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    CWG Issue 1952</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="1952"></A><H4>1952.
  
Constant expressions and library undefined behavior
</H4>
<B>Section: </B>7.7&#160; [<A href="https://wg21.link/expr.const">expr.const</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Daniel Kr&#252;gler
 &#160;&#160;&#160;

 <B>Date: </B>2014-06-22<BR>


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



<P>According to bullet 2.5 of 7.7 [<A href="https://wg21.link/expr.const">expr.const</A>],</P>

<BLOCKQUOTE>

<P>A <I>conditional-expression</I> <TT>e</TT> is a core constant
expression 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 operation that would have undefined behavior
[<I>Note:</I> including, for example, signed integer
overflow (Clause 7 [<A href="https://wg21.link/expr">expr</A>]), certain pointer
arithmetic (7.6.6 [<A href="https://wg21.link/expr.add">expr.add</A>]), division by zero
(7.6.5 [<A href="https://wg21.link/expr.mul">expr.mul</A>]), or certain shift operations
(7.6.7 [<A href="https://wg21.link/expr.shift">expr.shift</A>]) &#8212;<I>end note</I>];</P>
</LI>

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

</UL>

</BLOCKQUOTE>

<P>The definition of &#8220;operation&#8221; is unclear.  In
particular, is it intended to include use of library components
that are specified to produce undefined behavior, such as use of
the <TT>offsetof</TT> macro when applied to a non-standard-layout
class?</P>

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

<OL>
<LI><P>Change 7.7 [<A href="https://wg21.link/expr.const#2.5">expr.const</A>] bullet 2.5 as follows:</P></LI>

<UL><LI><P>an operation that would have undefined behavior
<INS>as specified in Clauses Clause 4 [<A href="https://wg21.link/intro">intro</A>] through
Clause 15 [<A href="https://wg21.link/cpp">cpp</A>] of this International Standard</INS>
[<I>Note:</I> including, for example, signed integer overflow (
Clause 7 [<A href="https://wg21.link/expr">expr</A>]), certain pointer arithmetic
(7.6.6 [<A href="https://wg21.link/expr.add">expr.add</A>]), division by zero
(7.6.5 [<A href="https://wg21.link/expr.mul">expr.mul</A>]), or certain shift operations
(7.6.7 [<A href="https://wg21.link/expr.shift">expr.shift</A>]) &#8212;<I>end note</I>];</P></LI></UL>

<LI><P>Add the following at the end of 7.7 [<A href="https://wg21.link/expr.const#2">expr.const</A>] paragraph 2:
</P></LI>

<BLOCKQUOTE>

<INS>If <TT>e</TT> satisfies the constraints of a core constant
expression, but evaluation of <TT>e</TT> would evaluate an operation
that has undefined behavior as specified in Clauses
Clause 16 [<A href="https://wg21.link/library">library</A>] through Clause 32 [<A href="https://wg21.link/thread">thread</A>] of this
International Standard, it is unspecified whether <TT>e</TT> is a
core constant expression.</INS>

</BLOCKQUOTE>

</OL>

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