<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P1302R1: Implicit Module Partition Lookup</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1302R1<br>Implicit Module Partition Lookup</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-01-21">2019-01-21</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/p1302r1">https://wg21.link/p1302r1</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="https://twitter.com/slurpsmadrips">Isabella Muerte</a>
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:richard@metafoo.co.uk">Richard Smith</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Current Render:
     <dd><a href="https://api.csswg.org/bikeshed/?force=1&amp;url=https://git.io/fxG98">P1302</a>
     <dt>Current Source:
     <dd><a href="https://git.io/fxGS1">slurps-mad-rips/papers/proposals/implicit-module-partition.bs</a>
    </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>We seek to enforce a particular form of module partition lookup as a

"fast path" opportunity for building modules, with minimal impact to
existing code, while allowing for incremental modularization of
user’s code bases. This is not a convention, but a requirement for
conformance for all vendors whose code will run on an abstract machine</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="#changelog"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r1"><span class="secno">1.1</span> <span class="content">Revision 1</span></a>
      <li><a href="#r0"><span class="secno">1.2</span> <span class="content">Revision 0</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li><a href="#design"><span class="secno">3</span> <span class="content">Design</span></a>
    <li>
     <a href="#examples"><span class="secno">4</span> <span class="content">Examples</span></a>
     <ol class="toc">
      <li><a href="#example-hierarchy"><span class="secno">4.1</span> <span class="content">Hierarchical</span></a>
      <li><a href="#example-splayed"><span class="secno">4.2</span> <span class="content">Splayed</span></a>
     </ol>
    <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="changelog"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="r1"><span class="secno">1.1. </span><span class="content">Revision 1</span><a class="self-link" href="#r1"></a></h3>
   <p>Added actionable language for inclusion into the standard. We now state that
splayed layouts are the minimum requirement and any hierarchy or module name
enforcement is solely the responsibility of the build system.</p>
   <h3 class="heading settled" data-level="1.2" id="r0"><span class="secno">1.2. </span><span class="content">Revision 0</span><a class="self-link" href="#r0"></a></h3>
   <p>Initial Release 🎉</p>
   <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>
   <p>As the advent of modules approaches, build systems and developers are still
waiting on the final bits that will make it into C++20. However, because of
various limitations, the standard cannot <em>enforce</em> specific convention, merely
encourage them. This paper seeks to provide a possible convention that reduces
work required by build systems, general effort from compilers (e.g., they will
not need to implement a local socket based server for information passing), and
to make the lives of developers easier as they will get to experiece a fairly
consistent development process when moving between or across projects.</p>
   <h2 class="heading settled" data-level="3" id="design"><span class="secno">3. </span><span class="content">Design</span><a class="self-link" href="#design"></a></h2>
   <p>The design for this behavior is as follows:</p>
   <p>A so-called <em>module entrypoint</em> is placed inside of a directory. A <em>module
entrypoint</em>, as defined in <a href="#wording">§5 Wording</a> is a file. The name of this <em>entrypoint</em> is user defined, with a fallback to a file with the base name <code class="highlight"><c- n>module</c-></code> and possibly some file extension. We do not enforce the requirement of
a file extension so that operating systems without them can do as they please.
This also allows the community to eventually select a "better" file extension.
That said, the author recommends <code class="highlight"><c- n>cxx</c-></code> as that is the environment variable used
by plenty of build systems to represent the C++ compiler, where as <code class="highlight"><c- n>CPP</c-></code> represents the <strong>C</strong> <strong>P</strong>re<strong>P</strong>rocessor. Additionally, moving libraries from
non-module APIs to modularized APIs can have a path of least resistance instead
of requiring something that has nothing to do with the name C++, like <code class="highlight"><c- n>cppm</c-></code> or <code class="highlight"><c- n>mxx</c-></code>.</p>
   <p>Therefore, if a module’s interface has <code class="highlight"><c- nl>import</c-> <c- p>:</c-><c- n>list</c-></code>, the compiler will look for a file with a base name of <code class="highlight"><c- n>list</c-></code>, and (optionally) the same file extension as <code class="highlight"><c- n>module</c-></code>.</p>
   <p>This behavior only occurs during the construction of a BMI. If dependent
modules are not yet compiled, or if users are expecting an object file to
magically pop out of the compiler, they will be surprised when the compiler
gives an error.</p>
   <p>The building of object files is still left up to build systems so that existing
distributed build system workflows are not interrupted (such as in the case of
tools like Icecream, distcc, or sccache).</p>
   <p>Where things get interesting is when a user desires to import <em>another module</em> into a <em>module entrypoint</em>. Given the following directory layout:</p>
<pre class="highlight"><c- p>.</c->
└── <c- n>src</c->
   └── <c- n>core</c->
       ├── <c- n>module</c-><c- p>.</c-><c- n>cxx</c->
       ├── <c- n>list</c-><c- p>.</c-><c- n>cxx</c->
       └── <c- n>io</c->
           ├── <c- n>module</c-><c- p>.</c-><c- n>cxx</c->
           └── <c- n>file</c-><c- p>.</c-><c- n>cxx</c->
</pre>
   <p>We can assume that, perhaps, the source for <code class="highlight"><c- n>core</c-><c- o>/</c-><c- n>module</c-><c- p>.</c-><c- n>cxx</c-></code> looks something
like:</p>
<pre class="language-cpp highlight"><c- k>export</c-> <c- n>module</c-> <c- n>name</c-><c- p>;</c->

<c- k>export</c-> <c- n>import</c-> <c- n>core</c-><c- p>.</c-><c- n>io</c-><c- p>;</c->
<c- nl>import</c-> <c- p>:</c-><c- n>list</c-><c- p>;</c->
</pre>
   <p>In other languages, this would imply that the compiler has to now recurse into
the <code class="highlight"><c- n>core</c-><c- o>/</c-><c- n>io</c-></code> directory. <strong>We do not do this</strong>. Instead, the build system is
required to have seen the <code class="highlight"><c- k>export</c-> <c- n>import</c-></code> and passed that directory along to
the compiler first. This is a combination of the various modules systems, but
has the following properties:</p>
   <ol>
    <li data-md>
     <p>It does not make the compiler a build system</p>
    <li data-md>
     <p>Existing work that has been done to handle dependency management does not
 need to be thrown away.</p>
    <li data-md>
     <p>The <code class="highlight"><c- n>core</c-><c- p>.</c-><c- n>io</c-></code> module <em>does not</em> have to exist in the directory <code class="highlight"><c- n>core</c-><c- o>/</c-><c- n>io</c-></code>.
 Rather, it can exist techically anywhere.</p>
    <li data-md>
     <p>Build systems are free to enforce the name of a module to its location on
 disk, while also permitting others to ignore it entirely.</p>
    <li data-md>
     <p>Build systems can have a guaranteed fallback location if developers don’t
 want to have to manually specify the location of each and every module.</p>
    <li data-md>
     <p>This doesn’t actually tie the compiler to a filesystem approach, as this
 is just a general convention.</p>
    <li data-md>
     <p>Build systems are free to implement, additional conventions, such as the
 Pitchfork or Coven filesystem layout and enforce it for modules having
 legacy non-module code in the same project layout.</p>
    <li data-md>
     <p>It allows developers to view modules as hierarchical, even if they aren’t.
 This means that, if treating modules as a hierarchy becomes widespread
 enough, the standard could possibly enforce modules as hierarchies in the
 future.</p>
    <li data-md>
     <p>Platforms where launching processes are expensive can take advantage of
 improved throughput when reading from files.</p>
    <li data-md>
     <p>Build systems and compilers are free to take an optimization where only
  the modified times of a directory are checked before the contents of each
  directory are checked. On every operating system (yes, every operating
  system), directories change their timestamp if any of the <em>files</em> contained within change, but do not update if child directories do as
  well. While some operating systems permit mounting drives and locations
  without modified times, doing so breaks nearly every other build system
  in existence. Thus we can safely assume that a build system does not need
  to reparse or rebuild a module if its containing directory has not
  changed.</p>
   </ol>
   <h2 class="heading settled" data-level="4" id="examples"><span class="secno">4. </span><span class="content">Examples</span><a class="self-link" href="#examples"></a></h2>
   <p>The following two examples show how implicit module partition lookup can be
used for both hierarchical and "splayed" directory layouts.</p>
   <h3 class="heading settled" data-level="4.1" id="example-hierarchy"><span class="secno">4.1. </span><span class="content">Hierarchical</span><a class="self-link" href="#example-hierarchy"></a></h3>
   <p>This sample borrows from the above example. Effectively, to import <code class="highlight"><c- n>core</c-><c- p>.</c-><c- n>io</c-></code>,
one must build it before building <code class="highlight"><c- n>core</c-></code> simply because the build system
assumes that <code class="highlight"><c- n>core</c-><c- p>.</c-><c- n>io</c-></code> refers to a directory named <code class="highlight"><c- n>core</c-><c- o>/</c-><c- n>io</c-></code> from the project
root.</p>
<pre class="highlight"><c- p>.</c->
└── <c- n>src</c->
   └── <c- n>core</c->
       ├── <c- n>module</c-><c- p>.</c-><c- n>cxx</c->
       ├── <c- n>list</c-><c- p>.</c-><c- n>cxx</c->
       └── <c- n>io</c->
           ├── <c- n>module</c-><c- p>.</c-><c- n>cxx</c->
           └── <c- n>file</c-><c- p>.</c-><c- n>cxx</c->
</pre>
   <p>This behavior is <em>not</em> enforced by the compiler, but rather by the build
system. If a build system does not support a <em>hierarchical</em> implicit lookup, it
can at least support a <em>splayed</em> implicit lookup</p>
   <h3 class="heading settled" data-level="4.2" id="example-splayed"><span class="secno">4.2. </span><span class="content">Splayed</span><a class="self-link" href="#example-splayed"></a></h3>
   <p>This approach is one that might be more commonly seen as C++ developers move
from headers to modules.</p>
<pre class="highlight"><c- p>.</c->
├── <c- n>core</c->
│   ├── <c- n>module</c-><c- p>.</c-><c- n>cxx</c->
│   └── <c- n>list</c-><c- p>.</c-><c- n>cxx</c->
└── <c- n>io</c->
    ├── <c- n>module</c-><c- p>.</c-><c- n>cxx</c->
    └── <c- n>file</c-><c- p>.</c-><c- n>cxx</c->
</pre>
   <p>In the above layout, <code class="highlight"><c- n>core</c-><c- p>.</c-><c- n>io</c-></code> is located in <code class="highlight"><c- p>.</c-><c- o>/</c-><c- n>io</c-></code>, rather than under the <code class="highlight"><c- p>.</c-><c- o>/</c-><c- n>core</c-></code> directory. A sufficiently simple build system could be told that <code class="highlight"><c- n>core</c-><c- p>.</c-><c- n>io</c-></code> resides under <code class="highlight"><c- p>.</c-><c- o>/</c-><c- n>io</c-></code> and not to rely on some kind of hierarchical
directory layout.</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>The following is to be placed into the current working draft at a location
within close proximity to, or adjacent to, the current merged modules wording:</p>
   <ins>
    <center><strong>General</strong></center>
    <p><sup>1</sup> This subclause describes operations to be supported regarding
modules, their <em>implicit</em> partitions, and interactions within the filesystem.</p>
    <p><sup>2</sup> A <em>module container</em> is a collection of files represented by a <em>directory</em>. [fs.general]</p>
    <p><sup>3</sup> A <em>module entrypoint</em> is a filesystem object within a <em>module container</em> that holds the purview of an <em>exported module</em>.</p>
    <p><sup>4</sup>An <em>implicit module partition</em> is a filesystem object adjacent to a <em>module entrypoint</em>.</p>
    <center><strong>Conformance</strong></center>
    <p><sup>1</sup>Conformance is required for all vendors whose implementations
create a final program to be run on the abstract machine</p>
    <p><sup>2</sup>Compilers must provide a flag for users to declare a <em>module
container</em> as a filesystem directory.</p>
    <p><sup>3</sup>Compilers must provide a flag for users to declare the name of a <em>module entrypoint</em></p>
    <p><sup>4</sup>Compilers must use a predefined <em>module entrypoint</em> if none is
provided by a user. This entrypoint must be a file with a base name of <em>module</em>. The extension of the file is currently left unspecified, but should
be one of the several file extensions that have been historically used. (e.g., <code class="highlight"><c- n>c</c-><c- o>++</c-></code>, <code class="highlight"><c- n>cpp</c-></code>, <code class="highlight"><c- n>cxx</c-></code>, and <code class="highlight"><c- n>cc</c-></code>, et. al.)</p>
    <center><strong>Behavior</strong></center>
    <p><sup>1</sup>In the <em>module entrypoint</em>'s <em>preamble</em>, when an implementation
encounters a <em>module partition</em> import, it needs to look for an <em>implicit
module partition</em> with the same base name as the <em>module partition</em></p>
   </ins>
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