<!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>P1498R0: Constrained Internal Linkage for Modules</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

  /* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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   <h1 class="p-name no-ref" id="title">P1498R0<br>Constrained Internal Linkage for Modules</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2019-02-20">2019-02-20</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p1498">http://wg21.link/p1498</a>
     <dt>Issue Tracking:
     <dd><a href="#issues-index">Inline In Spec</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:chandlerc@gmail.com">Chandler Carruth</a>
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:nathan@acm.org">Nathan Sidwell</a>
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:richard@metafoo.co.uk">Richard Smith</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>We propose constraints on the use of internal linkage names within module

  interface units to make them both reliable and useful. This is intended to
  avoid the need of linkage promotion and its associated problems while
  preserving the simple, teachable utility of internal linkage such as being
  able to factor non-inline function definitions into internal helpers without
  any non-local effects.</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="#problem"><span class="secno">1</span> <span class="content">Problem &amp; Motivation</span></a>
    <li>
     <a href="#proposal"><span class="secno">2</span> <span class="content">Proposal</span></a>
     <ol class="toc">
      <li><a href="#tu-local"><span class="secno">2.1</span> <span class="content">Translation Unit Local</span></a>
      <li>
       <a href="#restrictions"><span class="secno">2.2</span> <span class="content">Restrict Usage of Internal Linkage Names</span></a>
       <ol class="toc">
        <li><a href="#ex-functions"><span class="secno">2.2.1</span> <span class="content">Examples with basic functions</span></a>
        <li><a href="#ex-templates"><span class="secno">2.2.2</span> <span class="content">Examples with basic templates</span></a>
        <li><a href="#ex-class-mem-fun"><span class="secno">2.2.3</span> <span class="content">Examples with class member functions</span></a>
        <li><a href="#ex-class-templet-mem-fun"><span class="secno">2.2.4</span> <span class="content">Examples with class template member functions</span></a>
        <li><a href="#ex-variables"><span class="secno">2.2.5</span> <span class="content">Examples with variables</span></a>
        <li><a href="#ex-lambdas"><span class="secno">2.2.6</span> <span class="content">Examples with lambdas</span></a>
        <li><a href="#ex-local-classes"><span class="secno">2.2.7</span> <span class="content">Examples with function local classes</span></a>
        <li><a href="#ex-template-surprises"><span class="secno">2.2.8</span> <span class="content">Examples where template instantiation has interesting implications</span></a>
       </ol>
      <li><a href="#diagnostics"><span class="secno">2.3</span> <span class="content">Enforcing and Diagnosing the Restriction</span></a>
      <li><a href="#p1395"><span class="secno">2.4</span> <span class="content">Addressing <span>[p1395r0]</span> Issues With Linkage Promotion and Partitions</span></a>
      <li><a href="#p1347"><span class="secno">2.5</span> <span class="content">Addressing <span>[p1347r1]</span> Issues With Transitively Reachable Internal Names</span></a>
      <li><a href="#non-importable"><span class="secno">2.6</span> <span class="content">Non-importable Translation Units</span></a>
      <li><a href="#inline"><span class="secno">2.7</span> <span class="content">Inline</span></a>
      <li>
       <a href="#future"><span class="secno">2.8</span> <span class="content">Future Work</span></a>
       <ol class="toc">
        <li><a href="#hint-annotation"><span class="secno">2.8.1</span> <span class="content">Introduce a non-semantic inlining hint annotation</span></a>
        <li><a href="#deprecate-internal-inline"><span class="secno">2.8.2</span> <span class="content">Deprecate inline definitions of internal names</span></a>
        <li><a href="#deprecate-implementation-inline"><span class="secno">2.8.3</span> <span class="content">Deprecate declaring new inline entities within implementation units</span></a>
       </ol>
     </ol>
    <li>
     <a href="#alternatives"><span class="secno">3</span> <span class="content">Alternatives</span></a>
     <ol class="toc">
      <li><a href="#require-linkage-alt"><span class="secno">3.1</span> <span class="content">Require Names in Importable Units Have Non-Internal Linkage</span></a>
      <li><a href="#p1395-alt"><span class="secno">3.2</span> <span class="content">Alternative Proposed Solutions in <span>[p1395r0]</span></span></a>
      <li><a href="#p1347-alt"><span class="secno">3.3</span> <span class="content">Alternative Proposed Solutions in <span>[p1347r1]</span></span></a>
     </ol>
    <li><a href="#wording"><span class="secno">4</span> <span class="content">Wording</span></a>
    <li><a href="#acknowledgements"><span class="secno">5</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#changelog"><span class="secno">6</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#r0"><span class="secno">6.1</span> <span class="content">Revision 0</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>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="problem"><span class="secno">1. </span><span class="content">Problem &amp; Motivation</span><a class="self-link" href="#problem"></a></h2>
   <p>Internal linkage names for functions, variables, and types are routinely used
to factor and express implementation details of C++ interfaces today, and this
practice is useful and important for maintainability of this code. One
consequence of introducing modules into C++ is that it easily allows embedding
the implementation of an interface into a module’s interface unit without these
being exposed to the consumers of that interface unit. In effect, these
implementation details remain local to the translation unit, despite this being
a module interface unit. However, as currently specified, using internal
linkage names for components of these implementation details creates problems
because those names may also be used in ways that do <em>not</em> remain local to the
translation unit.</p>
   <h2 class="heading settled" data-level="2" id="proposal"><span class="secno">2. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>The overarching principle is that internal linkage names may only be used within
a context that is defined to remain local to the translation unit for importable
translation units.</p>
   <h3 class="heading settled" data-level="2.1" id="tu-local"><span class="secno">2.1. </span><span class="content">Translation Unit Local</span><a class="self-link" href="#tu-local"></a></h3>
   <p>Some parts of a module interface unit do not escape that translation unit. We
call these <i>translation unit local</i> or <i>TU-local</i>. The canonical
example is a non-inline function definition. The contents of this definition
have <em>no</em> visible effect beyond the translation unit. Our current understanding
of the TU-local constructs in C++:</p>
   <ul>
    <li data-md>
     <p>Types, functions, and variables whose names have internal linkage (including
those within anonymous namespaces)</p>
    <li data-md>
     <p>Types, functions, and variables whose names have no linkage</p>
     <p class="issue" id="issue-8bfedeb4"><a class="self-link" href="#issue-8bfedeb4"></a> Local class member functions appear to have no linkage even when they
occur within inline functions with external (or module) linkage. This seems
like a bug generally, but certainly such member functions are <strong>not</strong> TU-local.</p>
    <li data-md>
     <p>Function definitions which are not inline, non-templated, and not <code class="highlight"><c- n>consteval</c-></code></p>
    <li data-md>
     <p>Variable definitions which are not inline and non-templated</p>
    <li data-md>
     <p>Template specializations for which any template argument involves a TU-local
entity</p>
    <li data-md>
     <p>Template (or nested with a template) type definitions, function definitions,
and variable initializations for which implicit instantiation is precluded,
perhaps through an explicit instantiations</p>
     <p class="note" role="note"><span>Note:</span> an explicit instantiation of a class template might not preclude
implicit instantiation of some of its member function definitions!</p>
   </ul>
   <p>TODO(all): Are there missing items from this list?</p>
   <h3 class="heading settled" data-level="2.2" id="restrictions"><span class="secno">2.2. </span><span class="content">Restrict Usage of Internal Linkage Names</span><a class="self-link" href="#restrictions"></a></h3>
   <p>We propose restricting the usage of an internal linkage name to TU-local
contexts as defined above. This is not influenced by reachability making it both
easy to check and easy to teach. It also matches the underlying use case of
internal linkage names: organizing the implementation details of a translation
unit. If they are implementation details, the should remain local.</p>
   <p>We specifically mean to restrict <em>any</em> use except for the specific exception to
ODR-use afforded for non-volatile const objects in <a href="http://eel.is/c++draft/basic.def.odr#12.2.1"> [basic.def.odr]p12.2.1</a>.</p>
   <p class="note" role="note"><span>Note:</span> This stronger than ODR-use restriction is necessary to avoid forcing the
creation of unique names even in contexts where no symbol reference is required.
One example is referencing an internal linkage name from the signature of
a function.</p>
   <p>This restriction is only enforced for importable translation units which include
module interface units, partition units, and header units. One specific
advantage of this solution to the problems discovered with linkage promotion is
that this rule can be consistently used across all importable units including
header units. Non-importable units are discussed below.</p>
   <h4 class="heading settled" data-level="2.2.1" id="ex-functions"><span class="secno">2.2.1. </span><span class="content">Examples with basic functions</span><a class="self-link" href="#ex-functions"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="10-27" line-start="10" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="10"></span><span class="line"><c- k>static</c-> <c- b>int</c-> <c- nf>f_internal</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="11"></span><span class="line">       <c- b>int</c-> <c- nf>f_module</c-><c- p>()</c->   <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="12"></span><span class="line"><c- k>export</c-> <c- b>int</c-> <c- nf>f_exported</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="13"></span><span class="line"></span><span class="line-no" data-line="14"></span><span class="line"><c- k>static</c-> <c- kr>inline</c-> <c- b>int</c-> <c- nf>f_internal_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="15"></span><span class="line">       <c- kr>inline</c-> <c- b>int</c-> <c- nf>f_module_inline</c-><c- p>()</c->   <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="16"></span><span class="line"><c- k>export</c-> <c- kr>inline</c-> <c- b>int</c-> <c- nf>f_exported_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="17"></span><span class="line"></span><span class="line-no" data-line="18"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f_internal_constexpr</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="19"></span><span class="line">       <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f_module_constexpr</c-><c- p>()</c->   <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="20"></span><span class="line"><c- k>export</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f_exported_constexpr</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="21"></span><span class="line"></span><span class="line-no" data-line="22"></span><span class="line"><c- k>static</c-> <c- n>consteval</c-> <c- b>int</c-> <c- nf>f_internal_consteval</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="23"></span><span class="line">       <c- n>consteval</c-> <c- b>int</c-> <c- nf>f_module_consteval</c-><c- p>()</c->   <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="24"></span><span class="line"><c- k>export</c-> <c- n>consteval</c-> <c- b>int</c-> <c- nf>f_exported_consteval</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="25"></span><span class="line"></span><span class="line-no" data-line="26"></span><span class="line">       <c- k>decltype</c-><c- p>(</c-><c- n>f</c-><c- p>())</c-> <c- n>f_module_decltype</c-><c- p>()</c->   <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="27"></span><span class="line"><c- k>export</c-> <c- k>decltype</c-><c- p>(</c-><c- n>f</c-><c- p>())</c-> <c- n>f_exported_decltype</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span></pre>
   <h4 class="heading settled" data-level="2.2.2" id="ex-templates"><span class="secno">2.2.2. </span><span class="content">Examples with basic templates</span><a class="self-link" href="#ex-templates"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="30-43" line-start="30" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="30"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>static</c-> <c- b>int</c-> <c- n>ft_internal</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="31"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->        <c- b>int</c-> <c- n>ft_module</c-><c- p>()</c->   <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="32"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>export</c-> <c- b>int</c-> <c- n>ft_exported</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="33"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->        <c- b>int</c-> <c- n>ftei_module</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK for int</c-></span><span class="line-no" data-line="34"></span><span class="line"><c- k>template</c->                     <c- b>int</c-> <c- n>ftei_module</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>();</c-></span><span class="line-no" data-line="35"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>export</c-> <c- b>int</c-> <c- n>ftei_exported</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK for int</c-></span><span class="line-no" data-line="36"></span><span class="line"><c- k>template</c->                     <c- b>int</c-> <c- n>ftei_exported</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>();</c-></span><span class="line-no" data-line="37"></span><span class="line"></span><span class="line-no" data-line="38"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- kr>inline</c-> <c- b>int</c-> <c- n>ftei_module_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="39"></span><span class="line"><c- k>template</c->                     <c- b>int</c-> <c- n>ftei_module_inline</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>();</c-></span><span class="line-no" data-line="40"></span><span class="line"></span><span class="line-no" data-line="41"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-></span><span class="line-no" data-line="42"></span><span class="line"><c- k>constexpr</c-> <c- b>int</c-> <c- n>ftei_module_constexpr</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="43"></span><span class="line"><c- k>template</c-> <c- b>int</c-> <c- n>ftei_module_constexpr</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>();</c-></span></pre>
   <h4 class="heading settled" data-level="2.2.3" id="ex-class-mem-fun"><span class="secno">2.2.3. </span><span class="content">Examples with class member functions</span><a class="self-link" href="#ex-class-mem-fun"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="46-64" line-start="46" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="46"></span><span class="line"><c- k>namespace</c-> <c- p>{</c-></span><span class="line-no" data-line="47"></span><span class="line"><c- k>struct</c-> <c- n>c_internal</c-> <c- p>{</c-></span><span class="line-no" data-line="48"></span><span class="line">  <c- b>int</c-> <c- n>mf</c-><c- p>();</c-></span><span class="line-no" data-line="49"></span><span class="line">  <c- b>int</c-> <c- nf>mf_internal_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="50"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="51"></span><span class="line"><c- b>int</c-> <c- n>c_internal</c-><c- o>::</c-><c- n>mf</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="52"></span><span class="line"><c- p>}</c-> <c- c1>// namespace</c-></span><span class="line-no" data-line="53"></span><span class="line"></span><span class="line-no" data-line="54"></span><span class="line"><c- k>struct</c-> <c- n>c_module</c-> <c- p>{</c-></span><span class="line-no" data-line="55"></span><span class="line">  <c- b>int</c-> <c- n>mf_module</c-><c- p>();</c-></span><span class="line-no" data-line="56"></span><span class="line">  <c- b>int</c-> <c- nf>mf_module_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="57"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="58"></span><span class="line"><c- b>int</c-> <c- n>c_module</c-><c- o>::</c-><c- n>mf_module</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="59"></span><span class="line"></span><span class="line-no" data-line="60"></span><span class="line"><c- k>export</c-> <c- k>struct</c-> <c- n>c_exported</c-> <c- p>{</c-></span><span class="line-no" data-line="61"></span><span class="line">  <c- b>int</c-> <c- n>mf_exported</c-><c- p>();</c-></span><span class="line-no" data-line="62"></span><span class="line">  <c- b>int</c-> <c- nf>mf_exported_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="63"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="64"></span><span class="line"><c- b>int</c-> <c- n>c_exported</c-><c- o>::</c-><c- n>mf_exported</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span></pre>
   <h4 class="heading settled" data-level="2.2.4" id="ex-class-templet-mem-fun"><span class="secno">2.2.4. </span><span class="content">Examples with class template member functions</span><a class="self-link" href="#ex-class-templet-mem-fun"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="67-96" line-start="67" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="67"></span><span class="line"><c- k>namespace</c-> <c- p>{</c-></span><span class="line-no" data-line="68"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>struct</c-> <c- n>ct_internal</c-> <c- p>{</c-></span><span class="line-no" data-line="69"></span><span class="line">  <c- b>int</c-> <c- n>ct_mf</c-><c- p>();</c-></span><span class="line-no" data-line="70"></span><span class="line">  <c- b>int</c-> <c- nf>ct_mf_internal_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="71"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="72"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-></span><span class="line-no" data-line="73"></span><span class="line"><c- b>int</c-> <c- n>ct_internal</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>ct_mf</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="74"></span><span class="line"><c- p>}</c-></span><span class="line-no" data-line="75"></span><span class="line"></span><span class="line-no" data-line="76"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>struct</c-> <c- n>ct_module</c-> <c- p>{</c-></span><span class="line-no" data-line="77"></span><span class="line">  <c- b>int</c-> <c- n>ct_mf_module</c-><c- p>();</c-></span><span class="line-no" data-line="78"></span><span class="line">  <c- b>int</c-> <c- nf>ct_mf_module_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="79"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="80"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-></span><span class="line-no" data-line="81"></span><span class="line"><c- b>int</c-> <c- n>ct_module</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>ct_mf_module</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="82"></span><span class="line"></span><span class="line-no" data-line="83"></span><span class="line"><c- k>export</c-> <c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>struct</c-> <c- n>ct_exported</c-> <c- p>{</c-></span><span class="line-no" data-line="84"></span><span class="line">  <c- b>int</c-> <c- n>ct_mf_exported</c-><c- p>();</c-></span><span class="line-no" data-line="85"></span><span class="line">  <c- b>int</c-> <c- nf>ct_mf_exported_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="86"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="87"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-></span><span class="line-no" data-line="88"></span><span class="line"><c- b>int</c-> <c- n>ct_exported</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>ct_mf_exported</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="89"></span><span class="line"></span><span class="line-no" data-line="90"></span><span class="line"><c- k>export</c-> <c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>struct</c-> <c- n>ctei_exported</c-> <c- p>{</c-></span><span class="line-no" data-line="91"></span><span class="line">  <c- b>int</c-> <c- n>ctei_mf_exported</c-><c- p>();</c-></span><span class="line-no" data-line="92"></span><span class="line">  <c- b>int</c-> <c- nf>ctei_mf_exported_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="93"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="94"></span><span class="line"><c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-></span><span class="line-no" data-line="95"></span><span class="line"><c- b>int</c-> <c- n>ctei_exported</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>ctei_mf_exported</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK for int</c-></span><span class="line-no" data-line="96"></span><span class="line"><c- k>template</c-> <c- k>struct</c-> <c- n>ctei_exported</c-><c- o>&lt;</c-><c- b>int</c-><c- o>></c-><c- p>;</c-></span></pre>
   <h4 class="heading settled" data-level="2.2.5" id="ex-variables"><span class="secno">2.2.5. </span><span class="content">Examples with variables</span><a class="self-link" href="#ex-variables"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="99-111" line-start="99" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="99"></span><span class="line"><c- k>static</c-> <c- b>int</c-> <c- n>v_internal</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="100"></span><span class="line">       <c- b>int</c-> <c- n>v_module</c->   <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="101"></span><span class="line"><c- k>export</c-> <c- b>int</c-> <c- n>v_exported</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="102"></span><span class="line"></span><span class="line-no" data-line="103"></span><span class="line"><c- k>static</c-> <c- kr>inline</c-> <c- b>int</c-> <c- n>v_internal_inline</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="104"></span><span class="line">       <c- kr>inline</c-> <c- b>int</c-> <c- n>v_module_inline</c->   <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="105"></span><span class="line"><c- k>export</c-> <c- kr>inline</c-> <c- b>int</c-> <c- n>v_exported_inline</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="106"></span><span class="line"></span><span class="line-no" data-line="107"></span><span class="line"><c- k>struct</c-> <c- n>c_sdm_module</c-> <c- p>{</c-></span><span class="line-no" data-line="108"></span><span class="line">  <c- k>static</c-> <c- b>int</c-> <c- n>sdm_module</c-><c- p>;</c-></span><span class="line-no" data-line="109"></span><span class="line">  <c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- n>sdm_module_constexpr</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="110"></span><span class="line"><c- p>};</c-></span><span class="line-no" data-line="111"></span><span class="line"><c- b>int</c-> <c- n>c_sdm_module</c-><c- o>::</c-><c- n>sdm_module</c-> <c- o>=</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// OK</c-></span></pre>
   <p class="note" role="note"><span>Note:</span> variable templates follow identical patterns as function templates.</p>
   <h4 class="heading settled" data-level="2.2.6" id="ex-lambdas"><span class="secno">2.2.6. </span><span class="content">Examples with lambdas</span><a class="self-link" href="#ex-lambdas"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="115-117" line-start="115" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="115"></span><span class="line"><c- c1>// Note that this function is not inline, but the lambda's call operator *is*</c-></span><span class="line-no" data-line="116"></span><span class="line"><c- c1>// inline and that type becomes exported as the return type.</c-></span><span class="line-no" data-line="117"></span><span class="line"><c- k>export</c-> <c- k>auto</c-> <c- nf>f_exported_lambda</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- p>[]</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>};</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span></pre>
   <h4 class="heading settled" data-level="2.2.7" id="ex-local-classes"><span class="secno">2.2.7. </span><span class="content">Examples with function local classes</span><a class="self-link" href="#ex-local-classes"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="120-160" line-start="120" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="120"></span><span class="line"><c- k>static</c-> <c- b>int</c-> <c- nf>flc_internal</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="121"></span><span class="line">  <c- k>struct</c-> <c- n>lc_internal</c-> <c- p>{</c-></span><span class="line-no" data-line="122"></span><span class="line">    <c- b>int</c-> <c- n>lc_mf_internal</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="123"></span><span class="line">  <c- p>};</c-></span><span class="line-no" data-line="124"></span><span class="line">  <c- k>return</c-> <c- n>lc_internal</c-><c- p>().</c-><c- n>lc_mf_internal</c-><c- p>();</c-></span><span class="line-no" data-line="125"></span><span class="line"><c- p>}</c-></span><span class="line-no" data-line="126"></span><span class="line"></span><span class="line-no" data-line="127"></span><span class="line"><c- b>int</c-> <c- nf>flc_module</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="128"></span><span class="line">  <c- k>struct</c-> <c- n>lc_module</c-> <c- p>{</c-></span><span class="line-no" data-line="129"></span><span class="line">    <c- b>int</c-> <c- n>lc_mf_module</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="130"></span><span class="line">  <c- p>};</c-></span><span class="line-no" data-line="131"></span><span class="line">  <c- k>return</c-> <c- n>lc_module</c-><c- p>().</c-><c- n>lc_mf_module</c-><c- p>();</c-></span><span class="line-no" data-line="132"></span><span class="line"><c- p>}</c-></span><span class="line-no" data-line="133"></span><span class="line"></span><span class="line-no" data-line="134"></span><span class="line"><c- k>export</c-> <c- b>int</c-> <c- nf>flc_exported</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="135"></span><span class="line">  <c- k>struct</c-> <c- n>lc_exported</c-> <c- p>{</c-></span><span class="line-no" data-line="136"></span><span class="line">    <c- b>int</c-> <c- n>lc_mf_exported</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="137"></span><span class="line">  <c- p>};</c-></span><span class="line-no" data-line="138"></span><span class="line">  <c- k>return</c-> <c- n>lc_exported</c-><c- p>().</c-><c- n>lc_mf_exported</c-><c- p>();</c-></span><span class="line-no" data-line="139"></span><span class="line"><c- p>}</c-></span><span class="line-no" data-line="140"></span><span class="line"></span><span class="line-no" data-line="141"></span><span class="line"><c- k>static</c-> <c- kr>inline</c-> <c- b>int</c-> <c- nf>flc_internal_inline</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="142"></span><span class="line">  <c- k>struct</c-> <c- n>lc_internal_inline</c-> <c- p>{</c-></span><span class="line-no" data-line="143"></span><span class="line">    <c- b>int</c-> <c- n>lc_mf_internal_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// OK</c-></span><span class="line-no" data-line="144"></span><span class="line">  <c- p>};</c-></span><span class="line-no" data-line="145"></span><span class="line">  <c- k>return</c-> <c- n>lc_internal_inline</c-><c- p>().</c-><c- n>lc_mf_internal_inline</c-><c- p>();</c-></span><span class="line-no" data-line="146"></span><span class="line"><c- p>}</c-></span><span class="line-no" data-line="147"></span><span class="line"></span><span class="line-no" data-line="148"></span><span class="line"><c- kr>inline</c-> <c- b>int</c-> <c- nf>flc_module_inline</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="149"></span><span class="line">  <c- k>struct</c-> <c- n>lc_module_inline</c-> <c- p>{</c-></span><span class="line-no" data-line="150"></span><span class="line">    <c- b>int</c-> <c- n>lc_mf_module_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="151"></span><span class="line">  <c- p>};</c-></span><span class="line-no" data-line="152"></span><span class="line">  <c- k>return</c-> <c- n>lc_module_inline</c-><c- p>().</c-><c- n>lc_mf_module_inline</c-><c- p>();</c-></span><span class="line-no" data-line="153"></span><span class="line"><c- p>}</c-></span><span class="line-no" data-line="154"></span><span class="line"></span><span class="line-no" data-line="155"></span><span class="line"><c- k>export</c-> <c- kr>inline</c-> <c- b>int</c-> <c- nf>flc_exported_inline</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="156"></span><span class="line">  <c- k>struct</c-> <c- n>lc_exported_inline</c-> <c- p>{</c-></span><span class="line-no" data-line="157"></span><span class="line">    <c- b>int</c-> <c- n>lc_mf_exported_inline</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- p>}</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="158"></span><span class="line">  <c- p>};</c-></span><span class="line-no" data-line="159"></span><span class="line">  <c- k>return</c-> <c- n>lc_exported_inline</c-><c- p>().</c-><c- n>lc_mf_exported_inline</c-><c- p>();</c-></span><span class="line-no" data-line="160"></span><span class="line"><c- p>}</c-></span></pre>
   <h4 class="heading settled" data-level="2.2.8" id="ex-template-surprises"><span class="secno">2.2.8. </span><span class="content">Examples where template instantiation has interesting implications</span><a class="self-link" href="#ex-template-surprises"></a></h4>
<pre class="include-code highlight line-numbered" data-code-show="5-7" line-start="1" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="1"></span><span class="line"><c- k>export</c-> <c- n>module</c-> <c- n>M</c-><c- p>;</c-></span><span class="line-no" data-line="2"></span><span class="line"></span><span class="line-no" data-line="3"></span><span class="line"><c- k>static</c-> <c- k>constexpr</c-> <c- b>int</c-> <c- nf>f</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- mi>0</c-><c- p>;</c-> <c- p>}</c-></span></pre>
   <p>...</p>
<pre class="include-code highlight line-numbered" data-code-show="162-187" line-start="162" path="constrained_internal_restriction_examples.cpp"><span class="line-no" data-line="162"></span><span class="line"><c- c1>// An anonymous type with an internal name.</c-></span><span class="line-no" data-line="163"></span><span class="line"><c- k>namespace</c-> <c- p>{</c-> <c- k>struct</c-> <c- n>t_internal</c-> <c- p>{};</c-> <c- p>}</c-></span><span class="line-no" data-line="164"></span><span class="line"></span><span class="line-no" data-line="165"></span><span class="line"><c- k>export</c-> <c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- b>int</c-> <c- n>f_exported_instantiated</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="166"></span><span class="line">  <c- k>return</c-> <c- n>f</c-><c- p>();</c-> <c- c1>// ERROR</c-></span><span class="line-no" data-line="167"></span><span class="line"><c- p>}</c-></span><span class="line-no" data-line="168"></span><span class="line"></span><span class="line-no" data-line="169"></span><span class="line"><c- c1>// This instantiaties the exported template locally, but the important thing is</c-></span><span class="line-no" data-line="170"></span><span class="line"><c- c1>// that to allow this instantiation to succeed, even though it is local, would</c-></span><span class="line-no" data-line="171"></span><span class="line"><c- c1>// require either instatiating it with internal linkage, which makes the linkage</c-></span><span class="line-no" data-line="172"></span><span class="line"><c- c1>// contingent upon the contents of the function template definition. If we try</c-></span><span class="line-no" data-line="173"></span><span class="line"><c- c1>// to use the template definition linkage, we must compute a unique name for</c-></span><span class="line-no" data-line="174"></span><span class="line"><c- c1>// `t_internal` which would require linkage promotion.</c-></span><span class="line-no" data-line="175"></span><span class="line"><c- k>static</c-> <c- b>int</c-> <c- n>sink</c-> <c- o>=</c-> <c- n>f_exported_instantiated</c-><c- o>&lt;</c-><c- n>t_internal</c-><c- o>></c-><c- p>();</c-></span><span class="line-no" data-line="176"></span><span class="line"></span><span class="line-no" data-line="177"></span><span class="line"><c- c1>// If it is possible to detect the linkage of an entity, then this template</c-></span><span class="line-no" data-line="178"></span><span class="line"><c- c1>// breaks one of the previous options by choosing whether the instantiated</c-></span><span class="line-no" data-line="179"></span><span class="line"><c- c1>// definition contains a usage of an internal name by observing the linkage of</c-></span><span class="line-no" data-line="180"></span><span class="line"><c- c1>// the declaration. This suggests computing the linkage based on the</c-></span><span class="line-no" data-line="181"></span><span class="line"><c- c1>// instantiated definition is a bad strategy.</c-></span><span class="line-no" data-line="182"></span><span class="line"><c- k>export</c-> <c- k>template</c-> <c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- b>int</c-> <c- n>f_exported_circular_linkage</c-><c- p>()</c-> <c- p>{</c-></span><span class="line-no" data-line="183"></span><span class="line">  <c- k>if</c-> <c- k>constexpr</c-> <c- p>(</c-><c- d>/* sfinae or reflection test for external linkage */</c-><c- p>)</c-> <c- p>{</c-></span><span class="line-no" data-line="184"></span><span class="line">    <c- k>return</c-> <c- n>f</c-><c- p>();</c-></span><span class="line-no" data-line="185"></span><span class="line">  <c- p>}</c-></span><span class="line-no" data-line="186"></span><span class="line">  <c- k>return</c-> <c- mi>0</c-><c- p>;</c-></span><span class="line-no" data-line="187"></span><span class="line"><c- p>}</c-></span></pre>
   <h3 class="heading settled" data-level="2.3" id="diagnostics"><span class="secno">2.3. </span><span class="content">Enforcing and Diagnosing the Restriction</span><a class="self-link" href="#diagnostics"></a></h3>
   <p>We believe this restriction can be checked and enforced immediately for all
non-templated contexts. For templated contexts, the restriction can be checked
at the end of the translation unit in the vast majority of cases. In the case of
a templated context where there are explicit instantiations with the TU (or
potentially some other obscure corner cases), the error will need to be deferred
to instantiation. However, in those cases <em>all</em> instantiations outside of the TU
will require this diagnostic, allowing for extremely simple implementation
strategies. Implementations could simply emit deleted definitions rather than
the problematic ones, although a higher quality implementation would be likely
(including information to produce a good diagnostics).</p>
   <h3 class="heading settled" data-level="2.4" id="p1395"><span class="secno">2.4. </span><span class="content">Addressing <a data-link-type="biblio" href="#biblio-p1395r0">[p1395r0]</a> Issues With Linkage Promotion and Partitions</span><a class="self-link" href="#p1395"></a></h3>
   <p>The problems raised in <a data-link-type="biblio" href="#biblio-p1395r0">[p1395r0]</a> fundamentally arise from attempting to do linkage
promotion and organizing the code of a module within partitions. Names with
internal linkage or no linkage may <em>require</em> that property, and promoting them
conflicts with this. With this proposal, we preclude linkage promotion, which
precludes the issues in this paper.</p>
   <h3 class="heading settled" data-level="2.5" id="p1347"><span class="secno">2.5. </span><span class="content">Addressing <a data-link-type="biblio" href="#biblio-p1347r1">[p1347r1]</a> Issues With Transitively Reachable Internal Names</span><a class="self-link" href="#p1347"></a></h3>
   <p>The problems raised in <a data-link-type="biblio" href="#biblio-p1347r1">[p1347r1]</a> are described in terms of ADL-enabled
reaching of internal linkage names from exported interfaces or exported inline
function definitions. While the paper’s examples and presentation focused on
a particular mechanism of reaching this problematic behavior that appears to be
precluded by the current wording, that only precludes the discussed mechanism
for arriving at the problems -- the fundamental problem remains.</p>
   <p>Unfortunately, this means the analysis of the problem space is less complete
than we would like. It requires constructing much more complex examples to
explore the space, and we are still trying to complete this exploration.
However, so far every example we have thought of is addressed. At worst, we
expect to potentially still need a smaller and more narrow fix for any aspects
of this issue that remain even in the absence of linkage promotion. All of the
primary paths we have examined are addressed by this change.</p>
   <h3 class="heading settled" data-level="2.6" id="non-importable"><span class="secno">2.6. </span><span class="content">Non-importable Translation Units</span><a class="self-link" href="#non-importable"></a></h3>
   <p>We believe these rules are desirable even in non-importable translation units.
Within those contexts, ODR-uses of internal linkage names is a common source of
ODR violations in the wild. As a consequence, the rule we suggest for importable
translation units can and should be consistently taught to programmers for C++
as a whole.</p>
   <p>We propose to deprecate all of the usages that are restricted above for
importable units within non-importable units to put users on notice that C++ is
moving away from supporting these patterns as part of the move towards modules.</p>
   <h3 class="heading settled" data-level="2.7" id="inline"><span class="secno">2.7. </span><span class="content">Inline</span><a class="self-link" href="#inline"></a></h3>
   <p>We also suggest refining the <em>non-normative</em> meaning of the term <i>inline</i> for functions and variables within interface units. Currently, this is primarily
associated with a hint to the optimizer to inline more aggressively.
Increasingly, these hints are insufficient for peak performance and are replaced
with profile guided inlining or stronger and non-semantic vendor specific hints.
The hinting use case remains important, but we believe it would be better served
by a separate construct that does not carry additional semantic impact and can
be better tailored to the purpose of this hint.</p>
   <p>Within interface units of modules, we suggest converging on an interperation
more firmly rooted in the semantics: an inline entity (function definition or
variable initialization) is semantically incorporated into (or <em>inlined</em> into)
the interface of a module. The inlined code’s behavior is now <em>part of</em> the
interface and not an implementation detail. Changing its behavior <em>changes the
interface</em>. The module interface now includes the specific behavior of this
inlined code. Using this part of the interface may enable optimizations such as
inlining as well as other optimizations.</p>
   <p>We do not think this is a meaningfully different interpretation from the
reality of inline as it is used and implemented today. It does enable
optimization techniques (but not the fundamental optimizations). However, it
shifts to a <em>semantic</em> basis.</p>
   <p>We see this as a first (small) step on a longer path to decouple the semantic
decision of inlining an implementation into an interface from a non-semantic
decision of hinting to the optimizer about the utility and importance of
a particular (as-if) transformation. The remaining path toward fully arriving at
this more principled end state is outlined as future work.</p>
   <h3 class="heading settled" data-level="2.8" id="future"><span class="secno">2.8. </span><span class="content">Future Work</span><a class="self-link" href="#future"></a></h3>
   <h4 class="heading settled" data-level="2.8.1" id="hint-annotation"><span class="secno">2.8.1. </span><span class="content">Introduce a non-semantic inlining hint annotation</span><a class="self-link" href="#hint-annotation"></a></h4>
   <p>Currently, the <code class="highlight"><c- kr>inline</c-></code> specifier is used in contexts that shouldn’t be
restricted in their usage of internal names because it also provides a potential
hint to optimizers. This hint is often weaker than desired due to its pervasive
(semantic) usage. Vendors have experimented with an explicitly non-semantic hint
with the ability to also be a stronger hint. We should standardize such a hint,
likely as an attribute.</p>
   <p>This should also address the other problem with the <code class="highlight"><c- kr>inline</c-></code> specifier being
relied on for hinting to the optimizer -- often times that hint is needed on the <em>call</em> rather than the <em>declaration</em>, making a specifier completely
inapplicable.</p>
   <h4 class="heading settled" data-level="2.8.2" id="deprecate-internal-inline"><span class="secno">2.8.2. </span><span class="content">Deprecate inline definitions of internal names</span><a class="self-link" href="#deprecate-internal-inline"></a></h4>
   <p>The direction of this change also suggests deprecating the usage of <code class="highlight"><c- kr>inline</c-></code> when declaring names with internal or no linkage. We are happy to provide
a proposal to this effect if there is interest in EWG, but it should be done
much more slowly and cautiously. While this is a bug-prone pattern due to the
potential for ODR violations, it remains in use and we would need to carefully
evaluate the impact on existing code.</p>
   <h4 class="heading settled" data-level="2.8.3" id="deprecate-implementation-inline"><span class="secno">2.8.3. </span><span class="content">Deprecate declaring new inline entities within implementation units</span><a class="self-link" href="#deprecate-implementation-inline"></a></h4>
   <p>The direction of this change also suggests deprecating the usage of <code class="highlight"><c- kr>inline</c-></code> when declaring new names within an implementation unit. We are happy to provide
a proposal to this effect if there is interest in EWG, but it should be done
much more slowly and cautiously. This is not an especially bug-prone pattern, but
merely surprising and out of step with the semantic model. As a consequence, we
again would only suggest this with an appropriately long time horizon and
careful communication to users to understand and minimize negative impact.</p>
   <h2 class="heading settled" data-level="3" id="alternatives"><span class="secno">3. </span><span class="content">Alternatives</span><a class="self-link" href="#alternatives"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="require-linkage-alt"><span class="secno">3.1. </span><span class="content">Require Names in Importable Units Have Non-Internal Linkage</span><a class="self-link" href="#require-linkage-alt"></a></h3>
   <p>Rather than restricting the usage of names with internal or no linkage within
importable translation units, we could simply disallow names with internal or no
linkage to be declared at all within these units.</p>
   <p>However, definitions that are TU-local are an important facility introduced by
modules, and users expect to be able to leverage internal functions to factor
and manage code for such definitions. We should not create barriers to moving
code into modules if we can avoid it and this proposal does not seem
significantly more costly to achieve.</p>
   <h3 class="heading settled" data-level="3.2" id="p1395-alt"><span class="secno">3.2. </span><span class="content">Alternative Proposed Solutions in <a data-link-type="biblio" href="#biblio-p1395r0">[p1395r0]</a></span><a class="self-link" href="#p1395-alt"></a></h3>
   <p>There are two alternatives suggested in <a data-link-type="biblio" href="#biblio-p1395r0">[p1395r0]</a>:</p>
   <blockquote>
    <ol>
     <li data-md>
      <p>Unrestricted linkage promotion at the expense of restricting refactoring of
 module partition source that contains such promotion.</p>
     <li data-md>
      <p>Unrestricted module partition refactoring at the expense of linkage
 promotion collisions.</p>
    </ol>
   </blockquote>
   <p>Both of these are phrased as trade-offs and have significant disadvantages
described in the paper.</p>
   <h3 class="heading settled" data-level="3.3" id="p1347-alt"><span class="secno">3.3. </span><span class="content">Alternative Proposed Solutions in <a data-link-type="biblio" href="#biblio-p1347r1">[p1347r1]</a></span><a class="self-link" href="#p1347-alt"></a></h3>
   <p>One proposed solution is to simple make the internal names not visible outside
of the translation unit. This has significant downsides due to causing the same
code to be accepted both inside and outside of that translation unit but with
different overloads selected. This at least seems prone to ODR violations as
well as being deeply surprising.</p>
   <p>Another proposed solution is to make internal names visible, but when selected
by overload resolution the result be ill-formed. This avoids the risk of
surprising differences at the cost of increased complexity.</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>
   <p>TODO(herring, sidwell): Provide wording.</p>
   <h2 class="heading settled" data-level="5" id="acknowledgements"><span class="secno">5. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
   <p>These ideas were refined with help from members of EWG and other discussions
including at least Mathias Stearn, Davis Herring, Michael Spencer, and Daveed
Vandevoorde. For anyone I have missed, apologies, and don’t hesitate to suggest
an addition.</p>
   <h2 class="heading settled" data-level="6" id="changelog"><span class="secno">6. </span><span class="content">Revision History</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="6.1" id="r0"><span class="secno">6.1. </span><span class="content">Revision 0</span><a class="self-link" href="#r0"></a></h3>
   <p>Initially published during Kona 2019.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-p1347r1">[P1347R1]
   <dd>Nathan Sidwell, Davis Herring. <a href="https://wg21.link/p1347r1">Modules: ADL &amp; Internal Linkage</a>. 17 January 2019. URL: <a href="https://wg21.link/p1347r1">https://wg21.link/p1347r1</a>
   <dt id="biblio-p1395r0">[P1395R0]
   <dd>Nathan Sidwell. <a href="https://wg21.link/p1395r0">Modules: Partitions Are Not a Panacea</a>. 18 January 2019. URL: <a href="https://wg21.link/p1395r0">https://wg21.link/p1395r0</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> Local class member functions appear to have no linkage even when they
occur within inline functions with external (or module) linkage. This seems
like a bug generally, but certainly such member functions are <strong>not</strong> TU-local.<a href="#issue-8bfedeb4"> ↵ </a></div>
  </div>
