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    CWG Issue 368</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="368"></A><H4>368.
  
Uses of non-type parameters that should cause deduction to fail
</H4>
<B>Section: </B>13.10.3&#160; [<A href="https://wg21.link/temp.deduct">temp.deduct</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Jason Shirk
 &#160;&#160;&#160;

 <B>Date: </B>29 July 2002<BR>


<P>[Voted into WP at October 2003 meeting.]</P>

<P>I understand the rules in 13.10.3 [<A href="https://wg21.link/temp.deduct#2">temp.deduct</A>] paragraph 2
are meant to be an exhaustive list of what can cause type deduction to
fail.</P>

<P>Consider:</P>
<PRE>
  template&lt;typename U,U u&gt; struct wrap_t;

  template&lt;typename U&gt; static yes check( wrap_t&lt;U,U(0)&gt;* );

  struct X { X(int); };
  int main() {
    check&lt;X&gt;(0);
  }
</PRE>
<P>I can see 2 reasons this might cause type deduction to fail:</P>
<UL>
<LI>
Conversion is not a compile time constant.
</LI>
<LI>
Non-type parameter is a class type.
</LI>
</UL>
<P>Neither case is mentioned in 13.10.3 [<A href="https://wg21.link/temp.deduct#2">temp.deduct</A>] paragraph 2,
nor do I see a DR mentioning these.</P>

<P><B>Proposed resolution (October 2002):</B></P>

<P>Add after the fourth-to-last bullet of
13.10.3 [<A href="https://wg21.link/temp.deduct#2">temp.deduct</A>] paragraph 2:</P>
<BLOCKQUOTE>
<UL>
<LI>
Attempting to give an invalid type to a nontype template parameter.
[<I>Example:</I>
<PRE>
  template &lt;class T, T&gt; struct S {};
  template &lt;class T&gt; int f(S&lt;T, T()&gt;*);
  struct X {};
  int i0 = f&lt;X&gt;(0);
</PRE>
]
</LI>
</UL>
</BLOCKQUOTE>

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