<!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>P2413R1: Remove unsafe conversions of unique_ptr&lt;T></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>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version d765c696b, updated Fri Mar 8 15:58:52 2024 -0800" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
  <meta content="42b20cd62299695effe325bc839271dad82d7e74" name="revision">
  <meta content="dark light" name="color-scheme">
<style>
:root {
	/* Default (light) theme */
	--foreground-point: rgb(0,0,0);
	--background-point: rgb(255,255,255);

	--ins-text-color: rgb(0, 182, 15);
	--diff-ins-text-color: rgb(0, 176, 189);
	--del-text-color: rgb(255, 58, 58);
	--diff-del-text-color: rgb(139, 0, 194);
}

/* dark theme */
@media (prefers-color-scheme: dark) {
	:root {
		--foreground-point: rgb(255,255,255);
		--background-point: rgb(0,0,0);

		--ins-text-color: rgb(48, 255, 48);
		--diff-ins-text-color: rgb(91, 250, 255);
		--del-text-color: rgb(255, 52, 52);
		--diff-del-text-color: rgb(244, 129, 255);
	}
}

:root {
	--ins-text-background-color: color-mix(in srgb, var(--ins-text-color) 5%, var(--background-point));
	--ins-code-background-color: color-mix(in srgb, var(--ins-text-color) 15%, var(--background-point));
	--ins-text-decoration-color: color-mix(in srgb, var(--diff-ins-text-color) 10%, var(--foreground-point));
	--diff-ins-text-background-color: color-mix(in srgb, var(--diff-ins-text-color) 5%, var(--background-point));
	--diff-ins-code-background-color: color-mix(in srgb, var(--diff-ins-text-color) 15%, var(--background-point));
	--diff-ins-text-decoration-color: color-mix(in srgb, var(--diff-ins-text-color) 10%, var(--foreground-point));

	--del-text-background-color: color-mix(in srgb, var(--del-text-color) 5%, var(--background-point));
	--del-code-background-color: color-mix(in srgb, var(--del-text-color) 15%, var(--background-point));
	--del-text-decoration-color: color-mix(in srgb, var(--del-text-color) 10%, var(--foreground-point));
	--diff-del-text-background-color: color-mix(in srgb, var(--diff-del-text-color) 5%, var(--background-point));
	--diff-del-code-background-color: color-mix(in srgb, var(--diff-del-text-color) 15%, var(--background-point));
	--diff-del-text-decoration-color: color-mix(in srgb, var(--diff-del-text-color) 10%, var(--foreground-point));
}

@media print {
	.pagebreak { break-after: always }
}

ins .wording-section .highlight:not(.idl) { background: var(--ins-code-background-color); }
diff-ins .wording-section .highlight:not(.idl) { background: var(--diff-ins-code-background-color); }
del .wording-section .highlight:not(.idl) { background: var(--del-code-background-color); }
diff-del .wording-section .highlight:not(.idl) { background: var(--diff-del-code-background-color); }


.tg  {
	border-collapse: collapse;
	border-spacing:0;
}

.tg td{
	padding:10px 10px;
	border-style: solid;
	border-width:1px;
	overflow: hidden;
	word-break: normal;
	border-color: var(--table-border-color, unset);
}
.tg th{
	font-weight: normal;
	padding: 10px 10px;
	border-style: solid;
	border-width: 1px;
	overflow: hidden;
	word-break: normal;
	border-color: black;
}
.tg .tg-c3ow { 
	border-color: unset;
	text-align: center;
	vertical-align: top
}

.ins, ins, ins p, ins div, span.ins, span.ins * {
	background-color: var(--ins-text-background-color);
	background: var(--ins-text-background-color);
	color: var(--ins-text-color);
	text-decoration: underline;
	text-decoration-color: var(--ins-text-decoration-color);
}

.diff-ins, diff-ins, diff-ins p, diff-ins div, span.diff-ins, span.diff-ins * {
	background-color: var(--diff-ins-text-background-color);
	background: var(--diff-ins-text-background-color);
	color: var(--diff-ins-text-color);
	text-decoration: underline double;
	text-decoration-color: var(--diff-ins-text-decoration-color);
}

.del, del, del p, del div, span.del, span.del * {
	background-color: var(--del-text-background-color);
	background: var(--del-text-background-color);
	color: var(--del-text-color);
	text-decoration: line-through;
	text-decoration-color: var(--del-text-decoration-color);
}

.diff-del, diff-del, diff-del p, diff-del div, span.diff-del, span.diff-del * {
	background-color: var(--diff-del-text-background-color);
	background: var(--diff-del-text-background-color);
	color: var(--diff-del-text-color);
	text-decoration: line-through double;
	text-decoration-color: var(--diff-del-text-decoration-color);
}

code.c-kw {
	font-weight: bold;
}

math, span.math {
	font-family: serif;
	font-style: italic;
}

ul {
	list-style-type: "— ";
}

.wording-clause {
	counter-reset: wording-paragraph;
}

.wording-clause-header {
	font-size: 115%;
	display: block;
	padding-bottom: 1em;
	clear: both;
}

.wording-clause-number {
	font-weight: bold;
}

.wording-clause-title {
	font-weight: bold;
	margin: 2em;
}

.wording-clause-stable-tag {
	float: right;
}

.wording-clause-section-title {
	font-size: 105%;
	font-weight: bold;
}

.clause-stable-tag {
	float: right;
}

.footnote-ref {
	font-size: 75%;
	vertical-align: super;
}

.footnote {
	font-size: 75%;
	vertical-align: bottom;
}

div.wording-section {
	counter-reset: wording-paragraph;
}

div.wording-numbered, div.wording-newnumbered {
	margin-left: 2.5em;
	margin-top: 1em;
	margin-bottom: 1em;
}

div.wording-numbered:before, div.wording-newnumbered:before {
	position: absolute;
	margin-left: -2.5em;
	display: block;
}

div.wording-numbered:before {
	content: counter(wording-paragraph);
	counter-increment: wording-paragraph;
	font-size: smaller;
}

div.wording-newnumbered:before {
	content: counter(wording-paragraph)"✨";
	counter-increment: wording-paragraph;
	font-size: smaller;
}

div.wording-numbered-list>ul, div.wording-newnumbered ul {
	counter-reset: wording-list-item;
}

div.wording-numbered-list>ul>li>ul {
  counter-reset: wording-sublist-item;
}

div.wording-numbered-list li, div.wording-newnumbered li {
	margin-left: 4em;
}

div.wording-numbered-list li:before, div.wording-newnumbered li:before {
	position: absolute;
	margin-left: -4em;
	display: block;
}

div.wording-numbered-list>ul>li:before {
	font-size: smaller;
	content: "(" counter(wording-paragraph) "." counter(wording-list-item) ")";
	counter-increment: wording-list-item;
}

div.wording-numbered-list>ul>li>ul>li:before {
  margin-left: -5em;
	font-size: smaller;
	content: "(" counter(wording-paragraph) "." counter(wording-list-item) "." counter(wording-sublist-item) ")";
	counter-increment: wording-sublist-item;
}

div.wording-newnumbered-list li:before {
	content: "(✨" counter(wording-paragraph) "." counter(wording-list-item) ")";
	counter-increment: wording-list-item;
}

div.wording-numbered-0 {
	counter-reset: wording-paragraph -1;
}

div.wording-numbered-1 {
	counter-reset: wording-paragraph 0;
}

div.wording-numbered-2 {
	counter-reset: wording-paragraph 1;
}

div.wording-numbered-3 {
	counter-reset: wording-paragraph 2;
}

div.wording-numbered-4 {
	counter-reset: wording-paragraph 3;
}

div.wording-numbered-5 {
	counter-reset: wording-paragraph 4;
}

div.wording-numbered-6 {
	counter-reset: wording-paragraph 5;
}

div.wording-numbered-7 {
	counter-reset: wording-paragraph 6;
}

div.wording-numbered-8 {
	counter-reset: wording-paragraph 7;
}

div.wording-numbered-9 {
	counter-reset: wording-paragraph 8;
}

div.wording-numbered-10 {
	counter-reset: wording-paragraph 9;
}

div.wording-numbered-11 {
	counter-reset: wording-paragraph 10;
}

div.wording-numbered-12 {
	counter-reset: wording-paragraph 11;
}

div.wording-numbered-13 {
	counter-reset: wording-paragraph 12;
}

div.wording-numbered-14 {
	counter-reset: wording-paragraph 13;
}

div.wording-numbered-15 {
	counter-reset: wording-paragraph 14;
}

div.wording-numbered-16 {
	counter-reset: wording-paragraph 15;
}

div.wording-numbered-17 {
	counter-reset: wording-paragraph 16;
}

div.wording-numbered-18 {
	counter-reset: wording-paragraph 17;
}

div.wording-numbered-19 {
	counter-reset: wording-paragraph 18;
}

div.wording-numbered-20 {
	counter-reset: wording-paragraph 19;
}
</style>
<style>/* Boilerplate: 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 {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}

@media (prefers-color-scheme: dark) {
    :root {
        --selflink-text: black;
        --selflink-bg: silver;
        --selflink-hover-text: white;
    }
}
</style>
<style>/* Boilerplate: style-colors */
/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
:root {
    color-scheme: light dark;

    --text: black;
    --bg: white;

    --unofficial-watermark: url(https://www.w3.org/StyleSheets/TR/2016/logos/UD-watermark);

    --logo-bg: #1a5e9a;
    --logo-active-bg: #c00;
    --logo-text: white;

    --tocnav-normal-text: #707070;
    --tocnav-normal-bg: var(--bg);
    --tocnav-hover-text: var(--tocnav-normal-text);
    --tocnav-hover-bg: #f8f8f8;
    --tocnav-active-text: #c00;
    --tocnav-active-bg: var(--tocnav-normal-bg);

    --tocsidebar-text: var(--text);
    --tocsidebar-bg: #f7f8f9;
    --tocsidebar-shadow: rgba(0,0,0,.1);
    --tocsidebar-heading-text: hsla(203,20%,40%,.7);

    --toclink-text: var(--text);
    --toclink-underline: #3980b5;
    --toclink-visited-text: var(--toclink-text);
    --toclink-visited-underline: #054572;

    --heading-text: #005a9c;

    --hr-text: var(--text);

    --algo-border: #def;

    --del-text: red;
    --del-bg: transparent;
    --ins-text: #080;
    --ins-bg: transparent;

    --a-normal-text: #034575;
    --a-normal-underline: #bbb;
    --a-visited-text: var(--a-normal-text);
    --a-visited-underline: #707070;
    --a-hover-bg: rgba(75%, 75%, 75%, .25);
    --a-active-text: #c00;
    --a-active-underline: #c00;

    --blockquote-border: silver;
    --blockquote-bg: transparent;
    --blockquote-text: currentcolor;

    --issue-border: #e05252;
    --issue-bg: #fbe9e9;
    --issue-text: var(--text);
    --issueheading-text: #831616;

    --example-border: #e0cb52;
    --example-bg: #fcfaee;
    --example-text: var(--text);
    --exampleheading-text: #574b0f;

    --note-border: #52e052;
    --note-bg: #e9fbe9;
    --note-text: var(--text);
    --noteheading-text: hsl(120, 70%, 30%);
    --notesummary-underline: silver;

    --assertion-border: #aaa;
    --assertion-bg: #eee;
    --assertion-text: black;

    --advisement-border: orange;
    --advisement-bg: #fec;
    --advisement-text: var(--text);
    --advisementheading-text: #b35f00;

    --warning-border: red;
    --warning-bg: hsla(40,100%,50%,0.95);
    --warning-text: var(--text);

    --amendment-border: #330099;
    --amendment-bg: #F5F0FF;
    --amendment-text: var(--text);
    --amendmentheading-text: #220066;

    --def-border: #8ccbf2;
    --def-bg: #def;
    --def-text: var(--text);
    --defrow-border: #bbd7e9;

    --datacell-border: silver;

    --indexinfo-text: #707070;

    --indextable-hover-text: black;
    --indextable-hover-bg: #f7f8f9;

    --outdatedspec-bg: rgba(0, 0, 0, .5);
    --outdatedspec-text: black;
    --outdated-bg: maroon;
    --outdated-text: white;
    --outdated-shadow: red;

    --editedrec-bg: darkorange;
}

@media (prefers-color-scheme: dark) {
    :root {
        --text: #ddd;
        --bg: black;

        --unofficial-watermark: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='400' height='400'%3E%3Cg fill='%23100808' transform='translate(200 200) rotate(-45) translate(-200 -200)' stroke='%23100808' stroke-width='3'%3E%3Ctext x='50%25' y='220' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EUNOFFICIAL%3C/text%3E%3Ctext x='50%25' y='305' style='font: bold 70px sans-serif; text-anchor: middle; letter-spacing: 6px;'%3EDRAFT%3C/text%3E%3C/g%3E%3C/svg%3E");

        --logo-bg: #1a5e9a;
        --logo-active-bg: #c00;
        --logo-text: white;

        --tocnav-normal-text: #999;
        --tocnav-normal-bg: var(--bg);
        --tocnav-hover-text: var(--tocnav-normal-text);
        --tocnav-hover-bg: #080808;
        --tocnav-active-text: #f44;
        --tocnav-active-bg: var(--tocnav-normal-bg);

        --tocsidebar-text: var(--text);
        --tocsidebar-bg: #080808;
        --tocsidebar-shadow: rgba(255,255,255,.1);
        --tocsidebar-heading-text: hsla(203,20%,40%,.7);

        --toclink-text: var(--text);
        --toclink-underline: #6af;
        --toclink-visited-text: var(--toclink-text);
        --toclink-visited-underline: #054572;

        --heading-text: #8af;

        --hr-text: var(--text);

        --algo-border: #456;

        --del-text: #f44;
        --del-bg: transparent;
        --ins-text: #4a4;
        --ins-bg: transparent;

        --a-normal-text: #6af;
        --a-normal-underline: #555;
        --a-visited-text: var(--a-normal-text);
        --a-visited-underline: var(--a-normal-underline);
        --a-hover-bg: rgba(25%, 25%, 25%, .2);
        --a-active-text: #f44;
        --a-active-underline: var(--a-active-text);

        --borderedblock-bg: rgba(255, 255, 255, .05);

        --blockquote-border: silver;
        --blockquote-bg: var(--borderedblock-bg);
        --blockquote-text: currentcolor;

        --issue-border: #e05252;
        --issue-bg: var(--borderedblock-bg);
        --issue-text: var(--text);
        --issueheading-text: hsl(0deg, 70%, 70%);

        --example-border: hsl(50deg, 90%, 60%);
        --example-bg: var(--borderedblock-bg);
        --example-text: var(--text);
        --exampleheading-text: hsl(50deg, 70%, 70%);

        --note-border: hsl(120deg, 100%, 35%);
        --note-bg: var(--borderedblock-bg);
        --note-text: var(--text);
        --noteheading-text: hsl(120, 70%, 70%);
        --notesummary-underline: silver;

        --assertion-border: #444;
        --assertion-bg: var(--borderedblock-bg);
        --assertion-text: var(--text);

        --advisement-border: orange;
        --advisement-bg: #222218;
        --advisement-text: var(--text);
        --advisementheading-text: #f84;

        --warning-border: red;
        --warning-bg: hsla(40,100%,20%,0.95);
        --warning-text: var(--text);

        --amendment-border: #330099;
        --amendment-bg: #080010;
        --amendment-text: var(--text);
        --amendmentheading-text: #cc00ff;

        --def-border: #8ccbf2;
        --def-bg: #080818;
        --def-text: var(--text);
        --defrow-border: #136;

        --datacell-border: silver;

        --indexinfo-text: #aaa;

        --indextable-hover-text: var(--text);
        --indextable-hover-bg: #181818;

        --outdatedspec-bg: rgba(255, 255, 255, .5);
        --outdatedspec-text: black;
        --outdated-bg: maroon;
        --outdated-text: white;
        --outdated-shadow: red;

        --editedrec-bg: darkorange;
    }
    /* In case a transparent-bg image doesn't expect to be on a dark bg,
       which is quite common in practice... */
    img { background: white; }
}
</style>
<style>/* Boilerplate: style-counters */
body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}
</style>
<style>/* Boilerplate: style-issues */
a[href].issue-return {
    float: right;
    float: inline-end;
    color: var(--issueheading-text);
    font-weight: bold;
    text-decoration: none;
}
</style>
<style>/* Boilerplate: style-md-lists */
/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}
</style>
<style>/* Boilerplate: style-selflinks */
:root {
    --selflink-text: white;
    --selflink-bg: gray;
    --selflink-hover-text: black;
}
.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
.example > a.self-link,
.note > a.self-link,
.issue > a.self-link {
    /* These blocks are overflow:auto, so positioning outside
       doesn't work. */
    left: auto;
    right: 0;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: var(--selflink-bg);
    color: var(--selflink-text);
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: var(--selflink-hover-text);
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }
</style>
<style>/* Boilerplate: style-syntax-highlighting */
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }

.highlight:not(.idl) { background: rgba(0, 0, 0, .03); }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
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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">P2413R1<br>Remove unsafe conversions of unique_ptr&lt;T></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="2024-05-22">2024-05-22</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:cpp@lenardszolnoki.com">Lénárd Szolnoki</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
    </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="#revisions"><span class="secno">1</span> <span class="content">Revision history</span></a>
     <ol class="toc">
      <li><a href="#rev1"><span class="secno">1.1</span> <span class="content">R1</span></a>
      <li><a href="#rev0"><span class="secno">1.2</span> <span class="content">R0</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li>
     <a href="#problem"><span class="secno">3</span> <span class="content">Problematic constructs</span></a>
     <ol class="toc">
      <li><a href="#problematic_construction"><span class="secno">3.1</span> <span class="content">Problematic constructions of <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-></code></span></a>
      <li><a href="#odr_issue"><span class="secno">3.2</span> <span class="content">ODR-violation involving <code class="highlight"><c- n>unique_ptr</c-></code> conversion and incomplete class types</span></a>
     </ol>
    <li>
     <a href="#proposed_resolution"><span class="secno">4</span> <span class="content">Proposed resolution</span></a>
     <ol class="toc">
      <li><a href="#design_principles"><span class="secno">4.1</span> <span class="content">Design principles</span></a>
      <li><a href="#helper_concept"><span class="secno">4.2</span> <span class="content"><code><i><c- n>safely-convertible-for-delete</c-></i></code> helper concept</span></a>
      <li><a href="#default_delete_unique_ptr_changes"><span class="secno">4.3</span> <span class="content">Changes to <code class="highlight"><c- n>default_delete</c-></code> and <code class="highlight"><c- n>unique_ptr</c-></code></span></a>
     </ol>
    <li>
     <a href="#breaking_changes"><span class="secno">5</span> <span class="content">Breaking changes</span></a>
     <ol class="toc">
      <li><a href="#null_and_list_arguments"><span class="secno">5.1</span> <span class="content">Rejecting <code class="highlight"><c- mi>0</c-></code>, <code class="highlight"><c- p>{}</c-></code> as arguments for the pointer-taking member functions</span></a>
      <li><a href="#class_arguments"><span class="secno">5.2</span> <span class="content">Rejecting class types as arguments for the pointer-taking member functions</span></a>
     </ol>
    <li><a href="#test"><span class="secno">6</span> <span class="content">Testing the changes on LLVM</span></a>
    <li><a href="#prior_art"><span class="secno">7</span> <span class="content">Prior art</span></a>
    <li><a href="#acknowledgements"><span class="secno">8</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#wording"><span class="secno">9</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#uniqueptrdltrgeneral"><span class="secno">9.1</span> <span class="content">[unique.ptr.dltr.general]</span></a>
      <li><a href="#uniqueptrdltrdflt"><span class="secno">9.2</span> <span class="content">[unique.ptr.dltr.dflt]</span></a>
      <li><a href="#uniqueptrsinglegeneral"><span class="secno">9.3</span> <span class="content">[unique.ptr.single.general]</span></a>
      <li><a href="#uniqueptrsinglector"><span class="secno">9.4</span> <span class="content">[unique.ptr.single.ctor]</span></a>
      <li><a href="#uniqueptrsinglemodifiers"><span class="secno">9.5</span> <span class="content">[unique.ptr.single.modifiers]</span></a>
      <li><a href="#uniqueptrruntimector"><span class="secno">9.6</span> <span class="content">[unique.ptr.runtime.ctor]</span></a>
      <li><a href="#uniqueptrruntimemodifiers"><span class="secno">9.7</span> <span class="content">[unique.ptr.runtime.modifiers]</span></a>
     </ol>
    <li>
     <a href="#appendix"><span class="secno"></span> <span class="content">Appendix A</span></a>
     <ol class="toc">
      <li><a href="#safely-convertible-impl"><span class="secno"></span> <span class="content">Possible implementation of <code><c- n><i>safely-convertible-for-delete</i></c-></code></span></a>
      <li><a href="#destroying_delete"><span class="secno"></span> <span class="content">Destroying <code class="highlight"><c- k>operator</c-> <c- k>delete</c-></code></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="revisions"><span class="secno">1. </span><span class="content">Revision history</span><a class="self-link" href="#revisions"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="rev1"><span class="secno">1.1. </span><span class="content">R1</span><a class="self-link" href="#rev1"></a></h3>
   <ul>
    <li data-md>
     <p>Allow conversions where the target type has destroying <code class="highlight"><c- k>operator</c-> <c- k>delete</c-></code>.</p>
    <li data-md>
     <p>Constrain <code class="highlight"><c- n>default_delete</c-><c- o>::</c-><c- k>operator</c-><c- p>()</c-></code>.</p>
    <li data-md>
     <p>Constrain member functions of <code class="highlight"><c- n>unique_ptr</c-></code> that take raw pointer arguments.</p>
    <li data-md>
     <p>Handle incomplete types.</p>
    <li data-md>
     <p>Document breaking changes.</p>
   </ul>
   <h3 class="heading settled" data-level="1.2" id="rev0"><span class="secno">1.2. </span><span class="content">R0</span><a class="self-link" href="#rev0"></a></h3>
   <ul>
    <li data-md>
     <p>Constrain the conversion operator of <code class="highlight"><c- n>default_delete</c-></code>.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Smart pointers are a success story of modern C++ as they mitigate many dangers of manual memory management.
While smart pointers do provide "misusable" named functions (<code class="highlight"><c- n>reset</c-></code>, <code class="highlight"><c- n>release</c-></code>),
  their value-semantic operations (such as assignment and implicit conversions) are generally "easy to use, <em>impossible</em> to misuse."</p>
   <p>But there is one gap in <code class="highlight"><c- n>unique_ptr</c-></code>’s safety: sometimes it permits implicit conversions that are actually unsafe.
Consider the following program:</p>
<pre class="language-cpp highlight"><c- cp>#include</c-> &lt;memory>

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

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>base_ptr</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>make_unique</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-><c- p>();</c->
<c- p>}</c->
</pre>
   <p>The delete expression evaluated in the destructor of <code class="highlight"><c- n>base_ptr</c-></code> deletes an object with dynamic type <code class="highlight"><c- n>Derived</c-></code> and static type <code class="highlight"><c- n>Base</c-></code>.
As <code class="highlight"><c- n>Base</c-></code> does not have a virtual destructor the behavior is undefined.</p>
   <p>The goal of this proposal is to make this and other similar erroneous constructions of a <code class="highlight"><c- n>unique_ptr</c-></code> to a base type ill-formed.
As a result base classes with public non-virtual destructors become safer to use.</p>
   <h2 class="heading settled" data-level="3" id="problem"><span class="secno">3. </span><span class="content">Problematic constructs</span><a class="self-link" href="#problem"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="problematic_construction"><span class="secno">3.1. </span><span class="content">Problematic constructions of <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-></code></span><a class="self-link" href="#problematic_construction"></a></h3>
   <p>All of the following operations eventually result in undefined behavior,
unless the pointer gets released from the resulting <code class="highlight"><c- n>unique_ptr</c-></code> before its destruction:</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>NonPolyBase</c-> <c- p>{};</c->
<c- k>struct</c-> <c- nc>NonPolyDerived</c-> <c- o>:</c-> <c- n>NonPolyBase</c-> <c- p>{};</c->
  
<c- c1>// (1) conversion between unique_ptr types</c->
<c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>NonPolyBase</c-><c- o>></c-> <c- n>ptr1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>make_unique</c-><c- o>&lt;</c-><c- n>NonPolyDerived</c-><c- o>></c-><c- p>();</c->

<c- c1>// (2) raw pointer construction</c->
<c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>NonPolyBase</c-><c- o>></c-> <c- n>ptr2</c-><c- p>(</c-><c- k>new</c-> <c- n>NonPolyDerived</c-><c- p>{});</c->
<c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>NonPolyBase</c-><c- o>></c-> <c- n>ptr3</c-><c- p>(</c-><c- k>new</c-> <c- n>NonPolyDerived</c-><c- p>{},</c-> <c- n>std</c-><c- o>::</c-><c- n>default_delete</c-><c- o>&lt;</c-><c- n>NonPolyBase</c-><c- o>></c-><c- p>{});</c->

<c- c1>// (3) reset(raw_ptr)</c->
<c- b>void</c-> <c- nf>f</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>NonPolyBase</c-><c- o>>&amp;</c-> <c- n>uptr</c-><c- p>)</c-> <c- p>{</c->
  <c- n>uptr</c-><c- p>.</c-><c- n>reset</c-><c- p>(</c-><c- k>new</c-> <c- n>NonPolyDerived</c-><c- p>{});</c->
<c- p>}</c->
</pre>
   <h3 class="heading settled" data-level="3.2" id="odr_issue"><span class="secno">3.2. </span><span class="content">ODR-violation involving <code class="highlight"><c- n>unique_ptr</c-></code> conversion and incomplete class types</span><a class="self-link" href="#odr_issue"></a></h3>
   <p><code class="highlight"><c- n>unique_ptr</c-></code> and <code class="highlight"><c- n>default_delete</c-></code> are among the few class templates in the standard library that support incomplete types for their first template parameter.
Currently the convertiblity between specializations of these class templates is constrained on whether the corresponding raw pointer types are convertible, which is typically implemented with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>is_convertible</c-></code>.
As validity of the conversion between the pointer types can change when the types are completed this can result in ODR-violation.</p>
<pre class="language-cpp highlight"><c- k>struct</c-> <c- nc>Base</c-> <c- p>{</c->
    <c- k>virtual</c-> <c- o>~</c-><c- n>Base</c-><c- p>();</c->
<c- p>};</c->
<c- k>struct</c-> <c- nc>Derived</c-><c- p>;</c->

<c- k>struct</c-> <c- nc>S</c-> <c- p>{</c->
    <c- n>S</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>>&amp;&amp;</c-><c- p>)</c-> <c- p>{}</c->
<c- p>};</c->

<c- b>void</c-> <c- nf>foo</c-><c- p>(</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-><c- p>);</c-> <c- c1>// 1</c->
<c- b>void</c-> <c- nf>foo</c-><c- p>(</c-><c- b>long</c-><c- p>,</c-> <c- n>S</c-><c- p>);</c-> <c- c1>// 2</c->

<c- b>void</c-> <c- nf>bar</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>>&amp;&amp;</c-> <c- n>ptr</c-><c- p>)</c-> <c- p>{</c->
    <c- n>foo</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>ptr</c-><c- p>));</c-> <c- c1>// calls 2, ptr is not convertible to std::unique_ptr&lt;Base></c->
    <c- c1>// in the overload resolution std::is_convertible&lt;Derived*, Base*> gets instantiated</c->
    <c- c1>// with value "false"</c->
<c- p>}</c->

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

<c- b>void</c-> <c- nf>baz</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>>&amp;&amp;</c-> <c- n>ptr</c-><c- p>)</c-> <c- p>{</c->
    <c- n>foo</c-><c- p>(</c-><c- mi>1</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>ptr</c-><c- p>));</c-> <c- c1>// is_convertible&lt;Derived*, Base*> would change value, but it's already instantiated</c->
<c- p>}</c->
</pre>
   <p>If we can assume that <code class="highlight"><c- n>is_convertible</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>*</c-><c- p>,</c-> <c- n>Base</c-><c- o>*></c-></code> is used for the constraints, then the program has undefined behavior according to <a href="https://timsong-cpp.github.io/cppwp/n4950/meta.rqmts#5">[meta.rqmts]/5</a>:</p>
   <blockquote>
    <div class="wording-numbered wording-numbered-5">
      Unless otherwise specified, an incomplete type may be used to instantiate a template specified in [type.traits].
The behavior of a program is undefined if: 
     <div class="wording-numbered-list">
      <ul>
       <li> an instantiation of a template specified in [type.traits] directly or indirectly depends on an incompletely-defined object type <code>T</code>, and 
       <li> that instantiation could yield a different result were <code>T</code> hypothetically completed. 
      </ul>
     </div>
    </div>
   </blockquote>
   <p>This proposal makes the implicit conversion sequence from <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>ptr</c-><c- p>)</c-></code> to <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-></code> a hard error (ill-formed outside of immediate context) when <code class="highlight"><c- n>Derived</c-></code> is still incomplete, therefore the first call to <code class="highlight"><c- n>foo</c-></code> in the code above will be ill-formed with diagnostics required.</p>
   <h2 class="heading settled" data-level="4" id="proposed_resolution"><span class="secno">4. </span><span class="content">Proposed resolution</span><a class="self-link" href="#proposed_resolution"></a></h2>
   <p>The resolution avoids undefined behavior at the destruction of <code class="highlight"><c- n>unique_ptr</c-></code> due to an undefined delete expression as specified in <a href="https://timsong-cpp.github.io/cppwp/n4950/expr.delete#3">[expr.delete]/3</a>:</p>
   <blockquote>
    <div class="wording-numbered wording-numbered-3"> In a single-object delete expression, if the static type of the object to be deleted is not similar ([conv.qual]) to its dynamic type and the selected deallocation function (see below) is not a destroying operator delete, the static type shall be a base class of the dynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined. </div>
   </blockquote>
   <p>This requires the ability to check two traits for the target type:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>has_virtual_destructor_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, which checks whether the type has a virtual destructor declared.</p>
    <li data-md>
     <p><code><c- n><i>has-destroying-delete</i></c-></code><code class="highlight"><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>, which is satisfied when the expression <code class="highlight"><c- k>delete</c-> <c- n>p</c-></code> for a <code class="highlight"><c- n>p</c-></code> of type <code class="highlight"><c- n>T</c-><c- o>*</c-></code> would select a destroying operator delete for the deallocation function. <code><c- n><i>has-destroying-delete</i></c-></code> is used as an exposition-only concept throughout the proposal.</p>
   </ul>
   <p>Whether the resulting delete expression would be otherwise undefined due to operations within the destructor or selected deallocation function is out of scope of this proposal.</p>
   <h3 class="heading settled" data-level="4.1" id="design_principles"><span class="secno">4.1. </span><span class="content">Design principles</span><a class="self-link" href="#design_principles"></a></h3>
   <ul>
    <li data-md>
     <p>Avoid hard-coding <code class="highlight"><c- n>default_delete</c-></code> into preconditions of <code class="highlight"><c- n>unique_ptr</c-></code> member functions.</p>
     <ul>
      <li data-md>
       <p>Custom deleters should be able to constrain <code class="highlight"><c- n>unique_ptr</c-></code> conversions through a similar mechanism, if they wish to do so.</p>
     </ul>
    <li data-md>
     <p>Prefer SFINAE/concept friendliness.</p>
     <ul>
      <li data-md>
       <p>Type traits and concepts shouldn’t lie, if avoidable. <code class="highlight"><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>U</c-><c- o>>></c-></code> might be <code class="highlight">true</code> or <code class="highlight">false</code>. This matches the current design as well.</p>
     </ul>
    <li data-md>
     <p>Avoid ODR-issues related to incomplete types.</p>
     <ul>
      <li data-md>
       <p><code class="highlight"><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>U</c-><c- o>>></c-></code> might be unknown before <code class="highlight"><c- n>T</c-></code> and <code class="highlight"><c- n>U</c-></code> are completed. This should be a hard error.</p>
     </ul>
   </ul>
   <p>The goal of the proposal with these high level principles can be met with the following approach:</p>
   <ul>
    <li data-md>
     <p>Constrain both the converting constructor and <code class="highlight"><c- k>operator</c-><c- p>()</c-></code> member of single-object <code class="highlight"><c- n>default_delete</c-></code> the same way to disallow converting to a base class without a virtual destructor or destroying operator delete.</p>
     <ul>
      <li data-md>
       <p>This is similar to the design of array <code class="highlight"><c- n>default_delete</c-></code>, where only qualification conversions are allowed for both members.</p>
     </ul>
    <li data-md>
     <p>Constrain the raw pointer taking member functions of <code class="highlight"><c- n>unique_ptr</c-></code> to only accept pointers that can be deleted by the target deleter’s <code class="highlight"><c- k>operator</c-><c- p>()</c-></code> directly.</p>
     <ul>
      <li data-md>
       <p>This essentially makes <code class="highlight"><c- k>operator</c-><c- p>()</c-></code> of a deleter a customization point to control whether the raw pointer operations of <code class="highlight"><c- n>unique_ptr</c-></code> should be allowed for certain arguments.</p>
     </ul>
   </ul>
   <h3 class="heading settled" data-level="4.2" id="helper_concept"><span class="secno">4.2. </span><span class="content"><code><i><c- n>safely-convertible-for-delete</c-></i></code> helper concept</span><a class="self-link" href="#helper_concept"></a></h3>
   <p>This proposal introduces the following exposition-only helper concept:</p>
   <p><code class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>T</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>U</c-><c- o>></c-></code><br> <code class="highlight"><c- k>concept</c-> </code><code><i><c- n>safely-convertible-for-delete</c-></i></code><code class="highlight"> <c- o>=</c-> <c- d>/*see below*/</c-></code></p>
   <p><code><i><c- n>safely-convertible-for-delete</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>></c-></code> is <code class="highlight">true</code> for complete types <code class="highlight"><c- n>T</c-></code> and <code class="highlight"><c- n>U</c-></code> if and only if for some function <code class="highlight"><c- n>T</c-> <c- nf>create</c-><c- p>();</c-></code> the following code at block scope is well-formed and has defined behavior:</p>
<pre class="language-cpp highlight"><c- p>{</c->
  <c- n>U</c-><c- o>*</c-> <c- n>ptr</c-> <c- o>=</c-> <c- k>new</c-> <c- n>T</c-><c- p>(</c-><c- n>create</c-><c- p>());</c->
  <c- k>delete</c-> <c- n>ptr</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>otherwise it is <code class="highlight">false</code>.</p>
   <p>If either of <code class="highlight"><c- n>T</c-></code> and <code class="highlight"><c- n>U</c-></code> is incomplete then the concept evaluates to <code class="highlight">true</code> if for any possible completion of the types the concept evaluates to <code class="highlight">true</code>. It evaluates to <code class="highlight">false</code>, if for any possible completed types the concept evaluates to <code class="highlight">false</code>. Otherwise <code><i><c- n>safely-convertible-for-delete</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>U</c-><c- o>></c-></code> is ill-formed, outside of an immediate context.</p>
   <p>The most complex case is when <code class="highlight"><c- n>T</c-></code> and <code class="highlight"><c- n>U</c-></code> are distinct class types, where either or both of them are incomplete.
Some notable examples:</p>
   <ul>
    <li data-md>
     <p>If only <code class="highlight"><c- n>T</c-></code> is complete, then it can’t be possibly derived from <code class="highlight"><c- n>U</c-></code>, therefore the conversion from <code class="highlight"><c- n>T</c-><c- o>*</c-></code> to <code class="highlight"><c- n>U</c-><c- o>*</c-></code> is ill-formed, no matter how <code class="highlight"><c- n>U</c-></code> is completed.</p>
    <li data-md>
     <p>If only <code class="highlight"><c- n>U</c-></code> is complete and it’s a <code class="highlight"><c- k>final</c-></code> class then <code class="highlight"><c- n>T</c-></code> can’t be completed to derive from <code class="highlight"><c- n>U</c-></code>, therefore the conversion from <code class="highlight"><c- n>T</c-><c- o>*</c-></code> to <code class="highlight"><c- n>U</c-><c- o>*</c-></code> is ill-formed, no matter how <code class="highlight"><c- n>T</c-></code> is completed.</p>
    <li data-md>
     <p>If only <code class="highlight"><c- n>U</c-></code> is complete and it doesn’t have a virtual destructor or a destroying operator delete then <code class="highlight"><c- k>delete</c-> <c- n>ptr</c-><c- p>;</c-></code> above have undefined behavior, no matter how <code class="highlight"><c- n>T</c-></code> is completed.</p>
   </ul>
   <p>A possible implementation of this concept is in the <a href="#safely-convertible-impl">appendix</a>.</p>
   <h3 class="heading settled" data-level="4.3" id="default_delete_unique_ptr_changes"><span class="secno">4.3. </span><span class="content">Changes to <code class="highlight"><c- n>default_delete</c-></code> and <code class="highlight"><c- n>unique_ptr</c-></code></span><a class="self-link" href="#default_delete_unique_ptr_changes"></a></h3>
   <p>Apply the following constraint for single-object <code class="highlight"><c- n>default_delete</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>’s converting constructor and <code class="highlight"><c- k>operator</c-><c- p>()</c-></code>:</p>
   <p><code class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>U</c-><c- o>></c-></code><br> <code class="highlight"><c- k>requires</c-> </code><code><i><c- n>safely-convertible-for-delete</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>U</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-></code><br> <code class="highlight"><c- k>constexpr</c-> <c- n>default_delete</c-><c- p>(</c-><c- k>const</c-> <c- n>default_delete</c-><c- o>&lt;</c-><c- n>U</c-><c- o>>&amp;</c-> <c- n>d</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c-></code><br> <br> <code class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>U</c-><c- o>></c-></code><br> <code class="highlight"><c- k>requires</c-> </code><code><i><c- n>safely-convertible-for-delete</c-></i></code><code class="highlight"><c- o>&lt;</c-><c- n>U</c-><c- p>,</c-> <c- n>T</c-><c- o>></c-></code><br> <code class="highlight"><c- k>constexpr</c-> <c- b>void</c-> <c- nf>operator</c-><c- p>()(</c-><c- n>U</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c-></code></p>
   <p>Replace the raw-pointer taking member functions of single-object <code class="highlight"><c- n>unique_ptr</c-></code> with member function templates and apply the following constraints:</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>requires</c-> <c- k>requires</c-><c- p>(</c-><c- n>deleter_type</c-> <c- n>d</c-><c- p>,</c-> <c- n>U</c-> <c- n>p</c-><c- p>){</c-> <c- n>d</c-><c- p>(</c-><c- n>p</c-><c- p>);</c-> <c- p>}</c->
         <c- o>&amp;&amp;</c-> <c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>U</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>pointer</c-><c- o>></c->
<c- k>explicit</c-> <c- n>constepxr</c-> <c- n>unique_ptr</c-><c- p>(</c-><c- n>U</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>requires</c-> <c- k>requires</c-><c- p>(</c-><c- n>deleter_type</c-> <c- n>d</c-><c- p>,</c-> <c- n>U</c-> <c- n>p</c-><c- p>){</c-> <c- n>d</c-><c- p>(</c-><c- n>p</c-><c- p>);</c-> <c- p>}</c->
         <c- o>&amp;&amp;</c-> <c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>U</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>pointer</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>unique_ptr</c-><c- p>(</c-><c- n>U</c-> <c- n>p</c-><c- p>,</c-> <i><c- n>see</c-> <c- n>below</c-></i> <c- n>d1</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>requires</c-> <c- k>requires</c-><c- p>(</c-><c- n>deleter_type</c-> <c- n>d</c-><c- p>,</c-> <c- n>U</c-> <c- n>p</c-><c- p>){</c-> <c- n>d</c-><c- p>(</c-><c- n>p</c-><c- p>);</c-> <c- p>}</c->
         <c- o>&amp;&amp;</c-> <c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>U</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>pointer</c-><c- o>></c->
<c- k>constexpr</c-> <c- n>unique_ptr</c-><c- p>(</c-><c- n>U</c-> <c- n>p</c-><c- p>,</c-> <i><c- n>see</c-> <c- n>below</c-></i> <c- n>d2</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>U</c-> <c- o>=</c-> <c- n>pointer</c-><c- o>></c->
<c- k>requires</c-> <c- k>requires</c-><c- p>(</c-><c- n>deleter_type</c-> <c- n>d</c-><c- p>,</c-> <c- n>U</c-> <c- n>p</c-><c- p>){</c-> <c- n>d</c-><c- p>(</c-><c- n>p</c-><c- p>);</c-> <c- p>}</c->
         <c- o>&amp;&amp;</c-> <c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>U</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>pointer</c-><c- o>></c->
<c- k>constexpr</c-> <c- b>void</c-> <c- n>reset</c-><c- p>(</c-><c- n>U</c-> <c- n>p</c-> <c- o>=</c-> <c- n>pointer</c-><c- p>());</c->
</pre>
   <p>Some additional care need to be taken, so passing <code class="highlight"><c- k>nullptr</c-></code> remains working.</p>
   <p>Constrain further the raw-pointer taking member functions of array <code class="highlight"><c- n>unique_ptr</c-></code>:</p>
<pre class="language-cpp highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>U</c-><c- o>></c->
<c- k>requires</c-> <c- p>(</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>U</c-><c- p>,</c-> <c- n>pointer</c-><c- o>></c->
          <c- o>||</c-> <c- p>(</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>pointer</c-><c- p>,</c-> <c- n>element_type</c-><c- o>*></c->
              <c- o>&amp;&amp;</c-> <c- n>is_pointer_v</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c->
              <c- o>&amp;&amp;</c-> <c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>U</c-> <c- k>const</c-><c- o>*</c-><c- p>,</c-> <c- n>pointer</c-> <c- k>const</c-><c- o>*></c->
             <c- p>)</c->
         <c- p>)</c-> <c- o>&amp;&amp;</c-> <c- n>is_invocable_v</c-><c- o>&lt;</c-><c- n>deleter_type</c-><c- o>&amp;</c-><c- p>,</c-> <c- n>U</c-><c- o>&amp;></c->
<c- k>constexpr</c-> <c- k>explicit</c-> <c- n>unique_ptr</c-><c- p>(</c-><c- n>U</c-> <c- n>p</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->

<c- c1>// Similarly to the rest of the pointer-taking member functions</c->
<c- d>/*...*/</c->
</pre>
   <p>This is mainly for consistency with the single-object <code class="highlight"><c- n>unique_ptr</c-></code> changes. This causes no material changes to <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- p>[],</c-> <c- n>default_delete</c-><c- o>&lt;</c-><c- n>T</c-><c- p>[]</c-><c- o>>></c-></code>, but could be used by custom deleters.</p>
   <h2 class="heading settled" data-level="5" id="breaking_changes"><span class="secno">5. </span><span class="content">Breaking changes</span><a class="self-link" href="#breaking_changes"></a></h2>
   <h3 class="heading settled" data-level="5.1" id="null_and_list_arguments"><span class="secno">5.1. </span><span class="content">Rejecting <code class="highlight"><c- mi>0</c-></code>, <code class="highlight"><c- p>{}</c-></code> as arguments for the pointer-taking member functions</span><a class="self-link" href="#null_and_list_arguments"></a></h3>
   <p>Currently the pointer taking member functions for single-object <code class="highlight"><c- n>default_delete</c-></code> and <code class="highlight"><c- n>unique_ptr</c-></code> have varying support for taking <code class="highlight"><c- mi>0</c-></code> or <code class="highlight"><c- p>{}</c-></code> for their pointer arguments.</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- mi>0</c-><c- p>)</c-></code> and <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>({})</c-></code> are ill-formed (ambiguous for the <code class="highlight"><c- n>pointer</c-></code> and <code class="highlight"><c- n>nullptr_t</c-></code> taking members.</p>
    <li data-md>
     <p><code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>(</c-><c- mi>0</c-><c- p>,</c-> <c- n>default_delete</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{})</c-></code> and <code class="highlight"><c- n>unique_ptr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>({},</c-> <c- n>default_delete</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>{})</c-></code> are well-formed.</p>
    <li data-md>
     <p><code class="highlight"><c- n>uptr</c-><c- p>.</c-><c- n>reset</c-><c- p>(</c-><c- mi>0</c-><c- p>)</c-></code> and <code class="highlight"><c- n>uptr</c-><c- p>.</c-><c- n>reset</c-><c- p>({})</c-></code> are well-formed.</p>
   </ul>
   <p>This proposal makes no effort in preserving the current behavior here, and would possibly break usages depending on the well-formed constructs above.</p>
   <h3 class="heading settled" data-level="5.2" id="class_arguments"><span class="secno">5.2. </span><span class="content">Rejecting class types as arguments for the pointer-taking member functions</span><a class="self-link" href="#class_arguments"></a></h3>
   <p>Currently the pointer taking member functions for single-object <code class="highlight"><c- n>default_delete</c-></code> and <code class="highlight"><c- n>unique_ptr</c-></code> are non-templates, therefore they accept class types that convert to <code class="highlight"><c- n>pointer</c-></code>.</p>
   <p>This proposal replaces <code class="highlight"><c- n>default_delete</c-><c- o>::</c-><c- k>operator</c-><c- p>()</c-></code> with a function template that takes <code class="highlight"><c- n>U</c-><c- o>*</c-></code> with a deduced <code class="highlight"><c- n>U</c-></code> type. This parameter can’t take an argument that has class type and therefore break such usages. In this proposal the pointer-taking member functions of <code class="highlight"><c- n>unique_ptr</c-></code> check whether the deleter can be directly called with the argument, therefore those member functions also don’t take arguments of class type for specializations where the deleter is <code class="highlight"><c- n>default_delete</c-></code>.</p>
   <h2 class="heading settled" data-level="6" id="test"><span class="secno">6. </span><span class="content">Testing the changes on LLVM</span><a class="self-link" href="#test"></a></h2>
   <p>Arthur O’Dwyer made an <a href="https://github.com/Quuxplusone/llvm-project/tree/constrained-default-delete">LLVM project fork</a> where a similar constraint corresponding to the R0 revision of this paper is applied for the converting constructor of single-object <code class="highlight"><c- n>default_delete</c-></code> in libc++.
This fork was tested against compiling the LLVM project codebase.
One of the libc++ tests failed to compile, it originally had undefined behavior (<a href="https://reviews.llvm.org/D90536">https://reviews.llvm.org/D90536</a>).
Later one similar bug found in production code in LLVM https://reviews.llvm.org/D154776</p>
   <p>To my knowledge no false positives were found.</p>
   <h2 class="heading settled" data-level="7" id="prior_art"><span class="secno">7. </span><span class="content">Prior art</span><a class="self-link" href="#prior_art"></a></h2>
   <p><a href="https://www.boost.org/doc/libs/1_85_0/doc/html/move.html">Boost.Move</a> implements <code class="highlight"><c- n>unique_ptr</c-></code> and <code class="highlight"><c- n>default_delete</c-></code> that protects against missing virtual destructors.
Its implementation has some notable differences to this proposal:</p>
   <ul>
    <li data-md>
     <p>It might produce ill-formed code for otherwise correct conversions to target types with destroying <code class="highlight"><c- k>operator</c-> <c- k>delete</c-></code>.</p>
    <li data-md>
     <p>It’s not SFINAE-friendly.</p>
    <li data-md>
     <p>It only constrains the pointer-taking member functions of <code class="highlight"><c- n>unique_ptr</c-></code> if the deleter is <code class="highlight"><c- n>default_delete</c-></code>.</p>
   </ul>
   <h2 class="heading settled" data-level="8" id="acknowledgements"><span class="secno">8. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>I would like to thank Arthur O’Dwyer for his work to test the proposed changes on the LLVM codebase.
I would like to thank Peter Sommerlad for discussing the proposal and evaluating whether a library implementation would be possible for <code><c- n><i>has-destroying-delete</i></c-></code>.</p>
   <h2 class="heading settled" data-level="9" id="wording"><span class="secno">9. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>Wording is relative to <a data-link-type="biblio" href="#biblio-n4981" title="Working Draft, Standard for Programming Language C++">[N4981]</a>.</p>
   <h3 class="heading settled" data-level="9.1" id="uniqueptrdltrgeneral"><span class="secno">9.1. </span><span class="content">[unique.ptr.dltr.general]</span><a class="self-link" href="#uniqueptrdltrgeneral"></a></h3>
   <div> … </div>
   <ins>
    <div class="wording-numbered wording-numbered-3">
      The member functions of <code>default_delete</code> use the following exposition only concepts: 
<pre>template &lt;class T>
concept <i>has-destroying-delete</i> = <i>see below</i>;

template &lt;class T, classU>
concept <i>safely-convertible-for-delete</i> = <i>see below</i>;
</pre>
    </div>
    <div class="wording-numbered">
      For types <code>T</code> and <code>U</code>, <code><i>safely-convertible-for-delete</i>&lt;T, U></code> is: 
     <div class="wording-numbered-list">
      <ul>
       <li> <code>false</code> if either of <code>T</code> or <code>U</code> is not an object type, otherwise 
       <li> <code>true</code> if <code>T(*)[]</code> is convertible to <code>U(*)[]</code>, otherwise 
       <li> <code>false</code> if <code>reinterpret_cast&lt;U*>((T*)nullptr)</code> is ill-formed (casts away const), otherwise 
       <li> <code>false</code> if either of <code>T</code> or <code>U</code> is not a class type, otherwise 
       <li> ill-formed outside of immediate context if both <code>T</code> and <code>U</code> are incomplete types, otherwise 
       <li> <code>false</code> if <code>U</code> is an incomplete type, otherwise 
       <li> <code>false</code> if <code>T</code> is an incomplete type and <code>U</code> is final, otherwise 
       <li> <code>false</code> if <code>T</code> is an incomplete type and <code>has_virtual_destructor_v&lt;U> || </code><code><i>has-destroying-delete</i></code><code>&lt;U></code> is <code>false</code>, otherwise 
       <li> ill-formed outside of immediate context if <code>T</code> is an incomplete type, otherwise 
       <li> <code>true</code> if <code>T*</code> is convertible to <code>U*</code> and <code>has_virtual_destructor_v&lt;U> || </code><code><i>has-destroying-delete</i></code><code>&lt;U></code> is <code>true</code>, otherwise 
       <li> <code>false</code>. 
      </ul>
     </div>
    </div>
   </ins>
   <h3 class="heading settled" data-level="9.2" id="uniqueptrdltrdflt"><span class="secno">9.2. </span><span class="content">[unique.ptr.dltr.dflt]</span><a class="self-link" href="#uniqueptrdltrdflt"></a></h3>
<pre>namespace std {
  template&lt;class T> struct default_delete {
    constexpr default_delete() noexcept = default;
    template&lt;class U> constexpr default_delete(const default_delete&lt;U>&amp;) noexcept;
    <del>constexpr void operator()(T*) const;</del>
    <ins>template&lt;class U> constexpr void operator()(U*) const;</ins>
  };
}
</pre>
   <p><code>template<class u> constexpr default_delete(const default_delete&lt;U>&amp; other) noexcept;</class></code></p>
   <div class="wording-numbered">
     <i>Constraints</i>: 
    <del><code>U*</code> is implicitly convertible to <code>T*</code>.</del>
    <ins><code><i>safely-convertible-for-delete</i>&lt;U, T></code> is <code>true</code>.</ins>
   </div>
   <div class="wording-numbered wording-numbered-2"> <i>Effects</i>: Constructs a <code>default_delete</code> object from another <code>default_delete&lt;U></code> object. </div>
   <p><code><del>constexpr void operator()(T*) const;</del><br><ins>template&lt;class U> constexpr void operator()(U*) const;</ins></code></p>
   <div class="wording-numbered wording-numbered-3">
    <ins><i>Constraints</i>: <code><i>safely-convertible-for-delete</i>&lt;U, T></code> is <code>true</code>.</ins>
   </div>
   <div class="wording-numbered wording-numbered-4"> <i>Mandates</i>: <code>T</code> is a complete type. </div>
   <div class="wording-numbered wording-numbered-5"> <i>Effects</i>: Calls <code>delete</code> on <code>ptr</code>. </div>
   <h3 class="heading settled" data-level="9.3" id="uniqueptrsinglegeneral"><span class="secno">9.3. </span><span class="content">[unique.ptr.single.general]</span><a class="self-link" href="#uniqueptrsinglegeneral"></a></h3>
<pre>namespace std {
  template&lt;class T, class D = default_delete&lt;T>> class unique_ptr {
  public:
    using pointer      = <i>see below</i>;
    using element_type = T;
    using deleter_type = D;

    // [unique.ptr.single.ctor], constructors
    constexpr unique_ptr() noexcept;
    <del>constexpr explicit unique_ptr(type_identity_t&lt;pointer> p) noexcept;</del>
    <ins>template&lt;class U> explicit unique_ptr(U p) noexcept;</ins>
    <del>constexpr unique_ptr(type_identity_t&lt;pointer> p, <i>see below</i> d1) noexcept;</del>
    <del>constexpr unique_ptr(type_identity_t&lt;pointer> p, <i>see below</i> d2) noexcept;</del>
    <ins>template&lt;class U> constexpr unique_ptr(U p, <i>see below</i> d1) noexcept;</ins>
    <ins>template&lt;class U> constexpr unique_ptr(U p, <i>see below</i> d2) noexcept;</ins>
    constexpr unique_ptr(unique_ptr&amp;&amp; u) noexcept;
    constexpr unique_ptr(nullptr_t) noexcept;
    template&lt;class U, class E>
      constexpr unique_ptr(unique_ptr&lt;U, E>&amp;&amp; u) noexcept;

    // [unique.ptr.single.dtor], destructor
    constexpr ~unique_ptr();

    // [unique.ptr.single.asgn], assignment
    constexpr unique_ptr&amp; operator=(unique_ptr&amp;&amp; u) noexcept;
    template&lt;class U, class E>
      constexpr unique_ptr&amp; operator=(unique_ptr&lt;U, E>&amp;&amp; u) noexcept;
    constexpr unique_ptr&amp; operator=(nullptr_t) noexcept;

    // [unique.ptr.single.observers], observers
    constexpr add_lvalue_reference_t&lt;T> operator*() const noexcept(<i>see below</i>);
    constexpr pointer operator->() const noexcept;
    constexpr pointer get() const noexcept;
    constexpr deleter_type&amp; get_deleter() noexcept;
    constexpr const deleter_type&amp; get_deleter() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // [unique.ptr.single.modifiers], modifiers
    constexpr pointer release() noexcept;
    <del>constexpr void reset(pointer p = pointer()) noexcept;</del>
    <ins>constexpr void reset(nullptr_t = nullptr) noexcept;</ins>
    <ins>template&lt;class U> constexpr void reset(U p) noexcept;</ins>
    constexpr void swap(unique_ptr&amp; u) noexcept;

    // disable copy from lvalue
    unique_ptr(const unique_ptr&amp;) = delete;
    unique_ptr&amp; operator=(const unique_ptr&amp;) = delete;
  };
}
</pre>
   <div> … </div>
   <h3 class="heading settled" data-level="9.4" id="uniqueptrsinglector"><span class="secno">9.4. </span><span class="content">[unique.ptr.single.ctor]</span><a class="self-link" href="#uniqueptrsinglector"></a></h3>
   <div> … </div>
   <p><code><del>constexpr explicit unique_ptr(type_identity_t&lt;pointer> p) noexcept;</del><br><ins>template&lt;class U> constexpr explicit unique_ptr(U p) noexcept;</ins></code></p>
   <div class="wording-numbered wording-numbered-5">
     <i>Constraints</i>: <code>is_pointer_v&lt;deleter_type></code> is <code>false</code>
    <del> and</del>
    <ins>,</ins>
     <code>is_default_constructible_v&lt;deleter_type></code> is <code>true</code>
    <ins>, <code>get_deleter()(p)</code> is well-formed and <code>U</code> is convertible to <code>pointer</code></ins>
    . 
   </div>
   <div> … </div>
   <p><code><del>constexpr unique_ptr(type_identity_t&lt;pointer> p, const D&amp; d) noexcept;</del><br><del>constexpr unique_ptr(type_identity_t&lt;pointer> p, remove_reference_t&lt;D>&amp;&amp; d) noexcept;</del><br><ins>template&lt;class U> constexpr unique_ptr(U p, const D&amp; d1) noexcept;</ins><br><ins>template&lt;class U> constexpr unique_ptr(U p, remove_reference_t&lt;D>&amp;&amp; d2) noexcept;</ins></code></p>
   <div class="wording-numbered wording-numbered-9">
     <i>Constraints</i>: 
    <del><code>is_constructible_v&lt;D, decltype(d)></code> is <code>true</code>.</del>
    <ins>
     <div class="wording-numbered-list">
      <ul>
       <li> <code>is_constructible_v&lt;D, decltype(d)></code> is <code>true</code>, and
       <ul>
        <li> <code>U</code> is <code>nullptr_t</code>, or 
        <li> <code>get_deleter()(p)</code> is well-formed and <code>U</code> is convertible to <code>pointer</code>. 
       </ul>
      </ul>
     </div>
    </ins>
   </div>
   <div> … </div>
   <h3 class="heading settled" data-level="9.5" id="uniqueptrsinglemodifiers"><span class="secno">9.5. </span><span class="content">[unique.ptr.single.modifiers]</span><a class="self-link" href="#uniqueptrsinglemodifiers"></a></h3>
   <div> … </div>
   <p><code><del>constexpr void reset(pointer p = pointer()) noexcept;</del><br><ins>constexpr void reset(nullptr_t p = nullptr) noexcept;</ins></code></p>
   <ins>
    <div class="wording-numbered wording-numbered-3"> <i>Effects</i>: Equivalent to <code>reset(pointer())</code>. </div>
   </ins>
   <p><code><ins>template&lt;class U> constexpr void reset(U p) noexcept;</ins></code></p>
   <ins>
    <div class="wording-numbered wording-numbered-4"> <i>Constraints</i>: <code>get_deleter()(p)</code> is well-formed and <code>U</code> is convertible to <code>pointer</code>. </div>
   </ins>
   <div class="wording-numbered wording-numbered-5"> <i>Effects</i>: Assigns <code>p</code> to the stored pointer, and then, with the old value of the stored pointer, <code>old_p</code>, evaluates <code>if (old_p) get_deleter()(old_p);</code> </div>
   <div> … </div>
   <h3 class="heading settled" data-level="9.6" id="uniqueptrruntimector"><span class="secno">9.6. </span><span class="content">[unique.ptr.runtime.ctor]</span><a class="self-link" href="#uniqueptrruntimector"></a></h3>
   <p><span><code>template&lt;class U> constexpr explicit unique_ptr(U p) noexcept;</code></span></p>
   <div class="wording-numbered wording-numbered-1">
     This constructor behaves the same as the constructor in the primary template that takes a single parameter of type 
    <del><code>pointer</code></del>
    <ins><code>U</code></ins>
    . 
   </div>
   <div class="wording-numbered">
     <i>Constraints</i>: 
    <div class="wording-numbered-list">
     <ul>
      <li> <code>U</code> is the same type as <code>pointer</code>, or 
      <li>
        <code>pointer</code> is the same type as <code>element_type*</code>, <code>U</code> is a pointer type <code>V*</code>,
       <del> and</del>
        <code>V(*)[]</code> is convertible to <code>element_type(*)[]</code>
       <ins>, and <code>get_deleter()(p)</code> is well-formed</ins>
       . 
     </ul>
    </div>
   </div>
   <p><code>template&lt;class U> constexpr unique_ptr(U p, <i>see below</i> d) noexcept;<br>template&lt;class U> constexpr unique_ptr(U p, <i>see below</i> d) noexcept;</code></p>
   <div class="wording-numbered">
     These constructors behave the same as the constructors in the primary template that take a parameter of type 
    <del><code>pointer</code></del>
    <ins><code>U</code></ins>
     and a second parameter. 
   </div>
   <div class="wording-numbered">
     <i>Constraints</i>: 
    <div class="wording-numbered-list">
     <ul>
      <li> <code>U</code> is the same type as <code>pointer</code>, 
      <li> <code>U</code> is <code>nullptr_t</code>, or 
      <li>
        <code>pointer</code> is the same type as <code>element_type*</code>, <code>U</code> is a pointer type <code>V*</code>,
       <del> and</del>
        <code>V(*)[]</code> is convertible to <code>element_type(*)[]</code>
       <ins>, and <code>get_deleter()(p)</code> is well-formed</ins>
       . 
     </ul>
    </div>
   </div>
   <div> … </div>
   <h3 class="heading settled" data-level="9.7" id="uniqueptrruntimemodifiers"><span class="secno">9.7. </span><span class="content">[unique.ptr.runtime.modifiers]</span><a class="self-link" href="#uniqueptrruntimemodifiers"></a></h3>
   <div> … </div>
   <p><span><code>template&lt;class U> constexpr void reset(U p) noexcept;</code></span></p>
   <div class="wording-numbered wording-numbered-2"> This function behaves the same as the <code>reset</code> member of the primary template. </div>
   <div class="wording-numbered">
     <i>Constraints</i>: 
    <div class="wording-numbered-list">
     <ul>
      <li> <code>U</code> is the same type as <code>pointer</code>, or 
      <li>
        <code>pointer</code> is the same type as <code>element_type*</code>, <code>U</code> is a pointer type <code>V*</code>,
       <del> and</del>
        <code>V(*)[]</code> is convertible to <code>element_type(*)[]</code>
       <ins>, and <code>get_deleter()(p)</code> is well-formed</ins>
       . 
     </ul>
    </div>
   </div>
   <h2 class="heading settled" id="appendix"><span class="content">Appendix A</span><a class="self-link" href="#appendix"></a></h2>
   <h3 class="heading settled" id="safely-convertible-impl"><span class="content">Possible implementation of <code><c- n><i>safely-convertible-for-delete</i></c-></code></span><a class="self-link" href="#safely-convertible-impl"></a></h3>
<pre class="highlight language-cpp"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>From</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>To</c-><c- o>></c->
<c- k>concept</c-> <c- nc><i>casts-away-const</i></c-> <c- c1>// exposition-only</c->
<c- o>=</c-> <c- k>not</c-> <c- k>requires</c-> <c- p>(</c-><c- n>From</c-><c- o>*</c-> <c- n>ptr</c-><c- p>)</c-> <c- p>{</c->
  <c- k>reinterpret_cast</c-><c- o>&lt;</c-><c- n>To</c-><c- o>*></c-><c- p>(</c-><c- n>ptr</c-><c- p>);</c->
<c- p>};</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>From</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>To</c-><c- o>></c->
<c- k>concept</c-> <c- nc><i>safely-convertible-classes-for-delete</i></c-> <c- c1>// exposition-only</c->
<c- o>=</c-> <c- p>[]{</c->
  <c- k>static_assert</c-><c- p>(</c->
    <c- k>requires</c-> <c- p>{</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>From</c-><c- p>);</c-> <c- p>};</c->
    <c- o>||</c-> <c- k>requires</c-> <c- p>{</c->
        <c- k>sizeof</c-><c- p>(</c-><c- n>To</c-><c- p>);</c->
        <c- k>requires</c-> <c- n>is_final_v</c-><c- o>&lt;</c-><c- n>To</c-><c- o>></c->
                 <c- o>||</c-> <c- o>!</c-><c- p>(</c-><c- n>has_virtual_destructor_v</c-><c- o>&lt;</c-><c- n>To</c-><c- o>></c-> <c- o>||</c-> <c- n><i>has-destroying-delete</i></c-><c- o>&lt;</c-><c- n>To</c-><c- o>></c-><c- p>)</c->
    <c- p>}</c->
  <c- p>);</c->
  <c- k>return</c-> <c- k>requires</c-> <c- p>(</c-><c- n>From</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- b>void</c-><c- p>(</c-><c- o>*</c-><c- n>fn</c-><c- p>)(</c-><c- n>To</c-><c- o>*</c-><c- p>))</c-> <c- p>{</c->
    <c- n>fn</c-><c- p>(</c-><c- n>ptr</c-><c- p>);</c->
    <c- k>requires</c-> <c- n>has_virtual_destructor_v</c-><c- o>&lt;</c-><c- n>To</c-><c- o>></c-> <c- o>||</c-> <c- n><i>has-destroying-delete</i></c-><c- o>&lt;</c-><c- n>To</c-><c- o>></c-><c- p>;</c->
  <c- p>}</c->
<c- p>}();</c->

<c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- nc>From</c-><c- p>,</c-> <c- k>typename</c-> <c- nc>To</c-><c- o>></c->
<c- k>concept</c-> <c- nc><i>safely-convertible-for-delete</i></c-> <c- c1>// exposition-only</c->
<c- o>=</c-> <c- n>is_object_v</c-><c- o>&lt;</c-><c- n>From</c-><c- o>></c-> <c- o>&amp;&amp;</c-> <c- n>is_object_v</c-><c- o>&lt;</c-><c- n>To</c-><c- o>></c->
  <c- o>&amp;&amp;</c-> <c- p>(</c->
    <c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>From</c-><c- p>(</c-><c- o>*</c-><c- p>)[],</c-> <c- n>To</c-><c- p>(</c-><c- o>*</c-><c- p>)[]</c-><c- o>></c->
    <c- o>||</c-> <c- p>(</c->
      <c- k>not</c-> <c- n><i>casts-away-const</i></c-><c- o>&lt;</c-><c- n>From</c-><c- p>,</c-> <c- n>To</c-><c- o>></c->
      <c- o>&amp;&amp;</c-> <c- n>is_class_v</c-><c- o>&lt;</c-><c- n>From</c-><c- o>></c->
      <c- o>&amp;&amp;</c-> <c- n>is_class_v</c-><c- o>&lt;</c-><c- n>To</c-><c- o>></c->
      <c- o>&amp;&amp;</c-> <c- n><i>safely-convertible-classes-for-delete</i></c-><c- o>&lt;</c-><c- n>From</c-><c- p>,</c-> <c- n>To</c-><c- o>></c->
    <c- p>)</c->
  <c- p>);</c->
</pre>
   <h3 class="heading settled" id="destroying_delete"><span class="content">Destroying <code class="highlight"><c- k>operator</c-> <c- k>delete</c-></code></span><a class="self-link" href="#destroying_delete"></a></h3>
   <p>C++20 introduced destroying <code class="highlight"><c- k>operator</c-> <c- k>delete</c-></code>.
Because of this <code class="highlight"><c- k>delete</c-> <c- n>ptr</c-></code> might be defined even if the static type of the pointed object does not have a virtual destructor and the static type does not match the dynamic type of the pointed object.
Consider the following program:</p>
<pre class="language-cpp highlight"><c- cp>#include</c-> &lt;memory>
<c- cp>#include</c-> &lt;new>

<c- k>struct</c-> <c- nc>Base</c-> <c- p>{</c->
    <c- b>void</c-> <c- k>operator</c-> <c- k>delete</c-><c- p>(</c-><c- n>Base</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>destroying_delete_t</c-><c- p>);</c->
<c- p>};</c->

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

<c- b>void</c-> <c- n>Base</c-><c- o>::</c-><c- k>operator</c-> <c- nf>delete</c-><c- p>(</c-><c- n>Base</c-><c- o>*</c-> <c- n>ptr</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>destroying_delete_t</c-><c- p>)</c-> <c- p>{</c->
    <c- o>::</c-><c- k>delete</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>*></c-><c- p>(</c-><c- n>ptr</c-><c- p>);</c->
<c- p>}</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
    <c- n>std</c-><c- o>::</c-><c- n>unique_ptr</c-><c- o>&lt;</c-><c- n>Base</c-><c- o>></c-> <c- n>base_ptr</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>make_unique</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-><c- p>();</c->
<c- p>}</c->
</pre>
   <p>This program is well-formed in the current draft and has defined behavior. <a data-link-type="biblio" href="#biblio-p0722r1" title="Efficient sized delete for variable sized classes">[P0722R1]</a> provides a motivating example with a similar class hierarchy (section "Dynamic dispatch without vptrs").</p>
   <p>The R0 version of this proposal rejected the conversion above and suggested to add a customization point for users of destroying operator delete to optionally enable the conversion.</p>
   <p>The consensus of the mailing list review was that such a customization point is unnecessary and breaking correct code involving destroying operator delete by default is undesirable.</p>
   <p>The constraints added in the current proposal allow the conversion if the target type has destroying operator delete.</p>
  </main>
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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-n4981">[N4981]
   <dd>Thomas Köppe. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/n4981.pdf"><cite>Working Draft, Standard for Programming Language C++</cite></a>. URL: <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/n4981.pdf">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/n4981.pdf</a>
   <dt id="biblio-p0722r1">[P0722R1]
   <dd>Andrew Hunter; Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0722r1.html"><cite>Efficient sized delete for variable sized classes</cite></a>. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0722r1.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0722r1.html</a>
  </dl>