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    CWG Issue 968</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="968"></A><H4>968.
  
Syntactic ambiguity of the attribute notation
</H4>
<B>Section: </B>9.13.1&#160; [<A href="https://wg21.link/dcl.attr.grammar">dcl.attr.grammar</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD2
 &#160;&#160;&#160;

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

 <B>Date: </B>23 September, 2009<BR>


<P>[Voted into WP at March, 2010 meeting as document N3063.]</P>

<P>The <TT>[[ ... ]]</TT> notation for attributes was thought to be
completely unambiguous.  However, it turns out that two <TT>[</TT>
characters can be adjacent and not be an attribute-introducer: the
first could be the beginning of an array bound or subscript operator
and the second could be the beginning of a <I>lambda-introducer</I>.
This needs to be explored and addressed.</P>

<P>(See also <A HREF="951.html">issue 951</A>.)</P>

<P><B>Proposed resolution (November, 2009):</B></P>

<P>Add the following paragraph at the end of 9.3.3 [<A href="https://wg21.link/dcl.ambig.res">dcl.ambig.res</A>]:</P>

<BLOCKQUOTE>

<P><INS>Two consecutive left square bracket tokens shall appear
only when introducing an <I>attribute-specifier</I>.
[<I>Note:</I> If two consecutive left square brackets appear
where an <I>attribute-specifier</I> is not allowed, the program is
ill-formed even if the brackets match an alternative grammar
production. &#8212;<I>end note</I>] [<I>Example:</I></INS></P>

<PRE>
<INS>  int p[10];
  void f() {
    int x = 42;
    int(p[[x]{return x;}()]);  //<SPAN CLASS="cmnt"> Error: malformed attribute on a nested</SPAN>
                               //<SPAN CLASS="cmnt"> declarator-id and not a function-style cast of</SPAN>
                               //<SPAN CLASS="cmnt"> an element of </SPAN>p.
    new int[[]{return x;}()];  //<SPAN CLASS="cmnt"> Error even though attributes are not allowed</SPAN>
  }                            //<SPAN CLASS="cmnt"> in this context.</SPAN></INS>
</PRE>

<P><INS>&#8212;<I>end example</I>]</INS></P>

</BLOCKQUOTE>

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