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<title>Issue 3833: Remove specialization template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&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#C++23">C++23</a> status.</em></p>
<h3 id="3833"><a href="lwg-defects.html#3833">3833</a>. Remove specialization <code>template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;</code></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> 2022-11-27 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</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>
In the past I discussed with Victor and Charlie to remove
</p>
<blockquote><pre>
template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;;
</pre></blockquote>
<p>
Charlie disliked that since MSVC STL already shipped it and Victor
mentioned it was useful. Instead of proposing to remove the
specialization in LWG <a href="lwg-defects.html#3701" title="Make formatter&lt;remove_cvref_t&lt;const charT[N]&gt;, charT&gt; requirement explicit (Status: C++23)">3701</a><sup><a href="https://cplusplus.github.io/LWG/issue3701" title="Latest snapshot">(i)</a></sup> ("Make <code>formatter&lt;remove_cvref_t&lt;const charT[N]&gt;, charT&gt;</code> 
requirement explicit") I proposed to keep it. It's unused but it doesn't hurt. 
<p/>
That was until <a href="https://wg21.link/P2585R0" title=" Improving default container formatting">P2585R0</a> "Improve default container formatting". This paper makes it no 
longer possible to instantiate this specialization. See 
<a href="https://godbolt.org/z/bqjhhaexx">here for an example</a> and a possible work-around.
<p/>
The relevant wording is 28.5.1 <a href="https://wg21.link/format.syn">[format.syn]</a>:
</p>
<blockquote><pre>
template&lt;class R&gt;
  constexpr <i>unspecified</i> format_kind = <i>unspecified</i>;

template&lt;ranges::input_range R&gt;
    requires same_as&lt;R, remove_cvref_t&lt;R&gt;&gt;
  constexpr range_format format_kind&lt;R&gt; = <i>see below</i>;
</pre></blockquote>
<p>
combined with  28.5.7.1 <a href="https://wg21.link/format.range.fmtkind">[format.range.fmtkind]</a> p1:
</p>
<blockquote><p>
A program that instantiates the primary template of <code>format_kind</code> is
ill-formed.
</p></blockquote>
<p>
The issue is that <code>const charT[N]</code> does not satisfy the requirement
<code>same_as&lt;R, remove_cvref_t&lt;R&gt;&gt;</code>. So it tries to instantiate the primary
template, which is ill-formed.
<p/>
I see two possible solutions:
</p>
<ol>
<li><p><em>Removing</em> the specialization</p>
<blockquote><pre>
template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;;
</pre></blockquote></li>
<li><p><em>Adding</em> a specialization</p>
<blockquote><pre>
template&lt;class charT, size_t N&gt;
  constexpr range_format format_kind&lt;const charT[N]&gt; =
    range_format::disabled;
</pre></blockquote></li>
</ol>
<p>
I discussed this issue privately and got no objection for solution 1,
therefore I propose to take that route. Implementations can still
implement solution 2 as an extension until they are ready to ship an
API/ABI break.
</p>

<p><i>[2022-11-30; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>
<p>
"Rationale is not really convincing why the first option is the right one."
</p>
<p>
"The point is not that we would need to add a format_kind specialization,
it is that the specialization is inconsistent with the design of <code>formatter</code>,
which is supposed to be instantiated only for cv-unqualified non-reference types."
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">


<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</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. A 
<i>debug-enabled</i> specialization of <code>formatter</code> additionally provides a public, constexpr,
non-static member function <code>set_debug_format()</code> which modifies the state of the <code>formatter</code> 
to be as if the type of the <i>std-format-spec</i> parsed by the last call to <code>parse</code> were <code>?</code>. 
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 debug-enabled specializations [&hellip;]</p></li>
<li><p>(2.2) &mdash; For each <code>charT</code>, the debug-enabled 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;;
template&lt;size_t N&gt; struct formatter&lt;charT[N], charT&gt;;
<del>template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;;</del>
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>

<li><p>Add a new paragraph to C.2.10 <a href="https://wg21.link/diff.cpp20.utilities">[diff.cpp20.utilities]</a> as indicated:</p>

<blockquote>
<p>
<ins><b>Affected subclause</b>: 28.5.6.4 <a href="https://wg21.link/format.formatter.spec">[format.formatter.spec]</a></ins>
<p/>
<ins><b>Change</b>: Remove the <code>formatter</code> specialization
<code>template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;</code>.</ins>
<p/>
<ins><b>Rationale</b>: The <code>formatter</code> specialization was not used in the Standard library.
Keeping the specialization well-formed required an additional <code>format_kind</code>
specialization.</ins>
<p/>
<ins><b>Effect on original feature</b>: Valid C++ 2020 code that instantiated the removed
specialization is now ill-formed.</ins>
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2023-02-07 Tim provides updated wording]</i></p>


<p><i>[Issaquah 2023-02-08; LWG]</i></p>

<p>Unanimous consent to move to Immediate.</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3833"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</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. A 
<i>debug-enabled</i> specialization of <code>formatter</code> additionally provides a public, constexpr,
non-static member function <code>set_debug_format()</code> which modifies the state of the <code>formatter</code> 
to be as if the type of the <i>std-format-spec</i> parsed by the last call to <code>parse</code> were <code>?</code>. 
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 debug-enabled specializations [&hellip;]</p></li>
<li><p>(2.2) &mdash; For each <code>charT</code>, the debug-enabled 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;;
template&lt;size_t N&gt; struct formatter&lt;charT[N], charT&gt;;
<del>template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;;</del>
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>

<li><p>Add a new paragraph to C.2.10 <a href="https://wg21.link/diff.cpp20.utilities">[diff.cpp20.utilities]</a> as indicated:</p>

<blockquote>
<p>
<ins><b>Affected subclause</b>: 28.5.6.4 <a href="https://wg21.link/format.formatter.spec">[format.formatter.spec]</a></ins>
<p/>
<ins><b>Change</b>: Removed the <code>formatter</code> specialization
<code>template&lt;size_t N&gt; struct formatter&lt;const charT[N], charT&gt;</code>.</ins>
<p/>
<ins><b>Rationale</b>: The specialization is inconsistent with the design of <code>formatter</code>,
which is intended to be instantiated only with cv-unqualified object types.</ins>
<p/>
<ins><b>Effect on original feature</b>: Valid C++ 2020 code that instantiated the removed
specialization can become ill-formed.</ins>
</p>
</blockquote>
</li>
</ol>






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