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  <title>P1518R0: 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
 *
 * 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)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * 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
 *
 * 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)
 *
 ******************************************************************************/

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

	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;

		/* Colors */
		color: black;
		background: white 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-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		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;
		border-color: #c00;
	}

	/* 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: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

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

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* 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-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** 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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}
		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);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) 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);
		}

		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;
		background: transparent;
	}

	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;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

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

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		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;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; 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: medium;
	}
	dfn var {
		font-style: normal;
	}

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

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** 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;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	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;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

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

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

	img {
		border-style: none;
	}

	/* 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;
	}
	.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 {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

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

	blockquote {
		border-color: silver;
	}

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

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* 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;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* 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;
		background: #E9FBE9;
		overflow: auto;
	}

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

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

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

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

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

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** 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: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    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;
	}

	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-top-style: solid;
	}

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

	table.data  thead,
	table.index thead,
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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">P1518R0<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="2019-03-10">2019-03-10</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Authors:
     <dd>
      <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>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:mike_spertus@symantec.com">Mike Spertus</a>
     <dt>Audience:
     <dd>LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Current Source:
     <dd><a href="https://github.com/Quuxplusone/draft/blob/gh-pages/stack-ctad.bs">github.com/Quuxplusone/draft/blob/gh-pages/stack-ctad.bs</a>
     <dt>Current:
     <dd><a href="https://rawgit.com/Quuxplusone/draft/gh-pages/stack-ctad.html">rawgit.com/Quuxplusone/draft/gh-pages/stack-ctad.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="#overconst1"><span class="secno">1</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">1.1</span> <span class="content">The problem</span></a>
      <li><a href="#explanation1"><span class="secno">1.2</span> <span class="content">The explanation</span></a>
      <li><a href="#solution1"><span class="secno">1.3</span> <span class="content">The solution</span></a>
     </ol>
    <li>
     <a href="#overconst2"><span class="secno">2</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">2.1</span> <span class="content">The problem</span></a>
      <li><a href="#explanation2"><span class="secno">2.2</span> <span class="content">The explanation</span></a>
      <li>
       <a href="#solution2"><span class="secno">2.3</span> <span class="content">The solution</span></a>
       <ol class="toc">
        <li><a href="#option1"><span class="secno">2.3.1</span> <span class="content">Wording option 1</span></a>
        <li><a href="#option2"><span class="secno">2.3.2</span> <span class="content">Wording option 2</span></a>
       </ol>
      <li><a href="#implementation"><span class="secno">2.4</span> <span class="content">Implementation note</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="overconst1"><span class="secno">1. </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="1.1" id="problem1"><span class="secno">1.1. </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="1.2" id="explanation1"><span class="secno">1.2. </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- kr>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="1.3" id="solution1"><span class="secno">1.3. </span><span class="content">The solution</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><del>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.</del> </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 the <code class="highlight"><c- n>Container</c-></code> advertises that it can "use" the supplied allocator,
we shouldn’t interrogate it 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> 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.</p>
   <h2 class="heading settled" data-level="2" id="overconst2"><span class="secno">2. </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="2.1" id="problem2"><span class="secno">2.1. </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="2.2" id="explanation2"><span class="secno">2.2. </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>
   <h3 class="heading settled" data-level="2.3" id="solution2"><span class="secno">2.3. </span><span class="content">The solution</span><a class="self-link" href="#solution2"></a></h3>
   <p>We present two approaches for changing the wording that are equivalent in effect.</p>
   <h4 class="heading settled" data-level="2.3.1" id="option1"><span class="secno">2.3.1. </span><span class="content">Wording option 1</span><a class="self-link" href="#option1"></a></h4>
   <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, as is usual for function templates.
For example, 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>(Notice that the constructor <code class="highlight"><c- n>deque</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&amp;</c-><c- n>lt</c-><c- p>;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>const</c-> <c- n>Allocator</c-><c- o>&amp;</c-><c- p>)</c-></code> is not
changed, because its <code class="highlight"><c- n>Allocator</c-></code> parameter actually does contribute to deduction: without
that parameter’s type, we would not know what the <code class="highlight"><c- n>deque</c-></code>'s allocator type should be.)</p>
   <p>If this is not deemed to cause compatibility issues, we like this approach.</p>
   <h4 class="heading settled" data-level="2.3.2" id="option2"><span class="secno">2.3.2. </span><span class="content">Wording option 2</span><a class="self-link" href="#option2"></a></h4>
   <p>First modify <a href="http://eel.is/c++draft/sequence.reqmts#13"> [sequence.reqmts</a>]/13 as follows:</p>
   <p><small></small></p>
   <blockquote><small> A deduction guide for a sequence container shall not participate in overload resolution
if 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, or if 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<ins>,
or if it has an <code class="highlight"><c- n>Alloc</c-></code> template parameter and <code class="highlight"><c- n>uses_allocator_v</c-><c- o>&lt;</c-><c- n>X</c-><c- p>,</c-> <c- n>Alloc</c-><c- o>></c-></code> is <code class="highlight">false</code> where <code class="highlight"><c- n>X</c-></code> is the deduced type of the sequence container</ins>.<p></p> </small></blockquote>
   <p>(We think that change is not strictly necessary, but it clarifies our intent.)</p>
   <p>Add one new deduction guide for each sequence container. For example, 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- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>>></c->
    <c- n>deque</c-><c- p>(</c-><c- n>InputIterator</c-><c- p>,</c-> <c- n>InputIterator</c-><c- p>,</c-> <c- n>Allocator</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>())</c->
      <c- o>-></c-> <c- n>deque</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>></c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<ins><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Alloc</c-><c- o>></c->
    <c- n>deque</c-><c- p>(</c-><c- n>deque</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>,</c-> <c- n>Alloc</c-><c- p>)</c->
      <c- o>-></c-> <c- n>deque</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c-></ins>
</pre></small>
   </blockquote>
   <p>Then, modify <a href="http://eel.is/c++draft/associative.reqmts#15"> [associative.reqmts</a>]/15 as follows:</p>
   <p><small></small></p>
   <blockquote><small> A deduction guide for an associative container 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 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.</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><ins>It has an <code class="highlight"><c- n>Alloc</c-></code> template parameter and <code class="highlight"><c- n>uses_allocator_v</c-><c- o>&lt;</c-><c- n>X</c-><c- p>,</c-> <c- n>Alloc</c-><c- o>></c-></code> is <code class="highlight">false</code> where <code class="highlight"><c- n>X</c-></code> is the deduced type of the associative container</ins>. </li></ul> </small></blockquote>
   <p>(We think that change is not strictly necessary, but it clarifies our intent.)</p>
   <p>Add one new deduction guide for each associative container. For example, 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>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c->
         <c- k>class</c-> <c- nc>Compare</c-> <c- o>=</c-> <c- n>less</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>></c-><c- p>,</c->
         <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>>></c->
  <c- n>set</c-><c- p>(</c-><c- n>InputIterator</c-><c- p>,</c-> <c- n>InputIterator</c-><c- p>,</c->
      <c- n>Compare</c-> <c- o>=</c-> <c- n>Compare</c-><c- p>(),</c-> <c- n>Allocator</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>())</c->
    <c- o>-></c-> <c- n>set</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>></c-><c- p>,</c-> <c- n>Compare</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Key</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Compare</c-> <c- o>=</c-> <c- n>less</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>>></c->
  <c- n>set</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c-> <c- n>Compare</c-> <c- o>=</c-> <c- n>Compare</c-><c- p>(),</c-> <c- n>Allocator</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>())</c->
    <c- o>-></c-> <c- n>set</c-><c- o>&lt;</c-><c- n>Key</c-><c- p>,</c-> <c- n>Compare</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- o>></c->
  <c- n>set</c-><c- p>(</c-><c- n>InputIterator</c-><c- p>,</c-> <c- n>InputIterator</c-><c- p>,</c-> <c- n>Allocator</c-><c- p>)</c->
    <c- o>-></c-> <c- n>set</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>></c-><c- p>,</c->
           <c- n>less</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>></c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Key</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- o>></c->
  <c- n>set</c-><c- p>(</c-><c- n>initializer_list</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c-> <c- n>Allocator</c-><c- p>)</c-> <c- o>-></c-> <c- n>set</c-><c- o>&lt;</c-><c- n>Key</c-><c- p>,</c-> <c- n>less</c-><c- o>&lt;</c-><c- n>Key</c-><c- o>></c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<ins><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>Key</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Compare</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Alloc</c-><c- o>></c->
  <c- n>set</c-><c- p>(</c-><c- n>set</c-><c- o>&lt;</c-><c- n>Key</c-><c- p>,</c-> <c- n>Compare</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>,</c-> <c- n>Alloc</c-><c- p>)</c-> <c- o>-></c-> <c- n>set</c-><c- o>&lt;</c-><c- n>Key</c-><c- p>,</c-> <c- n>Compare</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c-></ins>
</pre></small>
   </blockquote>
   <p>Then, modify <a href="http://eel.is/c++draft/unord.req#18"> [unord.req</a>]/18 as follows:</p>
   <p><small></small></p>
   <blockquote><small> A deduction guide for an unordered associative container 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 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.</p> </li><li data-md><p>It has a <code class="highlight"><c- n>Hash</c-></code> template parameter and an integral type or 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>Pred</c-></code> template parameter and a type that qualifies as an allocator is
    deduced for that parameter.</p> </li><li data-md><ins>It has an <code class="highlight"><c- n>Alloc</c-></code> template parameter and <code class="highlight"><c- n>uses_allocator_v</c-><c- o>&lt;</c-><c- n>X</c-><c- p>,</c-> <c- n>Alloc</c-><c- o>></c-></code> is <code class="highlight">false</code> where <code class="highlight"><c- n>X</c-></code> is the deduced type of the associative container</ins>. </li></ul> </small></blockquote>
   <p>(We think that change is not strictly necessary, but it clarifies our intent.)</p>
   <p>Add one new deduction guide for each unordered associative container. For example, 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>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c->
         <c- k>class</c-> <c- nc>Hash</c-> <c- o>=</c-> <c- n>hash</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>></c-><c- p>,</c->
         <c- k>class</c-> <c- nc>Pred</c-> <c- o>=</c-> <c- n>equal_to</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>></c-><c- p>,</c->
         <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>>></c->
  <c- n>unordered_set</c-><c- p>(</c-><c- n>InputIterator</c-><c- p>,</c-> <c- n>InputIterator</c-><c- p>,</c-> <c- k>typename</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- o>::</c-><c- n>size_type</c-> <c- o>=</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- p>,</c->
                <c- n>Hash</c-> <c- o>=</c-> <c- n>Hash</c-><c- p>(),</c-> <c- n>Pred</c-> <c- o>=</c-> <c- n>Pred</c-><c- p>(),</c-> <c- n>Allocator</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>())</c->
    <c- o>-></c-> <c- n>unordered_set</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>></c-><c- p>,</c->
                     <c- n>Hash</c-><c- p>,</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Hash</c-> <c- o>=</c-> <c- n>hash</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c->
         <c- k>class</c-> <c- nc>Pred</c-> <c- o>=</c-> <c- n>equal_to</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-> <c- o>=</c-> <c- n>allocator</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>></c->
  <c- n>unordered_set</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>typename</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- o>::</c-><c- n>size_type</c-> <c- o>=</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- p>,</c->
                <c- n>Hash</c-> <c- o>=</c-> <c- n>Hash</c-><c- p>(),</c-> <c- n>Pred</c-> <c- o>=</c-> <c- n>Pred</c-><c- p>(),</c-> <c- n>Allocator</c-> <c- o>=</c-> <c- n>Allocator</c-><c- p>())</c->
    <c- o>-></c-> <c- n>unordered_set</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- o>></c->
  <c- n>unordered_set</c-><c- p>(</c-><c- n>InputIterator</c-><c- p>,</c-> <c- n>InputIterator</c-><c- p>,</c-> <c- k>typename</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- o>::</c-><c- n>size_type</c-><c- p>,</c-> <c- n>Allocator</c-><c- p>)</c->
    <c- o>-></c-> <c- n>unordered_set</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>></c-><c- p>,</c->
                     <c- n>hash</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>></c-><c- p>,</c->
                     <c- n>equal_to</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>></c-><c- p>,</c->
                     <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>InputIterator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Hash</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- o>></c->
  <c- n>unordered_set</c-><c- p>(</c-><c- n>InputIterator</c-><c- p>,</c-> <c- n>InputIterator</c-><c- p>,</c-> <c- k>typename</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- o>::</c-><c- n>size_type</c-><c- p>,</c->
                <c- n>Hash</c-><c- p>,</c-> <c- n>Allocator</c-><c- p>)</c->
    <c- o>-></c-> <c- n>unordered_set</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>></c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c->
                     <c- n>equal_to</c-><c- o>&lt;</c-><i><c- n>iter</c-><c- o>-</c-><c- n>value</c-><c- o>-</c-><c- n>type</c-></i><c- o>&lt;</c-><c- n>InputIterator</c-><c- o>>></c-><c- p>,</c->
                     <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- o>></c->
  <c- n>unordered_set</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>typename</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- o>::</c-><c- n>size_type</c-><c- p>,</c-> <c- n>Allocator</c-><c- p>)</c->
    <c- o>-></c-> <c- n>unordered_set</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>hash</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>equal_to</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Hash</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- o>></c->
  <c- n>unordered_set</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>typename</c-> <i><c- n>see</c-> <c- n>below</c-></i><c- o>::</c-><c- n>size_type</c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>Allocator</c-><c- p>)</c->
    <c- o>-></c-> <c- n>unordered_set</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>equal_to</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c->

<ins><c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Hash</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Pred</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Allocator</c-><c- p>,</c-> <c- k>class</c-> <c- nc>Alloc</c-><c- o>></c->
  <c- n>unordered_set</c-><c- p>(</c-><c- n>set</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>,</c-> <c- n>Alloc</c-><c- p>)</c->
    <c- o>-></c-> <c- n>unordered_set</c-><c- o>&lt;</c-><c- n>T</c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>></c-><c- p>;</c-></ins>
</pre></small>
   </blockquote>
   <h3 class="heading settled" data-level="2.4" id="implementation"><span class="secno">2.4. </span><span class="content">Implementation note</span><a class="self-link" href="#implementation"></a></h3>
    While implementers may of course implement this proposal exactly as described in the wording options above,
we wish to point out that a number of library implementations already behave in many cases as we are
proposing here as shown in the following table and <a href="https://godbolt.org/z/1QepHG">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</td> <td>libstdc++, libc++</td> <td></td> </tr><tr><td>std::multimap(pmm, &amp;mr)</td> <td>MSVC</td> <td>libstdc++, libc++</td> <td></td> </tr><tr><td>std::multiset(pms, &amp;mr)</td> <td>MSVC</td> <td>libstdc++, libc++</td> <td></td> </tr><tr><td>std::set(ps, &amp;mr)</td> <td>MSVC</td> <td>libstdc++, libc++</td> <td></td> </tr><tr><td>std::unordered_map(pum, &amp;mr)</td> <td>MSVC, libstdc++</td> <td>libc++</td> <td></td> </tr><tr><td>std::unordered_multimap(pumm, &amp;mr)</td> <td>MSVC, libstdc++</td> <td>libc++</td> <td></td> </tr><tr><td>std::unordered_multiset(pums, &amp;mr)</td> <td>MSVC, libstdc++</td> <td>libc++</td> <td></td> </tr><tr><td>std::unordered_set(pus, &amp;mr)</td> <td>MSVC, libstdc++</td> <td>libc++</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(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> 
   <p>The following analysis explains why MSVC, in effect, already implements
the approach described in <a href="#option1">wording option 1</a> above.
Consider 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>Key</c-><c- p>,</c-> <c- n>class</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>class</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>class</c-> <c- n>Allocator</c-><c- o>></c->
    <c- n>class</c-> <c- n>unordered_set</c-> <c- p>{</c->
        <c- n>unordered_set</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_set</c-><c- o>&lt;</c-><c- n>Key</c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>Pred</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>Key</c-><c- o>=</c-><c- b>int</c-></code>, <code class="highlight"><c- n>Hash</c-><c- o>=</c-><c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>, <code class="highlight"><c- n>Pred</c-><c- o>=</c-><c- n>std</c-><c- o>::</c-><c- n>equal_to</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></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 should 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>However, in practice, both MSVC and libstdc++ launder the <code class="highlight"><c- n>Allocator</c-></code> type through a typedef
that is opaque to deduction, so
that the constructor seen by the compiler in practice is equivalent to</p>
<pre class="language-c++ highlight">    <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>Key</c-><c- p>,</c-> <c- n>class</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>class</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>class</c-> <c- n>Allocator</c-><c- o>></c->
    <c- n>class</c-> <c- n>unordered_set</c-> <c- p>{</c->
        <c- n>using</c-> <c- n>allocator_type</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>type_identity</c-><c- o>&lt;</c-><c- n>Allocator</c-><c- o>>::</c-><c- n>type</c-><c- p>;</c->
        <c- n>unordered_set</c-><c- p>(</c-><c- k>const</c-> <c- n>unordered_set</c-><c- o>&lt;</c-><c- n>Key</c-><c- p>,</c-> <c- n>Hash</c-><c- p>,</c-> <c- n>Pred</c-><c- p>,</c-> <c- n>Allocator</c-><c- o>>&amp;</c-><c- p>,</c->
                      <c- k>const</c-> <c- n>allocator_type</c-><c- o>&amp;</c-><c- p>);</c->
    <c- p>};</c->
</pre>
   <p>From the first parameter, we deduce <code class="highlight"><c- n>Key</c-><c- o>=</c-><c- b>int</c-></code>, <code class="highlight"><c- n>Hash</c-><c- o>=</c-><c- n>std</c-><c- o>::</c-><c- n>hash</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-></code>, <code class="highlight"><c- n>Pred</c-><c- o>=</c-><c- n>std</c-><c- o>::</c-><c- n>equal_to</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></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 don’t deduce anything.
In practice, therefore, deduction succeeds.</p>
   <p>Implementers who have taken the above approach can use that to simplify their
implementation of this proposal, as an alternative to directly implementing the above wording
(which is of course also perfectly fine).</p>
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