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

2510. noexcept-specifier of friend function vs class completeness

According to 11.4.1 [class.mem.general] paragraph 7, a noexcept-specifier is a complete-class context. This raises an issue when the function is a friend function; for example, consider:

  using T = int;
  struct B {
    friend void g(B b) noexcept(sizeof(b.m) >= 4) { }
    T m = T();
  };
  int main() {
    B b;
    g(b);
  }

For friend declarations you need to be able to decide at the point of declaration whether it matches a prior declaration, and you can't do that if you treat the noexcept-specifier as a complete-class context.

There is implementation divergence in the treatment of this example.

Notes from the December, 2021 teleconference:

CWG questioned why the declaration matching couldn't be deferred until the end of the class.

CWG 2022-11-10

CWG believes that, in general, a "when needed" approach to parsing complete-class contexts is superior. In the present case, the existing wording clearly requires that the noexcept-specifier be delayed-parsed, which implies that matching the declaration of a friend function to declarations at namespace scope is also delayed.