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<title>Issue 4100: Default arguments and signatures of standard library non-member functions</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#New">New</a> status.</em></p>
<h3 id="4100"><a href="lwg-active.html#4100">4100</a>. Default arguments and signatures of standard library non-member functions</h3>
<p><b>Section:</b> 16.4.6.4 <a href="https://wg21.link/global.functions">[global.functions]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-05-11 <b>Last modified:</b> 2024-08-02</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#global.functions">issues</a> in [global.functions].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, <code>std::from_chars</code> and <code>std::to_chars</code> are specified with default arguments. 
Some implementors want to split them into more overloads to determine the base 10 at compile time 
(LLVM issue <a href="https://github.com/llvm/llvm-project/issues/91268">#91268</a>). However, the 
current standard wording doesn't clearly allow such technique.
</p>

<p><i>[2024-08-02; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
"It's a lot of new text just to say you can't do that, lose the example?"
"Might be NAD, is this already disallowed?" "Only for member functions."
</p>



<p id="res-4100"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>

<li><p>Modify 16.4.6.4 <a href="https://wg21.link/global.functions">[global.functions]</a> as indicated:</p>

<blockquote>
<p>
[&hellip;]
<p/>
-2- A call to a non-member function signature described in 17 <a href="https://wg21.link/support">[support]</a> through 
32 <a href="https://wg21.link/thread">[thread]</a> and D <a href="https://wg21.link/depr">[depr]</a> shall behave as if the implementation declared 
no additional non-member function signatures. <ins>However, for each non-member function specified 
with at least one default argument, an implementation may declare additional function signatures 
with one or more trailing parameters that have default arguments removed.</ins>
<p/>
<ins>[<i>Example</i> 1: The following program is possibly ill-formed because of ambiguity in overload 
resolution.</ins>
</p>
<blockquote><pre>
<ins>#include &lt;cstddef&gt;
#include &lt;string&gt;

// One overload of std::stoi ([string.conversions]) is specified as
// int stoi(const string&amp; str, size_t* idx = nullptr, int base = 10);
// An implementation can declare these overloads instead of using default arguments:
// int stoi(const string&amp; str, size_t* idx, int base);
// int stoi(const string&amp; str, size_t* idx);
// int stoi(const string&amp; str);

namespace usr {
  int stoi(const std::string&amp;);
  int stoi(const std::string&amp;, std::size_t*);
}

int main() {
  using std::stoi;
  using usr::stoi;
  int (*p1)(const std::string&amp;) = stoi;                 // possibly ill-formed
  int (*p2)(const std::string&amp;, std::size_t*) = stoi;   // possibly ill-formed
}</ins>
</pre></blockquote>
<p>
<ins>&mdash; end example]</ins>
</p>
</blockquote>

</li>
</ol>






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