VS2019 / C++ 17, signal::disconnect compile error: syntax error: 'template'

[johny7]

Using custom functor, Signals fail to compile with:
signal_template.hpp(537,50): error C2059: syntax error: 'template'

Here:

              // check for wrapped extended slot
              bound_extended_slot_function_type *fp;
              fp = (*it)->slot().slot_function().template target<bound_extended_slot_function_type>();

The reason behind is that the signal expects the functor to be the boost::function / std::function, that would contain the target() method.

Minimal example:

struct MyFunctorThisComparer
{
	MyFunctorThisComparer(const void* this_) {}
	bool operator==(const MyFunctorThisComparer& other) const { return true; }
};

struct MyFunctor
{
	typedef void result_type;

	template<typename F>
	void operator=(F) {}
	void operator()() {}

	// disconnect by comparison with custom predicate
	bool operator==(const MyFunctorThisComparer& other) const { return true; }
	operator MyFunctorThisComparer() const { return MyFunctorThisComparer { nullptr }; }
};

void signals_disconnect()
{
	boost::signals2::signal<
		void(), 
		//SignalSupport_::AllCall<typename boost::function_traits<void()>::result_type>,
		boost::signals2::optional_last_value<typename boost::function_traits<void()>::result_type>,
		int,
		std::less<int>,
		MyFunctor > s;

	s.connect( &signals_disconnect );
	s.disconnect( MyFunctorThisComparer(nullptr) );
}

I attempted here to provide the equivalence MyFunctorThisComparer predicate, that would filter out functors. In particular, it would know how to extract 'this' pointer from MyFunctor. However, it could be a more generic predicate.

Boost 1.72.0. However, I have checked the development version - it has the same lines there, so the issue is still present.

[fmhess]

I've never given much thought to allowing for the SlotFunction to be anything other than boost::function or std::function, although I do remember considering dropping the SlotFunction template parameter entirely. Recent changes to support c++20 means operator== won't even be used in the future (it uses the "contains" method instead). I guess it might be possible to get rid of the "target" call and just rely on "contains".