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<title>Issue 2974: Diagnose out of bounds tuple_element/variant_alternative</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++20">C++20</a> status.</em></p>
<h3 id="2974"><a href="lwg-defects.html#2974">2974</a>. Diagnose out of bounds <code>tuple_element</code>/<code>variant_alternative</code></h3>
<p><b>Section:</b> 22.3.4 <a href="https://wg21.link/pair.astuple">[pair.astuple]</a>, 22.6.4 <a href="https://wg21.link/variant.helper">[variant.helper]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Agust&iacute;n K-ballo Berg&eacute; <b>Opened:</b> 2017-06-10 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#pair.astuple">issues</a> in [pair.astuple].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Instantiating <code>tuple_element</code> with an out-of-bounds index requires a diagnostic for <code>tuple</code> and <code>array</code>, 
but not for <code>pair</code>. The specification requires an out-of-bounds index for <code>pair</code> to go to the base template 
instead, which is undefined.
<p/>
Similarly, instantiating <code>variant_alternative</code> with an out-of-bounds index violates a requirement, but it is not 
required to be diagnosed.
</p>

<p><i>[2017-06-12, Moved to Tentatively Ready after 5 positive votes on c++std-lib]</i></p>



<p id="res-2974"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.</p>

<ol>
<li>
<p>Modify 22.2.1 <a href="https://wg21.link/utility.syn">[utility.syn]</a>, header <code>&lt;utility&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]

<i>// 22.3.4 <a href="https://wg21.link/pair.astuple">[pair.astuple]</a>, tuple-like access to pair</i>
template &lt;class T&gt; class tuple_size;
template &lt;size_t I, class T&gt; class tuple_element;
template &lt;class T1, class T2&gt; struct tuple_size&lt;pair&lt;T1, T2&gt;&gt;;
<ins>template &lt;size_t I, class T1, class T2&gt; struct tuple_element&lt;I, pair&lt;T1, T2&gt;&gt;;</ins>
<del>template &lt;class T1, class T2&gt; struct tuple_element&lt;0, pair&lt;T1, T2&gt;&gt;;
template &lt;class T1, class T2&gt; struct tuple_element&lt;1, pair&lt;T1, T2&gt;&gt;;</del>

[&hellip;]
</pre>
</blockquote>
</li>

<li>
<p>Modify 22.3.4 <a href="https://wg21.link/pair.astuple">[pair.astuple]</a> as indicated:</p>

<blockquote>
<pre>
<del>tuple_element&lt;0, pair&lt;T1, T2&gt;&gt;::type</del>
</pre>
<blockquote>
<p>
<del>-1- <i>Value:</i> The type <code>T1</code>.</del>
</p>
</blockquote>
<pre>
<del>tuple_element&lt;1, pair&lt;T1, T2&gt;&gt;::type</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Value:</i> The type <code>T2</code>.</del>
</p>
</blockquote>
<pre>
<ins>tuple_element&lt;I, pair&lt;T1, T2&gt;&gt;::type</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> <code>I &lt; 2</code>. The program is ill-formed if <code>I</code> is out of bounds.</ins>
<p/>
<ins>-?- <i>Value:</i> The type <code>T1</code> if <code>I == 0</code>, otherwise the type <code>T2</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li>
<p>Modify 22.6.4 <a href="https://wg21.link/variant.helper">[variant.helper]</a> as indicated:</p>

<blockquote>
<pre>
variant_alternative&lt;I, variant&lt;Types...&gt;&gt;::type
</pre>
<blockquote>
<p>
-4- <i>Requires:</i> <code>I &lt; sizeof...(Types)</code>. <ins>The program is ill-formed if <code>I</code> is out of bounds.</ins>
<p/>
-5- <i>Value:</i> The type <code>T<sub><i>I</i></sub></code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>






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