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    CWG Issue 1824</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="1824"></A><H4>1824.
  
Completeness of return type vs point of instantiation
</H4>
<B>Section: </B>9.3.4.6&#160; [<A href="https://wg21.link/dcl.fct">dcl.fct</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Steve Clamage
 &#160;&#160;&#160;

 <B>Date: </B>2013-12-19<BR>


<P>[Moved to DR at the November, 2014 meeting.]</P>



<P>Consider the following example:</P>

<PRE>
  template&lt;typename T&gt; struct A {
    T t;
  };
  struct S {
    A&lt;S&gt; f() { return A&lt;S&gt;(); }
  };
</PRE>

<P>According to 9.3.4.6 [<A href="https://wg21.link/dcl.fct#9">dcl.fct</A>] paragraph 9,</P>

<BLOCKQUOTE>

The type of a parameter or the return type for a function
definition shall not be an incomplete class type (possibly
cv-qualified) unless the function is deleted
(9.6.3 [<A href="https://wg21.link/dcl.fct.def.delete">dcl.fct.def.delete</A>]) or the definition is nested
within the <I>member-specification</I> for that class
(including definitions in nested classes defined within the
class).

</BLOCKQUOTE>

<P>Thus type <TT>A&lt;S&gt;</TT> must be a complete type.
The requirement for a complete type triggers the instantiation
of the template, which requires that its template argument be
complete in order to use it as the type of a non-static data
member.</P>

<P>According to 13.8.4.1 [<A href="https://wg21.link/temp.point#4">temp.point</A>] paragraph 4,
the point of instantiation of <TT>A&lt;S&gt;</TT> is
&#8220;immediately preced[ing] the namespace scope
declaration or definition that refers to the specialization.&#8221;
Thus the point of instantiation precedes the definition of
<TT>S</TT>, making this example ill-formed.  Most or all current
implementations accept the example, however.</P>

<P>Perhaps the specification in 9.3.4.6 [<A href="https://wg21.link/dcl.fct">dcl.fct</A>] ought
to say that the completeness of the type is checked from the
context of the function body (at which S is a complete type)?</P>

<P><B>Proposed resolution (February, 2014):</B></P>

<P>Change 9.3.4.6 [<A href="https://wg21.link/dcl.fct#9">dcl.fct</A>] paragraph 9 as follows:</P>

<BLOCKQUOTE>

Types shall not be defined in return or parameter types. The type of a
parameter or the return type for a function definition shall not be an
incomplete class type (possibly cv-qualified) <INS>in the context of the
function definition</INS> unless the function is deleted
(9.6.3 [<A href="https://wg21.link/dcl.fct.def.delete">dcl.fct.def.delete</A>]) <DEL>or the definition is nested within
the <I>member-specification</I> for that class (including definitions in
nested classes defined within the class)</DEL>.

</BLOCKQUOTE>

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