So far as I have seen the weight parameter decides the priority of the nodes displayed in the pop-up. But they are not characterized by the socket which is calling them. For eg. If the Vector Socket of any node is dragged out, the pop-up shows all the possible socket connections of all the nodes in no defined manner. In my Opinion it should show all the Vector type sockets first and then all the other sockets. Right now the weights are assigned by :

const int weight = (&socket == main_socket) ? 0 : -1 - i;

which is the same for all the sockets no matter what their type is.

What we can do is we can assign the weights using the "type" of sockets like SOCK_VECTOR, SOCK_FLOAT, etc . We can check if the "type" of socket matches the "type" of calling socket then we assign the weight 0 to them and rest less than 0. For this we need to create a function bool matches_type(const bNodeSocket &socket) in the class SocketDeclaration which will return true if the type of both the socket matches. Something like this:

const int weight = (socket.matches_type(params.other_socket())) ? 0 : -1 - i;

Am I going in the right direction @Hans Goudey (HooglyBoogly) ?

Hmm, I think it would be simpler to just give the Combine XYZ and Combine RGB nodes special implementations of gather_link_search_ops that accounts for this. No need to generalize it further I don't think.

isn't this already done