This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-04-18

2557. Class member access referring to an unrelated class

Consider:

  struct A {
    static int x;
  };

  struct B {
    using type = A;
  };

  int y = B().type::x;

There seems to be no requirement that the member named in a class member access actually is a member of the class of the object expression. Subclause 7.5.4.1 [expr.prim.id.general] paragraph 3 does not cover static members:

An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

• as part of a class member access in which the object expression refers to the member's class or a class derived from that class, or
• ...

Suggested resolution:

1. Change in 7.6.1.5 [expr.ref] paragraph 4 as follows:

   Otherwise, the object expression shall be of class type. The class type shall be complete unless the class member access appears in the definition of that class.
   [Note: The program is ill-formed if the result differs from that when the class is complete (6.5.2 [class.member.lookup]). —end note]
   [Note: 6.5.5 [basic.lookup.qual] describes how names are looked up after the . and -> operators. —end note] If E2 is a qualified-id, the terminal name of its nested-name-specifier shall denote the type of E1 or a base class thereof.

   [Example:

     struct A {
       static int x;
     };
   
     struct B {
       static int x;
     };
   
     struct D : B {
       using type = A;
     };
   
     int y1 = D().B::x;         // OK, B is a base class of D
     int y2 = D().type::x;      // error: A is not a base class of D
     int y3 = D::type::x;       // OK, evaluates A::x
   

   —end example ]

2. Change in 7.6.1.5 [expr.ref] bullet 6.5 as follows:

   • ...
   • If E2 is a member an enumerator and the type of E2 is T, the expression E1.E2 is a prvalue. The type of E1.E2 is T.

     [Example:

       enum E { e };
       struct X {
         using E::e;
       };
       int f(X x) {
         return x.e;
       }
     

     —end example ]

3. Change in 7.5.4.1 [expr.prim.id.general] paragraph 3 as follows:

   An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

   • as part of a class member access (7.6.1.5 [expr.ref]) in which the object expression refers to the member's class [ Footnote: ... ] or a class derived from that class, or
   • to form a pointer to member (7.6.2.2 [expr.unary.op]), or
   • if that id-expression denotes a non-static data member and it appears in an unevaluated operand.