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  <title>P1502R0: Standard library header units for C++20</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,
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  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1502R0<br>Standard library header units for C++20</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-02-21">2019-03-15</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1502r0">http://wg21.link/p1502r0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:richard@metafoo.co.uk">Richard Smith</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>LEWG, EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper proposes minimal support for modular use of the C++ standard library via header units.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#background"><span class="secno">1</span> <span class="content">Background</span></a>
    <li><a href="#goals"><span class="secno">2</span> <span class="content">Goals</span></a>
    <li>
     <a href="#approach"><span class="secno">3</span> <span class="content">Approach</span></a>
     <ol class="toc">
      <li><a href="#standard-library-header-units"><span class="secno">3.1</span> <span class="content">Standard library header units</span></a>
      <li><a href="#std-module-land-grab"><span class="secno">3.2</span> <span class="content"><code class="highlight"><c- n>std</c-></code> module land-grab</span></a>
     </ol>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Wording</span></a>
     <ol class="toc">
      <li><a href="#101-module-units-and-purviews-moduleunit"><span class="secno">4.1</span> <span class="content">§10.1 Module units and purviews [module.unit]</span></a>
      <li><a href="#103-import-declaration-moduleimport"><span class="secno">4.2</span> <span class="content">§10.3 Import declaration [module.import]</span></a>
      <li><a href="#16512-headers-headers"><span class="secno">4.3</span> <span class="content">§16.5.1.2 Headers [headers]</span></a>
      <li><a href="#16522-headers-usingheaders"><span class="secno">4.4</span> <span class="content">§16.5.2.2 Headers [using.headers]</span></a>
     </ol>
    <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="background"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#background"></a></h2>
   <p>With the anticipated inclusion of modules in the C++20 langauge, we need a
natural and portable mechanism to import the standard library into a
modules-enabled compilation.</p>
   <p>This provides us with a unique opportunity: when converting the standard
library into a set of named modules (universally assumed to be given names
beginning <code class="highlight"><c- n>std</c-></code>), we can reconsider which pieces of the library
belong together and which can be split apart, with few backwards compatibility
constraints. (There are techniques that permit standard library vendors to
maintain ABI across such a reorganization, and we trust the them to use those
techniques as they see fit.)</p>
   <p>Indeed, various parties are investigating such reorganizations of the standard
(see <a data-link-type="biblio" href="#biblio-p0581r1">[P0581R1]</a>, <a data-link-type="biblio" href="#biblio-p1212r0">[P1212R0]</a>, <a data-link-type="biblio" href="#biblio-p1453r0">[P1453R0]</a>), and we anticipate such
investigation will lead to improvements in the overall structure of the
standard library as exposed to modern modular C++. If such a solution is ready
in time for C++20, we would welcome such a development.</p>
   <p>However, it is not clear that such a solution will be ready in time, and the
worst outcome is looming: that different implementations might expose the C++
standard library in different ways, fracturing the language and ecosystem.
While modules gives us an opportunity to improve upon the status quo in this
area, we can decouple such improvements from the addition of the modules
language feature and basic support for the standard library therein, and doing
so is good engineering practice.</p>
   <h2 class="heading settled" data-level="2" id="goals"><span class="secno">2. </span><span class="content">Goals</span><a class="self-link" href="#goals"></a></h2>
   <p>We seek to solve three problems:</p>
   <p>1) Provide a minimal, uninventive mechanism to expose the existing standard
   library organization to modular compilations.</p>
   <p>2) Do not get in the way of, or create any impediment to, a proper
   reorganization of the standard library into named modules.</p>
   <p>3) Reserve space for such a future reorgnization.</p>
   <h2 class="heading settled" data-level="3" id="approach"><span class="secno">3. </span><span class="content">Approach</span><a class="self-link" href="#approach"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="standard-library-header-units"><span class="secno">3.1. </span><span class="content">Standard library header units</span><a class="self-link" href="#standard-library-header-units"></a></h3>
   <p>We propose to guarantee that the C++ standard library headers can be imported
as header units:</p>
<pre class="highlight"><c- c1>// C++17 or C++20</c->
<c- cp>#include</c-> &lt;vector>
<c- cp>#include</c-> &lt;tuple>
</pre>
<pre class="highlight"><c- c1>// C++20</c->
<c- n>import</c-> <c- o>&lt;</c-><c- n>vector</c-><c- o>></c-><c- p>;</c->
<c- n>import</c-> <c- o>&lt;</c-><c- n>tuple</c-><c- o>></c-><c- p>;</c->
</pre>
   <p>This has a number of consequences, such as:</p>
   <ul>
    <li data-md>
     <p>No reorganization of any library contents nor ABI changes</p>
    <li data-md>
     <p>Existing <code class="highlight"><c- cp>#include</c-></code>s of standard library headers transparently turn into
module imports in C++20</p>
    <li data-md>
     <p>Additional names can "leak" out of standard library header units, just as
they can "leak" out of standard library headers today (for example, it’s
unspecified whether <code class="highlight"><c- o>&lt;</c-><c- n>vector</c-><c- o>></c-></code> includes <code class="highlight"><c- o>&lt;</c-><c- n>utility</c-><c- o>></c-></code> today)</p>
    <li data-md>
     <p>Some situations that were formerly undefined behavior now become ill-formed
or well-defined:</p>
     <ul>
      <li data-md>
       <p>Defining a keyword or library identifier then including or importing
a library header need not be undefined, as the <code class="highlight"><c- cp>#define</c-></code> does not affect
the standard library header unit.</p>
      <li data-md>
       <p>Including a standard library header inside a namespace becomes
ill-formed rather than undefined behavior.</p>
     </ul>
   </ul>
   <p>We propose that only the C++ standard library headers (excluding both the <code class="highlight"><c- o>&lt;</c-><c- n>cfoo</c-><c- o>></c-></code> headers and the deprecated <code class="highlight"><c- o>&lt;</c-><c- n>foo</c-><c- p>.</c-><c- n>h</c-><c- o>></c-></code> headers) are affected. The <code class="highlight"><c- o>&lt;</c-><c- n>foo</c-><c- p>.</c-><c- n>h</c-><c- o>></c-></code> headers are included from shared C / C++ header files, frequently in <code class="highlight"><c- k>extern</c-> <c- s>"C"</c-></code> blocks, for which <code class="highlight"><c- cp>#include</c-></code> translation would create trouble. In
addition, the underlying C standard library sometimes gives additional
semantics to macros defined before the <code class="highlight"><c- o>&lt;</c-><c- n>foo</c-><c- p>.</c-><c- n>h</c-><c- o>></c-></code> headers are included
(for example <code class="highlight"><c- n>__need_NULL</c-></code>, <code class="highlight"><c- n>_POSIX_SOURCE</c-></code>),
and the behavior of the <code class="highlight"><c- o>&lt;</c-><c- n>cassert</c-><c- o>></c-></code> header
differs based on which macros are
defined beforehand. Per 15.5.2.2 [using.headers]/2:</p>
   <blockquote>
    <p>A translation unit may include library headers in any order (Clause 5).
Each may be included more than once,
with no effect different from being included exactly once,
except that the effect of including either <code class="highlight"><c- o>&lt;</c-><c- n>cassert</c-><c- o>></c-></code> or <code class="highlight"><c- o>&lt;</c-><c- n>assert</c-><c- p>.</c-><c- n>h</c-><c- o>></c-></code> depends each time on the lexically current definition of <code class="highlight"><c- n>NDEBUG</c-></code>.</p>
   </blockquote>
   <h3 class="heading settled" data-level="3.2" id="std-module-land-grab"><span class="secno">3.2. </span><span class="content"><code class="highlight"><c- n>std</c-></code> module land-grab</span><a class="self-link" href="#std-module-land-grab"></a></h3>
   <p>We propose to reserve modules whose names begin with a <code class="highlight"><c- n>std</c-></code> name component
or <code class="highlight"><c- n>std</c-></code> followed by a sequence of digits.</p>
   <p>If any module name component is a reserved identifier, the module name is reserved
for implementations (eg, <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>_Vendor</c-><c- p>.</c-><c- n>something</c-></code>);
all other names are reserved for future standardization (eg, <code class="highlight"><c- n>std</c-><c- p>.</c-><c- n>vector</c-></code>).</p>
   <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>
   <h3 class="heading settled" data-level="4.1" id="101-module-units-and-purviews-moduleunit"><span class="secno">4.1. </span><span class="content">§10.1 Module units and purviews [module.unit]</span><a class="self-link" href="#101-module-units-and-purviews-moduleunit"></a></h3>
   <p>Change in 10.1 [module.unit] paragraph 1:</p>
   <blockquote>
    <p>
     A <em>module unit</em> is a translation unit that contains a <em>module-declaration</em>.
A <em>named module</em> is the collection of module units with the same <em>module-name</em>.
The <em>identifier</em>s <code class="highlight"><c- n>module</c-></code> and <code class="highlight"><c- n>import</c-></code> shall not appear as <em>identifier</em>s
in a <em>module-name</em> or <em>module-partition</em>. 
     <ins> All <em>module-name</em>s beginning with an <em>identifier</em> comprising <code class="highlight"><c- n>std</c-></code> followed by zero or more <em>digit</em>s are
reserved and shall not be specified in a <em>module-declaration</em>;
no diagnostic is required.
If any <em>identifier</em> in a reserved <em>module-name</em> is a reserved identifier (5.10 [lex.name]),
the module name is reserved for use by C++ implementations;
otherwise it is reserved for future standardization.</ins>
      The optional <em>attribute-specifier-seq</em> appertains to the <em>module-declaration</em>.
    </p>
   </blockquote>
   <h3 class="heading settled" data-level="4.2" id="103-import-declaration-moduleimport"><span class="secno">4.2. </span><span class="content">§10.3 Import declaration [module.import]</span><a class="self-link" href="#103-import-declaration-moduleimport"></a></h3>
   <p>Change in 10.3 [module.import] paragraph 3:</p>
   <blockquote>
    <p>
     [...] An <em>importable header</em> is a member of an implementation-defined set of headers 
     <ins> that includes all importable C++ library headers (16.5.1.2 [headers])</ins>
     .
[...]
    </p>
   </blockquote>
   <h3 class="heading settled" data-level="4.3" id="16512-headers-headers"><span class="secno">4.3. </span><span class="content">§16.5.1.2 Headers [headers]</span><a class="self-link" href="#16512-headers-headers"></a></h3>
   <p>Add a new paragraph after 16.5.1.2 [headers] paragraph 3:</p>
   <blockquote>
    <ins>
      The headers listed in Table 19 [C++ library headers],
or, for a freestanding implementation,
the subset of such headers that are provided by the implementation,
are collectively known as the <em>importable C++ library headers</em>.
[<em>Note</em>: Importable C++ library headers can be imported as
module units (10.3 [module.import]).
Including an importable C++ library header has the same effect
as importing the corresponding header unit. — <em>end note</em>]
[<em>Example</em>: 
<pre class="highlight"><c- n>import</c-> <c- o>&lt;</c-><c- n>vector</c-><c- o>></c-><c- p>;</c->         <c- c1>// imports the &lt;vector> header unit</c->
<c- n>std</c-><c- o>::</c-><c- n>vector</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-> <c- n>vi</c-><c- p>;</c->     <c- c1>// OK</c->
<c- cp>#include</c-> &lt;map>           // imports the &lt;map> header unit
<c- n>std</c-><c- o>::</c-><c- n>map</c-><c- o>&lt;</c-><c- b>int</c-><c- p>,</c-> <c- b>int</c-><c- o>></c-> <c- n>mii</c-><c- p>;</c->  <c- c1>// OK</c->
</pre>
     — <em>end example</em>] 
    </ins>
   </blockquote>
   <h3 class="heading settled" data-level="4.4" id="16522-headers-usingheaders"><span class="secno">4.4. </span><span class="content">§16.5.2.2 Headers [using.headers]</span><a class="self-link" href="#16522-headers-usingheaders"></a></h3>
   <p>Change in 16.5.2.2 [using.headers] paragraph 1:</p>
   <blockquote>
    <p>
     The entities in the C++ standard library are defined in headers,
whose contents are made available to a translation unit
when it contains the appropriate <code class="highlight"><c- cp>#include</c-></code> preprocessing directive (15.2)
     <ins>,
or, for an importable C++ library header (16.5.1.2 [headers]),
when it contains the appropriate <code class="highlight"><c- n>import</c-></code> declaration (10.3 [module.import])</ins>
     .
    </p>
   </blockquote>
   <p>Change in 16.5.2.2 [using.headers] paragraph 3:</p>
   <blockquote>
    <p>
     A translation unit shall include a header only outside of any declaration or definition 
     <ins> and only in the <em>global-module-fragment</em> of a module unit</ins>
     ,
and shall include the header 
     <ins>or import the corresponding header unit (if any)</ins>
      lexically before the first reference in that translation unit to any of the entities declared in that header.
No diagnostic is required.
    </p>
   </blockquote>
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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-p0581r1">[P0581R1]
   <dd>Marshall Clow, Beman Dawes, Gabriel Dos Reis, Stephan T. Lavavej, Billy O’Neal, Bjarne Stroustrup, Jonathan Wakely. <a href="https://wg21.link/p0581r1">Standard Library Modules</a>. 11 February 2018. URL: <a href="https://wg21.link/p0581r1">https://wg21.link/p0581r1</a>
   <dt id="biblio-p1212r0">[P1212R0]
   <dd>Ben Craig. <a href="https://wg21.link/p1212r0">Modules and Freestanding</a>. 6 October 2018. URL: <a href="https://wg21.link/p1212r0">https://wg21.link/p1212r0</a>
   <dt id="biblio-p1453r0">[P1453R0]
   <dd>Bryce Adelstein Lelbach. <a href="https://wg21.link/p1453r0">Modularizing the Standard Library is a Reorganization Opportunity</a>. 21 January 2019. URL: <a href="https://wg21.link/p1453r0">https://wg21.link/p1453r0</a>
  </dl>
