[Voted into WP at April, 2007 meeting.]

Proposed Resolution (October, 2006):

1. Replace the normative text in 6.5.4 [basic.lookup.argdep] paragraph 3 with the following (leaving the text of the note and example unchanged):

   Let X be the lookup set produced by unqualified lookup (6.5.3 [basic.lookup.unqual]) and let Y be the lookup set produced by argument dependent lookup (defined as follows). If X contains

   • a declaration of a class member, or
   • a block-scope function declaration that is not a using-declaration, or
   • a declaration that is neither a function nor a function template

   then Y is empty. Otherwise Y is the set of declarations found in the namespaces associated with the argument types as described below. The set of declarations found by the lookup of the name is the union of X and Y.

2. Change 6.5.3 [basic.lookup.unqual] paragraph 4 as indicated:

   When considering an associated namespace, the lookup is the same as the lookup performed when the associated namespace is used as a qualifier (6.5.5.3 [namespace.qual]) except that:

   • Any using-directives in the associated namespace are ignored.
   • Any namespace-scope friend functions or friend function templates declared in associated classes are visible within their respective namespaces even if they are not visible during an ordinary lookup (11.8.4 [class.friend]).
   • All names except those of (possibly overloaded) functions and function templates are ignored.

Additional Notes (September, 2006):

Recent discussion of this issue has emphasized the following points:

1. The concept of finding function pointers and function objects as part of argument-dependent lookup is not currently under active discussion in the Evolution Working Group.

2. The major area of concern with argument-dependent lookup is finding functions in unintended namespaces. There are current proposals to deal with this concern either by changing the definition of “associated namespace” so that fewer namespaces are considered or to provide a mechanism for enabling or disabling ADL altogether. Although this concern is conceptually distinct from the question of whether ADL finds function pointers and function objects, it is related in the sense that the current rules are perceived as finding too many functions (because of searching too many namespaces), and allowing function pointers and function objects would also increase the number of entities found by ADL.

3. Any expansion of ADL to include function pointers and function objects must necessarily update the overloading rules to specify how they interact with functions and function templates in the overload set. Current implementation experience (g++) is not helpful in making this decision because, although it performs a uniform lookup and finds non-function entities, it diagnoses an error in overload resolution if non-function entities are in the overload set.

4. There is a possible problem if types are found by ADL: it is not clear that overloading between callable entities (functions, function templates, function pointers, and function objects) and types (where the postfix syntax means a cast or construction of a temporary) is reasonable or useful.

James Widman:

There is a larger debate here about whether ADL should find object names; the proposed wording below is only intended to answer the request for wording to clarify the status quo (option 1 above) and not to suggest the outcome of the larger debate.

Notes from the 10/01 meeting:

There was unanimous agreement on one less controversial point: if the normal lookup of the identifier finds a non-function, argument-dependent lookup should not be done.

On the larger issue, the primary point of consensus is that making this change is an extension, and therefore it should wait until the point at which we are considering extensions (which could be very soon). There was also consensus on the fact that the standard as it stands is not clear: some introductory text suggests that argument-dependent lookup finds only functions, but the more detailed text that describes the lookup does not have any such restriction.

It was also noted that some existing implementations (e.g., g++) do find some non-functions in some cases.

The issue at this point is whether we should (1) make a small change to make the standard clear (presumably in the direction of not finding the non-functions in the lookup), and revisit the issue later as an extension, or (2) leave the standard alone for now and make any changes only as part of considering the extension. A straw vote favored option (1) by a strong majority.

Notes from 04/00 meeting:

Although many agreed that there were valid concerns motivating a desire for Koenig lookup to find non-function declarations, there was also concern that supporting this capability would be more dangerous than helpful in the absence of overload resolution for mixed function and non-function declarations.

A straw poll of the group revealed 8 in favor of Koenig lookup finding functions and function templates only, while 3 supported the broader result.

The original intent of the Committee when Koenig lookup was added to the language was apparently something like the following:

1. The name in the function call expression is looked up like any other unqualified name.
2. If the ordinary unqualified lookup finds nothing or finds the declaration of a (non-member) function, function template, or overload set, argument-dependent lookup is done and any functions found in associated namespaces are added to the result of the ordinary lookup.

This approach is not reflected in the current wording of the Standard. Instead, the following appears to be the status quo:

1. Lookup of an unqualified name used as the postfix-expression in the function call syntax always performs Koenig lookup (6.5.3 [basic.lookup.unqual] paragraph 3).
2. Unless ordinary lookup finds a class member function, the result of Koenig lookup always includes the declarations found in associated namespaces (6.5.4 [basic.lookup.argdep] paragraph 2), regardless of whether ordinary lookup finds a declaration and, if so, what kind of entity is found.
3. The declarations from associated namespaces are not limited to functions and template functions by anything in 6.5.4 [basic.lookup.argdep]. However, if Koenig lookup results in more than one declaration and at least one of the declarations is a non-function, the program is ill-formed (9.8.4 [namespace.udir], paragraph 4; although this restriction is in the description of the using-directive, the wording applies to any lookup that spans namespaces).

John Spicer: Argument-dependent lookup was created to solve the problem of looking up function names within templates where you don't know which namespace to use because it may depend on the template argument types (and was then expanded to permit use in nontemplates). The original intent only concerned functions. The safest and simplest change is to simply clarify the existing wording to that effect.

Bill Gibbons: I see no reason why non-function declarations should not be found. It would take a special rule to exclude "function objects", as well as pointers to functions, from consideration. There is no such rule in the standard and I see no need for one.

There is also a problem with the wording in 6.5.4 [basic.lookup.argdep] paragraph 2:

If the ordinary unqualified lookup of the name finds the declaration of a class member function, the associated namespaces and classes are not considered.

This implies that if the ordinary lookup of the name finds the declaration of a data member which is a pointer to function or function object, argument-dependent lookup is still done.

My guess is that this is a mistake based on the incorrect assumption that finding any member other than a member function would be an error. I would just change "class member function" to "class member" in the quoted sentence.

Mike Miller: In light of the issue of "short-circuiting" Koenig lookup when normal lookup finds a non-function, perhaps it should be written as "...finds the declaration of a class member, an object, or a reference, the associated namespaces..."?

Andy Koenig: I think I have to weigh in on the side of extending argument-dependent lookup to include function objects and pointers to functions. I am particularly concerned about [function objects], because I think that programmers should be able to replace functions by function objects without changing the behavior of their programs in fundamental ways.

Bjarne Stroustrup: I don't think we could seriously argue from first principles that [argument-dependent lookup should find only function declarations]. In general, C++ name lookup is designed to be independent of type: First we find the name(s), then, we consider its(their) meaning. 6.5 [basic.lookup] states "The name lookup rules apply uniformly to all names ..." That is an important principle.

Thus, I consider text that speaks of "function call" instead of plain "call" or "application of ()" in the context of koenig lookup an accident of history. I find it hard to understand how 7.6.1.3 [expr.call] doesn't either disallow all occurrences of x(y) where x is a class object (that's clearly not intended) or requires koenig lookup for x independently of its type (by reference from 6.5 [basic.lookup]). I suspect that a clarification of 7.6.1.3 [expr.call] to mention function objects is in order. If the left-hand operand of () is a name, it should be looked up using koenig lookup.

John Spicer: This approach causes otherwise well-formed programs to be ill-formed, and it does so by making names visible that might be completely unknown to the author of the program. Using-directives already do this, but argument-dependent lookup is different. You only get names from using-directives if you actually use using-directives. You get names from argument-dependent lookup whether you want them or not.

This basically breaks an important reason for having namespaces. You are not supposed to need any knowledge of the names used by a namespace.

But this example breaks if argument-dependent lookup finds non-functions and if the translation unit includes the <list> header somewhere.

    namespace my_ns {
        struct A {};
        void list(std::ostream&, A&);

        void f() {
            my_ns::A a;
            list(cout, a);
        }
    }

This really makes namespaces of questionable value if you still need to avoid using the same name as an entity in another namespace to avoid problems like this.

Erwin Unruh: Before we really decide on this topic, we should have more analysis on the impact on programs. I would also like to see a paper on the possibility to overload functions with function surrogates (no, I won't write one). Since such an extension is bound to wait until the next official update, we should not preclude any outcome of the discussion.

I would like to have a change right now, which leaves open several outcomes later. I would like to say that:

Koenig lookup will find non-functions as well. If it finds a variable, the program is ill-formed. If the primary lookup finds a variable, Koenig lookup is done. If the result contains both functions and variables, the program is ill-formed. [Note: A future standard will assign semantics to such a program.]

I myself are not comfortable with this as a long-time result, but it prepares the ground for any of the following long term solutions:

• Do overloading on mixed function/variable sets.
• Ignore variables on Koenig lookup.
• Don't do Koenig lookup if the primary lookup finds a variable.
• Find variables on Koenig lookup and give an error if there is a variable/function mix.

The note is there to prevent compiler vendors to put their own extensions in here.

(See also issues 113 and 143.)