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<title>Issue 3863: Is input_iterator guaranteed to have iter_const_reference_t?</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="3863"><a href="lwg-active.html#3863">3863</a>. Is <code>input_iterator</code> guaranteed to have <code>iter_const_reference_t</code>?</h3>
<p><b>Section:</b> 24.5.3.2 <a href="https://wg21.link/const.iterators.alias">[const.iterators.alias]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-01-26 <b>Last modified:</b> 2023-02-06</p>
<p><b>Priority: </b>2
</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 the C++20 iterator system, <code>input_iterator</code> is guaranteed to have a common reference, which 
reflects the <code>indirectly_readable</code> requires 
<code>common_reference_t&lt;iter_reference_t&lt;I&gt;&amp;&amp;, iter_value_t&lt;I&gt;&amp;&gt;</code> to be a valid type.
<p/>
However, for <code>iter_const_reference_t</code> which with a similar form:
</p>
<blockquote><pre>
template&lt;indirectly_readable It&gt;
  using iter_const_reference_t =
    common_reference_t&lt;const iter_value_t&lt;It&gt;&amp;&amp;, iter_reference_t&lt;It&gt;&gt;;
</pre></blockquote>
<p>
it is still theoretically possible to create an <code>input_iterator</code> that does not have a valid 
<code>iter_const_reference_t</code>, <a href="https://godbolt.org/z/oY3xoqPMa">for example</a>:
</p>
<blockquote><pre>
#include &lt;iterator&gt;

struct ref {
  ref(int&amp;);
};

struct rref {
  rref(const int&amp;);
  rref(const ref&amp;);
};

struct I {
  using value_type = int;
  using difference_type = std::ptrdiff_t;

  ref operator*() const;
  I&amp; operator++();
  I operator++(int);

  friend rref iter_move(const I&amp;);
};

static_assert(std::input_iterator&lt;I&gt;); // pass
using CR = std::iter_const_reference_t&lt;I&gt;;  // <span style="color:red;font-weight:bolder">error: no type named 'type' in 'struct std::common_reference&lt;const int&amp;&amp;, ref&gt;'</span>
</pre></blockquote>
<p>
which causes <code>basic_const_iterator&lt;I&gt;</code> to produce a hard error internally when it is instantiated.
</p>

<p><i>[2023-02-06; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
Seems contrived, should probably constrain not give a hard error.
The question of whether each range needs to have const reference to elems
is important.
</p>



<p id="res-3863"><b>Proposed resolution:</b></p>





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