<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P2003R0: Fixing Internal and External Linkage Entities in Header Units</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 {
		border-bottom: 2px solid;
	}

	table.data colgroup,
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		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
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		border-right: 2px solid;
		border-top: 1px solid silver;
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		text-align: center;
	}

	table.data.longlastcol td:last-child,
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	 vertical-align: baseline;
	 text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
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Possible extra rowspan handling

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	table.data  tbody td[rowspan]:first-child,
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		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
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		border-bottom-width: 3px !important;
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	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
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	.toc > li li li       { font-size:   95%;    }
	.toc > li li li li    { font-size:   90%;    }
	.toc > li li li li .secno { font-size: 85%; }
	.toc > li li li li li { font-size:   85%;    }
	.toc > li li li li li .secno { font-size: 100%; }

	/* @supports not (display:grid) { */
		.toc > li             { margin: 1.5rem 0;    }
		.toc > li li          { margin: 0.3rem 0;    }
		.toc > li li li       { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
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			float: left;
			width: 4rem;
			white-space: nowrap;
		}

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			clear: both;
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		.toc .secno                  { margin-left: -5rem; }
		.toc > li li li .secno       { margin-left: -7rem; }
		.toc > li li li li .secno    { margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc              { margin-left:  4rem; }
			.toc .secno                  { margin-left: -4rem; }
			.toc > li li li              { margin-left:  1rem; }
			.toc > li li li .secno       { margin-left: -5rem; }
			.toc > li li li li .secno    { margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			   comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
		}
		#toc .content:hover {
			background: rgba(75%, 75%, 75%, .25);
			border-bottom: 3px solid #054572;
			margin-bottom: -3px;
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index       { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li    { margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em }
	ul.index dl    { margin-top: 0; }
	ul.index dt    { margin: .2em 0 .2em 20px;}
	ul.index dd    { margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
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	table.index td,
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	}

	/* The link in the first column in the property table (formerly a TD) */
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		font-weight: bold;
	}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

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			margin: 0;
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			font-family: serif;
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		margin: 1.5cm 1.1cm;
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/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
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/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

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		margin-right: auto;
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			*/
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	}
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<style>
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dfn > a.self-link::before      { content: "#"; }</style>
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}</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P2003R0<br>Fixing Internal and External Linkage Entities in Header Units</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2020-01-13">2020-01-13</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p2003r0">http://wg21.link/p2003r0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:<michael_spencer@apple.com>">Michael Spencer</a> (<span class="p-org org">Apple</span>)
     <dt>Audience:
     <dd>EWG, SG2
     <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>Currently internal linkage entities (such as <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code> functions) and external linkage entities are allowed in header units, but there are two
  problems with how they are currently specified. First, C++ currently requires
  there to be a single header unit for a given header or source file
  ([module.import]/5). This means that regardless of how many times or from
  where a header is imported, there must be a single copy of any internal or
  external linkage entities within it. The second is that it is ill-formed to use internal linkage entities outside of that
  header unit. This is problematic as it is quite common for headers that want
  to be compatible with C to use <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code> for inline functions, as C’s
  version of inline works differently.</p>
   <p>This paper explores these problems and what current implementations do in

  these cases, and fixes it to be more compatible with current/expected usage by
  defining a model compatible with those implementations, which addresses the
  issues raised by US133 and US134.</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="#tony-table"><span class="secno">1</span> <span class="content">Effects of This Paper</span></a>
    <li><a href="#problem"><span class="secno">2</span> <span class="content">The Problem</span></a>
    <li>
     <a href="#solution"><span class="secno">3</span> <span class="content">The Solution</span></a>
     <ol class="toc">
      <li><a href="#ub"><span class="secno">3.1</span> <span class="content">Undefined Behavior</span></a>
     </ol>
    <li><a href="#target"><span class="secno">4</span> <span class="content">Ship Vehicle</span></a>
    <li><a href="#wording"><span class="secno">5</span> <span class="content">Wording</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="tony-table"><span class="secno">1. </span><span class="content">Effects of This Paper</span><a class="self-link" href="#tony-table"></a></h2>
   <table style="margin-left:auto;margin-right:auto;">
    <thead>
     <tr>
      <th>Status Quo
      <th>This Paper
    <tbody>
     <tr>
      <td>
        header.h 
<pre class="highlight"><c- cp>#ifndef HEADER_H</c->
<c- cp>#define HEADER_H</c->

<c- b>int</c-> <c- nf>ext</c-><c- p>();</c->

<c- c1>// Has address identity.</c->
<c- k>static</c-> <c- kr>inline</c-> <c- b>int</c-> <c- nf>abs</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- k>return</c-> <c- n>V</c-> <c- o>>=</c-> <c- mi>0</c-> <c- o>?</c-> <c- nl>V</c-> <c- p>:</c-> <c- o>-</c-><c- n>V</c-><c- p>;</c->
<c- p>}</c->

<c- b>int</c-> <c- nf>a</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->           <c- c1>// ✅</c->
  <c- k>return</c-> <c- n>abs</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->
<c- p>}</c->

<c- kr>inline</c-> <c- b>int</c-> <c- nf>b</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->    <c- c1>// ✅</c->
  <c- k>return</c-> <c- n>abs</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->         <c- c1>// ✅</c->
<c- p>}</c->

<c- c1>// Guaranteed to be called once.</c->
<c- k>static</c-> <c- b>int</c-> <c- n>glob</c-> <c- o>=</c-> <c- n>ext</c-><c- p>();</c-> <c- c1>// ✅</c->

<c- k>struct</c-> <c- n>c</c-> <c- p>{</c-> <c- b>int</c-> <c- n>var</c-><c- p>;</c-> <c- p>};</c->   <c- c1>// ✅</c->

<c- b>int</c-> <c- n>ext_glob</c-> <c- o>=</c-> <c- n>ext</c-><c- p>();</c->    <c- c1>// ✅</c->
<c- cp>#endif</c->
</pre>
       <p>func.cpp</p>
<pre class="highlight"><c- n>import</c-> <c- s>"header.h"</c->

<c- b>int</c-> <c- n>func</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ❌</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ❌</c->
<c- p>}</c->

<c- kr>inline</c-> <c- b>int</c-> <c- n>func2</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ❌</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ❌</c->
<c- p>}</c->
</pre>
       <p>m.cpp</p>
<pre class="highlight"><c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->
<c- n>import</c-> <c- s>"header.h"</c->

<c- k>export</c-> <c- b>int</c-> <c- n>mfunc</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ❌</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ❌</c->
<c- p>}</c->


<c- k>export</c-> <c- kr>inline</c-> <c- b>int</c-> <c- n>mfunc2</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ❌</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ❌</c->
<c- p>}</c->
</pre>
      <td>
        header.h 
<pre class="highlight"><c- cp>#ifndef HEADER_H</c->
<c- cp>#define HEADER_H</c->

<c- b>int</c-> <c- nf>ext</c-><c- p>();</c->

<c- c1>// Does not have address identity.</c->
<c- k>static</c-> <c- kr>inline</c-> <c- b>int</c-> <c- nf>abs</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- k>return</c-> <c- n>V</c-> <c- o>>=</c-> <c- mi>0</c-> <c- o>?</c-> <c- nl>V</c-> <c- p>:</c-> <c- o>-</c-><c- n>V</c-><c- p>;</c->
<c- p>}</c->

<c- b>int</c-> <c- nf>a</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->           <c- c1>// ☠️ ODR, linker error</c->
  <c- k>return</c-> <c- n>abs</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->
<c- p>}</c->

<c- kr>inline</c-> <c- b>int</c-> <c- nf>b</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->    <c- c1>// ✅</c->
  <c- k>return</c-> <c- n>abs</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->         <c- c1>// ☠️ ODR, different def</c->
<c- p>}</c->

<c- c1>// Multiple definitions.</c->
<c- k>static</c-> <c- b>int</c-> <c- n>glob</c-> <c- o>=</c-> <c- n>ext</c-><c- p>();</c-> <c- c1>// ✅</c->

<c- k>struct</c-> <c- n>c</c-> <c- p>{</c-> <c- b>int</c-> <c- n>var</c-><c- p>;</c-> <c- p>};</c->   <c- c1>// ✅</c->

<c- b>int</c-> <c- n>ext_glob</c-> <c- o>=</c-> <c- n>ext</c-><c- p>();</c->    <c- c1>// ☠️ ODR, linker error</c->
<c- cp>#endif</c->
</pre>
       <p>func.cpp</p>
<pre class="highlight"><c- n>import</c-> <c- s>"header.h"</c->

<c- b>int</c-> <c- n>func</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ✅</c->
<c- p>}</c->

<c- kr>inline</c-> <c- b>int</c-> <c- n>func2</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ☠️ ODR, different def</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ☠️ ODR, different def</c->
<c- p>}</c->
</pre>
       <p>m.cpp</p>
<pre class="highlight"><c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->
<c- n>import</c-> <c- s>"header.h"</c->

<c- k>export</c-> <c- b>int</c-> <c- n>mfunc</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ✅</c->
<c- p>}</c->

<c- c1>// These two would be ill-formed under P1815.</c->
<c- k>export</c-> <c- kr>inline</c-> <c- b>int</c-> <c- n>mfunc2</c-><c- p>(</c-><c- b>int</c-> <c- n>V</c-><c- p>)</c-> <c- p>{</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>glob</c-><c- p>;</c->             <c- c1>// ☠️ ODR, different def</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>ext_glob</c-><c- p>;</c->         <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>a</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- n>V</c-> <c- o>+=</c-> <c- n>b</c-><c- p>(</c-><c- n>V</c-><c- p>);</c->             <c- c1>// ✅</c->
  <c- k>return</c-> <c- nf>abs</c-><c- p>(</c-><c- n>V</c-><c- p>)</c-> <c- o>+</c-> <c- mi>4</c-><c- p>;</c->     <c- c1>// ☠️ ODR, different def</c->
<c- p>}</c->
</pre>
     <tr>
   </table>
   <p>Legend:</p>
   <ul>
    <li data-md>
     <p>✅ Works as expected.</p>
    <li data-md>
     <p>❌ Ill-formed.</p>
    <li data-md>
     <p>☠️ Undefined behavior</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="problem"><span class="secno">2. </span><span class="content">The Problem</span><a class="self-link" href="#problem"></a></h2>
   <p>[module.import]/5</p>
   <blockquote>
    <p>... Two <em>module-import-declarations</em> import the same header unit if and
only if their <em>header-names</em> identify the same header or source file.
[ ... ] A declaration of a name with internal linkage is permitted within a
header unit despite all declarations being implicitly exported. If such a
declaration declares an entity that is odr-used outside the header unit, or by
a template instantiation whose point of instantiation is outside the header
unit, the program is ill-formed.</p>
   </blockquote>
   <p>This requires a single header unit for each imported header and prohibits using
internal linkage entities from that header unit.</p>
   <p>The first problem with this is that requring a single header unit in a program
for a given header file borders on unimplementable. Some compiler invocation
has to produce that header unit, but there’s no reasonable way to decide which
one. For example, two different libraries developed by two unrelated groups use
a C library libO. Both of them <code class="highlight"><c- n>import</c-> <c- o>&lt;</c-><c- n>O</c-><c- o>/</c-><c- n>functionality</c-><c- p>.</c-><c- n>h</c-><c- o>></c-></code> which includes a
static local variable with a dynamic initalizer which observes how many
times it was called. A third group of developers comes along and tries to use
both of these libraries in the same program. How do you ensure that static local
variable is only initialized once? libO is a C library written before
C++20 modules existed, and so it doesn’t expect to need to build the header unit
for O/functionality.h, so it can’t be responsible. The two other libraries don’t
even know about each other, so neither one of those can assume it can build it.
The third group doesn’t even know this header exists as it’s an internal
implementation detail of the other two libraries.</p>
   <p>The end result is there’s no sane way to implement the requirement that there’s
a single header unit for each imported header or source file.</p>
   <p>The second problem is that <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code> functions are pretty common in
headers for two main reasons:</p>
   <ul>
    <li data-md>
     <p>C’s inline semantics are different from C++, and would require a non-inline
definition somewhere.</p>
    <li data-md>
     <p>They provide ABI isolation as you’ll never get a different version from what
the compiler saw. This means you can totally change how they work without
fear of breaking ABI.</p>
   </ul>
   <p>A search on github finds thousands of uses of static inline in C headers
including projects such as Wayland, OpenSSL, Clang builtins, mono. This
is also used pervasively on Apple platforms in the form of <code class="highlight"><c- n>NS_INLINE</c-></code> which is
defined as <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code>. Additionally, almost all inline functions in libc++
on Apple platforms are <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code> for ABI isolation reasons, even member
functions (via an extension). Even if both of these cases could be changed, just
fixing these isn’t enough. There are still at least 10s of thousands of
instances of <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code> in the wild, and it would be unfortunate if they
were not usable as header units.</p>
   <h2 class="heading settled" data-level="3" id="solution"><span class="secno">3. </span><span class="content">The Solution</span><a class="self-link" href="#solution"></a></h2>
   <p>Clang modules currently supports internal linkage entities by simply copying
them into whatever translation unit imports the module. This works quite well
as it’s the same semantics you would get by textually including the header the
Clang module was build from. However, this may not be the best way to specify
it.</p>
   <p>Instead, this paper proposes making two changes. The first is to say that
instead of there being a single header unit for each header or source file,
there may be multiple, and it’s unspecified which one is imported. The second is
that you may name imported internal linkage entities from header units with
the same ODR implications as today.</p>
   <p>The first item fixes the first problem above about not knowing who’s responsible
for building the header unit, as now every TU that imports the header can build
its own. This also fixes the issue with external linkage entities as you’ll
likely end up with multiple definitions of them, which would be an error. Note
that specifying it this was does not require that each importing TU build its
own header unit. A sufficently advanced build sytem could still build a single
one for the entire program.</p>
   <p>The second item fixes the second problem above by removing the restriction on
naming internal linkage entities. This is implementable by either copying the
definition into the importing TU, or by doing linkage promotion, as the spec
allows there to be one or more instances of any internal linkage entity in this
case.</p>
   <h3 class="heading settled" data-level="3.1" id="ub"><span class="secno">3.1. </span><span class="content">Undefined Behavior</span><a class="self-link" href="#ub"></a></h3>
   <p>This paper introduces two different types of UB. The first is that by allowing
multiple header units to exist for the same header or source file without
disallowing certain external linkage entities in header units, it allows for
violating the rule that <em>Every program shall contain exactly one definition
of every non-inline function or variable …</em>. While a violation of this rule
does not require a diagnostic, in the majority of situations a user will get
a multiple definition error. This could be made ill-formed instead, but it was
felt it was simpler at this time to stay closer to header semantics.</p>
   <p>The second type of UB is a more inconspicuous case. The one definition rule
(ODR) allows for multiple definitions of inline variables and functions, but
they must be the "same". Any inline definition that references an internal
linkage entitiy is a ODR violation waiting to happen. However, <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code> functions are extremely
common today, and a large portion of C headers would not be usable as header
units if they were not allowed to be referenced. As header units exists purely to support existing
code and code that will likely never move to modules (like C code), it would
significantly harm modules adoption to not allow this. The consequences are the
same as using <code class="highlight"><c- k>static</c-> <c- kr>inline</c-></code> from a textual header today. We’re not introducing
any new UB, just not stopping people from hitting it.</p>
   <h2 class="heading settled" data-level="4" id="target"><span class="secno">4. </span><span class="content">Ship Vehicle</span><a class="self-link" href="#target"></a></h2>
   <p>This paper is targeting C++20 as many C headers are not importable without it.</p>
   <h2 class="heading settled" data-level="5" id="wording"><span class="secno">5. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <p>TBD</p>
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