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    CWG Issue 2595</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="2595"></A><H4>2595.
  
"More constrained" for eligible special member functions
</H4>
<B>Section: </B>11.4.4&#160; [<A href="https://wg21.link/special">special</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Barry Revzin
 &#160;&#160;&#160;

 <B>Date: </B>2022-06-08<BR>


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

<P>Consider:</P>

<PRE>
  #include &lt;type_traits&gt;

  template&lt;typename T&gt;
  concept Int = std::is_same_v&lt;T, int&gt;;

  template&lt;typename T&gt;
  concept Float = std::is_same_v&lt;T, float&gt;;

  template&lt;typename T&gt;
  struct Foo {
    Foo() requires Int&lt;T&gt; = default; //<SPAN CLASS="cmnt"> #1</SPAN>
    Foo() requires Int&lt;T&gt; || Float&lt;T&gt; = default; //<SPAN CLASS="cmnt"> #2</SPAN>
  };
</PRE>

<P>Per the wording, #1 is not eligible for <TT>Foo&lt;float&gt;</TT>,
because the constraints are not satisfied.  But #2 also is not
eligible, because #1 is more constrained than #2.  The intent is that
#2 is eligible.</P>

<P><B>Proposed resolution (approved by CWG 2023-06-17):</B></P>

<P>Change in 11.4.4 [<A href="https://wg21.link/special#6">special</A>] paragraph 6 as follows:</P>

<BLOCKQUOTE>

An <I>eligible special member function</I> is a special member function for
which:
<UL>
<LI>the function is not deleted,</LI>

<LI>the associated constraints (13.5 [<A href="https://wg21.link/temp.constr">temp.constr</A>]), if any,
are satisfied, and</LI>

<LI>no special member function of the same kind <INS>whose associated
constraints, if any, are satisfied</INS> is more constrained
(13.5.5 [<A href="https://wg21.link/temp.constr.order">temp.constr.order</A>]).</LI>
</UL>

</BLOCKQUOTE>

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