Consider the following example:

    class B1 {};
    typedef void (B1::*PB1) (); // memptr to B1

    class B2 {};
    typedef void (B2::*PB2) (); // memptr to B2

    class D1 : public B1, public B2 {};
    typedef void (D1::*PD) (); // memptr to D1

    struct S {
         operator PB1(); // can be converted to PD
    } s;
    struct T {
         operator PB2(); // can be converted to PD
    } t;

    void foo() {
         s == t; // Is this an error?
    }

According to 12.5 [over.built] paragraph 16, there is an operator== for PD (“For every pointer to member type...”), so why wouldn't it be used for this comparison?

Mike Miller: The problem, as I understand it, is that 12.2.2.3 [over.match.oper] paragraph 3, bullet 3, sub-bullet 3 is broader than it was intended to be. It says that candidate built-in operators must “accept operand types to which the given operand or operands can be converted according to 12.2.4.2 [over.best.ics].” 12.2.4.2.3 [over.ics.user] describes user-defined conversions as having a second standard conversion sequence, and there is nothing to restrict that second standard conversion sequence.

My initial thought on addressing this would be to say that user-defined conversion sequences whose second standard conversion sequence contains a pointer conversion or a pointer-to-member conversion are not considered when selecting built-in candidate operator functions. They would still be applicable after the hand-off to Clause 5 (e.g., in bringing the operands to their common type, 7.6.10 [expr.eq], or composite pointer type, 7.6.9 [expr.rel]), just not in constructing the list of built-in candidate operator functions.

I started to suggest restricting the second standard conversion sequence to conversions having Promotion or Exact Match rank, but that would exclude the Boolean conversions, which are needed for !, &&, and ||. (It would have also restricted the floating-integral conversions, though, which might be a good idea. They can't be used implicitly, I think, because there would be an ambiguity among all the promoted integral types; however, none of the compilers I tested even tried those conversions because the errors I got were not ambiguities but things like “floating point operands not allowed for %”.)

Bill Gibbons: I recall seeing this problem before, though possibly not in committee discussions. As written this rule makes the set of candidate functions dependent on what classes have been defined, including classes not otherwise required to have been defined in order for "==" to be meaningful. For templates this implies that the set is dependent on what templates have been instantiated, e.g.

  template<class T> class U : public T { };
  U<B1> u;  // changes the set of candidate functions to include
            // operator==(U<B1>,U<B1>)?

There may be other places where the existence of a class definition, or worse, a template instantiation, changes the semantics of an otherwise valid program (e.g. pointer conversions?) but it seems like something to be avoided.

(See also issue 954.)