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  <title>P1208R0: Adopt source_location from Library Fundamentals V3 for C++20</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

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

	body {
		counter-reset: example figure issue;

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	p {
		margin: 1em 0;
	}

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

  /* Do something nice. */

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

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

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

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

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

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

	img {
		border-style: none;
	}

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

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

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

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

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

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

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

	blockquote {
		border-color: silver;
	}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	table.data  thead,
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1208R0<br>Adopt source_location from Library Fundamentals V3 for C++20</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-10-07">2018-10-07</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://rawgit.com/cor3ntin/CPPProposals/master/merge_source_location/P1208.html">https://rawgit.com/cor3ntin/CPPProposals/master/merge_source_location/P1208.html</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:corentin.jabot@gmail.com">Corentin Jabot</a>
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:rwdougla@gmail.com">Robert Douglas</a>
     <dt>Audience:
     <dd>LEWG, LWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This paper proposes that `source_location` (from Library Fundamentals V3) be adopted into the C++20 standard</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="#proposal"><span class="secno">1</span> <span class="content">Proposal</span></a>
    <li>
     <a href="#a-few-design-changes"><span class="secno">2</span> <span class="content">A few design changes</span></a>
     <ol class="toc">
      <li><a href="#enforcing-a-size-for-source_location-objects"><span class="secno">2.1</span> <span class="content">Enforcing a size for <code class="highlight"><c- n>source_location</c-></code> objects ?</span></a>
      <li><a href="#removing-current-altogether"><span class="secno">2.2</span> <span class="content">Removing <code class="highlight"><c- n>current</c-><c- p>()</c-></code> altogether ?</span></a>
     </ol>
    <li><a href="#proposed-wording"><span class="secno">3</span> <span class="content">Proposed Wording</span></a>
    <li><a href="#acknowledgments"><span class="secno">4</span> <span class="content">Acknowledgments</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="proposal"><span class="secno">1. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>This paper proposes that source_location from Library Fundamentals V3 <a data-link-type="biblio" href="#biblio-n4758">[N4758]</a> be adopted into the C++20 standard.</p>
   <p><code class="highlight"><c- n>source_location</c-></code> has been successfully implemented without difficulties in GCC, and Clang implementers almost completed their implementation of it.
As described by the original paper, it’s a necessary feature to implement logging without requiring macros, one that cannot be portably implemented
by non-standard libraries because it requires compiler support.</p>
   <p><code class="highlight"><c- n>source_location</c-></code> has been very favorably received by both EWG and LEWG <a data-link-type="biblio" href="#biblio-n4129">[n4129]</a>, <a data-link-type="biblio" href="#biblio-n4417">[n4417]</a> and has been part of library
fundamentals v2 <a data-link-type="biblio" href="#biblio-n4562">[n4562]</a> and v3 <a data-link-type="biblio" href="#biblio-n4758">[N4758]</a> since 2015 without changes to exposed interface.</p>
   <p class="note" role="note"><span>Note:</span> A proposal <a data-link-type="biblio" href="#biblio-p0555r0">[P0555R0]</a> in 2017 to make use of <code class="highlight"><c- n>string_view</c-></code> in <code class="highlight"><c- n>source_location</c-></code> has been withdrawn by their authors once <code class="highlight"><c- n>string_view</c-></code> gained a constexpr constructor, allowing the use of <code class="highlight"><c- n>source_location</c-></code> in constexpr contexts.</p>
   <h2 class="heading settled" data-level="2" id="a-few-design-changes"><span class="secno">2. </span><span class="content">A few design changes</span><a class="self-link" href="#a-few-design-changes"></a></h2>
   <p>During an early review of this papers, some interest in a few design changes arose:</p>
   <h3 class="heading settled" data-level="2.1" id="enforcing-a-size-for-source_location-objects"><span class="secno">2.1. </span><span class="content">Enforcing a size for <code class="highlight"><c- n>source_location</c-></code> objects ?</span><a class="self-link" href="#enforcing-a-size-for-source_location-objects"></a></h3>
   <p><code class="highlight"><c- n>source_location</c-></code> as currently implemented by Clang and GCC is a structure encapsulating all its members (file, function, line, column),
and so its size is roughly <code class="highlight"><c- mi>3</c-> <c- o>*</c-> <c- k>sizeof</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-><c- p>)</c-></code>.</p>
   <p>A note states:</p>
   <blockquote>
    <p>[ Note: The intent of source_location is to have a small size and efficient copying. — end note ]</p>
   </blockquote>
   <p>However, there seems to be some interest for having <code class="highlight"><c- n>source_location</c-></code> be guaranteed <code class="highlight"><c- k>sizeof</c-><c- p>(</c-><c- b>void</c-><c- o>*</c-><c- p>)</c-></code>.
Notably, reviewers wonder if <code class="highlight"><c- n>source_location</c-></code> should be embedded in a yet to be proposed <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>error</c-></code>.
Herb Sutter and Niall Douglas pointed out that <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>error</c-></code> is meant to handle recoverable errors,
rather than to communicate an error to developers (see Herb Sutter’s talk on error handling CppCon 2018).
And while we agree strongly with this sentiment, it’s worth noting that a subset of the C committee
is interested in having source information in whatever deterministic exception handling they are working towards;</p>
   <p>While, as pointed out by Niall, it’s unlikely the two languages will have directly interoperable mechanisms,
having a guaranteed size for <code class="highlight"><c- n>source_location</c-></code> might make such interoperability easier should a need for it arise.</p>
   <p>Barring that, <code class="highlight"><c- n>source_location</c-></code> is an error reporting tool targeted at human developers,
and the cost of any such error reporting system is many orders of magnitude larger than copying
3 pointers around. We do not think there is a strong incentive to guarantee the size of that type
or restricting the way it is implemented.</p>
   <p>For completeness, such modification could be achieved by adding one level of indirection:</p>
<pre class="highlight"><c- k>struct</c-> <c- n>__source_location_data</c-> <c- p>{</c->
  <c- d>/* implementation defined */</c->
<c- p>};</c->
<c- k>struct</c-> <c- n>source_location</c-> <c- p>{</c->
<c- k>private</c-><c- o>:</c->
    <c- k>const</c-> <c- n>__source_location_data</c-><c- o>*</c-> <c- n>__data</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>Alternatively, source_location could return a const reference:</p>
<pre class="highlight"><c- k>struct</c-> <c- n>source_location</c-> <c- p>{</c->
    <c- k>constexpr</c-> <c- k>const</c-> <c- n>source_location</c-> <c- o>&amp;</c-> <c- n>current</c-><c- p>()</c-> <c- k>noexcept</c-><c- p>;</c->
<c- p>};</c->
</pre>
   <p>The authors strongly prefer the first solution as we think retaining value semantics is important.
It is also important to note that, while not implemented, the first solution has been considered and is deemed
realistically implementable by both GCC and Clang developers (Jonathan Wakely, Eric Fiselier).</p>
   <h3 class="heading settled" data-level="2.2" id="removing-current-altogether"><span class="secno">2.2. </span><span class="content">Removing <code class="highlight"><c- n>current</c-><c- p>()</c-></code> altogether ?</span><a class="self-link" href="#removing-current-altogether"></a></h3>
   <p>If LEWG elects to keep value semantics, the authors would like to propose that the <code class="highlight"><c- n>current</c-></code> function is removed,
since, in its current form, <code class="highlight"><c- n>source_location</c-></code> has a default constructor that has no meaningful use.
Besides being harder to misuse, using the constructor of the type to acquire its value is also significantly less verbose.</p>
<pre class="highlight"><c- b>void</c-> <c- nf>log</c-><c- p>(</c-><c- k>auto</c-> <c- o>&amp;&amp;</c-> <c- n>message</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>source_location</c-> <c- n>sc</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>source_location</c-><c- o>::</c-><c- n>current</c-><c- p>());</c->
<c- c1>//or</c->
<c- b>void</c-> <c- nf>log</c-><c- p>(</c-><c- k>auto</c-> <c- o>&amp;&amp;</c-> <c- n>message</c-><c- p>,</c-> <c- n>std</c-><c- o>::</c-><c- n>source_location</c-> <c- n>sc</c-> <c- o>=</c-> <c- p>{});</c->
</pre>
   <h2 class="heading settled" data-level="3" id="proposed-wording"><span class="secno">3. </span><span class="content">Proposed Wording</span><a class="self-link" href="#proposed-wording"></a></h2>
   <p>
    Create a new header 
    <source_location> with the synopsis taken from paragraph ¶15.11 of <a data-link-type="biblio" href="#biblio-n4758">[N4758]</a>.</source_location>
   </p>
   <p>
    Add the <code class="highlight"><c- n>source_location</c-></code> class to the C++ Working paper using the content from <a data-link-type="biblio" href="#biblio-n4758">[N4758]</a> in the 
    <source_location> header.</source_location>
   </p>
   <p>Move this content from the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>fundamentals_v3</c-></code> namespace to the <code class="highlight"><c- n>std</c-></code> namespace.</p>
   <p>
    In section 15.5.1.3 <code class="highlight"><c- p>[</c-><c- n>library</c-><c- p>.</c-><c- n>requirements</c-><c- p>.</c-><c- n>organization</c-><c- p>.</c-><c- n>compliance</c-><c- p>]</c-></code>, add the 
    <source_location> header to the table 21 (<code class="highlight"><c- p>[</c-><c- nl>tab</c-><c- p>:</c-><c- n>cpp</c-><c- p>.</c-><c- n>headers</c-><c- p>.</c-><c- n>freestanding</c-><c- p>]</c-></code>).</source_location>
   </p>
   <p>NB: The new <code class="highlight"><c- o>&lt;</c-><c- n>source_location</c-><c- o>></c-></code> header would be best described in a new <code class="highlight"><c- n>Reflection</c-></code> library (as it is in <a data-link-type="biblio" href="#biblio-n4758">[N4758]</a>).
At the time of the publication of this category does not exist, and so this paper defers to LEWG and LWG to find the best
place to describe this header in the wording or to introduce a new Refection category in the wording.</p>
   <p>Similarly to the original <a data-link-type="biblio" href="#biblio-n4417">[n4417]</a> wording, we recommend the feature test macro <code class="highlight"><c- n>__cpp_lib_source_location</c-></code> for this feature.</p>
   <h2 class="heading settled" data-level="4" id="acknowledgments"><span class="secno">4. </span><span class="content">Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
   <p>The authors would like to thanks the people who reviewed early version of this proposal, notably Peter Dimov, Jonathan Wakely Arthur O’Dwyer
and Geoffrey Romer.
We would also like to thank Herb Sutter and Niall Douglas for their insightful remarks on std::error and errors handling in general.</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-n4129">[N4129]
   <dd>Robert Douglas. <a href="https://wg21.link/n4129">Source-Code Information Capture</a>. 10 October 2014. URL: <a href="https://wg21.link/n4129">https://wg21.link/n4129</a>
   <dt id="biblio-n4417">[N4417]
   <dd>Robert Douglas. <a href="https://wg21.link/n4417">Source-Code Information Capture</a>. 8 April 2015. URL: <a href="https://wg21.link/n4417">https://wg21.link/n4417</a>
   <dt id="biblio-n4562">[N4562]
   <dd>Geoffrey Romer. <a href="https://wg21.link/n4562">Working Draft, C++ Extensions for Library Fundamentals, Version 2</a>. 5 November 2015. URL: <a href="https://wg21.link/n4562">https://wg21.link/n4562</a>
   <dt id="biblio-n4758">[N4758]
   <dd>Thomas Köppe. <a href="https://wg21.link/n4758">Working Draft, C++ Extensions for Library Fundamentals, Version 3</a>. 25 June 2018. URL: <a href="https://wg21.link/n4758">https://wg21.link/n4758</a>
   <dt id="biblio-p0555r0">[P0555R0]
   <dd>Axel Naumann. <a href="https://wg21.link/p0555r0">string_view for source_location</a>. 20170130. URL: <a href="https://wg21.link/p0555r0">https://wg21.link/p0555r0</a>
  </dl>