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    CWG Issue 2079</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="2079"></A><H4>2079.
  
<TT>[[</TT> appearing in a <I>balanced-token-seq</I>
</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>CD4
 &#160;&#160;&#160;

 <B>Submitter: </B>Jonathan Caves
 &#160;&#160;&#160;

 <B>Date: </B>2015-02-03<BR>


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



<P>According to 9.13.1 [<A href="https://wg21.link/dcl.attr.grammar#6">dcl.attr.grammar</A>] paragraph 6,</P>

<BLOCKQUOTE>

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>]

</BLOCKQUOTE>

<P>In order to allow program fragments to appeae within
attributes, this restriction should not apply within the
<I>balanced-token-seq</I> of an attribute.</P>

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

<P>Change 9.13.1 [<A href="https://wg21.link/dcl.attr.grammar#6">dcl.attr.grammar</A>] paragraph 6 as follows:</P>

<BLOCKQUOTE>

<P>Two consecutive left square bracket tokens shall appear only when
introducing an <I>attribute-specifier</I> <INS>or within
the <I>balanced-token-seq</I> of
an <I>attribute-argument-clause</I></INS>. [<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>
</P>

<PRE>
  int p[10];
  void f() {
    int x = 42, y[5];
    int(p[[x] { return x; }()]);  //<SPAN CLASS="cmnt"> error: invalid 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<SPAN CLASS="cmnt">.</SPAN>
    y[[] { return 2; }()] = 2;    //<SPAN CLASS="cmnt"> error even though attributes are not allowed</SPAN>
                                  //<SPAN CLASS="cmnt"> in this context.</SPAN>

<INS>    int i [[vendor::attr([[]])]]; //<SPAN CLASS="cmnt"> well-formed implementation-defined attribute.</SPAN></INS>
  }
</PRE>

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

</BLOCKQUOTE>

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