Proposed resolution (approved by CWG 2024-10-11):

1. Change in 12.2.4.2.5 [over.ics.ref] paragraph 1 as follows:

   When a parameter of type “reference to cvT” binds directly (9.4.4 [dcl.init.ref]) to an argument expression:

   • If the argument expression has a type that is a derived class of the parameter type, the implicit conversion sequence is a derived-to-base conversion (12.2.4.2 [over.best.ics]).
   • Otherwise, if T is a function type, or if the type of the argument is possibly cv-qualified T, or if T is an array type of unknown bound with element type U and the argument has an array type of known bound whose element type is possibly cv-qualified U, the implicit conversion sequence is the identity conversion. [Note 1: When T is a function type, the type of the argument can differ only by the presence of noexcept. —end note]
   • Otherwise, if T is a function type, the implicit conversion sequence is a function pointer conversion.
   • Otherwise, the implicit conversion sequence is a qualification conversion.

   [Example 1:

     struct A {};
     struct B : public A {} b;
     int f(A&);
     int f(B&);
     int i = f(b);    // calls f(B&), an exact match, rather than f(A&), a conversion
   

      void g() noexcept;
      int h(void (&)() noexcept); // #1
      int h(void (&)());          // #2
      int j = h(g);               // calls #1, an exact match, rather than #2, a function pointer conversion
   

   —end example]

2. Change in 12.2.4.3 [over.ics.rank] bullet 3.2.6 as follows:

     int f(const int &);
     int f(int &);
     int g(const int &);
     int g(int);
     int i;
     int j = f(i);   // calls f(int &)
     int k = g(i);   // ambiguous
   
     struct X {
       void f() const;
       void f();
     };
     void g(const X& a, X b) {
       a.f();     // calls X::f() const
       b.f();     // calls X::f()
     }
   
     int h1(int (&)[]);
     int h1(int (&)[1]);
     int h2(void (&)());
     int h2(void (&)() noexcept);
     void g2() {
       int a[1];
       h1(a);
       extern void f2() noexcept;
       h2(f2);
     }
   

CWG 2024-06-26

Binding a reference to a function should not be considered an identity conversion if it strips a non-throwing exception specification. This amendment removes the ambiguity for the first example and makes the second example ambiguous, which is desirable.

Consider:

  void f() noexcept {}

  void g(void (*)() noexcept) {}
  void g(void (&)()) {}

  int main() {
    g(f);     // error: ambiguous
  }

In contrast:

  void f() noexcept {}

  void g(void (*)()) {} 
  void g(void (&)()) {}      // #1

  int main() {
    g(f);    // OK, calls #1
  }

In both cases, binding void(&)() to void() noexcept is considered an identity conversion, without further disambiguation by 12.2.4.3 [over.ics.rank].