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    CWG Issue 1188</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="1188"></A><H4>1188.
  
Type punning in constant expressions
</H4>
<B>Section: </B>7.7&#160; [<A href="https://wg21.link/expr.const">expr.const</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++11
 &#160;&#160;&#160;

 <B>Submitter: </B>Jason Merrill
 &#160;&#160;&#160;

 <B>Date: </B>2010-08-31<BR>


<P>[Voted into the WP at the March, 2011 meeting as part of paper N3262.]</P>



<P>The status of the following example is not clear:</P>

<PRE>
    union U { float f; unsigned long u; };

    constexpr U u1 = { 1.0 };
    constexpr unsigned long u2 = u1.u;
</PRE>

<P>This might be ill-formed because the aliasing causes undefined
behavior, which should make the expression not a constant expression.
However, a similar example using a permitted aliasing would
presumably be acceptable:</P>

<PRE>
    union U {
        unsigned char c[sizeof(double)];
        double d;
    };
    constexpr U c1u = { 0x12, 0x34 /* etc. */ };
    constexpr double c1 = c1u.d;
</PRE>

<P>One suggestion was that unions should not be considered literal
types, but some in the ensuing discussion deemed that excessive.  That
also would not address similar examples using
<TT>reinterpret_cast</TT>, which is currently also permitted in
constant expressions.</P>

<P><B>Proposed resolution (November, 2010) [SUPERSEDED]:</B></P>

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

<UL>
<LI><P>an lvalue-to-rvalue conversion...</P></LI>

<LI><P><INS>an lvalue-to-rvalue conversion (7.3.2 [<A href="https://wg21.link/conv.lval">conv.lval</A>])
that is applied to a glvalue that refers to a non-active member of a
union or a subobject thereof;</INS></P></LI>

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

<LI><P>
<DEL>a type conversion from a pointer or pointer-to-member type to a
literal type [<I>Note:</I> a user-defined conversion invokes a function
&#8212;<I>end note</I>]</DEL> <INS>a <TT>reinterpret_cast</TT>
(7.6.1.10 [<A href="https://wg21.link/expr.reinterpret.cast">expr.reinterpret.cast</A>])</INS>;</P></LI>

</UL>

<P><I>[Note: the proposed resolution of <A HREF="1098.html">issue 1098</A>
edits this bullet in an incompatible fashion.]</I></P>

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