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<title>Issue 3673: &sect;[locale.cons] Ambiguous argument in Throws for locale+name+category constructor</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#Resolved">Resolved</a> status.</em></p>
<h3 id="3673"><a href="lwg-defects.html#3673">3673</a>. &sect;[locale.cons] Ambiguous argument in <i>Throws</i> for locale+name+category constructor</h3>
<p><b>Section:</b> 28.3.3.1.3 <a href="https://wg21.link/locale.cons">[locale.cons]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Hubert Tong <b>Opened:</b> 2022-02-12 <b>Last modified:</b> 2023-03-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#locale.cons">issues</a> in [locale.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<blockquote><pre>
locale(const locale&amp; other, const char* std_name, category);
</pre></blockquote>
<p>
has
</p>
<blockquote>
<p>
<i>Throws:</i> <code>runtime_error</code> if the argument is not valid, or is null.
</p>
</blockquote>
<p>
There being three arguments, the statement is rather problematic. It looks like a copy/paste from
</p>
<blockquote><pre>
explicit locale(const char* std_name);
</pre></blockquote>
<p>
The conclusion, assuming that "the argument" is also <code>std_name</code> in the problem case, seems to be 
that the statement should be changed to read:
</p>
<blockquote>
<p>
<i>Throws:</i> <code>runtime_error</code> if <code>std_name</code> is not valid, or is null.
</p>
</blockquote>
<p>
However there is implementation divergence over whether or not values for the category argument not 
explicitly described as valid by 28.3.3.1.2.1 <a href="https://wg21.link/locale.category">[locale.category]</a> result in <code>runtime_error</code>.
<p/>
libc++ does not throw. libstdc++ does.
<p/>
Code:
</p>
<blockquote><pre>
#include &lt;locale&gt;
#include &lt;stdio.h&gt;
#include &lt;exception&gt;
int main(void) {
  std::locale Generic("C");
  try {
    std::locale Abomination(Generic, Generic, 0x7fff'ffff);
  } catch (std::runtime_error&amp;) {
    fprintf(stderr, "Threw\n");
  }
}
</pre></blockquote>
<p>
<a href="https://godbolt.org/z/3dossoqo6">Compiler Explorer link</a>.
</p>

<p><i>[2022-03-04; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>

<p><i>[2022-11-01; Jonathan comments]</i></p>

<p>The proposed resolution of <a href="lwg-defects.html#2295" title="Locale name when the provided Facet is a nullptr (Status: C++23)">2295</a><sup><a href="https://cplusplus.github.io/LWG/issue2295" title="Latest snapshot">(i)</a></sup> would resolve this too.</p>
<p>
The implementation divergence is not a problem.
28.3.3.1.2.1 <a href="https://wg21.link/locale.category">[locale.category]</a> p2 makes invalid <code>category</code> values
undefined, so silently ignoring them or throwing exceptions are both valid.
</p>

<p><i>[2023-03-22 LWG 2295 was approved in Issaquah. Status changed: New &rarr; Resolved.]</i></p>



<p id="res-3673"><b>Proposed resolution:</b></p>
<blockquote>

<p>
<ins><em>Preconditions</em>:
The <code>category</code> argument is a valid value 28.3.3.1.2.1 <a href="https://wg21.link/locale.category">[locale.category]</a>.
</ins></p>

<p>
<i>Throws:</i> <code>runtime_error</code> if
<del>the argument</del><ins><code>std_name</code></ins>
is not valid, or is null.
</p>

</blockquote>





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