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<title>Issue 4261: P1206R7 broke uses of container adaptors with old custom sequence containers</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="4261"><a href="lwg-active.html#4261">4261</a>. P1206R7 broke uses of container adaptors with old custom sequence containers</h3>
<p><b>Section:</b> 27.4.3.8.2 <a href="https://wg21.link/string.find">[string.find]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2025-05-07 <b>Last modified:</b> 2025-05-18</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#string.find">active issues</a> in [string.find].</p>
<p><b>View all other</b> <a href="lwg-index.html#string.find">issues</a> in [string.find].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<a href="https://wg21.link/P1206R7" title=" Conversions from ranges to containers">P1206R7</a> added some requirements for sequence containers, including program-defined ones.
<p/>
These requirements mean that if a custom C++20 sequence container type has no newly required member 
functions added in C++23 mode, it is no longer a sequence container, and uses of it with 
<code class='backtick'>std::stack</code>, <code class='backtick'>std::queue</code>, or <code class='backtick'>std::priority_queue</code> possibly have undefined behavior or 
make the program ill-formed. Additionally, some users want to use C++23 flat container adaptors 
with old sequence containers (<a href="https://github.com/llvm/llvm-project/issues/136656">llvm/llvm-project#136656</a>), 
it is unclear whether such uses are intentionally rejected.
<p/>
Perhaps the requirements on <code class='backtick'>from_range_t</code> constructor, <code class='backtick'>insert_range</code> and <code class='backtick'>assign_range</code> should be made optional, 
although these operations should have specified semantics whenever being well-formed in immediate contexts.
</p>


<p id="res-4261"><b>Proposed resolution:</b></p>





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