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    CWG Issue 2683</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="2683"></A><H4>2683.
  
Default arguments for member functions of templated nested classes
</H4>
<B>Section: </B>9.3.4.7&#160; [<A href="https://wg21.link/dcl.fct.default">dcl.fct.default</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD7
 &#160;&#160;&#160;

 <B>Submitter: </B>Matthew House
 &#160;&#160;&#160;

 <B>Date: </B>2023-01-11<BR>


<P>[Accepted as a DR at the June, 2023 meeting.]</P>

<P>Subclause 9.3.4.7 [<A href="https://wg21.link/dcl.fct.default#6">dcl.fct.default</A>] paragraph 6 specifies:</P>

<BLOCKQUOTE>

Except for member functions of class templates, the default arguments
in a member function definition that appears outside of the class
definition are added to the set of default arguments provided by the
member function declaration in the class definition; the program is
ill-formed if a default constructor (11.4.5.2 [<A href="https://wg21.link/class.default.ctor">class.default.ctor</A>]),
copy or move constructor (11.4.5.3 [<A href="https://wg21.link/class.copy.ctor">class.copy.ctor</A>]), or copy or
move assignment operator (11.4.6 [<A href="https://wg21.link/class.copy.assign">class.copy.assign</A>]) is so
declared. Default arguments for a member function of a class template
shall be specified on the initial declaration of the member function
within the class template.

</BLOCKQUOTE>

<P>That rule appears to allow adding default arguments for member
functions of classes that are nested within class templates, for
example:</P>

<PRE>
  template&lt;class&gt; struct A { struct B { void c(int); }; };
  template&lt;class T&gt; void A&lt;T&gt;::B::c(int = 0) {}
</PRE>

<P>MSVC accepts; gcc and clang reject.</P>

<P><B>Proposed resolution (approved by CWG 2023-03-03):</B></P>

<P>Change in 9.3.4.7 [<A href="https://wg21.link/dcl.fct.default#6">dcl.fct.default</A>] paragraph 6 as follows:</P>

<BLOCKQUOTE>

Except for member functions of <DEL>class
templates</DEL> <INS>templated classes</INS>, the default arguments in
a member function definition that appears outside of the class
definition are added to the set of default arguments provided by the
member function declaration in the class definition; the program is
ill-formed if a default constructor (11.4.5.2 [<A href="https://wg21.link/class.default.ctor">class.default.ctor</A>]),
copy or move constructor (11.4.5.3 [<A href="https://wg21.link/class.copy.ctor">class.copy.ctor</A>]), or copy or
move assignment operator (11.4.6 [<A href="https://wg21.link/class.copy.assign">class.copy.assign</A>]) is so
declared. Default arguments for a member function of
a <INS>templated</INS> class <DEL>template</DEL> shall be specified on
the initial declaration of the member function within
the <INS>templated</INS> class <DEL>template</DEL>.

</BLOCKQUOTE>

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