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<title>Issue 3619: Specification of vformat_to contains ill-formed formatted_size calls</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="3619"><a href="lwg-defects.html#3619">3619</a>. Specification of <code>vformat_to</code> contains ill-formed <code>formatted_size</code> calls</h3>
<p><b>Section:</b> 28.5.5 <a href="https://wg21.link/format.functions">[format.functions]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-10-17 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#format.functions">issues</a> in [format.functions].</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 specification of <code>vformat_to</code> says that it formats "into the range
<code>[out, out + N)</code>, where <code>N</code> is <code>formatted_size(fmt, args...)</code> for the
functions without a <code>loc</code> parameter and <code>formatted_size(loc, fmt, args...)</code> 
for the functions with a <code>loc</code> parameter".
<p/>
This is wrong in at least two ways:
</p>
<ul>
<li><p>First, <code>args</code> is a <code>(w)format_args</code>, not a pack, so it doesn't make
sense to use <code>...</code> to expand it.</p></li>
<li><p>Second, <code>fmt</code> is a <code>(w)string_view</code> parameter, and it is never a
constant expression, and so the call is ill-formed after <a href="https://wg21.link/P2216" title=" std::format improvements">P2216</a> added 
compile-time format string checking.</p></li>
</ul>

<p><i>[2022-01-29; Reflector poll]</i></p>

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

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3619"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 28.5.5 <a href="https://wg21.link/format.functions">[format.functions]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Out&gt;
  Out vformat_to(Out out, string_view fmt, format_args args);
template&lt;class Out&gt;
  Out vformat_to(Out out, wstring_view fmt, wformat_args args);
template&lt;class Out&gt;
  Out vformat_to(Out out, const locale&amp; loc, string_view fmt, format_args args);
template&lt;class Out&gt;
  Out vformat_to(Out out, const locale&amp; loc, wstring_view fmt, wformat_args args);
</pre>
<blockquote>
<p>
-12- Let <code>charT</code> be <code>decltype(fmt)::value_type</code>.
<p/>
-13- <i>Constraints:</i> <code>Out</code> satisfies <code>output_iterator&lt;const charT&amp;&gt;</code>.
<p/>
-14- <i>Preconditions:</i> <code>Out</code> models <code>output_iterator&lt;const charT&amp;&gt;</code>.
<p/>
-15- <i>Effects:</i> Places the character representation of formatting the arguments provided by 
<code>args</code>, formatted according to the specifications given in <code>fmt</code>, into the range 
<code>[out, out + N)</code>, where <code>N</code> is <del><code>formatted_size(fmt, args...)</code> for the 
functions without a <code>loc</code> parameter and <code>formatted_size(loc, fmt, args...)</code> for 
the functions with a <code>loc</code> parameter</del><ins>the number of characters in that
character representation</ins>. If present, <code>loc</code> is used for locale-specific formatting.
<p/>
-16- <i>Returns:</i> <code>out + N</code>.
<p/>
-17- [&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





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