<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P1180R0: Response to P1156</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

  /* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

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

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

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

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

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

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


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

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

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


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

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
		color: black;
		border-color: #3980B5;
		border-bottom-width: 3px !important;
		margin-bottom: 0px !important;
	}
	.toc a:visited {
		border-color: #054572;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

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

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

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

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

		.toc li {
			clear: both;
		}

		:not(li) > .toc              { margin-left:  5rem; }
		.toc .secno                  { margin-left: -5rem; }
		.toc > li li li .secno       { margin-left: -7rem; }
		.toc > li li li li .secno    { margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

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

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


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

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

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

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

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		background: #f7f8f9;
	}

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

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

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

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

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-left: calc(13px + 26.5rem - 50vw);
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-left: calc(40em - 50vw) !important;
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-left: 0 !important;
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			 * http://lea.verou.me/2012/04/background-attachment-local/
			 *
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
			            top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
			            top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style type="text/css">
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }
  </style>
  <meta content="Bikeshed version a04b880cde96bf0ae2d14a0edcf2bcdcb631548b" name="generator">
  <link href="http://wg21.link/p1180r0" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-syntax-highlighting */

.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1180R0<br>Response to P1156</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-09-18">2018-09-18</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1180r0">http://wg21.link/p1180r0</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>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 discusses some issues with the preamble as described in the merged modules proposal (P1103R0), particularly in light of P1156 and implementation concerns raised since P1103R0 was discussed.</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>
     <ol class="toc">
      <li><a href="#version"><span class="secno">1.1</span> <span class="content">Version of P1156</span></a>
      <li><a href="#purpose"><span class="secno">1.2</span> <span class="content">What is the preamble for?</span></a>
     </ol>
    <li>
     <a href="#3"><span class="secno">2</span> <span class="content">P1156R0 Section 3: Non-Modular Code</span></a>
     <ol class="toc">
      <li><a href="#3-proposal"><span class="secno">2.1</span> <span class="content">Proposal</span></a>
     </ol>
    <li>
     <a href="#4"><span class="secno">3</span> <span class="content">P1156R0 Section 4: Preamble End</span></a>
     <ol class="toc">
      <li><a href="#disallow"><span class="secno">3.1</span> <span class="content">Do not allow preprocessor action at the end of the preamble</span></a>
      <li><a href="#defermore"><span class="secno">3.2</span> <span class="content">Defer macro import slightly more</span></a>
      <li><a href="#nodefer"><span class="secno">3.3</span> <span class="content">Do not defer macro import</span></a>
      <li><a href="#4-propopsal"><span class="secno">3.4</span> <span class="content">Proposal</span></a>
     </ol>
    <li><a href="#4①"><span class="secno">4</span> <span class="content">P1156R0 Section 5: Module Partitions</span></a>
   </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>
   <h3 class="heading settled" data-level="1.1" id="version"><span class="secno">1.1. </span><span class="content">Version of P1156</span><a class="self-link" href="#version"></a></h3>
   <p>This paper is based on a draft of P1156R0 rather than the final version; the
reader has my apologies for any discrepancies between its claims and the
content of the final version of P1156R0.</p>
   <h3 class="heading settled" data-level="1.2" id="purpose"><span class="secno">1.2. </span><span class="content">What is the preamble for?</span><a class="self-link" href="#purpose"></a></h3>
   <p>It is a near-universal convention that translation units in C++ use <code class="highlight"><c- cp>#include</c-></code> directives to import the interfaces of other libraries, and
collect those <code class="highlight"><c- cp>#include</c-></code>s at the start of the translation unit. It is
reasonable to consider what benefit we gain by converting this from a mere
convention to a rule. There are several technical benefits, including:</p>
   <ul>
    <li data-md>
     <p>Tools that wish to perform automated management of imports can do so
more easily.</p>
    <li data-md>
     <p>Tools that wish to extract dependencies from source code do not need
to process the entire file.</p>
    <li data-md>
     <p>Collecting the imports into a single block enables compilation
strategies where the compiler identifies all the imports first, and
then triggers the compilation of all dependencies in parallel.</p>
    <li data-md>
     <p>In the case of macro importation, all macro imports can be delayed
until after the end of the preamble, enabling the set of imports of
a translation unit to be determined without having built any of its
dependencies.</p>
   </ul>
   <p>In addition, promoting this from convention to language rule makes C++
code more uniform.</p>
   <h2 class="heading settled" data-level="2" id="3"><span class="secno">2. </span><span class="content">P1156R0 Section 3: Non-Modular Code</span><a class="self-link" href="#3"></a></h2>
   <p>One important question that must be answered when determining how a module
system will fit into the C++ ecosystem is: how are the dependencies of a
translation unit determined?</p>
   <p>In traditional compilations of simple programs, each translation unit can
be compiled in isolation without an answer to this question. The set of
dependencies can be computed as a side-effect of the compilation, and used
only to decide when recompilation is necessary.</p>
   <p>However, once the need for more complexity arises (for instance, if the
program generates header files as part of its build process, or if a
distributed build process needs to be sent all the requisite inputs for a
compilation), a different strategy is needed. In practice, build systems
that tackle such problems either have explicitly-specified dependencies on
such headers, or perform <code class="highlight"><c- cp>#include</c-></code> scanning to determine the set of header
files used by a source file. (Some build systems combine both strategies, for
example requiring a superset of dependencies to be explicitly declared and
refined to an actual dependency set via include scanning.)</p>
   <p>We will not discuss systems with explicit specification of dependencies
further, as dependencies for modules can be explicitly specified in much
the same manner as for header files, and with the same benefits and
disadvantages. Instead, we focus on <code class="highlight"><c- cp>#include</c-></code> scanning.</p>
   <p>There are, broadly-speaking, two different approaches taken to perform <code class="highlight"><c- cp>#include</c-></code> scanning:</p>
   <ul>
    <li data-md>
     <p>approximate scanning just looks for lines in the source file that appear
to be <code class="highlight"><c- cp>#include</c-></code> directives, and assumes the named file is included</p>
    <li data-md>
     <p>precise scanning performs steps equivalent to a full preprocessing of
the source file, with the relevant configuration that will be used for
the compilation, and determines the set of files that are actually
included</p>
   </ul>
   <div class="example" id="example-d88379fa">
    <a class="self-link" href="#example-d88379fa"></a> 
<pre class="highlight"><c- cp>#define FOO "bar.h"</c->
<c- cp>#ifndef FOO</c->
<c- cp>#include</c-> "baz.h"
<c- cp>#endif</c->
<c- cp>#include</c-> FOO
</pre>
    <p>A precise scan would indicate that this file includes <code class="highlight"><c- s>"bar.h"</c-></code>, but an
approximate scan will likely determine that it includes <code class="highlight"><c- s>"baz.h"</c-></code>.</p>
   </div>
   <p>Approximate scanning needs no additional language support to work with
modules: the scanning tool can assume that the tokens after an <code class="highlight"><c- n>import</c-></code> are not subject to macro expansion, and that all <code class="highlight"><c- n>import</c-></code> declarations
are reachable, just as it likely does for <code class="highlight"><c- cp>#include</c-></code> directives.</p>
   <p>Precise scanning, however, needs to be able to scan (and therefore, in
general, preprocess) the entire portion of the file that might contain
imports, without already having a precompiled form of those imports. To
support this, we require all legacy header imports to precede the point
at which imported macros become visible, at least in translation units
that have preambles.</p>
   <p>Section 3 of P1156R0 suggests that the same rule should also apply to
non-module translation units, in order to allow precise dependency
scanning to be applied there too. However, this creates a problem.
Consider:</p>
<pre class="highlight"><c- c1>// a.h</c->
<c- cp>#ifndef A_H</c->
<c- cp>#define A_H</c->
<c- n>import</c-> <c- n>some</c-><c- p>.</c-><c- n>module</c-><c- p>;</c->
<c- c1>// ...</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- c1>// b.h</c->
<c- cp>#ifndef B_H</c->
<c- cp>#define B_H</c->
<c- cp>#include</c-> "x.h"
<c- cp>#include</c-> "a.h"
<c- c1>// ...</c->
<c- cp>#endif</c->
</pre>
   <p>The author of <code class="highlight"><c- s>"b.h"</c-></code> may have no idea that <code class="highlight"><c- s>"a.h"</c-></code> contains a module
import -- and nor should they! And yet, after preprocessing, <code class="highlight"><c- s>"b.h"</c-></code> expands to something like:</p>
<pre class="highlight"><c- c1>// ... declarations from x.h ...</c->
<c- n>import</c-> <c- n>some</c-><c- p>.</c-><c- n>module</c-><c- p>;</c->
<c- c1>// ... declarations from a.h ...</c->
<c- c1>// ... declarations from b.h ...</c->
</pre>
   <p>... in which notably the import of <code class="highlight"><c- n>some</c-><c- p>.</c-><c- n>module</c-></code> is in the middle of
the translation unit. We cannot disallow these scenarios without blocking
reasonable upgrade paths for existing legacy code.</p>
   <h3 class="heading settled" data-level="2.1" id="3-proposal"><span class="secno">2.1. </span><span class="content">Proposal</span><a class="self-link" href="#3-proposal"></a></h3>
   <p>In this paper, we propose a different solution for precise dependency
scanning for legacy translation units:</p>
   <ul>
    <li data-md>
     <p>Disallow the <code class="highlight"><c- n>import</c-></code> <em>header-name</em> <code class="highlight"><c- p>;</c-></code> syntax outside of a preamble.</p>
   </ul>
   <p>That is: within legacy code, we permit import of named modules, but not
import of legacy header files. For headers, the traditional <code class="highlight"><c- cp>#include</c-></code> syntax must be used, and it’s implementation-defined whether and when
those <code class="highlight"><c- cp>#include</c-></code>s are mapped into imports of legacy header units.</p>
   <p>A precise include scanner can then merely collect the named module imports
it sees, along with the names of header files that it enters, ambivalent of
whether a <code class="highlight"><c- cp>#include</c-></code> will be translated into a legacy header import by
the compiler.</p>
   <div class="note" role="note">
     A misconfigured build system might result in a non-modular header being
treated as a legacy header unit. One consequence of this is that the
"precise" include scanner may compute a different result than would be
determined by the compiler. For example: 
<pre class="highlight"><c- c1>// nonmodular.h</c->
<c- cp>#ifdef FOO</c->
<c- cp>#define BAR</c->
<c- cp>#endif</c->
</pre>
<pre class="highlight"><c- c1>// legacy.cpp</c->
<c- cp>#define FOO</c->
<c- cp>#include</c-> "nonmodular.h"
<c- cp>#ifndef BAR</c->
<c- cp>#include</c-> "other.h"
<c- cp>#endif</c->
</pre>
    <p>Here, if the build system is erroneously configured to treat <code class="highlight"><c- n>nonmodular</c-><c- p>.</c-><c- n>h</c-></code> as a legacy header unit, include scanning may determine
that <code class="highlight"><c- n>legacy</c-><c- p>.</c-><c- n>cpp</c-></code> does not include <code class="highlight"><c- s>"other.h"</c-></code>, but when actually compiled,
the macro <code class="highlight"><c- n>FOO</c-></code> from <code class="highlight"><c- n>legacy</c-><c- p>.</c-><c- n>cpp</c-></code> will not leak into <code class="highlight"><c- n>nonmodular</c-><c- p>.</c-><c- n>h</c-></code>, and
as a result, <code class="highlight"><c- n>other</c-><c- p>.</c-><c- n>h</c-></code> will be included.</p>
    <p>However, this is a consequence of a build system misconfiguration, and
is just one of many things that will go wrong if non-modular headers are
incorrectly configured as modular.</p>
    <p>Even in this case, a build system can detect the problem by inspecting a
list of dependencies produced by the compiler as a side-effect of
compilation (for example using GCC’s <code class="highlight"><c- o>-</c-><c- n>MD</c-></code> or MSVC’s <code class="highlight"><c- o>/</c-><c- n>showIncludes</c-></code>).</p>
   </div>
   <p>Such an include scanner would then transitively scan all headers reachable from
each visited legacy header, but this is no different from the status quo
before modules are introduced. Modular compilations would still benefit from
not needing to scan legacy header includes.</p>
   <h2 class="heading settled" data-level="3" id="4"><span class="secno">3. </span><span class="content">P1156R0 Section 4: Preamble End</span><a class="self-link" href="#4"></a></h2>
   <p>In corner cases where the preamble is immediately followed by the expansion
of an imported macro, it is non-trivial to determine where the preamble ends,
both for the compiler and for a human reader.</p>
   <div class="example" id="example-9fe297c6">
    <a class="self-link" href="#example-9fe297c6"></a> 
<pre class="highlight"><c- n>module</c-> <c- n>M</c-><c- p>;</c->
<c- n>import</c-> <c- s>"foo.h"</c-><c- p>;</c->
<c- cp>#ifdef FOO</c->
<c- b>int</c-> <c- n>x</c-><c- p>;</c->
<c- cp>#endif</c->
<c- b>int</c-> <c- n>y</c-><c- p>;</c->
</pre>
    If <code class="highlight"><c- s>"foo.h"</c-></code> exports a macro named <code class="highlight"><c- n>FOO</c-></code>, then <code class="highlight"><c- n>x</c-></code> should be declared according
to the rules in P1103R0. But an import <em>after</em> the <code class="highlight"><c- cp>#ifdef</c-></code> block changes this
behavior: 
<pre class="highlight"><c- n>module</c-> <c- n>M</c-><c- p>;</c->
<c- n>import</c-> <c- s>"foo.h"</c-><c- p>;</c->
<c- cp>#ifdef FOO</c->
<c- b>int</c-> <c- n>x</c-><c- p>;</c->
<c- cp>#endif</c->
<c- n>import</c-> <c- n>bar</c-><c- p>;</c->
<c- b>int</c-> <c- n>y</c-><c- p>;</c->
</pre>
    Now the preamble extends to the end of the <code class="highlight"><c- n>import</c-> <c- n>bar</c-><c- p>;</c-></code> declaration, and the <code class="highlight"><c- cp>#ifdef FOO</c-></code> block is not entered and <code class="highlight"><c- n>x</c-></code> is not declared. 
   </div>
   <p>Various parties have proposed to solve this problem with an explicit preamble
end marker. P1156R0 suggests <code class="highlight"><c- n>import</c-><c- p>;</c-></code>. The GCC modules implementation suggests
using simply <code class="highlight"><c- p>;</c-></code> (and only in the cases where the end of the preamble is not
otherwise obvious). These suggestions introduce verbosity into the preamble
that would be better avoided. We discuss some alternatives below.</p>
   <h3 class="heading settled" data-level="3.1" id="disallow"><span class="secno">3.1. </span><span class="content">Do not allow preprocessor action at the end of the preamble</span><a class="self-link" href="#disallow"></a></h3>
   <p>Perhaps the simplest solution would be to make the corner cases where
the end of the preamble is ambiguous ill-formed. More precisely:</p>
   <ul>
    <li data-md>
     <p>if the <em>token</em> following the <code class="highlight"><c- p>;</c-></code> that ends the preamble in the
preprocessed output (in phase 7 of translation) is not formed from
the <em>preprocessing-token</em> following the <code class="highlight"><c- p>;</c-></code> in the tokenized input
(in phase 3 of translation), the program is ill-formed</p>
   </ul>
   <p>This makes the first example above invalid, because the <em>preprocessing-token</em> after the <code class="highlight"><c- p>;</c-></code> that ends the preamble is the <code class="highlight"><c- cp>#</c-></code> <em>preprocessing-token</em> on line 3,
but the next <em>token</em> in the preprocessed output is the <code class="highlight"><c- b>int</c-></code> on line 6.</p>
   <h3 class="heading settled" data-level="3.2" id="defermore"><span class="secno">3.2. </span><span class="content">Defer macro import slightly more</span><a class="self-link" href="#defermore"></a></h3>
   <p>Instead of deferring macro import until just after the last token of
the preamble, we could instead defer macro import to just before the
first token <em>after</em> the preamble.</p>
   <p>This simplifies the technical problems with determining the end of the
preamble: because it is an error to form the <code class="highlight"><c- k>export</c-></code> and <code class="highlight"><c- n>import</c-></code> tokens of an import declaration by macro expansion, we can preprocess
the next token after the end of an <code class="highlight"><c- n>import</c-></code> declaration; if it is
neither of those, we can end the preamble, make imported macros
visible, and carry on from that token.</p>
   <p>Under this approach, the <code class="highlight"><c- cp>#ifdef FOO</c-></code> block would not be entered in
either of the examples above, because in both cases it is part of
the preamble. This does not completely remove the surprise from
the example.</p>
   <h3 class="heading settled" data-level="3.3" id="nodefer"><span class="secno">3.3. </span><span class="content">Do not defer macro import</span><a class="self-link" href="#nodefer"></a></h3>
   <p>Instead of deferring macro import until the end of the preamble, we could
specify:</p>
   <ul>
    <li data-md>
     <p>macros become visible immediately, but</p>
    <li data-md>
     <p>if an imported macro is expanded before the end of the preamble,
the program is ill-formed</p>
   </ul>
   <p>The first rule means that valid code always does what it looks like it does.
In the first example above, <code class="highlight"><c- n>FOO</c-></code> is imported and expanded. The second example
above is ill-formed, because <code class="highlight"><c- n>FOO</c-></code> was expanded before the end of the preamble.</p>
   <p>This also preserves the ability to precisely determine the set of imports
of a module unit without having precompiled module artifacts: if the program
is valid, then no imported macros can affect the set of imports.</p>
   <p>However, this loses the property that module imports can be reordered with
no semantic effect on the program: reordering a legacy module unit import
may change whether the program is ill-formed (but, if valid, not what it
means). Tooling wishing to automatically rearrange imports can still do so
by following the convention that legacy header imports are placed after all
other imports (and thus cannot export macros whose names conflict with the
names of any imported modules).</p>
   <p>This approach also means that a compiler cannot slurp up all the imports
first, hand them off to some other process (such as the build system) to
compile in parallel, and then resume and process the rest of the file.
However, there are good alternatives for an implementation to pursue. For
example, the following strategy can be used:</p>
   <ol>
    <li data-md>
     <p>while the input stream starts with <code class="highlight"><c- n>import</c-></code> or <code class="highlight"><c- k>export</c-> <c- n>import</c-></code> (without to any further preprocessing), preprocess and consume an
 import directive and add it to our list of imports</p>
    <li data-md>
     <p>take the list of imports, import those modules, make their macros
 visible, and preprocess until you can determine whether the next
 token(s) are <code class="highlight"><c- n>import</c-></code> or <code class="highlight"><c- k>export</c-> <c- n>import</c-></code>; if so, go to step 1</p>
    <li data-md>
     <p>we have reached the end of the preamble</p>
   </ol>
   <p>(An implementation can check after the fact if any of the identifiers it
encountered in this process would have corresponded to an imported macro.)</p>
   <h3 class="heading settled" data-level="3.4" id="4-propopsal"><span class="secno">3.4. </span><span class="content">Proposal</span><a class="self-link" href="#4-propopsal"></a></h3>
   <p>My preferred option is the final one above: make legacy header imports
behave as if they import macros immediately, but make the program ill-formed
if an imported macro would have been expanded within the preamble.</p>
   <p>This approach provides a reasonable user model: attempts to make the set
of imports depend on an imported macro are cleanly rejected rather than
being accepted but doing something unexpected.</p>
   <p>It also admits a relatively straightforward, single-pass implementation
strategy, and allows precise dependency extraction for valid code without
the need for pre-built module artifacts for dependencies.</p>
   <h2 class="heading settled" data-level="4" id="4①"><span class="secno">4. </span><span class="content">P1156R0 Section 5: Module Partitions</span><a class="self-link" href="#4①"></a></h2>
   <p>We wish to suggest a correction to P1156R0; D1156R0 section 5 says:</p>
   <blockquote>
    <p>The fact that there can (presumably) be both interface and implementation
partition units for the same partition further complicates things [...]</p>
   </blockquote>
   <p>However, P1103R0 <a href="10.7.1">dcl.module.unit</a>p3 disallows this situation:</p>
   <blockquote>
    <p>A named module shall not contain multiple module partitions with the same
sequence of <em>identifier</em>s in their <em>module-partition</em>s.</p>
   </blockquote>
   <p>That is, the names of module partitions are all distinct, regardless of
whether they are interface or implementation partitions. (This must be the
case, otherwise the <code class="highlight"><c- nl>import</c-> <c- p>:</c-><c- n>partition</c-><c- p>;</c-></code> syntax could not uniquely identify
the partition to import.)</p>
  </main>
<script>
(function() {
  "use strict";
  var collapseSidebarText = '<span aria-hidden="true">←</span> '
                          + '<span>Collapse Sidebar</span>';
  var expandSidebarText   = '<span aria-hidden="true">→</span> '
                          + '<span>Pop Out Sidebar</span>';
  var tocJumpText         = '<span aria-hidden="true">↑</span> '
                          + '<span>Jump to Table of Contents</span>';

  var sidebarMedia = window.matchMedia('screen and (min-width: 78em)');
  var autoToggle   = function(e){ toggleSidebar(e.matches) };
  if(sidebarMedia.addListener) {
    sidebarMedia.addListener(autoToggle);
  }

  function toggleSidebar(on) {
    if (on == undefined) {
      on = !document.body.classList.contains('toc-sidebar');
    }

    /* Don’t scroll to compensate for the ToC if we’re above it already. */
    var headY = 0;
    var head = document.querySelector('.head');
    if (head) {
      // terrible approx of "top of ToC"
      headY += head.offsetTop + head.offsetHeight;
    }
    var skipScroll = window.scrollY < headY;

    var toggle = document.getElementById('toc-toggle');
    var tocNav = document.getElementById('toc');
    if (on) {
      var tocHeight = tocNav.offsetHeight;
      document.body.classList.add('toc-sidebar');
      document.body.classList.remove('toc-inline');
      toggle.innerHTML = collapseSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, 0 - tocHeight);
      }
      tocNav.focus();
      sidebarMedia.addListener(autoToggle); // auto-collapse when out of room
    }
    else {
      document.body.classList.add('toc-inline');
      document.body.classList.remove('toc-sidebar');
      toggle.innerHTML = expandSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, tocNav.offsetHeight);
      }
      if (toggle.matches(':hover')) {
        /* Unfocus button when not using keyboard navigation,
           because I don’t know where else to send the focus. */
        toggle.blur();
      }
    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn’t exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn’t standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don’t have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we’re at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can’t find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>
