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<title>Issue 3179: subrange should always model Range</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="3179"><a href="lwg-defects.html#3179">3179</a>. <code>subrange</code> should always model <code>Range</code></h3>
<p><b>Section:</b> 25.5.4.2 <a href="https://wg21.link/range.subrange.ctor">[range.subrange.ctor]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2018-12-21 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</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>
The constructors of <code>subrange</code> place no requirements on the iterator and
sentinel values from which a <code>subrange</code> is constructed. They allow
constructions like <code>subrange{myvec.end(), myvec.begin()}</code> in which the
resulting <code>subrange</code> isn't in the domain of the <code>Range</code> concept
which requires that "<code>[ranges::begin(r), ranges::end(r))</code> denotes a
range" (<a href="https://wg21.link/range.range#3.1">[range.range]/3.1</a>).
We should forbid the construction of abominable values like this to enforce the
useful semantic that values of <code>subrange</code> are always in the domain of
<code>Range</code>.
</p><p>
(From <a href="https://github.com/ericniebler/stl2/issues/597">ericniebler/stl2#597</a>.)
</p>

<p><i>[2019-01-20 Reflector prioritization]</i></p>

<p>Set Priority to 0 and status to Tentatively Ready</p>


<p id="res-3179"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4791">N4791</a>.</p>

<ol>
<li><p>Change 25.5.4.2 <a href="https://wg21.link/range.subrange.ctor">[range.subrange.ctor]</a> as indicated:</p>
<blockquote>
<blockquote>
<pre>
constexpr subrange(I i, S s) requires (!StoreSize);
</pre>
</blockquote>
<p>
<ins>-?- <i>Expects:</i> <code>[i, s)</code> is a valid range.</ins>
</p><p>
-1- <i>Effects:</i> Initializes <code>begin_</code> with <code>i</code> and
<code>end_</code> with <code>s</code>.
</p>
<blockquote>
<pre>
constexpr subrange(I i, S s, iter_difference_t&lt;I&gt; n)
  requires (K == subrange_kind::sized);
</pre>
</blockquote>
<p>
-2- <i>Expects:</i> <ins><code>[i, s)</code> is a valid range, and</ins>
<code>n == ranges::distance(i, s)</code>.
</p>
</blockquote>
</li>
</ol>





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