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<title>Issue 3542: basic_format_arg mis-handles basic_string_view with custom traits</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="3542"><a href="lwg-defects.html#3542">3542</a>. <code>basic_format_arg</code> mis-handles <code>basic_string_view</code> with custom traits</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++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2021-04-20 <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.arg">issues</a> in [format.arg].</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>
<code>basic_format_arg</code> has a constructor that accepts a <code>basic_string_view</code> of an appropriate character type, 
with <em>any</em> <code>traits</code> type. The constructor is specified in 28.5.8.1 <a href="https://wg21.link/format.arg">[format.arg]</a> as:
</p>
<blockquote><pre>
template&lt;class traits&gt;
explicit basic_format_arg(basic_string_view&lt;char_type, traits&gt; s) noexcept;
</pre>
<blockquote>
<p>
-9- <i>Effects:</i> Initializes <code>value</code> with <code>s</code>.
</p>
</blockquote>
</blockquote>
<p>
Recall that <code>value</code> is a <code>variant&lt;monostate, bool, char_type, int, unsigned int, long long int, 
unsigned long long int, float, double, long double, const char_type*, basic_string_view&lt;char_type&gt;, const void*, handle&gt;</code> 
as specified earlier in the subclause. Since <code>basic_string_view&lt;<i>meow</i>, <i>woof</i>&gt;</code> cannot be 
initialized with an lvalue <code>basic_string_view&lt;<i>meow</i>, <i>quack</i>&gt;</code> &mdash; and certainly none 
of the other alternative types can be initialized by such an lvalue &mdash; the effects of this constructor are 
ill-formed when <code>traits</code> is not <code>std::char_traits&lt;char_type&gt;</code>.
<p/>
The <code>basic_string</code> constructor deals with this same issue by ignoring the deduced traits type when initializing 
<code>value</code>'s <code>basic_string_view</code> member:
</p>
<blockquote><pre>
template&lt;class traits, class Allocator&gt;
  explicit basic_format_arg(
    const basic_string&lt;char_type, traits, Allocator&gt;&amp; s) noexcept;
</pre>
<blockquote>
<p>
-10- <i>Effects:</i> Initializes <code>value</code> with <code>basic_string_view&lt;char_type&gt;(s.data(), s.size())</code>.
</p>
</blockquote>
</blockquote>
<p>
which immediately begs the question of "why doesn't the <code>basic_string_view</code> constructor do the same?"
</p>

<p><i>[2021-05-10; Reflector poll]</i></p>

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

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3542"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</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 traits&gt;
  explicit basic_format_arg(basic_string_view&lt;char_type, traits&gt; s) noexcept;
</pre>
<blockquote>
<p>
-9- <i>Effects:</i> Initializes <code>value</code> with <code><del>s</del><ins>basic_string_view&lt;char_type&gt;(s.data(), s.size())</ins></code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>





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