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    CWG Issue 2909</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="2909"></A><H4>2909.
  
Subtle difference between constant-initialized and constexpr
</H4>
<B>Section: </B>7.7&#160; [<A href="https://wg21.link/expr.const">expr.const</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>CWG
 &#160;&#160;&#160;

 <B>Date: </B>2024-06-24<BR>


<P>[Accepted as a DR at the November, 2024 meeting.]</P>

<P>Subclause 7.7 [<A href="https://wg21.link/expr.const#2">expr.const</A>] paragraph 2 defines
"constant-initialized" using the following rule:</P>

<BLOCKQUOTE>
A variable or temporary object o is <I>constant-initialized</I> if
<UL>
<LI>either it has an initializer or its default-initialization results in
some initialization being performed, and</LI>
<LI>...</LI>
</UL>

</BLOCKQUOTE>

<P>However, the rules for <TT>constexpr</TT> are slightly different,
per 9.2.6 [<A href="https://wg21.link/dcl.constexpr#6">dcl.constexpr</A>] paragraph 6:</P>

<BLOCKQUOTE>

A constexpr specifier used in an object declaration declares the
object as const. Such an object shall have literal type and shall be
initialized. ...

</BLOCKQUOTE>

<P>The difference manifests for an example such as:</P>

<PRE>
  struct S {};
  int main() {
    constexpr S s;       //<SPAN CLASS="cmnt"> OK</SPAN>
    constexpr S s2 = s;  //<SPAN CLASS="cmnt"> error: </SPAN>s<SPAN CLASS="cmnt"> is not constant-initialized</SPAN>
  }
</PRE>

<P>Is the difference intentional?</P>

<P><B>Proposed resolution (approved by CWG 2024-10-25):</B></P>

<P>Change in 7.7 [<A href="https://wg21.link/expr.const#2">expr.const</A>] paragraph 2 as follows:</P>

<BLOCKQUOTE>

A variable or temporary object o is constant-initialized if
<UL>
<LI>either it has an initializer or its <DEL>default-initialization
results in some initialization being performed</DEL> <INS>type is
const-default-constructible (9.5.1 [<A href="https://wg21.link/dcl.init.general">dcl.init.general</A>])</INS>,
and</LI>
<LI>...</LI>
</UL>

</BLOCKQUOTE>

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