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<title>Issue 3459: Why doesn't std::convertible_to have semantic requirement when To is reference-to-function type?</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="3459"><a href="lwg-active.html#3459">3459</a>. Why doesn't <code>std::convertible_to</code> have semantic requirement when <code>To</code> is reference-to-function type?</h3>
<p><b>Section:</b> 18.4.4 <a href="https://wg21.link/concept.convertible">[concept.convertible]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2020-06-30 <b>Last modified:</b> 2020-07-12</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#concept.convertible">active issues</a> in [concept.convertible].</p>
<p><b>View all other</b> <a href="lwg-index.html#concept.convertible">issues</a> in [concept.convertible].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
18.4.4 <a href="https://wg21.link/concept.convertible">[concept.convertible]</a> p2:
</p>
<blockquote>
<p>
Types <code>From</code> and <code>To</code> model <code>convertible_to&lt;From, To&gt;</code> only if:
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; <code>To</code> is not an object or reference-to-object type, or <code>static_cast&lt;To&gt;(f())</code> is 
equal to <code>test(f)</code>.</p></li>
<li><p>[&hellip;]</p></li>
</ol>
</blockquote>
<p>
This requires the implicit and explicit conversions to produce equal results.
<p/>
However, it seems that when <code>To</code> is a reference-to-function type, this restriction does not apply. 
This makes it possible to create a class that models <code>convertible_to</code>, but produces different 
results depending on the kind of conversion:
</p>
<blockquote><pre>
#include &lt;concepts&gt;

int foo() { return 0; }
int bar() { return 42; }

using FT = int();
struct A 
{
  operator FT&amp;() const { return foo; }
  explicit operator FT&amp;() { return bar; }
};

static_assert(std::convertible_to&lt;A, FT&amp;&gt;);

A a;
FT&amp; x = a;                    // x == foo
auto y = static_cast&lt;FT&amp;&gt;(a); // y == bar
</pre></blockquote>

<p><i>[2020-07-12; Reflector prioritization]</i></p>

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


<p id="res-3459"><b>Proposed resolution:</b></p>





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