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

136. Default arguments and friend declarations

[Moved to DR at 10/01 meeting.]

9.3.4.7 [dcl.fct.default] paragraph 4 says,

For non-template functions, default arguments can be added in later declarations of a function in the same scope. Declarations in different scopes have completely distinct sets of default arguments. That is, declarations in inner scopes do not acquire default arguments from declarations in outer scopes, and vice versa.

    void f(int, int, int=0);             // #1
    class C {
        friend void f(int, int=0, int);  // #2
    };
    void f(int=0, int, int);             // #3

There are several related questions involved with this issue:

1. Is the friend declaration in the scope of class C or in the surrounding namespace scope?

   Mike Miller: 9.3.4.7 [dcl.fct.default] paragraph 4 is speaking about the lexical location of the declaration... The friend declaration occurs in a different declarative region from the declaration at #1, so I would read [this paragraph] as saying that it starts out with a clean slate of default arguments.

   Bill Gibbons: Yes. It occurs in a different region, although it declares a name in the same region (i.e. a redeclaration). This is the same as with local externs and is intended to work the same way. We decided that local extern declarations cannot add (beyond the enclosing block) new default arguments, and the same should apply to friend declarations.

   John Spicer: The question is whether [this paragraph] does (or should) mean declarations that appear in the same lexical scope or declarations that declare names in the same scope. In my opinion, it really needs to be the latter. It seems somewhat paradoxical to say that a friend declaration declares a function in namespace scope yet the declaration in the class still has its own attributes. To make that work I think you'd have to make friends more like block externs that really do introduce a name into the scope in which the declaration is contained.

2. Should default arguments be permitted in friend function declarations, and what effect should they have?

   Bill Gibbons: In the absence of a declaration visible in class scope to which they could be attached, default arguments on friend declarations do not make sense. [They should be] ill-formed, to prevent surprises.

   John Spicer: It is important that the following case work correctly:

           class X {
                   friend void f(X x, int i = 1){}
           };
   
           int main()
           {
                   X x;
                   f(x);
           }
   

   In other words, a function first declared in a friend declaration must be permitted to have default arguments and those default arguments must be usable when the function is found by argument dependent lookup. The reason that this is important is that it is common practice to define functions in friend declarations in templates, and that definition is the only place where the default arguments can be specified.

3. What restrictions should be placed on default argument usage with friend declarations?

   John Spicer: We want to avoid instantiation side effects. IMO, the way to do this would be to prohibit a friend declaration from providing default arguments if a declaration of that function is already visible. Once a function has had a default specified in a friend declaration it should not be possible to add defaults in another declaration be it a friend or normal declaration.

   Mike Miller: The position that seems most reasonable to me is to allow default arguments in friend declarations to be used in Koenig lookup, but to say that they are completely unrelated to default arguments in declarations in the surrounding scope; and to forbid use of a default argument in a call if more than one declaration in the overload set has such a default, as in the proposed resolution for issue 1.

Notes from 10/99 meeting:

Four possible outcomes were identified:

1. If a friend declaration declares a default parameter, allow no other declarations of that function in the translation unit.
2. Same as preceding, but only allow the friend declaration if it is also a definition.
3. Disallow default arguments in friend declarations.
4. Treat the default arguments in each friend declaration as a distinct set, causing an error if the call would be ambiguous.

The core group eliminated the first and fourth options from consideration, but split fairly evenly between the remaining two.

A straw poll of the full committee yielded the following results (given as number favoring/could live with/"over my dead body"):

1. 0/14/5
2. 8/13/5
3. 11/7/14
4. 7/10/9

Additional discussion is recorded in the "Record of Discussion" for the meeting, J16/99-0036 = WG21 N1212. See also paper J16/00-0040 = WG21 N1263.

Proposed resolution (10/00):

In 9.3.4.7 [dcl.fct.default], add following paragraph 4:

If a friend declaration specifies a default argument expression, that declaration must be a definition and shall be the only declaration of the function or function template in the translation unit.