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    CWG Issue 2084</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="2084"></A><H4>2084.
  
NSDMIs and deleted union default constructors
</H4>
<B>Section: </B>11.4.5&#160; [<A href="https://wg21.link/class.ctor">class.ctor</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Daveed Vandevoorde
 &#160;&#160;&#160;

 <B>Date: </B>2015-02-12<BR>


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

<P>According to 11.4.5 [<A href="https://wg21.link/class.ctor#4">class.ctor</A>] paragraph 4 says,</P>

<BLOCKQUOTE>

<P>A defaulted default constructor for class <TT>X</TT> is defined as
deleted if:</P>

<UL>
<LI><P>
<TT>X</TT> is a union-like class that has a
variant member with a non-trivial default
constructor,</P></LI>

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

</UL>

</BLOCKQUOTE>

<P>This should make the following example ill-formed:</P>

<PRE>
  struct S {
    S();
  };
  union U {
    S s{};
  } u;
</PRE>

<P>because the default constructor of <TT>U</TT> is deleted.  However,
both clang and g++ accept this without error.  Should the rule be
relaxed for a union with an NSDMI?</P>

<P><B>Notes from the May, 2015 meeting:</B></P>

<P>An NSDMI is basically syntactic sugar for a
<I>mem-initializer</I>, so the presence of one should be
treated as if a user-declared default constructor were present.</P>

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

<P>Change 11.4.5 [<A href="https://wg21.link/class.ctor#4">class.ctor</A>] paragraph 4 as follows:</P>

<BLOCKQUOTE>

<P>...A defaulted default constructor for class <TT>X</TT> is defined as
deleted if:</P>

<UL>
<LI><P><INS><TT>X</TT> is a union that has a variant
member with a non-trivial default constructor and no variant
member of <TT>X</TT> has a default member
initializer,</INS></P></LI>

<LI><P>
<TT>X</TT> is
a <DEL>union-like</DEL> <INS>non-union</INS> class that has
a variant member <INS><TT>M</TT></INS> with a non-trivial default
constructor <INS>and no variant member of the anonymous union
containing <TT>M</TT> has a default member initializer</INS>,</P></LI>

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

</UL>

</BLOCKQUOTE>

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