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    CWG Issue 1612</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="1612"></A><H4>1612.
  
Implicit lambda capture and anonymous unions
</H4>
<B>Section: </B>7.5.6.3&#160; [<A href="https://wg21.link/expr.prim.lambda.capture">expr.prim.lambda.capture</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++14
 &#160;&#160;&#160;

 <B>Submitter: </B>Michael Wong
 &#160;&#160;&#160;

 <B>Date: </B>2013-01-31<BR><BR>


<A href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3903.html#CA17">N3690 comment
  CA&#160;17<BR></A>

<P>[Moved to DR at the February, 2014 meeting.]</P>



<P>It is not clear from the description of capturing in
7.5.6 [<A href="https://wg21.link/expr.prim.lambda">expr.prim.lambda</A>] whether an implicit capture resulting
from the odr-use of a member of an anonymous union captures that
member or the anonymous union, and there is implementation
divergence.  For example,</P>

<PRE>
  int main() {
    static int result;
    struct A { int x; };
    struct B { int y; };
    union {
      A a; B b;
    };
    a.x = 1;
    [=]() mutable { a.x = 2; result = b.y; }();
    if (result == 1) return 0;
    throw 0;
  }
</PRE>

<P><B>Notes from the April, 2013 meeting:</B></P>

<P>CWG decided that an attempt to capture a member of an anonymous union
should be ill-formed.</P>

<P><B>Proposed resolution (September, 2013):</B></P>

<OL>
<LI><P>Change 7.5.6 [<A href="https://wg21.link/expr.prim.lambda#15">expr.prim.lambda</A>] paragraph 15 as follows:</P></LI>

<BLOCKQUOTE>

...An array of runtime bound (9.3.4.5 [<A href="https://wg21.link/dcl.array">dcl.array</A>]) <INS>or a member
of an anonymous union</INS> shall not be captured by copy.

</BLOCKQUOTE>

<LI><P>Change 7.5.6 [<A href="https://wg21.link/expr.prim.lambda#16">expr.prim.lambda</A>] paragraph 16 as follows:</P></LI>

<BLOCKQUOTE>

...It is unspecified whether additional unnamed non-static data members are
declared in the closure type for entities captured by
reference. <INS>A member of an anonymous union shall not be captured by
reference.</INS> [<I>Note:</I>...

</BLOCKQUOTE>

</OL>

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