[sigc] default constructor needed for R in sigc::slot<R, A1, ...>

[Matthias Langer <mlangc gmx at>]

At least for libsigc++-2.0.15 the return type R in sigc::slot<R, ...>
needs to be default constructible. To verify this try

#include <sigc++/slot.h>
using sigc::slot; using sigc::ptr_fun;

class NoDefCtor
{
	public:
	NoDefCtor(int dummy) {}
};

NoDefCtor func() 
{ return NoDefCtor(3); }

int main()
{
	sigc::slot<NoDefCtor> tSlot(ptr_fun(func));
	tSlot();
}

Now, i don't want to say that this is a bug, but i think it would be
nicer if the above code fragment compiles ...

Any comments ?
Matthias