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

2596. Instantiation of constrained non-template friends

Consider:

  struct Base {};

  template<int N>
  struct S : public Base {
    friend int foo(Base&) requires (N == 1) { return 1; }
    friend int foo(Base&) requires (N == 2) { return 3; }
  };

  int main() {
    S<1> s1{};
    S<2> s2{};  // #1
  }

The current wording does not seem to cover what happens for this case. In particular, 13.9.2 [temp.inst] paragraph 17 does not cover constrained non-template friends.

See also the Itanium ABI issue 24.

Suggested resolution:

1. Change in 13.7.5 [temp.friend] paragraph 9 as follows:

   A non-template friend declaration with a requires-clause shall be a definition. A friend function template with a constraint that depends on a template parameter from an enclosing template shall be a definition. Such a constrained friend function or function template declaration does not declare the same function or function template as a declaration in inhabiting any other scope.

2. Change in 13.9.2 [temp.inst] paragraph 17 as follows:

   The type-constraints and requires-clause of a template specialization or member templated function are not instantiated along with the specialization or function itself, even for a member function of a local class; substitution into the atomic constraints formed from them is instead performed as specified in 13.5.3 [temp.constr.decl] and 13.5.2.3 [temp.constr.atomic] when determining whether the constraints are satisfied or as specified in 13.5.3 [temp.constr.decl] when comparing declarations.

   [ Note 7: ... ]

   [ Example 10: ... ]

   [ Example:

     struct Base {};
   
     template<int N>
     struct S : Base {
       friend int foo(Base&) requires (N == 1) { return 1; }  // #1
       friend int foo(Base&) requires (N == 2) { return 3; }  // #2
     };
     S<1> s1;
     S<2> s2;          // OK, no conflict between #1 and #2
     int x = foo(s1);  // OK, selects #1
     int y = foo(s2);  // OK, selects #2
   

   -- end example ]

   [ Example 11: ... ]

CWG 2022-11-10

The friend definitions should conflict with friend definitions from other instantiations of the same class template, consistent with how non-constrained friends would work. Note that the enclosing dependent class type does not appear in the friend function's signature, which is unusual.