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    CWG Issue 1981</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="1981"></A><H4>1981.
  
Implicit contextual conversions and <TT>explicit</TT>
</H4>
<B>Section: </B>7.3&#160; [<A href="https://wg21.link/conv">conv</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD4
 &#160;&#160;&#160;

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

 <B>Date: </B>2014-08-08<BR>


<P>[Moved to DR at the October, 2015 meeting.]</P>



<P>According to 7.3 [<A href="https://wg21.link/conv#5">conv</A>] paragraph 5,</P>

<BLOCKQUOTE>

Certain language constructs require conversion to a value
having one of a specified set of types appropriate to the
construct. An expression <TT>e</TT> of class type <TT>E</TT>
appearing in such a context is said to be <I>contextually
implicitly converted to</I> a specified type <TT>T</TT> and
is well-formed if and only if <TT>e</TT> can be implicitly
converted to a type <TT>T</TT> that is determined as
follows: <TT>E</TT> is searched for conversion functions
whose return type is <I>cv</I> <TT>T</TT> or reference
to <I>cv</I> <TT>T</TT> such that <TT>T</TT> is allowed by
the context. There shall be exactly one such <TT>T</TT>.

</BLOCKQUOTE>

<P>This description leaves open two questions: first, can
<TT>explicit</TT> conversion functions be used for this
conversion?  Second, assuming that they cannot, is the
restriction to &#8220;exactly one such <TT>T</TT>&#8221;
enforced before or after exclusion of <TT>explicit</TT>
conversion functions?</P>

<P><B>Notes from the November, 2014 meeting:</B></P>

<P>CWG felt that <TT>explicit</TT> conversion functions should
be removed from consideration before determining the set of
types for the conversion.</P>

<P><B>Proposed resolution (May, 2015):</B></P>

<P>Change 7.3 [<A href="https://wg21.link/conv#5">conv</A>] paragraph 5 as follows:</P>

<BLOCKQUOTE>

...An expression <TT>e</TT> of class type <TT>E</TT>
appearing in such a context is said to be <I>contextually
implicitly converted</I> to a specified type <TT>T</TT> and
is well-formed if and only if <TT>e</TT> can be implicitly
converted to a type <TT>T</TT> that is determined as
follows: <TT>E</TT> is searched for <INS>non-explicit</INS>
conversion functions whose return type
is <I>cv</I> <TT>T</TT> or reference to <I>cv</I> <TT>T</TT>
such that <TT>T</TT> is allowed by the context. There shall
be exactly one such <TT>T</TT>.

</BLOCKQUOTE>

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