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

2926. Lookup context for dependent qualified names

(From submission #599.)

Consider:

  template <typename U>
  constexpr bool foo = U::b;

  template <class T>
  struct A {
    A() requires(foo<A>) {}
    static constexpr bool b = true;
  };
  A<int> a;

All implementation accept, but there is no normative wording prescribing the lookup context for U::b.

Proposed resolution (approved by CWG 2024-09-13):

Change in 6.5.5.1 [basic.lookup.qual.general] paragraph 3 as follows:

Qualified name lookup in a class, namespace, or enumeration performs a search of the scope associated with it (6.5.2 [class.member.lookup]) except as specified below. Unless otherwise specified, a qualified name undergoes qualified name lookup in its lookup context from the point where it appears unless the lookup context either

• is dependent and is not the current instantiation (13.8.3.2 [temp.dep.type]), in which case the lookup is done from the point of instantiation (13.8.4.1 [temp.point]), or
• is not a class or class template a type that is not a class, in which case unqualified lookup is performed from the point where it appears. [ Note: Such a qualified name can be used to name a destructor (7.5.5.5 [expr.prim.id.dtor]). -- end note ]

If nothing is found by qualified lookup for a member-qualified name that is the terminal name (7.5.5.2 [expr.prim.id.unqual]) of a nested-name-specifier and is not dependent, it undergoes unqualified lookup.

Additional notes (September, 2024)

The proposed wording appears to make the following example unintentionally well-formed:

  int n;
  using T = int;
  int m = T::n;

CWG 2024-11-08

The example above is already ill-formed per 6.5.5.1 [basic.lookup.qual.general] paragraph 1.