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  <title>Views should not be required to be default constructible</title>
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<h1 class="title" style="text-align:center">Views should not be required to be default constructible</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2325R1</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2021-03-15</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a>
<ul>
<li><a href="#history"><span class="toc-section-number">2.1</span> History<span></span></a></li>
<li><a href="#uses-of-default-construction"><span class="toc-section-number">2.2</span> Uses of default construction<span></span></a></li>
<li><a href="#does-this-requirement-cause-harm"><span class="toc-section-number">2.3</span> Does this requirement cause harm?<span></span></a></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">3</span> Proposal<span></span></a>
<ul>
<li><a href="#timeline"><span class="toc-section-number">3.1</span> Timeline<span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">4</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2325R0">[<a href="#ref-P2325R0" role="doc-biblioref">P2325R0</a>]</span>, added discussion of the different treatments of lvalue vs rvalue fixed-extent <code class="sourceCode cpp">span</code> in pipelines.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>Currently, the <code class="sourceCode cpp">view</code> concept is defined in <span>24.4.4
 <a href="https://wg21.link/range.view">[range.view]</a></span> as:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">concept</span> view <span class="op">=</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>    range<span class="op">&lt;</span>T<span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb1-4"><a href="#cb1-4"></a>    movable<span class="op">&lt;</span>T<span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb1-5"><a href="#cb1-5"></a>    default_initializable<span class="op">&lt;</span>T<span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb1-6"><a href="#cb1-6"></a>    enable_view<span class="op">&lt;</span>T<span class="op">&gt;</span>;</span></code></pre></div>
<p>Three of these four criteria, I understand. A <code class="sourceCode cpp">view</code> clearly needs to be a <code class="sourceCode cpp">range</code>, and it’s important that they be <code class="sourceCode cpp">movable</code> for various operations to work. And the difference between a <code class="sourceCode cpp">view</code> and <code class="sourceCode cpp">range</code> is largely semantic, and so there needs to be an explicit opt-in in the form of <code class="sourceCode cpp">enable_view</code>.</p>
<p>But why does a view need to be <code class="sourceCode cpp">default_initializable</code>?</p>
<h2 data-number="2.1" id="history"><span class="header-section-number">2.1</span> History<a href="#history" class="self-link"></a></h2>
<p>The history of the design of Ranges is split between many papers and github issues in both the range-v3 <span class="citation" data-cites="range-v3">[<a href="#ref-range-v3" role="doc-biblioref">range-v3</a>]</span> and stl2 <span class="citation" data-cites="stl2">[<a href="#ref-stl2" role="doc-biblioref">stl2</a>]</span> libraries. However, I simply am unable to find much information that motivates this particular choice.</p>
<p>In <span class="citation" data-cites="N4128">[<a href="#ref-N4128" role="doc-biblioref">N4128</a>]</span>, we have (this paper predates the term <code class="sourceCode cpp">view</code>, at the time the term “range” instead was used to refer to what is now called a <code class="sourceCode cpp">view</code>. To alleviate confusion, I have editted this paragraph accordingly):</p>
<blockquote>
<p>We’ve already decided that [Views] are copyable and assignable. They are, in the terminology of <span class="citation" data-cites="EoP">[<a href="#ref-EoP" role="doc-biblioref">EoP</a>]</span> and <span class="citation" data-cites="N3351">[<a href="#ref-N3351" role="doc-biblioref">N3351</a>]</span>, Semiregular types. It follows that copies are independent, even though the copies are both aliases of the same underlying elements. The [views] are independent in the same way that a copy of a pointer or an iterator is independent from the original. Likewise, iterators from two [views] that are copies of each other are also independent. When the source [view] goes out of scope, it does not invalidate an iterator into the destination [view].</p>
<p>Semiregular also requires DefaultConstructible in <span class="citation" data-cites="N3351">[<a href="#ref-N3351" role="doc-biblioref">N3351</a>]</span>. We follow suit and require all [Views] to be DefaultConstructible. Although this complicates the implementation of some range types, it has proven useful in practice, so we have kept this requirement.</p>
</blockquote>
<p>There is also <span class="citation" data-cites="stl2-179">[<a href="#ref-stl2-179" role="doc-biblioref">stl2-179</a>]</span>, titled “Consider relaxing the DefaultConstructible requirements,” in which Casey Carter states (although the issue is about iterators rather than views):</p>
<blockquote>
<p>There’s concern in the community that relaxing type invariants to allow for default construction of a type that would not otherwise provide it is a horrible idea.</p>
<p>Relaxing the default construction requirement for iterators would also remove one of the few “breaking” differences between input and output iterators in the Standard (which do not require default construction) and Ranges (which currently do require default construction).</p>
</blockquote>
<p>Though, importantly, Casey points out one concern:</p>
<blockquote>
<p>The recent trend of making everything in the standard library <code class="sourceCode cpp"><span class="kw">constexpr</span></code> is in conflict with the desire to not require default construction. The traditional workaround for delayed initialization of a non-default-constructible <code class="sourceCode cpp">T</code> is to instead store an <code class="sourceCode cpp">optional<span class="op">&lt;</span>T<span class="op">&gt;</span></code>. Changing an <code class="sourceCode cpp">optional<span class="op">&lt;</span>T<span class="op">&gt;</span></code> from the empty to filled states is not possible in a constant expression</p>
</blockquote>
<p>This was true at the time of the writing of the issue, but has since been resolved first at the core language level by <span class="citation" data-cites="P1330R0">[<a href="#ref-P1330R0" role="doc-biblioref">P1330R0</a>]</span> and then at the library level by <span class="citation" data-cites="P2231R1">[<a href="#ref-P2231R1" role="doc-biblioref">P2231R1</a>]</span>. As such, I’m simply unsure what the motivation is for requiring default construction of views.</p>
<p>The motivation for default construction of <em>iterators</em> comes from <span class="citation" data-cites="N3644">[<a href="#ref-N3644" role="doc-biblioref">N3644</a>]</span>, although this doesn’t really apply to <em>output</em> iterators (which are also currently required to be default constructible).</p>
<h2 data-number="2.2" id="uses-of-default-construction"><span class="header-section-number">2.2</span> Uses of default construction<a href="#uses-of-default-construction" class="self-link"></a></h2>
<p>I couldn’t find any other motivation for default construction of views from the paper trail, so I tried to discover the motivation for it in range-v3. I did this with a large hammer: I removed all the default constructors and saw what broke.</p>
<p>And the answer is… not much. The commit can be found here: <span class="citation" data-cites="range-v3-no-dflt">[<a href="#ref-range-v3-no-dflt" role="doc-biblioref">range-v3-no-dflt</a>]</span>. The full list of breakage is:</p>
<ol type="1">
<li><p><code class="sourceCode cpp">join_view</code> and <code class="sourceCode cpp">join_with_view</code> need a default-constructed inner view. This clearly breaks if that view isn’t default constructible. I wrapped them in semiregular_box.</p></li>
<li><p><code class="sourceCode cpp">views<span class="op">::</span>ints</code> and <code class="sourceCode cpp">views<span class="op">::</span>indices</code> are interesting in range-v3 because it’s not just that <code class="sourceCode cpp">ints<span class="op">(</span><span class="dv">0</span>, <span class="dv">4</span><span class="op">)</span></code> gives you the range <code class="sourceCode cpp"><span class="op">[</span><span class="dv">0</span>,<span class="dv">4</span><span class="op">)</span></code> but also that <code class="sourceCode cpp">ints</code> by itself is also a range (from <code class="sourceCode cpp"><span class="dv">0</span></code> to infinity). These two inherit from iota, so once I removed the default constructor from iota, these uses break. So I added default constructors to <code class="sourceCode cpp">ints</code> and <code class="sourceCode cpp">indices</code>.</p></li>
<li><p>One of range-v3’s mechanisms for easier implementation of views and iterators is called <code class="sourceCode cpp">view_facade</code>. This is an implementation strategy that uses the view as part of the iterator as an implementation detail. As such, because the iterator has to be default constructible, the view must be as well. So <code class="sourceCode cpp">linear_distribute_view</code> and <code class="sourceCode cpp">chunk_view</code> (the specialization for input ranges) kept their defaulted default constructors. But this is simply an implementation strategy, there’s nothing inherent to these views that requires this approach.</p></li>
<li><p>There’s one test for <code class="sourceCode cpp">any_view</code> that just tests that it’s default constructible.</p></li>
</ol>
<p>That’s it. Broadly, just a few views that actually need default construction that can easily provide it, most simply don’t need this constraint.</p>
<h2 data-number="2.3" id="does-this-requirement-cause-harm"><span class="header-section-number">2.3</span> Does this requirement cause harm?<a href="#does-this-requirement-cause-harm" class="self-link"></a></h2>
<p>Rather than providing a benefit, it seems like the default construction requirement causes harm.</p>
<p>If the argument for default construction is that it enables efficient deferred initialization during view composition, then I’m not sure I buy that argument. <code class="sourceCode cpp">join_view</code> would have to use an optional where it wouldn’t have before, which makes it a little bigger. But conversely, right now, every range adaptor that takes a function has to use an optional: <code class="sourceCode cpp">transform_view</code>, <code class="sourceCode cpp">filter_view</code>, etc. all need to be default constructible so they have to wrap their callables in <code class="sourceCode cpp"><em>semiregular-box</em></code> to make them default constructible. If views didn’t have to be constructible, they wouldn’t have to do this. Or rather, they would still have to do some wrapping, but we’d only need the assignment parts of <code class="sourceCode cpp"><em>semiregular-box</em></code>, and not the default construction part, which means that <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">(</span><em>copyable-box</em><span class="op">&lt;</span>T<span class="op">&gt;)</span></code> would be equal to <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">(</span>T<span class="op">)</span></code>, whereas <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">(</span><em>semiregular-box</em><span class="op">&lt;</span>T<span class="op">&gt;)</span></code> could be larger.</p>
<p>My impression right now is that the default construction requirement actually adds storage cost to range adapters on the whole rather than removing storage cost.</p>
<p>Furthermore, there’s the question of <em>requiring</em> a partially formed state to types even they didn’t want to do that. This goes against the general advice of making bad states unrepresentable. Consider a type like <code class="sourceCode cpp">span<span class="op">&lt;</span><span class="dt">int</span>, <span class="dv">5</span><span class="op">&gt;</span></code>. This <em>should</em> be a <code class="sourceCode cpp">view</code>: it’s a non-owning, O(1)-everything range. But it’s not default constructible, so it’s not a <code class="sourceCode cpp">view</code>. The consequence of this choice is the difference in behavior when using fixed-extent <code class="sourceCode cpp">span</code> in pipelines that start with an lvalue vs an rvalue:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a>std<span class="op">::</span>span<span class="op">&lt;</span><span class="dt">int</span>, <span class="dv">5</span><span class="op">&gt;</span> s <span class="op">=</span> <span class="co">/* ... */</span>;</span>
<span id="cb2-2"><a href="#cb2-2"></a></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="co">// Because span&lt;int, 5&gt; is not a view, rather than copying s into</span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="co">// the resulting transform_view, this instead takes a</span></span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="co">// ref_view&lt;span&lt;int, 5&gt;&gt;. If s goes out of scope, this will dangle.</span></span>
<span id="cb2-6"><a href="#cb2-6"></a><span class="kw">auto</span> lvalue <span class="op">=</span> s <span class="op">|</span> views<span class="op">::</span>transform<span class="op">(</span>f<span class="op">)</span>;</span>
<span id="cb2-7"><a href="#cb2-7"></a></span>
<span id="cb2-8"><a href="#cb2-8"></a><span class="co">// Because span&lt;int, 5&gt; is a borrowed range, this compiles. We still</span></span>
<span id="cb2-9"><a href="#cb2-9"></a><span class="co">// don&#39;t copy the span&lt;int, 5&gt; directly, instead we end up with a</span></span>
<span id="cb2-10"><a href="#cb2-10"></a><span class="co">// subrange&lt;span&lt;int, 5&gt;::iterator&gt;.</span></span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="kw">auto</span> rvalue <span class="op">=</span> std<span class="op">::</span>move<span class="op">(</span>s<span class="op">)</span> <span class="op">|</span> views<span class="op">::</span>transform<span class="op">(</span>f<span class="op">)</span>;</span></code></pre></div>
<p>Both alternatives are less efficient than they could be. The lvalue case requires an extra indirection and exposes an opportunity for a dangling range. The rvalue case won’t dangle, but ends up requiring storing two iterators, which requires twice the storage as storing the single <code class="sourceCode cpp">span</code> would have. Either case is strictly worse than the behavior that would result from <code class="sourceCode cpp">span<span class="op">&lt;</span><span class="dt">int</span>, <span class="dv">5</span><span class="op">&gt;</span></code> having been a <code class="sourceCode cpp">view</code>.</p>
<p>But fixed-extent <code class="sourceCode cpp">span</code> isn’t default-constructible for good reason: if we were to add a default constructor that would make <code class="sourceCode cpp">span<span class="op">&lt;</span><span class="dt">int</span>, <span class="dv">5</span><span class="op">&gt;</span></code> partially formed, this adds an extra state that needs to be carefully checked by users, and suddenly every operation has additional preconditions that need to be documented. But this is true for every other view, too!</p>
<p><code class="sourceCode cpp">ranges<span class="op">::</span>ref_view</code> (see <span>24.7.4.2
 <a href="https://wg21.link/range.ref.view">[range.ref.view]</a></span>) is another such view. In the same way that <code class="sourceCode cpp">std<span class="op">::</span>reference_wrapper<span class="op">&lt;</span>T<span class="op">&gt;</span></code> is a rebindable reference to <code class="sourceCode cpp">T</code>, <code class="sourceCode cpp">ref_view<span class="op">&lt;</span>R<span class="op">&gt;</span></code> is a rebindable reference to the range <code class="sourceCode cpp">R</code>. Except <code class="sourceCode cpp">reference_wrapper<span class="op">&lt;</span>T<span class="op">&gt;</span></code> isn’t default constructible, but <code class="sourceCode cpp">ref_view<span class="op">&lt;</span>R<span class="op">&gt;</span></code> is — it’s just that as a user, I have no way to check to see if a particular <code class="sourceCode cpp">ref_view<span class="op">&lt;</span>R<span class="op">&gt;</span></code> is fully formed or not. All of its member functions have this precondition that it really does refer to a range that I as the user can’t check. This is broadly true of all the range adapters: you can’t do <em>anything</em> with a default constructed range adapter except assign to it.</p>
<p>If the default construction requirement doesn’t add benefit (and I’m not sure that it does) and it causes harm (both in the sense of requiring invalid states on types and adding to the storage requirements on all range adapters and further adding to user confusion when their types fail to model <code class="sourceCode cpp">view</code>), maybe we should get rid of it?</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Remove the <code class="sourceCode cpp">default_initializable</code> constraint from <code class="sourceCode cpp">view</code>, such that the concept becomes:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="kw">concept</span> view <span class="op">=</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>    range<span class="op">&lt;</span>T<span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb3-4"><a href="#cb3-4"></a>    movable<span class="op">&lt;</span>T<span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb3-5"><a href="#cb3-5"></a>    enable_view<span class="op">&lt;</span>T<span class="op">&gt;</span>;</span></code></pre></div>
<p>Remove the <code class="sourceCode cpp">default_initializable</code> constraint from <code class="sourceCode cpp">weakly_incrementable</code>. This ends up removing the default constructible requirement from input-only and output iterators, while still keeping it on forward iterators (<code class="sourceCode cpp">forward_iterator</code> requires <code class="sourceCode cpp">incrementable</code> which requires <code class="sourceCode cpp">regular</code>).</p>
<p>For <code class="sourceCode cpp">iota_view</code>, replace the <code class="sourceCode cpp">semiregular<span class="op">&lt;</span>W<span class="op">&gt;</span></code> constraint with <code class="sourceCode cpp">copyable<span class="op">&lt;</span>W<span class="op">&gt;</span></code>, and add a constraint on <code class="sourceCode cpp">iota_view<span class="op">&lt;</span>W, Bound<span class="op">&gt;::</span>iterator</code>’s default constructor. This allows an input-only <code class="sourceCode cpp">iota_view</code> with a non-default-constructible <code class="sourceCode cpp">W</code> while preserving the current behavior for all forward-or-better <code class="sourceCode cpp">iota_view</code>s.</p>
<p>Remove the default constructors from the standard library views and iterators for which they only exist to satisfy the requirement (<code class="sourceCode cpp">ref_view</code>, <code class="sourceCode cpp">istream_view</code>, <code class="sourceCode cpp">ostream_iterator</code>, <code class="sourceCode cpp">ostreambuf_iterator</code>, <code class="sourceCode cpp">back_insert_iterator</code>, <code class="sourceCode cpp">front_insert_iterator</code>, <code class="sourceCode cpp">insert_iterator</code>). Constrain the other standard library views’ default constructors on the underlying types being default constructible.</p>
<p>For <code class="sourceCode cpp">join_view</code>, store the inner view in a <code class="sourceCode cpp"><em>semiregular-box</em><span class="op">&lt;</span>views<span class="op">::</span>all_t<span class="op">&lt;</span><em>InnerRng</em><span class="op">&gt;&gt;</span></code>.</p>
<p>Make <code class="sourceCode cpp">span<span class="op">&lt;</span>ElementType, Extent<span class="op">&gt;</span></code> a <code class="sourceCode cpp">view</code> regardless of <code class="sourceCode cpp">Extent</code>. Currently, it is only a <code class="sourceCode cpp">view</code> when <code class="sourceCode cpp">Extent <span class="op">==</span> <span class="dv">0</span> <span class="op">||</span> Extent <span class="op">==</span> dynamic_extent</code>.</p>
<p>We currently use <code class="sourceCode cpp"><em>semiregular-box</em><span class="op">&lt;</span>T<span class="op">&gt;</span></code> to make types <code class="sourceCode cpp">semiregular</code> (see <span>24.7.3
 <a href="https://wg21.link/range.semi.wrap">[range.semi.wrap]</a></span>), which we use to wrap function objects throughout. We can do a little bit better by introducing a <code class="sourceCode cpp"><em>copyable-box</em><span class="op">&lt;</span>T<span class="op">&gt;</span></code> such that:</p>
<ul>
<li>If <code class="sourceCode cpp">T</code> is <code class="sourceCode cpp">copyable</code>, then <code class="sourceCode cpp"><em>copyable-box</em><span class="op">&lt;</span>T<span class="op">&gt;</span></code> is basically just <code class="sourceCode cpp">T</code></li>
<li>Otherwise, if <code class="sourceCode cpp">T</code> is <code class="sourceCode cpp">nothrow_copy_constructible</code> but not <code class="sourceCode cpp">copy_assignable</code>, then <code class="sourceCode cpp"><em>copyable-box</em><span class="op">&lt;</span>T<span class="op">&gt;</span></code> can be a thin wrapper around <code class="sourceCode cpp">T</code> that adds a copy assignment operator that does destroy-then-copy-construct.</li>
<li>Otherwise, <code class="sourceCode cpp"><em>copyable-box</em><span class="op">&lt;</span>T<span class="op">&gt;</span></code> is <code class="sourceCode cpp"><em>semiregular-box</em><span class="op">&lt;</span>T<span class="op">&gt;</span></code> (we still need <code class="sourceCode cpp">optional<span class="op">&lt;</span>T<span class="op">&gt;</span></code>’s empty state here to handle the case where copy construction can throw, to avoid double-destruction).</li>
</ul>
<p>Replace all function object <code class="sourceCode cpp"><em>semiregular-box</em><span class="op">&lt;</span>F<span class="op">&gt;</span></code> wrappers throughout <code class="sourceCode cpp"><span class="op">&lt;</span>ranges<span class="op">&gt;</span></code> with <code class="sourceCode cpp"><em>copyable-box</em><span class="op">&lt;</span>F<span class="op">&gt;</span></code> wrappers.</p>
<h2 data-number="3.1" id="timeline"><span class="header-section-number">3.1</span> Timeline<a href="#timeline" class="self-link"></a></h2>
<p>At the moment, only libstdc++ and MSVC provide an implementation of ranges (and MSVC’s is incomplete). We either have to make this change now and soon, or never.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-EoP">
<p>[EoP] Stepanov, A. and McJones, P. 2009. Elements of Programming. Addison-Wesley Professional. </p>
</div>
<div id="ref-N3351">
<p>[N3351] B. Stroustrup, A. Sutton. 2012-01-13. A Concept Design for the STL. <br />
<a href="https://wg21.link/n3351">https://wg21.link/n3351</a></p>
</div>
<div id="ref-N3644">
<p>[N3644] Alan Talbot. 2013-04-18. Null Forward Iterators. <br />
<a href="https://wg21.link/n3644">https://wg21.link/n3644</a></p>
</div>
<div id="ref-N4128">
<p>[N4128] E. Niebler, S. Parent, A. Sutton. 2014-10-10. Ranges for the Standard Library, Revision 1. <br />
<a href="https://wg21.link/n4128">https://wg21.link/n4128</a></p>
</div>
<div id="ref-P1330R0">
<p>[P1330R0] Louis Dionne, David Vandevoorde. 2018-11-10. Changing the active member of a union inside constexpr. <br />
<a href="https://wg21.link/p1330r0">https://wg21.link/p1330r0</a></p>
</div>
<div id="ref-P2231R1">
<p>[P2231R1] Barry Revzin. 2021. Missing <code class="sourceCode cpp"><span class="kw">constexpr</span></code> in <code class="sourceCode cpp">std<span class="op">::</span>optional</code> and <code class="sourceCode cpp">std<span class="op">::</span>variant</code>. <br />
<a href="https://wg21.link/p2231r1">https://wg21.link/p2231r1</a></p>
</div>
<div id="ref-P2325R0">
<p>[P2325R0] Barry Revzin. 2021-02-17. Views should not be required to be default constructible. <br />
<a href="https://wg21.link/p2325r0">https://wg21.link/p2325r0</a></p>
</div>
<div id="ref-range-v3">
<p>[range-v3] Eric Niebler and Casey Carter. 2013. range-v3 repo. <br />
<a href="https://github.com/ericniebler/range-v3/">https://github.com/ericniebler/range-v3/</a></p>
</div>
<div id="ref-range-v3-no-dflt">
<p>[range-v3-no-dflt] Barry Revzin. 2021. Removing default construction from range-v3 views. <br />
<a href="https://github.com/BRevzin/range-v3/commit/2e2c9299535211bc5417f9146eaed9945e596e83">https://github.com/BRevzin/range-v3/commit/2e2c9299535211bc5417f9146eaed9945e596e83</a></p>
</div>
<div id="ref-stl2">
<p>[stl2] Eric Niebler and Casey Carter. 2014. stl2 repo. <br />
<a href="https://github.com/ericniebler/stl2/">https://github.com/ericniebler/stl2/</a></p>
</div>
<div id="ref-stl2-179">
<p>[stl2-179] Casey Carter. 2016. Consider relaxing the DefaultConstructible requirements. <br />
<a href="https://github.com/ericniebler/stl2/issues/179">https://github.com/ericniebler/stl2/issues/179</a></p>
</div>
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