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<title>Issue 2767: not_fn call_wrapper can form invalid types</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++17">C++17</a> status.</em></p>
<h3 id="2767"><a href="lwg-defects.html#2767">2767</a>. <code>not_fn <i>call_wrapper</i></code> can form invalid types</h3>
<p><b>Section:</b> 22.10.13 <a href="https://wg21.link/func.not.fn">[func.not.fn]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2016-08-19 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#func.not.fn">issues</a> in [func.not.fn].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The definition of the <code><i>call_wrapper</i></code> type in the C++17 CD means this
fails to compile:
</p>
<blockquote><pre>
#include &lt;functional&gt;

struct abc { virtual void f() const = 0; };
struct derived : abc { void f() const { } };
struct F { bool operator()(abc&amp;) { return false; } };
derived d;
bool b = std::not_fn(F{})(static_cast&lt;abc&amp;&amp;&gt;(d));
</pre></blockquote>
<p>
The problem is that the return types use <code>result_of_t&lt;F(abc)&gt;</code> and
<code>F(abc)</code> is not a valid function type, because it takes an abstract
class by value.
<p/>
The return types should use <code>result_of_t&lt;F(Args<ins>&amp;&amp;</ins>...)&gt;</code> instead.
</p>

<p><i>[2016-09-09 Issues Resolution Telecon]</i></p>

<p>P0; move to Tentatively Ready</p>


<p id="res-2767"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4606.</p>

<ol>
<li><p>Modify  [func.not_fn], class <code><i>call_wrapper</i></code> synopsis, as indicated:</p>
<blockquote>
<pre>
class <i>call_wrapper</i>
{
  [&hellip;]
  template&lt;class... Args&gt;
    auto operator()(Args&amp;&amp;...) &amp;
      -&gt; decltype(!declval&lt;result_of_t&lt;FD&amp;(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
  template&lt;class... Args&gt;
    auto operator()(Args&amp;&amp;...) const&amp;
      -&gt; decltype(!declval&lt;result_of_t&lt;FD const&amp;(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
  template&lt;class... Args&gt;
    auto operator()(Args&amp;&amp;...) &amp;&amp;
      -&gt; decltype(!declval&lt;result_of_t&lt;FD(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
  template&lt;class... Args&gt;
    auto operator()(Args&amp;&amp;...) const&amp;&amp;
      -&gt; decltype(!declval&lt;result_of_t&lt;FD const(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
  [&hellip;]
};
</pre>
</blockquote>
</li>

<li><p>Modify the prototype declarations of  [func.not_fn] as indicated:</p>
<blockquote>
<pre>
template&lt;class... Args&gt;
  auto operator()(Args&amp;&amp;... args) &amp;
    -&gt; decltype(!declval&lt;result_of_t&lt;FD&amp;(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
template&lt;class... Args&gt;
  auto operator()(Args&amp;&amp;... args) const&amp;
    -&gt; decltype(!declval&lt;result_of_t&lt;FD const&amp;(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class... Args&gt;
  auto operator()(Args&amp;&amp;... args) &amp;&amp;
    -&gt; decltype(!declval&lt;result_of_t&lt;FD(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
template&lt;class... Args&gt;
  auto operator()(Args&amp;&amp;... args) const&amp;&amp;
    -&gt; decltype(!declval&lt;result_of_t&lt;FD const(Args<ins>&amp;&amp;</ins>...)&gt;&gt;());
</pre>
</blockquote>
</li>
</ol>





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