<html>
<head>
	<title>Pack expansion in lambda init-capture</title>

<style type="text/css">html {
	position: relative;
	max-width: 1024px;
	height: 100%;
}
body {
	font-family: Helvetica, arial, sans-serif;
	font-size: 14px;
	line-height: 1.6;
	padding-top: 10px;
	padding-bottom: 10px;
	background-color: white;
	padding: 30px;
}
body>*:first-child {
	margin-top: 0 !important;
}
body>*:last-child {
	margin-bottom: 0 !important;
}
a {
	color: #4183C4;
}
a.absent {
	color: #cc0000;
}
a.anchor {
	display: block;
	padding-left: 30px;
	margin-left: -30px;
	cursor: pointer;
	position: absolute;
	top: 0;
	left: 0;
	bottom: 0;
}
h1, h2, h3, h4, h5, h6 {
	margin: 20px 0 10px;
	padding: 0;
	font-weight: bold;
	-webkit-font-smoothing: antialiased;
	cursor: text;
	position: relative;
}
h1:hover a.anchor, h2:hover a.anchor, h3:hover a.anchor, h4:hover a.anchor, h5:hover a.anchor, h6:hover a.anchor {
	background: url(data:image/png;base64,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) no-repeat 10px center;
	text-decoration: none;
}
h1 tt, h1 code {
	font-size: inherit;
}
h2 tt, h2 code {
	font-size: inherit;
}
h3 tt, h3 code {
	font-size: inherit;
}
h4 tt, h4 code {
	font-size: inherit;
}
h5 tt, h5 code {
	font-size: inherit;
}
h6 tt, h6 code {
	font-size: inherit;
}
h1 {
	font-size: 28px;
	color: black;
}
h2 {
	font-size: 24px;
	border-bottom: 1px solid #cccccc;
	color: black;
}
h3 {
	font-size: 18px;
}
h4 {
	font-size: 16px;
}
h5 {
	font-size: 14px;
}
h6 {
	color: #777777;
	font-size: 14px;
}
p, blockquote, ol, dl, li, table, pre {
	margin: 15px 0;
}
hr {
	background: transparent url(data:image/png;base64,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) repeat-x 0 0;
	border: 0 none;
	color: #cccccc;
	height: 4px;
	padding: 0;
}
body>h2:first-child {
	margin-top: 0;
	padding-top: 0;
}
body>h1:first-child {
	margin-top: 0;
	padding-top: 0;
}
body>h1:first-child+h2 {
	margin-top: 0;
	padding-top: 0;
}
body>h3:first-child, body>h4:first-child, body>h5:first-child, body>h6:first-child {
	margin-top: 0;
	padding-top: 0;
}
a:first-child h1, a:first-child h2, a:first-child h3, a:first-child h4, a:first-child h5, a:first-child h6 {
	margin-top: 0;
	padding-top: 0;
}
h1 p, h2 p, h3 p, h4 p, h5 p, h6 p {
	margin-top: 0;
}
li p.first {
	display: inline-block;
}
li {
	margin: 0;
}
ol {
	padding-left: 30px;
}
ul :first-child, ol :first-child {
	margin-top: 0;
}
dl {
	padding: 0;
}
dl dt {
	font-size: 14px;
	font-weight: bold;
	font-style: italic;
	padding: 0;
	margin: 15px 0 5px;
}
dl dt:first-child {
	padding: 0;
}
dl dt> :first-child {
	margin-top: 0;
}
dl dt> :last-child {
	margin-bottom: 0;
}
dl dd {
	margin: 0 0 15px;
	padding: 0 15px;
}
dl dd> :first-child {
	margin-top: 0;
}
dl dd> :last-child {
	margin-bottom: 0;
}
blockquote {
	border-left: 4px solid #dddddd;
	padding: 0 15px;
	color: #777777;
}
blockquote> :first-child {
	margin-top: 0;
}
blockquote> :last-child {
	margin-bottom: 0;
}
table {
	padding: 0;
	border-collapse: collapse;
}
table tr {
	border-top: 1px solid #cccccc;
	background-color: white;
	margin: 0;
	padding: 0;
}
table tr:nth-child(2n) {
	background-color: #f8f8f8;
}
table tr th {
	font-weight: bold;
	border: 1px solid #cccccc;
	margin: 0;
	padding: 6px 13px;
}
table tr td {
	border: 1px solid #cccccc;
	margin: 0;
	padding: 6px 13px;
}
table tr th :first-child, table tr td :first-child {
	margin-top: 0;
}
table tr th :last-child, table tr td :last-child {
	margin-bottom: 0;
}
td {
	vertical-align: top;
}
img {
	max-width: 100%;
}
span.frame {
	display: block;
	overflow: hidden;
}
span.frame>span {
	border: 1px solid #dddddd;
	display: block;
	float: left;
	overflow: hidden;
	margin: 13px 0 0;
	padding: 7px;
	width: auto;
}
span.frame span img {
	display: block;
	float: left;
}
span.frame span span {
	clear: both;
	color: #333333;
	display: block;
	padding: 5px 0 0;
}
span.align-center {
	display: block;
	overflow: hidden;
	clear: both;
}
span.align-center>span {
	display: block;
	overflow: hidden;
	margin: 13px auto 0;
	text-align: center;
}
span.align-center span img {
	margin: 0 auto;
	text-align: center;
}
span.align-right {
	display: block;
	overflow: hidden;
	clear: both;
}
span.align-right>span {
	display: block;
	overflow: hidden;
	margin: 13px 0 0;
	text-align: right;
}
span.align-right span img {
	margin: 0;
	text-align: right;
}
span.float-left {
	display: block;
	margin-right: 13px;
	overflow: hidden;
	float: left;
}
span.float-left span {
	margin: 13px 0 0;
}
span.float-right {
	display: block;
	margin-left: 13px;
	overflow: hidden;
	float: right;
}
span.float-right>span {
	display: block;
	overflow: hidden;
	margin: 13px auto 0;
	text-align: right;
}
code, tt {
	margin: 0 2px;
	padding: 0 5px;
	white-space: nowrap;
	border: 1px solid #eaeaea;
	background-color: #f8f8f8;
	border-radius: 3px;
}
pre code {
	margin: 0;
	padding: 0;
	white-space: pre;
	border: none;
	background: transparent;
}
.highlight pre {
	background-color: #f8f8f8;
	border: 1px solid #cccccc;
	font-size: 13px;
	line-height: 19px;
	overflow: auto;
	padding: 6px 10px;
	border-radius: 3px;
}
pre {
	background-color: #f8f8f8;
	border: 1px solid #cccccc;
	font-size: 13px;
	line-height: 19px;
	overflow: auto;
	padding: 6px 10px;
	border-radius: 3px;
}
pre code, pre tt {
	background-color: transparent;
	border: none;
}
sup {
	font-size: 0.83em;
	vertical-align: super;
	line-height: 0;
}
kbd {
	display: inline-block;
	padding: 3px 5px;
	font-size: 11px;
	line-height: 10px;
	color: #555;
	vertical-align: middle;
	background-color: #fcfcfc;
	border: solid 1px #ccc;
	border-bottom-color: #bbb;
	border-radius: 3px;
	box-shadow: inset 0 -1px 0 #bbb
}
* {
	-webkit-print-color-adjust: exact;
}
ins {
	color: #00A000
}
del {
	color: #A00000
}
</style><style type="text/css">
/**
	* prism.js default theme for JavaScript, CSS and HTML
	* Based on dabblet (http://dabblet.com)
	* @author Lea Verou
	*/

code[class*="language-"], pre[class*="language-"] {
	color: black;
	background: none;
	text-shadow: 0 1px white;
	font-family: Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace;
	font-size: 11px;
	text-align: left;
	white-space: pre;
	word-spacing: normal;
	word-break: normal;
	word-wrap: normal;
	line-height: 1.5;
	-moz-tab-size: 4;
	-o-tab-size: 4;
	tab-size: 4;
	-webkit-hyphens: none;
	-moz-hyphens: none;
	-ms-hyphens: none;
	hyphens: none;
}
pre[class*="language-"]::-moz-selection, pre[class*="language-"] ::-moz-selection, code[class*="language-"]::-moz-selection, code[class*="language-"] ::-moz-selection {
	text-shadow: none;
	background: #b3d4fc;
}
pre[class*="language-"]::selection, pre[class*="language-"] ::selection, code[class*="language-"]::selection, code[class*="language-"] ::selection {
	text-shadow: none;
	background: #b3d4fc;
}
@media print {
	code[class*="language-"], pre[class*="language-"] {
		text-shadow: none;
	}
}

/* Code blocks */

pre[class*="language-"] {
	padding: 1em;
	margin: .5em 0;
	overflow: auto;
}
:not(pre)>code[class*="language-"], pre[class*="language-"] {
	background: #f8f8f8;
}

/* Inline code */

:not(pre)>code[class*="language-"] {
	padding: .1em;
	border-radius: .3em;
	white-space: normal;
}
.token.comment, .token.prolog, .token.doctype, .token.cdata {
	color: slategray;
}
.token.punctuation {
	color: #999;
}
.namespace {
	opacity: .7;
}
.token.property, .token.tag, .token.boolean, .token.number, .token.constant, .token.symbol, .token.deleted {
	color: #905;
}
.token.selector, .token.attr-name, .token.string, .token.char, .token.builtin, .token.inserted {
	color: #690;
}
.token.operator {
	color: #a67f59;
}
.token.entity, .token.url, .language-css .token.string, .style .token.string {
	color: #a67f59;
	background: hsla(0, 0%, 100%, .5);
}
.token.atrule, .token.attr-value, .token.keyword {
	color: #07a;
}
.token.function {
	color: #DD4A68;
}
.token.regex, .token.important, .token.variable {
	color: #e90;
}
.token.important, .token.bold {
	font-weight: bold;
}
.token.italic {
	font-style: italic;
}
.token.entity {
	cursor: help;
}
</style>

<script type="text/javascript">
var _self="undefined"!=typeof window?window:"undefined"!=typeof WorkerGlobalScope&&self instanceof WorkerGlobalScope?self:{},Prism=function(){var e=/\blang(?:uage)?-(\w+)\b/i,t=0,n=_self.Prism={util:{encode:function(e){return e instanceof a?new a(e.type,n.util.encode(e.content),e.alias):"Array"===n.util.type(e)?e.map(n.util.encode):e.replace(/&/g,"&amp;").replace(/</g,"&lt;").replace(/\u00a0/g," ")},type:function(e){return Object.prototype.toString.call(e).match(/\[object (\w+)\]/)[1]},objId:function(e){return e.__id||Object.defineProperty(e,"__id",{value:++t}),e.__id},clone:function(e){var t=n.util.type(e);switch(t){case"Object":var a={};for(var r in e)e.hasOwnProperty(r)&&(a[r]=n.util.clone(e[r]));return a;case"Array":return e.map&&e.map(function(e){return n.util.clone(e)})}return e}},languages:{extend:function(e,t){var a=n.util.clone(n.languages[e]);for(var r in t)a[r]=t[r];return a},insertBefore:function(e,t,a,r){r=r||n.languages;var l=r[e];if(2==arguments.length){a=arguments[1];for(var i in a)a.hasOwnProperty(i)&&(l[i]=a[i]);return l}var o={};for(var s in l)if(l.hasOwnProperty(s)){if(s==t)for(var i in a)a.hasOwnProperty(i)&&(o[i]=a[i]);o[s]=l[s]}return n.languages.DFS(n.languages,function(t,n){n===r[e]&&t!=e&&(this[t]=o)}),r[e]=o},DFS:function(e,t,a,r){r=r||{};for(var l in e)e.hasOwnProperty(l)&&(t.call(e,l,e[l],a||l),"Object"!==n.util.type(e[l])||r[n.util.objId(e[l])]?"Array"!==n.util.type(e[l])||r[n.util.objId(e[l])]||(r[n.util.objId(e[l])]=!0,n.languages.DFS(e[l],t,l,r)):(r[n.util.objId(e[l])]=!0,n.languages.DFS(e[l],t,null,r)))}},plugins:{},highlightAll:function(e,t){var a={callback:t,selector:'code[class*="language-"], [class*="language-"] code, code[class*="lang-"], [class*="lang-"] code'};n.hooks.run("before-highlightall",a);for(var r,l=a.elements||document.querySelectorAll(a.selector),i=0;r=l[i++];)n.highlightElement(r,e===!0,a.callback)},highlightElement:function(t,a,r){for(var l,i,o=t;o&&!e.test(o.className);)o=o.parentNode;o&&(l=(o.className.match(e)||[,""])[1],i=n.languages[l]),t.className=t.className.replace(e,"").replace(/\s+/g," ")+" language-"+l,o=t.parentNode,/pre/i.test(o.nodeName)&&(o.className=o.className.replace(e,"").replace(/\s+/g," ")+" language-"+l);var s=t.textContent,u={element:t,language:l,grammar:i,code:s};if(!s||!i)return n.hooks.run("complete",u),void 0;if(n.hooks.run("before-highlight",u),a&&_self.Worker){var c=new Worker(n.filename);c.onmessage=function(e){u.highlightedCode=e.data,n.hooks.run("before-insert",u),u.element.innerHTML=u.highlightedCode,r&&r.call(u.element),n.hooks.run("after-highlight",u),n.hooks.run("complete",u)},c.postMessage(JSON.stringify({language:u.language,code:u.code,immediateClose:!0}))}else u.highlightedCode=n.highlight(u.code,u.grammar,u.language),n.hooks.run("before-insert",u),u.element.innerHTML=u.highlightedCode,r&&r.call(t),n.hooks.run("after-highlight",u),n.hooks.run("complete",u)},highlight:function(e,t,r){var l=n.tokenize(e,t);return a.stringify(n.util.encode(l),r)},tokenize:function(e,t){var a=n.Token,r=[e],l=t.rest;if(l){for(var i in l)t[i]=l[i];delete t.rest}e:for(var i in t)if(t.hasOwnProperty(i)&&t[i]){var o=t[i];o="Array"===n.util.type(o)?o:[o];for(var s=0;s<o.length;++s){var u=o[s],c=u.inside,g=!!u.lookbehind,h=!!u.greedy,f=0,d=u.alias;u=u.pattern||u;for(var p=0;p<r.length;p++){var m=r[p];if(r.length>e.length)break e;if(!(m instanceof a)){u.lastIndex=0;var y=u.exec(m),v=1;if(!y&&h&&p!=r.length-1){var b=r[p+1].matchedStr||r[p+1],k=m+b;if(p<r.length-2&&(k+=r[p+2].matchedStr||r[p+2]),u.lastIndex=0,y=u.exec(k),!y)continue;var w=y.index+(g?y[1].length:0);if(w>=m.length)continue;var _=y.index+y[0].length,P=m.length+b.length;if(v=3,P>=_){if(r[p+1].greedy)continue;v=2,k=k.slice(0,P)}m=k}if(y){g&&(f=y[1].length);var w=y.index+f,y=y[0].slice(f),_=w+y.length,S=m.slice(0,w),O=m.slice(_),j=[p,v];S&&j.push(S);var A=new a(i,c?n.tokenize(y,c):y,d,y,h);j.push(A),O&&j.push(O),Array.prototype.splice.apply(r,j)}}}}}return r},hooks:{all:{},add:function(e,t){var a=n.hooks.all;a[e]=a[e]||[],a[e].push(t)},run:function(e,t){var a=n.hooks.all[e];if(a&&a.length)for(var r,l=0;r=a[l++];)r(t)}}},a=n.Token=function(e,t,n,a,r){this.type=e,this.content=t,this.alias=n,this.matchedStr=a||null,this.greedy=!!r};if(a.stringify=function(e,t,r){if("string"==typeof e)return e;if("Array"===n.util.type(e))return e.map(function(n){return a.stringify(n,t,e)}).join("");var l={type:e.type,content:a.stringify(e.content,t,r),tag:"span",classes:["token",e.type],attributes:{},language:t,parent:r};if("comment"==l.type&&(l.attributes.spellcheck="true"),e.alias){var i="Array"===n.util.type(e.alias)?e.alias:[e.alias];Array.prototype.push.apply(l.classes,i)}n.hooks.run("wrap",l);var o="";for(var s in l.attributes)o+=(o?" ":"")+s+'="'+(l.attributes[s]||"")+'"';return"<"+l.tag+' class="'+l.classes.join(" ")+'" '+o+">"+l.content+"</"+l.tag+">"},!_self.document)return _self.addEventListener?(_self.addEventListener("message",function(e){var t=JSON.parse(e.data),a=t.language,r=t.code,l=t.immediateClose;_self.postMessage(n.highlight(r,n.languages[a],a)),l&&_self.close()},!1),_self.Prism):_self.Prism;var r=document.currentScript||[].slice.call(document.getElementsByTagName("script")).pop();return r&&(n.filename=r.src,document.addEventListener&&!r.hasAttribute("data-manual")&&document.addEventListener("DOMContentLoaded",n.highlightAll)),_self.Prism}();"undefined"!=typeof module&&module.exports&&(module.exports=Prism),"undefined"!=typeof global&&(global.Prism=Prism);
</script>

<script type="text/javascript">
Prism.languages.clike={comment:[{pattern:/(^|[^\\])\/\*[\w\W]*?\*\//,lookbehind:!0},{pattern:/(^|[^\\:])\/\/.*/,lookbehind:!0}],string:{pattern:/(["'])(\\(?:\r\n|[\s\S])|(?!\1)[^\\\r\n])*\1/,greedy:!0},"class-name":{pattern:/((?:\b(?:class|interface|extends|implements|trait|instanceof|new)\s+)|(?:catch\s+\())[a-z0-9_\.\\]+/i,lookbehind:!0,inside:{punctuation:/(\.|\\)/}},keyword:/\b(if|else|while|do|for|return|in|instanceof|function|new|try|throw|catch|finally|null|break|continue)\b/,"boolean":/\b(true|false)\b/,"function":/[a-z0-9_]+(?=\()/i,number:/\b-?(?:0x[\da-f]+|\d*\.?\d+(?:e[+-]?\d+)?)\b/i,operator:/--?|\+\+?|!=?=?|<=?|>=?|==?=?|&&?|\|\|?|\?|\*|\/|~|\^|%/,punctuation:/[{}[\];(),.:]/};
</script>

<script type="text/javascript">
Prism.languages.c=Prism.languages.extend("clike",{keyword:/\b(asm|typeof|inline|auto|break|case|char|const|continue|default|do|double|else|enum|extern|float|for|goto|if|int|long|register|return|short|signed|sizeof|static|struct|switch|typedef|union|unsigned|void|volatile|while)\b/,operator:/\-[>-]?|\+\+?|!=?|<<?=?|>>?=?|==?|&&?|\|?\||[~^%?*\/]/,number:/\b-?(?:0x[\da-f]+|\d*\.?\d+(?:e[+-]?\d+)?)[ful]*\b/i}),Prism.languages.insertBefore("c","string",{macro:{pattern:/(^\s*)#\s*[a-z]+([^\r\n\\]|\\.|\\(?:\r\n?|\n))*/im,lookbehind:!0,alias:"property",inside:{string:{pattern:/(#\s*include\s*)(<.+?>|("|')(\\?.)+?\3)/,lookbehind:!0},directive:{pattern:/(#\s*)\b(define|elif|else|endif|error|ifdef|ifndef|if|import|include|line|pragma|undef|using)\b/,lookbehind:!0,alias:"keyword"}}},constant:/\b(__FILE__|__LINE__|__DATE__|__TIME__|__TIMESTAMP__|__func__|EOF|NULL|stdin|stdout|stderr)\b/}),delete Prism.languages.c["class-name"],delete Prism.languages.c["boolean"];
</script>

<script type="text/javascript">
Prism.languages.cpp=Prism.languages.extend("c",{keyword:/\b(alignas|alignof|asm|auto|bool|break|case|catch|char|char16_t|char32_t|class|compl|const|constexpr|const_cast|continue|decltype|default|delete|do|double|dynamic_cast|else|enum|explicit|export|extern|float|for|friend|goto|if|inline|int|long|mutable|namespace|new|noexcept|nullptr|operator|private|protected|public|register|reinterpret_cast|return|short|signed|sizeof|static|static_assert|static_cast|struct|switch|template|this|thread_local|throw|try|typedef|typeid|typename|union|unsigned|using|virtual|void|volatile|wchar_t|while)\b/,"boolean":/\b(true|false)\b/,operator:/[-+]{1,2}|!=?|<{1,2}=?|>{1,2}=?|\->|:{1,2}|={1,2}|\^|~|%|&{1,2}|\|?\||\?|\*|\/|\b(and|and_eq|bitand|bitor|not|not_eq|or|or_eq|xor|xor_eq)\b/}),Prism.languages.insertBefore("cpp","keyword",{"class-name":{pattern:/(class\s+)[a-z0-9_]+/i,lookbehind:!0}});
</script>
</head>
<body>

<address align=right>
Document number: P0780R0 <br />
Date: 2017-10-08 <br />
Audience: Evolution Working Group <br />
Reply-To: Barry Revzin &lt;barry.revzin@gmail.com>
</address>
<hr/>
<h1 align=center>Allow pack expansion in lambda init-capture</h1>

<h2>Contents</h2>

<ul>
<li><a href="#Motivation">Motivation</a></li>
<li><a href="#Restriction">Init-capture restriction history</a></li>
<li><a href="#Proposal">Proposal</a></li>
<li><a href="#Acks">Acknowledgements</a></li>
<li><a href="#References">References</a></li>
</ul>

<a name="Motivation"> </a><h2>Motivation</h2>

<p>With the introduction of generalized lambda capture [1], lambda captures can be nearly arbitrarily complex and solve nearly all problems. However, there is still an awkward hole in the capabilities of lambda capture when it comes to parameter packs: you can only capture packs by copy, by reference, or by... <code class="language-cpp">std::tuple</code>?

<p>Consider the simple example of trying to wrap a function and its arguments into a callable to be accessed later. If we copy everything, the implementation is both easy to write and read:

<pre><code class="language-cpp">template&lt;class F, class... Args>
auto delay_invoke(F f, Args... args) {
    // the capture here can also be just [=]
    return [f, args...]() -> decltype(auto) {
        return std::invoke(f, args...);
    };
}</code></pre>

<p>But if we try to be more efficient about the implementation and try to <code class="language-cpp">move</code> all the arguments into the lambda? It seems like you should be able to use an <i>init-capture</i> and write:

<pre><code class="language-cpp">template&lt;class F, class... Args>
auto delay_invoke(F f, Args... args) {
    return [f=std::move(f), <font color="#0000A0">args=std::move(args)...</font>]() -> decltype(auto) {
        return std::invoke(f, args...);
    };
}</code></pre>

<p>But this runs afoul of very explicit wording from <a href="http://eel.is/c++draft/expr.prim.lambda#capture-17.sentence-2">[expr.prim.lambda.capture]/17</a>, emphasis mine:

<blockquote class="std">A <i>simple-capture</i> followed by an ellipsis is a pack expansion. <b>An <i>init-capture</i> followed by an ellipsis is ill-formed.</b>
</blockquote>

<p>As a result of this restriction, our only option is to put all the <code class="language-cpp">args...</code> into a <code class="language-cpp">std::tuple</code>. But once we do that, we don't have access to the arguments as a parameter pack, so we need to pull them back out of the tuple in the body, using something like <code class="language-cpp">std::apply()</code>:

<pre><code class="language-cpp">template&lt;class F, class... Args>
auto delay_invoke(F f, Args... args) {
    return [f=std::move(f), tup=std::make_tuple(std::move(args)...)]() -> decltype(auto) {
        return std::apply(f, tup);
    };
}</code></pre>

<p>Which gets even worse if what we wanted to do with that captured parameter pack was invoke a <i>named</i> function rather than a captured object. At that point, all semblance of comprehension goes out the window:

<table style="width:100%">
<tr><th style="width:50%">By copy</th><th style="width:50%">By move</th></tr>
<tr><td><pre style="background:transparent;border:0px"><code class="language-cpp">template &lt;class... Args>
auto delay_invoke_foo(Args... args) {
    return [args...]() -> decltype(auto) {
    
        return foo(args...);
        
    };
}</code></pre></td>
<td><pre style="background:transparent;border:0px"><code class="language-cpp">template &lt;class... Args>
auto delay_invoke_foo(Args... args) {
    return [tup=std::make_tuple(std::move(args)...)]() -> decltype(auto) {
        return std::apply([](auto const&... args) -> decltype(auto) {
            return foo(args...);
        }, tup);
    };
}</code></pre></td></tr>
</table>

We can do better.

<a name="Restriction"></a><h2>Init-capture restriction history</h2>

The explicit restriction on pack expansion in <i>init-capture</i> was in the initial wording paper for generalized lambda capture [2], due in part to the rules proposed in that paper as to how <i>init-capture</i> would work. The original wording read:

<blockquote class="std">For every <i>init-capture</i> a non-static data member named by the <i>identifier</i> of the <i>init-capture</i> is declared in the closure type. This member is not a bit-field and not mutable. The type of that member corresponds to the type of a hypothetical variable declaration of the form "auto <i>init-capture</i> ;", except that the variable name (i.e., the <i>identifier</i> of the <i>init-capture</i>) is replaced by a unique identifier.</blockquote>

Which introduces a problem, as explained by Richard Smith in [3]:

<blockquote class="std">One problem here is that an init-capture introduces a *named* member of the closure type. A class member name that names a pack would be a new notion, and would bring with it significant additional complications (such as the inability to determine syntactically whether a construct contains an unexpanded parameter pack).

<p>[...] 

<p>Consider this: <pre style="background:transparent;border:none"><code class="language-cpp" style="width:95%">template &lt;typename T> void call_f(T t) {
    f(t.x...);
}</code></pre>

Right now, this is ill-formed (no diagnostic required) because "t.x" does not contain an unexpanded parameter pack. But if we allow class members to be pack expansions, this code could be valid -- we'd lose any syntactic mechanism to determine whether an expression contains an unexpanded pack. This is fatal to at least one implementation strategy for variadic templates. It also admits the possibility of pack expansions occurring outside templates, which current implementations are not well-suited to handle.

<p>Since init-captures add named members to the closure type, allowing init-captures to be pack expansions risks introducing the same problem if those names are visible in *any* context outside the body of the lambda-expression itself.</blockquote>

However, this problem went away with the adoption of CWG 1760 [4], which changed the wording from <i>init-capture</i> introducing a <i>named</i> member (of unspecified access) to introducing an <i>unnamed</i> member. Once <i>init-capture</i> doesn't give us named members, the problem that was pointed out in [3] is no longer a problem. There would be no named pack member to give complications in parsing, so this proposal claims that this restriction is no longer necessary.

<a name="Proposal"></a><h2>Proposal</h2>

<p>The proposal is to simply remove the restriction on pack expansions in <i>init-capture</i> in [expr.prim.lambda.capture]/17:

<blockquote class="std">A <i>simple-capture</i> <ins>or <i>init-capture</i></ins> followed by an ellipsis is a pack expansion. <del>An <i>init-capture</i> followed by an ellipsis is ill-formed.</del> <i>[ Example: </i>
<pre><code class="codeblock" style="width:95%">template&lt;class... Args>
void f(Args... args) {
  auto lm = [&, args...] { return g(args...); };
  lm();
  
  <ins>auto lm2 = [xs=std::move(args)...] { return g(xs...); };
  lm2();</ins>
}</code></pre>
<i>- end example ]</i>
</blockquote>

<p>It is likely that some additional wording change would be necessary in [expr.prim.lambda.capture]/6.

<p>This would simplify all the code that currently relies on <code class="language-cpp">std::tuple</code> just to solve this problem, in a way that we are already used to seeing pack expansion:

<table style="width:100%">
<tr><th style="width:50%">C++17 today</th><th style="width:50%">This proposal</th></tr>
<tr><td><pre style="background:transparent;border:none"><code class="language-cpp">template &lt;class... Args>
auto delay_invoke_foo(Args... args) {
    return [tup=std::make_tuple(std::move(args)...)]() -> decltype(auto) {
        return std::apply([](auto const&... args) -> decltype(auto) {
            return foo(args...);
        }, tup);
    };
}</code></pre></td><td><pre style="background:transparent;border:none"><code class="language-cpp">template &lt;class... Args>
auto delay_invoke_foo(Args... args) {
    return [args=std::move(args)...]() -> decltype(auto) {
    
        return foo(args...);
        
    };
}</code></pre></td></tr>
</table>

<a name="Acks"></a><h2>Acknowledgements</h2>

Thanks to T.C. for suggesting this solution and pointing out the history of the <i>init-capture</i> restriction.

<p>Thanks to Richard Smith, John Spicer, and Daveed Vandevoorde for considering the viability of this change.

<a name="References"></a><h2>References</h2>

<p> [1] <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3610.html">N3610</a>: "Generic lambda-capture initializers, supporting capture-by-move"
<p> [2] <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3648.html">N3648</a>: "Wording Changes for Generalized Lambda-capture"
<p> [3] <a href="https://groups.google.com/a/isocpp.org/d/msg/std-discussion/ePRzn4K7VcM/q734vAlL-moJ">A problem with generalized captures and pack expansion</a>
<p> [4] <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1760">CWG 1760</a>: Access of member corresponding to <i>init-capture</i>


</body>
</html>