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<title>Issue 3246: What are the constraints on the template parameter of basic_format_arg?</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++20">C++20</a> status.</em></p>
<h3 id="3246"><a href="lwg-defects.html#3246">3246</a>. What are the constraints on the template parameter of <code>basic_format_arg</code>?</h3>
<p><b>Section:</b> 28.5.8.1 <a href="https://wg21.link/format.arg">[format.arg]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2019-08-01 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#format.arg">issues</a> in [format.arg].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In <a href="https://wg21.link/p0645r10">P0645R10</a> 20.?.5.1/ we find:
</p>
<blockquote><p>
<i>Constraints:</i> <code>typename Context::template formatter_type&lt;T&gt;</code> is enabled.
</p></blockquote>
<p>
&hellip; which doesn't mean anything, because that is an arbitrary type. Presumably the intent 
is that that type will always be a specialization of formatter, but there appear to be 
no constraints whatsoever on the <code>Context</code> template parameter, so there seems to be no 
requirement that that is the case.
<p/>
Should <code>basic_format_arg</code> place some constraints on its <code>Context</code> template parameter? 
E.g., should it be required to be a specialization of <code>basic_format_context</code>?
<p/>
<b>Victor Zverovich:</b>
<p/>
The intent here is to allow different context types provide their own formatter extension types. 
The default formatter context and extension are <code>basic_format_context</code> and formatter 
respectively, but it's possible to have other. For example, in the <code>fmt</code> library there 
is a formatter context that supports <code>printf</code> formatting for legacy code. It cannot use 
the default formatter specializations because of the different syntax <code>(%...</code> vs 
<code>{...})</code>.
<p/>
<b>Richard Smith:</b>
<p/>
In either case, the specification here seems to be missing the rules for what is a valid 
<code>Context</code> parameter.
<p/>
I'm happy to editorially change "is enabled" to "is an enabled specialization of formatter", 
since there's nothing else that this could mean, but we still need a wording fix for the 
broader issue here. Here's what I have so far for this wording:
</p>
<blockquote><p>
<i>Constraints:</i> The template specialization <code>typename Context::template 
formatter_type&lt;T&gt;</code> is an enabled specialization of formatter 
([formatter.requirements]).
</p></blockquote>
<p>
<b>Tim Song:</b>
<p/>
I think what we actually want here is "<code>typename Context::template formatter_type&lt;T&gt;</code> 
meets the <i>Formatter</i> requirements".
</p>

<p><i>[2019-08-17 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.</p>


<p id="res-3246"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 28.5.8.1 <a href="https://wg21.link/format.arg">[format.arg]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt; explicit basic_format_arg(const T&amp; v) noexcept;
</pre>
<blockquote>
<p>
-4- <i>Constraints:</i> The template specialization
</p>
<blockquote>
<pre>
typename Context::template formatter_type&lt;T&gt;
</pre>
</blockquote>
<p>
<del>is an enabled specialization of <code>formatter</code></del><ins>meets the
<i>Formatter</i> requirements</ins> (28.5.6 <a href="https://wg21.link/format.formatter">[format.formatter]</a>). The extent 
to which an implementation determines that the specialization <del>is 
enabled</del><ins>meets the <i>Formatter</i> requirements</ins> is unspecified, 
except that as a minimum the expression
</p>
<blockquote>
<pre>
typename Context::template formatter_type&lt;T&gt;()
  .format(declval&lt;const T&amp;&gt;(), declval&lt;Context&amp;&gt;())
</pre>
</blockquote>
<p>
shall be well-formed when treated as an unevaluated operand.
</p>
</blockquote>
</blockquote>
</li>
</ol>




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