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  <title>p0945r0: Generalizing alias declarations</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,
	table.data  tbody,
	table.index tbody {
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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">p0945r0<br>Generalizing alias declarations</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-02-10">10 February 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p0945r0">http://wg21.link/p0945r0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:richard@metafoo.co.uk">Richard Smith</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper proposes extending <em>alias-declaration</em>s to allow the formation of aliases for all names, not only type names.</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="#problem"><span class="secno">1</span> <span class="content">Problem</span></a>
    <li><a href="#design-principles"><span class="secno">2</span> <span class="content">Design principles</span></a>
    <li>
     <a href="#proposal"><span class="secno">3</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#member-aliases"><span class="secno">3.1</span> <span class="content">Member aliases</span></a>
      <li><a href="#templated-aliases"><span class="secno">3.2</span> <span class="content">Templated aliases</span></a>
     </ol>
    <li><a href="#interactions-with-other-proposals"><span class="secno">4</span> <span class="content">Interactions with other proposals</span></a>
    <li><a href="#acknowledgements"><span class="secno">5</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="problem"><span class="secno">1. </span><span class="content">Problem</span><a class="self-link" href="#problem"></a></h2>
   <p>Large-scale interface refactorings often follow a three-step process:</p>
   <ul>
    <li data-md="">
     <p>Change the interface so both the old version and the new version are concurrently available</p>
    <li data-md="">
     <p>Change all users of the old version to use the new version (for externally-released libraries, marking the old version as <code class="highlight"><span class="p">[[</span><span class="n">deprecated</span><span class="p">]]</span></code> is a useful tool for this)</p>
    <li data-md="">
     <p>Remove the old version</p>
   </ul>
   <p>C++ provides some language features to help out with the first step of this process. <em>using-declaration</em>s provide a tool to allow a name to be exposed in two different namespaces. <em>alias-declaration</em>s and <code class="highlight"><span class="k">typedef</span></code> declarations provide a tool to allow a type to be exposed with two different names. Function overloading provides a tool to allow a function to present two different signatures. However, C++ lacks a general mechanism to allow an entity to be exposed with two different names.</p>
   <p>There are a set of workarounds for this problem, for renaming different kinds of entities:</p>
   <ul>
    <li data-md="">
     <p>A type can be renamed using an <em>alias-declaration</em>.</p>
    <li data-md="">
     <p>A function (or function template) can be renamed by providing a forwarding function. However, it is not possible to perfectly forward parameters passed by value, so this may introduce additional moves of function parameters. Also, the functions can be distinguished by address, although this seldom matters.</p>
    <li data-md="">
     <p>A variable (or variable template) can be renamed by providing a (<code class="highlight"><span class="k">constexpr</span> <span class="kr">inline</span></code>) reference variable: <code class="highlight"><span class="k">constexpr</span> <span class="kr">inline</span> <span class="k">auto</span> <span class="o">&amp;</span><span class="n">old_name</span> <span class="o">=</span> <span class="n">new_name</span><span class="p">;</span></code></p>
    <li data-md="">
     <p>A non-static data member cannot be renamed without occupying additional storage within the object (for instance, by adding a reference member bound to another member; this also prevents the structure from being trivially-copyable).</p>
    <li data-md="">
     <p>An enumerator can be renamed by using a <code class="highlight"><span class="k">constexpr</span> <span class="kr">inline</span></code> variable.</p>
    <li data-md="">
     <p>A namespace can be renamed by using a <em>namespace-alias</em> declaration: <code class="highlight"><span class="k">namespace</span> <span class="n">old_name</span> <span class="o">=</span> <span class="n">new_name</span><span class="p">;</span></code></p>
    <li data-md="">
     <p>A type template can be renamed by using an alias template (but the default arguments must be duplicated, and the alias is distinguishable from the original when used as a template template argument).</p>
    <li data-md="">
     <p>A concept can be renamed by defining another concept in terms of it: <code class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="n">concept</span> <span class="n">OldName</span> <span class="o">=</span> <span class="n">NewName</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="p">;</span></code></p>
   </ul>
   <p>However, aside from the special-case <em>alias-declaration</em> and <em>namespace-alias</em> syntaxes, none of these directly represents the intent of simply binding another name to an entity.</p>
   <h2 class="heading settled" data-level="2" id="design-principles"><span class="secno">2. </span><span class="content">Design principles</span><a class="self-link" href="#design-principles"></a></h2>
   <div class="note" role="note"> <em>Note:</em> These principles are copied from Herb’s <a data-link-type="biblio" href="#biblio-p0515r3">[P0515R3]</a> at the encouragement of that paper. Thanks, Herb! </div>
   <p>The primary design goal is <a href="https://en.wikipedia.org/wiki/The_Mythical_Man-Month">conceptual integrity</a>, which means that the design is coherent and reliably does what the user expects it to do. Conceptual integrity’s major supporting principles are:</p>
   <ul>
    <li data-md="">
     <p><em>Be consistent:</em> Don’t make similar things different, including in spelling, behavior, or capability. Don’t make different things appear similar when they have different behavior or capability. -- For example, this paper follows this principle in using the exact same syntax to imbue an entity with a new name, regardless of the kind of entity, and semantic differences between the newly-proposed feature and the pre-existing <em>using-declaration</em> are removed.</p>
    <li data-md="">
     <p><em>Be orthogonal:</em> Avoid arbitrary coupling. Let features be used freely in combination. -- For example, this paper permits templated <em>alias-declaration</em>s.</p>
    <li data-md="">
     <p><em>Be general:</em> Don’t restrict what is inherent. Don’t arbitrarily restrict a complete set of uses. Avoid special cases and partial features. -- For example, this paper permits arbitrary renaming between members and non-members, except in cases where the result would be meaningless or problematic (such as a namespace-scope class member, or a class-scope namespace name).</p>
   </ul>
   <h2 class="heading settled" data-level="3" id="proposal"><span class="secno">3. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>This paper proposes to extend <em>alias-declaration</em>s to permit aliasing any entity. This is not a novel idea; the original paper proposing alias templates (<a data-link-type="biblio" href="#biblio-n1449">[N1449]</a>) described this as an extension of the base functionality, and indeed "[the] motivation for using any keyword at all [for <em>alias-declarations</em>] stemmed partly from the desire to use a syntax that might be compatible with [aliasing more general entities]".</p>
   <p>The specific syntax proposed is:</p>
   <p><em>alias-declaration:</em><br>         <code class="highlight"><span class="k">using</span></code> <em>identifier</em> <em>attribute-specifier-seq</em><sub><em>opt</em></sub> <code class="highlight"><span class="o">=</span></code> <em>id-expression</em> <code class="highlight"><span class="p">;</span></code></p>
   <p>The <em>identifier</em> is declared as a name for the entity or overload set named by the <em>id-expression</em>. The program is ill-formed if this results in a conflicting meaning for the name within its declarative region, but this syntax may be used to redeclare a name to refer to the same entity to which it already referred.</p>
   <p>The resulting declared identifier is declared as the same kind of name as the <em>id-expression</em>: it is a <em>class-name</em> if the <em>id-expression</em> was a <em>class-name</em>, is a <em>namespace-name</em> if the <em>id-expression</em> was a <em>namespace-name</em>, is a <em>typedef-name</em> if the <em>id-expression</em> was a <em>typedef-name</em>, and so on.</p>
   <p>As with the existing <em>alias-declaration</em>, this new form is permitted at namespace scope, block scope, and within class definitions. An <em>alias-declaration</em> that names a non-static data member or non-static member function may only appear as a <em>member-declaration</em> of the same class or one of its derived classes. An <em>alias-declaration</em> that names a namespace may not appear at class scope. No other restrictions are proposed on the kind of entity that may be named by the <em>id-expression</em>.</p>
   <p>For consistency, we propose removing those restrictions from <em>using-declaration</em>s that are not present for this form of <em>alias-declaration</em>:</p>
   <ul>
    <li data-md="">
     <p>A class-scope <em>using-declaration</em> may name a non-class member</p>
    <li data-md="">
     <p>A non-class-scope <em>using-declaration</em> may name a class member other than a non-static data member or non-static member function</p>
    <li data-md="">
     <p>A <em>using-declaration</em> may name a namespace or a scoped enumerator</p>
   </ul>
   <p>The general principle is to make the following declarations equivalent, so the former is merely a shorthand for the latter:</p>
<pre class="highlight"><span class="k">using</span> <span class="n">A</span><span class="o">::</span><span class="n">B</span><span class="p">;</span>
<span class="k">using</span> <span class="n">B</span> <span class="o">=</span> <span class="n">A</span><span class="o">::</span><span class="n">B</span><span class="p">;</span>
</pre>
   <p>We further propose deprecating <em>namespace-alias</em> declarations, as the more general <em>alias-declaration</em> syntax can be used in its stead.</p>
   <h3 class="heading settled" data-level="3.1" id="member-aliases"><span class="secno">3.1. </span><span class="content">Member aliases</span><a class="self-link" href="#member-aliases"></a></h3>
   <p>When an <em>alias-declaration</em> or <em>using-declaration</em> at class scope names a non-member, that non-member may be accessed by explicit qualification of the class name (<code class="highlight"><span class="n">Class</span><span class="o">::</span><span class="n">Member</span></code>). If the name does not name a type nor a type template, class member access syntax may also be used (<code class="highlight"><span class="n">instance</span><span class="p">.</span><span class="n">member</span></code>). Non-member functions and variables brought into a class by such a declaration behave analogously to static class members:</p>
<pre class="highlight"><span class="kt">void</span> <span class="nf">f</span><span class="p">(</span><span class="kt">int</span> <span class="n">a</span><span class="p">,</span> <span class="kt">int</span> <span class="n">b</span><span class="p">);</span>
<span class="k">struct</span> <span class="n">Y</span> <span class="p">{};</span>
<span class="kt">void</span> <span class="nf">g</span><span class="p">(</span><span class="n">Y</span><span class="p">);</span>

<span class="k">struct</span> <span class="n">X</span> <span class="p">{</span>
  <span class="k">using</span> <span class="n">g</span> <span class="o">=</span> <span class="n">f</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">h</span><span class="p">(</span><span class="n">Y</span> <span class="n">y</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">g</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">);</span>  <span class="c1">// OK, calls f(1, 2)</span>
<span class="c1"></span>    <span class="n">g</span><span class="p">(</span><span class="n">y</span><span class="p">);</span>     <span class="c1">// error, lookup finds X::g (== ::f)</span>
<span class="c1"></span>  <span class="p">}</span>
<span class="p">};</span>

<span class="kt">void</span> <span class="nf">h</span><span class="p">(</span><span class="n">X</span> <span class="n">x</span><span class="p">)</span> <span class="p">{</span>
  <span class="n">x</span><span class="p">.</span><span class="n">g</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">);</span>  <span class="c1">// OK, same as X::g(1, 2), same as ::f(1, 2)</span>
<span class="c1"></span><span class="p">}</span>
</pre>
   <p>Member <em>alias-declaration</em>s or member <em>using-declaration</em>s introduce member names of the enclosing class, but do not make the nominated entities members of the class. In particular, the nominated entities have no special access to non-public members of the class.</p>
   <p>Member <em>alias-declaration</em>s can be used to provide aliases for non-static data members and non-static member functions. For example:</p>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">K</span><span class="p">,</span> <span class="k">typename</span> <span class="n">V</span><span class="o">></span>
<span class="k">struct</span> <span class="n">map</span><span class="o">&lt;</span><span class="n">K</span><span class="p">,</span> <span class="n">V</span><span class="o">>::</span><span class="nl">value_type</span> <span class="p">:</span> <span class="k">public</span> <span class="n">std</span><span class="o">::</span><span class="n">pair</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">K</span><span class="p">,</span> <span class="n">V</span><span class="o">></span> <span class="p">{</span>
  <span class="k">using</span> <span class="n">pair</span><span class="o">::</span><span class="n">pair</span><span class="p">;</span>
  <span class="k">using</span> <span class="n">key</span> <span class="o">=</span> <span class="n">first</span><span class="p">;</span>
  <span class="k">using</span> <span class="n">value</span> <span class="o">=</span> <span class="n">second</span><span class="p">;</span>
<span class="p">};</span>
</pre>
   <p>For consistency with class-scope <em>using-declaration</em>s, functions introduced by class-scope <em>alias-declaration</em>s are hidden by declared class members with the same signature:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">B</span> <span class="p">{</span>
  <span class="kt">void</span> <span class="n">f</span><span class="p">(</span><span class="kt">int</span><span class="p">);</span>
  <span class="kt">void</span> <span class="nf">f</span><span class="p">(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">int</span><span class="p">);</span>
<span class="p">};</span>
<span class="k">struct</span> <span class="nl">C</span> <span class="p">:</span> <span class="n">B</span> <span class="p">{</span>
  <span class="k">using</span> <span class="n">B</span><span class="o">::</span><span class="n">f</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">f</span><span class="p">(</span><span class="kt">int</span><span class="p">);</span> <span class="c1">// hides B::f(int) but not B::f(int, int)</span>
<span class="c1"></span><span class="p">};</span>
<span class="k">struct</span> <span class="nl">D</span> <span class="p">:</span> <span class="n">B</span> <span class="p">{</span>
  <span class="k">using</span> <span class="n">g</span> <span class="o">=</span> <span class="n">B</span><span class="o">::</span><span class="n">f</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">g</span><span class="p">(</span><span class="kt">int</span><span class="p">);</span> <span class="c1">// hides B::f(int) but not B::f(int, int)</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   <p>An aliased virtual function may be overridden using either its original name or its aliased name. As usual, if there are multiple final overriders for a single virtual function, the program is ill-formed. Example:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">B1</span> <span class="p">{</span>
  <span class="k">virtual</span> <span class="kt">void</span> <span class="n">f</span><span class="p">();</span>
  <span class="k">using</span> <span class="n">f1</span> <span class="o">=</span> <span class="n">f</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">struct</span> <span class="n">B2</span> <span class="p">{</span>
  <span class="k">virtual</span> <span class="kt">void</span> <span class="n">f</span><span class="p">();</span>
  <span class="k">using</span> <span class="n">f2</span> <span class="o">=</span> <span class="n">f</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">struct</span> <span class="nl">D</span> <span class="p">:</span> <span class="n">B1</span><span class="p">,</span> <span class="n">B2</span> <span class="p">{</span>
  <span class="kt">void</span> <span class="n">f1</span><span class="p">()</span> <span class="k">override</span><span class="p">;</span>  <span class="c1">// OK, overrides B1::f but not B2::f</span>
<span class="c1"></span>  <span class="kt">void</span> <span class="nf">f</span><span class="p">()</span> <span class="k">override</span><span class="p">;</span>   <span class="c1">// error, multiple final overriders for B1::f in D</span>
<span class="c1"></span><span class="p">};</span>
</pre>
   <h3 class="heading settled" data-level="3.2" id="templated-aliases"><span class="secno">3.2. </span><span class="content">Templated aliases</span><a class="self-link" href="#templated-aliases"></a></h3>
   <p>For consistency with existing <em>alias-declaration</em>s and for maximal flexibility, we propose to allow the generalized <em>alias-declaration</em> to also be templated. For example, this permits a static data member of a class template to be renamed to a non-class-member template:</p>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">X</span> <span class="p">{</span> <span class="k">static</span> <span class="kt">int</span> <span class="n">y</span><span class="p">;</span> <span class="p">};</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="k">using</span> <span class="n">Y</span> <span class="o">=</span> <span class="n">X</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">y</span><span class="p">;</span>
<span class="k">static_assert</span><span class="p">(</span><span class="o">&amp;</span><span class="n">Y</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">></span> <span class="o">==</span> <span class="o">&amp;</span><span class="n">X</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">>::</span><span class="n">y</span><span class="p">);</span>
</pre>
   <p>… and permits an alias to reorganize the parameter list of a template, for instance if a new mandatory template parameter is needed:</p>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="p">,</span> <span class="k">typename</span> <span class="n">U</span><span class="o">></span> <span class="n">pair</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="n">U</span><span class="o">></span> <span class="n">my_pair</span><span class="p">;</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="k">using</span> <span class="n">old_pair</span> <span class="o">=</span> <span class="n">my_pair</span><span class="o">&lt;</span><span class="n">T</span><span class="p">,</span> <span class="kt">int</span><span class="o">></span><span class="p">;</span>
</pre>
   <p>For the purpose of template argument deduction, when an alias template names a function template specialization, the template arguments from the alias are first substituted into the function template(s) to form another set of function templates prior to deduction:</p>
<pre class="highlight"><span class="k">namespace</span> <span class="n">New</span> <span class="p">{</span>
  <span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">Iterator</span><span class="o">></span> <span class="kt">void</span> <span class="n">process</span><span class="p">(</span><span class="n">Iterator</span> <span class="n">begin</span><span class="p">,</span> <span class="n">Iterator</span> <span class="n">end</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">namespace</span> <span class="n">Old</span> <span class="p">{</span>
  <span class="c1">// Substitutes Iterator = T* into New::process, forming</span>
<span class="c1"></span>  <span class="c1">//   template&lt;typename T> void process(T *begin, T *end);</span>
<span class="c1"></span>  <span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="k">using</span> <span class="n">process</span> <span class="o">=</span> <span class="n">New</span><span class="o">::</span><span class="n">process</span><span class="o">&lt;</span><span class="n">T</span><span class="o">*></span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">void</span> <span class="n">f</span><span class="p">()</span> <span class="p">{</span>
  <span class="kt">int</span> <span class="n">arr</span><span class="p">[</span><span class="mi">5</span><span class="p">];</span>
  <span class="n">Old</span><span class="o">::</span><span class="n">process</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">begin</span><span class="p">(</span><span class="n">arr</span><span class="p">),</span> <span class="n">std</span><span class="o">::</span><span class="n">end</span><span class="p">(</span><span class="n">arr</span><span class="p">));</span> <span class="c1">// ok, deduces T = int, so Iterator = int*</span>
<span class="c1"></span><span class="p">}</span>
</pre>
   <p>When a templated <em>alias-declaration</em> has a dependent <em>nested-name-specifier</em>, it is not possible in general to determine whether it names a function (or set of overloaded functions), nor to perform template argument deduction against it; such a declaration is therefore not permitted to form part of an overload set and must instead be the only declaration with its name in its declarative region.</p>
   <p>As with existing alias templates, the generalized form cannot be explicitly or partially specialized nor explicitly instantiated, and instead acts as a transparent alias for its (substituted) <em>id-expression</em>. The <em>id-expression</em> in a templated <em>alias-declaration</em> shall not name a template.</p>
   <h2 class="heading settled" data-level="4" id="interactions-with-other-proposals"><span class="secno">4. </span><span class="content">Interactions with other proposals</span><a class="self-link" href="#interactions-with-other-proposals"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0634r1">[P0634R1]</a> proposes that the <code class="highlight"><span class="k">typename</span></code> keyword be made optional in a number of contexts. One of these is the right-hand side of an <em>alias-declaration</em>. That portion of that proposal conflicts with this proposal; in a case such as</p>
<pre class="highlight"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">X</span> <span class="p">{</span>
  <span class="k">using</span> <span class="n">A</span> <span class="o">=</span> <span class="n">T</span><span class="o">::</span><span class="n">something</span><span class="p">;</span>
  <span class="kt">void</span> <span class="nf">f</span><span class="p">()</span> <span class="p">{</span> <span class="n">A</span> <span class="o">*</span> <span class="n">a</span><span class="p">;</span> <span class="p">}</span>
<span class="p">};</span>
</pre>
   <p>… correctly parsing the definition of <code class="highlight"><span class="n">X</span><span class="o">&lt;</span><span class="n">T</span><span class="o">>::</span><span class="n">f</span><span class="p">()</span></code> requires knowledge of whether <code class="highlight"><span class="n">A</span></code> is a type. Therefore we still require the <code class="highlight"><span class="k">typename</span></code> keyword in dependent <em>alias-declaration</em>s if we choose to move forward with both proposals.</p>
   <h2 class="heading settled" data-level="5" id="acknowledgements"><span class="secno">5. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to Gabriel Dos Reis and Mat Marcus for proposing <em>alias-declaration</em>s in <a data-link-type="biblio" href="#biblio-n1449">[N1449]</a>. Thanks to Jorg Brown for providing the <code class="highlight"><span class="n">map</span><span class="o">::</span><span class="n">value_type</span></code> example, and to Matt Calabrese for providing early feedback on this proposal.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-n1449">[N1449]
   <dd>G. Dos ReisM. Marcus. <a href="https://wg21.link/n1449">Proposal to add template aliases to C++</a>. URL: <a href="https://wg21.link/n1449">https://wg21.link/n1449</a>
   <dt id="biblio-p0515r3">[P0515R3]
   <dd>Herb Sutter, Jens Maurer, Walter E. Brown. <a href="https://wg21.link/p0515r3">Consistent comparison</a>. URL: <a href="https://wg21.link/p0515r3">https://wg21.link/p0515r3</a>
   <dt id="biblio-p0634r1">[P0634R1]
   <dd>Daveed Vandevoorde, Nina Ranns. <a href="https://wg21.link/p0634r1">Down with `typename`!</a>. URL: <a href="https://wg21.link/p0634r1">https://wg21.link/p0634r1</a>
  </dl>
