Trait object incorrect method selection bug

gistfile1.rs

trait VisitorBase<T> {
    fn visit(&mut self, node: &T);
}

trait Visitor : VisitorBase<Num> + VisitorBase<Add> {}
impl<T> Visitor for T where T: VisitorBase<Num> + VisitorBase<Add> {}

trait Node {
    fn accept(&self, visitor: &mut Visitor );
}

struct Add {
    lhs: Box<Node>,
    rhs: Box<Node>
}

impl Node for Add {
    fn accept(&self, visitor: &mut Visitor) {
        println!("accept Add");
        visitor.visit(self);
    }
}

struct Num {
    val: i32
}

impl Node for Num {
    fn accept(&self, visitor: &mut Visitor) {
        println!("accept Num");
        visitor.visit(self);
    }
}

struct EvalVisitor {
    result: i32
}

impl EvalVisitor {
    fn new() -> EvalVisitor {
        EvalVisitor{result:0}
    }
}

impl VisitorBase<Add> for EvalVisitor {
    fn visit(&mut self, node: &Add) {
        println!("visit Add");
        node.lhs.accept(self);
        let temp = self.result;
        node.rhs.accept(self);
        println!("lhs: {} rhs: {}", temp, self.result);
        self.result = temp + self.result;
    }
}

impl VisitorBase<Num> for EvalVisitor {
    fn visit(&mut self, node: &Num) {
        println!("visit Num");
        println!("val {}", node.val);
        self.result = node.val;
    }
}

fn main() {
    let top = Num{val:5};
    let mut visitor = EvalVisitor::new();
    top.accept(&mut visitor);
    let res = visitor.result;

    println!("value: {}", res);
}