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    CWG Issue 2104</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="2104"></A><H4>2104.
  
Internal-linkage <TT>constexpr</TT> references and ODR requirements
</H4>
<B>Section: </B>6.3&#160; [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>James Widman
 &#160;&#160;&#160;

 <B>Date: </B>2015-03-17<BR>


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



<P>In an example like:</P>

<PRE>
  extern int i;
  namespace {
    constexpr int&amp; r = i;
  }
  inline int f() { return r; }
</PRE>

<P>use of <TT>f()</TT> in multiple translation units results in an
ODR violation because of use of the internal-linkage reference
<TT>r</TT>.  It would be helpful if 6.3 [<A href="https://wg21.link/basic.def.odr#6">basic.def.odr</A>] paragraph 6
could be amended to &#8220;look through&#8221; a
<TT>constexpr</TT> reference in determining whether an inline
function violates the ODR or not.</P>

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

<P>Change 6.3 [<A href="https://wg21.link/basic.def.odr#6.2">basic.def.odr</A>] bullet 6.2 as follows,
dividing the running text into a bulleted list:</P>

<BLOCKQUOTE>

<P>...Given such an entity named <TT>D</TT> defined in more than one
translation unit, then</P>

<UL>
<LI><P>each definition of <TT>D</TT> shall consist of
the same sequence of tokens; and</P></LI>

<LI><P>in each definition of <TT>D</TT>, corresponding
names, looked up according to 6.5 [<A href="https://wg21.link/basic.lookup">basic.lookup</A>],
shall refer to an entity defined within the definition
of <TT>D</TT>, or shall refer to the same entity, after
overload resolution (12.2 [<A href="https://wg21.link/over.match">over.match</A>]) and after
matching of partial template specialization
(13.10.4 [<A href="https://wg21.link/temp.over">temp.over</A>]), except that a name can refer
to</P></LI>

<UL>
<LI><P>a non-volatile const object with internal or no
linkage if the object</P></LI>

<UL>
<LI><P>has the same literal type in all definitions of
<TT>D</TT>, <DEL>and</DEL>
</P></LI>

<LI><P>
<DEL>the object</DEL> is initialized with a constant
expression (7.7 [<A href="https://wg21.link/expr.const">expr.const</A>]), <DEL>and</DEL>
</P></LI>

<LI><P>
<DEL>the object</DEL> is not odr-used, and</P></LI>

<LI><P>
<DEL>the object</DEL> has the same value in all
definitions of <TT>D</TT><INS>,</INS>
</P></LI>

</UL>

<P><INS>or</INS></P>

<LI><P>
<INS>a reference with internal or no linkage
initialized with a constant expression such that the reference
refers to the same entity in all definitions of <TT>D</TT></INS>;</P></LI>

</UL>

<P>and</P>

<LI><P>in each definition of <TT>D</TT>, corresponding
entities...</P></LI>

</UL>

</BLOCKQUOTE>

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