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  <title>P3031R0: Resolve CWG2561, conversion function for lambda with explicit object parameter</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
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * 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)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * 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
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
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 *     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)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

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

	html {
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		counter-reset: example figure issue;

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

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

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

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

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

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	@media not print {
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			height: 1.33em;
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			margin-bottom: -1.9em;
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		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
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		#toc-nav:not(:hover) > a:not(:focus) > span + span {
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/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
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			width: 23.5em;
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			font-variant: small-caps;
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		/* See also Overflow section at the bottom */

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			position: absolute;
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/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

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

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

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

	h1 + h2,
	#profile-and-date {
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	h2 + h3,
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	h5 + h6 {
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/** Section divider ***********************************************************/

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		border: transparent solid 0;
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	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
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/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
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	li {
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	dl dd {
		margin: 0 0 .5em 2em;
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	.head dd + dd { /* compact for header */
		margin-top: -.5em;
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	/* Style for algorithms */
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	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
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	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 border-radius: .5em;
	 margin: .5em calc(-0.5em - 1px);
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	[data-algorithm]:not(.heading) > :first-child {
	 margin-top: 0;
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	[data-algorithm]:not(.heading) > :last-child {
	 margin-bottom: 0;
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	/* Style for switch/case <dl>s */
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	margin-left: 0;
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	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;
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	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: inherit;
	}
	dfn var {
		font-style: normal;
	}

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

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
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	.amendment del, .correction del, .addition del,
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	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
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	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
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		text-decoration-style: double;
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/** Miscellaneous improvements to inline formatting ***************************/

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

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

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

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		text-align: left;
		text-align: start;
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	pre {
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		margin-bottom: 1em;
		overflow: auto;
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/** Inline Code fragments *****************************************************/

	/* Do something nice. */

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

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

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	a[href] {
		color: #034575;
		color: var(--a-normal-text);
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		text-decoration: underline var(--a-normal-underline);
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	/* Backout above styling for W3C logo */
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/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
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		/* Intentionally not color-scheme aware. */
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	/* For autogen numbers, add
	  .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

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

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

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		border-right-style: solid;
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	blockquote > :first-child,
	.note  > p:first-child,
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	.amendment > p:first-child,
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		margin-top: 0;
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	blockquote > :last-child,
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	.amendment > p:last-child,
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	.addition > p:last-child {
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	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
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	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
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/** Blockquotes ***************************************************************/

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		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
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/** Open issue ****************************************************************/

	.issue {
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		background: #fbe9e9;
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		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
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	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
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	/* 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;
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		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
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	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
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	/* 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;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

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

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

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

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

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

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** 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: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

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

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

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    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;
		border-bottom: 1px solid var(--defrow-border);
	}

	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-color: var(--datacell-border);
		border-top-style: solid;
	}

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

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

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

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

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

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

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

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


/*
Alternate table alignment rules

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

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

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

Possible extra rowspan handling

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

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

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


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

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

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

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

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

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

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

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

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


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

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

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

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

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

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

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

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

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

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

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}



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

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

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

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style>
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <meta content="Bikeshed version 82ce88815, updated Thu Sep 7 16:33:55 2023 -0700" name="generator">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
hilite  {background-color: #FFFF00; font-weight: bold;}
ins     {background-color: #CCFFCC; text-decoration: underline;}
del     {background-color: #FFCACA; text-decoration: line-through;}
</style>
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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">P3031R0<br>Resolve CWG2561, conversion function for lambda with explicit object parameter</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Published Proposal, <time class="dt-updated" datetime="2023-11-12">2023-11-12</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt class="editor">Author:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:arthur.j.odwyer@gmail.com">Arthur O'Dwyer</a>
     <dt>Audience:
     <dd>EWG, CWG
     <dt>Project:
     <dd>ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
     <dt>Draft Revision:
     <dd>7
    </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>Let <code class="highlight"><c- n>T</c-></code> be the closure type of <code class="highlight"><c- p>[](</c-><c- k>this</c-> <c- k>auto</c-><c- p>,</c-> <c- b>int</c-><c- p>){}</c-></code>.

  CWG2561 points out that the meaning of <code class="highlight"><c- o>+</c-><c- n>T</c-><c- p>()</c-></code> is unclear.
  Microsoft (non-conformingly) treats that expression the same as <code class="highlight"><c- o>+</c-><c- p>[](</c-><c- b>int</c-><c- p>){}</c-></code>.
  Clang (conformingly) treats the lambda as a generic lambda and <code class="highlight"><c- o>+</c-><c- n>T</c-><c- p>()</c-></code> as ill-formed,
  but (non-conformingly and awkwardly) accepts <code class="highlight"><c- b>void</c-> <c- p>(</c-><c- o>*</c-><c- n>pf</c-><c- p>)(</c-><c- n>T</c-><c- p>,</c-> <c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>T</c-><c- p>()</c-></code>.
  The (February 2022) P/R of CWG2561 proposes Clang’s direction, but this paper proposes
  either the more user-friendly and consistent direction taken by Microsoft, or
  else to eliminate the function-pointer conversion for explicit-object lambdas
  altogether.</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">Note</span></a>
    <li>
     <a href="#background"><span class="secno">2</span> <span class="content">Background</span></a>
     <ol class="toc">
      <li>
       <a href="#generic"><span class="secno">2.1</span> <span class="content">Is <code class="highlight"><c- p>[](</c-><c- k>this</c-> <c- k>auto</c-><c- p>){}</c-></code> generic?</span></a>
       <ol class="toc">
        <li><a href="#generic-paramlist"><span class="secno">2.1.1</span> <span class="content">Is <code class="highlight"><c- k>this</c-> <c- k>auto</c-></code> different from <code class="highlight"><c- k>this</c-> <c- n>T</c-></code>?</span></a>
        <li><a href="#act-different"><span class="secno">2.1.2</span> <span class="content">Should we allow <code class="highlight"><c- p>(</c-><c- o>+</c-><c- n>c</c-><c- p>)()</c-></code> to act differently from <code class="highlight"><c- n>c</c-><c- p>()</c-></code>?</span></a>
        <li><a href="#generic-constrained"><span class="secno">2.1.3</span> <span class="content">This lambda can only be passed its own type</span></a>
       </ol>
     </ol>
    <li><a href="#experience"><span class="secno">3</span> <span class="content">Implementation experience</span></a>
    <li>
     <a href="#wording"><span class="secno">4</span> <span class="content">Options for proposed wording</span></a>
     <ol class="toc">
      <li><a href="#wording-punt"><span class="secno">4.1</span> <span class="content">Punt for now</span></a>
      <li><a href="#wording-clang"><span class="secno">4.2</span> <span class="content">Clang’s approach</span></a>
      <li><a href="#wording-msvc"><span class="secno">4.3</span> <span class="content">MSVC’s approach</span></a>
     </ol>
    <li><a href="#straw-polls"><span class="secno">5</span> <span class="content">Straw polls taken in EWG, 2023-11-07</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="changelog"><span class="secno">1. </span><span class="content">Note</span><a class="self-link" href="#changelog"></a></h2>
   <p>R0 is the initial, and only, revision of this paper.
D3031R0 (draft revision 6 dated 2023-11-03) was discussed by EWG
in Kona 2023 (<a href="https://wiki.edg.com/bin/view/Wg21kona2023/NotesEWGCWG2561">minutes</a>),
and EWG wisely decided to take the "punt" option as presented. MSVC’s
behavior is cool and nice, but requires a design paper if we actually want
to pursue it. The post-Kona mailing contains P3031R0 (draft revision 7,
i.e., including the note you are now reading) only for the official record;
this paper demands no further action from anyone.</p>
   <h2 class="heading settled" data-level="2" id="background"><span class="secno">2. </span><span class="content">Background</span><a class="self-link" href="#background"></a></h2>
   <p>"Deducing this" allows lambdas to have explicit object parameters.
Microsoft and Clang both support the feature already, but with differing semantics.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>Any</c-> <c- p>{</c-> <c- n>Any</c-><c- p>(</c-><c- k>auto</c-><c- p>)</c-> <c- p>{</c-> <c- n>puts</c-><c- p>(</c-><c- s>"called"</c-><c- p>);</c-> <c- p>}</c-> <c- p>};</c->

<c- k>auto</c-> <c- n>a1</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>a2</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- n>Any</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>};</c->
</pre>
   <p>Here <code class="highlight"><c- n>a1</c-></code>’s closure type is certainly equivalent to</p>
<pre class="language-c++ highlight"><c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- n>using</c-> <c- n>Just</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->

<c- k>struct</c-> <c- nc>A1</c-> <c- p>{</c->
  <c- b>int</c-> <c- nf>operator</c-><c- p>()(</c-><c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>}</c->
  <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>+</c-><c- p>[](</c-><c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>A1</c-><c- p>()(</c-><c- n>x</c-><c- p>);</c-> <c- p>};</c-> <c- p>}</c->
<c- p>};</c->
</pre>
   <p>But it’s unclear which of the following two options should correspond to <code class="highlight"><c- n>a2</c-></code>’s closure type:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>A2_MSVC</c-> <c- p>{</c->
  <c- b>int</c-> <c- nf>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>Any</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>}</c->
  <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>+</c-><c- p>[](</c-><c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>A2_MSVC</c-><c- p>()(</c-><c- n>x</c-><c- p>);</c-> <c- p>};</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>A2_Clang</c-> <c- p>{</c->
  <c- b>int</c-> <c- nf>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>Any</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>}</c->
  <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>Any</c-><c- p>,</c-> <c- b>int</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>&amp;</c-><c- n>A2_Clang</c-><c- o>::</c-><c- n>operator</c-><c- p>();</c-> <c- p>}</c->
<c- p>};</c->
</pre>
   <p>MSVC’s version is friendlier: it allows us to refactor an implicit-object-parameter lambda
into an explicit-object-parameter lambda (or vice versa) without changing the type of
the expression <code class="highlight"><c- o>+</c-><c- n>a2</c-></code>. MSVC also allows us to write a recursive captureless lambda taking
itself as a function pointer; this is impossible on Clang, because the function pointer’s
type would recursively involve its own type.</p>
   <table>
    <tbody>
     <tr>
      <th>Before
      <th>After
     <tr>
      <td>
<pre class="highlight"><c- b>void</c-> <c- nf>f</c-><c- p>(</c-><c- k>auto</c-> <c- n>c</c-><c- p>);</c->
<c- k>auto</c-> <c- n>a1</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>};</c->
<c- n>f</c-><c- p>(</c-><c- n>a1</c-><c- p>);</c->  <c- c1>// OK</c->
<c- n>f</c-><c- p>(</c-><c- o>+</c-><c- n>a1</c-><c- p>);</c-> <c- c1>// OK</c->
</pre>
      <td>
<pre class="highlight"><c- b>void</c-> <c- nf>f</c-><c- p>(</c-><c- k>auto</c-> <c- n>pf</c-><c- p>);</c->
<c- k>auto</c-> <c- n>a2</c-> <c- o>=</c-> <c- p>[](</c-><c- k>this</c-> <c- n>Any</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>};</c->
<c- n>f</c-><c- p>(</c-><c- n>a2</c-><c- p>);</c->  <c- c1>// OK</c->
<c- n>f</c-><c- p>(</c-><c- o>+</c-><c- n>a2</c-><c- p>);</c-> <c- c1>// Error on Clang, OK on MSVC</c->
</pre>
     <tr>
      <td>
<pre class="highlight"><c- c1>// OK</c->
<c- k>auto</c-> <c- n>fib</c-> <c- o>=</c-> <c- p>[](</c-><c- k>this</c-> <c- k>auto</c-> <c- n>fib</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- o>-></c-> <c- b>int</c-> <c- p>{</c->
  <c- k>return</c-> <c- n>x</c-> <c- o>&lt;</c-> <c- mi>2</c-> <c- o>?</c-> <c- n>x</c-> <c- o>:</c-> <c- n>fib</c-><c- p>(</c-><c- n>x</c-><c- mi>-1</c-><c- p>)</c-> <c- o>+</c-> <c- n>fib</c-><c- p>(</c-><c- n>x</c-><c- mi>-2</c-><c- p>);</c->
<c- p>};</c->
<c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- n>fib</c-><c- p>(</c-><c- mi>5</c-><c- p>);</c->
</pre>
      <td>
<pre class="highlight"><c- c1>// Error on Clang, OK on MSVC</c->
<c- k>auto</c-> <c- n>fib</c-> <c- o>=</c-> <c- p>[](</c-><c- k>this</c-> <c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>fib</c-><c- p>)(</c-><c- b>int</c-><c- p>),</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c->
  <c- k>return</c-> <c- n>x</c-> <c- o>&lt;</c-> <c- mi>2</c-> <c- o>?</c-> <c- n>x</c-> <c- o>:</c-> <c- n>fib</c-><c- p>(</c-><c- n>x</c-><c- mi>-1</c-><c- p>)</c-> <c- o>+</c-> <c- n>fib</c-><c- p>(</c-><c- n>x</c-><c- mi>-2</c-><c- p>);</c->
<c- p>};</c->
<c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- n>fib</c-><c- p>(</c-><c- mi>5</c-><c- p>);</c->
</pre>
   </table>
   <p>But if you try <code class="highlight"><c- n>A2_MSVC</c-></code> as written, you’ll find that it is ambiguous:</p>
<pre class="language-c++ highlight"><c- n>A2_MSVC</c-> <c- n>a2</c-><c- p>;</c->
<c- n>a2</c-><c- p>(</c-><c- mi>1</c-><c- p>);</c-> <c- c1>// ill-formed, ambiguous</c->
</pre>
   <p>There are two candidates for this call: (1) call <code class="highlight"><c- n>Any</c-><c- p>(</c-><c- n>A2_MSVC</c-><c- p>)</c-></code> and then <code class="highlight"><c- n>A2_MSVC</c-><c- o>::</c-><c- k>operator</c-><c- p>()(</c-><c- k>this</c-> <c- n>Any</c-><c- p>,</c-> <c- b>int</c-><c- p>)</c-></code>,
or (2) call <code class="highlight"><c- n>A2_MSVC</c-><c- o>::</c-><c- k>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-></code> and then the built-in <code class="highlight"><c- k>operator</c-><c- p>()</c-></code> on that function pointer.
We need new core wording to prefer (1) over (2).</p>
   <p>Why don’t ordinary (implicit-object-parameter) lambdas suffer from this ambiguity? I think it’s because
the identity conversion binding <code class="highlight"><c- k>this</c-></code> always wins over the user-defined conversion to function pointer.
But for the explicit-object-parameter lambda <code class="highlight"><c- n>a2</c-></code>, binding <code class="highlight"><c- k>this</c-></code> to <code class="highlight"><c- k>this</c-> <c- n>Any</c-> <c- n>self</c-></code> is <em>also</em> a user-defined
conversion.</p>
   <h3 class="heading settled" data-level="2.1" id="generic"><span class="secno">2.1. </span><span class="content">Is <code class="highlight"><c- p>[](</c-><c- k>this</c-> <c- k>auto</c-><c- p>){}</c-></code> generic?</span><a class="self-link" href="#generic"></a></h3>
   <p>According to <a href="https://eel.is/c++draft/expr.prim.lambda#def:generic_lambda">[expr.prim.lambda.general]/6</a>,
a lambda becomes "generic" when any of its parameters involve placeholder types — even when the
only placeholder type is the explicit object parameter’s! In other words, <code class="highlight"><c- n>b</c-></code> below is technically
considered a generic lambda.</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>b</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>};</c->
</pre>
   <p>It is unclear which of the following three options should correspond to <code class="highlight"><c- n>b</c-></code>’s closure type.</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>B_Clang</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- b>int</c-> <c- n>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>T</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>}</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>T</c-><c- p>,</c-> <c- b>int</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>&amp;</c-><c- n>B_Clang</c-><c- o>::</c-><c- n>operator</c-><c- p>();</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>B_Generic</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- b>int</c-> <c- n>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>T</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>}</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>+</c-><c- p>[](</c-><c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>B_Generic</c-><c- p>()(</c-><c- n>x</c-><c- p>);</c-> <c- p>};</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>B_MSVC</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- b>int</c-> <c- n>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>T</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>}</c->
  <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>+</c-><c- p>[](</c-><c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>B_MSVC</c-><c- p>()(</c-><c- n>x</c-><c- p>);</c-> <c- p>};</c-> <c- p>}</c-> 
<c- p>};</c->
</pre>
   <p>MSVC’s version is friendliest:</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>b</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>x</c-><c- o>+</c-><c- mi>1</c-><c- p>;</c-> <c- p>};</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>pb1</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>b</c-><c- p>;</c-> <c- c1>// Generic+MSVC: OK; Clang: error</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>pb2</c-><c- p>)(</c-><c- n>decltype</c-><c- p>(</c-><c- n>b</c-><c- p>),</c-> <c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>b</c-><c- p>;</c-> <c- c1>// Clang: OK; Generic+MSVC: error</c->
<c- k>auto</c-> <c- n>pb3</c-> <c- o>=</c-> <c- o>+</c-><c- n>b</c-><c- p>;</c-> <c- c1>// MSVC: OK; Generic+Clang: error</c->
</pre>
   <p>This suggests that a <code class="highlight"><c- k>this</c-> <c- k>auto</c-></code> parameter shouldn’t suffice to make a lambda "generic."
But see the next section.</p>
   <h4 class="heading settled" data-level="2.1.1" id="generic-paramlist"><span class="secno">2.1.1. </span><span class="content">Is <code class="highlight"><c- k>this</c-> <c- k>auto</c-></code> different from <code class="highlight"><c- k>this</c-> <c- n>T</c-></code>?</span><a class="self-link" href="#generic-paramlist"></a></h4>
   <p>Zhihao Yuan argues that it is counterintuitive for MSVC to treat these two spellings
differently:</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>a</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- k>auto</c-><c- p>)</c-> <c- p>{};</c-> <c- c1>// "non-generic" on MSVC</c->
<c- k>auto</c-> <c- n>b</c-> <c- o>=</c-> <c- p>[]</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>this</c-> <c- n>T</c-><c- p>)</c-> <c- p>{};</c-> <c- c1>// generic</c->

<c- k>auto</c-> <c- n>pa</c-> <c- o>=</c-> <c- o>+</c-><c- n>a</c-><c- p>;</c-> <c- c1>// OK on MSVC</c->
<c- k>auto</c-> <c- n>pb</c-> <c- o>=</c-> <c- o>+</c-><c- n>b</c-><c- p>;</c-> <c- c1>// error on MSVC</c->
<c- b>void</c-> <c- p>(</c-><c- o>*</c-><c- n>qa</c-><c- p>)()</c-> <c- o>=</c-> <c- n>a</c-><c- p>;</c-> <c- c1>// OK on MSVC</c->
<c- b>void</c-> <c- p>(</c-><c- o>*</c-><c- n>qb</c-><c- p>)()</c-> <c- o>=</c-> <c- n>b</c-><c- p>;</c-> <c- c1>// error on MSVC</c->
</pre>
   <p>If <code class="highlight"><c- p>[]</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-><c- p>(</c-><c- k>this</c-> <c- n>T</c-><c- p>){}</c-></code> is considered generic, then so should be <code class="highlight"><c- p>[](</c-><c- k>this</c-> <c- k>auto</c-><c- p>){}</c-></code>; we shouldn’t
carve out an exception for the latter.</p>
   <h4 class="heading settled" data-level="2.1.2" id="act-different"><span class="secno">2.1.2. </span><span class="content">Should we allow <code class="highlight"><c- p>(</c-><c- o>+</c-><c- n>c</c-><c- p>)()</c-></code> to act differently from <code class="highlight"><c- n>c</c-><c- p>()</c-></code>?</span><a class="self-link" href="#act-different"></a></h4>
   <p>Gašper Ažman provides this example of a lambda that can’t be called directly:</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>c</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>)</c-> <c- o>-></c-> <c- b>int</c-> <c- p>{</c-> <c- k>return</c-> <c- n>self</c-><c- p>.</c-><c- n>value</c-><c- p>;</c-> <c- p>};</c->
<c- n>using</c-> <c- n>C</c-> <c- o>=</c-> <c- n>decltype</c-><c- p>(</c-><c- n>c</c-><c- p>);</c->

<c- k>struct</c-> <c- nc>Derived</c-> <c- o>:</c-> <c- n>C</c-> <c- p>{</c-> <c- b>int</c-> <c- n>value</c-> <c- o>=</c-> <c- mi>2</c-><c- p>;</c-> <c- p>};</c->
</pre>
   <p>Here it is a (SFINAE-unfriendly) error to instantiate <code class="highlight"><c- n>C</c-><c- o>::</c-><c- k>operator</c-><c- p>()</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-></code>,
but it is OK to instantiate <code class="highlight"><c- n>C</c-><c- o>::</c-><c- k>operator</c-><c- p>()</c-><c- o>&lt;</c-><c- n>Derived</c-><c- o>></c-></code>. Therefore both <code class="highlight"><c- n>c</c-><c- p>()</c-></code> and <code class="highlight"><c- o>+</c-><c- n>c</c-></code> are errors, but this is OK:</p>
<pre class="language-c++ highlight"><c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>C</c-><c- p>,</c-> <c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)()</c-><c- o>></c-><c- p>);</c-> <c- c1>// OK on MSVC</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>is_convertible_v</c-><c- o>&lt;</c-><c- n>C</c-><c- p>,</c-> <c- b>int</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>C</c-><c- p>)</c-><c- o>></c-><c- p>);</c-> <c- c1>// OK on Clang</c->

<c- n>Derived</c-> <c- n>d</c-><c- p>;</c->
<c- b>int</c-> <c- n>two</c-> <c- o>=</c-> <c- n>d</c-><c- p>();</c-> <c- c1>// OK</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>p</c-><c- p>)()</c-> <c- o>=</c-> <c- n>d</c-><c- p>;</c-> <c- c1>// error on Clang+MSVC</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>p</c-><c- p>)(</c-><c- n>Derived</c-><c- p>)</c-> <c- o>=</c-> <c- n>d</c-><c- p>;</c-> <c- c1>// OK on Clang, error on MSVC</c->
</pre>
   <p>On Clang, <code class="highlight"><c- o>+</c-><c- n>d</c-></code> fails to deduce the template parameter to <code class="highlight"><c- n>C</c-></code>’s conversion function template (SFINAE-friendly).
On MSVC, <code class="highlight"><c- o>+</c-><c- n>d</c-></code> unambiguously calls the non-template conversion function inherited from <code class="highlight"><c- n>C</c-></code>,
which hard-errors during instantiation of <code class="highlight"><c- n>C</c-><c- o>::</c-><c- k>operator</c-><c- p>()</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-></code>.</p>
   <p>A more problematic variation is:</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>c2</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>self</c-><c- p>);</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>Derived2</c-> <c- o>:</c-> <c- n>decltype</c-><c- p>(</c-><c- n>c2</c-><c- p>)</c-> <c- p>{</c-> <c- b>int</c-> <c- n>pad</c-><c- p>;</c-> <c- p>}</c-> <c- n>d2</c-><c- p>;</c->
<c- n>assert</c-><c- p>(</c-><c- n>d2</c-><c- p>()</c-> <c- o>==</c-> <c- mi>4</c-><c- p>);</c->
<c- n>assert</c-><c- p>((</c-><c- o>+</c-><c- n>d2</c-><c- p>)()</c-> <c- o>==</c-> <c- mi>1</c-><c- p>);</c->
</pre>
   <p>Here <code class="highlight"><c- n>d2</c-><c- p>()</c-> <c- o>==</c-> <c- mi>4</c-></code>, but <code class="highlight"><c- o>+</c-><c- n>d2</c-></code> points to a function that returns <code class="highlight"><c- mi>1</c-></code>.
This example suggests that the conversion function should
not exist for explicit-object lambdas (i.e., the "Punt" wording option below).</p>
   <p>Two other problematic cases are <code class="highlight"><c- n>c3</c-></code> (no inheritance):</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>c3</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- k>auto</c-><c- o>&amp;&amp;</c-> <c- n>self</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>is_rvalue_reference_v</c-><c- o>&lt;</c-><c- n>decltype</c-><c- p>(</c-><c- n>self</c-><c- p>)</c-><c- o>></c-><c- p>;</c-> <c- p>};</c->
<c- n>assert</c-><c- p>(</c-><c- n>c3</c-><c- p>()</c-> <c- o>==</c-> false<c- p>);</c->
<c- n>assert</c-><c- p>((</c-><c- o>+</c-><c- n>c3</c-><c- p>)()</c-> <c- o>==</c-> true<c- p>);</c->
</pre>
   <p>and <code class="highlight"><c- n>d4</c-></code> (a non-generic lambda):</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>Evil</c-> <c- p>{</c-> <c- b>int</c-> <c- n>i</c-><c- p>;</c-> <c- n>Evil</c-><c- p>(</c-><c- k>auto</c-> <c- n>x</c-><c- p>)</c-> <c- o>:</c-> <c- n>i</c-><c- p>(</c-><c- k>sizeof</c-><c- p>(</c-><c- n>x</c-><c- p>))</c-> <c- p>{}</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>c4</c-> <c- o>=</c-> <c- p>[](</c-><c- n>this</c-> <c- n>Evil</c-> <c- n>self</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>self</c-><c- p>.</c-><c- n>i</c-><c- p>;</c-> <c- p>};</c->
<c- k>struct</c-> <c- nc>Derived4</c-> <c- o>:</c-> <c- n>decltype</c-><c- p>(</c-><c- n>c4</c-><c- p>)</c-> <c- p>{</c-> <c- b>int</c-> <c- n>pad</c-><c- p>;</c-> <c- p>}</c-> <c- n>d4</c-><c- p>;</c->
<c- n>assert</c-><c- p>(</c-><c- n>d4</c-><c- p>()</c-> <c- o>==</c-> <c- mi>4</c-><c- p>);</c->
<c- n>assert</c-><c- p>((</c-><c- o>+</c-><c- n>d4</c-><c- p>)()</c-> <c- o>==</c-> <c- mi>1</c-><c- p>);</c->
</pre>
   <p>Maybe these are obscure enough problems that we don’t care? Or, on the other hand, maybe
we should make a solid rule that <em>whenever <code class="highlight"><c- p>(</c-><c- o>+</c-><c- n>lambda</c-><c- p>)()</c-></code> is well-formed, it is guaranteed to have the same behavior
as <code class="highlight"><c- n>lambda</c-><c- p>()</c-></code>.</em> Explicit-object lambdas cannot provide that guarantee, and therefore we cannot give them conversion functions
(i.e., the "Punt" wording option below).</p>
   <h4 class="heading settled" data-level="2.1.3" id="generic-constrained"><span class="secno">2.1.3. </span><span class="content">This lambda can only be passed its own type</span><a class="self-link" href="#generic-constrained"></a></h4>
   <p>Consider this case, which (having a <em>template-parameter-list</em>) is clearly a generic lambda:</p>
<pre class="language-c++ highlight"><c- k>auto</c-> <c- n>c</c-> <c- o>=</c-> <c- p>[]</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-><c- p>(</c-><c- n>this</c-> <c- n>T</c-> <c- n>self</c-><c- p>,</c-> <c- n>T</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>x</c-><c- p>;</c-> <c- p>};</c->
</pre>
   <p>It is unclear which of the following three options should correspond to <code class="highlight"><c- n>c</c-></code>’s closure type:</p>
<pre class="language-c++ highlight"><c- k>struct</c-> <c- nc>C_Clang</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- b>void</c-> <c- n>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>T</c-> <c- n>self</c-><c- p>,</c-> <c- n>T</c-> <c- n>x</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>x</c-><c- p>;</c-> <c- p>}</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>void</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>T</c-><c- p>,</c-> <c- n>T</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> <c- k>return</c-> <c- o>&amp;</c-><c- n>C_Clang</c-><c- o>::</c-><c- n>operator</c-><c- p>();</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>C_MSVC</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- b>void</c-> <c- n>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>T</c-> <c- n>self</c-><c- p>,</c-> <c- n>T</c-> <c- n>x</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>x</c-><c- p>;</c-> <c- p>}</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>void</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>T</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c->
  <c- p>{</c-> <c- k>return</c-> <c- o>+</c-><c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>x</c-><c- p>;</c-> <c- p>};</c-> <c- p>}</c->
<c- p>};</c->

<c- k>struct</c-> <c- nc>C_Constrained</c-> <c- p>{</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- b>void</c-> <c- n>operator</c-><c- p>()(</c-><c- n>this</c-> <c- n>T</c-> <c- n>self</c-><c- p>,</c-> <c- n>T</c-> <c- n>x</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>x</c-><c- p>;</c-> <c- p>}</c->
  <c- n>template</c-><c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c-> <c- n>operator</c-> <c- n>Just</c-><c- o>&lt;</c-><c- b>void</c-><c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>T</c-><c- p>)</c-><c- o>></c-><c- p>()</c-> <c- k>const</c->
    <c- n>requires</c-> <c- n>std</c-><c- o>::</c-><c- n>is_same_v</c-><c- o>&lt;</c-><c- n>C_Constrained</c-><c- p>,</c-> <c- n>T</c-><c- o>></c->
  <c- p>{</c-> <c- k>return</c-> <c- o>+</c-><c- p>[](</c-><c- k>auto</c-> <c- n>x</c-><c- p>)</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>x</c-><c- p>;</c-> <c- p>};</c-> <c- p>}</c->
<c- p>};</c->
</pre>
   <p>Here MSVC’s version is friendly, but confusing, because MSVC rightly rejects <code class="highlight"><c- n>c</c-><c- p>(</c-><c- mi>1</c-><c- p>)</c-></code> but accepts <code class="highlight"><c- p>(</c-><c- o>+</c-><c- n>c</c-><c- p>)(</c-><c- mi>1</c-><c- p>)</c-></code>! So the function pointer that MSVC returns from <code class="highlight"><c- o>+</c-><c- n>c</c-></code> is not
in fact "invoking the closure type’s function call operator on a default-constructed instance
of the closure type" — that wouldn’t compile! Do we need the conversion function template
to be constrained? and if so, should it be constrained as in <code class="highlight"><c- n>C_Constrained</c-></code>, or otherwise?</p>
   <h2 class="heading settled" data-level="3" id="experience"><span class="secno">3. </span><span class="content">Implementation experience</span><a class="self-link" href="#experience"></a></h2>
   <p>As far as I can tell, we have implementation experience of both Clang’s approach (in Clang)
and MSVC’s approach (in MSVC) — although I don’t fully understand what MSVC is doing
internally to avoid the overload-resolution ambiguity.
But MSVC’s approach seems to be implementable, since it’s been implemented.</p>
   <h2 class="heading settled" data-level="4" id="wording"><span class="secno">4. </span><span class="content">Options for proposed wording</span><a class="self-link" href="#wording"></a></h2>
   <p class="note" role="note"><span class="marker">Note:</span> Throughout, the Standard’s chosen examples rarely seem on-point. I’d like to add more
relevant examples and eliminate some of the examples already present.</p>
   <h3 class="heading settled" data-level="4.1" id="wording-punt"><span class="secno">4.1. </span><span class="content">Punt for now</span><a class="self-link" href="#wording-punt"></a></h3>
   <p>Modify <a href="https://eel.is/c++draft/expr.prim.lambda#general-6">[expr.prim.lambda.general]/6</a> as follows:</p>
   <p><small></small></p>
   <blockquote><small> 6․ A lambda is a <i>generic lambda</i> if the <i>lambda-expression</i> has any generic parameter type placeholders ([dcl.spec.auto]),
or if the lambda has a <i>template-parameter-list</i>.<br> [<i>Example 4:</i><br><p></p> <pre class="highlight"><del><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>}(</c-><c- mi>3</c-><c- p>,</c-> <c- mi>4</c-><c- p>);</c->                  <c- c1>// OK, a generic lambda</c-><br><c- c1>int j = []&lt;class T>(T t, int i) { return i; }(3, 4);            // OK, a generic lambda</c-></del>
<ins><c- k>auto</c-> <c- n>x</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                          <c- c1>// OK, a generic lambda</c-></ins>
<ins><c- k>auto</c-> <c- n>y</c-> <c- o>=</c-> <c- p>[](</c-><c- k>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                  <c- c1>// OK, a generic lambda</c-></ins>
<ins><c- k>auto</c-> <c- n>z</c-> <c- o>=</c-> <c- p>[]</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                         <c- c1>// OK, a generic lambda</c-></ins>
</pre><i>— end example</i>]</small></blockquote>
   <p>Modify <a href="https://eel.is/c++draft/expr.prim.lambda#closure-9">[expr.prim.lambda.closure]/9</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>9․ The closure type for a non-generic <em>lambda-expression</em> with no <em>lambda-capture</em> <ins>and
no explicit object parameter ([dcl.fct])</ins> whose constraints (if any) are satisfied
has a conversion function to pointer to function with C++ language linkage
having the same parameter and return types as the closure type’s function call operator.
The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification.
If the function call operator is a static member function, then the value returned by this conversion function is the address
of the function call operator. Otherwise, the value returned by this conversion function is the address of a function <code class="highlight"><c- n>F</c-></code> that,
when invoked, has the same effect as invoking the closure type’s function call operator on a default-constructed instance
of the closure type. <code class="highlight"><c- n>F</c-></code> is a constexpr function if the function call operator is a constexpr function
and an immediate function if the function call operator is an immediate function. </p><p>10․ For a generic lambda with no <em>lambda-capture</em> <ins>and no explicit object parameter</ins>,
the closure type has a conversion function template to pointer to function.
The conversion function template has the same invented template parameter list, and the pointer to function has the same parameter types,
as the function call operator template. The return type of the pointer to function shall behave as if it were
a <em>decltype-specifier</em> denoting the return type of the corresponding function call operator template specialization. </p><p>11․ [<i>Note 4:</i> If the generic lambda has no <em>trailing-return-type</em> or the <em>trailing-return-type</em> contains a placeholder type,
return type deduction of the corresponding function call operator template specialization has to be done.
The corresponding specialization is that instantiation of the function call operator template with the same template arguments
as those deduced for the conversion function template. Consider the following: </p><pre class="highlight"><c- k>auto</c-> <c- n>glambda</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>fp</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>glambda</c-><c- p>;</c->
</pre><p>The behavior of the conversion function of <code class="highlight"><c- n>glambda</c-></code> above is like that of the following conversion function:</p> <pre class="highlight"><c- k>struct</c-> <c- nc>Closure</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>auto</c-> <c- k>operator</c-><c- p>()(</c-><c- n>T</c-> <c- n>t</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>static</c-> <c- k>auto</c-> <c- n>lambda_call_operator_invoker</c-><c- p>(</c-><c- n>T</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c->
    <c- c1>// forwards execution to operator()(a) and therefore has</c->
    <c- c1>// the same return type deduced</c->
    <c- d>/* ... */</c->
  <c- p>}</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>using</c-> <c- n>fptr_t</c-> <c- o>=</c->
     <c- k>decltype</c-><c- p>(</c-><c- n>lambda_call_operator_invoker</c-><c- p>(</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>()))</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>T</c-><c- p>);</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>operator</c-> <c- n>fptr_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>()</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- o>&amp;</c-><c- n>lambda_call_operator_invoker</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->
</pre><i>— end note</i>]<p>[<i>Example 6:</i></p><pre class="highlight"><c- b>void</c-> <c- nf>f1</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->   <c- p>{</c-> <c- p>}</c->
<c- b>void</c-> <c- nf>f2</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->  <c- p>{</c-> <c- p>}</c-><br>
<c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->    <c- p>{</c-> <c- p>}</c->     <c- c1>// #1</c->
<c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>char</c-><c- p>))</c->  <c- p>{</c-> <c- p>}</c->     <c- c1>// #2</c-><br>
<c- b>void</c-> <c- nf>h</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->    <c- p>{</c-> <c- p>}</c->     <c- c1>// #3</c->
<c- b>void</c-> <c- nf>h</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->   <c- p>{</c-> <c- p>}</c->     <c- c1>// #4</c-><br>
<c- k>auto</c-> <c- n>glambda</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- n>f1</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                    <c- c1>// OK</c->
<c- n>f2</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                    <c- c1>// error: ID is not convertible</c->
<c- n>g</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                     <c- c1>// error: ambiguous</c->
<c- n>h</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                     <c- c1>// OK, calls #3 since it is convertible from ID</c->
<c- b>int</c-><c- o>&amp;</c-> <c- p>(</c-><c- o>*</c-><c- n>fpi</c-><c- p>)(</c-><c- b>int</c-><c- o>*</c-><c- p>)</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-><c- o>*</c-> <c- n>a</c-><c- p>)</c-> <c- o>-></c-> <c- k>auto</c-><c- o>&amp;</c-> <c- p>{</c-> <c- k>return</c-> <c- o>*</c-><c- n>a</c-><c- p>;</c-> <c- p>};</c->        <c- c1>// OK</c->
</pre><i>— end example</i>] <p>12․ If the function call operator template is a static member function template, then the value returned
by any given specialization of this conversion function template is the address of the corresponding function call operator template specialization.
Otherwise, the value returned by any given specialization of this conversion function template is
the address of a function <code class="highlight"><c- n>F</c-></code> that, when invoked, has the same effect as invoking the generic lambda’s corresponding function call
operator template specialization on a default-constructed instance of the closure type. <code class="highlight"><c- n>F</c-></code> is a constexpr function if the corresponding specialization is a constexpr function
and an immediate function if the function call operator template specialization is an immediate function. </p><p>[<i>Note 5:</i> This will result in the implicit instantiation of the generic lambda’s body.
The instantiated generic lambda’s return type and parameter types are required to match the return type and parameter types of the pointer to function. <i>— end note</i>] </p><p>[<i>Example 7:</i><br> </p><pre class="highlight"><c- k>auto</c-> <c- n>GL</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>a</c-><c- p>;</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>GL_int</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>GL</c-><c- p>;</c->        <c- c1>// OK, through conversion function template</c->
<c- n>GL_int</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->                      <c- c1>// OK, same as GL(3)</c->
</pre><i>— end example</i>]<p>13․ The conversion function or conversion function template is public, constexpr, non-virtual, non-explicit, const, and has a non-throwing exception specification. </p><p>[<i>Example 8:</i><br></p><pre class="highlight"><c- k>auto</c-> <c- n>Fwd</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>fp</c-><c- p>)(</c-><c- b>int</c-><c- p>),</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>fp</c-><c- p>(</c-><c- n>a</c-><c- p>);</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>C</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c-><br>
<c- k>static_assert</c-><c- p>(</c-><c- n>Fwd</c-><c- p>(</c-><c- n>C</c-><c- p>,</c-><c- mi>3</c-><c- p>)</c-> <c- o>==</c-> <c- mi>3</c-><c- p>);</c->   <c- c1>// OK</c-><br>
<c- c1>// No specialization of the function call operator template can be constexpr (due to the local static).</c->
<c- k>auto</c-> <c- n>NC</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>static</c-> <c- b>int</c-> <c- n>s</c-><c- p>;</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>Fwd</c-><c- p>(</c-><c- n>NC</c-><c- p>,</c-><c- mi>3</c-><c- p>)</c-> <c- o>==</c-> <c- mi>3</c-><c- p>);</c->  <c- c1>// error</c->
</pre><i>— end example</i>] </small>
   </blockquote>
   <h3 class="heading settled" data-level="4.2" id="wording-clang"><span class="secno">4.2. </span><span class="content">Clang’s approach</span><a class="self-link" href="#wording-clang"></a></h3>
   <p>Modify <a href="https://eel.is/c++draft/expr.prim.lambda#general-6">[expr.prim.lambda.general]/6</a> as follows:</p>
   <p><small></small></p>
   <blockquote><small> 6․ A lambda is a <i>generic lambda</i> if the <i>lambda-expression</i> has any generic parameter type placeholders ([dcl.spec.auto]),
or if the lambda has a <i>template-parameter-list</i>.<br> [<i>Example 4:</i><br><p></p> <pre class="highlight"><del><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>}(</c-><c- mi>3</c-><c- p>,</c-> <c- mi>4</c-><c- p>);</c->                  <c- c1>// OK, a generic lambda</c-><br><c- c1>int j = []&lt;class T>(T t, int i) { return i; }(3, 4);            // OK, a generic lambda</c-></del>
<ins><c- k>auto</c-> <c- n>x</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                          <c- c1>// OK, a generic lambda</c-></ins>
<ins><c- k>auto</c-> <c- n>y</c-> <c- o>=</c-> <c- p>[](</c-><c- k>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                  <c- c1>// OK, a generic lambda</c-></ins>
<ins><c- k>auto</c-> <c- n>z</c-> <c- o>=</c-> <c- p>[]</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                         <c- c1>// OK, a generic lambda</c-></ins>
</pre><i>— end example</i>]</small></blockquote>
   <p>Modify <a href="https://eel.is/c++draft/expr.prim.lambda#closure-9">[expr.prim.lambda.closure]/9</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>9․ The closure type for a non-generic <em>lambda-expression</em> with no <em>lambda-capture</em> whose constraints (if any) are satisfied
has a conversion function to pointer to function with C++ language linkage
having the same parameter and return types as the closure type’s function call operator<ins>,
except that if the function call operator has an explicit object parameter of type <code class="highlight"><c- n>T</c-></code>, then
the function type has a leading parameter of type <code class="highlight"><c- n>T</c-></code></ins>.
The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification.
If the function call operator is a static member function <ins>or explicit object member function</ins>, then the value returned by this conversion function is the address
of the function call operator. Otherwise, the value returned by this conversion function is the address of a function <code class="highlight"><c- n>F</c-></code> that,
when invoked, has the same effect as invoking the closure type’s function call operator on a default-constructed instance
of the closure type. <code class="highlight"><c- n>F</c-></code> is a constexpr function if the function call operator is a constexpr function
and an immediate function if the function call operator is an immediate function. </p><p>10․ For a generic lambda with no <em>lambda-capture</em>,
the closure type has a conversion function template to pointer to function.
The conversion function template has the same invented template parameter list, and the pointer to function has the same parameter types,
as the function call operator template<ins>, except that if the function call operator template has an explicit object parameter of type <code class="highlight"><c- n>T</c-></code>, then 
the function type has a leading parameter of type <code class="highlight"><c- n>T</c-></code></ins>. The return type of the pointer to function shall behave as if it were
a <em>decltype-specifier</em> denoting the return type of the corresponding function call operator template specialization. </p><p>11․ [<i>Note 4:</i> If the generic lambda has no <em>trailing-return-type</em> or the <em>trailing-return-type</em> contains a placeholder type,
return type deduction of the corresponding function call operator template specialization has to be done.
The corresponding specialization is that instantiation of the function call operator template with the same template arguments
as those deduced for the conversion function template. Consider the following: </p><pre class="highlight"><c- k>auto</c-> <c- n>glambda</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>fp</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>glambda</c-><c- p>;</c->
</pre><p>The behavior of the conversion function of <code class="highlight"><c- n>glambda</c-></code> above is like that of the following conversion function:</p> <pre class="highlight"><c- k>struct</c-> <c- nc>Closure</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>auto</c-> <c- k>operator</c-><c- p>()(</c-><c- n>T</c-> <c- n>t</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>static</c-> <c- k>auto</c-> <c- n>lambda_call_operator_invoker</c-><c- p>(</c-><c- n>T</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c->
    <c- c1>// forwards execution to operator()(a) and therefore has</c->
    <c- c1>// the same return type deduced</c->
    <c- d>/* ... */</c->
  <c- p>}</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>using</c-> <c- n>fptr_t</c-> <c- o>=</c->
     <c- k>decltype</c-><c- p>(</c-><c- n>lambda_call_operator_invoker</c-><c- p>(</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>()))</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>T</c-><c- p>);</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>operator</c-> <c- n>fptr_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>()</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- o>&amp;</c-><c- n>lambda_call_operator_invoker</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->
</pre><i>— end note</i>]<p>[<i>Example 6:</i></p><pre class="highlight"><c- b>void</c-> <c- nf>f1</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->   <c- p>{</c-> <c- p>}</c->
<c- b>void</c-> <c- nf>f2</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->  <c- p>{</c-> <c- p>}</c-><br>
<c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->    <c- p>{</c-> <c- p>}</c->     <c- c1>// #1</c->
<c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>char</c-><c- p>))</c->  <c- p>{</c-> <c- p>}</c->     <c- c1>// #2</c-><br>
<c- b>void</c-> <c- nf>h</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->    <c- p>{</c-> <c- p>}</c->     <c- c1>// #3</c->
<c- b>void</c-> <c- nf>h</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->   <c- p>{</c-> <c- p>}</c->     <c- c1>// #4</c-><br>
<c- k>auto</c-> <c- n>glambda</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- n>f1</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                    <c- c1>// OK</c->
<c- n>f2</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                    <c- c1>// error: ID is not convertible</c->
<c- n>g</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                     <c- c1>// error: ambiguous</c->
<c- n>h</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                     <c- c1>// OK, calls #3 since it is convertible from ID</c->
<c- b>int</c-><c- o>&amp;</c-> <c- p>(</c-><c- o>*</c-><c- n>fpi</c-><c- p>)(</c-><c- b>int</c-><c- o>*</c-><c- p>)</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-><c- o>*</c-> <c- n>a</c-><c- p>)</c-> <c- o>-></c-> <c- k>auto</c-><c- o>&amp;</c-> <c- p>{</c-> <c- k>return</c-> <c- o>*</c-><c- n>a</c-><c- p>;</c-> <c- p>};</c->        <c- c1>// OK</c->
</pre><i>— end example</i>] <p>12․ If the function call operator template is a static member function template <ins>or explicit object member function</ins>, then the value returned
by any given specialization of this conversion function template is the address of the corresponding function call operator template specialization.
Otherwise, the value returned by any given specialization of this conversion function template is
the address of a function <code class="highlight"><c- n>F</c-></code> that, when invoked, has the same effect as invoking the generic lambda’s corresponding function call
operator template specialization on a default-constructed instance of the closure type. <code class="highlight"><c- n>F</c-></code> is a constexpr function if the corresponding specialization is a constexpr function
and an immediate function if the function call operator template specialization is an immediate function. </p><p>[<i>Note 5:</i> This will result in the implicit instantiation of the generic lambda’s body.
The instantiated generic lambda’s return type and parameter types are required to match the return type and parameter types of the pointer to function. <i>— end note</i>] </p><p>[<i>Example 7:</i><br> </p><pre class="highlight"><c- k>auto</c-> <c- n>GL</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>a</c-><c- p>;</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>GL_int</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>GL</c-><c- p>;</c->        <c- c1>// OK, through conversion function template</c->
<c- n>GL_int</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->                      <c- c1>// OK, same as GL(3)</c->
</pre><i>— end example</i>]<p>13․ The conversion function or conversion function template is public, constexpr, non-virtual, non-explicit, const, and has a non-throwing exception specification. </p><p>[<i>Example 8:</i><br></p><pre class="highlight"><c- k>auto</c-> <c- n>Fwd</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>fp</c-><c- p>)(</c-><c- b>int</c-><c- p>),</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>fp</c-><c- p>(</c-><c- n>a</c-><c- p>);</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>C</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c-><br>
<c- k>static_assert</c-><c- p>(</c-><c- n>Fwd</c-><c- p>(</c-><c- n>C</c-><c- p>,</c-><c- mi>3</c-><c- p>)</c-> <c- o>==</c-> <c- mi>3</c-><c- p>);</c->   <c- c1>// OK</c-><br>
<c- c1>// No specialization of the function call operator template can be constexpr (due to the local static).</c->
<c- k>auto</c-> <c- n>NC</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>static</c-> <c- b>int</c-> <c- n>s</c-><c- p>;</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>Fwd</c-><c- p>(</c-><c- n>NC</c-><c- p>,</c-><c- mi>3</c-><c- p>)</c-> <c- o>==</c-> <c- mi>3</c-><c- p>);</c->  <c- c1>// error</c->
</pre><i>— end example</i>] </small>
   </blockquote>
   <h3 class="heading settled" data-level="4.3" id="wording-msvc"><span class="secno">4.3. </span><span class="content">MSVC’s approach</span><a class="self-link" href="#wording-msvc"></a></h3>
   <p class="note" role="note"><span class="marker">Note:</span> This proposed wording doesn’t explain why <code class="highlight"><c- p>[](</c-><c- k>this</c-> <c- k>auto</c-><c- p>){}()</c-></code> should prefer to call the user-defined <code class="highlight"><c- k>operator</c-><c- p>()</c-></code> instead of using the builtin operator on the result of the lambda’s non-template conversion function.
I’m hoping someone at Microsoft can shed light on how MSVC tiebreaks this internally.</p>
   <p>Modify <a href="https://eel.is/c++draft/expr.prim.lambda#general-6">[expr.prim.lambda.general]/6</a> as follows:</p>
   <p><small></small></p>
   <blockquote><small> 6․ A lambda is a <i>generic lambda</i> if the <i>lambda-expression</i> has any generic <ins>non-object</ins> parameter type placeholders ([dcl.spec.auto]),
or if the lambda has a <i>template-parameter-list</i>.<br> [<i>Example 4:</i><br><p></p> <pre class="highlight"><del><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>}(</c-><c- mi>3</c-><c- p>,</c-> <c- mi>4</c-><c- p>);</c->                  <c- c1>// OK, a generic lambda</c-><br><c- c1>int j = []&lt;class T>(T t, int i) { return i; }(3, 4);            // OK, a generic lambda</c-></del>
<ins><c- k>auto</c-> <c- n>w</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- n>i</c-><c- p>,</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                          <c- c1>// OK, a generic lambda</c-></ins>
<ins><c- k>auto</c-> <c- n>x</c-> <c- o>=</c-> <c- p>[](</c-><c- k>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>,</c-> <c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                  <c- c1>// OK, a non-generic lambda</c-></ins>
<ins><c- k>auto</c-> <c- n>y</c-> <c- o>=</c-> <c- p>[](</c-><c- k>this</c-> <c- k>auto</c-> <c- n>self</c-><c- p>,</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                 <c- c1>// OK, a generic lambda</c-></ins>
<ins><c- k>auto</c-> <c- n>z</c-> <c- o>=</c-> <c- p>[]</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>i</c-><c- p>;</c-> <c- p>};</c->                         <c- c1>// OK, a generic lambda</c-></ins>
</pre><i>— end example</i>]</small></blockquote>
   <p>Modify <a href="https://eel.is/c++draft/expr.prim.lambda#closure-4">[expr.prim.lambda.closure]/4</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>The closure type for a <em>lambda-expression</em> has a public inline function call operator <del>(for a non-generic lambda)</del> or function call operator template <del>(for a generic lambda)</del> ([over.call]) whose parameters and return type are those
of the <i>lambda-expression</i>’s <em>parameter-declaration-clause</em> and <em>trailing-return-type</em> respectively,
and whose <em>template-parameter-list</em> consists of the specified <em>template-parameter-list</em>, if any. The <em>requires-clause</em> of
the function call operator template is the <em>requires-clause</em> immediately following <code class="highlight"><c- o>&lt;</c-></code> <em>template-parameter-list</em> <code class="highlight"><c- o>></c-></code>, if any.
The trailing <em>requires-clause</em> of the function call operator or operator template is the <em>requires-clause</em> of the <em>lambda-declarator</em>, if any. </p><p>[<i>Note 2:</i> The function call operator template <del>for a generic lambda</del> can be an abbreviated function template ([dcl.fct]). <i>— end note</i>] </p></small>
   </blockquote>
   <p>Modify <a href="https://eel.is/c++draft/expr.prim.lambda#closure-9">[expr.prim.lambda.closure]/9</a> as follows:</p>
   <p><small></small></p>
   <blockquote>
    <p></p>
    <small> <p>9․ The closure type for a non-generic <em>lambda-expression</em> with no <em>lambda-capture</em> whose constraints (if any) are satisfied
has a conversion function to pointer to function with C++ language linkage
having the same parameter and return types as the closure type’s function call operator <ins>(omitting the object parameter, if any)</ins>.
The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification.
If the function call operator is a static member function, then the value returned by this conversion function is the address
of the function call operator. Otherwise, the value returned by this conversion function is the address of a function <code class="highlight"><c- n>F</c-></code> that,
when invoked, has the same effect as invoking the closure type’s function call operator on a default-constructed instance
of the closure type. <code class="highlight"><c- n>F</c-></code> is a constexpr function if the function call operator is a constexpr function
and an immediate function if the function call operator is an immediate function. </p><p>10․ For a generic lambda with no <em>lambda-capture</em>,
the closure type has a conversion function template to pointer to function.
The conversion function template has the same invented template parameter list <ins>as the function call operator template
(omitting the invented <i>template-parameter</i> corresponding to the function call operator’s explicit
object parameter, if any)</ins>, and the pointer to function has the same parameter types<del>,</del> as
the function call operator template <ins>(omitting the object parameter, if any)</ins>. The return type of the pointer to function shall behave as if it were
a <em>decltype-specifier</em> denoting the return type of the corresponding function call operator template specialization. </p><p>11․ [<i>Note 4:</i> If the generic lambda has no <em>trailing-return-type</em> or the <em>trailing-return-type</em> contains a placeholder type,
return type deduction of the corresponding function call operator template specialization has to be done.
The corresponding specialization is that instantiation of the function call operator template with the same template arguments
as those deduced for the conversion function template. Consider the following: </p><pre class="highlight"><c- k>auto</c-> <c- n>glambda</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>fp</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>glambda</c-><c- p>;</c->
</pre><p>The behavior of the conversion function of <code class="highlight"><c- n>glambda</c-></code> above is like that of the following conversion function:</p> <pre class="highlight"><c- k>struct</c-> <c- nc>Closure</c-> <c- p>{</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>auto</c-> <c- k>operator</c-><c- p>()(</c-><c- n>T</c-> <c- n>t</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>static</c-> <c- k>auto</c-> <c- n>lambda_call_operator_invoker</c-><c- p>(</c-><c- n>T</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c->
    <c- c1>// forwards execution to operator()(a) and therefore has</c->
    <c- c1>// the same return type deduced</c->
    <c- d>/* ... */</c->
  <c- p>}</c->
  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>using</c-> <c- n>fptr_t</c-> <c- o>=</c->
     <c- k>decltype</c-><c- p>(</c-><c- n>lambda_call_operator_invoker</c-><c- p>(</c-><c- n>declval</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>()))</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- n>T</c-><c- p>);</c->

  <c- k>template</c-><c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>operator</c-> <c- n>fptr_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>()</c-> <c- k>const</c->
    <c- p>{</c-> <c- k>return</c-> <c- o>&amp;</c-><c- n>lambda_call_operator_invoker</c-><c- p>;</c-> <c- p>}</c->
<c- p>};</c->
</pre><i>— end note</i>]<p>[<i>Example 6:</i></p><pre class="highlight"><c- b>void</c-> <c- nf>f1</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->   <c- p>{</c-> <c- p>}</c->
<c- b>void</c-> <c- nf>f2</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->  <c- p>{</c-> <c- p>}</c-><br>
<c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->    <c- p>{</c-> <c- p>}</c->     <c- c1>// #1</c->
<c- b>void</c-> <c- nf>g</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>char</c-><c- p>))</c->  <c- p>{</c-> <c- p>}</c->     <c- c1>// #2</c-><br>
<c- b>void</c-> <c- nf>h</c-><c- p>(</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->    <c- p>{</c-> <c- p>}</c->     <c- c1>// #3</c->
<c- b>void</c-> <c- nf>h</c-><c- p>(</c-><c- b>char</c-> <c- p>(</c-><c- o>*</c-><c- p>)(</c-><c- b>int</c-><c- p>))</c->   <c- p>{</c-> <c- p>}</c->     <c- c1>// #4</c-><br>
<c- k>auto</c-> <c- n>glambda</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- n>f1</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                    <c- c1>// OK</c->
<c- n>f2</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                    <c- c1>// error: ID is not convertible</c->
<c- n>g</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                     <c- c1>// error: ambiguous</c->
<c- n>h</c-><c- p>(</c-><c- n>glambda</c-><c- p>);</c->                     <c- c1>// OK, calls #3 since it is convertible from ID</c->
<c- b>int</c-><c- o>&amp;</c-> <c- p>(</c-><c- o>*</c-><c- n>fpi</c-><c- p>)(</c-><c- b>int</c-><c- o>*</c-><c- p>)</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-><c- o>*</c-> <c- n>a</c-><c- p>)</c-> <c- o>-></c-> <c- k>auto</c-><c- o>&amp;</c-> <c- p>{</c-> <c- k>return</c-> <c- o>*</c-><c- n>a</c-><c- p>;</c-> <c- p>};</c->        <c- c1>// OK</c->
</pre><i>— end example</i>] <p>12․ If the function call operator template is a static member function template, then the value returned
by any given specialization of this conversion function template is the address of the corresponding function call operator template specialization.
Otherwise, the value returned by any given specialization of this conversion function template is
the address of a function <code class="highlight"><c- n>F</c-></code> that, when invoked, has the same effect as invoking the generic lambda’s corresponding function call
operator template specialization on a default-constructed instance of the closure type. <code class="highlight"><c- n>F</c-></code> is a constexpr function if the corresponding specialization is a constexpr function
and an immediate function if the function call operator template specialization is an immediate function. </p><p>[<i>Note 5:</i> This will result in the implicit instantiation of the generic lambda’s body.
The instantiated generic lambda’s return type and <ins>non-object</ins> parameter types are required to match the return type and parameter types of the pointer to function. <i>— end note</i>] </p><p>[<i>Example 7:</i><br> </p><pre class="highlight"><c- k>auto</c-> <c- n>GL</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- n>a</c-><c- p>;</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>GL_int</c-><c- p>)(</c-><c- b>int</c-><c- p>)</c-> <c- o>=</c-> <c- n>GL</c-><c- p>;</c->        <c- c1>// OK, through conversion function template</c->
<c- n>GL_int</c-><c- p>(</c-><c- mi>3</c-><c- p>);</c->                      <c- c1>// OK, same as GL(3)</c->
</pre><i>— end example</i>]<p>13․ The conversion function or conversion function template is public, constexpr, non-virtual, non-explicit, const, and has a non-throwing exception specification. </p><p>[<i>Example 8:</i><br></p><pre class="highlight"><c- k>auto</c-> <c- n>Fwd</c-> <c- o>=</c-> <c- p>[](</c-><c- b>int</c-> <c- p>(</c-><c- o>*</c-><c- n>fp</c-><c- p>)(</c-><c- b>int</c-><c- p>),</c-> <c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>fp</c-><c- p>(</c-><c- n>a</c-><c- p>);</c-> <c- p>};</c->
<c- k>auto</c-> <c- n>C</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c-><br>
<c- k>static_assert</c-><c- p>(</c-><c- n>Fwd</c-><c- p>(</c-><c- n>C</c-><c- p>,</c-><c- mi>3</c-><c- p>)</c-> <c- o>==</c-> <c- mi>3</c-><c- p>);</c->   <c- c1>// OK</c-><br>
<c- c1>// No specialization of the function call operator template can be constexpr (due to the local static).</c->
<c- k>auto</c-> <c- n>NC</c-> <c- o>=</c-> <c- p>[](</c-><c- k>auto</c-> <c- n>a</c-><c- p>)</c-> <c- p>{</c-> <c- k>static</c-> <c- b>int</c-> <c- n>s</c-><c- p>;</c-> <c- k>return</c-> <c- n>a</c-><c- p>;</c-> <c- p>};</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>Fwd</c-><c- p>(</c-><c- n>NC</c-><c- p>,</c-><c- mi>3</c-><c- p>)</c-> <c- o>==</c-> <c- mi>3</c-><c- p>);</c->  <c- c1>// error</c->
</pre><i>— end example</i>] </small>
   </blockquote>
   <h2 class="heading settled" data-level="5" id="straw-polls"><span class="secno">5. </span><span class="content">Straw polls taken in EWG, 2023-11-07</span><a class="self-link" href="#straw-polls"></a></h2>
   <table class="def">
    <tbody>
     <tr>
      <th style="width: 70%;">
      <th><strong>SF</strong>
      <th><strong>F</strong>
      <th><strong>N</strong>
      <th><strong>A</strong>
      <th><strong>SA</strong>
     <tr>
      <th><small>Move forward with the option to "punt" from D3031, forwarding to CWG for C++26.<br>A subsequent paper can explore other solutions.</small> 
      <th> 9 
      <th> 11 
      <th> 3 
      <th> 0 
      <th> 0 
   </table>
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