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    CWG Issue 575</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="575"></A><H4>575.
  
Criteria for deduction failure
</H4>
<B>Section: </B>13.10.3&#160; [<A href="https://wg21.link/temp.deduct">temp.deduct</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++11
 &#160;&#160;&#160;

 <B>Submitter: </B>James Widman
 &#160;&#160;&#160;

 <B>Date: </B>19 April 2006<BR>


<P>[Voted into WP at August, 2010 meeting.]</P>



<P>The last two sentences of 13.10.3 [<A href="https://wg21.link/temp.deduct#5">temp.deduct</A>] paragraph 5
read:</P>

<BLOCKQUOTE>

When all template arguments have been deduced or obtained from default
template arguments, all uses of template parameters in non-deduced
contexts are replaced with the corresponding deduced or default
argument values. If the substitution results in an invalid type, as
described above, type deduction fails.

</BLOCKQUOTE>

<P>Shouldn't the substitution occur for all uses of the parameters, so
that any of them could result in deduction failure?</P>

<P><B>Proposed resolution (October, 2006):</B></P>

<P>Change 13.10.3 [<A href="https://wg21.link/temp.deduct#5">temp.deduct</A>] paragraph 5 as follows:</P>

<BLOCKQUOTE>

...When all template arguments have been deduced or obtained from
default template arguments, all uses of template parameters in
<DEL>non-deduced contexts</DEL> <INS>the function type</INS> are replaced with
the corresponding deduced or default argument values. If the
substitution results in an invalid type, as described above, type
deduction fails.

</BLOCKQUOTE>

<P><B>Notes from the September, 2008 meeting:</B></P>

<P>This issue was returned to "drafting" status in order to
coordinate the wording with the concepts proposal.</P>

<P><B>Proposed resolution (March, 2010):</B></P>

<P>Change 13.10.3 [<A href="https://wg21.link/temp.deduct#5">temp.deduct</A>] paragraph 5 as follows:</P>

<BLOCKQUOTE>

When all template arguments have been deduced or obtained from
default template arguments, all uses of template parameters in
<DEL>non-deduced contexts</DEL> <INS>the template parameter list
of the template and the function type</INS> are replaced with the
corresponding deduced or default argument values. If the
substitution results in an invalid type, as described above, type
deduction fails.

</BLOCKQUOTE>

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