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<title>Issue 3701: Make formatter&lt;remove_cvref_t&lt;const charT[N]&gt;, charT&gt; requirement explicit</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++23">C++23</a> status.</em></p>
<h3 id="3701"><a href="lwg-defects.html#3701">3701</a>. Make <code>formatter&lt;remove_cvref_t&lt;const charT[N]&gt;, charT&gt;</code> requirement explicit</h3>
<p><b>Section:</b> 28.5.6.4 <a href="https://wg21.link/format.formatter.spec">[format.formatter.spec]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Mark de Wever <b>Opened:</b> 2022-05-17 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#format.formatter.spec">active issues</a> in [format.formatter.spec].</p>
<p><b>View all other</b> <a href="lwg-index.html#format.formatter.spec">issues</a> in [format.formatter.spec].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The wording in 28.5.5 <a href="https://wg21.link/format.functions">[format.functions]</a>/20 and 28.5.5 <a href="https://wg21.link/format.functions">[format.functions]</a>/25 both contain
</p>
<blockquote>
<p>
<code>formatter&lt;remove_cvref_t&lt;T<sub><i>i</i></sub>&gt;, charT&gt;</code> meets the <i>BasicFormatter</i> 
requirements (28.5.6.1 <a href="https://wg21.link/formatter.requirements">[formatter.requirements]</a>) for each <code>T<sub><i>i</i></sub></code> in <code>Args</code>.
</p>
</blockquote>
<p>
The issue is that <code>remove_cvref_t&lt;const charT[N]&gt;</code> becomes <code>charT[N]</code>. 
28.5.6.4 <a href="https://wg21.link/format.formatter.spec">[format.formatter.spec]</a>/2.2 requires a specialization for
</p>
<blockquote>
<pre>
template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;;
</pre>
</blockquote>
<p>
but there's no requirement to provide
</p>
<blockquote>
<pre>
 template&lt;size_t N&gt; struct formatter&lt;charT[N], charT&gt;;
</pre>
</blockquote>
<p>
There's no wording preventing library vendors from providing additional specializations. 
So it's possible to implement the current specification but the indirect requirement is odd. 
I noticed this while implementing a <code>formattable</code> concept. The concept is based on the 
<code>formattable</code> concept of <a href="https://wg21.link/P2286" title=" Formatting Ranges">P2286</a> "Formatting Ranges" (This paper is targeting C++23.)
<p/>
It could be argued that the specialization
</p>
<blockquote>
<pre>
template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;
</pre>
</blockquote>
<p>
is not needed and should be removed from the Standard. This will be an API break. Vendors 
can decide to keep the no longer required specialization as an extension; which would lead 
to implementation divergence. Microsoft is already shipping this specialization as stable
and Victor doesn't like the removal too.
<p/>
Therefore I only propose to add the required <code>formatter</code> specialization.
</p>

<p><i>[2022-06-21; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3701"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 28.5.6.4 <a href="https://wg21.link/format.formatter.spec">[format.formatter.spec]</a> as indicated:</p>

<blockquote>
<p>
-2- Let <code>charT</code> be either <code>char</code> or <code>wchar_t</code>. Each specialization of 
<code>formatter</code> is either enabled or disabled, as described below. Each header that declares the 
template <code>formatter</code> provides the following enabled specializations:
</p>
<ol style="list-style-type:none">
<li><p>(2.1) &mdash; The specializations [&hellip;]</p></li>
<li><p>(2.2) &mdash; For each <code>charT</code>, the string type specializations</p>
<blockquote><pre>
template&lt;&gt; struct formatter&lt;charT*, charT&gt;;
template&lt;&gt; struct formatter&lt;const charT*, charT&gt;;
<ins>template&lt;size_t N&gt; struct formatter&lt;charT[N], charT&gt;;</ins>
template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;;
template&lt;class traits, class Allocator&gt;
  struct formatter&lt;basic_string&lt;charT, traits, Allocator&gt;, charT&gt;;
template&lt;class traits&gt;
  struct formatter&lt;basic_string_view&lt;charT, traits&gt;, charT&gt;;
</pre></blockquote>
</li>
<li><p>(2.3) &mdash; [&hellip;]</p></li>
<li><p>(2.4) &mdash; [&hellip;]</p></li>
</ol>
</blockquote>
</li>

</ol>





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