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    CWG Issue 2437</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="2437"></A><H4>2437.
  
Conversion of <TT>std::strong_ordering</TT> in a defaulted <TT>operator&lt;=&gt;</TT>
</H4>
<B>Section: </B>11.10.3&#160; [<A href="https://wg21.link/class.spaceship">class.spaceship</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++20
 &#160;&#160;&#160;

 <B>Submitter: </B>Richard Smith
 &#160;&#160;&#160;

 <B>Date: </B>2019-08-14<BR>


<P>[Adopted as a DR at the November, 2019 meeting.]</P>

<P>According to 11.10.3 [<A href="https://wg21.link/class.spaceship#3">class.spaceship</A>] paragraph 3,</P>

<BLOCKQUOTE>

The return value <TT>V</TT> of type <TT>R</TT> of the
defaulted three-way comparison operator function with
parameters <TT>x</TT> and <TT>y</TT> of the same type is
determined by comparing corresponding elements
<TT>x</TT><I><SUB>i</SUB></I>
and <TT>y</TT><I><SUB>i</SUB></I> in the expanded lists of
subobjects for <TT>x</TT> and <TT>y</TT> (in increasing
index order) until the first index <I>i</I> where the
synthesized three-way comparison for comparison category
type <TT>R</TT> between <TT>x</TT><I><SUB>i</SUB></I>
and <TT>y</TT><I><SUB>i</SUB></I> yields a result
value <TT>v</TT><I><SUB>i</SUB></I>
where <TT>v</TT><I><SUB>i</SUB></I>
<TT>!=</TT> 0, contextually converted to <TT>bool</TT>,
yields <TT>true</TT>; <TT>V</TT>
is <TT>v</TT><I><SUB>i</SUB></I>
converted to <TT>R</TT>. If no such
index exists, <TT>V</TT>
is <TT>std::strong_ordering::equal</TT>
converted to
<TT>R</TT>.

</BLOCKQUOTE>

<P>This is meaningless, however, because the kind of conversion
is not specified. According to bullet 1.1,</P>

<UL>
<LI><P>If overload resolution for <TT>a &lt;=&gt; b</TT>
finds a usable
function (12.2 [<A href="https://wg21.link/over.match">over.match</A>]), <TT>static_cast&lt;R&gt;(a
&lt;=&gt; b)</TT>.</P></LI>

</UL>

<P>so consistency would suggest that
<TT>static_cast&lt;R&gt;(std::strong_ordering::equal)</TT> is
probably the right answer.</P>

<P><B>Proposed resolution (October, 2019):</B></P>

<P>Change 11.10.3 [<A href="https://wg21.link/class.spaceship#3">class.spaceship</A>] paragraph 3 as follows:</P>

<BLOCKQUOTE>

...If no such index exists, <TT>V</TT> is
<INS><TT>static_cast&lt;R&gt;(</TT></INS><TT>std::strong_ordering::equal</TT><INS><TT>)</TT></INS>
<DEL>converted to <TT>R</TT></DEL>.

</BLOCKQUOTE>

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