<!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>P3105R1: constexpr std::uncaught_exceptions()</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;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#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>
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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">P3105R1<br><code class="highlight"><c- k>constexpr</c-> <c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-><c- p>()</c-></code></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="2024-03-06">2024-03-06</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://eisenwave.github.io/cpp-proposals/constexpr-uncaught-exceptions.html">https://eisenwave.github.io/cpp-proposals/constexpr-uncaught-exceptions.html</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:janschultke@gmail.com">Jan Schultke</a>
     <dt>Audience:
     <dd>SG18, LEWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
     <dt>Source:
     <dd><a href="https://github.com/Eisenwave/cpp-proposals/blob/master/src/enum-direct-init.bs">eisenwave/cpp-proposals</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper marks <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>current_exception</c-></code> <code class="highlight"><c- k>constexpr</c-></code>.</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="#revision-history"><span class="secno">1</span> <span class="content">Revision history</span></a>
     <ol class="toc">
      <li><a href="#changes-since-r0"><span class="secno">1.1</span> <span class="content">Changes since R0</span></a>
     </ol>
    <li><a href="#overview"><span class="secno">2</span> <span class="content">Overview</span></a>
    <li><a href="#motivation"><span class="secno">3</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#possible-implementation"><span class="secno">4</span> <span class="content">Possible implementation</span></a>
     <ol class="toc">
      <li><a href="#constexpr-uncaught-exceptions"><span class="secno">4.1</span> <span class="content"><code class="highlight"><c- k>constexpr</c-> <c- n>uncaught_exceptions</c-></code></span></a>
      <li><a href="#constexpr-current-exception"><span class="secno">4.2</span> <span class="content"><code class="highlight"><c- k>constexpr</c-> <c- n>current_exception</c-></code></span></a>
      <li><a href="#constexpr-exception-ptr"><span class="secno">4.3</span> <span class="content"><code class="highlight"><c- k>constexpr</c-></code> <code class="highlight"><c- n>exception_ptr</c-></code></span></a>
      <li><a href="#non-trivial-implementations"><span class="secno">4.4</span> <span class="content">Non-trivial implementations</span></a>
      <li><a href="#impact-on-abi"><span class="secno">4.5</span> <span class="content">Impact on ABI</span></a>
     </ol>
    <li><a href="#proposed-wording"><span class="secno">5</span> <span class="content">Proposed wording</span></a>
    <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="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="revision-history"><span class="secno">1. </span><span class="content">Revision history</span><a class="self-link" href="#revision-history"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="changes-since-r0"><span class="secno">1.1. </span><span class="content">Changes since R0</span><a class="self-link" href="#changes-since-r0"></a></h3>
   <ul>
    <li data-md>
     <p>Mention <a data-link-type="biblio" href="#biblio-p3068r0" title="Allowing exception throwing in constant-evaluation.">[P3068R0]</a>.</p>
    <li data-md>
     <p>Bring up future-proofing in <a href="#motivation">§ 3 Motivation</a>.</p>
    <li data-md>
     <p>Add missing synopsis edits to <a href="#proposed-wording">§ 5 Proposed wording</a>.</p>
    <li data-md>
     <p>Minor editorial fixes/improvements.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="overview"><span class="secno">2. </span><span class="content">Overview</span><a class="self-link" href="#overview"></a></h2>
   <p>I propose to mark the function <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code> <code class="highlight"><c- k>constexpr</c-></code>.</p>
   <p>Currently, this can be done because there can never be an active exception during constant evaluation. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code> would always return zero during constant evaluation.
Similarly, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>current_exception</c-></code> would always return a null pointer.</p>
   <p><a data-link-type="biblio" href="#biblio-p2996r1" title="Reflection for C++26">[P2996R1]</a> recommends exceptions as an error handling mechanism for reflections,
during constant evaluation. <a data-link-type="biblio" href="#biblio-p3068r0" title="Allowing exception throwing in constant-evaluation.">[P3068R0]</a> proposes allowing exception throwing in constant expressions.
If exceptions were throwable in constant expressions, marking these functions <code class="highlight"><c- k>constexpr</c-></code> would simply be part of exception support. <b>However, these proposals is not a prerequisite to this proposal.</b></p>
   <h2 class="heading settled" data-level="3" id="motivation"><span class="secno">3. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>The motivation is the same as allowing <code class="highlight"><c- k>try</c-></code>-<code class="highlight"><c- k>catch</c-></code> blocks in <code class="highlight"><c- k>constexpr</c-></code> functions,
a feature added to C++20 thanks to <a data-link-type="biblio" href="#biblio-p1002r1" title="Try-catch blocks in constexpr functions">[P1002R1]</a>.
Allowing the use of <code class="highlight"><c- n>uncaught_exceptions</c-><c- p>()</c-></code> in constant expressions makes
metaprogramming easier and eliminates special <code class="highlight"><c- k>if</c-> <c- k>consteval</c-></code> cases.</p>
   <p>A common use case for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code> is in the implementation of RAII types which
execute a function upon destruction, but only if an exception was (not) thrown in the current
scope.
This is utilized by <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>scope_success</c-></code> and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>scope_failure</c-></code>; see <a data-link-type="biblio" href="#biblio-n4806" title="Working Draft, C++ Extensions for Library Fundamentals, Version 3">[N4806]</a>.</p>
   <div class="example" id="example-58e7c9e9">
    <a class="self-link" href="#example-58e7c9e9"></a> In the implementation of a <code class="highlight"><c- k>constexpr</c-></code> stack, one may write: 
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>value_type</c-> <c- nf>pop</c-><c- p>()</c-> <c- p>{</c->
    <c- c1>// Only decrease the size of the stack if no exception was thrown during</c->
    <c- c1>// copy/move construction of the returned object.</c->
    <c- c1>// This ensures a strong exception guarantee.</c->
    <c- n>std</c-><c- o>::</c-><c- n>scope_success</c-> <c- n>_</c-><c- p>{[</c-><c- k>this</c-><c- p>]</c-> <c- p>{</c-> <c- n>m_size</c-><c- o>--</c-><c- p>;</c-> <c- p>}};</c->
    <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>top</c-><c- p>());</c->
<c- p>}</c->
</pre>
    <p>It is reasonable to mark such code <code class="highlight"><c- k>constexpr</c-></code>, and ideally <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code> would not be an
obstacle to this.</p>
   </div>
   <p>Besides the quality-of-life aspect, we want to future-proof code.
If the user circumvents <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code> by guarding its use with an <code class="highlight"><c- k>if</c-> <c- o>!</c-><c- k>consteval</c-></code> block, this makes the assumption that exceptions aren’t throwable in
constant expression.
That may be true now, but could change in the future, in which case the user will have to rewrite
their code to avoid this assumption.</p>
   <p>Furthermore, it makes sense to mark <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code>' sister function, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>current_exception</c-></code> <code class="highlight"><c- k>constexpr</c-></code>.
This is done purely for the purpose of consistency.
I am not aware of any concrete example of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>current_exception</c-></code>’s
lack of <code class="highlight"><c- k>constexpr</c-></code> being an obstacle.</p>
   <h2 class="heading settled" data-level="4" id="possible-implementation"><span class="secno">4. </span><span class="content">Possible implementation</span><a class="self-link" href="#possible-implementation"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="constexpr-uncaught-exceptions"><span class="secno">4.1. </span><span class="content"><code class="highlight"><c- k>constexpr</c-> <c- n>uncaught_exceptions</c-></code></span><a class="self-link" href="#constexpr-uncaught-exceptions"></a></h3>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- b>int</c-> <c- nf>uncaught_exceptions</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c->
    <c- k>if</c-> <c- k>consteval</c-> <c- p>{</c->
        <c- k>return</c-> <c- mi>0</c-><c- p>;</c->
    <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>__uncaught_exceptions_impl</c-><c- p>();</c->
    <c- p>}</c->
<c- p>}</c->
</pre>
   <p>It is obviously possible for the user to wrap <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>uncaught_exceptions</c-></code> like this themselves (e.g. <a data-link-type="biblio" href="#biblio-scopelite" title="uncaught_exceptions() wrapper in scope-lite">[ScopeLite]</a>), but this is an unnecessary burden.</p>
   <h3 class="heading settled" data-level="4.2" id="constexpr-current-exception"><span class="secno">4.2. </span><span class="content"><code class="highlight"><c- k>constexpr</c-> <c- n>current_exception</c-></code></span><a class="self-link" href="#constexpr-current-exception"></a></h3>
<pre class="language-cpp highlight"><c- k>constexpr</c-> <c- n>exception_ptr</c-> <c- nf>current_exception</c-><c- p>()</c-> <c- k>noexcept</c-> <c- p>{</c->
    <c- k>if</c-> <c- k>consteval</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>exception_ptr</c-><c- p>(</c-><c- k>nullptr</c-><c- p>);</c->
    <c- p>}</c-> <c- k>else</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>__current_exception_impl</c-><c- p>();</c->
    <c- p>}</c->
<c- p>}</c->
</pre>
   <h3 class="heading settled" data-level="4.3" id="constexpr-exception-ptr"><span class="secno">4.3. </span><span class="content"><code class="highlight"><c- k>constexpr</c-></code> <code class="highlight"><c- n>exception_ptr</c-></code></span><a class="self-link" href="#constexpr-exception-ptr"></a></h3>
   <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>exception_ptr</c-></code> would also need to be made a literal type.
All major three standard libraries implement <code class="highlight"><c- n>exception_ptr</c-></code> as a wrapper class for <code class="highlight"><c- b>void</c-><c- o>*</c-></code>,
which makes this easily possible.</p>
   <ul>
    <li data-md>
     <p>See <a href="https://github.com/llvm/llvm-project/blob/main/libcxx/include/__exception/exception_ptr.h">libc++, exception_ptr.h</a>.</p>
    <li data-md>
     <p>See <a href="https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/libsupc%2B%2B/exception_ptr.h">libstdc++, exception_ptr.h</a>.</p>
    <li data-md>
     <p>See <a href="https://github.com/microsoft/STL/blob/main/stl/inc/exception">MSVC STL, exception</a>.</p>
   </ul>
   <p>Simply mark all special member functions <code class="highlight"><c- k>constexpr</c-></code> and if necessary, guard their implementation
with an <code class="highlight"><c- k>if</c-> <c- o>!</c-><c- k>consteval</c-></code> block.
It is impossible to create an <code class="highlight"><c- n>exception_ptr</c-></code> that is not a null pointer during constant
evaluations.</p>
   <h3 class="heading settled" data-level="4.4" id="non-trivial-implementations"><span class="secno">4.4. </span><span class="content">Non-trivial implementations</span><a class="self-link" href="#non-trivial-implementations"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p2996r1" title="Reflection for C++26">[P2996R1]</a> suggests allowing <code class="highlight"><c- k>throw</c-></code> in constant expressions.
This would mean that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>active_exceptions</c-></code>, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>current_exception</c-></code>,
and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>exception_ptr</c-></code> would no longer have such trivial implementations,
and further functions such as <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>make_exception_ptr</c-></code> may be marked <code class="highlight"><c- k>constexpr</c-></code>.</p>
   <p>The bare minimum compiler support needed for this is:</p>
   <ol>
    <li data-md>
     <p>The compiler must track all active exceptions "magically", so that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>active_exceptions</c-><c- p>()</c-></code> returns the correct amount, and <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>current_exception</c-><c- p>()</c-></code> returns
the current exception.
This needs compiler support because such mutable global state normally doesn’t exist in
constant expressions.</p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>exception_ptr</c-></code> behaves like a type-erased, reference-counted smart pointer. <a data-link-type="biblio" href="#biblio-p2738r1" title="constexpr cast from void*: towards constexpr type-erasure">[P2738R1]</a> has been accepted into C++26, adding <code class="highlight"><c- k>constexpr</c-></code> cast from <code class="highlight"><c- b>void</c-><c- o>*</c-></code>.
This makes the implementation of such type-erasure in <code class="highlight"><c- k>constexpr</c-> <c- n>std</c-><c- o>::</c-><c- n>exception_ptr</c-></code> feasible.</p>
   </ol>
   <h3 class="heading settled" data-level="4.5" id="impact-on-abi"><span class="secno">4.5. </span><span class="content">Impact on ABI</span><a class="self-link" href="#impact-on-abi"></a></h3>
   <p>Multiple functions, including member functions of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>exception_ptr</c-></code> would become
inline functions if marked <code class="highlight"><c- k>constexpr</c-></code>.
To remain ABI-compatible with existing software, it is necessary to emit these inline function
into the runtime library.</p>
   <p>libstdc++ already conditionally does this by marking member functions of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>exception_ptr</c-></code> <code class="highlight"><c- n>__attribute__</c-><c- p>((</c-><c- n>__used__</c-><c- p>))</c-></code>.
Therefore:</p>
   <ul>
    <li data-md>
     <p>libstdc++ is not affected.</p>
    <li data-md>
     <p>MSVC STL already defines all member function of <code class="highlight"><c- n>exception_ptr</c-></code> as inline functions,
and is also not affected.</p>
    <li data-md>
     <p>libc++ would need to apply similar compatibility measures as libstdc++.</p>
   </ul>
   <h2 class="heading settled" data-level="5" id="proposed-wording"><span class="secno">5. </span><span class="content">Proposed wording</span><a class="self-link" href="#proposed-wording"></a></h2>
   <p>The proposed changes are relative to the working draft of the standard as of <a data-link-type="biblio" href="#biblio-n4917" title="Working Draft, Standard for Programming Language C++">[N4917]</a>.</p>
   <p>Update subclause 17.3.2 [version.syn], paragraph 2 as follows:</p>
   <blockquote>
<pre class="highlight"><code><c- cp>#define __cpp_lib_constexpr_current_exception   </c-><ins><c- cp>202401L</c-></ins><c- cp> </c-><c- c1>// freestanding, also in &lt;exception></c->
<c- p>[...]</c->
<c- cp>#define __cpp_lib_uncaught_exceptions           </c-><del><c- cp>201411L</c-></del><ins><c- cp>202401L</c-></ins><c- cp> </c-><c- c1>// freestanding, also in &lt;exception></c->
</code></pre>
   </blockquote>
   <p>Update subclause 17.9.2 [exception.syn] as follows:</p>
   <blockquote>
<pre class="highlight"><ins><c- k>constexpr</c-></ins> <c- b>int</c-> <c- nf>uncaught_exceptions</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->    <br>
<c- k>using</c-> <c- n>exception_ptr</c-> <c- o>=</c-> <i><c- n>unspecified</c-></i><c- p>;</c->

<ins><c- k>constexpr</c-></ins> <c- n>exception_ptr</c-> <c- nf>current_exception</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c-></pre>
   </blockquote>
   <p>Update subclause 17.9.6 [uncaught.exceptions] as follows:</p>
   <blockquote>
<pre class="highlight"><code><ins><c- k>constexpr</c-></ins> <c- b>int</c-> <c- nf>uncaught_exceptions</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c-></code></pre>
   </blockquote>
   <p>Update subclause 17.9.7 [propagation], paragraph 2 as follows:</p>
   <blockquote>
     <code class="highlight"><c- n>exception_ptr</c-></code> 
    <ins>is a literal type([basic.types.general])</ins>
     which meets the requirements of <em>Cpp17NullablePointer</em> (Table 36). 
    <ins>All expressions which must be valid for a <em>Cpp17NullablePointer</em> are constant expressions for a null value of type <code class="highlight"><c- n>exception_ptr</c-></code>.</ins>
   </blockquote>
   <p class="note" role="note"><span class="marker">Note:</span> This wording is slightly work-in-progress.</p>
   <p>Update subclause 17.9.7 [propagation], <code class="highlight"><c- n>current_exception</c-></code> as follows:</p>
   <blockquote>
<pre class="highlight"><code><ins><c- k>constexpr</c-></ins> <c- n>exception_ptr</c-> <c- nf>current_exception</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c-></code></pre>
   </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>The original idea for this paper and a portion of its content have been adopted from a
proposal draft by Morwenn.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-n4917">[N4917]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4917"><cite>Working Draft, Standard for Programming Language C++</cite></a>. 5 September 2022. URL: <a href="https://wg21.link/n4917">https://wg21.link/n4917</a>
  </dl>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-n4806">[N4806]
   <dd>Thomas Köppe. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4806.html"><cite>Working Draft, C++ Extensions for Library Fundamentals, Version 3</cite></a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4806.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4806.html</a>
   <dt id="biblio-p1002r1">[P1002R1]
   <dd>Louis Dionne. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1002r1.pdf"><cite>Try-catch blocks in constexpr functions</cite></a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1002r1.pdf">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1002r1.pdf</a>
   <dt id="biblio-p2738r1">[P2738R1]
   <dd>Corentin Jabot; David Ledger. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2738r1.pdf"><cite>constexpr cast from void*: towards constexpr type-erasure</cite></a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2738r1.pdf">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2738r1.pdf</a>
   <dt id="biblio-p2996r1">[P2996R1]
   <dd>Wyatt Childers; et al. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2996r1.html"><cite>Reflection for C++26</cite></a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2996r1.html">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2996r1.html</a>
   <dt id="biblio-p3068r0">[P3068R0]
   <dd>Hana Dusíková. <a href="https://wg21.link/p3068r0"><cite>Allowing exception throwing in constant-evaluation.</cite></a>. URL: <a href="https://wg21.link/p3068r0">https://wg21.link/p3068r0</a>
   <dt id="biblio-scopelite">[ScopeLite]
   <dd>Martin Moene. <a href="https://github.com/martinmoene/scope-lite/blob/89b274a106363101ea258cb9555a9c6a47ae2928/include/nonstd/scope.hpp#L586-L597"><cite>uncaught_exceptions() wrapper in scope-lite</cite></a>. URL: <a href="https://github.com/martinmoene/scope-lite/blob/89b274a106363101ea258cb9555a9c6a47ae2928/include/nonstd/scope.hpp#L586-L597">https://github.com/martinmoene/scope-lite/blob/89b274a106363101ea258cb9555a9c6a47ae2928/include/nonstd/scope.hpp#L586-L597</a>
  </dl>