This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

386. Friend declaration of name brought in by using-declaration

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

The following came up recently on comp.lang.c++.moderated (edited for brevity):

  namespace N1 {
    template<typename T> void f( T* x ) {
      // ... other stuff ...
      delete x;
    }
  }

  namespace N2 {
    using N1::f;

    template<> void f<int>( int* ); // A: ill-formed

    class Test {
      ~Test() { }
      friend void f<>( Test* x );   // B: ill-formed?
    };
  }

I strongly suspect, but don't have standardese to prove, that the friend declaration in line B is ill-formed. Can someone show me the text that allows or disallows line B?

Here's my reasoning: Writing "using" to pull the name into namespace N2 merely allows code in N2 to use the name in a call without qualification (per 9.9 [namespace.udecl]). But just as declaring a specialization must be done in the namespace where the template really lives (hence line A is ill-formed), I suspect that declaring a specialization as a friend must likewise be done using the original namespace name, not obliquely through a "using". I see nothing in 9.9 [namespace.udecl] that would permit this use. Is there?

Andrey Tarasevich: 13.7.5 [temp.friend] paragraph 2 seems to get pretty close: "A friend declaration that is not a template declaration and in which the name of the friend is an unqualified 'template-id' shall refer to a specialization of a function template declared in the nearest enclosing namespace scope".

Herb Sutter: OK, thanks. Then the question in this is the word "declared" -- in particular, we already know we cannot declare a specialization of a template in any other namespace but the original one.

John Spicer: This seems like a simple question, but it isn't.

First of all, I don't think the standard comments on this usage one way or the other.

A similar example using a namespace qualified name is ill-formed based on 9.3.4 [dcl.meaning] paragraph 1:

  namespace N1 {
        void f();
  }

  namespace N2 {
        using N1::f;
        class A {
                friend void N2::f();
        };
  }

Core issue 138 deals with this example:

  void foo();
  namespace A{
    using ::foo;
    class X{
      friend void foo();
    };
  }

The proposed resolution (not yet approved) for issue 138 is that the friend declares a new foo that conflicts with the using-declaration and results in an error.

Your example is different than this though because the presence of the explicit argument list means that this is not declaring a new f but is instead using a previously declared f.

One reservation I have about allowing the example is the desire to have consistent rules for all of the "declaration like" uses of template functions. Issue 275 (in DR status) addresses the issue of unqualified names in explicit instantiation and explicit specialization declarations. It requires that such declarations refer to templates from the namespace containing the explicit instantiation or explicit specialization. I believe this rule is necessary for those directives but is not really required for friend declarations -- but there is the consistency issue.

Notes from April 2003 meeting:

This is related to issue 138. John Spicer is supposed to update his paper on this topic. This is a new case not covered in that paper. We agreed that the B line should be allowed.