Operator overloading only considers the method name, not which trait is implemented

Currently this code compiles, but it shouldn't:

``` rust
trait MyMul<RHS, Res>
{
    fn mul(&self, rhs: &RHS) -> Res;
}

fn foo<T: MyMul<f64, f64>>(a: &T, b: f64) -> f64
{
    a * b
}

fn main()
{

}
```
