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

2634. Avoid circularity in specification of scope for friend class declarations

[Accepted as a DR at the March, 2024 meeting.]

Consider:

auto f(struct X* ptr) {
  struct D {
    private:
      int d;
      friend class X;      // #1
  };
  return D{};
}
X* b = 0;
struct X {
  void show() {
    auto t = f(0);
    t.d = 10;              // #2 error: ::X is not a friend of f::D
  }
};

The target scope for #2 is f's block scope, making ::X not a friend of f::D. Thus the access at #2 is ill-formed. Clang disagrees.

Subclause 9.2.9.5 [dcl.type.elab] paragraph 3 specifies:

... If E contains an identifier but no nested-name-specifier and (unqualified) lookup for the identifier finds nothing, E shall not be introduced by the enum keyword and declares the identifier as a class-name. The target scope of E is the nearest enclosing namespace or block scope.

If an elaborated-type-specifier appears with the friend specifier as an entire member-declaration, the member-declaration shall have one of the following forms:

friend class-key nested-name-specifieropt identifier ;
...

Any unqualified lookup for the identifier (in the first case) does not consider scopes that contain the target scope; no name is bound.

This specification is circular in that the target scope that limits unqualified lookup is defined only if the identifier is actually declared, but the identifier is declared only if lookup finds nothing.

Proposed resolution (approved by CWG 2023-11-11):

Change in 9.2.9.5 [dcl.type.elab] paragraph 4 as follows:

... Any unqualified lookup for the identifier (in the first case) does not consider scopes that contain the target nearest enclosing namespace or block scope; no name is bound. [ Note: ... ]