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    CWG Issue 2464</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="2464"></A><H4>2464.
  
Constexpr launder and unions
</H4>
<B>Section: </B>17.6.5&#160; [<A href="https://wg21.link/ptr.launder">ptr.launder</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD6
 &#160;&#160;&#160;

 <B>Submitter: </B>Hubert Tong
 &#160;&#160;&#160;

 <B>Date: </B>2020-11-07<BR>




<P>According to 17.6.5 [<A href="https://wg21.link/ptr.launder">ptr.launder</A>], referring to
<TT>std::launder</TT>,</P>

<BLOCKQUOTE>

An invocation of this function may be used in a core
constant expression whenever the value of its argument may
be used in a core constant expression.

</BLOCKQUOTE>

<P>It is not clear whether this wording is intended to
permit an example like</P>

<PRE>
  #include &lt;new&gt;

  struct A { char x; };

  union U {
    A a;
    A a2;
  };

  constexpr A foo() {
    U u = {{42}};
    A *ap = &amp;u.a2;
    return *std::launder(ap);
  }

  extern constexpr A globA = foo();
</PRE>

<P>In particular, is the wording intended to restrict use of
<TT>std::launder</TT> in a constant expression to cases in
which the function returns its argument unchanged? As a
further example, consider</P>

<PRE>
  #include &lt;new&gt;

  struct A { char x; };
  struct B { A a; };
  struct BytesAndMore {
    unsigned char bytes[sizeof(A)];
    unsigned char more;
  };

  union U {
    BytesAndMore bytes;
    A a;
    B b;
  };

  constexpr B foo() {
    U u;
    A *ap = &amp;u.a;
    B *bp = &amp;u.b;
    u.bytes.more = 0;
    std::launder(ap)-&gt;x = 42;
    return *std::launder(bp);
  }

  extern constexpr B globB = foo();
</PRE>

<P><B>Notes from the December, 2020 teleconference:</B></P>

<P>See also
<A HREF="https://cplusplus.github.io/LWG/issue3495">LWG issue 3495</A>.</P>

<BR><BR>
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