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 <head>
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  <title>P1181R0: Proposing &lt;code>unless&lt;/code></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,
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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">P1181R0<br>Proposing <code class="highlight"><c- n>unless</c-></code></h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Proposal, <time class="dt-updated" datetime="2018-09-08">2018-09-08</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wg21.link/P1181R0">https://wg21.link/P1181R0</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:morwenn29@hotmail.fr">Morwenn</a>
     <dt>Audience:
     <dd>EWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/Morwenn/if-not-proposal/blob/master/P1181.bs">github.com/Morwenn/if-not-proposal/blob/master/P1181.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>We propose <code class="highlight"><c- k>if</c-> <c- n>not</c-> <c- p>(</c-><c- n>condition</c-><c- p>)</c-></code> and a few consistent additions to the C++ language, mostly as a convenience feature, but also because it might help to avoid some subtle classes of problems. The goal is rather simple: applying a negation to the condition passed to an <code class="highlight"><c- k>if</c-> <c- n>not</c-></code> statement <em>after</em> having applied the contextual conversion of the condition to <code class="highlight"><c- b>bool</c-></code>.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#changelog"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#changelog-r0"><span class="secno">1.1</span> <span class="content">Revision 0</span></a>
     </ol>
    <li>
     <a href="#motivation"><span class="secno">2</span> <span class="content">Motivation</span></a>
     <ol class="toc">
      <li><a href="#declaration"><span class="secno">2.1</span> <span class="content">Declaring a variable with <code class="highlight"><c- k>if</c-></code></span></a>
      <li><a href="#operator-not"><span class="secno">2.2</span> <span class="content">Avoiding tricky overloads of <code class="highlight"><c- k>operator</c-><c- o>!</c-></code></span></a>
     </ol>
    <li>
     <a href="#extensions"><span class="secno">3</span> <span class="content">Clarifications &amp; Extensions</span></a>
     <ol class="toc">
      <li><a href="#while-not"><span class="secno">3.1</span> <span class="content"><code class="highlight"><c- k>while</c-> <c- n>not</c-></code></span></a>
      <li><a href="#if-constexpr"><span class="secno">3.2</span> <span class="content"><code class="highlight"><c- k>if</c-> <c- k>constexpr</c-></code></span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#changelog"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="changelog-r0"><span class="secno">1.1. </span><span class="content">Revision 0</span><a class="self-link" href="#changelog-r0"></a></h3>
   <p>Initial release.</p>
   <h2 class="heading settled" data-level="2" id="motivation"><span class="secno">2. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Introducing <code class="highlight"><c- k>if</c-> <c- n>not</c-></code> is mainly a matter of convenience: for most purposes <code class="highlight"><c- k>if</c-> <c- p>(</c-><c- n>not</c-> <c- n>condition</c-><c- p>)</c-></code> is enough to not take a branch when <code class="highlight"><c- n>condition</c-></code> evaluates to <code class="highlight">true</code>. The main idea is to lift the negation into the boolean domain since that is the domain conditional statements typically deal with.</p>
   <p>Moreover there are a few corner cases where it is not possible to negate the <code class="highlight"><c- k>if</c-></code> condition naively without a workaround, which this proposal intends to make easier to deal with. Consider the examples in the following subsections.</p>
   <h3 class="heading settled" data-level="2.1" id="declaration"><span class="secno">2.1. </span><span class="content">Declaring a variable with <code class="highlight"><c- k>if</c-></code></span><a class="self-link" href="#declaration"></a></h3>
   <p>When <code class="highlight"><c- k>if</c-></code> is used to declare a variable, there was no easy and clean way to negate the condition before C++17:</p>
<pre class="language-c++ highlight"><c- c1>// C++14, best effort, might be confusing</c->
<c- k>if</c-> <c- p>(</c-><c- k>auto</c-> <c- n>opt</c-> <c- o>=</c-> <c- n>some_optional</c-><c- p>());</c-> <c- k>else</c-> <c- p>{</c->
    <c- c1>// Do stuff</c->
<c- p>}</c->

<c- c1>// C++17, separate declaration and condition</c->
<c- k>if</c-> <c- p>(</c-><c- k>auto</c-> <c- n>opt</c-> <c- o>=</c-> <c- n>some_optional</c-><c- p>()</c-> <c- p>;</c-> <c- n>not</c-> <c- n>opt</c-><c- p>)</c-> <c- p>{</c->
    <c- c1>// Do stuff</c->
<c- p>}</c-> 
</pre>
   <p>With the proposed extensions, the condition above could be written as follows:</p>
<pre class="language-c++ highlight"><c- k>if</c-> <c- nf>not</c-> <c- p>(</c-><c- k>auto</c-> <c- n>opt</c-> <c- o>=</c-> <c- n>some_optional</c-><c- p>())</c-> <c- p>{</c->
    <c- c1>// Do stuff</c->
<c- p>}</c->
</pre>
   <p>We believe that the notation itself is clear enough and should be rather obvious to everyone reading the conditional statement.</p>
   <h3 class="heading settled" data-level="2.2" id="operator-not"><span class="secno">2.2. </span><span class="content">Avoiding tricky overloads of <code class="highlight"><c- k>operator</c-><c- o>!</c-></code></span><a class="self-link" href="#operator-not"></a></h3>
   <p>If <code class="highlight"><c- k>operator</c-><c- o>!</c-></code> is overloaded, <code class="highlight"><c- k>if</c-> <c- p>(</c-><c- n>not</c-> <c- n>condition</c-><c- p>)</c-></code> is not guaranteed to have a result contextually convertible to <code class="highlight"><c- b>bool</c-></code>, which might result in tricky corner cases when writing generic algorithms. Consider the following naive implementation of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>find_if_not</c-></code>:</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>InputIterator</c-><c- p>,</c-> <c- kr>typename</c-> <c- n>Predicate</c-><c- o>></c->
<c- n>InputIterator</c-> <c- n>find_if_not</c-><c- p>(</c-><c- n>InputIterator</c-> <c- n>first</c-><c- p>,</c-> <c- n>InputIterator</c-> <c- n>last</c-><c- p>,</c-> <c- n>Predicate</c-> <c- n>pred</c-><c- p>)</c->
<c- p>{</c->
    <c- k>for</c-> <c- p>(;</c-> <c- n>first</c-> <c- o>!=</c-> <c- n>last</c-><c- p>;</c-> <c- o>++</c-><c- n>first</c-><c- p>)</c-> <c- p>{</c->
        <c- c1>// Fails to compile when operator! doesn’t return a</c->
        <c- c1>// result contextually convertible to bool</c->
        <c- k>if</c-> <c- p>(</c-><c- o>!</c-><c- n>pred</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>))</c-> <c- p>{</c->
            <c- k>break</c-><c- p>;</c->
        <c- p>}</c->
    <c- p>}</c->
    <c- k>return</c-> <c- n>first</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>If <code class="highlight"><c- k>operator</c-><c- o>!</c-></code> is overloaded in such a way that it doesn’t return a value contextually convertibe to <code class="highlight"><c- b>bool</c-></code>, the algorithm above will fail to compile, despite <code class="highlight"><c- n>pred</c-></code> not violating the <em>Predicate</em> requirements (the issue came up while reviewing the inclusion of the Ranges TS in C++20, see also <a data-link-type="biblio" href="#biblio-lwg2114">[LWG2114]</a>). We propose that <code class="highlight"><c- k>if</c-> <c- n>not</c-></code> applies the negation <em>after</em> the contextual conversion to <code class="highlight"><c- b>bool</c-></code>, avoiding this subtle class of problems altogether:</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>InputIterator</c-><c- p>,</c-> <c- kr>typename</c-> <c- n>Predicate</c-><c- o>></c->
<c- n>InputIterator</c-> <c- n>find_if_not</c-><c- p>(</c-><c- n>InputIterator</c-> <c- n>first</c-><c- p>,</c-> <c- n>InputIterator</c-> <c- n>last</c-><c- p>,</c-> <c- n>Predicate</c-> <c- n>pred</c-><c- p>)</c->
<c- p>{</c->
    <c- k>for</c-> <c- p>(;</c-> <c- n>first</c-> <c- o>!=</c-> <c- n>last</c-><c- p>;</c-> <c- o>++</c-><c- n>first</c-><c- p>)</c-> <c- p>{</c->
        <c- c1>// Safe even when operator! is overloaded: the contextual</c->
        <c- c1>// conversion to bool is applied before the negation</c->
        <c- k>if</c-> <c- n>not</c-> <c- p>(</c-><c- n>pred</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>))</c-> <c- p>{</c->
            <c- k>break</c-><c- p>;</c->
        <c- p>}</c->
    <c- p>}</c->
    <c- k>return</c-> <c- n>first</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>The current workaround is to use <code class="highlight"><c- k>static_cast</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-><c- p>(</c-><c- n>pred</c-><c- p>(</c-><c- o>*</c-><c- n>first</c-><c- p>))</c-></code> before <code class="highlight"><c- o>!</c-></code> is called which, as far as I know, is not what standard library implementations of algorithms do. Using <code class="highlight"><c- k>if</c-> <c- n>not</c-></code> would be an idiomatic solution to avoid this kind of problem and produce more correct generic code without extra effort. It would ensure that the negation always happens in the boolean domain.</p>
   <h2 class="heading settled" data-level="3" id="extensions"><span class="secno">3. </span><span class="content">Clarifications &amp; Extensions</span><a class="self-link" href="#extensions"></a></h2>
   <p>First of all, while I use <code class="highlight"><c- n>not</c-></code> through the proposal because of personal preference, it is intended that <code class="highlight"><c- k>if</c-> <c- o>!</c-><c- p>(</c-><c- n>condition</c-><c- p>)</c-></code> works too. Second, <code class="highlight"><c- k>if</c-> <c- n>not</c-></code> should also negate the condition even in the presence of an <em>init-statement</em> as introduced in C++17.</p>
   <h3 class="heading settled" data-level="3.1" id="while-not"><span class="secno">3.1. </span><span class="content"><code class="highlight"><c- k>while</c-> <c- n>not</c-></code></span><a class="self-link" href="#while-not"></a></h3>
   <p>The control flow statements <code class="highlight"><c- k>if</c-></code> and <code class="highlight"><c- k>while</c-></code> tend to evolve in a somewhat symmetrical manner, we propose to allow <code class="highlight"><c- k>while</c-></code> and <code class="highlight"><c- k>do</c-> <c- k>while</c-></code> to benefit from the same augmentation in a similar manner.</p>
   <h3 class="heading settled" data-level="3.2" id="if-constexpr"><span class="secno">3.2. </span><span class="content"><code class="highlight"><c- k>if</c-> <c- k>constexpr</c-></code></span><a class="self-link" href="#if-constexpr"></a></h3>
   <p>It would be logical to allow the same extension <code class="highlight"><c- k>if</c-> <c- k>constexpr</c-></code> to benefit from the same extension. The form <code class="highlight"><c- k>if</c-> <c- k>constexpr</c-> <c- n>not</c-></code> would be preferred to <code class="highlight"><c- k>if</c-> <c- n>not</c-> <c- k>constexpr</c-></code> because it subjectively reads better when spelled with an exclamation mark. On the other hand <a data-link-type="biblio" href="#biblio-p1073">[P1073]</a> introduces a <code class="highlight"><c- k>constexpr</c-><c- o>!</c-></code> token which would probably be preferred by max munch rule unless special cased if one tried to write <code class="highlight"><c- k>if</c-> <c- k>constexpr</c-><c- o>!</c-><c- p>(</c-><c- n>condition</c-><c- p>)</c-></code>, which might be seen as a drawback. This will probably need some bikeshed.</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-lwg2114">[LWG2114]
   <dd>Daniel Krügler. <a href="https://wg21.link/LWG2114">Incorrect contextually convertible to bool requirements</a>. 09 December 2011. URL: <a href="https://wg21.link/LWG2114">https://wg21.link/LWG2114</a>
   <dt id="biblio-p1073">[P1073]
   <dd>Richard Smith; Andrew Sutton; Daveed Vandevoorde. <a href="https://wg21.link/P1073R1">constexpr! functions</a>. 22 June 2018. URL: <a href="https://wg21.link/P1073R1">https://wg21.link/P1073R1</a>
  </dl>