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<title>Issue 4171: P2609R3 breaks code that uses views::zip and get&lt;T&gt;</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="4171"><a href="lwg-active.html#4171">4171</a>. P2609R3 breaks code that uses <code class='backtick'>views::zip</code> and <code>get&lt;T&gt;</code></h3>
<p><b>Section:</b> 24.3.6.3 <a href="https://wg21.link/indirectcallable.indirectinvocable">[indirectcallable.indirectinvocable]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2024-11-01 <b>Last modified:</b> 2024-11-03</p>
<p><b>Priority: </b>Not Prioritized
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following use of <code class='backtick'>std::ranges::for_each</code> is valid before <a href="https://wg21.link/P2609R3" title=" Relaxing Ranges Just A Smidge">P2609R3</a> and invalid after that.
</p>
<blockquote><pre>
#include &lt;algorithm&gt;
#include &lt;ranges&gt;
#include &lt;tuple&gt;
using namespace std::ranges;

void f() {
  int a[1];
  auto fun = [](auto t) {
    [[maybe_unused]] auto x = std::get&lt;int&amp;&gt;(t);
  };
  for_each(views::zip(a), fun);
}
</pre></blockquote>
<p>
The reason is that, <a href="https://wg21.link/P2609R3" title=" Relaxing Ranges Just A Smidge">P2609R3</a> requires <code class='backtick'>fun</code> to be <code class='backtick'>invocable</code> with <code>iter_value_t&lt;I&gt;&amp;</code>, 
which is <code>tuple&lt;int&gt;&amp;</code> when <code class='backtick'>I</code> is <code class='backtick'>zip_view</code>'s iterator, and <code>tuple&lt;int&gt;&amp;</code> 
doesn't support <code>std::get&lt;int&amp;&gt;(t)</code> because there isn't a <code>int&amp;</code> member.
<p/>
P2609R3 argues that "The actual consequence on user code seems small", but I believe that this code pattern is 
common enough, and it hurts if we cannot use <code>get&lt;int&amp;&gt;(t)</code> in the lambda body.
<p/>
Note that <code class='backtick'>for_each</code> doesn't actually call <code class='backtick'>fun</code> with <code>iter_value_t&lt;I&gt;</code>, as can be seen by adding 
an explicit return type to <code class='backtick'>fun</code>.
<p/>
Did LWG foresee this impact of <a href="https://wg21.link/P2609R3" title=" Relaxing Ranges Just A Smidge">P2609R3</a>? Could P2609R3 be reverted to unbreak this code pattern?
</p>


<p id="res-4171"><b>Proposed resolution:</b></p>




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