From test_lambda.cc:

  // auto-disconnect
  //
  // The functor returns std::ostream&, but the compiler dislikes both
  // slot<std::ostream>, slot<std::ostream&> and slot<std::ostream*>.
  // std::ostream can be implicitly converted to bool, because it has either
  // operator void*() (before C++11) or operator bool() (since C++11).
  sigc::slot<bool> sl1;
  {
    book guest_book("karl");
    sl1 = (sigc::var(result_stream) << sigc::ref(guest_book) << sigc::var("\n"));
    sl1();
    util->check_result(result_stream, "karl\n");
  } // auto-disconnect
  sl1(); // :-)
  util->check_result(result_stream, "");

basic_ios::operator bool is an explicit conversion operator, so it requires an explicit conversion.  See http://en.cppreference.com/w/cpp/language/explicit and examine the cases of nb1 and nb2 (or Stroustrup §18.4.2).

2>  test_lambda.cc
2>E:\src\git\libsigc++2\sigc++/functors/slot.h(103): error C2440: 'return' : cannot convert from 'std::ostream' to 'bool'
2>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Without thinking about this much (still haven't had my coffee), adding static_cast<T_return>() to slot.h will get rid of the error, but I have no idea what side effects, if any, this may have.

  static T_return call_it(slot_rep* rep)
    {
      typedef typed_slot_rep<T_functor> typed_slot;
      typed_slot *typed_rep = static_cast<typed_slot*>(rep);
      return static_cast<T_return>((typed_rep->functor_)());
    }