<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P1837R0: Remove NTTPs of class type from C++20</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">P1837R0<br>Remove NTTPs of class type from C++20</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-07-28">2019-07-28</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Author:
     <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>
     <dt>Audience:
     <dd>EWG
     <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/d1837-remove-class-type-nttps.bs">github.com/Quuxplusone/draft/blob/gh-pages/d1837-remove-class-type-nttps.bs</a>
     <dt>Current:
     <dd><a href="https://rawgit.com/Quuxplusone/draft/gh-pages/d1837-remove-class-type-nttps.html">rawgit.com/Quuxplusone/draft/gh-pages/d1837-remove-class-type-nttps.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><a data-link-type="biblio" href="#biblio-p0732">[P0732]</a> "Class types in non-type template parameters"

  was accepted into C++20 in June 2018.
  Newly discovered issues with its premise, and new exploration in the area,
  suggest that we should postpone this feature until C++2b.</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="#p0732"><span class="secno">1</span> <span class="content">A summary of P0732</span></a>
    <li>
     <a href="#killer-app"><span class="secno">2</span> <span class="content">P0732’s killer app: template parameters of "string" type</span></a>
     <ol class="toc">
      <li><a href="#buggy-app"><span class="secno">2.1</span> <span class="content">Slight variation remains ill-formed</span></a>
      <li><a href="#subtlety"><span class="secno">2.2</span> <span class="content">Subtle wording</span></a>
     </ol>
    <li><a href="#unsame"><span class="secno">3</span> <span class="content">Identity and equality are not the same thing in C++17</span></a>
    <li>
     <a href="#ongoing"><span class="secno">4</span> <span class="content">There is ongoing exploration of the NTTP space</span></a>
     <ol class="toc">
      <li><a href="#float"><span class="secno">4.1</span> <span class="content"><code class="highlight"><c- b>float</c-></code> as NTTP</span></a>
      <li><a href="#operator-template"><span class="secno">4.2</span> <span class="content">A mangling or serialization operator</span></a>
     </ol>
    <li><a href="#conclusion"><span class="secno">5</span> <span class="content">Conclusion</span></a>
    <li><a href="#polls"><span class="secno"></span> <span class="content">Appendix A: Proposed straw polls</span></a>
    <li><a href="#wording"><span class="secno"></span> <span class="content">Appendix B: Proposed wording</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <p>This paper repeats some material from <a data-link-type="biblio" href="#biblio-enums">[Enums]</a>, and adds new material as well.</p>
   <h2 class="heading settled" data-level="1" id="p0732"><span class="secno">1. </span><span class="content">A summary of P0732</span><a class="self-link" href="#p0732"></a></h2>
   <p>C++ does not support non-type template parameters (NTTPs) of arbitrary types.
For example, <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- b>int</c-> <c- n>I</c-><c- o>></c-></code> is valid, but <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>string</c-> <c- n>S</c-><c- o>></c-></code> is
not valid.</p>
   <p>Jeff Snyder’s <a data-link-type="biblio" href="#biblio-p0732">[P0732]</a> "Class types in non-type template parameters"
(first revision, 2018-02-11) observed that the essential problem preventing
C++ from supporting arbitrary-typed NTTPs is that the compiler can’t tell
when two arbitrary NTTPs are "identical." "Identity" is important because it
is how the compiler determines (<a href="http://eel.is/c++draft/temp.type#1">[temp.type]/1</a>)
whether two templates are the same entity. For example:</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- n>using</c-> <c- n>SomeType</c-> <c- o>=</c-> <c- p>[...];</c->
    <c- n>template</c-><c- o>&lt;</c-><c- n>SomeType</c-> <c- n>X</c-><c- o>></c-> <c- b>void</c-> <c- n>foo</c-><c- p>();</c->
    <c- n>template</c-><c- o>&lt;></c-> <c- b>void</c-> <c- n>foo</c-><c- o>&lt;</c-><c- n>SomeType</c-><c- p>(</c-><c- mi>100</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-><c- p>;</c-> <c- p>}</c->
    <c- n>template</c-><c- o>&lt;></c-> <c- b>void</c-> <c- n>foo</c-><c- o>&lt;</c-><c- n>SomeType</c-><c- p>(</c-><c- mi>356</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-><c- p>;</c-> <c- p>}</c->
</pre></small> 
   <p>Here, if <code class="highlight"><c- n>SomeType</c-></code> is <code class="highlight"><c- b>int</c-></code>, then we have a valid program with two explicit
specializations of <code class="highlight"><c- n>foo</c-></code>. But if <code class="highlight"><c- n>SomeType</c-></code> is <code class="highlight"><c- b>unsigned</c-> <c- b>char</c-></code>, then there is
only one explicit specialization of <code class="highlight"><c- n>foo</c-></code>, multiply defined; and so the program
is ill-formed.</p>
   <p>P0732 proposed that in present-day C++, determining the "identity" of two
NTTP values is easy because the only supported NTTP types are simple scalar
types where identity means equality, and where it is "obvious" to the compiler
when two such values are equal.
In C++2a, <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> can be defaulted; this increases the number
of types for which it is "obvious" what identity means.</p>
   <p>The rest of this paper will argue that "obviousness" is more subtle
than we thought.</p>
   <h2 class="heading settled" data-level="2" id="killer-app"><span class="secno">2. </span><span class="content">P0732’s killer app: template parameters of "string" type</span><a class="self-link" href="#killer-app"></a></h2>
   <p>Thanks to Louis Dionne for this example using P0732 NTTPs.</p>
   <p>We create a function template <code class="highlight"><c- n>foo</c-></code> that takes a <code class="highlight"><c- n>FixedLengthString</c-><c- o>&lt;???></c-></code> of deduced class type as its NTTP.
(The CTAD syntax we’re using here is homonymous with the syntaxes for plain old NTTPs
and for concept-constrained type parameters, but it is not either of those.)
When the programmer of <code class="highlight"><c- n>main</c-></code> writes <code class="highlight"><c- n>foo</c-><c- o>&lt;</c-><c- s>"hello"</c-><c- o>></c-></code>, the compiler will perform
overload resolution and class template
argument deduction to determine that the right candidate is <code class="highlight"><c- n>foo</c-><c- o>&lt;</c-><c- n>FixedLengthString</c-><c- o>&lt;</c-><c- mi>6</c-><c- o>></c-><c- p>(</c-><c- s>"hello"</c-><c- p>)</c-><c- o>></c-></code>.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- n>template</c-><c- o>&lt;</c-><c- b>int</c-> <c- n>N</c-><c- o>></c->
    <c- k>struct</c-> <c- n>FixedLengthString</c-> <c- p>{</c->
        <c- b>char</c-> <c- n>data_</c-><c- p>[</c-><c- n>N</c-><c- p>]</c-> <c- p>{};</c->
        <c- n>constexpr</c-> <c- nf>FixedLengthString</c-><c- p>(</c-><c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- n>p</c-><c- p>)</c-> <c- p>{</c->
            <c- k>for</c-> <c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- o>=</c-><c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>)</c-> <c- p>{</c->
                <c- n>data_</c-><c- p>[</c-><c- n>i</c-><c- p>]</c-> <c- o>=</c-> <c- n>p</c-><c- p>[</c-><c- n>i</c-><c- p>];</c->
            <c- p>}</c->
        <c- p>}</c->
        <c- k>auto</c-> <c- n>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>FixedLengthString</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- p>};</c->
    <c- n>template</c-><c- o>&lt;</c-><c- b>int</c-> <c- n>N</c-><c- o>></c-> <c- n>FixedLengthString</c-><c- p>(</c-><c- k>const</c-> <c- b>char</c-> <c- p>(</c-><c- o>&amp;</c-><c- p>)[</c-><c- n>N</c-><c- p>])</c-> <c- o>-></c-> <c- n>FixedLengthString</c-><c- o>&lt;</c-><c- n>N</c-><c- o>></c-><c- p>;</c->

    <c- n>template</c-><c- o>&lt;</c-><c- n>FixedLengthString</c-> <c- n>S</c-><c- o>></c->
    <c- b>int</c-> <c- n>foo</c-><c- p>()</c-> <c- p>{</c->
        <c- k>static</c-> <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
        <c- k>return</c-> <c- o>++</c-><c- n>i</c-><c- p>;</c->
    <c- p>}</c->

    <c- b>int</c-> <c- n>main</c-><c- p>()</c-> <c- p>{</c->
        <c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- n>foo</c-><c- o>&lt;</c-><c- s>"hello"</c-><c- o>></c-><c- p>();</c->
        <c- b>int</c-> <c- n>y</c-> <c- o>=</c-> <c- n>foo</c-><c- o>&lt;</c-><c- s>"hello"</c-><c- o>></c-><c- p>();</c->
        <c- k>return</c-> <c- n>y</c-><c- p>;</c->
    <c- p>}</c->
</pre></small> 
   <p><code class="highlight"><c- n>FixedLengthString</c-><c- o>&lt;</c-><c- mi>6</c-><c- o>></c-></code> has a structural comparison operator.
Comparison on <code class="highlight"><c- n>FixedLengthString</c-><c- o>&lt;</c-><c- mi>6</c-><c- o>></c-><c- p>(</c-><c- s>"hello"</c-><c- p>)</c-></code> compares exactly the six bytes <code class="highlight"><c- s>"hello"</c-></code> which are stored in <code class="highlight"><c- k>this</c-><c- o>-></c-><c- n>data_</c-></code>.
This program has well-defined behavior and returns <code class="highlight"><c- mi>2</c-></code>.</p>
   <p>My understanding is that this is the "killer app" for P0732 NTTPs: we can
use literal class types to "smuggle" contraband such as string literals
and floating-point values which would otherwise not be allowed in template
arguments.</p>
   <h3 class="heading settled" data-level="2.1" id="buggy-app"><span class="secno">2.1. </span><span class="content">Slight variation remains ill-formed</span><a class="self-link" href="#buggy-app"></a></h3>
   <p>On the other hand, in this next example we create a function template <code class="highlight"><c- n>bar</c-></code> that takes a concrete <code class="highlight"><c- n>VariableLengthString</c-></code> as its NTTP.
When the programmer of <code class="highlight"><c- n>main</c-></code> writes <code class="highlight"><c- n>bar</c-><c- o>&lt;</c-><c- s>"hello"</c-><c- o>></c-></code>, the compiler will
perform overload resolution to determine that the right candidate is <code class="highlight"><c- n>bar</c-><c- o>&lt;</c-><c- n>VariableLengthString</c-><c- p>(</c-><c- s>"hello"</c-><c- p>)</c-><c- o>></c-></code>.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- k>struct</c-> <c- n>VariableLengthString</c-> <c- p>{</c->
        <c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- n>data_</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->
        <c- n>constexpr</c-> <c- nf>VariableLengthString</c-><c- p>(</c-><c- k>const</c-> <c- b>char</c-> <c- o>*</c-><c- n>p</c-><c- p>)</c-> <c- o>:</c-> <c- n>data_</c-><c- p>(</c-><c- n>p</c-><c- p>)</c-> <c- p>{}</c->
        <c- k>auto</c-> <c- n>operator</c-><c- o>&lt;=></c-><c- p>(</c-><c- k>const</c-> <c- n>VariableLengthString</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- p>};</c->

    <c- n>template</c-><c- o>&lt;</c-><c- n>VariableLengthString</c-> <c- n>S</c-><c- o>></c->
    <c- b>int</c-> <c- n>bar</c-><c- p>()</c-> <c- p>{</c->
        <c- k>static</c-> <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
        <c- k>return</c-> <c- o>++</c-><c- n>i</c-><c- p>;</c->
    <c- p>}</c->

    <c- b>int</c-> <c- n>main</c-><c- p>()</c-> <c- p>{</c->
        <c- b>int</c-> <c- n>x</c-> <c- o>=</c-> <c- n>bar</c-><c- o>&lt;</c-><c- s>"hello"</c-><c- o>></c-><c- p>();</c-> <c- c1>// ERROR</c->

        <c- k>auto</c-><c- o>&amp;</c-> <c- n>hello</c-> <c- o>=</c-> <c- s>"hello"</c-><c- p>;</c->
        <c- b>int</c-> <c- n>y</c-> <c- o>=</c-> <c- n>bar</c-><c- o>&lt;</c-><c- n>hello</c-><c- o>></c-><c- p>();</c-> <c- c1>// ERROR</c->
        <c- k>return</c-> <c- n>y</c-><c- p>;</c->
    <c- p>}</c->
</pre></small> 
   <p><code class="highlight"><c- n>VariableLengthString</c-></code> has a structural comparison operator, but when
the compiler goes to mangle the name of <code class="highlight"><c- n>bar</c-><c- o>&lt;</c-><c- n>VariableLengthString</c-><c- p>(</c-><c- s>"hello"</c-><c- p>)</c-><c- o>></c-></code>,
it finds that it cannot produce a mangling of the <code class="highlight"><c- n>data_</c-></code> member’s value
because <code class="highlight"><c- s>"hello"</c-></code> is not a named variable. Therefore the program above
is ill-formed.
(The relevant wording is <a href="http://eel.is/c++draft/temp.arg.nontype#2">[temp.arg.nontype]/2</a>).</p>
   <p>The following <code class="highlight"><c- n>main</c-></code> function would be well-formed:</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
        <c- k>static</c-> <c- k>const</c-> <c- b>char</c-> <c- n>hello_array</c-><c- p>[]</c-> <c- o>=</c-> <c- s>"hello"</c-><c- p>;</c->
        <c- k>auto</c-><c- o>&amp;</c-> <c- n>hello</c-> <c- o>=</c-> <c- n>hello_array</c-><c- p>;</c->
        <c- b>int</c-> <c- n>y</c-> <c- o>=</c-> <c- n>bar</c-><c- o>&lt;</c-><c- n>hello</c-><c- o>></c-><c- p>();</c->
        <c- k>return</c-> <c- n>y</c-><c- p>;</c->
    <c- p>}</c->
</pre></small> 
   <p>because <code class="highlight"><c- n>hello</c-></code> refers to <code class="highlight"><c- n>hello_array</c-></code>, and <code class="highlight"><c- n>hello_array</c-></code> is a named variable.</p>
   <h3 class="heading settled" data-level="2.2" id="subtlety"><span class="secno">2.2. </span><span class="content">Subtle wording</span><a class="self-link" href="#subtlety"></a></h3>
   <p>The above-described behavior for template arguments involving pointers
has been present since C++11 or earlier. It is quite subtle.
It was implied by <a data-link-type="biblio" href="#biblio-n4700">[N4700]</a>'s old wording
(<a href="http://eel.is/c++draft/temp.type#1">[temp.type]/1</a>):</p>
   <blockquote>
    <p>Two <i>template-id</i>s refer to the same class, function, or variable if [...]
their corresponding non-type <i>template-argument</i>s of pointer type
refer to the same object or function or are both the null pointer value [...]</p>
   </blockquote>
   <p><a data-link-type="biblio" href="#biblio-p0732">[P0732]</a>'s changed wording, IMHO, obscures and possibly breaks the intent
for template arguments involving pointers. <a data-link-type="biblio" href="#biblio-n4810">[N4810]</a>'s new wording:</p>
   <blockquote>
    <p>Two <i>template-id</i>s refer to the same class, function, or variable if [...]
corresponding non-type <i>template-argument</i>s have the same type and value
after conversion to the type of the <i>template-parameter</i>,
where they are considered to have the same value if they compare equal
with the <code class="highlight"><c- o>==</c-></code> operator [...]</p>
   </blockquote>
   <p>The new wording does not clearly say what happens if the comparison with the <code class="highlight"><c- o>==</c-></code> operator is not a constant expression. This can happen if it attempts to
compare beyond-the-end pointers. <a data-link-type="biblio" href="#biblio-n4700">[N4700]</a> handled this because
a beyond-the-end pointer does not "refer to an object."
There is also relevant wording in <a href="http://eel.is/c++draft/temp.arg.nontype#2">[temp.arg.nontype/2]</a>, introduced
in <a data-link-type="biblio" href="#biblio-n4268">[N4268]</a>; but it does not mention beyond-the-end pointers.</p>
   <p>Nor does the new wording say how the lookup for the <code class="highlight"><c- o>==</c-></code> operator should be done.
That’s the next thing we’ll look at.</p>
   <h2 class="heading settled" data-level="3" id="unsame"><span class="secno">3. </span><span class="content">Identity and equality are not the same thing in C++17</span><a class="self-link" href="#unsame"></a></h2>
   <p>Consider the following valid C++17 code:</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- k>enum</c-> <c- n>E</c-> <c- p>{</c->
        <c- n>ONE</c-><c- p>,</c-> <c- n>TWO</c->
    <c- p>};</c->
    <c- n>namespace</c-> <c- n>N</c-> <c- p>{</c->
        <c- n>template</c-><c- o>&lt;</c-><c- n>E</c-><c- o>></c-> <c- b>int</c-> <c- n>foo</c-><c- p>()</c-> <c- p>{</c->
            <c- k>static</c-> <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
            <c- k>return</c-> <c- o>++</c-><c- n>i</c-><c- p>;</c->
        <c- p>}</c->
        <c- b>void</c-> <c- n>test</c-><c- p>()</c-> <c- p>{</c->
            <c- n>foo</c-><c- o>&lt;</c-><c- n>ONE</c-><c- o>></c-><c- p>();</c->
            <c- n>foo</c-><c- o>&lt;</c-><c- n>TWO</c-><c- o>></c-><c- p>();</c->
        <c- p>}</c->
    <c- p>}</c->
</pre></small> 
   <p>This code is valid and well-defined in C++17 today. It causes two
distinct specializations of <code class="highlight"><c- n>foo</c-></code> to be instantiated.</p>
   <p>We can add the following overload of <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> anywhere in this
code — before or after the definition of <code class="highlight"><c- n>foo</c-></code>, inside namespace <code class="highlight"><c- n>N</c-></code> or the global
namespace — and the compiler won’t care.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>E</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> true<c- p>;</c-> <c- p>}</c->
</pre></small> 
   <p>Thus, in C++17, it is perfectly possible that <code class="highlight"><c- k>static_assert</c-><c- p>(</c-><c- n>ONE</c-> <c- o>==</c-> <c- n>TWO</c-><c- p>)</c-></code> and yet <code class="highlight"><c- k>static_assert</c-><c- p>(</c-><c- o>&amp;</c-><c- n>foo</c-><c- o>&lt;</c-><c- n>ONE</c-><c- o>></c-> <c- o>!=</c-> <c- o>&amp;</c-><c- n>foo</c-><c- o>&lt;</c-><c- n>TWO</c-><c- o>></c-><c- p>)</c-></code>. (<a href="https://godbolt.org/z/aav4CQ">Godbolt.</a>)</p>
   <p>With P0732 in the Committee Draft, two bad things happen. First, a wording issue:
it’s unclear whether, when <a href="http://eel.is/c++draft/temp.type#1.5">[temp.type]/1.5</a> asks if the arguments "compare equal with the <code class="highlight"><c- o>==</c-></code> operator,"
the compiler will use our overloaded <code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>E</c-><c- p>)</c-></code>.
(In practice, it will not.)</p>
   <p>Second, because <code class="highlight"><c- n>E</c-></code> is an enum type, it has strong structural equality
(<a href="http://eel.is/c++draft/class.compare.default#3">[class.compare.default]/3</a>).
So under P0732’s rules we can create a literal class type <code class="highlight"><c- n>A</c-></code> that can be used as an NTTP.</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- k>enum</c-> <c- n>E</c-> <c- p>{</c->
        <c- n>ONE</c-><c- p>,</c-> <c- n>TWO</c->
    <c- p>};</c->
    <c- n>namespace</c-> <c- n>N</c-> <c- p>{</c->
        <c- k>struct</c-> <c- n>A</c-> <c- p>{</c->
            <c- n>E</c-> <c- n>e_</c-><c- p>;</c->
            <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- k>const</c-> <c- n>A</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>const</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
        <c- p>};</c->
        <c- n>template</c-><c- o>&lt;</c-><c- n>A</c-><c- o>></c-> <c- b>int</c-> <c- n>foo</c-><c- p>()</c-> <c- p>{</c->
            <c- k>static</c-> <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
            <c- k>return</c-> <c- o>++</c-><c- n>i</c-><c- p>;</c->
        <c- p>}</c->
        <c- b>void</c-> <c- n>test</c-><c- p>()</c-> <c- p>{</c->
            <c- n>foo</c-><c- o>&lt;</c-><c- n>A</c-><c- p>{</c-><c- n>ONE</c-><c- p>}</c-><c- o>></c-><c- p>();</c->
            <c- n>foo</c-><c- o>&lt;</c-><c- n>A</c-><c- p>{</c-><c- n>TWO</c-><c- p>}</c-><c- o>></c-><c- p>();</c->
        <c- p>}</c->
        <c- n>constexpr</c-> <c- b>bool</c-> <c- n>operator</c-><c- o>==</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>E</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> true<c- p>;</c-> <c- p>}</c->
    <c- p>}</c->
</pre></small> 
   <p>Again, we can insert the overload of <code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>E</c-><c- p>)</c-></code> anywhere in this
code — before or after the definition of <code class="highlight"><c- n>foo</c-></code>, before or after the definition of <code class="highlight"><c- n>A</c-></code>,
inside namespace <code class="highlight"><c- n>N</c-></code> or the global namespace.</p>
   <p>It is not clear how <code class="highlight"><c- n>A</c-><c- o>::</c-><c- k>operator</c-><c- o>==</c-></code> should call <code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>E</c-><c- p>)</c-></code>, if <code class="highlight"><c- k>operator</c-><c- o>==</c-><c- p>(</c-><c- n>E</c-><c- p>,</c-> <c- n>E</c-><c- p>)</c-></code> was not yet declared when <code class="highlight"><c- n>A</c-><c- o>::</c-><c- k>operator</c-><c- o>==</c-></code> was defined.</p>
   <p>No vendor has implemented P0732 NTTPs. But we can get some hint of the
subtleties involved by looking at MSVC’s in-progress implementation of <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code>. (<a href="https://godbolt.org/z/ENOQDb">Godbolt.</a>)</p>
   <h2 class="heading settled" data-level="4" id="ongoing"><span class="secno">4. </span><span class="content">There is ongoing exploration of the NTTP space</span><a class="self-link" href="#ongoing"></a></h2>
   <p><a data-link-type="biblio" href="#biblio-p0732">[P0732]</a> has done a great service by stimulating new exploration of the NTTP space.
After P0732 was discussed and adopted, the following new work appeared:</p>
   <h3 class="heading settled" data-level="4.1" id="float"><span class="secno">4.1. </span><span class="content"><code class="highlight"><c- b>float</c-></code> as NTTP</span><a class="self-link" href="#float"></a></h3>
   <p>Jorg Brown’s <a data-link-type="biblio" href="#biblio-p1714">[P1714]</a> "NTTP are incomplete without float, double, and long double!"
was discussed by EWG (<a data-link-type="biblio" href="#biblio-p1714discussion">[P1714discussion]</a>), and the reception was
favorable (1–14–9–3–3). Proponents want something like</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- n>template</c-><c- o>&lt;</c-><c- b>double</c-> <c- n>Exponent</c-><c- o>></c->
    <c- b>double</c-> <c- n>pow</c-><c- p>(</c-><c- b>double</c-> <c- n>base</c-><c- p>);</c->
</pre></small> 
   <p>P0732 does not permit this, because <code class="highlight"><c- b>double</c-></code> does not have strong structural equality. <code class="highlight"><c- b>double</c-></code> has <code class="highlight"><c- n>partial_ordering</c-></code>, because of NaN. So Jorg’s workaround for C++2a is
similar to the <code class="highlight"><c- n>FixedLengthString</c-><c- o>&lt;</c-><c- mi>6</c-><c- o>></c-></code> hack above: where <code class="highlight"><c- n>FixedLengthString</c-><c- o>&lt;</c-><c- mi>6</c-><c- o>></c-></code> smuggles a string literal through an array of char, Jorg’s <code class="highlight"><c- n>AsTemplateArg</c-><c- o>&lt;</c-><c- b>double</c-><c- o>></c-></code> smuggles a <code class="highlight"><c- b>double</c-></code> through an array of char.</p>
   <p>Two observations:</p>
   <ul>
    <li data-md>
     <p>It seems that "smuggling contraband through arrays of char" is in practice
the main use-case for P0732 NTTPs. I have not seen anyone excited about
P0732 who isn’t planning to use them in this way.</p>
    <li data-md>
     <p>P0732 is not compatible with EWG’s interest in <code class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- b>double</c-> <c- n>Exponent</c-><c- o>></c-></code>. <code class="highlight"><c- b>double</c-></code> will never have strong structural equality, and P0732-based NTTPs
will never support types with less-than-strong structural equality.</p>
   </ul>
   <p>It seems that EWG is interested in exploring avenues which P0732 cuts off.</p>
   <p>In <a data-link-type="biblio" href="#biblio-p1714discussion">[P1714discussion]</a>, one participant is quoted as saying,
"My proposal is that we take it [i.e., P0732] out and try again."</p>
   <h3 class="heading settled" data-level="4.2" id="operator-template"><span class="secno">4.2. </span><span class="content">A mangling or serialization operator</span><a class="self-link" href="#operator-template"></a></h3>
   <p>Richard Smith writes:</p>
   <blockquote>
    <p>Broadly, I think that attempting to make NTTP identity be the same thing
as equality is an evolutionary dead end for C++. They’re fundamentally
different operations, with different constraints and different goals.</p>
   </blockquote>
   <p>In EWG reflector thread "[isocpp-ext] Can we have float/double as template parameters now?",
he informally explored the notion of an overloadable <code class="highlight"><c- k>operator</c-> <c- k>template</c-></code> which would allow us to pass <em>any</em> literal type <code class="highlight"><c- n>T</c-></code> as an NTTP, as long as it
provides a way to "serialize" its value into
some serialized form that the compiler knows how to mangle (such as a POD
struct), and a way to "deserialize" from that mangled representation
back into an object of type <code class="highlight"><c- n>T</c-></code>.</p>
   <p>This approach is premised on the idea that the fundamental building block
for NTTPs should not be <code class="highlight"><c- o>==</c-></code> equality, but rather some sort of <em>identity</em> operation. This plays well with the bare fact that <code class="highlight"><c- o>==</c-></code> equality is already
irrelevant to NTTP-identity when the type <code class="highlight"><c- n>T</c-></code> is an enum type... or a reference type.
Consider:</p>
   <p><small></small></p>
   <small> <pre class="language-c++ highlight">    <c- n>constexpr</c-> <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>1</c-><c- p>;</c->
    <c- n>constexpr</c-> <c- b>int</c-> <c- n>j</c-> <c- o>=</c-> <c- mi>1</c-><c- p>;</c->

    <c- n>template</c-><c- o>&lt;</c-><c- k>const</c-> <c- b>int</c-><c- o>&amp;></c->
    <c- b>void</c-> <c- n>foo</c-><c- p>()</c-> <c- p>{}</c->

    <c- n>static_assert</c-><c- p>(</c->     <c- n>i</c->  <c- o>==</c->      <c- n>j</c-> <c- p>,</c-> <c- s>""</c-><c- p>);</c->
    <c- n>static_assert</c-><c- p>(</c-><c- o>&amp;</c-><c- n>foo</c-><c- o>&lt;</c-><c- n>i</c-><c- o>></c-> <c- o>!=</c-> <c- o>&amp;</c-><c- n>foo</c-><c- o>&lt;</c-><c- n>j</c-><c- o>></c-><c- p>,</c-> <c- s>""</c-><c- p>);</c->
</pre></small> 
   <p>Today, in C++17, it is perfectly possible that <code class="highlight"><c- k>static_assert</c-><c- p>(</c-><c- n>i</c-> <c- o>==</c-> <c- n>j</c-><c- p>)</c-></code> and yet <code class="highlight"><c- k>static_assert</c-><c- p>(</c-><c- o>&amp;</c-><c- n>foo</c-><c- o>&lt;</c-><c- n>i</c-><c- o>></c-> <c- o>!=</c-> <c- o>&amp;</c-><c- n>foo</c-><c- o>&lt;</c-><c- n>j</c-><c- o>></c-><c- p>)</c-></code>. (<a href="https://godbolt.org/z/32k8Uw">Godbolt.</a>)</p>
   <p>The importance of this difference between "equality" and "identity"
was not widely known during the original discussion of P0732.
Had it been known, our approach to NTTPs might have taken a different form.</p>
   <h2 class="heading settled" data-level="5" id="conclusion"><span class="secno">5. </span><span class="content">Conclusion</span><a class="self-link" href="#conclusion"></a></h2>
   <p>P0732 was premised on an erroneous conflation of "<code class="highlight"><c- o>==</c-></code> equality" and
"NTTP identity." These are similar — but distinguishable — notions. Conflating them
causes subtle inconsistencies which will be very hard for
any future work in the area to fix.</p>
   <p>We should not ship class-typed NTTPs in C++20 without thoroughly exploring
the consequences. Once P0732 has appeared in a published standard, it will
be too late to fix it.</p>
   <p>Incidentally, in hindsight, it was probably a bad idea to allow users to overload <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> for enums. It was maybe even a bad idea to allow NTTPs of reference type.
But we cannot fix these things (even if we wanted to), because they have already shipped.
In this paper, I’m trying to apply foresight (not hindsight) to prevent
a feature from shipping <em>before</em> we regret it.</p>
   <p>The situation with NTTPs in C++17 is subtle and confusing,
but at least it’s been relatively stable since C++03. P0732 makes some
existing issues easier to run into, and causes new issues of its own.
Finally, it permanently cuts off potentially fruitful avenues of exploration
(such as <code class="highlight"><c- b>float</c-></code> NTTPs, and user-defined mechanisms for NTTP-identity
beyond <code class="highlight"><c- o>==</c-></code>).</p>
   <p>I propose that WG21 remove "class types in non-type template parameters"
from C++20, with the expectation that it — or something even
better! — may return in C++2b.</p>
   <p class="note" role="note"><span>Note:</span> This paper (P1837) proposes to remove class-typed NTTPs but leaves <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code> out of scope.
Another paper in this mailing, ADAM David Alan Martin’s <a data-link-type="biblio" href="#biblio-p1821r0">[P1821R0]</a> "Spaceship needs to be grounded,"
proposes to remove <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code> but leaves class-typed NTTPs out of scope.
In Arthur’s opinion, it is conceivable to remove both <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code> and class-typed NTTPs
(that is, these papers are compatible), or to remove just class-typed NTTPs, but it’s
unlikely that we could remove just <code class="highlight"><c- k>operator</c-><c- o>&lt;=></c-></code> without also either removing or redesigning
class-typed NTTPs.</p>
   <h2 class="heading settled" id="polls"><span class="content">Appendix A: Proposed straw polls</span><a class="self-link" href="#polls"></a></h2>
   <table class="def">
    <tbody>
     <tr>
      <th style="width: 70%;">
      <th><strong>SF</strong>
      <th><strong>F</strong>
      <th><strong>N</strong>
      <th><strong>A</strong>
      <th><strong>SA</strong>
     <tr>
      <th><small>Revert P0732, with the expectation that it or something better will return in C++2b.</small>
      <th>_
      <th>_
      <th>_
      <th>_
      <th>_
   </table>
   <h2 class="heading settled" id="wording"><span class="content">Appendix B: Proposed wording</span><a class="self-link" href="#wording"></a></h2>
   <p class="note" role="note"><span>Note:</span> Arthur will draft wording for the removal of P0732, if called upon to do so.
I don’t foresee any difficulty with the wording. As far as I know, P0732 is still
a "leaf feature" with no library users.</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="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-n4700">[N4700]
   <dd>Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4700.pdf">Working Draft, Standard for Programming Language C++</a>. October 2017. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4700.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4700.pdf</a>
   <dt id="biblio-n4810">[N4810]
   <dd>Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4810.pdf">Working Draft, Standard for Programming Language C++</a>. March 2019. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4810.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/n4810.pdf</a>
  </dl>
  <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-enums">[Enums]
   <dd>Arthur O'Dwyer. <a href="https://quuxplusone.github.io/blog/2019/07/04/strong-structural-equality-is-broken/">Enums break strong structural equality</a>. July 2019. URL: <a href="https://quuxplusone.github.io/blog/2019/07/04/strong-structural-equality-is-broken/">https://quuxplusone.github.io/blog/2019/07/04/strong-structural-equality-is-broken/</a>
   <dt id="biblio-n4268">[N4268]
   <dd>Richard Smith. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4268.html">Allow constant evaluation for all non-type template arguments</a>. November 2014. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4268.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4268.html</a>
   <dt id="biblio-p0732">[P0732]
   <dd>Jeff Snyder; Louis Dionne. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0732r2.pdf">Class Types in Non-Type Template Parameters</a>. June 2018. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0732r2.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0732r2.pdf</a>
   <dt id="biblio-p1714">[P1714]
   <dd>Jorg Brown. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1714r0.html">NTTP are incomplete without float, double, and long double!</a>. June 2019. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1714r0.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1714r0.html</a>
   <dt id="biblio-p1714discussion">[P1714discussion]
   <dd><a href="http://wiki.edg.com/bin/view/Wg21cologne2019/P1714R0-EWG">Notes from discussion of P1714R0 (private wiki)</a>. July 2019. URL: <a href="http://wiki.edg.com/bin/view/Wg21cologne2019/P1714R0-EWG">http://wiki.edg.com/bin/view/Wg21cologne2019/P1714R0-EWG</a>
   <dt id="biblio-p1821r0">[P1821R0]
   <dd>ADAM David Alan Martin. <a href="https://wg21.link/p1821r0">The Spaceship Needs to be Grounded: Pull Spaceship from C++20</a>. July 2019. URL: <a href="https://wg21.link/p1821r0">https://wg21.link/p1821r0</a>
  </dl>