namespace detail {
  struct A {
    constexpr A () { }
    friend constexpr int adl_flag (A);
  };

  template<class Tag>
  struct writer {
    friend constexpr int adl_flag (Tag) {
      return 0;
    }
  };
}

template<class Tag, int = adl_flag (Tag {})>
constexpr bool is_flag_usable (int) {
  return true;
}

template<class Tag>
constexpr bool is_flag_usable (...) {
  return false;
}

template<bool B, class Tag = detail::A>
struct dependent_writer : detail::writer<Tag> { };

template<
  class Tag = detail::A,
  bool    B = is_flag_usable<Tag> (0),
  int       = sizeof (dependent_writer<B>)
>
constexpr int f () {
  return B;
}

int main () {
  constexpr int a = f ();
  constexpr int b = f ();

  static_assert (a != b, "fail");
}