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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="138"></A><H4>138.
  
Friend declaration name lookup
</H4>
<B>Section: </B>_N4868_.9.8.2.3&#160; [<A href="https://wg21.link/namespace.memdef">namespace.memdef</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD6
 &#160;&#160;&#160;

 <B>Submitter: </B>Martin von Loewis
 &#160;&#160;&#160;

 <B>Date: </B>14 Jul 1999<BR>



<P>[Accepted at the November, 2020 meeting as part of paper P1787R6 and
moved to DR at the February, 2021 meeting.]</P>



<P>_N4868_.9.8.2.3 [<A href="https://wg21.link/namespace.memdef#3">namespace.memdef</A>] paragraph 3
says,</P>

<BLOCKQUOTE>
If a <TT>friend</TT> declaration in a non-local class first declares a
class or function the friend class or function is a member of the
innermost enclosing namespace...  When looking for a prior declaration
of a class or a function declared as a <TT>friend</TT>, scopes outside
the innermost enclosing namespace scope are not considered.
</BLOCKQUOTE>

It is not clear from this passage how to determine whether an entity
is "first declared" in a <TT>friend</TT> declaration.  One question is
whether a <I>using-declaration</I> influences this determination.
For instance:

<PRE>
    void foo();
    namespace A{
      using ::foo;
      class X{
	friend void foo();
      };
    }
</PRE>

Is the <TT>friend</TT> declaration a reference to <TT>::foo</TT> or
a different <TT>foo</TT>?

<P>Part of the question involves determining the meaning of
the word "synonym" in
9.10 [<A href="https://wg21.link/namespace.udecl#1">namespace.udecl</A>] paragraph 1:
</P>

<BLOCKQUOTE>
A <I>using-declaration</I> introduces a name into the declarative
region in which the <I>using-declaration</I> appears.  That name
is a synonym for the name of some entity declared elsewhere.
</BLOCKQUOTE>

Is "<TT>using ::foo;</TT>" the declaration of a function or not?

<P>More generally, the question is how to describe the lookup of the
name in a <TT>friend</TT> declaration.</P>

<P>
<U>John Spicer</U>:
When a declaration specifies an unqualified name, that
name is declared, not looked up.  There is a mechanism in which that
declaration is linked to a prior declaration, but that mechanism is
not, in my opinion, via normal name lookup.  So, the friend always
declares a member of the nearest namespace scope regardless of how
that name may or may not already be declared there.</P>

<P>
<U>Mike Miller</U>:
6.5.3 [<A href="https://wg21.link/basic.lookup.unqual#7">basic.lookup.unqual</A>] paragraph 7
says:</P>

<BLOCKQUOTE>
A name used in the definition of a class <TT>X</TT> outside of a
member function body or nested class definition shall be declared in
one of the following ways:...  [<I>Note:</I> when looking for a
prior declaration of a class or function introduced by a <TT>friend</TT>
declaration, scopes outside of the innermost enclosing namespace scope
are not considered.]
</BLOCKQUOTE>

The presence of this note certainly implies that this paragraph
describes the lookup of names in <TT>friend</TT> declarations.

<P>
<U>John Spicer</U>:
It most certainly does not.  If that section described the friend
lookup it would yield the incorrect results for the friend declarations
of <TT>f</TT> and <TT>g</TT> below.
I don't know why that note is there, but it can't
be taken to mean that that is how the friend lookup is done.</P>

<PRE>
    void f(){}
    void g(){}
    class B {
        void g();
    };
    class A : public B {
        void f();
        friend void f(); // ::f not A::f
        friend void g(); // ::g not B::g
    };
</PRE>

<P>
<U>Mike Miller</U>:
If so, the lookups for friend functions and classes behave differently.
Consider the example in
6.5.6 [<A href="https://wg21.link/basic.lookup.elab#3">basic.lookup.elab</A>] paragraph 3:
</P>

<PRE>
    struct Base {
        struct Data;         // <I>OK: declares nested</I> Data
        friend class Data;   // <I>OK: nested</I> Data <I>is a friend</I>
    };
</PRE>

<P>If the <TT>friend</TT> declaration is <I>not</I> a reference to
<TT>::foo</TT>, there is a related but separate question: does the
<TT>friend</TT> declaration introduce a conflicting (albeit "invisible")
declaration into namespace <TT>A</TT>, or is it simply a reference
to an as-yet undeclared (and, in this instance, undeclarable)
<TT>A::foo</TT>?  Another part of the example in
6.5.6 [<A href="https://wg21.link/basic.lookup.elab#3">basic.lookup.elab</A>] paragraph 3
is
related:</P>

<PRE>
    struct Data {
        friend struct Glob;  // <I>OK: Refers to (as yet) undeclared</I> Glob
                             // <I>at global scope.</I>
    };
</PRE>

<P>
<U>John Spicer</U>:
You can't refer to something that has not yet been declared.  The friend
is a declaration of <TT>Glob</TT>,
it just happens to declare it in a such a way that
its name cannot be used until it is redeclared.</P>

<P>(A somewhat similar question has been raised in connection with
<A HREF="36.html">issue 36</A>.  Consider:</P>

<PRE>
    namespace N {
        struct S { };
    }
    using N::S;
    struct S;          // legal?
</PRE>

<P>According to 11.3 [<A href="https://wg21.link/class.name#2">class.name</A>] paragraph 2,</P>

<BLOCKQUOTE>

A declaration consisting solely of <I>class-key identifier ;</I>
is either a redeclaration of the name in the current scope or a
forward declaration of the identifier as a class name.

</BLOCKQUOTE>

<P>Should the elaborated type declaration in this example be
considered a redeclaration of <TT>N::S</TT> or an invalid
forward declaration of a different class?)</P>

<P>(See also issues
<A HREF="95.html">95</A>,
<A HREF="136.html">136</A>,
<A HREF="139.html">139</A>,
<A HREF="143.html">143</A>,
<A HREF="165.html">165</A>, and
<A HREF="166.html">166</A>, as well as paper J16/00-0006 = WG21
N1229.)</P>





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