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<title>Issue 3699: lexically_relative on UNC drive paths (\\?\C:\...) results in a default-constructed value</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="3699"><a href="lwg-active.html#3699">3699</a>. <code>lexically_relative</code> on UNC drive paths (<code>\\?\C:\...</code>) results in a default-constructed value</h3>
<p><b>Section:</b> 31.12.6.5.11 <a href="https://wg21.link/fs.path.gen">[fs.path.gen]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Nicole Mazzuca <b>Opened:</b> 2022-05-12 <b>Last modified:</b> 2022-05-17</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#fs.path.gen">issues</a> in [fs.path.gen].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
As a resolution to LWG <a href="lwg-defects.html#3070" title="path::lexically_relative causes surprising results if a filename can also be a 
root-name (Status: C++20)">3070</a><sup><a href="https://cplusplus.github.io/LWG/issue3070" title="Latest snapshot">(i)</a></sup>, in <code>path lexically_relative(const path&amp; base) const</code>, 
bullet 3.4 was added:
</p>
<blockquote><p>
If: [&hellip;] any <i>filename</i> in <code>relative_path()</code> or <code>base.relative_path()</code> can be interpreted as 
a <i>root-name</i>, [&hellip;] returns <code>path()</code>.
</p></blockquote>
<p>
This resolution was correct when we have really weird paths like <code>abc\X:\c</code>, but the MSVC standard library 
implementation treats UNC drive-relative paths as:
</p>
<blockquote><pre>
\\?\C:\foo\bar = { <i>root-name</i> = \\?, <i>root-directory</i> = \, <i>relative-path</i> = C:\foo\bar }
</pre></blockquote>
<p>
If we were able to go back in time, we might have <code><i>root-name</i> = \\?\C:</code>, but we can't make that 
change at that point without silently breaking users; therefore, we believe it would be best to instead change 
<code>lexically_relative()</code> to work around this issue.
<p/>
There exists a <a href="https://github.com/microsoft/STL/issues/2256">related github issue</a>.
<p/>
I don't yet have standard wording, but I think it would be reasonable to do something like:
</p>
<blockquote><p>
If <code>relative_path().has_root_path() &amp;&amp; base.relative_path().has_root_path()</code>, and 
<code>relative_path().root_path() == base.relative_path().root_path()</code>, then 
<code>return relative_path().lexically_relative(base.relative_path())</code>.
</p></blockquote>

<p><i>[2022-05-17; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3699"><b>Proposed resolution:</b></p>





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