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  <title>P2490R0: Zero-overhead exception stacktraces</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
	}

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

	/* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

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

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

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

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version dfbc2b297, updated Thu Nov 11 15:52:32 2021 -0800" name="generator">
  <link href="https://wg21.link/P2490" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
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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">P2490R0<br>Zero-overhead exception stacktraces</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="2021-11-13">2021-11-13</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P2490">https://wg21.link/P2490</a>
     <dt>Issue Tracking:
     <dd><a href="https://github.com/ecatmur/stacktrace-from-exception/issues">GitHub</a>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ed@catmur.uk">Ed Catmur</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/ecatmur/stacktrace-from-exception/blob/main/paper.md">github.com/ecatmur/stacktrace-from-exception/blob/main/paper.md</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#overview"><span class="secno">1</span> <span class="content">Overview</span></a>
     <ol class="toc">
      <li><a href="#abstract"><span class="secno">1.1</span> <span class="content">Abstract</span></a>
      <li>
       <a href="#background"><span class="secno">1.2</span> <span class="content">Background</span></a>
       <ol class="toc">
        <li><a href="#runtime-cost"><span class="secno">1.2.1</span> <span class="content">Runtime cost</span></a>
        <li><a href="#old-third-party-libraries"><span class="secno">1.2.2</span> <span class="content">Old third-party libraries</span></a>
        <li><a href="#conflicting-requirements"><span class="secno">1.2.3</span> <span class="content">Conflicting requirements</span></a>
       </ol>
      <li><a href="#alternatives"><span class="secno">1.3</span> <span class="content">Alternatives</span></a>
      <li><a href="#acknowledgements"><span class="secno">1.4</span> <span class="content">Acknowledgements</span></a>
      <li><a href="#revision-history"><span class="secno">1.5</span> <span class="content">Revision history</span></a>
     </ol>
    <li>
     <a href="#possible-syntaxes"><span class="secno">2</span> <span class="content">Possible syntaxes</span></a>
     <ol class="toc">
      <li><a href="#special-function"><span class="secno">2.1</span> <span class="content">Special function</span></a>
      <li><a href="#default-parameter"><span class="secno">2.2</span> <span class="content">Default parameter</span></a>
      <li><a href="#catch-with-init"><span class="secno">2.3</span> <span class="content">"catch-with-init"</span></a>
      <li><a href="#expose-search-phase"><span class="secno">2.4</span> <span class="content">Expose search phase</span></a>
     </ol>
    <li><a href="#concerns"><span class="secno">3</span> <span class="content">Concerns</span></a>
    <li>
     <a href="#implementation-experience"><span class="secno">4</span> <span class="content">Implementation experience</span></a>
     <ol class="toc">
      <li><a href="#windows"><span class="secno">4.1</span> <span class="content">Windows</span></a>
      <li><a href="#itanium"><span class="secno">4.2</span> <span class="content">Itanium</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="overview"><span class="secno">1. </span><span class="content">Overview</span><a class="self-link" href="#overview"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="abstract"><span class="secno">1.1. </span><span class="content">Abstract</span><a class="self-link" href="#abstract"></a></h3>
   <p>This paper identifies concerns with part of the <strong>Stacktrace from exception</strong> <a data-link-type="biblio" href="#biblio-p2370">[p2370]</a> proposal.  We suggest alternate approaches and offer implementation experience of the
techniques that could underly such alternatives.</p>
   <h3 class="heading settled" data-level="1.2" id="background"><span class="secno">1.2. </span><span class="content">Background</span><a class="self-link" href="#background"></a></h3>
   <p>The paper <strong>Stacktrace from exception</strong> <a data-link-type="biblio" href="#biblio-p2370">[p2370]</a> amply sets out why it is desired to be able to access a stacktrace from exception; that is, when <em>handling</em> an exception it should be
possible to retrieve a stacktrace from the (most recent) <code class="highlight"><c- k>throw</c-></code> point of the exception, through the point of handling; and that this should be <em>transparent</em> to and not require <em>cooperation by</em> or <em>modification of</em> throwing code.  That paper acknowledges that the cost of taking a stacktrace on <em>every</em> exception throw would be prohibitive and proposes a
mechanism to disable it via a standard library routine <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>this_thread</c-><c- o>::</c-><c- n>set_capture_stacktraces_at_throw</c-></code> that will set a thread-local flag.</p>
   <p>We argue that this mechanism still imposes a runtime cost and would not achieve the aims of the paper for the proposed facility.</p>
   <h4 class="heading settled" data-level="1.2.1" id="runtime-cost"><span class="secno">1.2.1. </span><span class="content">Runtime cost</span><a class="self-link" href="#runtime-cost"></a></h4>
   <p>Accessing a thread-local variable has a cost in instructions and memory access; at present this could be argued to be lost in the "noise" of the existing exception handling
machinery, particularly as this currently involves memory allocation, but in future if and when the <strong>Zero-overhead deterministic exceptions: Throwing values</strong> <a data-link-type="biblio" href="#biblio-p0709">[p0709]</a> proposal is adopted this will become relatively more
significant.</p>
   <p>In any case even a <em>de minimis</em> runtime cost is not zero.</p>
   <h4 class="heading settled" data-level="1.2.2" id="old-third-party-libraries"><span class="secno">1.2.2. </span><span class="content">Old third-party libraries</span><a class="self-link" href="#old-third-party-libraries"></a></h4>
   <p>Under the proposed mechanism, throw sites would need recompilation and/or relinking to participate in the facility.  It is entirely possible that third-party library code is
shipped with its own implementations of the exception-raising mechanism, such that it would not participate in the facility until such time as the vendor recompiles and relinks
the library, which may not come for years or even decades.</p>
   <h4 class="heading settled" data-level="1.2.3" id="conflicting-requirements"><span class="secno">1.2.3. </span><span class="content">Conflicting requirements</span><a class="self-link" href="#conflicting-requirements"></a></h4>
   <p>Third-party library vendors who use exceptions for control flow may be expected to view the proposed facility negatively; if user code enables it via the proposed mechanism the
cost will be considerable even for exceptions that are caught and handled successfully entirely within the third-party library.  Thus they are likely to disable the facility at
API entry points, both negating the point of the facility for any exceptions that <em>do</em> leak out of the third-party library, and interfering with user code that expects it to
remain enabled.</p>
   <h3 class="heading settled" data-level="1.3" id="alternatives"><span class="secno">1.3. </span><span class="content">Alternatives</span><a class="self-link" href="#alternatives"></a></h3>
   <p>We note that C++ exception handling is typically built on top of a lower-level, language-agnostic facility.  On Windows this is structured exception handling <a data-link-type="biblio" href="#biblio-seh">[seh]</a>,
while on the Itanium ABI (used by most Unix-style OSes on x64-64) it is the Level I Base ABI <a data-link-type="biblio" href="#biblio-itanium">[itanium]</a>.  This lower-level facility uses <em>two-phase</em> exception handling; in 
the first, "search" phase the stack is walked from the throw point to identify a suitable handler, while in the second, "unwind" phase it is walked again from the 
throw point to the selected handler, this time invoking cleanup (i.e., destructors) along the way. Importantly,</p>
   <ul>
    <li data-md>
     <p>during the whole of the search phase the stack is still intact, and</p>
    <li data-md>
     <p>identifying a handler is <em>dynamic</em>, calling into a compiler- or library-generated match function.</p>
   </ul>
   <p>This suggests a possible alternative mechanism; viz.:</p>
   <ul>
    <li data-md>
     <p>user code can mark a <code class="highlight"><c- k>catch</c-></code> block as requiring a stacktrace, either via a special function or via new syntax; and</p>
    <li data-md>
     <p>on recognising <code class="highlight"><c- k>catch</c-></code> blocks so marked, the compiler can emit suitable code or data such that <em>if and when</em> that <code class="highlight"><c- k>catch</c-></code> block is selected as a handler for a thrown
  exception, it takes a stacktrace during search phase, immediately before nominating that <code class="highlight"><c- k>catch</c-></code> block as a suitable handler; then</p>
    <li data-md>
     <p>the user code can retrieve that stored stacktrace during exception handling, after stack unwinding.</p>
   </ul>
   <p>The advantages of this approach are twofold:</p>
   <ul>
    <li data-md>
     <p><em>transparency</em>: there is no need whatsoever for throwing code to be modified, recompiled or relinked.  Indeed, since this mechanism relies solely on changes to the catch
  site, code using this mechanism may be introduced into existing (perhaps even running) programs without any need for those programs to be recompiled or relinked, as long as that new code has access
  to any necessary support libraries;</p>
    <li data-md>
     <p><em>zero-cost</em>: if the search phase does not reach a <code class="highlight"><c- k>catch</c-></code> block so marked (i.e., if the exception is caught and handled internally) then behavior is entirely unaffected.</p>
   </ul>
   <h3 class="heading settled" data-level="1.4" id="acknowledgements"><span class="secno">1.4. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h3>
   <p>Thank you especially to Antony Peacock for getting this paper ready for initial submission, and to Mathias Gaunard for inspiration, review and feedback.</p>
   <h3 class="heading settled" data-level="1.5" id="revision-history"><span class="secno">1.5. </span><span class="content">Revision history</span><a class="self-link" href="#revision-history"></a></h3>
   <dl>
    <dt data-md>R0
    <dd data-md>
     <p>Initial revision; incorporated informal feedback.</p>
   </dl>
   <h2 class="heading settled" data-level="2" id="possible-syntaxes"><span class="secno">2. </span><span class="content">Possible syntaxes</span><a class="self-link" href="#possible-syntaxes"></a></h2>
   <p>We now propose a number of possible syntaxes for user code to opt in to the mechanism and retrieve the stored stacktrace.</p>
   <h3 class="heading settled" data-level="2.1" id="special-function"><span class="secno">2.1. </span><span class="content">Special function</span><a class="self-link" href="#special-function"></a></h3>
   <p>Under this mechanism, a <code class="highlight"><c- k>catch</c-></code> block that contains a potentially-evaluated call to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>stacktrace</c-><c- o>::</c-><c- n>from_current_exception</c-><c- p>()</c-></code> is marked as requiring a stacktrace to be taken
during search phase:</p>
<pre class="language-c++ highlight"><c- n>try</c-> <c- p>{</c->
    <c- p>...</c->
<c- p>}</c-> <c- n>catch</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>exception</c-><c- o>&amp;</c-> <c- n>ex</c-><c- p>)</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>ex</c-><c- p>.</c-><c- n>what</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>stacktrace</c-><c- o>::</c-><c- n>from_current_exception</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>Drawback: if the call is moved out of the <code class="highlight"><c- k>catch</c-></code> block, e.g. to a helper function, or even perhaps to a lambda within it, the facility will cease to work.
Possible workaround: make <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>stacktrace</c-><c- o>::</c-><c- n>from_current_exception</c-><c- p>()</c-></code> ill-formed when called from anywhere other than a <code class="highlight"><c- k>catch</c-></code> block - this could be ugly.</p>
   <h3 class="heading settled" data-level="2.2" id="default-parameter"><span class="secno">2.2. </span><span class="content">Default parameter</span><a class="self-link" href="#default-parameter"></a></h3>
<pre class="language-c++ highlight"><c- n>try</c-> <c- p>{</c->
    <c- p>...</c->
<c- p>}</c-> <c- n>catch</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>exception</c-><c- o>&amp;</c-> <c- n>ex</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>stacktrace</c-> <c- n>st</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stacktrace</c-><c- o>::</c-><c- n>from_current_exception</c-><c- p>())</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>ex</c-><c- p>.</c-><c- n>what</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-> <c- o>&lt;&lt;</c-> <c- n>st</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>This can be understood by analogy to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>source_location</c-><c- o>::</c-><c- n>current</c-><c- p>()</c-></code> which similarly has special behavior in particular contexts.</p>
   <p>Drawbacks: new syntax, could be understood as providing multiple types to be caught.</p>
   <h3 class="heading settled" data-level="2.3" id="catch-with-init"><span class="secno">2.3. </span><span class="content">"catch-with-init"</span><a class="self-link" href="#catch-with-init"></a></h3>
<pre class="language-c++ highlight"><c- n>try</c-> <c- p>{</c->
    <c- p>...</c->
<c- p>}</c-> <c- n>catch</c-><c- p>(</c-><c- k>auto</c-> <c- n>st</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stacktrace</c-><c- o>::</c-><c- n>current</c-><c- p>();</c-> <c- n>std</c-><c- o>::</c-><c- n>exception</c-><c- o>&amp;</c-> <c- n>ex</c-><c- p>)</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>ex</c-><c- p>.</c-><c- n>what</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-> <c- o>&lt;&lt;</c-> <c- n>st</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>Here we add an (optional) <em>init-statement</em> to the <code class="highlight"><c- k>catch</c-></code> clause, to be executed before unwind (if the catch clause is selected).</p>
   <p>Drawbacks: new syntax, may not be safe to run general user code during search phase.</p>
   <h3 class="heading settled" data-level="2.4" id="expose-search-phase"><span class="secno">2.4. </span><span class="content">Expose search phase</span><a class="self-link" href="#expose-search-phase"></a></h3>
<pre class="language-c++ highlight"><c- n>try</c-> <c- p>{</c->
    <c- p>...</c->
<c- p>}</c-> <c- n>catch</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>exception</c-><c- o>&amp;</c-> <c- n>ex</c-><c- p>)</c-> <c- k>if</c-> <c- p>(</c-><c- k>auto</c-> <c- n>st</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stacktrace</c-><c- o>::</c-><c- n>current</c-><c- p>();</c-> true<c- p>)</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>ex</c-><c- p>.</c-><c- n>what</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- s>"</c-><c- se>\n</c-><c- s>"</c-> <c- o>&lt;&lt;</c-> <c- n>st</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>Drawbacks: new syntax, open to abuse, may not be safe to run general user code during search phase.</p>
   <h2 class="heading settled" data-level="3" id="concerns"><span class="secno">3. </span><span class="content">Concerns</span><a class="self-link" href="#concerns"></a></h2>
   <p>We do not know whether this mechanism is indeed implementible on all platforms.  We do know that (and, indeed, have practical experience <a href="#implementation-experience">§ 4 Implementation experience</a> to show that) it is
implementable on two major platforms (i.e. Windows on Intel, and Unix-like on x86-64) that between them cover a dominant proportion of the market.  We would welcome 
information regarding alternative platforms.</p>
   <p>Third-party vendors who view secrecy as a virtue may be tempted to put <code class="highlight"><c- k>catch</c-> <c- p>(...)</c-></code> blocks at API entry points to prevent information on their library internals leaking
out.  In practice they can achieve much the same end by stripping debug symbols and obfuscating object names, and are likely to do so; meanwhile the same information is 
available by attaching a debugger.</p>
   <p>Some of the proposed mechanisms <a href="#possible-syntaxes">§ 2 Possible syntaxes</a> are potentially confusing or open to abuse.</p>
   <p>For a <em>rethrown</em> exception (using <code class="highlight"><c- k>throw</c-><c- p>;</c-></code> or <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>rethrow_exception</c-></code>) the stacktrace will only extend as far as the rethrow point.  We could provide mechanisms to alleviate
this, either opt-in or opt-out; for example, adding <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>current_exception_with_stacktrace</c-></code> or marking <code class="highlight"><c- k>catch</c-></code> blocks containing <code class="highlight"><c- k>throw</c-><c- p>;</c-></code> as requiring stacktrace.</p>
   <h2 class="heading settled" data-level="4" id="implementation-experience"><span class="secno">4. </span><span class="content">Implementation experience</span><a class="self-link" href="#implementation-experience"></a></h2>
   <p>The following implementations are provided solely to demonstrate implementability; we anticipate that any Standard implementation would be significantly less ugly in both
internals and in use.</p>
   <h3 class="heading settled" data-level="4.1" id="windows"><span class="secno">4.1. </span><span class="content">Windows</span><a class="self-link" href="#windows"></a></h3>
   <p>It is well known that the vendor-specific <code class="highlight"><c- kr>__try</c-></code> and <code class="highlight"><c- kr>__except</c-></code> keywords <a data-link-type="biblio" href="#biblio-try-except">[try-except]</a> (present in Visual Studio and compatible compilers) permit arbitrary code to be invoked during search
phase, since the argument to the <code class="highlight"><c- kr>__except</c-></code> keyword is a <em>filter-expression</em> evaluated during search phase, to an enumeration indicating whether the consequent code block is to 
be selected as the handler.  We present a proof-of-concept implementation <a data-link-type="biblio" href="#biblio-poc">[poc]</a> (32-bit and 64-bit) adapted from an article by Howard Jeng <a data-link-type="biblio" href="#biblio-jeng">[jeng]</a>.</p>
   <h3 class="heading settled" data-level="4.2" id="itanium"><span class="secno">4.2. </span><span class="content">Itanium</span><a class="self-link" href="#itanium"></a></h3>
   <p>Although exception handling on Itanium is also two-phase, the handler selection mechanism is largely hidden from the user.  However, there is a workaround involving creating a
type whose run-time type information (that is, its <code class="highlight"><c- k>typeid</c-></code>) refers to an instance of a user-defined subclass of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>type_info</c-></code>.  This technique is not particularly widely known, but has been used 
in several large proprietary code bases to good effect for some time.  We present a proof-of-concept implementation <a data-link-type="biblio" href="#biblio-poc">[poc]</a>.</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-itanium">[ITANIUM]
   <dd><a href="https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html"><cite>C++ ABI for Itanium: Exception Handling</cite></a>. URL: <a href="https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html">https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html</a>
   <dt id="biblio-jeng">[JENG]
   <dd><a href="https://www.gamedev.net/tutorials/programming/general-and-gameplay-programming/the-visual-c-exception-model-r2488/"><cite>The Visual C++ Exception Model</cite></a>. URL: <a href="https://www.gamedev.net/tutorials/programming/general-and-gameplay-programming/the-visual-c-exception-model-r2488/">https://www.gamedev.net/tutorials/programming/general-and-gameplay-programming/the-visual-c-exception-model-r2488/</a>
   <dt id="biblio-p0709">[P0709]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0709r4.pdf"><cite>Zero-overhead deterministic exceptions: Throwing values</cite></a>. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0709r4.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0709r4.pdf</a>
   <dt id="biblio-p2370">[P2370]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2370r1.html"><cite>Stacktrace from exception</cite></a>. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2370r1.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2370r1.html</a>
   <dt id="biblio-poc">[POC]
   <dd><a href="https://github.com/ecatmur/stacktrace-from-exception/blob/main/stacktrace-from-exception.cpp"><cite>Proof-of-concept implementation.</cite></a>. URL: <a href="https://github.com/ecatmur/stacktrace-from-exception/blob/main/stacktrace-from-exception.cpp">https://github.com/ecatmur/stacktrace-from-exception/blob/main/stacktrace-from-exception.cpp</a>
   <dt id="biblio-seh">[SEH]
   <dd><a href="https://docs.microsoft.com/en-us/cpp/cpp/structured-exception-handling-c-cpp"><cite>Structured Exception Handling (C/C++)</cite></a>. URL: <a href="https://docs.microsoft.com/en-us/cpp/cpp/structured-exception-handling-c-cpp">https://docs.microsoft.com/en-us/cpp/cpp/structured-exception-handling-c-cpp</a>
   <dt id="biblio-try-except">[TRY-EXCEPT]
   <dd><a href="https://docs.microsoft.com/en-us/cpp/cpp/try-except-statement"><cite>`try-except` statement</cite></a>. URL: <a href="https://docs.microsoft.com/en-us/cpp/cpp/try-except-statement">https://docs.microsoft.com/en-us/cpp/cpp/try-except-statement</a>
  </dl>