> <c- n>h2</c-><c- p>);</c->
</pre>
   <h2 class="heading settled" data-level="5" id="technicalspecifications"><span class="secno">5. </span><span class="content">Technical Specifications</span><a class="self-link" href="#technicalspecifications"></a></h2>
   <p>All the proposed changes are relative to <a data-link-type="biblio" href="#biblio-n4910">[N4910]</a>.</p>
   <h3 class="heading settled" data-level="5.1" id="feature-testing-macro"><span class="secno">5.1. </span><span class="content">Feature testing macro</span><a class="self-link" href="#feature-testing-macro"></a></h3>
   <p>In [version.syn], modify</p>
<pre class="highlight"><c- cp>#define __cpp_lib_chrono </c-><del><c- cp>201907L</c-></del><ins><c- cp>YYYYMML</c-></ins><c- cp> </c-><c- c1>// also in &lt;chrono></c->
</pre>
   <p>by replacing the existing value with the year and month of adoption of the
present proposal.</p>
   <h3 class="heading settled" data-level="5.2" id="proposed-wording"><span class="secno">5.2. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposed-wording"></a></h3>
   <p>Add the following at the end of [time.syn]:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- nn>std</c-> <c- p>{</c->
  <c- c1>// ???, hash support</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- nc>hash</c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Rep</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Period</c-><c- o>></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>duration</c-><c- o>&lt;</c-><c- n>Rep</c-><c- p>,</c-> <c- n>Period</c-><c- o>>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Clock</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Duration</c-><c- o>></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>time_point</c-><c- o>&lt;</c-><c- n>Clock</c-><c- p>,</c-> <c- n>Duration</c-><c- o>>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>weekday</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>weekday_indexed</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>weekday_last</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_day</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_day_last</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_weekday</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_weekday_last</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_day</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_day_last</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_weekday</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_weekday_last</c-><c- o>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Duration</c-><c- p>,</c-> <c- k>class</c-> <c- nc>TimeZonePtr</c-><c- o>></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>zoned_time</c-><c- o>&lt;</c-><c- n>Duration</c-><c- p>,</c-> <c- n>TimeZonePtr</c-><c- o>>></c-><c- p>;</c->
  <c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>leap_second</c-><c- o>></c-><c- p>;</c->
<c- p>}</c->
</pre>
   </blockquote>
   <p>Add a new subclause after [time.parse], with the following content:</p>
   <blockquote>
     <strong>??? Hash support [time.hash]</strong> 
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Rep</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Period</c-><c- o>></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>duration</c-><c- o>&lt;</c-><c- n>Rep</c-><c- p>,</c-> <c- n>Period</c-><c- o>>></c-><c- p>;</c->
</pre>
    <p>(1) Letting D be <code class="highlight"><c- n>chrono</c-><c- o>::</c-><c- n>duration</c-><c- o>&lt;</c-><c- n>Rep</c-><c- p>,</c-> <c- n>Period</c-><c- o>></c-></code>, the specialization <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-></code> is enabled ([unord.hash]) if and only if <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Rep</c-><c- o>></c-></code> is
enabled. When enabled, for an object <code class="highlight"><c- n>d</c-></code> of type <code class="highlight"><c- n>D</c-></code>, <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>D</c-><c- o>></c-><c- p>()(</c-><c- n>d</c-><c- p>)</c-></code> evaluates to the same value as <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Rep</c-><c- o>></c-><c- p>()(</c-><c- n>d</c-><c- p>.</c-><c- n>count</c-><c- p>())</c-></code>. The member
functions are not guaranteed to be <code class="highlight"><c- k>noexcept</c-></code>.</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Clock</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Duration</c-><c- o>></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>time_point</c-><c- o>&lt;</c-><c- n>Clock</c-><c- p>,</c-> <c- n>Duration</c-><c- o>>></c-><c- p>;</c->
</pre>
    <p>(2) Letting TP be <code class="highlight"><c- n>chrono</c-><c- o>::</c-><c- n>time_point</c-><c- o>&lt;</c-><c- n>Clock</c-><c- p>,</c-> <c- n>Duration</c-><c- o>></c-></code>, the specialization <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>TP</c-><c- o>></c-></code> is enabled ([unord.hash]) if and only if <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Duration</c-><c- o>></c-></code> is
enabled. When enabled, for an object <code class="highlight"><c- n>tp</c-></code> of type <code class="highlight"><c- n>TP</c-></code>, <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>TP</c-><c- o>></c-><c- p>()(</c-><c- n>tp</c-><c- p>)</c-></code> evaluates to the same value as <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Duration</c-><c- o>></c-><c- p>()(</c-><c- n>tp</c-><c- p>.</c-><c- n>time_since_epoch</c-><c- p>())</c-></code>.
The member functions are not guaranteed to be <code class="highlight"><c- k>noexcept</c-></code>.</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>day</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>weekday</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>weekday_indexed</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>weekday_last</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_day</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_day_last</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_weekday</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>month_weekday_last</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_day</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_day_last</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_weekday</c-><c- o>></c-><c- p>;</c->
<c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>year_month_weekday_last</c-><c- o>></c-><c- p>;</c->
</pre>
    <p>(3) The specialization is enabled ([unord.hash]).</p>
    <p>(4) Given an object <code class="highlight"><c- n>h</c-></code> of type <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-></code> (where <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-></code> is one of the specializations listed above) and an object <code class="highlight"><c- n>k</c-></code> of type
(possibly cv-qualified) <code class="highlight"><c- n>Key</c-></code>, if <code class="highlight"><c- n>k</c-><c- p>.</c-><c- n>ok</c-><c- p>()</c-> <c- o>==</c-> false</code>, then the
evaluation of <code class="highlight"><c- n>h</c-><c- p>(</c-><c- n>k</c-><c- p>)</c-></code> returns an unspecified value.</p>
    <p>[<em>Note</em> 1: These <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-></code> specializations still meet the <em>Cpp17Hash</em> requirements,
even for objects <code class="highlight"><c- n>k</c-></code> such that <code class="highlight"><c- n>k</c-><c- p>.</c-><c- n>ok</c-><c- p>()</c-> <c- o>==</c-> false</code>. — <em>end note</em>]</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Duration</c-><c- p>,</c-> <c- k>class</c-> <c- nc>TimeZonePtr</c-><c- o>></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>zoned_time</c-><c- o>&lt;</c-><c- n>Duration</c-><c- p>,</c-> <c- n>TimeZonePtr</c-><c- o>>></c-><c- p>;</c->
</pre>
    <p>(5) Letting ZT be <code class="highlight"><c- n>chrono</c-><c- o>::</c-><c- n>zoned_time</c-><c- o>&lt;</c-><c- n>Duration</c-><c- p>,</c-> <c- n>TimeZonePtr</c-><c- o>></c-></code>, the specialization <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>ZT</c-><c- o>></c-></code> is enabled ([unord.hash]) if and only if <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>Duration</c-><c- o>></c-></code> is
enabled and <code class="highlight"><c- n>hash</c-><c- o>&lt;</c-><c- n>TimeZonePtr</c-><c- o>></c-></code> is enabled.
The member functions are not guaranteed to be <code class="highlight"><c- k>noexcept</c-></code>.</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;></c-> <c- k>struct</c-> <c- nc>hash</c-><c- o>&lt;</c-><c- n>chrono</c-><c- o>::</c-><c- n>leap_second</c-><c- o>></c-><c- p>;</c->
</pre>
    <p>(6) The specialization is enabled ([unord.hash]).</p>
   </blockquote>
   <h2 class="heading settled" data-level="6" id="acknowledgements"><span class="secno">6. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to KDAB for supporting this work.</p>
   <p>Thanks to Howard Hinnant for designing <code class="highlight"><c- o>&lt;</c-><c- n>chrono</c-><c- o>></c-></code> and for discussing
why hash support had not been added in the past.</p>
   <p>All remaining errors are ours and ours only.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-n4910">[N4910]
   <dd>Thomas Köppe. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf"><cite>Working Draft, Standard for Programming Language C++</cite></a>. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf</a>
  </dl>