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    CWG Issue 1872</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="1872"></A><H4>1872.
  
Instantiations of <TT>constexpr</TT> templates that cannot appear in constant expressions
</H4>
<B>Section: </B>9.2.6&#160; [<A href="https://wg21.link/dcl.constexpr">dcl.constexpr</A>]
 &#160;&#160;&#160;

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

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

 <B>Date: </B>2014-02-17<BR>


<P>[Adopted at the February, 2016 meeting.]</P>



<P>According to 9.2.6 [<A href="https://wg21.link/dcl.constexpr#6">dcl.constexpr</A>] paragraph 6,</P>

<BLOCKQUOTE>

If the instantiated template specialization of a <TT>constexpr</TT>
function template or member function of a class template would fail to
satisfy the requirements for a <TT>constexpr</TT> function
or <TT>constexpr</TT> constructor, that specialization is still
a <TT>constexpr</TT> function or <TT>constexpr</TT> constructor, even
though a call to such a function cannot appear in a constant expression.

</BLOCKQUOTE>

<P>The restriction on appearing in a constant expression assumes the
previous wording that made such a specialization non-<TT>constexpr</TT>,
and a call to a non-<TT>constexpr</TT> function cannot appear in a
constant expression.  With the current wording, however, there is no
normative restriction against calls to such specializations.
7.7 [<A href="https://wg21.link/expr.const">expr.const</A>] should be updated to include such a
prohibition.</P>

<P><B>Proposed resolution (January, 2016):</B></P>

<P>Add the following bullet following 7.7 [<A href="https://wg21.link/expr.const">expr.const</A>]
bulllet 2.3:</P>

<BLOCKQUOTE>

<P>A <I>conditional-expression</I> e is a <I>core constant
expression</I> unless the evaluation of <TT>e</TT>, following the
rules of the abstract machine (6.10.1 [<A href="https://wg21.link/intro.execution">intro.execution</A>]),
would evaluate one of the following expressions:</P>

<UL>
<LI><P>...</P></LI>

<LI><P>an invocation of an undefined <TT>constexpr</TT> function or an
undefined <TT>constexpr</TT> constructor;</P></LI>

<LI><P><INS>an invocation of an instantiated <TT>constexpr</TT>
function or <TT>constexpr</TT> constructor that fails to
satisfy the requirements for a <TT>constexpr</TT> function or
<TT>constexpr</TT> constructor
(9.2.6 [<A href="https://wg21.link/dcl.constexpr">dcl.constexpr</A>]);</INS></P></LI>

<LI><P>...</P></LI>

</UL>

</BLOCKQUOTE>

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