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    CWG Issue 300</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="300"></A><H4>300.
  
References to functions in template argument deduction
</H4>
<B>Section: </B>13.10.3.6&#160; [<A href="https://wg21.link/temp.deduct.type">temp.deduct.type</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD1
 &#160;&#160;&#160;

 <B>Submitter: </B>Andrei Iltchenko
 &#160;&#160;&#160;

 <B>Date: </B>11 Jul 2001<BR>


<P>[Moved to DR at October 2002 meeting.]</P>

<P>Paragraph 9 of 13.10.3.6 [<A href="https://wg21.link/temp.deduct.type">temp.deduct.type</A>] enumerates the
forms that the types P and A need to have in order for template
argument deduction to succeed.</P>

<P>For P denoting a pointer to function the paragraph lists the following
forms as allowing for template argument deduction:</P>
<PRE>
type(*)(T)
T(*)()
T(*)(T)
</PRE>

<P>On the other hand, no provision has been made to accommodate similar
cases for references to functions, which in light of the wording of
13.10.3.6 [<A href="https://wg21.link/temp.deduct.type#11">temp.deduct.type</A>] paragraph 11 means that the program below
is ill-formed (some of the C++ compilers do not reject it however):</P>

<PRE>
    template&lt;typename Arg, typename Result, typename T&gt;
    Result  foo_r(Result(&amp; rf)(Arg), T x)
    {   return  rf(Arg(x));   }

    template&lt;typename Arg, typename Result, typename T&gt;
    Result  foo_p(Result(* pf)(Arg), T x)
    {   return  pf(Arg(x));   }

    #include &lt;iostream&gt;
    int  show_arg(char c)
    {
       std::cout &lt;&lt; c &lt;&lt; ' ';
       if(std::cout)  return  0;
       return  -1;
    }

    int  main()
    {
                                                   // The deduction
       int  (&amp; rf1)(int(&amp;)(char), double) = foo_r; // shall fail here
                                                   // While here
       int  (&amp; rf2)(int(*)(char), double) = foo_p; // it shall succeed
       return  rf2(show_arg, 2);
    }
</PRE>

<P><B>Proposed resolution (10/01, same as suggested resolution):</B></P>

<P>In the list of allowable forms for the types <TT>P</TT> and
<TT>A</TT> in paragraph 9 of
13.10.3.6 [<A href="https://wg21.link/temp.deduct.type">temp.deduct.type</A>] replace</P>
<PRE>
type(*)(T)
T(*)()
T(*)(T)
</PRE>
<P>by</P>
<PRE>
type(T)
T()
T(T)
</PRE>

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