Title
Conflicting ambiguity rules
Status
cd1
Section
6.5.2 [class.member.lookup]
Submitter
Neal M Gafter

Created on 1998-08-20.00:00:00 last changed 161 months ago

Messages

Date: 2005-04-15.00:00:00

[Voted into WP at April 2005 meeting.]

Date: 2004-03-15.00:00:00

Notes from the March 2004 meeting:

We discussed this further and agreed with these latest recommendations. Mike Miller has produced a paper N1626 that gives just the final collected set of changes.

(This resolution also resolves isssue 306.)

Date: 2003-10-15.00:00:00

Notes from October 2003 meeting:

Mike Miller raised some new issues in N1543, and we adjusted the proposed resolution as indicated in that paper.

Further information from Mike Miller (January 2004):

Unfortunately, I've become aware of a minor glitch in the proposed resolution for issue 39 in N1543, so I'd like to suggest a change that we can discuss in Sydney.

A brief review and background of the problem: the major change we agreed on in Kona was to remove detection of multiple-subobject ambiguity from class lookup (6.5.2 [class.member.lookup]) and instead handle it as part of the class member access expression. It was pointed out in Kona that 11.8.3 [class.access.base]/5 has this effect:

If a class member access operator, including an implicit "this->," is used to access a nonstatic data member or nonstatic member function, the reference is ill-formed if the left operand (considered as a pointer in the "." operator case) cannot be implicitly converted to a pointer to the naming class of the right operand.

After the meeting, however, I realized that this requirement is not sufficient to handle all the cases. Consider, for instance,

    struct B {
        int i;
    };

    struct I1: B { };
    struct I2: B { };

    struct D: I1, I2 {
        void f() {
            i = 0;    // not ill-formed per 11.2p5
        }
    };

Here, both the object expression ("this") and the naming class are "D", so the reference to "i" satisfies the requirement in 11.8.3 [class.access.base]/5, even though it involves a multiple-subobject ambiguity.

In order to address this problem, I proposed in N1543 to add a paragraph following 7.6.1.5 [expr.ref]/4:

If E2 is a non-static data member or a non-static member function, the program is ill-formed if the class of E1 cannot be unambiguously converted (10.2) to the class of which E2 is directly a member.

That's not quite right. It does diagnose the case above as written; however, it breaks the case where qualification is used to circumvent the ambiguity:

    struct D2: I1, I2 {
        void f() {
            I2::i = 0;    // ill-formed per proposal
        }
    };

In my proposed wording, the class of "this" can't be converted to "B" (the qualifier is ignored), so the access is ill-formed. Oops.

I think the following is a correct formulation, so the proposed resolution we discuss in Sydney should contain the following paragraph instead of the one in N1543:

If E2 is a nonstatic data member or a non-static member function, the program is ill-formed if the naming class (11.2) of E2 cannot be unambiguously converted (10.2) to the class of which E2 is directly a member.

This reformulation also has the advantage of pointing readers to 11.8.3 [class.access.base], where the the convertibility requirement from the class of E1 to the naming class is located and which might otherwise be overlooked.

Date: 2002-10-15.00:00:00

Proposed resolution (October 2002):

Replace 6.5.2 [class.member.lookup] paragraph 2 with:

The following steps define the result of name lookup for a member name f in a class scope C.

The lookup set for f in C, called S(f,C), consists of two component sets: the declaration set, a set of members named f; and the subobject set, a set of subobjects where declarations of these members (possibly including using-declarations) were found. In the declaration set, using-declarations are replaced by the members they designate, and type declarations (including injected-class-names) are replaced by the types they designate. S(f,C) is calculated as follows.

If C contains a declaration of the name f, the declaration set contains every declaration of f in C (excluding bases), the subobject set contains C itself, and calculation is complete.

Otherwise, S(f,C) is initially empty. If C has base classes, calculate the lookup set for f in each direct base class subjobject Bi, and merge each such lookup set S(f,Bi) in turn into S(f,C).

The following steps define the result of merging lookup set S(f,Bi) into the intermediate S(f,C):

  • If each of the subobject members of S(f,Bi) is a base class subobject of at least one of the subobject members of S(f,C), S(f,C) is unchanged and the merge is complete. Conversely, if each of the subobject members of S(f,C) is a base class subobject of at least one of the subobject members of S(f,Bi), the new S(f,C) is a copy of S(f,Bi).
  • Otherwise, if the declaration sets of S(f,Bi) and S(f,C) differ, the merge is ambiguous: the new S(f,C) is a lookup set with an invalid declaration set and the union of the subobject sets. In subsequent merges, an invalid declaration set is considered different from any other.
  • Otherwise, consider each declaration d in the set, where d is a member of class A. If d is a nonstatic member, compare the A base class subobjects of the subobject members of S(f,Bi) and S(f,C). If they do not match, the merge is ambiguous, as in the previous step. [Note: It is not necessary to remember which A subobject each member comes from, since using-declarations don't disambiguate. ]
  • Otherwise, the new S(f,C) is a lookup set with the shared set of declarations and the union of the subobject sets.

The result of name lookup for f in C is the declaration set of S(f,C). If it is an invalid set, the program is ill-formed.

[Example:

    struct A { int x; };                    // S(x,A) = {{ A::x }, { A }}
    struct B { float x; };                  // S(x,B) = {{ B::x }, { B }}
    struct C: public A, public B { };       // S(x,C) = { invalid, { A in C, B in C }}
    struct D: public virtual C { };         // S(x,D) = S(x,C)
    struct E: public virtual C { char x; }; // S(x,E) = {{ E::x }, { E }}
    struct F: public D, public E { };       // S(x,F) = S(x,E)

    int main() {
      F f;
      f.x = 0;   // OK, lookup finds { E::x }
    }
S(x,F) is unambiguous because the A and B base subobjects of D are also base subobjects of E, so S(x,D) is discarded in the first merge step. --end example]

Turn 6.5.2 [class.member.lookup] paragraphs 5 and 6 into notes.

Date: 2001-10-15.00:00:00

Notes from 10/01 meeting (Jason Merrill):

The example in the issues list:

    struct A {
        int x(int);
    };
    struct B: A {
        using A::x;
        float x(float);
    };

    int f(B* b) {
        b->x(3);  // ambiguous
    }
Is broken under the existing wording:
... Each of these declarations that was introduced by a using-declaration is considered to be from each sub-object of C that is of the type containing the declaration designated by the using-declaration. If the resulting set of declarations are not all from sub-objects of the same type, or the set has a nonstatic member and includes members from distinct sub-objects, there is an ambiguity and the program is ill-formed.
Since the two x's are considered to be "from" different objects, looking up x produces a set including declarations "from" different objects, and the program is ill-formed. Clearly this is wrong. The problem with the existing wording is that it fails to consider lookup context.

The first proposed solution:

The resulting set of declarations shall all be from sub-objects of the same type, or there shall be a set of declarations from sub-objects of a single type that contains using-declarations for the declarations found in all other sub-object types. Furthermore, for nonstatic members, the resulting set of declarations shall all be from a single sub-object, or there shall be a set of declarations from a single sub-object that contains using-declarations for the declarations found in all other sub-objects. Otherwise, there is an ambiguity and the program is ill-formed.
breaks this testcase:
    struct A { static void f(); };

    struct B1: virtual A {
        using A::f;
    };

    struct B2: virtual A {
        using A::f;
    };

    struct C: B1, B2 { };

    void g() {
        C::f();        // OK, calls A::f()
    }
because it considers the lookup context, but not the definition context; under this definition of "from", the two declarations found are the using-declarations, which are "from" B1 and B2.

The solution is to separate the notions of lookup and definition context. I have taken an algorithmic approach to describing the strategy.

Incidentally, the earlier proposal allows one base to have a superset of the declarations in another base; that was an extension, and my proposal does not do that. One algorithmic benefit of this limitation is to simplify the case of a virtual base being hidden along one arm and not another ("domination"); if we allowed supersets, we would need to remember which subobjects had which declarations, while under the following resolution we need only keep two lists, of subobjects and declarations.

Date: 2001-04-15.00:00:00

Notes from 04/01 meeting:

The following example should be accepted but is rejected by the wording above:

    struct A { static void f(); };

    struct B1: virtual A {
        using A::f;
    };

    struct B2: virtual A {
        using A::f;
    };

    struct C: B1, B2 { };

    void g() {
        C::f();        // OK, calls A::f()
    }
Date: 2000-10-15.00:00:00

Proposed Resolution (10/00):

  1. Replace the two cited sentences from 6.5.2 [class.member.lookup] paragraph 2 with the following:

    The resulting set of declarations shall all be from sub-objects of the same type, or there shall be a set of declarations from sub-objects of a single type that contains using-declarations for the declarations found in all other sub-object types. Furthermore, for nonstatic members, the resulting set of declarations shall all be from a single sub-object, or there shall be a set of declarations from a single sub-object that contains using-declarations for the declarations found in all other sub-objects. Otherwise, there is an ambiguity and the program is ill-formed.
  2. Replace the examples in 6.5.2 [class.member.lookup] paragraph 3 with the following:

        struct A {
            int x(int);
            static int y(int);
        };
        struct V {
            int z(int);
        };
        struct B: A, virtual V {
            using A::x;
            float x(float);
            using A::y;
            static float y(float);
            using V::z;
            float z(float);
        };
        struct C: B, A, virtual V {
        };
    
        void f(C* c) {
            c->x(3);    // ambiguous -- more than one sub-object A
            c->y(3);    // not ambiguous
            c->z(3);    // not ambiguous
        }
    
Date: 2004-09-10.00:00:00

The ambiguity text in 6.5.2 [class.member.lookup] may not say what we intended. It makes the following example ill-formed:

    struct A {
        int x(int);
    };
    struct B: A {
        using A::x;
        float x(float);
    };

    int f(B* b) {
        b->x(3);  // ambiguous
    }
This is a name lookup ambiguity because of 6.5.2 [class.member.lookup] paragraph 2:
... Each of these declarations that was introduced by a using-declaration is considered to be from each sub-object of C that is of the type containing the declaration designated by the using-declaration. If the resulting set of declarations are not all from sub-objects of the same type, or the set has a nonstatic member and includes members from distinct sub-objects, there is an ambiguity and the program is ill-formed.
This contradicts the text and example in paragraph 12 of 9.9 [namespace.udecl] .
History
Date User Action Args
2008-10-05 00:00:00adminsetstatus: wp -> cd1
2005-10-22 00:00:00adminsetstatus: dr -> wp
2005-05-01 00:00:00adminsetmessages: + msg1192
2005-05-01 00:00:00adminsetstatus: ready -> dr
2004-11-07 00:00:00adminsetstatus: review -> ready
2004-04-09 00:00:00adminsetmessages: + msg975
2003-11-15 00:00:00adminsetstatus: ready -> review
2003-04-25 00:00:00adminsetstatus: review -> ready
2002-05-10 00:00:00adminsetmessages: + msg625
2002-03-11 00:00:00adminsetmessages: + msg583
2002-03-11 00:00:00adminsetmessages: + msg582
2001-05-20 00:00:00adminsetmessages: + msg486
2000-11-18 00:00:00adminsetmessages: + msg410
2000-11-18 00:00:00adminsetstatus: drafting -> review
1998-08-20 00:00:00admincreate