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  <title>P1518R1: Stop overconstraining allocators in container deduction guides</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 {
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	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;			  /* limit line length to 50em for readability   */
		margin: 0 auto;				/* center text within page                    */
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		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
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/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

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

	div.head { margin-bottom: 1em; }
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	div.head h2 { margin-bottom: 1.5em;}

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

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		border-color: var(--logo-active-bg);
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	/* see also additional rules in Link Styling section */

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

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

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

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			display: none;
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	@media not print {
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			box-shadow: 0 0 2px;
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			height: 1.33em;
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			margin: 0;

			box-shadow: 0 0 2px;
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			color: #707070;
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			background: var(--tocnav-hover-bg);
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		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
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		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
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			padding-bottom: 1.5rem;
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		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
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/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
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			left: 0;
			width: 23.5em;
			max-width: 80%;
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			overflow: auto;
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			color: var(--tocsidebar-text);
			background: inherit;
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			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
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			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
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			padding-left: calc(1em + 26px);
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			color: var(--tocsidebar-text);
			background: inherit;
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			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
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		body:not(.toc-inline) #toc h2 {
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			font-variant: small-caps;
			font-variant: all-small-caps;
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			color: gray;
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		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;
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		body:not(.toc-inline) {
			margin: 0 4em;
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/******************************************************************************/
/*                                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 */
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	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
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	h1, h2, h3 {
		color: #005A9C;
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	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

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

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

	:not(.head) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
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		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;
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	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
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	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;
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	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: none;
		border-bottom: 1px solid #707070;
		border-bottom: 1px solid var(--a-normal-underline);
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		border-bottom-color: #bbb;
		border-bottom-color: var(--a-visited-underline);
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	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		border-color: #c00;
		border-color: var(--a-active-underline);
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	/* 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;
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	.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;
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	.figure table, figure table {
		margin: auto;
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	@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-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* 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:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

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

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

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

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

		.toc li {
			clear: both;
		}

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

.outdated-warning span {
	display: block;
}

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

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

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

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

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



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

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

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

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

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

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version b94c7e755, updated Fri Feb 19 16:28:59 2021 -0800" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
ins  {background-color: #CCFFCC; text-decoration: underline;}
del  {background-color: #FFCACA; text-decoration: line-through;}
</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
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   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1518R1<br>Stop overconstraining allocators in container deduction guides</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2021-02-24">2021-02-24</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Authors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:arthur.j.odwyer@gmail.com">Arthur O'Dwyer</a>
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:msspertu@amazon.com">Mike Spertus</a>
     <dt>Audience:
     <dd>LWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Draft Revision:
     <dd>7
     <dt>Current Source:
     <dd><a href="https://github.com/Quuxplusone/draft/blob/gh-pages/d1518-container-deduction-guides.bs">github.com/Quuxplusone/draft/blob/gh-pages/d1518-container-deduction-guides.bs</a>
     <dt>Current:
     <dd><a href="https://rawgit.com/Quuxplusone/draft/gh-pages/d1518-container-deduction-guides.html">rawgit.com/Quuxplusone/draft/gh-pages/d1518-container-deduction-guides.html</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Discussion of flatmap’s deduction guides revealed that the deduction guides for sequence

  containers and container adaptors are needlessly overconstrained, making use cases such as
  pmr containers unnecessarily difficult. We fix this.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#changelog"><span class="secno">1</span> <span class="content">Changelog</span></a>
    <li>
     <a href="#overconst1"><span class="secno">2</span> <span class="content">Stop overconstraining allocators that do not participate in deduction</span></a>
     <ol class="toc">
      <li><a href="#problem1"><span class="secno">2.1</span> <span class="content">The problem</span></a>
      <li><a href="#explanation1"><span class="secno">2.2</span> <span class="content">The explanation</span></a>
      <li><a href="#solution1"><span class="secno">2.3</span> <span class="content">Proposed wording</span></a>
     </ol>
    <li>
     <a href="#overconst2"><span class="secno">3</span> <span class="content">Stop deducing from allocators that should not participate in deduction</span></a>
     <ol class="toc">
      <li><a href="#problem2"><span class="secno">3.1</span> <span class="content">The problem</span></a>
      <li><a href="#explanation2"><span class="secno">3.2</span> <span class="content">The explanation</span></a>
      <li><a href="#solution2"><span class="secno">3.3</span> <span class="content">Proposed wording</span></a>
      <li><a href="#implementation"><span class="secno">3.4</span> <span class="content">Implementation note</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="changelog"><span class="secno">1. </span><span class="content">Changelog</span><a class="self-link" href="#changelog"></a></h2>
   <ul>
    <li data-md>
     <p>R1: Incorporate LWG wording feedback. Mention <a data-link-type="biblio" href="#biblio-lwg3506">[LWG3506]</a> and <a data-link-type="biblio" href="#biblio-p1425r3">[P1425R3]</a>.</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="overconst1"><span class="secno">2. </span><span class="content">Stop overconstraining allocators that do not participate in deduction</span><a class="self-link" href="#overconst1"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="problem1"><span class="secno">2.1. </span><span class="content">The problem</span><a class="self-link" href="#problem1"></a></h3>
   <p>Consider this code:</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>monotonic_buffer_resource</c-> <c- n>mr</c-><c- p>;</c->
<c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>polymorphic_allocator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>a</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>mr</c-><c- p>;</c->
<c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>pv</c-><c- p>(</c-><c- n>a</c-><c- p>);</c->

<c- k>auto</c-> <c- n>s1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stack</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-><c- p>(</c-><c- n>pv</c-><c- p>);</c->
<c- k>auto</c-> <c- n>s2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stack</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- n>a</c-><c- p>);</c->
<c- k>auto</c-> <c- n>s3</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stack</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>mr</c-><c- p>);</c->

<c- k>auto</c-> <c- n>ds1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stack</c-><c- p>(</c-><c- n>pv</c-><c- p>);</c->
<c- k>auto</c-> <c- n>ds2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stack</c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- n>a</c-><c- p>);</c->
<c- k>auto</c-> <c- n>ds3</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>stack</c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>mr</c-><c- p>);</c->
</pre>
   <p>The initializers of <code class="highlight"><c- n>s1</c-></code>, <code class="highlight"><c- n>s2</c-></code>, and <code class="highlight"><c- n>s3</c-></code> (which do not use CTAD) are all well-formed,
as are the initializers of <code class="highlight"><c- n>ds1</c-></code> and <code class="highlight"><c- n>ds2</c-></code> (which do).</p>
   <p>However, the natural and useful <code class="highlight"><c- n>ds3</c-></code> is ill-formed, even though the <code class="highlight"><c- o>&amp;</c-><c- n>mr</c-></code> argument is irrelevant to determining the
template arguments for <code class="highlight"><c- n>stack</c-></code>! It seems clearly wrong that we give all the right information to unambiguously deduce
the desired specialization of stack and then reject what would be a perfectly valid constructor invocation for that
class. The allocator parameter’s type does not <em>contribute</em> to class template argument deduction in these cases;
it brings no new information, and therefore should be treated no differently than it is in the non-CTAD case.
Indeed, we believe this is an oversight and should simply be fixed.</p>
   <h3 class="heading settled" data-level="2.2" id="explanation1"><span class="secno">2.2. </span><span class="content">The explanation</span><a class="self-link" href="#explanation1"></a></h3>
   <p><code class="highlight"><c- n>stack</c-></code> has one relevant deduction guide.</p>
<pre class="language-c++ highlight">    <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>Container</c-><c- p>,</c-> <c- n>class</c-> <c- n>Allocator</c-><c- o>></c->
    <c- n>stack</c-><c- p>(</c-><c- n>Container</c-><c- p>,</c-> <c- n>Allocator</c-><c- p>)</c-> <c- o>-></c-> <c- n>stack</c-><c- o>&lt;</c-><c- n>typename</c-> <c- n>Container</c-><c- o>::</c-><c- n>value_type</c-><c- p>,</c-> <c- n>Container</c-><c- o>></c-><c- p>;</c->
</pre>
   <p>This deduction guide satisfies the following constraints.</p>
   <p>[container.adaptors.general] says: <small></small></p>
   <blockquote><small> A deduction guide for a container adaptor shall not participate in overload resolution if
any of the following are true: ...<p></p> <ul><li data-md><p><b>It has an <code class="highlight"><c- n>Allocator</c-></code> template parameter and a type that does not qualify as an allocator
  is deduced for that parameter.</b></p> </li><li data-md><p>It has both <code class="highlight"><c- n>Container</c-></code> and <code class="highlight"><c- n>Allocator</c-></code> template parameters, and <code class="highlight"><c- n>uses_allocator_v</c-><c- o>&lt;</c-><c- n>Container</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-></code> is <code class="highlight">false</code>.</p> </li></ul> </small></blockquote>
   <p>To qualify as an allocator, a type <code class="highlight"><c- n>A</c-></code> must have a nested typedef <code class="highlight"><c- n>A</c-><c- o>::</c-><c- n>value_type</c-></code> <small>([container.requirements.general]/17)</small>.
Since <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>monotonic_buffer_resource</c-><c- o>*</c-></code> has no such nested typedef, <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>monotonic_buffer_resource</c-><c- o>*</c-></code> doesn’t qualify as an allocator.
However, <code class="highlight"><c- n>uses_allocator_v</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>monotonic_buffer_resource</c-><c- o>*></c-></code> is <code class="highlight">true</code>.</p>
   <p>Because the type of <code class="highlight"><c- o>&amp;</c-><c- n>mr</c-></code> doesn’t qualify as an allocator, the deduction guide drops out
of overload resolution, failing deduction even though it has all the information
needed to safely deduce the correct template arguments for <code class="highlight"><c- n>stack</c-></code>.</p>
   <h3 class="heading settled" data-level="2.3" id="solution1"><span class="secno">2.3. </span><span class="content">Proposed wording</span><a class="self-link" href="#solution1"></a></h3>
   <p>To fix all three container adaptors, modify <a href="http://eel.is/c++draft/container.adaptors.general">[container.adaptors.general</a>] as follows:</p>
   <p><small></small></p>
   <blockquote><small> A deduction guide for a container adaptor shall not participate in overload resolution if any of the following are true:<p></p> <ul><li data-md><p>It has an <code class="highlight"><c- n>InputIterator</c-></code> template parameter and a type that does not qualify as an
 input iterator is deduced for that parameter.</p> </li><li data-md><p>It has a <code class="highlight"><c- n>Compare</c-></code> template parameter and a type that qualifies as an allocator is deduced
 for that parameter.</p> </li><li data-md><p>It has a <code class="highlight"><c- n>Container</c-></code> template parameter and a type that qualifies as an allocator is deduced
 for that parameter.</p> </li><li data-md><p>It has <ins>no <code class="highlight"><c- n>Container</c-></code> template parameter, and it has</ins> an <code class="highlight"><c- n>Allocator</c-></code> template parameter<ins>,</ins> and a type that does not qualify as an allocator is deduced for that parameter.</p> </li><li data-md><p>It has both <code class="highlight"><c- n>Container</c-></code> and <code class="highlight"><c- n>Allocator</c-></code> template parameters, and <code class="highlight"><c- n>uses_allocator_v</c-><c- o>&lt;</c-><c- n>Container</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-></code> is <code class="highlight">false</code>.</p> </li></ul> </small></blockquote>
   <p class="note" role="note"><span>Note:</span> The rationale is that if it does have a <code class="highlight"><c- n>Container</c-></code> parameter, and the <code class="highlight"><c- n>Container</c-></code> advertises
that it can "use" the supplied allocator, we shouldn’t interrogate the allocator parameter any more
closely. Trust the <code class="highlight"><c- n>Container</c-></code>'s specialization of <code class="highlight"><c- n>uses_allocator</c-></code>.</p>
   <p class="note" role="note"><span>Note:</span> As of C++20, no container-adaptor deduction guide has an <code class="highlight"><c- n>Allocator</c-></code> template parameter without
also having a <code class="highlight"><c- n>Container</c-></code> template parameter. But, as the resolution of <a data-link-type="biblio" href="#biblio-lwg3506">[LWG3506]</a> will add such deduction guides,
LWG asked that we future-proof this wording.</p>
   <p class="note" role="note"><span>Note:</span> <a data-link-type="biblio" href="#biblio-p1425r3">[P1425R3]</a> also proposes new container-adaptor deduction guides which have an <code class="highlight"><c- n>Allocator</c-></code> parameter
without a <code class="highlight"><c- n>Container</c-></code> parameter. (These guides were proposed in P1425R1, dropped in P1425R2, and restored
in P1425R3.)</p>
   <h2 class="heading settled" data-level="3" id="overconst2"><span class="secno">3. </span><span class="content">Stop deducing from allocators that should not participate in deduction</span><a class="self-link" href="#overconst2"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="problem2"><span class="secno">3.1. </span><span class="content">The problem</span><a class="self-link" href="#problem2"></a></h3>
   <p>Consider the following code:</p>
<pre class="language-c++ highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>monotonic_buffer_resource</c-> <c- n>mr</c-><c- p>;</c->
<c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>polymorphic_allocator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>a</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>mr</c-><c- p>;</c->
<c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>pv</c-><c- p>(</c-><c- n>a</c-><c- p>);</c->

<c- k>auto</c-> <c- n>v1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>polymorphic_allocator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-><c- p>(</c-><c- n>pv</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>polymorphic_allocator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- n>a</c-><c- p>);</c->
<c- k>auto</c-> <c- n>v3</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>polymorphic_allocator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>mr</c-><c- p>);</c->

<c- k>auto</c-> <c- n>dv1</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>(</c-><c- n>pv</c-><c- p>);</c->
<c- k>auto</c-> <c- n>dv2</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- n>a</c-><c- p>);</c->
<c- k>auto</c-> <c- n>dv3</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- p>(</c-><c- n>pv</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>mr</c-><c- p>);</c->
</pre>
   The initializers of <code class="highlight"><c- n>v1</c-></code>, <code class="highlight"><c- n>v2</c-></code>, and <code class="highlight"><c- n>v3</c-></code> (which do not use CTAD) are all well-formed,
as are the initializers of <code class="highlight"><c- n>dv1</c-></code> and <code class="highlight"><c- n>dv2</c-></code> (which do). 
   <p>But the initializer of <code class="highlight"><c- n>dv3</c-></code> is ill-formed!</p>
   <p>Again, we know from the <code class="highlight"><c- n>pv</c-></code> argument that the correct type for the vector is <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>pv</c-><c- p>)</c-></code>, i.e. <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>. Therefore we
know what is the possible range of types for the allocator parameter. The allocator parameter’s type does not <em>contribute</em> to class template argument deduction in these cases; it brings no new information, and therefore should be treated no differently
than it is in the non-CTAD case.</p>
   <p>The problem we see with <code class="highlight"><c- n>vector</c-></code> also occurs for all other sequence containers, associative containers, and unordered associative containers.</p>
   <h3 class="heading settled" data-level="3.2" id="explanation2"><span class="secno">3.2. </span><span class="content">The explanation</span><a class="self-link" href="#explanation2"></a></h3>
   <p><code class="highlight"><c- n>vector</c-></code> has only one deduction guide, and it’s not relevant to what happens here. Here, we end up
looking at the implicit guide generated from the constructor</p>
<pre class="language-c++ highlight">    <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- p>,</c-> <c- n>class</c-> <c- n>Allocator</c-><c- o>></c->
    <c- n>class</c-> <c- n>vector</c-> <c- p>{</c->
        <c- n>vector</c-><c- p>(</c-><c- k>const</c-> <c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
    <c- p>};</c->
</pre>
   <p>From the first parameter, we deduce <code class="highlight"><c- n>T</c-><c- o>=</c-><c- b>int</c-></code> and <code class="highlight"><c- n>Allocator</c-><c- o>=</c-><c- n>std</c-><c- o>::</c-><c- n>pmr</c-><c- o>::</c-><c- n>polymorphic_allocator</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>.
From the second parameter, we deduce <code class="highlight"><c- n>Allocator</c-><c- o>=</c-><c- n>std</c-><c- o>::</c-><c- n>monotonic_buffer_resource</c-><c- o>*</c-></code>.
We’ve deduced conflicting types for <code class="highlight"><c- n>Allocator</c-></code>, so deduction fails.</p>
   <p>In this case, the second argument unnecessarily participates in deduction, and again unexpectedly
prevents natural and useful code from working as desired.</p>
   <p>Observe that even in the absence of CTAD, the signature above
permits construction only from the same specialization of <code class="highlight"><c- n>vector</c-></code>,
not from any arbitrary specialization of <code class="highlight"><c- n>vector</c-></code>. For example:</p>
<pre class="language-c++ highlight">    <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>A1</c-><c- o>></c-> <c- n>v1</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>A2</c-><c- o>></c-> <c- n>v2</c-><c- p>(</c-><c- n>v1</c-><c- p>,</c-> <c- n>A2</c-><c- p>());</c->  <c- c1>// ill-formed</c->
</pre>
   <h3 class="heading settled" data-level="3.3" id="solution2"><span class="secno">3.3. </span><span class="content">Proposed wording</span><a class="self-link" href="#solution2"></a></h3>
   <p>If we were introducing the containers from scratch today, the natural approach would be for the
constructors to indicate which arguments should be considered deducible.
LWG has indicated their satisfaction with this approach, so that’s what we propose here.</p>
   <p>Modify <a href="http://eel.is/c++draft/deque.overview"> [deque.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- n>deque</c-><c- p>(</c-><c- n>deque</c-><c- o>&amp;&amp;</c-><c- p>);</c->
<del><c- n>deque</c-><c- p>(</c-><c- k>const</c-> <c- n>deque</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>deque</c-><c- p>(</c-><c- k>const</c-> <c- n>deque</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>deque</c-><c- p>(</c-><c- n>deque</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>deque</c-><c- p>(</c-><c- n>deque</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>deque</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/forwardlist.overview"> [forwardlist.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- n>forward_list</c-><c- p>(</c-><c- n>forward_list</c-><c- o>&amp;&amp;</c-><c- p>);</c->
<del><c- n>forward_list</c-><c- p>(</c-><c- k>const</c-> <c- n>forward_list</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>forward_list</c-><c- p>(</c-><c- k>const</c-> <c- n>forward_list</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>forward_list</c-><c- p>(</c-><c- n>forward_list</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>forward_list</c-><c- p>(</c-><c- n>forward_list</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>forward_list</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/list.overview"> [list.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- n>list</c-><c- p>(</c-><c- n>list</c-><c- o>&amp;&amp;</c-><c- p>);</c->
<del><c- n>list</c-><c- p>(</c-><c- k>const</c-> <c- n>list</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>list</c-><c- p>(</c-><c- k>const</c-> <c- n>list</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>list</c-><c- p>(</c-><c- n>list</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>list</c-><c- p>(</c-><c- n>list</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>list</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/vector.overview"> [vector.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>vector</c-><c- o>&amp;&amp;</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<del><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- k>const</c-> <c- n>vector</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- k>const</c-> <c- n>vector</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>vector</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>vector</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/vector.bool"> [vector.bool</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>vector</c-><c- o>&amp;&amp;</c-> <c- n>x</c-><c- p>);</c->
<del><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- k>const</c-> <c- n>vector</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- k>const</c-> <c- n>vector</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>vector</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>vector</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- k>constexpr</c-> <c- nf>vector</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/map.overview"> [map.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>map</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>map</c-><c- p>(</c-><c- k>const</c-> <c- n>map</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>map</c-><c- p>(</c-><c- k>const</c-> <c- n>map</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>map</c-><c- p>(</c-><c- n>map</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>map</c-><c- p>(</c-><c- n>map</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>map</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-><c- p>,</c->
  <c- k>const</c-> <c- n>Compare</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Compare</c-><c- p>,</c->
  <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/multimap.overview"> [multimap.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>multimap</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>multimap</c-><c- p>(</c-><c- k>const</c-> <c- n>multimap</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>multimap</c-><c- p>(</c-><c- k>const</c-> <c- n>multimap</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>multimap</c-><c- p>(</c-><c- n>multimap</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>multimap</c-><c- p>(</c-><c- n>multimap</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>multimap</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-><c- p>,</c->
  <c- k>const</c-> <c- n>Compare</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Compare</c-><c- p>,</c->
  <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/set.overview"> [set.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>set</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>set</c-><c- p>(</c-><c- k>const</c-> <c- n>set</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>set</c-><c- p>(</c-><c- k>const</c-> <c- n>set</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>set</c-><c- p>(</c-><c- n>set</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>set</c-><c- p>(</c-><c- n>set</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>set</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Compare</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Compare</c-><c- p>(),</c->
    <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/multiset.overview"> [multiset.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>multiset</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>multiset</c-><c- p>(</c-><c- k>const</c-> <c- n>multiset</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>multiset</c-><c- p>(</c-><c- k>const</c-> <c- n>multiset</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>multiset</c-><c- p>(</c-><c- n>multiset</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>multiset</c-><c- p>(</c-><c- n>multiset</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>multiset</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Compare</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Compare</c-><c- p>(),</c->
    <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/unord.map.overview"> [unord.map.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>unordered_map</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>unordered_map</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_map</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_map</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_map</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>unordered_map</c-><c- p>(</c-><c- n>unordered_map</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_map</c-><c- p>(</c-><c- n>unordered_map</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>unordered_map</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-> <c- n>il</c-><c- p>,</c->
              <c- n>size_type</c-> <c- n>n</c-> <c- o>=</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- p>,</c->
              <c- k>const</c-> <c- n>hasher</c-><c- o>&amp;</c-> <c- n>hf</c-> <c- o>=</c-> <c- n>hasher</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>key_equal</c-><c- o>&amp;</c-> <c- n>eql</c-> <c- o>=</c-> <c- n>key_equal</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>allocator_type</c-><c- o>&amp;</c-> <c- n>a</c-> <c- o>=</c-> <c- n>allocator_type</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/unord.multimap.overview"> [unord.multimap.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>unordered_multimap</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>unordered_multimap</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_multimap</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_multimap</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_multimap</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>unordered_multimap</c-><c- p>(</c-><c- n>unordered_multimap</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_multimap</c-><c- p>(</c-><c- n>unordered_multimap</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>unordered_multimap</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-> <c- n>il</c-><c- p>,</c->
              <c- n>size_type</c-> <c- n>n</c-> <c- o>=</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- p>,</c->
              <c- k>const</c-> <c- n>hasher</c-><c- o>&amp;</c-> <c- n>hf</c-> <c- o>=</c-> <c- n>hasher</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>key_equal</c-><c- o>&amp;</c-> <c- n>eql</c-> <c- o>=</c-> <c- n>key_equal</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>allocator_type</c-><c- o>&amp;</c-> <c- n>a</c-> <c- o>=</c-> <c- n>allocator_type</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/unord.set.overview"> [unord.set.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>unordered_set</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>unordered_set</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_set</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_set</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_set</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>unordered_set</c-><c- p>(</c-><c- n>unordered_set</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_set</c-><c- p>(</c-><c- n>unordered_set</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>unordered_set</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-> <c- n>il</c-><c- p>,</c->
              <c- n>size_type</c-> <c- n>n</c-> <c- o>=</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- p>,</c->
              <c- k>const</c-> <c- n>hasher</c-><c- o>&amp;</c-> <c- n>hf</c-> <c- o>=</c-> <c- n>hasher</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>key_equal</c-><c- o>&amp;</c-> <c- n>eql</c-> <c- o>=</c-> <c- n>key_equal</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>allocator_type</c-><c- o>&amp;</c-> <c- n>a</c-> <c- o>=</c-> <c- n>allocator_type</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <p>Modify <a href="http://eel.is/c++draft/unord.multiset.overview"> [unord.multiset.overview</a>] as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <pre class="highlight"><c- k>explicit</c-> <c- nf>unordered_multiset</c-><c- p>(</c-><c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c->
<del><c- n>unordered_multiset</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_multiset</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_multiset</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_multiset</c-><c- o>&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<del><c- n>unordered_multiset</c-><c- p>(</c-><c- n>unordered_multiset</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>);</c-></del>
<ins><c- n>unordered_multiset</c-><c- p>(</c-><c- n>unordered_multiset</c-><c- o>&amp;&amp;</c-><c- p>,</c-> <c- k>const</c-> <c- n>type_identity_t</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>&amp;</c-><c- p>);</c-></ins>
<c- n>unordered_multiset</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-> <c- n>il</c-><c- p>,</c->
              <c- n>size_type</c-> <c- n>n</c-> <c- o>=</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- p>,</c->
              <c- k>const</c-> <c- n>hasher</c-><c- o>&amp;</c-> <c- n>hf</c-> <c- o>=</c-> <c- n>hasher</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>key_equal</c-><c- o>&amp;</c-> <c- n>eql</c-> <c- o>=</c-> <c- n>key_equal</c-><c- p>(),</c->
              <c- k>const</c-> <c- n>allocator_type</c-><c- o>&amp;</c-> <c- n>a</c-> <c- o>=</c-> <c- n>allocator_type</c-><c- p>());</c->
</pre></small>
   </blockquote>
   <h3 class="heading settled" data-level="3.4" id="implementation"><span class="secno">3.4. </span><span class="content">Implementation note</span><a class="self-link" href="#implementation"></a></h3>
    Some of the changes proposed in this paper have already been implemented "accidentally" by
one or more library implementations,
as shown in the following table and <a href="https://godbolt.org/z/5oKKGd">this Godbolt</a>: 
   <p><small></small></p>
   <small> <table class="def"> <tbody><tr><th>Construct</th><th>Well-formed</th><th>SFINAE-friendly<br> ill-formed</th><th>Hard error</th></tr> <tr><td>std::deque(pd, &amp;mr)</td> <td></td> <td>MSVC, libstdc++, libc++</td> <td></td> </tr><tr><td>std::forward_list(pfl, &amp;mr)</td> <td></td> <td>MSVC, libstdc++, libc++</td> <td></td> </tr><tr><td>std::list(pl, &amp;mr)</td> <td></td> <td>MSVC, libstdc++, libc++</td> <td></td> </tr><tr><td>std::vector(pv, &amp;mr)</td> <td></td> <td>MSVC, libstdc++, libc++</td> <td></td> </tr><tr><td>std::map(pm, &amp;mr)</td> <td>MSVC, libc++</td> <td>libstdc++</td> <td></td> </tr><tr><td>std::multimap(pmm, &amp;mr)</td> <td>MSVC, libc++</td> <td>libstdc++</td> <td></td> </tr><tr><td>std::multiset(pms, &amp;mr)</td> <td>MSVC, libc++</td> <td>libstdc++</td> <td></td> </tr><tr><td>std::set(ps, &amp;mr)</td> <td>MSVC, libc++</td> <td>libstdc++</td> <td></td> </tr><tr><td>std::unordered_map(pum, &amp;mr)</td> <td>MSVC, libstdc++, libc++</td> <td></td> <td></td> </tr><tr><td>std::unordered_multimap(pumm, &amp;mr)</td> <td>MSVC, libstdc++, libc++</td> <td></td> <td></td> </tr><tr><td>std::unordered_multiset(pums, &amp;mr)</td> <td>MSVC, libstdc++, libc++</td> <td></td> <td></td> </tr><tr><td>std::unordered_set(pus, &amp;mr)</td> <td>MSVC, libstdc++, libc++</td> <td></td> <td></td> </tr><tr></tr> <tr><td>std::priority_queue(ppq, &amp;mr)</td> <td>MSVC, libstdc++, libc++</td> <td></td> <td></td> </tr><tr><td>std::queue(pq, &amp;mr)</td> <td>MSVC, libstdc++, libc++</td> <td></td> <td></td> </tr><tr><td>std::stack(ps, &amp;mr)</td> <td>MSVC, libstdc++, libc++</td> <td></td> <td></td> </tr><tr></tr> <tr><td>std::priority_queue(comp, pv, &amp;mr)</td> <td></td> <td>MSVC, libstdc++, libc++</td> <td></td> </tr><tr><td>std::queue(pd, &amp;mr)</td> <td></td> <td>MSVC, libstdc++, libc++</td> <td></td> </tr><tr><td>std::stack(pv, &amp;mr)</td> <td></td> <td>MSVC, libstdc++, libc++</td> <td></td> </tr></tbody></table> </small> 
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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-lwg3506">[LWG3506]
   <dd>Tim Song. <a href="https://cplusplus.github.io/LWG/issue3506">LWG 3506: Missing allocator-extended constructors for priority_queue</a>. November 2020. URL: <a href="https://cplusplus.github.io/LWG/issue3506">https://cplusplus.github.io/LWG/issue3506</a>
   <dt id="biblio-p1425r3">[P1425R3]
   <dd>Corentin Jabot. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1425r3.pdf">P1425R3: Iterator-pair constructors for stack and queue</a>. February 2021. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1425r3.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1425r3.pdf</a>
  </dl>