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    CWG Issue 634</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="634"></A><H4>634.
  
Conditionally-supported behavior for non-POD objects passed to ellipsis redux
</H4>
<B>Section: </B>7.6.1.3&#160; [<A href="https://wg21.link/expr.call">expr.call</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Howard Hinnant
 &#160;&#160;&#160;

 <B>Date: </B>6 Jun 2007<BR>


<P>[Voted into the WP at the September, 2008 meeting.]</P>



<P>
<A HREF="506.html">Issue 506</A> changed passing a non-POD
class type to an ellipsis from undefined behavior to
conditionally-supported behavior.  As a result, an implementation could
conceivably reject code like the following:</P>

<PRE>
    struct two {char _[2];};

    template &lt;class From, class To&gt;
    struct is_convertible
    {
    private:
            static From f;

            template &lt;class U&gt; static char test(const U&amp;);
            template &lt;class U&gt; static two test(...);
    public:
            static const bool value = sizeof(test&lt;To&gt;(f)) == 1;
    };

    struct A
    {
         A();
    };

    int main()
    {
         const bool b = is_convertible&lt;A,int&gt;::value;  // b == false
    }
</PRE>

<P>This technique has become popular in template metaprogramming,
and no non-POD object is actually passed at runtime.  Concepts
will eliminate much (perhaps not all) of the need for this
kind of programming, but legacy code will persist.</P>

<P>Perhaps this technique should be officially supported by allowing
implementations to reject passing a non-POD type to ellipsis only if
it appears in a potentially-evaluated expression?</P>

<P><B>Notes from the July, 2007 meeting:</B></P>

<P>The CWG agreed with the suggestion to allow such calls in
unevaluated contexts.</P>

<P><B>Proposed resolution (September, 2007):</B></P>

<P>Change 7.6.1.3 [<A href="https://wg21.link/expr.call#7">expr.call</A>] paragraph 7 as follows:</P>

<BLOCKQUOTE>

...Passing <DEL>an</DEL> <INS>a potentially-evaluated</INS> argument of
non-trivial class type (Clause 11 [<A href="https://wg21.link/class">class</A>]) with no
corresponding parameter is conditionally-supported, with
implementation-defined semantics...

</BLOCKQUOTE>

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