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 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P1766R0: Mitigating minor modules maladies</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">P1766R0<br>Mitigating minor modules maladies</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-06-16">2019-06-16</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1766r0">http://wg21.link/p1766r0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:richard@metafoo.co.uk">Richard Smith</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/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>This paper identifies three minor issues relevant to modules and suggests resolutions</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="#typedef-names-for-linkage-purposes"><span class="secno">1</span> <span class="content">Typedef names for linkage purposes</span></a>
     <ol class="toc">
      <li><a href="#compiler-misbehavior"><span class="secno">1.1</span> <span class="content">Compiler misbehavior</span></a>
      <li><a href="#modules-impact"><span class="secno">1.2</span> <span class="content">Modules impact</span></a>
      <li><a href="#proposed-resolution"><span class="secno">1.3</span> <span class="content">Proposed resolution</span></a>
      <li><a href="#rationale-for-fixing-for-c20"><span class="secno">1.4</span> <span class="content">Rationale for fixing for C++20</span></a>
     </ol>
    <li>
     <a href="#static-assertions-and-empty-declarations-in-export"><span class="secno">2</span> <span class="content">Static assertions and empty declarations in <code class="highlight"><c- k>export</c-></code></span></a>
     <ol class="toc">
      <li><a href="#problem"><span class="secno">2.1</span> <span class="content">Problem</span></a>
      <li><a href="#proposed-resolution①"><span class="secno">2.2</span> <span class="content">Proposed resolution</span></a>
      <li><a href="#rationale-for-fixing-for-c20①"><span class="secno">2.3</span> <span class="content">Rationale for fixing for C++20</span></a>
     </ol>
    <li>
     <a href="#default-argument-inconsistency"><span class="secno">3</span> <span class="content">Default argument inconsistency</span></a>
     <ol class="toc">
      <li><a href="#problem①"><span class="secno">3.1</span> <span class="content">Problem</span></a>
      <li><a href="#proposal"><span class="secno">3.2</span> <span class="content">Proposal</span></a>
      <li><a href="#rationale-for-fixing-for-c20②"><span class="secno">3.3</span> <span class="content">Rationale for fixing for C++20</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="typedef-names-for-linkage-purposes"><span class="secno">1. </span><span class="content">Typedef names for linkage purposes</span><a class="self-link" href="#typedef-names-for-linkage-purposes"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="compiler-misbehavior"><span class="secno">1.1. </span><span class="content">Compiler misbehavior</span><a class="self-link" href="#compiler-misbehavior"></a></h3>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- b>int</c-> <c- n>X</c-><c- p>;</c->
<c- k>typedef</c-> <c- k>struct</c-> <c- p>{</c->
  <c- b>int</c-> <c- o>*</c-><c- n>n</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>X</c-><c- o>&lt;</c-><c- k>decltype</c-><c- p>(</c-><c- k>this</c-><c- p>)</c-><c- o>></c-><c- p>;</c->
<c- p>}</c-> <c- n>Y</c-><c- p>;</c->
<c- n>Y</c-> <c- n>y</c-><c- p>;</c->
</pre>
   <p>GCC miscompiles this (incorrectly giving <code class="highlight"><c- n>X</c-><c- o>&lt;</c-><c- n>Y</c-><c- o>></c-></code> internal linkage).</p>
   <p>Clang rejects, giving a hint as to what’s going on:</p>
<pre class="highlight"><c- o>&lt;</c-><c- n>source</c-><c- o>>:</c-><c- mi>4</c-><c- o>:</c-><c- mi>3</c-><c- o>:</c-> <c- nl>error</c-><c- p>:</c-> <c- nl>unsupported</c-><c- p>:</c-> <c- k>typedef</c-> <c- n>changes</c-> <c- n>linkage</c-> <c- n>of</c-> <c- n>anonymous</c-> <c- n>type</c-><c- p>,</c-> <c- n>but</c-> <c- n>linkage</c-> <c- n>was</c-> <c- n>already</c-> <c- n>computed</c->
<c- p>}</c-> <c- n>Y</c-><c- p>;</c->
  <c- o>^</c->
<c- o>&lt;</c-><c- n>source</c-><c- o>>:</c-><c- mi>2</c-><c- o>:</c-><c- mi>15</c-><c- o>:</c-> <c- nl>note</c-><c- p>:</c-> <c- n>use</c-> <c- n>a</c-> <c- n>tag</c-> <c- n>name</c-> <c- n>here</c-> <c- n>to</c-> <c- n>establish</c-> <c- n>linkage</c-> <c- n>prior</c-> <c- n>to</c-> <c- n>definition</c->
<c- k>typedef</c-> <c- k>struct</c-> <c- p>{</c->
              <c- o>^</c->
               <c- n>Y</c->
</pre>
   <p>This is not an isolated example: lots of constructs within an anonymous struct
can trigger a linkage computation, but the linkage computation cannot be
performed until <em>after</em> we reach the end of the struct and find out whether it
has a typedef name for linkage purposes.</p>
   <p>The <code class="highlight"><c- k>typedef</c-></code> keyword doesn’t help us here. This case does not have a typedef
name for linkage purposes:</p>
<pre class="highlight"><c- k>typedef</c-> <c- k>struct</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c-> <c- o>*</c-><c- n>p</c-><c- p>;</c->
</pre>
   <p>... and this case does:</p>
<pre class="highlight"><c- k>struct</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c-> <c- k>typedef</c-> <c- n>x</c-><c- p>;</c-> <c- c1>// ?!!</c->
</pre>
   <h3 class="heading settled" data-level="1.2" id="modules-impact"><span class="secno">1.2. </span><span class="content">Modules impact</span><a class="self-link" href="#modules-impact"></a></h3>
   <p>When such a construct appears in a header unit, the problem is exacerbated. It
is possible for the compiler to be aware of another definition of a struct from
a different translation unit, but for that other definition to not be
reachable:</p>
<pre class="highlight"><c- c1>// foo.h</c->
<c- cp>#ifndef FOO_H</c->
<c- cp>#define FOO_H</c->
<c- k>typedef</c-> <c- k>struct</c-> <c- p>{</c-> <c- d>/*...*/</c-> <c- p>}</c-> <c- n>X</c-><c- p>;</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#include</c-> "foo.h"
<c- k>export</c-> <c- n>module</c-> <c- n>A</c-><c- p>;</c->
<c- n>X</c-> <c- n>x</c-><c- p>;</c->
</pre>
<pre class="highlight"><c- k>export</c-> <c- n>module</c-> <c- n>B</c-><c- p>;</c->
<c- n>import</c-> <c- n>A</c-><c- p>;</c->
</pre>
<pre class="highlight"><c- n>import</c-> <c- n>B</c-><c- p>;</c->
<c- cp>#include</c-> "foo.h"
</pre>
   <p>In the final translation unit above, the definition of <code class="highlight"><c- n>X</c-></code> is not (necessarily)
reachable, so including a definition of <code class="highlight"><c- n>X</c-></code> is acceptable. However, a compiler
may already know of the definition of <code class="highlight"><c- n>X</c-></code> in module <code class="highlight"><c- n>A</c-></code>. This results in a need
to "merge" the new definition with the old one. There are various techniques
for this, but they typically rely on knowing that the definition is in fact a
redeclaration of the already-known definition before anything "too complicated"
happens.</p>
   <h3 class="heading settled" data-level="1.3" id="proposed-resolution"><span class="secno">1.3. </span><span class="content">Proposed resolution</span><a class="self-link" href="#proposed-resolution"></a></h3>
   <p>Typedef names for linkage purposes exist for C compatibility. Therefore it
would be reasonable to restrict their use to C-compatible structures.</p>
   <p>We propose disallowing any case where a class is given a typedef
name for linkage and its members include anything other than:</p>
   <ul>
    <li data-md>
     <p>non-static data members with no default member initializers
(and no <em>lambda-expression</em>s in the type of the member)</p>
    <li data-md>
     <p><code class="highlight"><c- k>static_assert</c-></code> declarations</p>
    <li data-md>
     <p>empty declarations</p>
   </ul>
   <p>This is intended to ensure that structs with typedef names for linkage
purposes are always simple enough that no linkage calculation is necessary
before the typedef name for linkage is encountered, and likewise that the
post-facto "merge" step is simple.</p>
   <h3 class="heading settled" data-level="1.4" id="rationale-for-fixing-for-c20"><span class="secno">1.4. </span><span class="content">Rationale for fixing for C++20</span><a class="self-link" href="#rationale-for-fixing-for-c20"></a></h3>
   <p>This is a bugfix; the issue is longstanding but is expected to become more
significant in the presence of modules in C++20.</p>
   <h2 class="heading settled" data-level="2" id="static-assertions-and-empty-declarations-in-export"><span class="secno">2. </span><span class="content">Static assertions and empty declarations in <code class="highlight"><c- k>export</c-></code></span><a class="self-link" href="#static-assertions-and-empty-declarations-in-export"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="problem"><span class="secno">2.1. </span><span class="content">Problem</span><a class="self-link" href="#problem"></a></h3>
   <p>In the Modules TS and all subsequent proposals, there is a restriction that an <code class="highlight"><c- k>export</c-></code> declaration or <code class="highlight"><c- k>export</c-></code> block can contain only declarations that
introduce at least one name.</p>
   <p>For individual <code class="highlight"><c- k>export</c-></code> declarations, this makes sense:</p>
<pre class="highlight"><c- k>export</c-> <c- nf>static_assert</c-><c- p>(</c->true<c- p>);</c-> <c- c1>// error: what do you mean, 'export'?</c->
</pre>
   <p>But for <code class="highlight"><c- k>export</c-></code> blocks, this rule introduces some awkwardness:</p>
<pre class="highlight"><c- k>export</c-> <c- p>{</c->
  <c- k>struct</c-> <c- n>Foo</c-> <c- p>{</c-> <c- d>/*...*/</c-> <c- p>};</c->
  <c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_trivially_copyable_v</c-><c- o>&lt;</c-><c- n>Foo</c-><c- o>></c-><c- p>);</c-> <c- c1>// error: can’t export this!</c->

  <c- k>struct</c-> <c- n>Bar</c-> <c- p>{</c-> <c- d>/*...*/</c-> <c- p>};</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>struct</c-> <c- n>X</c-> <c- p>{</c-> <c- n>T</c-> <c- n>t</c-><c- p>;</c-> <c- p>};</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>X</c-><c- p>(</c-><c- n>T</c-><c- p>)</c-> <c- o>-></c-> <c- n>X</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>;</c-> <c- c1>// error: can’t export this either!</c->

  <c- c1>// ...</c->

<c- cp>#define STR(x) constexpr char x[] = #x;</c->
  <c- n>STR</c-><c- p>(</c-><c- n>foo</c-><c- p>);</c->  <c- c1>// error: can’t export empty-declaration ';'</c->
  <c- n>STR</c-><c- p>(</c-><c- n>bar</c-><c- p>);</c->  <c- c1>// error: can’t export empty-declaration ';'</c->
<c- cp>#undef X</c->
<c- p>}</c->
</pre>
   <p>To avoid this, the code must be reorganized: the <code class="highlight"><c- k>export</c-></code> block could be split
into pieces, or the <code class="highlight"><c- k>static_assert</c-></code> and deduction guide could be moved elsewhere
(and the redundant <code class="highlight"><c- p>;</c-></code>s after the macros could be removed).</p>
   <h3 class="heading settled" data-level="2.2" id="proposed-resolution①"><span class="secno">2.2. </span><span class="content">Proposed resolution</span><a class="self-link" href="#proposed-resolution①"></a></h3>
   <p>Remove the rule that a declaration in an <code class="highlight"><c- k>export</c-></code> block introduces at least one
name. Retain the rule for the single-declaration form of <code class="highlight"><c- k>export</c-></code>.</p>
   <h3 class="heading settled" data-level="2.3" id="rationale-for-fixing-for-c20①"><span class="secno">2.3. </span><span class="content">Rationale for fixing for C++20</span><a class="self-link" href="#rationale-for-fixing-for-c20①"></a></h3>
   <p>This fixes a usability problem with a new C++20 feature.</p>
   <h2 class="heading settled" data-level="3" id="default-argument-inconsistency"><span class="secno">3. </span><span class="content">Default argument inconsistency</span><a class="self-link" href="#default-argument-inconsistency"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="problem①"><span class="secno">3.1. </span><span class="content">Problem</span><a class="self-link" href="#problem①"></a></h3>
   <p>For non-inline function declarations, C++ permits default arguments to be
different in different translation units. Likewise, for templates, C++ permits
default template arguments to be different in different translation units:</p>
<pre class="highlight"><c- c1>// a.h</c->
<c- b>int</c-> <c- nf>f</c-><c- p>(</c-><c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- mi>123</c-><c- p>);</c->
</pre>
<pre class="highlight"><c- c1>// b.h</c->
<c- b>int</c-> <c- nf>f</c-><c- p>(</c-><c- b>int</c-> <c- n>a</c-> <c- o>=</c-> <c- mi>45</c-><c- p>);</c->
</pre>
   <p>It would be an error if both declarations were included into the same
translation unit. But modules creates situations where both declarations
may be simultaneously reachable (even though only one of them is visible)
and it is unduly burdensome on an implementation to require that only the
visible default argument is used, or even that the mismatch is detected
and diagnosed.</p>
<pre class="highlight"><c- n>import</c-> <c- n>A</c-><c- p>;</c->     <c- c1>// indirectly uses a.h, does not export it</c->
<c- n>import</c-> <c- s>"b.h"</c-><c- p>;</c->
<c- b>int</c-> <c- n>n</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c->  <c- c1>// ??</c->
</pre>
   <h3 class="heading settled" data-level="3.2" id="proposal"><span class="secno">3.2. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h3>
   <p>Disallow giving the same function parameter different default arguments in two
declarations in the same namespace scope in different translation units, and
likewise for default template arguments. No diagnostic is required unless one
such function declaration is reachable where the other is encountered.</p>
   <p>Note that we already have a similar rule for inline functions.</p>
   <h3 class="heading settled" data-level="3.3" id="rationale-for-fixing-for-c20②"><span class="secno">3.3. </span><span class="content">Rationale for fixing for C++20</span><a class="self-link" href="#rationale-for-fixing-for-c20②"></a></h3>
   <p>This fixes an area of underspecification and friction in a new C++20 feature.</p>
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