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  <title>P1616R1: Using unconstrained template template parameters with constrained templates</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,
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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">P1616R1<br>Using unconstrained template template parameters with constrained templates</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-03-11">2019-07-17</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:mike_spertus@symantec.com">Mike Spertus</a>
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:rbock@eudoxos.de">Roland Bock</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: 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>Unconstrained template template parameters cannot accept constrained templates.

    This blocks important use cases and causes legacy templates to behave in
    unexpected ways as pointed out in Concepts TS issue 14. We propose 
    addressing this.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#problem"><span class="secno">1</span> <span class="content">The problem</span></a>
    <li><a href="#solution"><span class="secno">2</span> <span class="content">The solution</span></a>
    <li><a href="#faq0"><span class="secno">3</span> <span class="content">Without a language change, can we accept a general template as a template template parameter?</span></a>
    <li><a href="#faq1"><span class="secno">4</span> <span class="content">Why not create a new notation that matches any template?</span></a>
    <li><a href="#faq2"><span class="secno">5</span> <span class="content">What if we want to only match unconstrained templates?</span></a>
    <li><a href="#faq2"><span class="secno">6</span> <span class="content">Wording</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="problem"><span class="secno">1. </span><span class="content">The problem</span><a class="self-link" href="#problem"></a></h2>
    It is our understanding that any template accepting an unconstrained template template parameter
cannot accept a constrained template as pointed out in <a href="http://cplusplus.github.io/concepts-ts/ts-active.html#14">Concepts TS Issue 14</a>. 
This creates problems with both legacy and new code. 
   <p>To illustrate the problem, define the following classes</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- p>,</c-> <c- n>class</c-> <c- n>U</c-><c- o>></c-> 
<c- k>struct</c-> <c- n>Unconstrained</c-> <c- p>{</c-> <c- n>Unconstrained</c-><c- p>(</c-><c- n>U</c-><c- p>,</c-> <c- n>V</c-><c- p>);};</c->

<c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- p>,</c-> <c- n>class</c-> <c- n>U</c-><c- p>,</c-> <c- n>class</c-> <c- o>=</c-> <c- n>enable_if_t</c-><c- o>&lt;</c-><c- n>is_integral_v</c-><c- o>&lt;</c-><c- n>U</c-><c- o>>>></c-> 
<c- k>struct</c-> <c- n>Sfinae</c-> <c- p>{</c-> <c- n>Sfinae</c-><c- p>(</c-><c- n>U</c-><c- p>,</c-> <c- n>V</c-><c- p>);</c-> <c- p>};</c->

<c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- p>,</c-> <c- n>class</c-> <c- n>U</c-><c- o>></c-> <c- n>requires</c-> <c- n>Integral</c-><c- o>&lt;</c-><c- n>U</c-><c- o>></c-> 
<c- k>struct</c-> <c- n>Conceptized</c-> <c- p>{</c-> <c- n>Conceptized</c-><c- p>(</c-><c- n>U</c-><c- p>,</c-> <c- n>V</c-><c- p>);</c-> <c- p>};</c->
</pre>
   <p>In Concepts TS issue 14, the following was a blocker to adding concepts to sqlpp11.</p>
<pre class="language-c++ highlight"><c- c1>// A template that takes a tuple and copies its arguments into another template (sink)</c->
<c- n>template</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>Tuple</c-><c- p>,</c-> <c- n>template</c-><c- o>&lt;</c-><c- kr>typename</c-><c- p>...</c-><c- o>></c-> <c- n>class</c-> <c- n>Sink</c-><c- o>></c->
<c- n>using</c-> <c- n>copy_tuple_args</c-> <c- o>=</c-> <c- p>...</c->

<c- n>copy_tuple_args</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>char</c-> <c- o>*</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>Unconstrained</c-><c- o>></c-> <c- n>u1</c-><c- p>;</c-> <c- c1>// OK. Unconstrained&lt;char *, int></c->
<c- n>copy_tuple_args</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>char</c-> <c- o>*</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>Sfinae</c-><c- o>></c-> <c- n>s1</c-><c- p>;</c->        <c- c1>// OK. Sfinae&lt;char *, int></c->
<c- n>copy_tuple_args</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>char</c-> <c- o>*</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>Conceptized</c-><c- o>></c-> <c- n>c1</c-><c- p>;</c->   <c- c1>// Ill-formed instead of desired Conceptized&lt;char *, int> :(</c->
</pre>
   <p>As C++17 template template parameters are incompatible with concept-constrained
templates, any attempt to concept constrain templates in existing code bases that make use of
template template parameters will be likely
to cause difficult or even impossible to fix breakage, discouraging the adoption of Concepts:</p>
<pre class="language-C++ highlight"><c- c1>// Valid C++17 template</c->
<c- n>template</c-><c- o>&lt;</c-><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-><c- p>,</c-> <c- n>class</c-><c- o>></c-> <c- n>class</c-> <c- n>TT</c-><c- p>,</c-> <c- n>class</c-> <c- n>A</c-><c- o>></c->
<c- k>struct</c-> <c- n>curry</c-> <c- p>{</c->    
    <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>B</c-><c- o>></c->
    <c- n>using</c-> <c- n>type</c-> <c- o>=</c-> <c- n>TT</c-><c- o>&lt;</c-><c- n>A</c-><c- p>,</c-> <c- n>B</c-><c- o>></c-><c- p>;</c-> 
<c- p>};</c-> 

<c- n>curry</c-><c- o>&lt;</c-><c- n>Unconstrained</c-><c- p>,</c-> <c- b>char</c-> <c- o>*>::</c-><c- n>type</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>u3</c-><c- p>;</c->  <c- c1>// OK. Unconstrained&lt;char *, int></c->
<c- n>curry</c-><c- o>&lt;</c-><c- n>Sfinae</c-><c- p>,</c-> <c- b>char</c-> <c- o>*>::</c-><c- n>type</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c->  <c- n>s3</c->         <c- c1>// OK. Sfinae&lt;char *, int></c->
<c- n>curry</c-><c- o>&lt;</c-><c- n>Conceptized</c-><c- p>,</c-> <c- b>char</c-> <c- o>*>::</c-><c- n>type</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>c3</c-><c- p>;</c->    <c- c1>// Ill-formed instead of desired Conceptized&lt;char *, int></c->
</pre>
   There is not even a way to write new code with template template parameters without introducing restrictions on constraints, hindering use cases like <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1069r1.html">Inferencing heap objects (P1069)</a>: 
<pre class="language-c++ highlight"><c- c1>// Notation of P1069</c->
<c- k>auto</c-> <c- n>u4</c-> <c- o>=</c-> <c- n>make_unique</c-><c- o>&lt;</c-><c- n>Unconstrained</c-><c- o>></c-><c- p>(</c-><c- mi>3</c-><c- p>,</c-> <c- mi>5</c-><c- p>);</c-> <c- c1>// OK. unique_ptr&lt;Unconstrained&lt;int, int>></c->
<c- k>auto</c-> <c- n>s4</c-> <c- o>=</c-> <c- n>make_unique</c-><c- o>&lt;</c-><c- n>Sfinae</c-><c- o>></c-><c- p>(</c-><c- mi>3</c-><c- p>,</c-> <c- mi>5</c-><c- p>);</c->        <c- c1>// OK. unique_ptr&lt;Unconstrained&lt;int, int>></c->
<c- k>auto</c-> <c- n>c4</c-> <c- o>=</c-> <c- n>make_unique</c-><c- o>&lt;</c-><c- n>Conceptized</c-><c- o>></c-><c- p>(</c-><c- mi>3</c-><c- p>,</c-> <c- mi>5</c-><c- p>);</c->   <c- c1>// Ill-formed instead of desired Conceptized&lt;int, int></c->
</pre>
   <p>Likewise, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1206r1.pdf">ranges::to (P1206)</a> awkwardly relies on standard library containers being unconstrained:</p>
<pre class="language-c++ highlight"><c- c1>// copy a list to a vector of the same type, deducing value_type</c->
<c- n>Same</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>>></c-> <c- k>auto</c-> <c- n>c</c-> <c- o>=</c-> <c- n>ranges</c-><c- o>::</c-><c- n>to</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>></c-><c- p>(</c-><c- n>l</c-><c- p>);</c-> <c- c1>// OK, vector is not concept-constrained</c->
</pre>
   <h2 class="heading settled" data-level="2" id="solution"><span class="secno">2. </span><span class="content">The solution</span><a class="self-link" href="#solution"></a></h2>
    We propose simply that an unconstrained template
template parameter match concept-constrained classes. E.g., 
<pre class="language-c++ highlight"><c- n>copy_tuple_args</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>char</c-> <c- o>*</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-><c- p>,</c-> <c- n>Conceptized</c-><c- o>></c-> <c- n>c1</c-><c- p>;</c->   <c- c1>// Proposed: Conceptized&lt;char *, int></c->
<c- n>curry</c-><c- o>&lt;</c-><c- n>Conceptized</c-><c- p>,</c-> <c- b>char</c-> <c- o>*>::</c-><c- n>type</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>c3</c-><c- p>;</c->              <c- c1>// Proposed: Conceptized&lt;char *, int></c->
<c- k>auto</c-> <c- n>c4</c-> <c- o>=</c-> <c- n>make_unique</c-><c- o>&lt;</c-><c- n>Conceptized</c-><c- o>></c-><c- p>(</c-><c- mi>3</c-><c- p>,</c-> <c- mi>5</c-><c- p>);</c->              <c- c1>// Proposed: Conceptized&lt;int, int></c->
</pre>
   Note that violations of the constraints will still produce concept-aware error messages 
<pre class="language-c++ highlight"><c- c1>// Proposed: Ill-formed with concept violation as desired</c->
<c- n>copy_tuple_args</c-><c- o>&lt;</c-><c- n>tuple</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>char</c-> <c- o>*></c-><c- p>,</c-> <c- n>Conceptized</c-><c- o>></c-> <c- n>c5</c-><c- p>;</c-> 
</pre>
   <h2 class="heading settled" data-level="3" id="faq0"><span class="secno">3. </span><span class="content">Without a language change, can we accept a general template as a template template parameter?</span><a class="self-link" href="#faq0"></a></h2>
    As far as we can tell, you can’t. So use cases like the above are simply impossible. (Did we
miss something? We’d love to be proven wrong). 
   <h2 class="heading settled" data-level="4" id="faq1"><span class="secno">4. </span><span class="content">Why not create a new notation that matches any template?</span><a class="self-link" href="#faq1"></a></h2>
    While one could imagine some new notation for template template parameters
that works with and without constraints, we believe that would still be an unsatisfactory solution
for the following reasons 
   <ul>
    <li data-md>
     <p>All pre-C++20 template template parameters that were written in the past with the expectation that they would match any templates will fail to work with concept-constrained templates, discouraging existing code bases from conceptizing classes</p>
    <li data-md>
     <p>Teachers will have to teach the subtle point that any constrained class that might ever need to match a template template parameter should be SFINAE constrained rather than concept-constrained. We would much rather stop teaching SFINAE altogether instead of a new subtlety...</p>
    <li data-md>
     <p>A new notation could likely not be added before C++23. Our proposal could easily apply to C++20 as a wording fix to Concepts.</p>
   </ul>
   <h2 class="heading settled" data-level="5" id="faq2"><span class="secno">5. </span><span class="content">What if we want to only match unconstrained templates?</span><a class="self-link" href="#faq2"></a></h2>
    We are unaware of use cases for the current behavior of only matching unconstrained concepts
(especially compared to the significant use cases for this proposal). However, 
note that natural notations could easily be developed and added, e.g., by making the lack of constraints explicit: 
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-><c- p>,</c-> <c- n>class</c-><c- o>></c-> <c- n>requires</c-> <c- p>{}</c-> <c- n>class</c-> <c- n>X</c-><c- o>></c-> <c- c1>// Not proposed at this time</c->
<c- n>class</c-> <c- n>OnlyMatchesUnconstrained</c-> <c- p>{};</c->

<c- n>OnlyMatchesUnconstrained</c-><c- o>&lt;</c-><c- n>Unconstrained</c-><c- o>></c-> <c- n>u5</c-><c- p>;</c-> <c- c1>// OK</c->
<c- n>OnlyMatchesUnconstrained</c-><c- o>&lt;</c-><c- n>Sfinae</c-><c- o>></c-> <c- n>s5</c-><c- p>;</c->        <c- c1>// OK</c->
<c- n>OnlyMatchesUnconstrained</c-><c- o>&lt;</c-><c- n>Conceptized</c-><c- o>></c-> <c- n>s5</c-><c- p>;</c->   <c- c1>// Ill-formed as desired</c->
</pre>
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">6. </span><span class="content">Wording</span>  
      </h2>
      Change the beginning of temp.arg.template/3 to say
      <blockquote>A <em>template-argument</em> matches a template <em>template-parameter</em> <tt>P</tt> when <tt>P</tt> is at least as specialized as the template-argument <tt>A</tt>. <ins>In this comparison, if <tt>P</tt> is unconstrained, the constraints on <tt>A</tt> are not considered.</ins> If <tt>P</tt> contains...</blockquote>
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