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<title>Issue 3325: Constrain return type of transformation function for transform_view</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="3325"><a href="lwg-defects.html#3325">3325</a>. Constrain return type of transformation function for <code>transform_view</code></h3>
<p><b>Section:</b> 25.7.9.2 <a href="https://wg21.link/range.transform.view">[range.transform.view]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2019-11-06 <b>Last modified:</b> 2022-01-15</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><b>Addresses <a href="https://github.com/cplusplus/nbballot/issues/299">US 303</a></b></p>

<p>
The <code>transform_view</code> does not constrain the return type of the transformation function. It
is invalid to pass a <code>void</code>-returning transformation function to the <code>transform_view</code>, 
which would cause its <code>iterator</code>s' <code>operator*</code> member to return <code>void</code>.
<p/>
Proposed change:
</p>
<p>
Change the constraints on <code>transform_view</code> to the following:
</p>
<blockquote><pre>
template&lt;input_range V, copy_constructible F&gt;
  requires view&lt;V&gt; &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
           regular_invocable&lt;F&amp;, range_reference_t&lt;V&gt;&gt; &amp;&amp;
           <i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;V&gt;&gt;&gt;
class transform_view;
</pre></blockquote>
<p>
<b>Jonathan Wakely:</b>
<p/>
The NB comment says "The <code>transform_view</code> does not constrain the return type of the transformation 
function. It is invalid to pass a <code>void</code>-returning transformation function to the <code>transform_view</code>, 
which would cause its <code>iterator</code>s' <code>operator*</code> member to return <code>void</code>."
</p>

<p><i>[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]</i></p>



<p id="res-3325"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4835">N4835</a>.</p>

<ol>
<li><p>Modify 25.7.9.2 <a href="https://wg21.link/range.transform.view">[range.transform.view]</a>, class template <code>transform_view</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, copy_constructible F&gt;
    requires view&lt;V&gt; &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
             regular_invocable&lt;F&amp;, range_reference_t&lt;V&gt;&gt; <ins>&amp;&amp;
             <i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;V&gt;&gt;&gt;</ins>
  class transform_view : public view_interface&lt;transform_view&lt;V, F&gt;&gt; {
    [&hellip;]
  };
[&hellip;]
}
</pre>
</blockquote>
</li>

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