initializer_list and argument-dependent lookup

Question

I'm trying to use an std::initializer_list as an argument in a function that uses argument-dependent lookup (ADL). But I don't get it to work and I don't understand why. The following is a minimal failing example:

    #include <initializer_list>
    #include <iostream>

    class Foo {
    public:
      inline friend void bar(std::initializer_list<Foo> v) {
        std::cout << "size = " << v.size() << std::endl;
      }
    };

    void baz(std::initializer_list<Foo> v) {
      std::cout << "size = " << v.size() << std::endl;
    }

    int main(){
      Foo a;
      //bar({a,a});   // error: use of undeclared identifier 'bar'
      baz({a,a});   // works

      return 0;
    }

As seen above, an equivalent global function works just fine. Why does the above not work?

I'm using clang on OS X 10.10.

Answer 1

I believe that the problem is that the subexpression¹{ a, a } does not really have a type, and as such it does not have associated types or namespaces which in turn means that ADL does not kick in. If you have a function in the global namespace, normal lookup will find it, and it will find that { a, a } can match the function call as the initializer for std::initializer_list<Foo>.

¹ The syntax { a, a } is called braced-init-list and it is not really an expression (or subexpression) in the language.