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  <title>P1857R1: Modules Dependency Discovery</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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/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


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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">P1857R1<br>Modules Dependency Discovery</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-10-07">2019-10-07</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P1857r1">http://wg21.link/P1857r1</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, SG15
     <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 changes to <code class="highlight"><c- n>import</c-></code> in <a data-link-type="biblio" href="#biblio-p1703r1">[P1703r1]</a> were both too much

  and not enough to enable fast dependency scanning via partial preprocessing.
  It made <code class="highlight"><c- n>import</c-></code> at the start of a line anywhere in the program an <em>import-keyword</em>, breaking things like <code class="highlight"><c- n>import</c-><c- o>-></c-><c- n>doImport</c-><c- p>();</c-></code>.
  Also, no similar changes were applied to <code class="highlight"><c- n>module</c-></code>.  This paper
  fine-tunes the context-sensitivity of <code class="highlight"><c- n>import</c-></code> and <code class="highlight"><c- n>module</c-></code>, applying the fixes discussed on the reflector.</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="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#fast-dep-scan"><span class="secno">2.1</span> <span class="content">Fast Dependency Scanning</span></a>
      <li><a href="#import-relaxed"><span class="secno">2.2</span> <span class="content"><code class="highlight"><c- n>import</c-></code> is Too Relaxed</span></a>
     </ol>
    <li>
     <a href="#discussion"><span class="secno">3</span> <span class="content">Discussion</span></a>
     <ol class="toc">
      <li><a href="#implementation"><span class="secno">3.1</span> <span class="content">Implementation</span></a>
      <li><a href="#one-line"><span class="secno">3.2</span> <span class="content">One Line Restriction</span></a>
      <li><a href="#extra-dep"><span class="secno">3.3</span> <span class="content">Extra Dependencies</span></a>
      <li><a href="#ship-vehicle"><span class="secno">3.4</span> <span class="content">Ship Vehicle</span></a>
     </ol>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#lex-pptoken"><span class="secno">4.1</span> <span class="content">[lex.pptoken]</span></a>
      <li><a href="#basic-link"><span class="secno">4.2</span> <span class="content">[basic.link]</span></a>
      <li><a href="#module-unit"><span class="secno">4.3</span> <span class="content">[module.unit]</span></a>
      <li><a href="#module-global"><span class="secno">4.4</span> <span class="content">[module.global]</span></a>
      <li><a href="#cpp"><span class="secno">4.5</span> <span class="content">[cpp]</span></a>
      <li><a href="#cpp-module"><span class="secno">4.6</span> <span class="content">[cpp.module]</span></a>
      <li><a href="#cpp-glob-frag"><span class="secno">4.7</span> <span class="content">[cpp.glob.frag]</span></a>
     </ol>
    <li><a href="#polls"><span class="secno">5</span> <span class="content">Suggested Polls</span></a>
    <li><a href="#acknowledgments"><span class="secno">6</span> <span class="content">Acknowledgments</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </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>
   <p>At the start of phase 4 an <code class="highlight"><c- n>import</c-></code> or <code class="highlight"><c- n>module</c-></code> token is treated as starting a directive and are converted
to their respective keywords iff:</p>
   <ul>
    <li data-md>
     <p>After skipping horizontal whitespace are</p>
     <ul>
      <li data-md>
       <p>at the start of a logical line, or</p>
      <li data-md>
       <p>preceded by an <code class="highlight"><c- k>export</c-></code> at the start of the logical line.</p>
     </ul>
    <li data-md>
     <p>Are followed by an identifier pp token (before macro expansion), or</p>
     <ul>
      <li data-md>
       <p><code class="highlight"><c- o>&lt;</c-></code>, <code class="highlight"><c- s>"</c-></code><span style="display: none;">"</span>, or <code class="highlight"><c- o>:</c-></code> (but not <code class="highlight"><c- o>::</c-></code>) pp tokens for <code class="highlight"><c- n>import</c-></code>, or</p>
      <li data-md>
       <p><code class="highlight"><c- p>;</c-></code> for <code class="highlight"><c- n>module</c-></code></p>
     </ul>
   </ul>
   <p>Otherwise the token is treated as an identifier.</p>
   <p>Additionally:</p>
   <ul>
    <li data-md>
     <p>The entire <code class="highlight"><c- n>import</c-></code> or <code class="highlight"><c- n>module</c-></code> directive (including the closing <code class="highlight"><c- p>;</c-></code>) must be on a single logical line and for <code class="highlight"><c- n>module</c-></code> must not come from an <code class="highlight"><c- cp>#include</c-></code>.</p>
    <li data-md>
     <p>The expansion of macros must not result in an <code class="highlight"><c- n>import</c-></code> or <code class="highlight"><c- n>module</c-></code> directive
introducer that was not there prior to macro expansion.</p>
    <li data-md>
     <p>A <code class="highlight"><c- n>module</c-></code> directive may only appear as the first preprocessing tokens in
a file (excluding the global module fragment.)</p>
   </ul>
   <p>Failure to meet these additional requirements makes the program ill-formed
(rather than not being interpreted as a directive).</p>
   <table style="margin-left:auto;margin-right:auto;">
    <thead>
     <tr>
      <th>Status Quo
      <th>This Paper
    <tbody>
     <tr>
      <td>
<pre class="highlight"><c- k>export</c->
<c- n>module</c-> <c- n>x</c->
<c- p>;</c->                      <c- c1>// ✅</c->
</pre>
<pre class="highlight"><c- c1>// -Dm="export module x;"</c->
<c- n>m</c->                      <c- c1>// ✅</c->
</pre>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#define m x</c->
<c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->       <c- c1>// ✅</c->
</pre>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#if FOO</c->
<c- k>export</c-> <c- n>module</c-> <c- n>foo</c-><c- p>;</c->     <c- c1>// ✅</c->
<c- cp>#else</c->
<c- k>export</c-> <c- n>module</c-> <c- n>bar</c-><c- p>;</c->     <c- c1>// ✅</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#define EMPTY</c->
<c- n>EMPTY</c-> <c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c-> <c- c1>// ✅</c->
</pre>
<pre class="highlight"><c- cp>#if MODULES</c->
<c- n>module</c-><c- p>;</c->
<c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->       <c- c1>// ❌</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- cp>#if MODULES</c->
<c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->       <c- c1>// ✅</c->
<c- cp>#endif</c->
</pre>
      <td>
<pre class="highlight"><c- k>export</c->
<c- n>module</c-> <c- n>x</c->
<c- p>;</c->                      <c- c1>// ⛔</c->
</pre>
<pre class="highlight"><c- c1>// -Dm="export module x;"</c->
<c- n>m</c->                      <c- c1>// ❌</c->
</pre>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#define m x</c->
<c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->       <c- c1>// ✅</c->
</pre>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#if FOO</c->
<c- k>export</c-> <c- n>module</c-> <c- n>foo</c-><c- p>;</c->     <c- c1>// ✅</c->
<c- cp>#else</c->
<c- k>export</c-> <c- n>module</c-> <c- n>bar</c-><c- p>;</c->     <c- c1>// ✅</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- cp>#define EMPTY</c->
<c- n>EMPTY</c-> <c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c-> <c- c1>// ❌</c->
</pre>
<pre class="highlight"><c- cp>#if MODULES</c->
<c- n>module</c-><c- p>;</c->
<c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->       <c- c1>// ❌</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- cp>#if MODULES</c->
<c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->       <c- c1>// ❌</c->
<c- cp>#endif</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- n>module</c-> <c- n>y</c-> <c- o>=</c-> <c- p>{};</c->         <c- c1>// ❌</c->

<c- o>::</c-><c- n>import</c-> <c- n>x</c-> <c- o>=</c-> <c- p>{};</c->       <c- c1>// ✅</c->
<c- o>::</c-><c- n>module</c-> <c- n>y</c-> <c- o>=</c-> <c- p>{};</c->       <c- c1>// ✅</c->

<c- n>import</c-><c- o>::</c-><c- n>inner</c-> <c- n>xi</c-> <c- o>=</c-> <c- p>{};</c-> <c- c1>// ⛔</c->
<c- n>module</c-><c- o>::</c-><c- n>inner</c-> <c- n>yi</c-> <c- o>=</c-> <c- p>{};</c-> <c- c1>// ✅</c->

<c- k>namespace</c-> <c- n>N</c-> <c- p>{</c->
  <c- n>module</c-> <c- n>a</c-><c- p>;</c->            <c- c1>// ✅</c->
  <c- n>import</c-> <c- n>b</c-><c- p>;</c->            <c- c1>// ⛔</c->
<c- p>}</c->

<c- cp>#define MAYBE_IMPORT(x) x</c->
<c- n>MAYBE_IMPORT</c-><c- p>(</c->
  <c- n>import</c-> <c- o>&lt;</c-><c- n>a</c-><c- o>></c-><c- p>;</c->          <c- c1>// ☠️</c->
<c- p>)</c->
<c- cp>#define EAT(x)</c->
<c- n>EAT</c-><c- p>(</c->
  <c- n>import</c-> <c- o>&lt;</c-><c- n>a</c-><c- o>></c-><c- p>;</c->          <c- c1>// ☠️</c->
<c- p>)</c->

<c- b>void</c-> <c- n>f</c-><c- p>(</c-><c- n>Import</c-> <c- o>*</c-><c- n>import</c-><c- p>)</c-> <c- p>{</c->
  <c- n>import</c-><c- o>-></c-><c- n>doImport</c-><c- p>();</c->  <c- c1>// ⛔</c->
<c- p>}</c->
</pre>
      <td>
<pre class="highlight"><c- n>module</c-> <c- n>y</c-> <c- o>=</c-> <c- p>{};</c->         <c- c1>// ⛔</c->

<c- o>::</c-><c- n>import</c-> <c- n>x</c-> <c- o>=</c-> <c- p>{};</c->       <c- c1>// ✅</c->
<c- o>::</c-><c- n>module</c-> <c- n>y</c-> <c- o>=</c-> <c- p>{};</c->       <c- c1>// ✅</c->

<c- n>import</c-><c- o>::</c-><c- n>inner</c-> <c- n>xi</c-> <c- o>=</c-> <c- p>{};</c-> <c- c1>// ✅</c->
<c- n>module</c-><c- o>::</c-><c- n>inner</c-> <c- n>yi</c-> <c- o>=</c-> <c- p>{};</c-> <c- c1>// ✅</c->

<c- k>namespace</c-> <c- n>N</c-> <c- p>{</c->
  <c- n>module</c-> <c- n>a</c-><c- p>;</c->            <c- c1>// ⛔</c->
  <c- n>import</c-> <c- n>b</c-><c- p>;</c->            <c- c1>// ⛔</c->
<c- p>}</c->

<c- cp>#define MAYBE_IMPORT(x) x</c->
<c- n>MAYBE_IMPORT</c-><c- p>(</c->
  <c- n>import</c-> <c- o>&lt;</c-><c- n>a</c-><c- o>></c-><c- p>;</c->          <c- c1>// ☠️</c->
<c- p>)</c->
<c- cp>#define EAT(x)</c->
<c- n>EAT</c-><c- p>(</c->
  <c- n>import</c-> <c- o>&lt;</c-><c- n>a</c-><c- o>></c-><c- p>;</c->          <c- c1>// ☠️</c->
<c- p>)</c->

<c- b>void</c-> <c- n>f</c-><c- p>(</c-><c- n>Import</c-> <c- o>*</c-><c- n>import</c-><c- p>)</c-> <c- p>{</c->
  <c- n>import</c-><c- o>-></c-><c- n>doImport</c-><c- p>();</c->  <c- c1>// ✅</c->
<c- p>}</c->
</pre>
   </table>
   <p>Legend:</p>
   <ul>
    <li data-md>
     <p>✅ The declaration was accepted, but may not be a modules declaration.</p>
    <li data-md>
     <p>⛔ A token was identified as a modules keyword, but the program is later found
 to be ill-formed.</p>
    <li data-md>
     <p>❌ Ill-formed.</p>
    <li data-md>
     <p>☠️ Undefined behaivor</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="fast-dep-scan"><span class="secno">2.1. </span><span class="content">Fast Dependency Scanning</span><a class="self-link" href="#fast-dep-scan"></a></h3>
    Fast dependency scanning relies on partial preprocessing.  Several people are
working on fast dependency scanning for modules, and they all share the common
trait of skipping over non-directives. <a href="https://github.com/llvm/llvm-project/tree/master/clang/tools/clang-scan-deps">clang-scan-deps</a> is one such tool and is a representative motivating usecase for these changes. 
   <p>clang-scan-deps works by minimizing source code via partial preprocessing.
Partial preprocessing works by running phases 1-3 of translation.  Minimization
proceeds by throwing away every line which is not a preprocessing directive,
then removing all preprocessing directives that can’t impact the set of dependencies for that file.
Up to C++17 this means it only needs to keep <code class="highlight"><c- cp>#define</c-></code>, <code class="highlight"><c- cp>#undef</c-></code>, <code class="highlight"><c- cp>#if*</c-></code>, <code class="highlight"><c- cp>#else</c-></code>, <code class="highlight"><c- cp>#elif</c-></code>, <code class="highlight"><c- cp>#endif</c-></code>, and <code class="highlight"><c- cp>#include</c-></code>. And only the bodies of <code class="highlight"><c- cp>#if</c-></code>s that contain a
preprocessing directive that could impact dependencies.  This reduces most files down to their header guards and list of <code class="highlight"><c- cp>#include</c-></code>s, and is correct for standard C++ except for abuse of <code class="highlight"><c- n>__LINE__</c-></code> such
as <code class="highlight"><c- cp>#if __LINE__ > 456</c-></code>.  The only other things that break it are compiler
extensions which do not follow the parsing rules of <code class="highlight"><c- cp>#</c-></code> directives such as <code class="highlight"><c- n>_Pragma</c-><c- p>(</c-><c- s>"push_macro(</c-><c- se>\"</c-><c- s>X</c-><c- se>\"</c-><c- s>)"</c-><c- p>)</c-></code>, or <code class="highlight"><c- p>.</c-><c- n>incbin</c-></code> in inline assembly.  Additionally,
abuse of the compiler extension <code class="highlight"><c- n>__COUNTER__</c-></code> can break this.</p>
   <p>This minimization is context free, and is done <em>once</em> for each file in the
entire build.</p>
   <p>Then the minimized source file is fully preprocessed by running phases 1-4, with <code class="highlight"><c- cp>#include</c-></code>s resolving to their minimized equivalent, macros expanded, and all
preprocessing directives executed.</p>
   <p>Minimization followed by using clang’s full preprocessor currently provides
about a 9x speed up (~3 seconds vs ~28 seconds out of a 15m build on a 18 core
iMac Pro) over no minimization at scanning all of llvm and clang (~7k files and ~3.8m LoC).  We do not
have access to a large C++20 modules only (no header units) codebase to do tests
on, but that case still requires scanning the entire file due to the potential
for <code class="highlight"><c- cp>#include</c-></code> based <a href="https://en.wikibooks.org/wiki/C_Programming/Preprocessor_directives_and_macros#X-Macros">x-macros</a> which will not be going away with modules, as they are intentionally non-modular
includes.  There will still be a difference
in that we no longer have normal includes, and thus don’t need to preprocess a
header for each TU it’s used in.</p>
   <p>Of the 3s only 33ms are spent minimizing, while the majority of the time is spent in the full lexer, preprocessor, and header
search.  We believe we can reduce this overhead.
This overhead is important as it’s on the critical path.  Any
speedup here turns into a direct reduction in build latency, which is important
for extremely parallel builds.</p>
   <p>C++20 currently breaks this approach. <a data-link-type="biblio" href="#biblio-p1703r1">[P1703r1]</a> resolved the issue for <code class="highlight"><c- n>import</c-></code>, but dependency scanning also needs to find <code class="highlight"><c- n>module</c-></code> declarations, and
the current rules are not enough.</p>
   <h3 class="heading settled" data-level="2.2" id="import-relaxed"><span class="secno">2.2. </span><span class="content"><code class="highlight"><c- n>import</c-></code> is Too Relaxed</span><a class="self-link" href="#import-relaxed"></a></h3>
    <a data-link-type="biblio" href="#biblio-p1703r1">[P1703r1]</a> went too far in fixing the dependency scanning issue with <code class="highlight"><c- n>import</c-></code>.
Now any line starting with <code class="highlight"><c- n>import</c-></code> is treated as an import directive, even if
it obviously couldn’t be.  This breaks a decent amount of real code with
function arguments or local variables named <code class="highlight"><c- n>import</c-></code>, as any use of them without
a prefix is treated as an import directive, and prepending <code class="highlight"><c- o>::</c-></code> doesn’t work,
you must wrap some part of the expression in <code class="highlight"><c- p>()</c-></code>. 
   <h2 class="heading settled" data-level="3" id="discussion"><span class="secno">3. </span><span class="content">Discussion</span><a class="self-link" href="#discussion"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="implementation"><span class="secno">3.1. </span><span class="content">Implementation</span><a class="self-link" href="#implementation"></a></h3>
    There were some concerns expressed during the discussion of <a data-link-type="biblio" href="#biblio-p1703r1">[P1703r1]</a> that
there may be performance or complexity issues.  I have implemented this fix
in Clang.  It was rather simple to implement, with the most complex part being
doing token lookahead while lexing (which isn’t that complicated).  Measurment
did not show any performance impact, which I expect to be due to the codepath
only occuring with the <code class="highlight"><c- n>import</c-></code> and <code class="highlight"><c- n>module</c-></code> tokens. 
   <h3 class="heading settled" data-level="3.2" id="one-line"><span class="secno">3.2. </span><span class="content">One Line Restriction</span><a class="self-link" href="#one-line"></a></h3>
    To be fully resiliant, a modules directive must be entirely on a single
logical line.  You could have a rule that the <code class="highlight"><c- p>;</c-></code> must not come from a macro and
say that the directive extends to the next <code class="highlight"><c- p>;</c-></code>, but that is incorrect: 
<pre class="highlight"><c- cp>#define f(x) "blah" #x "blah"</c->
<c- n>import</c-> <c- n>f</c-><c- p>(</c->
  <c- p>;</c->
<c- p>);</c->
</pre>
   <h3 class="heading settled" data-level="3.3" id="extra-dep"><span class="secno">3.3. </span><span class="content">Extra Dependencies</span><a class="self-link" href="#extra-dep"></a></h3>
<pre class="highlight"><c- cp>#define eat(x)</c->
<c- n>eat</c-><c- p>(</c->
  <c- n>import</c-> <c- o>&lt;</c-><c- n>a</c-><c- o>></c-><c- p>;</c->
<c- p>);</c->
</pre>
   <p>In the WD this has undefined behaivor due to <a href="http://eel.is/c++draft/cpp#1.sentence-1">[cpp]/2</a></p>
   <blockquote>
    <p>A preprocessing directive consists of a sequence of preprocessing tokens that
satisfies the following constraints: The first token in the sequence, referred
to as a directive-introducing token, is a # preprocessing token, an import
preprocessing token, or an export preprocessing token immediately followed by
an import preprocessing token, that (at the start of translation phase 4)
either begins with the first character in the source file (optionally after
white space containing no new-line characters) or follows white space
containing at least one new-line character.</p>
   </blockquote>
   <p>and <a href="http://eel.is/c++draft/cpp#replace-11.sentence-3">[cpp.replace]/11</a></p>
   <blockquote>
    <p>If there are sequences of preprocessing tokens within the list of [macro]
arguments that would otherwise act as preprocessing directives, the behavior
is undefined.</p>
   </blockquote>
   <p>A minimizer will transform this into just <code class="highlight"><c- n>import</c-> <c- o>&lt;</c-><c- n>a</c-><c- o>></c-><c- p>;</c-></code> which when preprocessed
will form a valid module import.  There are two outcomes of this:</p>
   <ul>
    <li data-md>
     <p>The dependency scanner emits an error because <code class="highlight"><c- o>&lt;</c-><c- n>a</c-><c- o>></c-></code> is not a header unit or
it is able to detect the usage in a macro.</p>
    <li data-md>
     <p>The dependency scanner succeeds and then the compiler sees the code and
i̴̝̍ĝ̸̠̳͚̻ņ̸̱̗̅́̾̐͠o̶̲̳̫͒͊͊ȑ̵̺̱͚̩͎͌͑͒̕e̸̗̾s̸̮̰̻̥͑̑͊̂̆ ̴̤͍̄ȉ̸̬͝m̵̞͈̿́p̷̢̛̣̹̒̑̽̀ǫ̴̜̖̱̈̏̈͝r̸̨̻̖̪̔̍̾ͅt̵̙̑s̸̩͎̜̼͑́̚ ̵̤̒͑ȩ̶̹̫̥͗̂r̵͍͈̦͗r̵̩̲̊̿͝ǫ̸̞̙́͊͐͗͗r̷̝̥̬̲͇̀͋̚s̷̰̠͙̤̉ͅ due to undefined behavior.</p>
   </ul>
   <p>This is not the best outcome, as an implementation is not guaranteed to emit a
diagnosic, from my tests of clang, msvc, gcc, and icc, only clang rejectes <code class="highlight"><c- cp>#include</c-></code> in this case, and no compiler rejects an import directive (I plan
to fix this in clang as part of implementing P1703 and this paper).  This should
be fixed by changing <a href="http://eel.is/c++draft/cpp#replace-11.sentence-3">[cpp.replace]/11</a> to be ill-formed instead of UB, but that deserves a separate paper.  With that
change though the possible outcomes would be:</p>
   <ul>
    <li data-md>
     <p>The dependency scanner emits an error because <code class="highlight"><c- o>&lt;</c-><c- n>a</c-><c- o>></c-></code> is not a header unit or
it is able to detect the usage in a macro.</p>
    <li data-md>
     <p>The dependency scanner succeeds and then the compiler sees the code and
rejects it because directives cannot appear as a macro argument.</p>
   </ul>
   <p>Both of these outcomes are fine as their result is the same.  If a partial
preprocessing would ever get the dependencies wrong, the compiler will reject
the program as ill-formed.</p>
   <h3 class="heading settled" data-level="3.4" id="ship-vehicle"><span class="secno">3.4. </span><span class="content">Ship Vehicle</span><a class="self-link" href="#ship-vehicle"></a></h3>
    This needs to ship with modules as this is a backwards incompatible change.
Thus it needs to be in C++20. 
   <h2 class="heading settled" data-level="4" id="wording"><span class="secno">4. </span><span class="content">Wording</span><a class="self-link" href="#wording"></a></h2>
   <div class="cwg-note"> This is a wording note.  Its purpose is to help clarify the intent of the
wording in this document to the members of the committee.  It is not an
instruction to the editor. </div>
   <h3 class="heading settled" data-level="4.1" id="lex-pptoken"><span class="secno">4.1. </span><span class="content">[lex.pptoken]</span><a class="self-link" href="#lex-pptoken"></a></h3>
   <blockquote>
<pre class="highlight"><em><c- n>preprocessing</c-><c- o>-</c-><c- nl>token</c-><c- p>:</c-></em>
    <em><c- n>header</c-><c- o>-</c-><c- n>name</c-></em>
    <em><c- n>import</c-><c- o>-</c-><c- n>keyword</c-></em>
    <ins><em><c- n>module</c-><c- o>-</c-><c- n>keyword</c-></em></ins>
    <em><c- p>...</c-></em>
</pre>
   </blockquote>
   <blockquote>
     4 The <em>import-keyword</em> is produced by processing an import directive
([cpp.import])
    <ins>, and the <em>module-keyword</em> is produced by preprocessing a
module directive ([cpp.module]).</ins>
    <del>and has no</del>
    <ins>[<em>Note</em>: Neither has any</ins>
     associated grammar productions. 
    <ins>—<em>end note</em>]</ins>
   </blockquote>
   <h3 class="heading settled" data-level="4.2" id="basic-link"><span class="secno">4.2. </span><span class="content">[basic.link]</span><a class="self-link" href="#basic-link"></a></h3>
   <blockquote>
<pre class="highlight"><em><c- n>translation</c-><c- o>-</c-><c- nl>unit</c-></em><c- p>:</c->
    <em><c- n>top</c-><c- o>-</c-><c- n>level</c-><c- o>-</c-><c- n>declaration</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></em>
    <em><c- n>global</c-><c- o>-</c-><c- n>module</c-><c- o>-</c-><c- n>fragment</c-><sub><c- n>opt</c-></sub> <c- n>module</c-><c- o>-</c-><c- n>declaration</c-> <c- n>top</c-><c- o>-</c-><c- n>level</c-><c- o>-</c-><c- n>declaration</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub> <c- k>private</c-><c- o>-</c-><c- n>module</c-><c- o>-</c-><c- n>fragment</c-><sub><c- n>opt</c-></sub></em>
<em><c- k>private</c-><c- o>-</c-><c- n>module</c-><c- o>-</c-><c- nl>fragment</c-></em><c- p>:</c->
    <del><strong><c- n>module</c-></strong></del> <ins><em><c- n>module</c-><c- o>-</c-><c- nl>keyword</c-></em></ins> <strong><c- p>:</c-> <c- k>private</c-> <c- p>;</c-></strong> <em><c- n>top</c-><c- o>-</c-><c- n>level</c-><c- o>-</c-><c- n>declaration</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></em>
</pre>
    <p>...</p>
    <del>3 A token sequence beginning with <code class="highlight"><strong><c- n>export</c-></strong><sub><em><c- n>opt</c-> </em></sub> <strong><c- n>module</c-></strong></code> and not immediately followed by <code class="highlight"><c- o>::</c-></code> is never interpreted as the <em>declaration</em> of a <em>top-level-declaration</em>.</del>
   </blockquote>
   <h3 class="heading settled" data-level="4.3" id="module-unit"><span class="secno">4.3. </span><span class="content">[module.unit]</span><a class="self-link" href="#module-unit"></a></h3>
   <blockquote>
<pre class="highlight"><em><c- n>module</c-><c- o>-</c-><c- nl>declaration</c-></em><c- p>:</c->
    <strong><c- n>export</c-></strong><em><sub><c- n>opt</c-></sub></em> <del><strong><c- n>module</c-></strong></del> <ins><em><c- n>module</c-><c- o>-</c-><c- n>keyword</c-></em></ins> <em><c- n>module</c-><c- o>-</c-><c- n>name</c-></em> <em><c- n>module</c-><c- o>-</c-><c- n>partition</c-><sub><c- n>opt</c-></sub></em> <em><c- n>attribute</c-><c- o>-</c-><c- n>specifier</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></em> <strong><c- p>;</c-></strong>
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="4.4" id="module-global"><span class="secno">4.4. </span><span class="content">[module.global]</span><a class="self-link" href="#module-global"></a></h3>
   <blockquote>
<pre class="highlight"><em><c- n>global</c-><c- o>-</c-><c- n>module</c-><c- o>-</c-><c- nl>fragment</c-></em><c- p>:</c->
    <del><strong><c- n>module</c-></strong></del> <ins><em><c- n>module</c-><c- o>-</c-><c- n>keyword</c-></em></ins> <strong><c- p>;</c-></strong> <em><c- n>top</c-><c- o>-</c-><c- n>level</c-><c- o>-</c-><c- n>declaration</c-><c- o>-</c-><c- n>seq</c-><sub><c- n>opt</c-></sub></em>
</pre>
   </blockquote>
   <h3 class="heading settled" data-level="4.5" id="cpp"><span class="secno">4.5. </span><span class="content">[cpp]</span><a class="self-link" href="#cpp"></a></h3>
   <blockquote>
     1 A <em>preprocessing directive</em> consists of a sequence of preprocessing tokens that
satisfies the following constraints: The first <span>token<ins>s</ins></span> in the sequence, referred
to as a <em>directive-introducing token <ins>subsequence</ins></em>, is a <strong>#</strong> preprocessing
token, an <strong><code class="highlight"><c- n>import</c-></code></strong>
    <ins> or <strong><code class="highlight"><c- n>module</c-></code></strong></ins>
     preprocessing token, or an <strong><code class="highlight"><c- k>export</c-></code></strong> preprocessing token
immediately followed by an <strong><code class="highlight"><c- n>import</c-></code></strong>
    <ins> or <strong><code class="highlight"><c- n>module</c-></code></strong></ins>
     preprocessing token, that 
    <ins>is followed by a <em>header-name</em>, <em>identifier</em>, or <code class="highlight"><c- o>:</c-></code> preprocessing tokens for <strong><code class="highlight"><c- n>import</c-></code></strong>, or is followed
by an <em>identifier</em> or <code class="highlight"><c- p>;</c-></code> preprocessing tokens for <strong><code class="highlight"><c- n>module</c-></code> </strong>, and that</ins>
     (at the start of translation phase 4)
either begins with the first character in the source file (optionally after
white space containing no new-line characters) or follows white space containing
at least one new-line character. The last token in the sequence is the first new-line character
that follows the first token in the sequence. A new-line character ends the
preprocessing directive even if it occurs within what would otherwise be an
invocation of a function-like macro. 
    <ins>[<em>Example</em>:</ins>
<pre class="highlight"><ins><c- c1>// These are examples of </c-><em><c- c1>directive-introducing token subsequences</c-></em>
<c- cp>#</c->
<c- n>module</c-> <c- p>;</c->
<c- k>export</c-> <c- n>module</c-> <c- n>leftpad</c->
<c- n>import</c-> <c- o>&lt;</c-><c- n>string</c-><c- o>></c->
<c- k>export</c-> <c- n>import</c-> <c- s>"squee"</c->
<c- n>import</c-> <c- n>rightpad</c->
<c- nl>import</c-> <c- p>:</c->
<c- c1>// These are not </c-><em><c- c1>directive-introducing token subsequences</c-></em>
<c- n>module</c->
<c- p>;</c->
<c- n>import</c-> <c- o>::</c->
<c- n>import</c-> <c- o>-></c->
</ins></pre>
    <ins>—<em>end example</em>]</ins>
    <p></p>
<pre class="highlight"><em><c- n>preprocessing</c-><c- o>-</c-><c- nl>file</c-><c- p>:</c-></em>
    <em><c- p>...</c-></em>

<em><c- n>control</c-><c- o>-</c-><c- nl>line</c-></em><c- p>:</c->
    <strong><c- cp># include</c-></strong><c- cp> </c-><em><c- cp>pp-tokens new-line</c-></em>
    <strong><c- n>export</c-></strong><em><sub><c- n>opt</c-></sub></em> <strong><c- n>import</c-></strong> <em><ins><c- n>header</c-><c- o>-</c-><c- n>name</c-></ins> <c- n>pp</c-><c- o>-</c-><c- n>tokens</c-> <c- k>new</c-><c- o>-</c-><c- n>line</c-></em>
    <ins><strong><c- n>export</c-></strong><em><sub><c- n>opt</c-></sub></em> <strong><c- n>import</c-></strong> <em><c- n>identifier</c-> <c- n>pp</c-><c- o>-</c-><c- n>tokens</c-> <c- k>new</c-><c- o>-</c-><c- n>line</c-></em></ins>
    <ins><strong><c- n>export</c-></strong><em><sub><c- n>opt</c-></sub></em> <strong><c- nl>import</c-></strong> <strong><c- p>:</c-></strong> <em><c- n>pp</c-><c- o>-</c-><c- n>tokens</c-> <c- k>new</c-><c- o>-</c-><c- n>line</c-></em></ins>
    <ins><strong><c- n>export</c-></strong><em><sub><c- n>opt</c-></sub></em> <strong><c- n>module</c-></strong> <em><c- n>identifier</c-> <c- n>pp</c-><c- o>-</c-><c- n>tokens</c-> <c- k>new</c-><c- o>-</c-><c- n>line</c-></em></ins>
    <ins><strong><c- n>export</c-></strong><em><sub><c- n>opt</c-></sub></em> <strong><c- n>module</c-></strong> <strong><c- p>;</c-></strong> <em><c- k>new</c-><c- o>-</c-><c- n>line</c-></em></ins>
    
    <em><c- p>...</c-></em>
</pre>
    <p></p>
    <p></p>
    <div class="cwg-note">
      This is intended to match any logical lines that start with: 
     <ul>
      <li>#
      <li>export<em><sub>opt</sub></em> module ;
      <li>export<em><sub>opt</sub></em> module identifier
      <li>export<em><sub>opt</sub></em> import :
      <li>export<em><sub>opt</sub></em> import &lt;
      <li>export<em><sub>opt</sub></em> import "
      <li>export<em><sub>opt</sub></em> import identifier
     </ul>
     <p>And that the entirety of the above is a <em>directive-introducing token
  subsequence</em>.</p>
     <p>This should only require examining up to two characters after skipping
  horizontal whitespace.</p>
    </div>
    <p></p>
    <p></p>
    <div class="cwg-note">
      There’s a potential issue here with: 
<pre class="highlight"><c- k>export</c-> <c- n>module</c-> <c- n>m</c-><c- p>;</c->
<c- cp>#define export static</c->
<c- k>using</c-> <c- n>import</c-> <c- o>=</c-> <c- b>int</c-><c- p>;</c->
<c- k>export</c-> <c- nf>import</c-><c- p>(</c-><c- o>*</c-><c- n>a</c-><c- p>);</c->
</pre>
     <p>If this is allowed, the lexer needs to do two token lookahead when it sees an <code class="highlight"><c- k>export</c-></code> due to <a href="http://eel.is/c++draft/cpp#6">[cpp]/6</a> to tell if
  this is a directive or not.</p>
     <p>During discussion at the SG15 meeting after CppCon Gaby and Boris said that
  this lookahead/backtracking was fine for MSVC and GCC respectively. I have
  implemented this in Clang.</p>
    </div>
    <p></p>
    <p>
     2 A text line shall not begin with a 
     <del># preprocessing token</del>
     <ins> <em>directive-introducing token subsequence</em></ins>
     . A <em>conditionally-supported-directive</em> shall not begin with any of the
directive names appearing in the syntax. A <em>conditionally-supported-directive </em> is conditionally-supported with implementation-defined semantics.
    </p>
   </blockquote>
   <h3 class="heading settled" data-level="4.6" id="cpp-module"><span class="secno">4.6. </span><span class="content">[cpp.module]</span><a class="self-link" href="#cpp-module"></a></h3>
    Add a new section. 
   <blockquote>
<pre class="highlight"><ins><em><c- n>pp</c-><c- o>-</c-><c- nl>module</c-></em><c- p>:</c->
    <strong><c- n>export</c-></strong><em><sub><c- n>opt</c-></sub></em> <strong><c- n>module</c-></strong> <em><c- n>pp</c-><c- o>-</c-><c- n>tokens</c-><sub><c- n>opt</c-></sub></em> <c- p>;</c-> <em><c- k>new</c-><c- o>-</c-><c- n>line</c-></em>
</ins></pre>
    <p>
     <ins> 1 No part of <em>pp-module</em> shall be produced directly or indirectly via source file
inclusion ([cpp.include]). </ins>
    </p>
    <div class="cwg-note"> This replaces the part in [cpp.glob.frag] and makes it apply even without a
  global module fragment. </div>
    <div class="cwg-note"> Do we need "directly or indirectly"?  It was copied from the wording in
  [cpp.glob.frag]/1 </div>
    <p>
     <ins> 2 At the start of phase 4 of translation a <em>pp-module</em> directive shall
appear only as the first preprocessing tokens in the translation unit or
as the second <em>pp-module</em> in the <em>pp-global-module-fragment</em>. </ins>
    </p>
    <p>
     <ins> 3 Any preprocessing tokens after the <strong>module</strong> preprocessing
token in the <strong>module</strong> <em>control-line</em> are
processed just as in normal text. [<em>Note</em>: Each identifier currently defined as a
macro name is replaced by its replacement list of preprocessing tokens. —<em>end note</em>] </ins>
    </p>
    <p>
     <ins> 4 The <strong>module</strong> preprocessing token is replaced by the <em>module-keyword</em> preprocessing token. [<em>Note</em>: This makes the line no longer a
directive so it is not removed at the end of phase 4. —<em>end note</em>] </ins>
    </p>
   </blockquote>
   <h3 class="heading settled" data-level="4.7" id="cpp-glob-frag"><span class="secno">4.7. </span><span class="content">[cpp.glob.frag]</span><a class="self-link" href="#cpp-glob-frag"></a></h3>
   <blockquote>
<pre class="highlight"><em><c- n>pp</c-><c- o>-</c-><c- n>global</c-><c- o>-</c-><c- n>module</c-><c- o>-</c-><c- nl>fragment</c-></em><c- p>:</c->
    <del><c- n>module</c-> <c- p>;</c-> <c- n>pp</c-><c- o>-</c-><c- n>balanced</c-><c- o>-</c-><c- n>token</c-><c- o>-</c-><c- n>seq</c-> <c- n>module</c-></del>
    <ins><em><c- n>pp</c-><c- o>-</c-><c- n>module</c-> <c- n>pp</c-><c- o>-</c-><c- n>balanced</c-><c- o>-</c-><c- n>token</c-><c- o>-</c-><c- n>seq</c-> <c- k>new</c-><c- o>-</c-><c- n>line</c-> <c- n>pp</c-><c- o>-</c-><c- n>module</c-></em></ins>
</pre>
    <div class="cwg-note"> I’m not sure this <em>new-line</em> is needed, but nothing in the grammar for <em>pp-module</em> requires it otherwise. </div>
    <p>
     1 <span>If<ins>,</ins></span> 
     <ins>at</ins>
      the 
     <del>first two preprocessing tokens at the</del>
      start of
phase 4 of translation 
     <del>are <code class="highlight"><c- n>module</c-> <c- p>;</c-></code> </del>
     <ins>,</ins>
     <ins>the source file begins with a <em>pp-module</em> of the form <code class="highlight"><c- n>module</c-> <c- p>;</c-></code></ins>
     , the result of
preprocessing shall begin with a <em>pp-global-module-fragment</em> for which all
preprocessing-tokens in the <em>pp-balanced-token-seq</em> were produced directly or
indirectly by source file inclusion ([cpp.include]) 
     <del>, and for which the
second module preprocessing-token was not produced by source file inclusion or
macro replacement (15.5)</del>
     . Otherwise, the first two preprocessing tokens
at the end of phase 4 of translation shall not be <code class="highlight"><c- n>module</c-> <c- p>;</c-></code>.
    </p>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="polls"><span class="secno">5. </span><span class="content">Suggested Polls</span><a class="self-link" href="#polls"></a></h2>
   <ul>
    <li data-md>
     <p>C++ dependencies should be discoverable without full preprocessing.</p>
    <li data-md>
     <p>Forward P1857R1 to CWG for inclusion in C++20 as a resolution to NB comments &lt;comments>.</p>
   </ul>
   <h2 class="heading settled" data-level="6" id="acknowledgments"><span class="secno">6. </span><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
   <p>Thanks to Boris Kolpackov for writing P1703, Mathias Stearn and Walter Brown for feedback on this
paper and to Richard Smith, Corentin, Gabriel Dos Reis, Ben Craig, Matthew
Woehlke, and Nathan Sidwell for discussion regarding this issue.</p>
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      }
    }, false);
  }
  else {
    console.warn("Can’t find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

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  var numTables = tables.length;
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    var table = tables[i];
    var wrapper = document.createElement('div');
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    table.parentNode.insertBefore(wrapper, table);
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})();
</script>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-p1703r1">[P1703r1]
   <dd>Boris Kolpackov. <a href="https://wg21.link/p1703r1">Recognizing Header Unit Imports Requires Full Preprocessing</a>. 19 July 2019. URL: <a href="https://wg21.link/p1703r1">https://wg21.link/p1703r1</a>
  </dl>