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 <head>
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  <title>P1811R0: Relaxing redefinition restrictions for re-exportation robustness</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">P1811R0<br>Relaxing redefinition restrictions for re-exportation robustness</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-07-18">2019-07-18</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1811r0">http://wg21.link/p1811r0</a>
     <dt>Authors:
     <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>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:gdr@microsoft.com">Gabriel Dos Reis</a> (<span class="p-org org">Microsoft</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>The current redefinition rules for entities attached to the global module unnecessarily disallows some natural code patterns. We propose to relax these rules to permit the relevant code patterns.</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="#background"><span class="secno">1</span> <span class="content">Background</span></a>
    <li><a href="#problem"><span class="secno">2</span> <span class="content">Problem</span></a>
    <li>
     <a href="#proposal"><span class="secno">3</span> <span class="content">Proposed solution</span></a>
     <ol class="toc">
      <li><a href="#proposal-translation"><span class="secno">3.1</span> <span class="content">Making <code class="highlight"><c- cp>#include</c-></code> translation optional</span></a>
     </ol>
    <li><a href="#alternatives"><span class="secno">4</span> <span class="content">Alternatives</span></a>
    <li><a href="#impl"><span class="secno">5</span> <span class="content">Implementation experience</span></a>
    <li>
     <a href="#wording"><span class="secno">6</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#allow-redefinition-when-a-prior-definition-is-reachable"><span class="secno">6.1</span> <span class="content">Allow redefinition when a prior definition is reachable</span></a>
      <li><a href="#cleanup-remove-now-unused-definition-of-necessarily-reachable-for-declarations"><span class="secno">6.2</span> <span class="content">Cleanup: remove now-unused definition of "necessarily reachable" for declarations</span></a>
      <li><a href="#make-include-translation-optional"><span class="secno">6.3</span> <span class="content">Make <code class="highlight"><c- cp>#include</c-></code> translation optional</span></a>
      <li><a href="#feature-test-macro"><span class="secno">6.4</span> <span class="content">Feature test macro</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="background"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#background"></a></h2>
   <p>In C++ code prior to C++20, interfaces are exposed by textual inclusion. This
results in definitions of the same entity appearing in multiple translation
units. We require (though unfortunately do not consistently enforce in
practice) that all such definitions are "the same". ("The same" is a complex
issue, but the details aren’t salient to this paper.)</p>
   <p>In addition, we require that there is at most one definition of each entity in
each translation unit. From an implementation perspective, this (necessarily)
weak enforcement of the One Definition Rule serves a number of purposes,
notably:</p>
   <ul>
    <li data-md>
     <p>it permits simpler implementations, as there is no need to cope with
encountering a definition of an entity when another definition is already
known</p>
    <li data-md>
     <p>it ensures that compilation time is not wasted processing redundant
redefinitions</p>
     <div class="example" id="example-de162996">
      <a class="self-link" href="#example-de162996"></a> 
<pre class="highlight">    <c- c1>// foo.h -- user forgot the include guards</c->
    <c- k>struct</c-> <c- n>X</c-> <c- p>{</c-> <c- b>int</c-> <c- n>foo</c-><c- p>;</c-> <c- p>};</c->
</pre>
<pre class="highlight">    <c- cp>#include</c-> "foo.h"
    <c- cp>#include</c-> "foo.h" // error, redefinition
</pre>
     </div>
    <li data-md>
     <p>it catches some bugs where there is an accidental name collision between
two different entities of the same kind (eg, two different <code class="highlight"><c- k>struct</c-> <c- n>X</c-></code>s)</p>
     <div class="example" id="example-5ada4e67">
      <a class="self-link" href="#example-5ada4e67"></a> 
<pre class="highlight">    <c- c1>// foo.h</c->
    <c- k>struct</c-> <c- n>X</c-> <c- p>{</c-> <c- b>int</c-> <c- n>foo</c-><c- p>;</c-> <c- p>};</c->
</pre>
<pre class="highlight">    <c- c1>// bar.h</c->
    <c- k>struct</c-> <c- n>X</c-> <c- p>{</c-> <c- b>int</c-> <c- n>bar</c-><c- p>;</c-> <c- p>};</c->
</pre>
<pre class="highlight">    <c- cp>#include</c-> "foo.h"
    <c- cp>#include</c-> "bar.h" // error, redefinition
</pre>
     </div>
   </ul>
   <p>However, it is important to note that the requirement of only one definition
per translation unit is not necessary for the soundness of the language: the
requirement that all definitions are "the same" is sufficient. We could accept
example 1 and consider example 2 to be ill-formed (likely with no diagnostic
required), but it is preferable to reject such redefinition cases when there is
not a compelling reason to permit them.</p>
   <p>In the current C++20 draft, we continue to allow multiple definitions of
entities that are attached to the global module (entities attached to named
modules can only have one definition). We now allow definitions to be made
reachable from translation units other than the one in which they are defined
(via various forms of <code class="highlight"><c- n>import</c-></code>), and so in an attempt to preserve the above
useful properties to the extent possible, we disallow redefinition of an entity
if a prior definition is "necessarily reachable" -- which, for names attached
to the global module, means the definition can be found through a path of zero
or more <code class="highlight"><c- n>import</c-></code> declarations, where each one other than the first is exported.</p>
   <p>Note that this necessary reachability rule does not satisfy the first goal
above: implementations do need to cope with encountering a definition of an
entity when another definition is already known, because there are cases where
such a definition must be available to template instantiation in some contexts,
despite not being necessarily reachable. (In such cases, we might describe the
definition as "incidentally reachable", although the standard wording doesn’t
use that term.)</p>
   <div class="example" id="example-59164974">
    <a class="self-link" href="#example-59164974"></a> 
<pre class="highlight"><c- c1>// foo.h</c->
<c- cp>#ifndef FOO_H</c->
<c- cp>#define FOO_H</c->
<c- k>struct</c-> <c- n>Foo</c-> <c- p>{</c->
  <c- k>constexpr</c-> <c- b>int</c-> <c- n>get</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>42</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- c1>// bar.h</c->
<c- cp>#include</c-> "foo.h"
<c- c1>// ...</c->
</pre>
<pre class="highlight"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c->
<c- n>import</c-> <c- s>"bar.h"</c-><c- p>;</c->

<c- k>export</c-> <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>constexpr</c-> <c- n>T</c-> <c- n>make</c-><c- p>()</c-> <c- p>{</c->
  <c- k>return</c-> <c- n>T</c-><c- p>(</c-><c- n>Foo</c-><c- p>().</c-><c- n>get</c-><c- p>());</c->
<c- p>}</c->
</pre>
<pre class="highlight"><c- n>import</c-> <c- n>M</c-><c- p>;</c->

<c- c1>// OK, array bound is 42</c->
<c- c1>// Note that make&lt;int> can be instantiated and used here, even though Foo is</c->
<c- c1>// not necessarily reachable from this context.</c->
<c- b>int</c-> <c- n>arr</c-><c- p>[</c-><c- n>make</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>()];</c->

<c- c1>// OK, definition of Foo is valid, because prior definition is not necessarily</c->
<c- c1>// reachable in this context. (Note that the include guard macro FOO_H is not</c->
<c- c1>// made visible by the import of M.)</c->
<c- cp>#include</c-> "foo.h"
</pre>
   </div>
   <h2 class="heading settled" data-level="2" id="problem"><span class="secno">2. </span><span class="content">Problem</span><a class="self-link" href="#problem"></a></h2>
   <p>We permit entities attached to the global module to be made reachable through a
named module, in one of several ways:</p>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#include</c-> "a.h" // struct A {};
<c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c->
<c- k>export</c-> <c- n>import</c-> <c- s>"b.h"</c-><c- p>;</c-> <c- c1>// struct B {}; (in importable header)</c->
<c- k>export</c-> <c- n>A</c-> <c- nf>f</c-><c- p>();</c->
</pre>
   <p>Here, the definitions of <code class="highlight"><c- n>A</c-></code> and <code class="highlight"><c- n>B</c-></code> are reachable in an importer of module <code class="highlight"><c- n>M</c-></code>:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>A</c-></code> is reachable implicitly because it’s used in the declaration of <code class="highlight"><c- n>f</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>B</c-></code> is reachable explicitly because it’s defined in an exported header unit</p>
   </ul>
   <p>This re-export of entities attached to the global module divorces the entities
from their include guard macros, which exposes the possibiltiy that a consumer
of this module interface will encounter a redefinition error with the current
rule:</p>
<pre class="highlight"><c- n>import</c-> <c- n>M</c-><c- p>;</c->
<c- cp>#include</c-> "a.h" // error, redefinition of A
<c- cp>#include</c-> "b.h" // OK, but see below
</pre>
   <p>Due to the <code class="highlight"><c- cp>#include</c-></code> translation rule, the <code class="highlight"><c- cp>#include</c-> "b.h"</code> directive is
translated to <code class="highlight"><c- n>import</c-> <c- s>"b.h"</c-><c- p>;</c-></code> because <code class="highlight"><c- s>"b.h"</c-></code> is an importable header, so
there is no redefinition error in that case. But this does not remove all risk:</p>
<pre class="highlight"><c- c1>// b-impl.h, not an importable header</c->
<c- cp>#ifndef B_IMPL_H</c->
<c- cp>#define B_IMPL_H</c->
<c- k>struct</c-> <c- n>B</c-> <c- p>{};</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- c1>// b.h, importable header</c->
<c- cp>#include</c-> "b-impl.h"
</pre>
<pre class="highlight"><c- n>import</c-> <c- n>M</c-><c- p>;</c->
<c- cp>#include</c-> "b-impl.h" // error, redefinition of B
</pre>
   <p>In both cases, disassociating the <code class="highlight"><c- cp>#include</c-></code> guard macro from the definitions
that it is guarding has removed the protection from redefinition errors that we
were traditionally relying upon.</p>
   <h2 class="heading settled" data-level="3" id="proposal"><span class="secno">3. </span><span class="content">Proposed solution</span><a class="self-link" href="#proposal"></a></h2>
   <p>Change the redefinition restriction as follows: for entities attached to the
global module, permit at most one definition per translation unit, regardless
of whether any definition of the entity is already reachable.</p>
   <p>What we gain:</p>
   <ul>
    <li data-md>
     <p>increased robustness in the presence of re-exportation of entities attached
to the global module</p>
    <li data-md>
     <p>a slightly simpler rule (we can entirely remove the notion of "necessarily
reachable" and replace it with "in the same translation unit"</p>
    <li data-md>
     <p>a rule that is in some sense closer to the pre-C++20 rule</p>
    <li data-md>
     <p>a lower burden on implementations, as there is no longer any requirement to
precisely track which definitions are necessarily reachable</p>
   </ul>
   <p>What we lose:</p>
   <ul>
    <li data-md>
     <p>in principle, there are some cases where we would previously have caught
a name collision between two different definitions of the same type, and
will no be guaranteed to give a diagnostic; as usual, if there are multiple
definitions that are not "the same" (according to the ODR), the program is
ill-formed with no diagnostic required</p>
   </ul>
   <p>Note that this change should apply to all constructs that are permitted to be
repeated across translation units but not within a single translation unit,
including redeclarations of default arguments and default template arguments
in addition to redefinitions of classes, functions, variables, and
enumerations.</p>
   <h3 class="heading settled" data-level="3.1" id="proposal-translation"><span class="secno">3.1. </span><span class="content">Making <code class="highlight"><c- cp>#include</c-></code> translation optional</span><a class="self-link" href="#proposal-translation"></a></h3>
   <p>Assuming the main proposal is accepted, there is another question we can now
consider: should we require implementations to perform <code class="highlight"><c- cp>#include</c-></code> translation
for all headers for which they would permit an <code class="highlight"><c- n>import</c-></code>?</p>
   <p>Previously, we required include translation in part in order to mitigate the
effects of the problem that is solved by this proposal. In the simple <code class="highlight"><c- n>b</c-><c- p>.</c-><c- n>h</c-></code> example above, <code class="highlight"><c- cp>#include</c-></code> translation saves us from a redefinition error unless
a definition in an imported header can also be found in a textual header.</p>
   <p>With the acceptance of this proposal, we can reconsider whether we want to give
implementations the freedom to choose whether to map <code class="highlight"><c- cp>#include</c-></code> directives to <code class="highlight"><c- n>import</c-></code> declarations separately from choosing which header files are
importable. At least one implementation vendor believes that migration to
modules would be eased by permitting these two decisions to be decoupled.</p>
   <p>We propose that implementations be permitted to choose not to perform <code class="highlight"><c- cp>#include</c-></code> translation for importable headers.</p>
   <h2 class="heading settled" data-level="4" id="alternatives"><span class="secno">4. </span><span class="content">Alternatives</span><a class="self-link" href="#alternatives"></a></h2>
   <p>If we keep the current redefinition rule, we will need to ensure that current
header file techniques properly prevent redefinition. This would likely entail</p>
   <ul>
    <li data-md>
     <p>exporting macros from named modules whenever any entity attached to the
global module is exported</p>
    <li data-md>
     <p>ensuring that export of entities from the global module has the same level
of granularity as include guard macros (the export must be "all or nothing"
because an include guard macro can’t be half-defined)</p>
   </ul>
   <p>If we do not wish to allow macro export from named modules and want to keep the
current rule for redefinitions, we conclude that we would need to disallow
re-exportation of entities attached to the global module entirely.</p>
   <p>We don’t consider any of the alternatives in this area to be acceptable, as
they would require us to give up important properties (the ability for a module
to export declarations from a non-modular library, or the guarantee that a
named module import does not bring in macros).</p>
   <h2 class="heading settled" data-level="5" id="impl"><span class="secno">5. </span><span class="content">Implementation experience</span><a class="self-link" href="#impl"></a></h2>
   <p>This approach has been implemented in the Microsoft compiler for 3+ years.</p>
   <p>This approach has also been implemented in a branch of the Clang compiler, and
confirmed to fix the redefinition errors in the above examples.</p>
   <h2 class="heading settled" data-level="6" id="wording"><span class="secno">6. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="allow-redefinition-when-a-prior-definition-is-reachable"><span class="secno">6.1. </span><span class="content">Allow redefinition when a prior definition is reachable</span><a class="self-link" href="#allow-redefinition-when-a-prior-definition-is-reachable"></a></h3>
   <p>Change in 6.2 [basic.def.odr] paragraph 1:</p>
   <blockquote>
    <ins>No translation unit shall contain more than one definition of any</ins>
    <del>A</del>
     variable, function, class type, enumeration type, or template 
    <del>shall not be defined where a prior definition is necessarily reachable
(10.6); no diagnostic is required if the prior declaration is in another more
than once in a single translation unit</del>
    . 
   </blockquote>
   <p>Change in 6.2 [basic.def.odr] paragraph 12:</p>
   <blockquote>
    <p>
     There can be more than one definition of a class type (Clause 11),
enumeration type (9.6), inline function with external linkage (9.1.6), inline
variable with external linkage (9.1.6), class template (Clause 13),
non-static function template (13.6.6), concept (13.6.8), static data member
of a class template (13.6.1.3), member function of a class template
(13.6.1.1), or template specialization for which some template parameters are
not specified (13.8, 13.6.5) in a program provided that 
     <del>no prior definition
is necessarily reachable (10.6) at the point where a definition appears</del>
     <ins>each
definition appears in a different translation unit</ins>
     , and
provided the definitions satisfy the following requirements [...].
    </p>
   </blockquote>
   <p><em>Drafting note:</em> The two prior changes revert edits from P1103R3.</p>
   <h3 class="heading settled" data-level="6.2" id="cleanup-remove-now-unused-definition-of-necessarily-reachable-for-declarations"><span class="secno">6.2. </span><span class="content">Cleanup: remove now-unused definition of "necessarily reachable" for declarations</span><a class="self-link" href="#cleanup-remove-now-unused-definition-of-necessarily-reachable-for-declarations"></a></h3>
   <p>Change in 10.5 [module.context] paragraph 7:</p>
   <blockquote>
    <p>
     [<em>Example</em>: …
so the definition of <code class="highlight"><c- n>X</c-></code> 
     <del>is not necessarily</del>
     <ins>need not
be</ins>
      reachable, as described in [module.reach].
—<em>end example</em>]
    </p>
   </blockquote>
   <p>Change in 10.6 [module.reach] paragraph 3:</p>
   <blockquote>
    <p>
     A declaration <em>D</em> is reachable 
     <del>or necessarily reachable, respectively,</del>
      if,
for any point <em>P</em> in the instantiation context (10.5),
    </p>
    <ul>
     <li data-md>
      <p><em>D</em> appears prior to <em>P</em> in the same translation unit, or</p>
     <li data-md>
      <p>
       <em>D</em> is not discarded (10.4), appears in a translation unit that is
reachable 
       <del>or necessarily reachable</del>
        from <em>P</em>
       <del>,
respectively,</del>
        and either does not appear within a <em>private-module-fragment</em> or appears in a <em>private-module-fragment</em> of the
module containing <em>P</em>.
      </p>
    </ul>
   </blockquote>
   <h3 class="heading settled" data-level="6.3" id="make-include-translation-optional"><span class="secno">6.3. </span><span class="content">Make <code class="highlight"><c- cp>#include</c-></code> translation optional</span><a class="self-link" href="#make-include-translation-optional"></a></h3>
   <p>Change in 15.2 [cpp.include] paragraph 7:</p>
   <blockquote>
    <p>
     If the header identified by the <em>header-name</em> denotes an importable header (10.3), 
     <ins>it
is implementation-defined whether</ins>
      the preprocessing directive is
instead replaced by the <em>preprocessing-token</em>s
    </p>
<pre class="highlight"><c- n>import</c-> <c- n>header</c-><c- o>-</c-><c- n>name</c-> <c- p>;</c->
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="6.4" id="feature-test-macro"><span class="secno">6.4. </span><span class="content">Feature test macro</span><a class="self-link" href="#feature-test-macro"></a></h3>
   <p>While a feature test macro for this functionality is not obviously useful, a
feature test macro for the Modules feature overall does seem useful.</p>
   <p>Add a feature test macro <code class="highlight"><c- n>__cpp_modules</c-></code> with a suitable value to Table 17 in
15.10 [cpp.predefined].</p>
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