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<title>Issue 3812: [fund.ts.v3] Incorrect constraint on propagate_const conversion function</title>
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<h3 id="3812"><a href="lwg-active.html#3812">3812</a>. [fund.ts.v3] Incorrect constraint on <code>propagate_const</code> conversion function</h3>
<p><b>Section:</b> 6.1.2.6 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#propagate_const.const_observers">[fund.ts.v3::propagate_const.const_observers]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Giuseppe D'Angelo <b>Opened:</b> 2022-11-04 <b>Last modified:</b> 2022-11-12</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: fund.ts.v3</b></p>
<p>
This issue has its origin in the discussion of <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107525">gcc issue 107525</a>.
<p/>
The current draft of LFTSv3 specifies this conversion function for <code>propagate_const</code> in 
6.1.2.6 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#propagate_const.const_observers">[fund.ts.v3::propagate_const.const_observers]</a>:
</p>
<blockquote>
<pre>
constexpr operator const element_type*() const;
</pre>
<blockquote>
<p>
-7- <i>Returns</i>: <code>get()</code>.
<p/>
-8- <i>Remarks</i>: This function shall not participate in overload resolution unless <code>T</code> is an 
object pointer type or has an implicit conversion to <code>const element_type*</code>. 
</p>
</blockquote>
</blockquote>
<p>
The constraint should however specify that <code>const T</code> (and not <code>T</code>) needs
to have an implicit conversion to <code>const element_type *</code>.
<p/>
Basically: if <code>const T</code> cannot do the conversion, then neither <code>const propagate_const&lt;T&gt;</code> 
should be able to.
<p/>
One can design a type <code>X</code> such as a <code>const X</code> cannot convert to <code>const element_type *</code> 
(for instance, by <code>=delete</code>ing the corresponding conversion function). If now one asks whether 
<code>const propagate_const&lt;X&gt;</code> is convertible to <code>const element_type *</code>, the answer is 
(surprisingly) "yes".
</p>

<p><i>[Kona 2022-11-12; Set priority to 3]</i></p>




<p id="res-3812"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4840" title=" Working Draft, C++ Extensions for Library Fundamentals, Version 3">N4840</a>.
</p>

<ol>
<li><p>Modify 6.1.2.6 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#propagate_const.const_observers">[fund.ts.v3::propagate_const.const_observers]</a> as indicated:</p>

<blockquote>
<pre>
constexpr operator const element_type*() const;
</pre>
<blockquote>
<p>
-7- <i>Returns</i>: <code>get()</code>.
<p/>
-8- <i>Remarks</i>: This function shall not participate in overload resolution unless <code>T</code> is an 
object pointer type or <ins><code>const T</code></ins> has an implicit conversion to <code>const element_type*</code>. 
</p>
</blockquote>
</blockquote>

</li>

</ol>





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