<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P1130R1: Module Resource Requirement Propagation</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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/*
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Possible extra rowspan handling

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*/

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


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

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/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

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/*                             Overflow Control                               */
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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">P1130R1<br>Module Resource Requirement Propagation</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>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:phdofthehouse@gmail.com">JeanHeyd Meneide</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Latest:
     <dd><a href="https://thephd.github.io/vendor/future_cxx/papers/d1130.html">https://thephd.github.io/vendor/future_cxx/papers/d1130.html</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>This paper attempts to provide a level of indirection upon which build system and package management tools build sane, higher-level abstractions.</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="#changelog-r1"><span class="secno">1.1</span> <span class="content">Revision 1 - January 21st, 2019</span></a>
      <li><a href="#changelog-r0"><span class="secno">1.2</span> <span class="content">Revision 0 - November 26th, 2018</span></a>
     </ol>
    <li><a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
    <li><a href="#solution"><span class="secno">3</span> <span class="content">Proposed Solution</span></a>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Proposed Changes to the Standard</span></a>
     <ol class="toc">
      <li><a href="#wording-feature"><span class="secno">4.1</span> <span class="content">Proposed Feature Test Macro</span></a>
      <li><a href="#wording-intent"><span class="secno">4.2</span> <span class="content">Intent</span></a>
      <li><a href="#wording-language"><span class="secno">4.3</span> <span class="content">Proposed Wording</span></a>
     </ol>
    <li><a href="#acknowledgements"><span class="secno">5</span> <span class="content">Acknowledgements</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="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="changelog-r1"><span class="secno">1.1. </span><span class="content">Revision 1 - January 21st, 2019</span><a class="self-link" href="#changelog-r1"></a></h3>
   <ul>
    <li data-md>
     <p>Add words of power, "resource location".</p>
    <li data-md>
     <p>Clarify wording around it not being a hard error.</p>
    <li data-md>
     <p>Track changes to <a data-link-type="biblio" href="#biblio-p1103">[p1103]</a> accordingly.</p>
    <li data-md>
     <p>Fix description in motivation of the use of <code class="highlight"><c- n>require</c-></code>.</p>
    <li data-md>
     <p>Add HRESOURCE example.</p>
   </ul>
   <h3 class="heading settled" data-level="1.2" id="changelog-r0"><span class="secno">1.2. </span><span class="content">Revision 0 - November 26th, 2018</span><a class="self-link" href="#changelog-r0"></a></h3>
   <ul>
    <li data-md>
     <p>Initial release.</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>
   <p>The only way to declare a dependency in C++ currently is to use a <code class="highlight"><c- cp>#include</c-></code> statement. With Modules coming to C++, dependency information was greatly enhanced by preambles and globule module fragments which allow for both the compiler and build system to understand physical and semantic layout of code. However, there is still a problem area that C++ has not addressed that people in the brave new Modules ecosystem want to answer: external dependency information for Modular C++. There is a huge opportunity to add a small directive to C++ which can be transiently ignored by the compiler but allows a build system or dependency graph generator to stay up to date without compiler-specific and tool-specific hacks.</p>
   <p>In particular, consider a resource file on Windows (<code class="highlight"><c- p>.</c-><c- n>rc</c-></code>) or an injected resource on Linux (with <code class="highlight"><c- n>objcopy</c-></code>) or Apple’s Bundles. At present, there’s no way to inform C++ of these dependencies or inform the build system in a way that keeps both the source code and the build system in-sync: it is easy to fall out of lock-step with each other, and often requires custom rules on the part of the build system vendor or the application author. While resource files are a problem that <a data-link-type="biblio" href="#biblio-p1040">[p1040]</a> plan to solve, San Diego discussion around such a proposal demonstrated that people were conflicted with the idea that there might not be a way to communicate without doing full Semantic Analysis the actual source dependencies.</p>
   <p>Reducing the complexity of the build system and its dependency on vendor-specific extensions and tools for handling source dependency information is of high priority. This proposal wants to add 2 new statements to the Preprocessing Module Tokens for <a data-link-type="biblio" href="#biblio-p1103">[p1103]</a>, in particular <code class="highlight"><c- n>requires</c-> <c- p>{</c-> <c- n>identifier</c-> <c- o>=</c-> <c- n>requires</c-> <c- s>"blah.txt"</c-> <c- p>}</c-></code> and <code class="highlight"><c- n>requires</c-> <c- s>"blah.txt"</c-></code> in the Module Declaration.</p>
   <h2 class="heading settled" data-level="3" id="solution"><span class="secno">3. </span><span class="content">Proposed Solution</span><a class="self-link" href="#solution"></a></h2>
   <p>The proposed solution to the problem is to add a general purpose marker for communication of dependencies in the preamble. There is also an addition for making the string literal used to show dependencies as a global variable, allowing it to be used in <code class="highlight"><c- k>constexpr</c-></code> contexts. It will be used as part of the typical modules declaration, making it available in public and private modules. For example:</p>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- c1>// communicate this module</c->
<c- c1>// relies on a file "foo.txt"</c->
<c- n>module</c-> <c- n>bar</c-> <c- n>requires</c-> <c- s>"foo.txt"</c-><c- p>;</c->
</pre>
   <p>For multiple dependencies for a single module, we use a multi-clause requires with brackets:</p>
<pre class="highlight"><c- n>module</c-><c- p>;</c->
<c- c1>// communicate this module</c->
<c- c1>// relies on multiple files / assets</c->
<c- k>export</c-> <c- n>module</c-> <c- n>mega_bar</c-> <c- n>requires</c-> <c- p>{</c-> <c- s>"qux.txt"</c-><c- p>,</c-> <c- s>"meow.jpg"</c-><c- p>,</c-> <c- s>"uuids.csv"</c-> <c- p>};</c->
</pre>
   <p>This allows us to inform the build system of dependencies with no new keywords. This also lets the compiler export dependency information with user-added information at no additional cost to the compiler, not occurring semantic analysis. Note that there is no error if the compiler cannot find the file: it will be up to the build system to determine that information.</p>
   <p>A program may also set one or more of the string literals to an identifier made available after the preamble. The value of that identifier will be the string literal set equal to the identifier in the module requires:</p>
<pre class="highlight"><c- n>module</c-> <c- n>bar</c-> <c- n>requires</c-> <c- p>{</c-> <c- n>bar_ico_name</c-> <c- o>=</c-> L<c- s>"bar.ico"</c-> <c- p>};</c->

<c- c1>// later, same module unit ...</c->

<c- c1>// using, for example, </c->
<c- c1>// Windows’s RC system</c->
<c- n>HRSRC</c-> <c- n>hResource</c-> <c- o>=</c-> <c- n>FindResourceExW</c-><c- p>(</c-><c- n>m_hmoduleInstance</c-><c- p>,</c-> 
	<c- n>bar_ico_name</c-><c- p>,</c->
	<c- n>m_hresource_type</c-><c- p>,</c-> 
	<c- n>MAKELANGID</c-><c- p>(</c-><c- n>LANG_NEUTRAL</c-><c- p>,</c-> <c- n>SUBLANG_NEUTRAL</c-><c- p>));</c->
</pre>
   <p>This allows individuals to not suffer from the multi-location-updating problem: specify the name in the preamble, use that name everywhere else without having to update multiple places or keep 2 names in sync.</p>
   <p>The proposal’s wording also introduces a "Word of Power" — <em>resource-locations</em> — to specify and allow the rest of the C++ Standard to all reference the same place about resource locations and lookup. It is important that this is distinct from inclusion paths, because the two lookup specifications are inherently separate and serve different purposes. Having it as something that can be referenced means that we can keep all resource interaction attempts -- from <a data-link-type="biblio" href="#biblio-p1040">p1040 - std::embed</a> to any upcoming initiatives later in the standard -- perfectly in-sync with one another.</p>
   <h2 class="heading settled" data-level="4" id="wording"><span class="secno">4. </span><span class="content">Proposed Changes to the Standard</span><a class="self-link" href="#wording"></a></h2>
   <p>These changes are relative to <a data-link-type="biblio" href="#biblio-p1103">[p1103]</a>. If this is successful in being merged to C++20, then this paper will be rebased on the C++ Working Draft.</p>
   <h3 class="heading settled" data-level="4.1" id="wording-feature"><span class="secno">4.1. </span><span class="content">Proposed Feature Test Macro</span><a class="self-link" href="#wording-feature"></a></h3>
   <p>The proposed feature test macro for this is <code class="highlight"><c- n>__cpp_module_dependency_requires</c-></code>.</p>
   <h3 class="heading settled" data-level="4.2" id="wording-intent"><span class="secno">4.2. </span><span class="content">Intent</span><a class="self-link" href="#wording-intent"></a></h3>
   <p>The intent of this wording is to:</p>
   <ul>
    <li data-md>
     <p>provide a Word of Power called "<em>resource-locations</em>" in the Standard that allow other places to reference the same lookup mechanism;</p>
    <li data-md>
     <p>allow a <code class="highlight"><c- n>requires</c-></code> clause at the end of a module declaration to specify resource identifiers;</p>
    <li data-md>
     <p>and, allow resource identifiers to be set to a variable name that can be used throughout the <i>module-unit</i>.</p>
   </ul>
   <p>This proposal wants to ensure that future and concurrent proposals (such as <a data-link-type="biblio" href="#biblio-p1040">std::embed</a>) can refer to this same lookup mechanism by using the specified word of power ("<em>resource-locations</em>").</p>
   <h3 class="heading settled" data-level="4.3" id="wording-language"><span class="secno">4.3. </span><span class="content">Proposed Wording</span><a class="self-link" href="#wording-language"></a></h3>
   <p>Modify section §100.1 Modules units and purview [<strong>module.unit</strong>] to also include the following:</p>
   <blockquote>
    <dl>
     <dt><i>module-declaration:</i>
     <dd>
       export<sub>opt</sub> module <i>module-name</i> <i>module-partition<sub>opt</sub></i> <i>attribute-specifier-seq<sub>opt</sub></i> 
      <ins> <i>module-requires<sub>opt</sub></i></ins>
       ; 
    </dl>
   </blockquote>
   <p>Add a section §100.� Resource requirement propagation [<strong>module.requires</strong>] to §100 [<strong>modules</strong>]:</p>
   <blockquote>
    <ins>
      <b>100.� Resource requirement propagation <span style="float: right">[module.requires]</span></b> 
     <dl style="text-indent: 5em">
      <dt><i>module-requires-name:</i>
      <dd> <i>string-literal</i> 
      <dd> <i>identifier</i> = <i>string-literal</i> 
      <dt><i>module-requires-name-seq:</i>
      <dd> <i>module-requires-name</i> 
      <dd> <i>module-requires-name</i> , <i>module-requires-name-seq<sub>opt</sub></i> 
      <dt><i>module-requires:</i>
      <dd> requires { <i>module-requires-name-seq<sub>opt</sub></i> } 
      <dd> requires <i>string-literal</i> 
     </dl>
     <p><sup>1</sup> A resource requirement is a way for a module to communicate its dependency on other resources for the program. A program may specify one or more <i>string-literal</i>s in the requirements clause to make clear this dependency in the <i>module-declaration</i>.</p>
     <p><sup>2</sup> When the <i>module-requires-name</i> specifies an <i>identifier</i> set to a value, then the <i>module-requires-name</i>’s <i>identifier</i> is made available to the module unit as-if by <code class="highlight"><c- k>decltype</c-><c- p>(</c-><c- n>string</c-><c- o>-</c-><c- n>literal</c-><c- p>)</c-><c- o>&amp;</c-> <c- n>identifier</c-> <c- o>=</c-> <c- n>string</c-><c- o>-</c-><c- n>literal</c-><c- p>;</c-></code>.</p>
     <p><sup>3</sup> The locations the implementation may search for the resource uniquely identified by the <code class="highlight"><c- n>string</c-><c- o>-</c-><c- n>literal</c-></code> is implementation defined, and is called the <i>resource-locations</i>.</p>
     <p><sup>4</sup> This information does not affect the well-formedness of the program. [ <i>Note:</i> the implementation may choose to issue a warning if it cannot find the resource specified. <i>— end note</i> ]</p>
    </ins>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="acknowledgements"><span class="secno">5. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>Thanks to Isabella Muerte for helping to select the right type of syntax for this feature.</p>
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  <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-p1040">[P1040]
   <dd>JeanHeyd Meneide. <a href="https://wg21.link/p1040">std::embed</a>. October 12th, 2018. URL: <a href="https://wg21.link/p1040">https://wg21.link/p1040</a>
   <dt id="biblio-p1103">[P1103]
   <dd>Richard Smith. <a href="https://wg21.link/p1103">Merging Modules</a>. November 26th, 2018. URL: <a href="https://wg21.link/p1103">https://wg21.link/p1103</a>
  </dl>