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    CWG Issue 2634</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="2634"></A><H4>2634.
  
Avoid circularity in specification of scope for friend class declarations
</H4>
<B>Section: </B>9.2.9.5&#160; [<A href="https://wg21.link/dcl.type.elab">dcl.type.elab</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD7
 &#160;&#160;&#160;

 <B>Submitter: </B>Jim X
 &#160;&#160;&#160;

 <B>Date: </B>2022-07-04<BR>


<P>[Accepted as a DR at the March, 2024 meeting.]</P>

<P>Consider:</P>

<PRE>
auto f(struct X* ptr) {
  struct D {
    private:
      int d;
      friend class X;      //<SPAN CLASS="cmnt"> #1</SPAN>
  };
  return D{};
}
X* b = 0;
struct X {
  void show() {
    auto t = f(0);
    t.d = 10;              //<SPAN CLASS="cmnt"> #2 error: </SPAN>::X<SPAN CLASS="cmnt"> is not a friend of </SPAN>f::D
  }
};
</PRE>

<P>The target scope for #2 is <TT>f</TT>'s block scope,
making <TT>::X</TT> not a friend of <TT>f::D</TT>.  Thus the access at
#2 is ill-formed. Clang disagrees.</P>

<P>Subclause 9.2.9.5 [<A href="https://wg21.link/dcl.type.elab#3">dcl.type.elab</A>] paragraph 3 specifies:</P>

<BLOCKQUOTE>

<P>
... If E contains an identifier but
no <I>nested-name-specifier</I> and (unqualified) lookup for the
identifier finds nothing, E shall not be introduced by the enum
keyword and declares the identifier as a <I>class-name</I>. The target
scope of E is the nearest enclosing namespace or block scope.
</P>

<P>If an <I>elaborated-type-specifier</I> appears with the friend
specifier as an entire <I>member-declaration</I>, the
<I>member-declaration</I> shall have one of the following forms:

<pre>
friend <I>class-key</I> <I>nested-name-specifier<sub>opt</sub></I> identifier ;
...
</pre>
Any unqualified lookup for the <I>identifier</I> (in the first case)
does not consider scopes that contain the target scope; no name is
bound.</P>

</BLOCKQUOTE>

<P>This specification is circular in that the target scope that limits
unqualified lookup is defined only if the <I>identifier</I> is
actually declared, but the <I>identifier</I> is declared only if
lookup finds nothing.</P>

<P><B>Proposed resolution (approved by CWG 2023-11-11):</B></P>

<P>Change in 9.2.9.5 [<A href="https://wg21.link/dcl.type.elab#4">dcl.type.elab</A>] paragraph 4 as follows:</P>

<BLOCKQUOTE>

... Any unqualified lookup for the <I>identifier</I> (in the first
case) does not consider scopes that contain the
<DEL>target</DEL> <INS>nearest enclosing namespace or block </INS>
scope; no name is bound. [ Note: ... ]

</BLOCKQUOTE>

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