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<title>Issue 3350: Simplify return type of lexicographical_compare_three_way</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="3350"><a href="lwg-defects.html#3350">3350</a>. Simplify return type of <code>lexicographical_compare_three_way</code></h3>
<p><b>Section:</b> 26.8.12 <a href="https://wg21.link/alg.three.way">[alg.three.way]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2019-12-04 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.three.way">issues</a> in [alg.three.way].</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 current return type is:
</p>
<blockquote><pre>
common_comparison_category_t&lt;decltype(comp(*b1, *b2)), strong_ordering&gt;
</pre></blockquote>
<p>
Finding the common category with <code>strong_ordering</code> doesn't do anything. The common category of <code>X</code> 
and <code>strong_ordering</code> is always <code>X</code>, so we can simplify it to:
</p>
<blockquote><pre>
common_comparison_category_t&lt;decltype(comp(*b1, *b2))&gt;
</pre></blockquote>
<p>
This can further be simplified, because the common category of any comparison category type is just that type. 
If it's not a comparison category then the result would be <code>void</code>, but the function would be ill-formed 
in that case anyway, as we have:
</p>
<blockquote><p>
<i>Mandates:</i> <code>decltype(comp(*b1, *b2))</code> is a comparison category type.
</p></blockquote>
<p>
So the only effect of the complicated return type seems to be to cause the return type to be deduced as 
<code>void</code> for specializations of the function template that are ill-formed if called. That doesn't seem useful.
</p>

<p><i>[2019-12-12 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.</p>


<p id="res-3350"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4842">N4842</a>.</p>

<ol>
<li><p>Modify 26.4 <a href="https://wg21.link/algorithm.syn">[algorithm.syn]</a> as indicated:</p>
<blockquote>
<pre>
[&hellip;]
<i>// 26.8.12 <a href="https://wg21.link/alg.three.way">[alg.three.way]</a>, three-way comparison algorithms</i>
template&lt;class InputIterator1, class InputIterator2, class Cmp&gt;
  constexpr auto
    lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
                                      InputIterator2 b2, InputIterator2 e2,
                                      Cmp comp)
      -&gt; <del>common_comparison_category_t&lt;</del>decltype(comp(*b1, *b2))<del>, strong_ordering&gt;</del>;
template&lt;class InputIterator1, class InputIterator2&gt;
  constexpr auto
    lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
                                      InputIterator2 b2, InputIterator2 e2);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 26.8.12 <a href="https://wg21.link/alg.three.way">[alg.three.way]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class InputIterator1, class InputIterator2, class Cmp&gt;
  constexpr auto
    lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1,
                                      InputIterator2 b2, InputIterator2 e2,
                                      Cmp comp)
      -&gt; <del>common_comparison_category_t&lt;</del>decltype(comp(*b1, *b2))<del>, strong_ordering&gt;</del>;
</pre>
<blockquote>
<p>
-1- <i>Mandates:</i> <code>decltype(comp(*b1, *b2))</code> is a comparison category type.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>




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