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<title>Issue 3215: variant default constructor has vague constexpr requirements</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#New">New</a> status.</em></p>
<h3 id="3215"><a href="lwg-active.html#3215">3215</a>. <code>variant</code> default constructor has vague <code>constexpr</code> requirements</h3>
<p><b>Section:</b> 22.6.3.2 <a href="https://wg21.link/variant.ctor">[variant.ctor]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Louis Dionne <b>Opened:</b> 2019-06-04 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#variant.ctor">active issues</a> in [variant.ctor].</p>
<p><b>View all other</b> <a href="lwg-index.html#variant.ctor">issues</a> in [variant.ctor].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In 22.6.3.2 <a href="https://wg21.link/variant.ctor">[variant.ctor]</a> p5, we say:
</p>
<blockquote><p>
<i>Remarks:</i> This function shall be <code>constexpr</code> if and only if the value-initialization of
the alternative type <code>T<sub>0</sub></code> would satisfy the requirements for a constexpr function.
[&hellip;]
</p></blockquote>
<p>
First of all, I find it confusing that we say "This function shall be <code>constexpr</code> if [&hellip;]",
when the declaration of the function clearly has the <code>constexpr</code> keyword on it
unconditionally. Instead, I would use the wording "This function shall be usable in a constexpr context
if [&hellip;]".
<p/>
Secondly, I think we shouldn't be using if-and-only-if since it restricts whether implementations can be
<code>constexpr</code>-friendly as an extension. Instead, it seems better to just say "if".
<p/>
Finally, I think the condition under which the function must be <code>constexpr</code>-friendly is not
something we can test for because it says "value-initialization of the alternative type
<code>T<sub>0</sub></code> would satisfy the requirements for a constexpr function", which doesn't imply
the value initialization can actually be be performed inside a constexpr context (for example the
default constructor could be <code>constexpr</code> friendly <em>but</em> not marked with the
<code>constexpr</code> keyword).
</p>

<p><i>[2017-06-17, Tim Song comments]</i></p>

<p>This issue is related to LWG <a href="lwg-active.html#2833" title="Library needs to specify what it means when it declares a function constexpr (Status: Open)">2833</a><sup><a href="https://cplusplus.github.io/LWG/issue2833" title="Latest snapshot">(i)</a></sup>.</p>

<p><i>[2019-07 Issue Prioritization]</i></p>

<p>Priority to 2 after discussion on the reflector.</p>

<strong>Previous resolution from Daniel [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4810">N4810</a>.</p>

<ol>
<li><p>Modify 22.6.3.2 <a href="https://wg21.link/variant.ctor">[variant.ctor]</a> as indicated:</p>

<blockquote>
<pre>
constexpr variant() noexcept(<i>see below</i>);
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> [&hellip;]
<p/>
-3- <i>Ensures:</i> [&hellip;]
<p/>
-4- <i>Throws:</i> [&hellip;]
<p/>
-5- <i>Remarks:</i> This function shall be <ins>usable in a context that requires constant
evaluation if the alternative type <code>T<sub>0</sub></code> can be value-initialized in a context
that requires constant evaluation</ins><del><code>constexpr</code> if and only if the
value-initialization of the alternative type <code>T<sub>0</sub></code> would satisfy the requirements
for a constexpr function</del>. [&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2020-06-08 Nina Dinka Ranns comments]</i></p>

<p>
The revised wording provided by LWG <a href="lwg-active.html#2833" title="Library needs to specify what it means when it declares a function constexpr (Status: Open)">2833</a><sup><a href="https://cplusplus.github.io/LWG/issue2833" title="Latest snapshot">(i)</a></sup> should resolve this issue as well.
</p>


<p id="res-3215"><b>Proposed resolution:</b></p>




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