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    CWG Issue 2052</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="2052"></A><H4>2052.
  
Template argument deduction vs overloaded operators
</H4>
<B>Section: </B>12.4&#160; [<A href="https://wg21.link/over.oper">over.oper</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD4
 &#160;&#160;&#160;

 <B>Submitter: </B>Richard Smith
 &#160;&#160;&#160;

 <B>Date: </B>2014-12-03<BR>


<P>[Moved to DR at the October, 2015 meeting.]</P>



<P>In an example like</P>

<PRE>
  struct A { operator int(); };
  template&lt;typename T&gt; T operator&lt;&lt;(T, int);
  void f(A a) { 1 &lt;&lt; a; }
</PRE>

<P>Template argument deduction succeeds for the operator template,
producing the signature <TT>operator&lt;&lt;(int,int)</TT>.  The
resulting declaration is synthesized and added to the overload
set, per 13.10.4 [<A href="https://wg21.link/temp.over#1">temp.over</A>] paragraph 1.  However, this
violates the requirement of 12.4 [<A href="https://wg21.link/over.oper#6">over.oper</A>] paragraph 6,</P>

<BLOCKQUOTE>

An operator function shall either be a non-static member
function or be a non-member function that has at least one
parameter whose type is a class, a reference to a class, an
enumeration, or a reference to an enumeration.

</BLOCKQUOTE>

<P>This is not a SFINAE context, so the program is
ill-formed, rather than selecting the built-in operator.</P>

<P><B>Proposed resolution (May, 2015):</B></P>

<P>Change 13.10.4 [<A href="https://wg21.link/temp.over#1">temp.over</A>] paragraph 1 as follows,</P>

<BLOCKQUOTE>

...If, for a given function template, argument deduction fails
<INS>or the synthesized function template specialization
would be ill-formed</INS>, no such function is added to the
set of candidate functions for that template.

</BLOCKQUOTE>

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